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Life Cycle Assessment of district heat production ina straw fired CHP plant

Ranjan Parajuli a,*, Søren Løkke b, Poul Alberg Østergaard b,Marie Trydeman Knudsen a, Jannick H. Schmidt b, Tommy Dalgaard a

a Department of Agroecology, Aarhus University, Blichers All�e 20, DK-8830, Tjele, Denmarkb Department of Development and Planning, Aalborg University, Vestre Havnepromenade 9, DK-9000, Aalborg,

Denmark

a r t i c l e i n f o

Article history:

Received 12 November 2013

Received in revised form

31 May 2014

Accepted 6 June 2014

Available online

Keywords:

Life Cycle Assessment

Straw

CHP

Boiler

Environmental performances

* Corresponding author. Tel.: þ45 71606831.E-mail addresses: ranjan.parajuli@agrsci.

http://dx.doi.org/10.1016/j.biombioe.2014.06.0961-9534/© 2014 Elsevier Ltd. All rights rese

a b s t r a c t

Due to concerns about the sustainability of the energy sector, conversion of biomass to

energy is increasing its hold globally. Life Cycle Impact Assessment (LCIA) is being adopted

as an analytical tool to assess the environmental impacts in the entire cycle of biomass

production and conversions to different products. This study deals with the LCIA of straw

conversion to district heat in a Combined Heat and Power (CHP) plant and in a district

heating boiler (producing heat only). Environmental impact categories are Global Warming

Potential (GWP), Acidification Potential (AP), aquatic and terrestrial Eutrophication Poten-

tial (EP) and Non-Renewable Energy (NRE) use. In the case of CHP, the co-produced elec-

tricity is assumed to displace the marginal Danish electricity mix. The current study

showed that straw fired in the CHP plant would lead to a GWP of �187 g CO2-eq, AP

0.01 m2 UES (un-protected ecosystem), aquatic EP 0.16 g NO3-eq, terrestrial EP 0.008 m2 UES,

and NRE use �0.14 MJ-primary per 1 MJ heat production. Straw conversion to heat in the

CHP plant showed better environmental performances compared to the district heating

boiler. Furthermore, removing straw from the field is related to the consequence e.g.

decline in soil carbon sequestration, limiting soil nutrient availability, and when compared

with natural gas the conversion of straw to heat would lead to a higher aquatic and

terrestrial EP and AP. The study also outlays spaces for the detail sustainability assessment

of straw conversion in a biorefinery and compare with the current study.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The predictions on the global energy demand have drawn

attentions to a wider diversification of biomass and bioenergy

use [1]. Political concerns that articulate about energy secu-

rity, global climate change issues, land and water manage-

ment and rural income are among the factors driving for the

dk, parajuliranjan@yahoo005rved.

maximized use of biomass in the society [2]. Since production

of agricultural biomass is related to fulfill the multi-fold de-

mands including in food, fuel and feed sectors, it is essential

to assess a wider environmental and economic sustainability

(e.g. land use impact, soil nutrient loss, biodiversity loss) [3,4]

when prioritizing them for any specific purposes.

Manure, grass and lignocellulosic biomass (e.g. wood,

straw) [5,6] are among the most popular sources of biomass in

.com (R. Parajuli).

Abbreviations

AP Acidification Potential

C Carbon

CHP Combined Heat and Power

CoNG Combustion with Natural Gas

CoWS Combustion with Wheat Straw

DM Dry Matter

EP Eutrophication Potential

GHG Greenhouse Gas

GWP Global Warming Potential

LCA Life Cycle Assessment

LCI Life Cycle Inventory

LCIA Life Cycle Impact Assessment

MJ Mega Joule

Mha Million hectatre

Mt Million ton

m2 UES Square Meter of Un-protected Ecosystem

m2-a Square Meter-annual

N Nitrogen

N2eN Nitrogen Emission from Nitrogen Fertilizer

Application

NH3eN AmmmonicaleNitrogen

NO3eN NitrateeNitrogen

N2OeN Nitrous OxideeNitrogen

NOxeN Nitrogen Oxide as NO2

NRE Non-Renewable Energy

tDM Ton of Dry Matter

PJ Peta Joule

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4116

the European Union [7], particularly in the context of

achieving the EU 2020 goals targeting to upscale the renewable

energy by 20% by the 2020 [8]. Denmark is the EU pioneer

country for the use of straw as a fuel source, being the result of

dedicated policies started from the first oil crisis of 1973 and

onwards [9,10]. Today Denmark uses approximately 1.8

Million ton (Mt) of straw each year for energy [11]. In the

Danish agricultural system, cereals cover majority of the

agricultural area and straw is dominantly based on these

crops, in particular wheat [12]. The Danish agricultural sta-

tistics shows that of the total area covered by cereals in 2011,

i.e. 1.48 million hectare (Mha), the area covered by wheat was

about 50%. Including other cereals, total straw productionwas

from an area of about 0.96 Mha, with a productivity of about

3.29 ton per hectare (t/ha) [12] making an annual Danish straw

production of nearly 3.3 Mt.

In general, Denmark has relatively limited availability of

renewable energy sources, of which one of the most impor-

tant wind power inherently is of a fluctuating nature [13,14],

and thus needs to be supplemented by fuel-based technolo-

gies [15] in order to ensure that the energy system has the

required load-following capability. This might be very rele-

vant in light of Danish future sustainable energy goals, which

emphasizes integration of wind power in a larger scale to

implement a greater use of renewable energy. In the context,

it is also relevant to address the underlying issues in relation

to the demand side management [13], in the operational

management of existing CHP plants and other district heat

production facilities. District heating represents about 11% of

the gross energy consumption of the country in 2011 and 36%

of the gross energy consumption of the household sector [16].

In 2012, of the total district heat production (136 PJ), 60% was

produced from CHP plants alone (including both large and

small scale), 23% was from district heating boiler, and rest by

auto producers [16], illustrating that CHP plant has a signifi-

cant impact on the sustainability of Danish energy system

[10]. The CHP plant referred in the current study indicates the

production of both heat and electricity simultaneously, and

the district heating boiler represents production of heat only.

Biomass, in particular straw and wood has a larger share in

the total district heat production of the country, almost

covering 39% of the total production in 2012. The analysis of

the Danish Ministry of Climate and Energy [17], as well as in

independent analyses from the Danish Society of Engineers

[18], and in studies including those of Refs [19e21] have also

stressed on the prudent utilization of this finite resource to

meet the country's long term sustainability goal of energy

sector.

Compared to other biomass sources, some of the strong

advantages of straw include: its minimum competition with

food and feed industries and lower land use change impacts

[22]. Regardless of these advantages, there are also some de-

bates particularly related to the wider environmental sus-

tainability, including consequences connected to their

removal from an agriculture field [23e25]. Consequences

include mainly a decline in soil carbon pools with the subse-

quent effect on the soil's water holding capacity, structure

etc., a loss of soil carbon sequestration potential [25], and a

loss of availability of nutrients from straw to the soil [26]. In

order to have a proper basis for decision-making on the large-

scale use of straw for energy purposes, it is thus important to

assess the entire sets of impacts from the biomass generation

to their utilization. This might also bring attention towards

the perspectives, such as (i) what is the optimal use of biomass

for energy and the environmental impacts associated with

them and (ii) what could be the basis for choosing alternative

means of biomass conversions, besides heat and power pro-

duction, etc. In order to work with such perspectives the

concept of Life Cycle Assessment (LCA) is relevant. LCA is a

tool for the assessment of environmental loadings of entire

life cycle processes related to a production system, covering

all the processes, activities and resources used [27]. In the

current study, Life Cycle Impact Assessment (LCIA) of straw

conversion to district heat is discussed. Since, wheat is the

most common crop and the most important source for both

grain and straw in Denmark [28], the source of straw consid-

ered is wheat. The important aspects that have been dis-

cussed in the current paper are: assessment of related

environmental consequences of straw removal, assessment

of time based biomass decay process and their relation to the

soil carbon (C) and Nitrogen (N) build-up. In most of the LCA

studies, when considering the soil C sequestration potential

(related to crop production system and land management),

the time horizon used is mostly less than 100 years. Petersen

et al. [29] suggested to account for the soil carbon changes also

considering the partial carbon budget for individual crops and

combining it with the biomass decay process, which often

changes with the time perspectives on the release of CO2 from

Fig. 1 e System boundary the reference flow of wheat straw for the CHP plant (use left) and for the Boiler use (use right).

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 117

the soil to the atmosphere. In the current paper, while

assessing the consequences of straw removal such aspects

have been accounted for, and are discussed in Section 2.3.1.

Regarding the conversion of biomass, two distinct cases of

straw conversion to district heat are discussed: (i) conversion

in the CHP plant and (ii) in the boiler (producing heat only).

Generally the district heating network is primarily supplied by

CHP plants, which are supplemented by thermal storage and

peak-load boilers [30]. Thus it is relevant to assess the envi-

ronmental loadings of the district heating boiler and compare

how they differ with the CHP plant. Generally literature

available in such pathways of biomass conversion, also

considering the consequential behavior of straw removal is

very limited. Furthermore, the study also compares straw

with natural gas as a fuel alternative to produce district heat

in the CHP plant. Finally research perspectives are drawn in

relation to the opportunities of reducing the assessed impacts

and optimizing the biomass conversion efficiency, e.g. in a

biorefinery. The aim of this study is to assess consequences of

straw removal as an alternative fuel to produce district heat in

a CHP plant and a district heating boiler.

2. Material and methods

2.1. Scope, functional unit and LCA method

The functional unit of the LCIA is 1 MJ of heat production, and

the impact categories are: Global Warming Potential (GWP),

Acidification Potential (AP), aquatic and terrestrial Eutrophica-

tion Potentials (EPs), and Non-Renewable Energy (NRE) use.

Units for the selected impact categories, unless otherwise

stated in the text are: GWP (g CO2-eq), AP (m2 UES), EP-aquatic

(gNO3-eq), EP-terrestrial (m2 UES) andNRE use (MJ-primary) per

functional unit. In the case AP and EP (terrestrial), m2 UES is the

site-generic as well as the site-dependent acidification and

terrestrial Eutrophication Potentials of an emission from a

functional unit, which is expressed as the area of ecosystem

within the full deposition area and brought to exceed the crit-

ical load of acidification and eutrophication as consequences of

emission [31,32]. The critical load in the case of terrestrial

eutrophication indicates the availability of excess amount of

nutrients in the ecosystems leading to a change of the species

composition and hence to an unwanted change in the char-

acter of the given ecosystem [32]. In the case of calculating the

EP (aquatic), it is assumed that impact of nitrate leaching is on

the lake ecosystem. Even thoughmost Danish lakes are already

regarded highly eutrophic [33], but with a defined aim to reduce

the present eutrophication status.

The “Stepwise2006” method [34,35] is used to assess the

environmental impacts for the selected impact categories.

GWP factors of methane (CH4), and di-nitrogen monoxide

(N2O) found in the method are adjusted to 25 and 298

respectively, as per IPCC [36], corresponding to a time horizon

of 100 years. The method is implemented in the PC tool

SimaPro 7.3.3 [37].

2.2. Approach of the assessment

With regard to formulation of LCIA model, system expansion

approach is used. In this approach, we have looked into the

effects and influences of a particular product in the overall

Fig. 2 e Decay profile of straw left in field.

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4118

system. The general idea is that such product(s) could play a

significant role in displacing the environmental impacts of

similar kind of product available in the market. In the current

study, it is considered that the conversion of straw in the CHP

plant leads to production of heat and electricity, and the co-

produced electricity from the plant is assumed to displace

the marginal Danish electricity mix and thus the related

environmental impacts. It is not possible in the case of the

district heating boiler because of the absence of co-production

of electricity. In the current study, the composition of mar-

ginal Danish electricity mix is based on Schmidt and Brandao

[38]. The methodology to estimate the mix is elaborated in

Schmidt et al. [39], which revealed that it was built considering

the future possible outlook of Danish electricity supplies

based on the difference between 2008 and 2020.

The idea behind the consideration of the system expansion

approach is that it allows to assess the changes in the envi-

ronmental consequences with the changes in the demand of

the marginal product [40]. Furthermore, as Schmidt [41]

highlighted, the system expansion approach is also useful to

assess and compare the environmental consequences be-

tween two situations of the material flow, such as; ‘a product

being at a place’ and ‘product being removed’. In the current

study, such cases are (i) straw incorporated into the soil, or,

removed for the energy purpose, (ii) CHP operated to produce

both heat and electricity: the electricity displacing the mar-

ginal supply and (iii) recycling nutrient content of slag back to

field displacing the equivalent amount of mineral fertilizers

(see Section 2.3.3).

2.3. System boundary, process descriptions and basicassumptions

The reference flow of the biomass is 1 ton (t), with 85% dry

matter (DM), which is subjected to produce the district heat

and further analyzed in relation to the functional unit. Straw

harvested from the field is regarded as mostly dried, with dry

matter (DM) content over 85% [42,43]. The system boundary

(Fig. 1) comprises of energy conversion stages: (i) straw

removal, (ii) collection and pre-processing, (iii) combustion

and (iv) management of fly ash and bottom ash (slag), in both

cases of heat production. It is assumed that heat is the main

product and electricity as a co-product of the CHP plant, while

particularly in northern and western European countries

electricity is sold as an additional product [44,45]. Other out-

puts of the system are slag and fly ash, the former is consid-

ered to be reused as fertilizer because of its nutrient value, and

the latter is supposed to be disposed in landfill sites because of

the presence of heavy metals [46,47]. Energy conversion pro-

cesses and basic assumptions employed in this study are

discussed in the Sections 2.3.1e2.3.3.

2.3.1. Straw removalWhen identifying the effects of removing straw, following

questions are crucial:

i. How does the removal of straw limit the soil C sequestra-

tion potential compared to the situation if alternatively

straw is incorporated into the soil in each second cropping

cycle?

ii. How it will have an impact on the availability of soil nu-

trients, and in turn what are the consequences associated

with it?

When crop residues are used as biomass for energy, the

effect is that the carbon content of the straw is emitted as CO2

instantaneously, while a long decay time in the soil causing

the same CO2 emissions but stretched over a longer period of

time is avoided. The decay function for straw can be estab-

lished using different models, e.g. the ‘C-tool’ [48] and the

‘ROTHC-26.3’ [49], and have been used for straw in Denmark,

in Petersen et al. [29] and Schmidt and Brandao [38] respec-

tively. The decay function for straw in Danish climatic con-

ditions based on Ref. [29] is illustrated in Fig. 2. The climatic

effect of affecting the timing of CO2 emission was modeled

using time weighted GWP. In common practice when calcu-

lating carbon footprints (and LCAs), no distinction is made

between different timings of emissions. The IPCC Global

Warming Potentials (GWPs) [36] are normally used for

expressing the relative importance of different GHG-

emissions. Most often (or always) this is done relative to

CO2. The GWP of a GHG emission is calculated based on the

Table 1 e Environmental consequences of straw removal at farm gate.

Description Unit Amount Comments

Reference flow t 1 1 t (85% DM)

Emission to air (kg CO2-eq) kg �1331 (¼Soil C sequestration lossa,b e CO2 from,

avoiding residue decay process)c 100 years

perspectives ¼ 143e1474 kg CO2

Additional fertilizer (inputs)

Calcium Ammonium Nitrate (N)-application kg 1.53 30%*FSOMd ¼ 1.53 kg ¼ FSN (see text)

Triple Superphosphate (P2O5)-application kg 1.75 Estimated P valuee ¼ 0.765 kg

Potassium Chloride (K2O)-application kg 15.36 Estimated K valuef ¼ 12.75 kg

Emissions

(a) Calculation steps for N2O(Direct)eN emissions

N2OeN from extra N application as a fertilizer kg 0.0153 0.01*FSN [54]. See equation (i)

N2OeN from crop residues kg �0.051 0.01*FSOMd [54]. See equation (i)

(b) Calculation steps for N2O(Indirect)eN emissions

NH3eN from added fertilizer-N application kg 0.0306 0.02g*FSN [55,57]

NOxeN from additional fertilizer-N application kg 0.0107 0.007*FSN [58]

N2eN from additional fertilizer-N application kg 0.0719 0.047*FSN [58]

NO3eN from additional fertilizer (potential

changes in the leaching due to straw removal)

kg 0.299 Calculated also considering the changes in

the leaching due to straw removal. See text belowh

Indirect N2OeN kg 0.0026 0.0075*NO3eN þ 0.01*(NH3eN þ NOxeN) [54]. See text belowj

Assumptions:a Composition of straw (85% DM); C ¼ 47.3%, N ¼ 0.6%, P ¼ 0.09%, K ¼ 1.5% [51].b Soil C sequestration ¼ C content in straw*0.85*emission reduction potential ¼ 47.3%*1 t*0.85*9.7% ¼ 38.99 kg C ¼ 143 kg CO2-eq.c CO2 from avoiding residue decay ¼ 85% DM in straw*47.3% C in straw*44/12 ¼ 1474 kg CO2/t straw.d FSOM ¼ Total N output in the straw removed ¼ 0.6%*1 t straw*0.85 ¼ 5.1 kg. See equation (ii).e P2O5 ¼ kg of P in the crop residue (1 t straw, 85% DM)*(Ratio of mol. wt) ¼ 0.09%*1 t straw*0.85*(142/62).f K2O ¼ kg of K in 1 t of straw (85% DM)*(Ratio of mol. wt) ¼ 1.5%*1 (kg)*0.85*(94/78).g Emission factors for total NH3 emissions from soils due to N fertilizer volatilization ¼ 0.02, based on EEA [55].h NO3eN (from additional fertilizer)¼ Additional fertilizer (as N application) e FSOM eN2eN from additional fertilizer-N application e (eN2OeN

from crop residues e N build-up in soili) ¼ 1.53e5.1 e 0.0719 e (�0.051e3.89). Calculation based on Ref. [53] and see equations (ii) and (iii).i N build-up in soil ¼ Soil C build-up*(1/10) ¼ 3.89 kg N.j Indirect N2OeN ¼ 0.0075*0.299 þ 0.01*(0.0306 þ 0.0107). See equations (ii) and (iii).

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 119

decay rate of CO2 and associated radiative forcing over a

period (time horizon) against which the cumulative radiative

forcing of that GHG emission over the same period is calcu-

lated. The same procedure can be used for calculating the

GWP of emitting CO2 in a given year relative to year zero. This

is referred to as time weighted GWP. The principle is also

described in Refs. [29,38,50].

Petersen et al. [29] calculated the emission reduction po-

tential of straw retention in field as 19.8% and 21.3% of the

total C content in the crop residue in a 20 year perspective for

the Danish loamy sand and sandy loam soil respectively.

Likewise, the emission reduction potential in a 100 years

perspective is 9.7% of the total C content in the crop residue

for the sandy loam soil. In a similar way Schmidt and Brandao

[38] have calculated that the emission reduction potential as

16.7% and 2.9% in a 20 year and 100 year perspectives

respectively for average soils. The difference in the values of

emission reduction potential as such presented in Petersen

et al. [29] and Schmidt and Brandao [38] is due to use of

different models for the estimation of crop residue decay,

which show different decay rates.

In the current study to calculate the soil C sequestration

potential, a 100-year time perspective based on Petersen et al.

[29] is used. The C content of the straw is assumed to be 47.3%

per tDM [51]. 1 t (85% DM) straw contains 402 kg C which

corresponds to 1474 kg CO2. When this straw is removed, it

causes avoided sequestration, which is calculated as 9.7% of

soil Cminus 1474 kg CO2 leading to�1331 kg CO2-eq (see Table

1). When straw is combusted it causes biogenic CO2 emissions

and is equivalent to 1474 kg (see Table 2).

In addition to this, the straw removal process also limits

the process of building-up the organic nitrogen (N) content,

which is assumed to take place with the ratio of 1:10 of the

carbon [52]. The soil C build-up over a 20-year time perspective

in the case of loamy and soil is calculated to be 79.6 kg C (19.8%

*0.85 tDM*47.3% C/tDM) and the resulting soil N build-up is

equivalent to 7.9 kg N. In 100-years soil C build-up is 38.9 kg C

(9.7%*0.85 tDM*47.3% C/tDM) resulting to the soil N build-up

equivalent to 3.89 kg N. The straw removal process hence re-

stricts these phenomena that are suitable to maintain the soil

health. However, its availability also leads to changes in the

leaching of nitrate to the aquatic ecosystem. We have

considered these effects, while discussing about the conse-

quences of the straw removal (Table 1).

Similarly, as a consequence of the straw removal process,

additional fertilizer should be applied to compensate the

nutrient that would have been available from residue if is

incorporated into the soil [38,53]. The equivalent amount of

fertilizers is calculated based on the elemental composition of

the straw in the form of Nitrogen (N), Phosphorous (P) and

Table 2 e LCI of district heat production with straw fired in the CHP plant and boiler.

Process Unit Amount Comments/remarks LCI data

Collection and pre-processing

Inputs

Amount of straw t 1

Baling and handlinga MJ 61 Based on Dalgaard et al. [59]

Chopping straw at power plantb MJ 6.46 Based on Dalgaard et al. [59]

Transport (to power plant) tkm 170 0.85 tDM*200 km (Lorry (>32 t) [65]

Outputs t 1 1 t (85% DM) at Power plant

Combustion (1 t, 85% DM)

Inputs

Straw, 85% DM t 1 LHV ¼ 14.5 GJ/t

Heat (own product)c MJ 40 Assumed to be used from the system

CHP: Electricity is used from the system

Electricityc kWh 110 Boiler: assumed to be externally feed-in (for marginal [65]

Danish electricity mix based on Schmidt et al. [39])

Outputs (CHP)

Heat MJ 8700 Net heat outputc ¼ gross heat output-heat

Net heat output MJ 8660

Electricity kWh 1006 Net electricity outputc ¼ gross electricity output-electricity input

Net electricity output kWh 897

Outputs (Boiler)

Heat MJ 12,325 Net heat outputc ¼ gross heat output-heat input

Net heat output MJ 12,285

Net straw input (DM)

CHP t 0.995 Estimated based on the net heat output

Boiler t 0.996

Bottom ash (slag) recyclingd 86% of the bottom ash collectable [53]

CHP kg 53.73

Boiler kg 53.78

Transport to proper sites, by truck Lorry (>16 ton) for 200 km

CHP tkm 10.74 [65]

Boiler tkm 10.75

Nutriente Fertilizer value

CHP [65]

P fertilizer value kg 0.779

K fertilizer value kg 8.59

Boiler

P fertilizer value kg 0.779

K fertilizer value kg 8.60

Fly ash disposal in landfills Estimated based on the fly ash deposits per tDM [64] [65]

CHP kg 8.25

Boiler kg 8.26

Direct emissions (fuel input) (g) Total emissions calculated based on heat

content of net fuel input for the CHP and boiler

CHP

CO2 (biogenic)f kg 1474

CH4 g 7.21

N2O g 20.2

SO2 g 677.74

NOx g 1889

HCl g 663.32

Assumptions:a Estimated diesel consumption for baling and handling ¼ 2 lt�1 [59]. LHV of diesel ¼ 35.9 MJ l�1.b Estimated diesel consumption for chopping 1 t of straw ¼ 0.18 l.c Heat and electricity required during combustion process per tDM [64]. Electricity input in case of the boiler is assumed as marginal electricity

mix of Denmark [39,65].d Bottom ash per 1 tDM of wheat straw ¼ 54 kg [64].e Nutrient value ¼ total bottom ash*P and K content in the ash.f For biogenic CO2 emission, see Table 1.

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4120

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 121

Potassium (K) [51]. It is further assumed that about 30% of the N

in straw is available to crop, as N is immobilized instead of

being mineralized at least for first few years [53]. In contrast, it

is assumed that 100% of the fertilizer value available in the

form of P and K in the straw is available to the crop [53]. The P

and K values are further transformed to P2O5 and K2O, by fac-

torizing with the ratio of their molecular weight (see Table 1).

The resulting emissions related to the straw removal pro-

cess are calculated with the aid of following equations,

adapted from IPCC [54] Tier 2 methodology:

(i) Direct N2OeN: It is calculated for N-fertilizer application

and from the crop residues (see equation (i)). In Table 1 of

the current study, N2OeN (from additional N-fertilizer

application) and N2OeN (avoided N2OeN from crop residue

removal) are separately calculated.

N2O�NðDirectÞ ¼ ðFSN þ FSOMÞ*EF1 (i)

where,

N2OeN(Direct) ¼ Direct N2OeN emissions produced from

managed soils, kg N2OeN/yr.

FSN ¼ synthetic fertilizer N applied to soils ¼ additional N-

fertilizer application, kg N/yr.

FSOM ¼ Amount of N in mineral soils that is mineralized

(related to a loss of soil C from soil organic matter as a

result of changes to land use or management, kg N/

yr) ¼ Total N in straw (in the current study).

EF1 ¼ Emission factor for additional N-fertilizer applica-

tion, crop residues, and Nmineralized frommineral soil as

a result of loss of soil carbon [kg N2OeN (kgN)�1]¼ 0.01 [54].

(ii) Indirect N2OeN

N2O�NðLeachÞ ¼ ðFSN þ FSOMÞ*FracLEACH�ðHÞ*EF5 (ii)

where,

N2OeN(Leach) ¼ Amount of N2OeN produced from leaching

and runoff of additional N-fertilizer application (kg N2OeN/

yr) to a managed soils in regions where leaching/runoff

occurs.

FracLEACH-(H) ¼ fraction of all N added to/mineralised in

managed soils in regions where leaching/runoff occurs

that is lost through leaching and runoff, kg N per kg of

additional N-fertilizer application ¼ 0.1 [54]. The potential

changes in the leaching due to straw removal process is

shown in Table 1 and discussed below.

EF5¼ emission factor forN2Oemissions fromN leaching and

runoff, kg N2OeN per kg N leached and runoff ¼ 0.0075 [54].

(iii) N2OeN from atmospheric deposition of N-volatilized

from managed soil:

N2OðATDÞ �N ¼ ðFSN*FracGASFi*EF4Þ (iii)

where,

N2O(ATD)eN ¼ Amount of N2OeN produced from atmo-

spheric deposition of N volatilized from managed soils,

kg N2OeN/yr.

FracGASFi ¼ fraction of synthetic fertilizer N-application (i.e.

FSN) that volatilizes as NH3 and NOx, kg N volatilized per kg

of additional N-fertilizer application under different con-

ditions i, ¼ 0.02 [55]. EF4 ¼ emission factor for N2O emis-

sions from atmospheric deposition of N on soils and water

surfaces, [kg NeN2O per (kg NH3eN þ NOxeN

volatilized)] ¼ 0.01 [54].

Nitrate leaching [56] is calculated considering the amount

of nitrogeneous fertilizer applied, and is based on the equation

(ii) [54]. In Table 1, NO3eN represents the changes in the ni-

trate leaching accounted due to straw removal process.

Furthermore, the emissions related to fertilizer applications,

such as NH3eN is based on the Ref. [55,57] and NOxeN is based

on Ref. [58] (Table 1).

2.3.2. Collection and pre-processingFuel inputs for the processes such as baling and handling of

biomass are calculated based on the methods developed by

Dalgaard et al. [59]. This stage of the energy conversion of the

straw also includes the transportation of the biomass at a

distance of 200 km to deliver it at the power plant.

2.3.3. Heat production and waste managementAt first, the gross district heat output is calculated in relation

to the reference flow of feedstock (i.e. 1 t, 85% DM of straw).

The thermal and electrical efficiencies of the CHP plant is

assumed to be 60% and 25% respectively [60,61]. Similarly, the

thermal efficiency of boiler is taken to be 85% [62]. These ef-

ficiencies are relativelymodest compared to units operated on

fossil fuels due to the corrosiveness of combusting straw at

high temperatures. Generally in the CHP plant operating in the

back-pressure mode, production of heat and electricity takes

place at an almost constant ratio, whilst if is operated in an

extraction mode the ratio is flexible [60]. This may infer to a

situation that in the steam extraction mode production of

heat and electricity primarily depend on the market demand.

For instance, Lund et al. [63] have discussed that the operation

of CHP plant normally responds to the changes in the elec-

tricity demand in a number of ways depending on the load

factor at each hour of operation. This indicates that at all

hours, the different energy plants compete on marginal pro-

duction costs on the electricity market. CHP plants can

decrease production by replacing heat production by heat

from the heat storage or the peak-load boiler. They further

argued that such situations normally occur if the marginal

production costs exceed the market price and if such change

does not violate the restriction of maintaining grid stability in

the system and thus are regulated. In the current study, it is

assumed that the CHP plant will work in an extraction mode,

and district heat production is the prime objective along with

selling electricity to the grid. It further entails that the mar-

ginal electricity production is not based solely on themarginal

change in capacity but can be characterized as a complex set

of affected electricity and heat technologies. Based on these

argumentswe have assumed that (i) partial load efficiencies to

be identical to nominal load efficiencies and (ii) extraction

plants to operate with back-pressure mode efficiencies.

As input to the combustion process, electricity and heat

required are assumed to be 40 MJ and 110 kWh respectively

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4122

per t (85% DM) [64]. It is assumed that both heat and electricity

required for the combustion are utilized from the produced

power of the straw fired CHP plant. In the case of boiler,

electricity required during the combustion process is assumed

to be from the marginal Danish electricity mix (see Section 2.2

for the marginal electricity mix). Life Cycle Inventory (LCI) for

the marginal technology mix is based on Ecoinvent Centre

[65], but the fuel mix is adjusted as indicated in Schmidt and

Brandao [38]. In the case of biomass fired power plants, the LCI

is based on the process related to “Wood chips, burned in

cogen 6400 kWth, emission control/CH U” [65]. Emissions

relevant to the combustion of straw, as per the selected

impact categories are presented in Table 2 and are based on

Nielsen [64]. The biogenic CO2 emission from the combustion

of straw is calculated as 1474 kg (see Table 2).

Net heat output is calculated by subtracting the heat input

required in the combustion process from the gross heat

output, and thus the net fuel input is also re-calculated. The

net electricity output is calculated accordingly, subtracting

the electricity input required during the combustion process

from the gross electricity produced from the CHP plant.

Furthermore, in the case of CHP plant co-produced electricity

is assumed to displace the marginal electricity mix (i.e. the

marginal Danish electricity mix, as stated above). This

displacement effect also indicates that the environmental

impacts related to their production can also be reduced while

accounting the net environmental impacts (e.g. GWP, NRE

use).

The total amount of bottom ash/slag produced from the

combustion process is assumed to be 54 kg tDM�1 [64].

Nutrient values of the slag are calculated based on the average

weight of P and K content, which are 1.45% and 16% respec-

tively [62] (Table 2). 100% of the nutrient values present in the

slag is assumed to be collected, as also suggested in Nguyen

et al. [53]. We have not accounted the N content of the bottom

ash, as most of the Nitrogen is lost in the combustion process

[53]. The nutrients values (P and K) available in the bottom ash

are assumed to displace the equivalent amount of fertilizer (P

and K fertilizer values). This partial substitution of the fertil-

izer thus displaces the related environmental impact of pro-

ducing them. Transport distance to deliver the bottom ash to

Fig. 3 e Environmental impacts in relatio

the agricultural field is assumed similar to the distance of

transporting the DM (i.e. 200 km). Similarly, fly ash (deposit) is

assumed to be 8.3 kg per t (85% DM) of the residue [64], and is

assumed to be disposed in a landfill site.

3. Results

3.1. Consequence of the straw removal process

Table 1 shows the consequences of removing 1 t (85% DM) of

the straw from the agricultural field, where it relates with

impacts: (i) constraining the soil C sequestration potential and

(ii) limiting the relevant N build-up that would have occurred

due to the biomass decay process if alternatively the residues

were incorporated to the soil. It also accounts the negative CO2

emission to atmosphere, equivalent to the biogenic CO2

release that would have been possible with the biomass

incorporation to the soil and followed by the decay process, as

discussed in Section 2.3.1. The overall consequences of the

straw removal are:

� If straw is not taken away from field, the emission reduc-

tion potential in relation to 1 t (85% DM) is equivalent to

39 kg C (i.e. þ143 kg CO2-eq) in a 100-year time frame. This

much of emission reduction potential is avoided, if the C

content in straw is converted to CO2 immediately after the

removal process (e.g. when used for energy purposes).

Moreover, if the negative CO2 emission is accounted while

estimating the total GWP the straw removal process avoids

the release of 1474 kg CO2-eq per reference flow of straw to

the atmosphere, making the net GWP at �1331 kg CO2-eq

per reference flow of straw (Table 1). At this level the net

GWP does not include emission related to compensating

fertilizer.

� Compensation of fertilizers (i.e. additional input) is

required to fulfill the nutrients that are generally removed

along with the straw. The nutrient values of straw are

calculated as 1.53, 0.765 and 12.75 kg (N, P, K) per reference

flow of straw. The net GWP including emission related to

manufacturing of compensating amount of chemical

n to removal of 1 t (85% DM) straw.

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 123

fertilizers is calculated as �1312 kg CO2-eq per reference

quantity of straw.

� Hence, emissions from the fertilizer applications, primarily

from their manufacturing process are the added impact to

the environment.

In addition to the above mentioned consequences,

0.0153 kg of N2OeN is the calculated emission related to N-

fertilizer application. Likewise, in-direct N2OeN is 0.0026 kg

per 1 t (85% DM) straw (Table 1). Likewise, consequence of

limiting the soil C sequestration potential on the soil N build-

up is calculated to be at the rate of 3.89 kg-N per 1 t (85%DM) of

Fig. 4 e Environmental impacts of the straw fired district heat pr

(A): processes at farm gate to the power plant; downstream pro

straw (see Table 1). The potential changes in leaching are

accounted to be 0.299 kg NO3eN per reference flow of straw.

N2O emissions related to the removal of 1tDM of straw, as

stated in Ref. [66] is in the range of 0.1e0.25 kg N, which is

comparable with the current study presenting for 85% DM of

straw.

Based on the current study, if only the avoided soil C build-

up is accounted then the consequence of constraining the

emission reduction potential would cover 85% of the gross

GWP (i.6167 kg CO2-eq) and the rest is related to the emissions

from the manufacturing process of compensating chemical

fertilizers. The figure is comparable to the study [66], stating in

oduction in a CHP plant and in a boiler (upstream processes

cesses (B): combustion, management of slag and fly ash).

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4124

the range calculated of about 183e366 kg CO2-eq, but Wilhelm

et al. [67] stated it in the range of 36.67e146 kg CO2-eq per tDM

strawDM. It should be noted that the range given is dependent

on a major interaction with tillage and also these studies

calculated the GWP on 30 years perspective for 1 tDM of straw.

The straw removal process also leads to an AP at

3.6 m2 UES, EP (aquatic) 1.1 kg NO3-eq, EP (terrestrial)

10.3 m2 UES and NRE use at 290 MJ-primary per reference flow

of straw. These impacts are primarily because of the conse-

quences related to the fertilizer compensation. 100% of the

NRE use is related to the manufacturing process of the

compensating fertilizers (Fig. 3). Likewise, 38% of the gross AP

is due to processes related to the application of chemical

fertilizers and rest is due to emissions from the

manufacturing process of the compensating fertilizers. 16% of

the aquatic EP is related to the leaching of nitrate from the

added N-fertilizer and rest is emissions related to

manufacturing process of chemical fertilizers. Similarly, 62%

of the calculated terrestrial EP is related to the applied N-

Fig. 5 e Environmental hotspots of straw fired district he

fertilizer and the rest of the impact is related to the

manufacturing process of the calculated additional fertilizers.

These consequences are irrespective of how the application of

the removed straw is taking place, and in Section 3.2 we have

discussed the consequences in relation to the conversion of

the removed straw to district heat.

3.2. Comparison of environmental performance of strawconversion in the CHP plant and the boiler

In Fig. 4, environmental impacts of producing district heat in

the CHP plant and the boiler are presented, where we have

categorized: (A) the upstream processes followed by (Ai) straw

removal, (Aii) collection and pre-processing and (Aiii) trans-

portation of straw to the power plant; and (B) the downstream

processes followed by (Bi) combustion, (Bii) re-use of nutrient

content in the collected slag and their transportation and (Biii)

transportation of fly ash and related emission taking place in

the landfill site. It should be noted that the ‘straw removal

at production (a) CHP versus (b) boiler for heat only.

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 125

process’, as mentioned in this section incorporates all the

consequences, as discussed in Section 3.1. Thus the net im-

pacts related to the straw removal process are accounted in

the calculation of entire biomass conversion process and

discussed in this section. The gross value represents the

added (þ) impacts related to the energy conversion processes,

and the net value represents “gross impact minus impacts

that are displaced”. Here displaceable impacts are the envi-

ronmental impacts related to both co-products; electricity and

nutrients values of slag, and thus they are subtracted from the

gross impact. In the case of the CHP plant, these co-products

are accounted, otherwise in the case of the boiler only the

nutrient values of slag are accounted. In the case of the straw

fired CHP plant the equivalent amount of electricity generated

while generating 1 MJ of heat is calculated to be 0.416 MJ

(considering the electricity efficiency of the CHP plant, see

Section 2.3.3). Furthermore, it should be noted that in Fig. 4,

the downstream side represents only the gross values of the

impact and the displaced impacts because of the co-products

(both electricity and nutrient content in the slag in the case of

CHP and only the latter product in the case of boiler) which

also occur at the same stream is separately presented in the

same figure.

3.2.1. Global Warming PotentialThe gross GWPper 1MJ of district heat production in the boiler

is calculated as 4.28 g CO2-eq, whilst the net GWP is

�102 g CO2-eq. The reduction in the GWP is primarily due to

avoidance of CO2 release that would have been possible due to

the biomass decay process if alternately straw is incorporated

into the soil, as discussed in Section 3.1. The reduction in the

calculated gross GWP due to stated (i) consequences of the

straw removal process, accounting both avoidance of soil C

sequestration and avoidance of negative CO2 emission to the

atmosphere is calculated to be �106 g CO2-eq/MJ heat, and (ii)

due to the nutrients available in the slag is calculated as

�0.03 g CO2-eq/MJ heat (Fig. 4a). 56% of the gross impact is

related to the GHG emissions related to the activities involved

in the upstream processes (baling and handling, chopping,

and transportation). It is calculated that 40% of the gross

impact is related to the transportation process, followed by

the combined processes: baling and handling (15%), and the

rest related to the chopping of residues. The downstream side,

in particular electricity input for the initial combustion is

calculated to cover 29% (i.e. 1.23 g CO2-eq/MJ heat) of the gross

GWP. It should be noted that we have assumed that the input

electricity is based on the marginal electricity mix of

Denmark, as stated in Table 2. In the case of the district

heating boiler, nutrient available in the collectable slag is

found insignificantly reducing the GHGs with respect to the

gross value. The detail breakdown of the calculated GWP at

the different stages of energy conversion processes are shown

in Fig. 4a and environmental hotspots are presented in Fig. 5b.

‘Environmental hotspots’ are defined as the most susceptible

stage of the energy conversion processes leading to increase

the respective environmental loadings.

In the samemanner, the net GWP related to the straw fired

CHP plant is calculated as �160 g CO2-eq, whilst the gross

value is 4.31 g CO2-eq per 1 MJ of heat production. Of this gross

GWP, 79% is contributed by upstream process and rest by the

downstream process (see Figs. 4a and 5a). The upstream side

relates to the activities involved at the collection and pre-

processing stages. The GWP related to the transportation of

the biomass is calculated as 2.45 g CO2-eq/MJ heat, which is

57% of the gross GWP of the entire energy conversion process.

Electricity as a co-product is found displacing �13.4 g CO2-eq/

MJ heat, which is as a result of substituting the equivalent

amount of the marginal electricity mix. Likewise, the reduc-

tion in the GWP due to nutrient content in the slag is calcu-

lated taking place at the rate of �0.04 g CO2-eq/MJ heat.

From the comparison of calculated GWP related to the

district heating boiler and the CHP plant, it is found that in the

former case the calculated GWP is higher by 1.6-fold than the

latter. Potential reduction of GWP in the case of the CHP plant

is primarily due to displacement of environmental impacts

related to the marginal electricity mix, which is displaced by

co-produced electricity. Furthermore, these characteristics

may signify the importance of understanding the substitution

effects in a LCA process, particularly when alternatives are

available in the market and have some displacement effects.

3.2.2. Acidification PotentialThe net AP per 1 MJ of heat production in the district heating

boiler is calculated as 0.008 m2 UES, and in the CHP plant it is

0.01 m2 UES. It is found that the AP is mostly related to the

combustion process covering about 94% of the gross impact,

calculated in the similar range for the both CHP plant and

boiler (Fig. 5a and b). In the case of CHP plant, co-produced

electricity would displace the impact at �0.0004 m2 UES/MJ

heat (Fig. 4b).

3.2.3. Aquatic and terrestrial Eutrophication PotentialThe net EP (aquatic) per 1 MJ of heat production in the boiler is

calculated as 0.10 g NO3-eq, where the gross impact is

0.104 g NO3-eq, making no such differences. The gross impact

is basically higher in the upstream activities of the energy

conversion processes, covering 89% of it. The straw removal

process is the primary cause of the EP (aquatic) compared to

other processes. It is calculated that the straw removal pro-

cess would lead to cover 88% of the gross impact (i.e.

0.092 g NO3-eq/MJ heat) (Fig. 5b). This impact is primarily

related to emission related to the compensating fertilizers

(both production and application). Nutrient values in the

bottom ash have insignificant effect to reduce the impact,

which is calculated at �0.004 g NO3-eq/MJ heat (Fig. 4c).

Similarly, in the case of the CHP plant, the calculated net

aquatic EP is 0.14 g NO3-eq/MJ heat, which is higher by 1.4-fold

compared to the boiler. The EP (aquatic) is related to the

quantity of the residue removed and its consequences, which

is relatively higher in CHP plant than in boiler (Table 2), since

the amount of biomass required in the former case is higher

than in the latter because of the lower conversion efficiency to

heat. Moreover, the co-produced electricity in the case of the

CHP plant would make the conversion system more efficient

and system expansion of the co-produced products is

accounted in the LCIA. Nevertheless, in the case of lowering

the EP (aquatic) only the nutrient values in the slag is effective.

In the current study, collectively both co-products are found

able to marginally displace the impact, which is at the rate of

�0.009 g NO3-eq/MJ heat (Fig. 4c). This also reveals that the

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4126

impact is more related to the N-fertilizer related emissions

compared to the biomass conversion processes.

The net terrestrial EP in the case of the boiler is found to be

0.0066 m2 UES/MJ heat (Fig. 4d). Terrestrial EP is primarily

related to the downstream process, contributing 80% of the

gross impact (0.007m2 UES/MJ heat). Combustion process, as a

part of the downstream processes increases the impact by

0.0052 m2 UES/MJ heat. In the case of CHP plant, the net

terrestrial EP is calculated as 0.008 m2 UES/MJ heat, where co-

products marginally displaces the impact by �0.001 m2 UES/

MJ heat from the gross value (Fig. 4d). 20% of the gross

terrestrial EP is due to upstream process, where the straw

removal process has the highest contribution, covering 13% of

the gross impact. Likewise, the downstream process is found

covering the rest of the impact, where combustion alone

covers 78% of the gross impact (Fig. 5a).

3.2.4. Non-Renewable Energy useThe gross NRE use calculated in the case of straw fired boiler is

0.084 MJ-primary/MJ heat (Fig. 4e). The upstream processes

are calculated to be covering 74% of the gross impact and the

rest by the downstream processes. Collection and pre-

processing of the biomass alone is found to be covering 46%

of the gross NRE use, followed by 28% by the straw removal

process (mainly related to the manufacturing process of

compensating fertilizers), and 23% of the NRE use is related to

the impact of electricity input to the combustion process.

Among the processes involved in the collection and pre-

processing of the biomass, transportation of the biomass

from farm gate to the power plant is calculated at 35% of the

gross impact; baling 10% and rest of the impact is related to

chopping of straw. The nutrient value of the slag has insig-

nificant displacement of the NRE use and is calculated as

0.0004 MJ-primary/MJ heat.

In the case of the CHP plant, the gross and the net NRE use

is calculated as 0.092 MJ-primary/MJ heat and �0.14 MJ-pri-

mary/MJ heat respectively (Fig. 4e). In contrast to the boiler, in

the CHP scenario, 36% of the gross NRE use is due to straw

Fig. 6 e Variations in GWP related to straw removal

process, considering 20 and 100 year time perspectives

with ROTH-C and C-tool models.

removal process which is as a part of the consequence of

removing relatively higher residue for the fuel purpose (Table

2), but the co-produced electricity since has an substitution

effects to the marginal electricity reduction of the impact also

takes place. Similarly rest of the gross impact is related to the

collection and pre-processing processes covering 60%. Among

the processes involved in the stage of collection and pre-

processing, 13% is related to the baling and handling pro-

cesses, 46% is related to the transportation of straw from farm

gate to power plant, and 1.5% is because of energy consump-

tion in the process of chopping the biomass. A substantial

amount of NRE use is found to be displaced due to the co-

produced electricity, which is calculated as �0.228 MJ-pri-

mary/MJ heat. Transportation of slag and fly ash also has a

determinant role to increase the NRE use, covering respec-

tively 2.6% and 3.4% of the gross impact calculated in the case

of the boiler and the CHP plant.

3.3. Sensitivity analysis

3.3.1. Consequences of straw removal including soil carbonchanges using different models or time horizonsThe sensitivity analysis on the GWP, in relation to the removal

of 1 t straw (85% DM), discussed herewith, is attempted to

compare the results of C-tool and ROTHC model (Fig. 6). For

this, we have considered the emission reduction potential for

100 and 20 years perspectives, as reported in Petersen et al.

[29] to be taking place at the rate of 9.7% and 21.3% respec-

tively, and in Schmidt and Brandao [38] at 2.9% and 16.7%

respectively.

It is found that while using the C-tool decay model, the net

GWP100 and GWP20 is �1474*(1 � 0.097) ¼ �1331 and

�1474*(1 � 0.213) ¼ �1160 kg CO2-eq respectively per 1 t

straw removed. In the same manner, with the ROTHC decay

model, the calculated GWP100 and GWP20 is �14

74*(1� 0.029)¼�1431 and�1474*(1� 0.167)¼�1228 kg CO2-eq

respectively per t straw removed. The differences in the re-

sults between C-tool and ROTHC decay model could be

because of dissimilarities in the features of the twomodels: (i)

variations in soil depth: in the C-tool model soil C pool turn-

over is assumed to occur from topsoil pool (0e25 cm) to the

corresponding subsoil pool (25e100 cm), but in the ROTHC

model only topsoil was accounted [48]. The assumption for

Schmidt and Brandao [38] based ROTHCmodel is based on the

argument provided in Refs. [29,48] for the same model and (ii)

effects of temperature: the consequence of soil depth and

possible relation with temperature also change the soil C

turnover, e.g. Petersen et al. [29] argued that the increase in

temperature increases the soil turnover and thereby reduces

the emission reduction potential. Consequences of using

different soil depth could also be linked with the rate of

decomposition process of residues, which is relatively quickly

if residues are left above the ground, while the decay time in

soil typically takes significantly longer time, and sometimes

some of the carbon stays in the soil permanently [38]. In

addition to this, the production of CO2 in soil is entirely from

root respiration and microbial decomposition of organic

matter, and these processes are dependent on temperature

and water availability [29,68]. Most importantly it has been

argued that the soil water function in ROTHC model is the

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 127

cause of the large build-up of C [48]. Similarly, the ROTHC

model discussed in Schmidt and Brandao [38] also ignored the

changes in the soil C turnover that would be possible due to

changes in the land management (e.g. crop rotation), whilst it

has been considered in C-tool model.

3.3.2. Environmental performances under differentsubstitutable marginal electricity scenariosIn Section 3.2, we discussed about the potential benefits of

straw fired in the CHP plant in comparison to the boiler. We

made an assumption that the co-produced electricity dis-

places the environmental impacts of marginal Danish elec-

tricity mix. Realizing about the debates on the marginal

electricity productions [69e71], this section deals with the

assessment of environmental impacts considering different

types of possible marginal electricity production differing by

fuel types. In the debates, a long term/short term effects of

electricity substitution have been regarded as a measure for

defining the marginal electricity [70,71], whereas Ekvall and

Weidema [70] discussed from the perspectives of market

proportion and competitive alternatives available in the

market. Mathiesen et al. [69] argued on the basis of cost of

production of the alternatives and future energymanagement

perspectives. Weidema et al. [71], argued from the standpoint

of an environmental regulations, and have also highlighted

that due to lower capital cost, natural gas fired power plants

were regarded asmarginal technology in the Nordic electricity

market. The official Danish energy policies of 2003 has

stressed natural gas as the marginal production, where fuel

prices were relatively lower [69]. Likewise, coal can be con-

strained from the environmental regulations, such as carbon

quotas, but differs in individual countries [71,72]. As per the

Danish 2005 energy policy, which had CO2-quota prices

included on top of the expected three different fuel prices,

wind power was identified as the marginal technology, but in

Mathiesen et al. [69], wind power is argued to be disregarded

as the marginal technology from its responses to changes in

demand and its spatial production i.e. differing from one re-

gion to another.

Considering these uncertainties, the assessment is simu-

lated further in three different scenarios of substitutable

marginal electricity: (i) natural gas fired power plant (me1), (ii)

coal fired power plant (me2), (iii) wind power (me3). Table 3

presents the technical information about the power plants

Table 3 e Information about the marginal electricityscenarios.

Marginalelectricityscenarios

Technologyinformationand LCI data

source

Adjustment inthe datasets

me1 “Electricity, natural gas,

at power plant/DE U” [65]

Efficiency of natural

gas fired power plant

adjusted to 46% [60]

me2 “Electricity, hard coal,

at power plant/DE U” [65]

Efficiency of coal fired

power plant adjusted

to 35% [60]

me3 “Electricity, at wind power

plant/RER U’’ [65]

e

and adjustments made in the LCI provided in Ecoinvent

Centre [65].

Table 4 depicts that the environmental performances of

district heat production in the CHP plant favors positively, if

the co-produced electricity is assumed to displace coal as

the substitutable marginal electricity production, compared

to natural gas and wind power. For instance, while dis-

placing coal as the marginal electricity, combustion with

natural gas (CoNG) and combustion with wheat straw

(CoWS) (see Table 3) leads to a GWP at �13 g CO2-eq/MJ heat

and �262 g CO2-eq/MJ heat respectively. In the same

manner, if coal is the substitutable marginal electricity then

NRE use in the case of natural gas and the straw fired dis-

trict heat production are 1.96 MJ-primary/MJ heat and

�1.26 MJ-primary/MJ heat respectively. Other impact cate-

gories are lower in CoNG compared to CoWS with coal as the

displaceable marginal electricity production. Environmental

impacts of both CoWS and CoNG when considering natural

gas and wind power as the substitutable marginal electricity

production are shown in Table 4, where from the standpoint

of GWP and NRE use the former is favorable option

compared to the latter, whilst for other impact categories

CoNG is favorable than the CoWS.

4. Discussions

4.1. Integrating effects of the identified environmentalhotspots in the biomass conversion

In the earlier sections, it is discussed that straw as a fuel input

for district heat production is attractive than natural gas,

primarily from the standpoint of lowering the GWP and NRE

use. Despite this, it is also argued that utilization of this pru-

dent source of renewable energy has higher EP and AP. For

instance, aquatic EP is primarily associated with the conse-

quences of the straw removal process (Fig. 5), in particular due

nitrate leaching and emissions from the manufacturing pro-

cess of the compensating N fertilizers. Some studies

including, McLenaghen et al. [73] and Kirchmann et al. [74]

reported significant potentials for improvement of agricul-

tural management practices thus controlling the nitrate

leaching and reducing aquatic EP. Martinez and Guiraud [75]

have highlighted that an integration of winter “catch crops”

in the agricultural system, following the harvest of the wheat

crop (for instance) and the straw removal prevents fields for

not remained bare and control nitrate leaching. A bare land

without vegetation leads to more leaching compared to fields

with catch crops [76]. The advantage of catch crops, e.g. rye

grass (Lolium perenne) normally decrease leaching loss by

absorbing much of the mineralized N back into the organic

pool [77]. These potential solutions to lower the leaching los-

ses not only help to reduce the environmental impact, but also

ensure the production of additional biomass either for energy

purposes or as animal feed etc. The latter opportunity could

mitigate another crucial consequence of residue removal for

other purposes rather than using as animal feed. Neverthe-

less, LCIA of such additional integration in the agricultural

management practices can be carried out to assess: to what

level aquatic EP may be controlled.

Table 4 e Environmental impacts of straw and natural gas fired in a CHP plant with three situations of marginal electricity(me1eme3).

Impact categories Units CoWS CoNG

me1 me2 me3 me1 me2 me3

GWP g CO2-eq/MJ heat �201 �261.68 �147.28 87.94 �13.39 101.11

AP m2 UES/MJ heat 0.01 0.01 0.01 0.00 �0.002 0.001

EP, aquatic g NO3-eq/MJ heat 0.14 0.13 0.14 0.005 �0.0024 0.0070

EP, terrestrial m2 UES/MJ heat 0.01 0.01 0.01 0.002 �0.0002 0.0035

NRE use MJ-primary/MJ heat �0.96 �1.26 0.07 3.09 1.96 3.3

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4128

Likewise, the terrestrial ecosystem is affected via the

fertilization of crops with Nitrogen (N) as the major yield

limiting nutrient and consequently the concentration of NOx

and NH3 increases in the ecosystem [78] making the envi-

ronment eutrophic. From the environmental hotspots anal-

ysis (Fig. 5), we have found that terrestrial EP is mainly due to

emissions from the combustion process; hence improvement

in the combustion system to reduce NOx in particular can

reduce this effect.

From Fig. 5, it is also found that AP is primarily related to the

downstream processes of energy conversion, and in particular

from the combustion of the straw. The AP is higher in the straw

fired heating plant due to higher emissions of chlorine, sulfur

and NOx compared to wood fuel [79]. Furthermore, it is in

particular, triggered via emissions related to inorganic fertilizer

application and other material inputs during the processing of

the biomass. One of the possible means of controlling the

acidification at a technological level (i.e. with the improvement

of the combustion element) is adopting the desulphurization

technology [80]. Generally, desulphurization technologies: the

wet scrubber is found able to reduce the SO2 emissions at the

rate of 92%e98%, while the spray dry scrubber technology re-

duces at the rate of 85e92% [81]. It has also been suggested that

it may facilitate the reduction of the respective impacts pro-

portionately with the stated reduction, as above and described

in Refs. [80,81]. Based on such prospects, as a research

perspective it can be inferred that sensitivity of adopting such

technologies in relation to the economic and environmental

performances can be analyzed.

Similarly, NOx emissions may be controlled by adopting

modifications in the combustion process, e.g. separating the

combustion process in stages which partially delays the

combustion process and results in a cooler flame suppressing

the thermal NOx formation [80]. There are other possible al-

ternatives to lower the NOx emissions, such as combustion in

low excess air and recirculation of parts of the flue gases into

the combustion air that prevents NOx formation [82]. Flue gas

condensation technologies are also popular for straw firing

technologies in Denmark, andmoreover are also found able to

increase the thermal efficiency with 5e10% while reducing

SO2 emissions to a minimum level. Likewise, NOx emissions

may be reduced at the rate of 60e70% by adopting a non-

catalytic reduction (SNCR) technology [60].

4.2. CHP in the fluctuating renewable energy scenariosof Denmark

The research project “Coherent Energy and Environmental

System Analysis” targeting the Danish future energy mix

conveys that the share of fluctuating renewable energy sour-

ces, particularly wind power, photovoltaic, solar thermal, and

wave energy will be increased in the future total energy pro-

duction of Denmark [15]. Biomass is also expected to cover

higher share on the total primary energy (TPE) consumption of

the country [15,18]. For instance, it has been reported that

Denmark could be fossil fuel free by 2050 with a TPE con-

sumption of about 480 PJ, of which biomass and waste could

cover 49%, and the rest e approximately 245 PJ e by wind

turbines, solar, geothermal and wave energy [15]. Biomass

thus could play a vital role in conjunction with such fluctu-

ating sources, since it acts as a storable form of energy [83,84].

Nevertheless, having these opportunities, it also reveals the

challenges for balancing electricity production with such a

high degree of fluctuating energy sources in the fuel mix. This

also connects with a challenge, whether operation of biomass

based CHP plants and boilers should be facilitated in instances

having low electricity price at the spotmarket, whichmight be

an indication of high availability of wind power. Nielsen et al.

[85] reported that the correlation between wind power and

electricity spot prices is not always evident, but in average, the

price decreaseswithmorewind in the system, and is expected

to be occurring at a higher frequency in future [86], thus

making it more competitive at least compared to electricity

production in the CHP plants. It has also been realized that

balance of electricity production from a CHP plants with re-

strictions in biomass fuels, grid connections and consumer

demands is also pertaining issues [87e89]. Ummels et al. [90],

argued that operation of heat boiler operation, enabling the

shutdown of CHP at a lower minimum load may be regarded

as a preferable solution compared to wasting available wind

resources. Further balancing options lie in e.g. the integration

of wind power and heat via heat pumps [91], integration of

wind power and biomass [92], integration of power generation

and transport [93,94], the use of storages [95], and the flexible

operation of CHP plants and loads [14,95]. However as pointed

out in Østergaard [83] energy systems with the best perfor-

mance according to load-following criteria do not necessarily

perform best according to primary energy use.

One of the first steps is simply to exploit the regulating

capability inherent in CHP plants that may operate in

condensing mode operation, extraction mode with varying

degrees of steam being bled for heat generation or back

pressuremode. This may offer a better solution to balance the

electricity production while covering an accompanying heat

demand. Nevertheless, a rational prioritization of the possible

pathways of energy conversionmight be necessary in the case

of Danish energy structure, most importantly in a situation

where there are constrained biomass resources and multiple

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 129

sustainable outlets of their utilization. The different pathways

of biomass energy conversion chain includes, direct com-

bustion (e.g. combustion in a CHP plant), thermo-chemical

process (e.g. gasification, carbonization), and bio-chemical

processes (e.g. biogas, methane, biofuels), but sustainability

assessment of these energy conversion pathways is necessary

before coming to any conclusion. In Section 4.3, we will

elaborate on relevant environmental impacts related to the

biomass end-uses.

4.3. Resource constraints and perspectives

Parajuli [96] calculated that the levelised cost of district heat

production in the straw fired CHP plant (with coal as substi-

tutable marginal electricity) is about 0.008 V/MJ, which is 69%

and 41% lower compared to production based on natural gas

and imported Wood pellets. Despite biomass, in particular

straw is one of the important sources of renewable energy and

economically attractive, expansions of such power plants are

not gaining proper momentum. For instance, in the Danish

gross district heat production sector, straw fired power plants

have merely increased from 5% to 7%, whereas the wood has

increased from 4% to 19% between 2000 and 2011 [16]. Ea En-

ergy Analyses [97] reported that the barrier is primarily related

with the economic perspectives (e.g. particularly with respect

to biogas), as well as availability of alternative technologies/

fuels in the energy market, such as biogas and other energy

crops. Regardless of the price of straw is lower compared to

other available biomass, the handling and transporting pro-

cess makes straw less favorable compared to wood chips and

pellets [97], particularly due to density of the biomass. The

density of the biomass is important parameter as it influences

the conveniences of collecting, transporting, storing and for

feeding into the combustion chamber. This in turn will influ-

ence the overall efficiency of the biomass conversion [98].

Based on these arguments, production of straw pellets could

support to mitigate the issues associated with the density,

however LCIA of such conversions is yet to present.

Irrespective of these issues, biomass fired in a CHP plant

can be important not only from the perspective of balancing

the electricity, but also ensuring better environmental per-

formance compared to other non-renewable based heat and

power production. As discussed in Section 4.2, it is also

equally relevant to identify the sustainable pathways of

biomass to energy conversions and also about integrating

potential perennial energy crops and grasses if can be utilized

as a feedstock in such pathways.

In Nguyen et al. [99] environmental performance of straw

to produce electricity in a gasification and CHP technology are

discussed, where it is found that the former technology while

producing 1 kWh of electricity leads to a GWP of 80 g CO2-eq,

total EP �1.9 g NO3-eq, and NRE Use 0.2 MJ-primary. For the

equivalent output, straw fired in a CHP plant with heat as a co-

product is reported accompanying a higher environmental

impact than the gasification technology, e.g. 20% higher GWP

in the latter than the former case [99].

Similarly, Parajuli et al. [100] argued that if autumn harvest

Miscanthus is used as a feedstock in a CHP plant, the calcu-

lated GWP is�71 g CO2-eq/MJ heat, and aNRE use is�0.767MJ-

primary/MJ heat, and in the case natural gas was assumed as

the substitutable marginal electricity supply. This indicates

that when larger heat/power demand has to be satisfied with

Miscanthus, as a potential biomass source, it could exacerbate

the land use competition among the energy crops and also

among the cereals. This also shows the importance of crop

residues in the sustainable energy and agriculture manage-

ment. When conversion of straw and Miscanthus to district

heat are compared, the former case leads tomore reduction in

the GWP than the latter. However, it should be noted that in

the case of straw removal both avoided soil C sequestration

loss and avoided CO2 emissions are accounted for, whilst in

the case of Miscanthus only the soil C sequestration gain was

accounted for. Regarding the savings in the NRE use, Mis-

canthus is reported additionally saving it by 0.53 MJ-primary

compared to the straw, since the net heat output and elec-

tricity from the conversion of Miscanthus was higher by 50 MJ

and 25 kWh respectively. The displacement effect of produc-

ing additional 25 kWh of electricity compared to straw thus

makes higher reduction in the NRE use, e.g.�0.76 MJ-primary/

MJ heat (in the case of Miscanthus) and �0.23 MJ-primary/MJ

heat (in the case of straw). At the same time, land occupation

with the cultivation of Miscanthus as alternative fuel was

identified as a disadvantageous element, which accounts at

the rate of 0.09 m2-a/MJ heat (m2-a represents occupancy of

land, as square meter in a year), whereas straw is regarded as

a residual product of cereal crops.

Likewise, Blengini et al. [101] argued with a range of NRE

use while producing 1 MJ of electricity/heat using promising

energy crops such as Maize, Sorghum, Triticale and Mis-

canthus mixed with cow manure in a biogas plant, and is re-

ported to be between 0.2 and 0.3 MJ-primary. The ranges are

due to the consideration of different co-products in the sys-

tem [101]. Similarly, Concawe et al. [102] reported that

biomass use for electricity production enhances larger GHG

savings, especially when compared to first generation bio-

fuels. Searcy and Flynn [103] showed that while producing

electricity from unit input of biomass such as agricultural

residues through the processes such as direct firing or gasifi-

cation can save about three times the amount of GHG emis-

sions compared to the amount saved by bioethanol and

FischereTropsch diesel (i.e. production from switch grass via

gasification of biomass). In this comparison, marginal substi-

tution of electricity was based on coal. Furthermore, Cher-

ubini et al. [104] and Kaltschmitt et al. [105] argued for a

greater GHG savings per hectare of land in the pathway of

utilizing biomass for heat production compared to the con-

ventional biofuels and bioelectricity production systems. In

the conversion of bagasse to either electricity or bioethanol,

electricity was found favorable when indicators as such en-

ergy consumption, GHG, eutrophication and acidification

were considered. Furthermore, bioethanol production was

preferable if indicators such as resource depletion and toxicity

concerns were considered for the evaluation [106]. In the

process of generating electricity willow and hybrid poplar

have been regarded more favorable if their high yield is

compared to food crops like corn and soybean, and also if their

residues are not harvested. Moreover, electricity generation

from the former is less efficient than converting the latter to

ethanol through ‘‘biorefinery’’ process. Biomass power plants

convert only 23e37% of biomass energy content into

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4130

electricity [107,108], while ethanol contains 53e56% of

biomass energy content [93,109,110]. In addition to the crop

land, a biomass power plant requires 7e11m2/TJ of direct land

area assuming a lifetime of 30 years [108,110].

Despite having potential benefits of different biomass

feedstock in heat and power production, it is essential tomake

an appropriate selection for their best utilization

[103,104,106,111]. The selection should ensure whether

biomass would be used for direct combustion, such as in CHP

plant, or have to be regarded as principal feedstock for

transport fuels production. An indication towards a situation

leading to insignificantly limited availability of biomass is

highlighted in Ref. [20] based on the existing land use pattern

of Denmark and other potential processing of biomass for

alternative transport fuels. It is argued that an increased de-

mand of biomass for more ‘valuable’ transport fuels is ex-

pected in the Danish future energy mix [15,21]. In contrast the

recent ‘ten-million ton plan’ of Denmark claims to addition-

ally produce biomass at 10 Mt per annum without substan-

tially affecting the existing food and feed productions and also

supporting in the reduction of environmental impact

compared to the current level [112]. Moreover for the sus-

tainability of biomass feedstock supply it is important to

judiciously utilize them in days ahead. Most importantly, in

today's world, it is also very relevant to consider the complex

science and market of material demand (chemicals, fuels,

proteins and nutrients to maintain soil health) for the sus-

tainability of agro-ecosystem [3,4,113]. It means that if any

biomass has to be taken into account as a fuel source, it is also

necessary to assess if alternatively they can be processed to

deliver a spectrum of bio-based products including fuel/en-

ergy. This further draws attention towards the biorefinery

technology, which has been regarded as one of the innovative

means of biomass conversions and is taking ground in many

developed economies [114,115]. Furthermore, one may argue

that the demand of biomass (e.g. straw) would bemuch higher

in the biorefinery value chains (e.g. to deliver 2nd generation

bioethanol) if a larger demand of biofuel has to be maintained

leading to a competition among the existing application of

straw e.g. as nutrient supply to soil, fuel for heat and power

production, feed for ruminants and bedding materials for

livestocks. This might also lead to create less attention to-

wards its use in heat and power sector, or may have compe-

tition on the available biomass, between the cases of directly

converting biomass to heat/power and in biorefinery. None-

theless, another corner of argument is that to some extent this

potential competition could be mitigated through the inte-

gration of a biorefinery value chains in the energy system. For

instance, Cherubini and Ulgiati [115] argued that the use of

crop residues in a biorefinery could save GHG emissions by

50% and more than 80% of non-renewable energy could be

saved in comparison to the conventional fossil basedmaterial

processing. Likewise, switch grass use in a biorefinery process

is reported to reduce GHG emissions by 79% and saves NRE use

by 80% with respect to the alternative fossil fuel [116].

It is thus relevant to choose the most appropriate sus-

tainable feedstock supply in the biorefinery value chains,

demanding a comparative assessment of different types of

lignocellulosic feedstocks, and also assess opportunities to

promote both green (grasses) and yellow biomass (straw) [117]

as biorefinery feedstocks. Such assessment will be important

to ensure the sustainable year round supply of biomass [100].

Hence, as a future perspective, assessment of economic and

environmental efficiency of the production of biobased prod-

ucts via biorefineries can be carried out considering the po-

tential feedstocks supply scenarios and be compared with the

case of conversion to heat and power as discussed in the

current study.

5. Summary and conclusions

The assessment of environmental impacts in relation to 1 MJ

of district heat production discussed in this study can be

summarized in following points:

� For 1 MJ of district heat production in the straw fired CHP

plant and assuming the co-produced electricity substitutes

the marginal Danish electricity mix, the net GWP is

calculated as �160 g CO2-eq, AP is at 0.01 m2 UES, EP

(aquatic) 0.14 g NO3-eq, EP (terrestrial) 0.008 m2 UES, and

NRE Use at �0.14 MJ-primary. For the same amount of heat

output, straw fired in the district heating boiler leads to a

GWP of �101 g CO2-eq, AP 0.008 m2 UES, aquatic and

terrestrial EP 0.1 g NO3-eq and 0.0065 m2 UES respectively

and NRE use as 0.084 MJ-primary.

� For 1 MJ of district heat production in the straw fired CHP

plant and assuming coal as the displaceable marginal

electricity, the calculated net GWP is �261 g CO2-eq, AP

0.007 m2 UES, aquatic and terrestrial EP at 0.13 g NO3-eq

and 0.005 m2 UES respectively, and NRE use �1.26 MJ-

primary.

� Straw fired district heat production in the CHP plant is

better than fired with natural gas, e.g. the former leads to a

lower GWP and NRE use. In the latter case, the GWP and

NRE use are�13.4 g CO2-eq/MJ heat and 1.96 MJ primary/MJ

heat respectively assuming coal as the substitutable mar-

ginal electricity production. But for other impact cate-

gories, natural gas fired district heat production shows

better environmental performances. For e.g. compared to

straw it leads to AP of �0.002 m2 UES/MJ heat, and the

aquatic and terrestrial EP of �0.002 g NO3-eq/MJ heat and

0.0002 m2 UES/MJ heat respectively.

Conversion of straw to district heat production in the CHP

plant is better than in district heating boiler for all impact

categories. Furthermore, based on the current study it is also

concluded that sustainability of straw conversion to a com-

bined production of heat and power might be determined by

the potential substitution of marginal electricity by the co-

produced electricity assumed to be taking place somewhere

else. Straw is found attractive as a fuel input to CHP plant

compared to natural gas from the standpoint of lowering GWP

and NRE-use, but also leads to higher AP, EPs compared to the

former case. It is thus necessary to integrate additional system

that would help to mitigate the exhaust emissions of major

pollutants leading to increase AP and EP in the case of straw

conversion to heat and power. One of the prospects to lower

such impacts could be expanding the system boundary of the

assessment, primarily integrating the catch crop at the farm

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 131

gate level (beyond straw removal process in the upstream

side) to lower the nitrogen related emissions and the adoption

of desulphurization and flue gas condensation technologies

(in the downstream side) to reduce the AP. This opens area for

the further studies in the related sector.

Nevertheless, straw fired in the CHP plant could play an

important role in conjunction with other fluctuating renew-

able energy technologies, such as wind power, in light of

maximizing the share of renewable energy, e.g. in the Danish

future energy goals. This finite source of biomass is not only

important to maintain the short term energy demand but also

to serve as a storable form of energy carriers. This also means

the good performance of the system depends on the precon-

dition that the future energy system needs the heat input.

Apart from the abovementioned advantages of straw as a fuel

input, there are other potential applications of straw such as

animal feed and feedstock to future biorefineries, but in the

current study we have not compared the environmental per-

formances of such applications. This opens avenues for a

wider sustainability assessment of straw also in the sectors

other than heat/power production.

Acknowledgments

This article is based on the M.Sc. thesis entitled “Life Cycle

Assessment of wheat straw as a fuel input for district heat

production” [96], carried out by the first author at the Depart-

ment of Development and Planning, Aalborg University,

Denmark during the period of FebruaryeJune 2013. The article

is further re-developed as part of the PhD study at the Depart-

ment of Agroecology, Aarhus University (AU), Denmark, which

is co-funded by the Bio-Value Platform (http://biovalue.dk/),

funded under the SPIR initiative by The Danish Council for

Strategic Research and TheDanish Council for Technology and

Innovation, case no: 0603-00522B. The first authorwould like to

thank theGraduate School of Science andTechnology (GSST) of

AU for the PhD scholarship. This article is prepared as a case of

baseline scenario of straw conversion and is aimed to compare

with the cases of conversions in biorefinery product scenarios.

Feedbacks received from the editor and three anonymous

reviewers are highly acknowledged.

r e f e r e n c e s

[1] Volk TA, Verwijst T, Tharakan PJ, Abrahamson LP,White EH. Growing fuel: a sustainability assessment ofwillow biomass crops. Front Ecol Environ 2004;2(8):411e8.

[2] Petersen J-E. Energy production with agricultural biomass:environmental implications and analytical challenges. EurRev Agric Econ 2008;35(3):385e408.

[3] Lin Y, Rujiang G, Chong S. Food and energy of the collisionalarm. Outlook Wkly 2006;50:44e5.

[4] Watson HK. Potential to expand sustainable bioenergy fromsugarcane in southern Africa. Energy Policy2011;39(10):5746e50.

[5] Dale BE. Lignocellulose conversion and the future offermentation biotechnology. Trends Biotechnol1987;5(10):287e91.

[6] Uihlein A, Schebek L. Environmental impacts of alignocellulose feedstock biorefinery system: an assessment.Biomass Bioenergy 2009;33(5):793e802.

[7] Tonini D, Astrup T. LCA of biomass-based energy systems: acase study for Denmark. Appl Energy 2012;99(0):234e46.

[8] Lund H, Mathiesen BV. Energy system analysis of 100%renewable energy systemsdthe case of Denmark in years2030 and 2050. Energy 2009;34(5):524e31.

[9] Skøtt T, BioPress. Straw to energydstatus, technologies andinnovation in Denmark 2011. Niels Pedersens All�e 2, 8830,Tjele: Agro Business Park A/S; 2011. pp. 1e40.

[10] Parajuli R. Looking into the Danish energy system: lesson tobe learned by other communities. Renew Sustain EnergyRev 2012;16(4):2191e9.

[11] Giuntoli J, Boulamanti AK, Corrado S, Motegh M, Agostini A,Baxter D. Environmental impacts of future bioenergypathways: the case of electricity from wheat straw balesand pellets. Glob Chang Biol Bioenergy 2012:497e512.

[12] Statistics Denmark. Statistical year book 2012. Copenhagen:Statitistics Denmark; 2012. pp. 1e542.

[13] Lund H. Large-scale integration of wind power into differentenergy systems. Energy 2005;30(13):2402e12.

[14] Lund H, Munster E. Management of surplus electricity-production from a fluctuating renewable-energy source.Appl Energy 2003;76(1e3):65e74.

[15] Hvelplund F, Lund H, Schaltz E, Pillay JR, Nielsen MP,Felby C, et al. Coherent energy and environmental systemanalysis. Aalborg University; 2011. pp. 1e93.

[16] Danish Energy Agency. Annual energy statistics.Copenhagen: Danish Energy Agency; 2012.

[17] Danish Energy Agency. Energy strategy 2050 e from coal, oiland gas to green energy. 1470 Copenhagen K, Denmark: TheDanish Ministry of Climate and Energy; 2011. pp. 1e66.

[18] IDA. The IDA climate plan. Copenhagen, Denmark: TheDanish Society of Engineers (IDA); 2009. pp. 1e253.

[19] Østergaard PA, Lund H. A renewable energy system inFrederikshavn using low-temperature geothermal energyfor district heating. Appl Energy 2011;88(2):479e87.

[20] Østergaard PA, Mathiesen BV, M€oller B, Lund H. Arenewable energy scenario for Aalborg Municipality basedon low-temperature geothermal heat, wind power andbiomass. Energy 2010;35(12):4892e901.

[21] Kwon PS, Østergaard PA. Comparison of future energyscenarios for Denmark: IDA 2050, CEESA (Coherent Energyand Environmental System Analysis), and ClimateCommission 2050. Energy 2012;46(1):275e82.

[22] Fan Y-T, Zhang Y-H, Zhang S-F, Hou H-W, Ren B-Z. Efficientconversion of wheat straw wastes into biohydrogen gas bycow dung compost. Bioresour Technol 2006;97(3):500e5.

[23] Lal R. Crop residues as soil amendments and feedstock forbioethanol production. Waste Manag 2008;28(4):747e58.

[24] Clapp CE, Allmaras RR, Layese MF, Linden DR, Dowdy RH.Soil organic carbon and 13C abundance as related to tillage,crop residue, and nitrogen fertilization under continuouscorn management in Minnesota. Soil Tillage Res2000;55(3e4):127e42.

[25] Dick WA, Blevins RL, Frye WW, Peters SE, Christenson DR,Pierce FJ, et al. Impacts of agricultural managementpractices on C sequestration in forest-derived soils of theeastern Corn Belt. Soil Tillage Res 1998;47(3e4):235e44.

[26] Christensen BT, Olesen JE. Nitrogen mineralizationpotential of organomineral size separates from soils withannual straw incorporation. Eur J Soil Sci 1998;49(1):25e36.

[27] Mogensen L, Hermansen JE, Halberg N, Dalgaard R, Vis J,Smith BG. Life cycle assessment across the food supplychain. Sustain Food Ind 2009:115e44.

[28] Statistics Denmark. Agricultural statistics. Copenhagen:Statistics Denmark; 2010. p. 182.

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4132

[29] Petersen BM, Knudsen MT, Hermansen JE, Halberg N. Anapproach to include soil carbon changes in life cycleassessments. J Clean Prod 2013;52(0):217e24.

[30] Connolly D, Mathiesen BV. A technical and economicanalysis of one potential pathway to a 100% renewableenergy system. Int J Sustain Energy Plann Manag2014;1:7e28.

[31] Hauschild M. Spatial differentiation in life cycle impactassessment: a decade of method development to increasethe environmental realism of LCIA. Int J Life Cycle Assess2006;11(1):11e3.

[32] Hauschild M, Potting J. Spatial differentiation in life cycleimpact assessment e the EDIP2003 methodology. DanishMinistry of Environment; 2005. pp. 9e191.

[33] Yang X-E, Wu X, Hao H-L, He Z-L. Mechanisms andassessment of water eutrophication. J Zhejiang Univ Sci B2008;9(3):197e209.

[34] Weidema BP. Using the budget constraint to monetariseimpact assessment results. Ecol Econ 2009;68(6):1591e8.

[35] Weidema BP, Wesnæs M, Hermansen J, Kristensen T,Halberg N, Eder P, et al. Environmental improvementpotentials of meat and dairy products. Sevilla: Institute forProspective Technological Studies; 2008.

[36] IPCC. IPCC fourth assessment report: climate change 2007;2007.

[37] PR�e Consultants. SimaPro 7.3.3. Amersfort, TheNetherlands: Pre Consultants; 2012.

[38] Schmidt JH, Brandao M. LCA screening of biofuels-iLUC,biomass manipulation and soil carbon. Copenhagen,Denmark; 2013. pp. 3e97.

[39] Schmidt JH, Merciai S, Thrane M, Dalgaard R. Inventory ofcountry specific electricity in LCA e consequential andattributional scenarios [Methodology report v2]. Aalborg: 2.-0 LCA Consultants; 2011.

[40] Dalgaard R, Schmidt J, Halberg N, Christensen P, Thrane M,Pengue W. LCA of soybean meal. Int J Life Cycle Assess2008;13(3):240e54.

[41] Schmidt JH. System delimitation in agriculturalconsequential LCA. Int J Life Cycle Assess 2008;13(4):350e64.

[42] L�opez S, Davies DR, Gir�aldez FJ, Dhanoa MS, Dijkstra J,France J. Assessment of nutritive value of cereal and legumestraws based on chemical composition and in vitrodigestibility. J Sci Food Agric 2005;85(9):1550e7.

[43] Jenkins BM, Baxter LL, Miles Jr TR, Miles TR. Combustionproperties of biomass. Fuel Process Technol1998;54(1e3):17e46.

[44] Rasburskis N, Gudzinskas J, Gylys J. Combined heat andpower production: socio-economic and sustainabledevelopment aspects. J Civ Eng Manag 2006;12(1):29e36.

[45] Lund H, Andersen AN. Optimal designs of small CHP plantsin a market with fluctuating electricity prices. EnergyConvers Manag 2005;46(6):893e904.

[46] Cenni R, Janisch B, Spliethoff H, Hein KRG. Legislative andenvironmental issues on the use of ash from coal andmunicipal sewage sludge co-firing as construction material.Waste Manag 2001;21(1):17e31.

[47] P€oyki€o R, R€onkk€om€aki H, Nurmesniemi H, Per€am€aki P,Popov K, V€alim€aki I, et al. Chemical and physicalproperties of cyclone fly ash from the grate-fired boilerincinerating forest residues at a small municipal districtheating plant (6 MW). J Hazard Mater2009;162(2e3):1059e64.

[48] Petersen BM, Knudsen MT. Consequences of straw removalfor soil carbon sequestration of agricultural fields, using soilcarbon in a time frame perspective. Faculty of AgriculturalSciences, Aarhus University. Aarhus, Denmark: AarhusUniversity; 2010. pp. 1e47.

[49] Coleman K, Jenkinson DS. A model for the turnover ofcarbon in soil model description and windows users guide.Harpenden Herts AL5 2JQ, United Kingdom: RothamstedResearch; 2008. pp. 1e43.

[50] Kløverpris J, Mueller S. Baseline time accounting:considering global land use dynamics when estimating theclimate impact of indirect land use change caused bybiofuels. Int J Life Cycle Assess 2013;18(2):319e30.

[51] Møller J, Thøgersen R, Kjeldsen A, Weisbjer M, Søegaard K,Hvelplund T, et al. Fodermiddltabel 2005. Sammensætningog foderværdi af fodermidler til kvæg. LandbrugetsRådgivningscenter; 2005.

[52] Petersen B, Berntsen J. The turnover of soil organic matteron different farm types. 3rd ed. DARCOFenews; 2003.

[53] Nguyen TLT, Hermansen JE, Mogensen L. Environmentalperformance of crop residues as an energy source forelectricity production: the case of wheat straw in Denmark.Appl Energy 2013;104(0):633e41.

[54] IPCC. 2006 IPCC guidelines for National Greenhouse GasInventories; 200611.1e11.24.

[55] EEA. EMEP/EEA air pollutant emission inventory guidebook2013. Copenhagen, Denmark: European EnvironmentAgency; 2013. pp. 1e43.

[56] Cherubini F, Strømman AH. Life cycle assessment ofbioenergy systems: state of the art and future challenges.Bioresour Technol 2011;102(2):437e51.

[57] Nemecek T, K€agi T. Life cycle inventories of agriculturalproduction systems. Duebendorf, Switzerland: Swiss Centrefor Life Cycle Inventories; 2007.

[58] Nguyen TLT, Hermansen JE, Mogensen L. Fossil energy andGHG saving potentials of pig farming in the EU. EnergyPolicy 2010;38(5):2561e71.

[59] Dalgaard T, Halberg N, Porter JR. A model for fossil energyuse in Danish agriculture used to compare organic andconventional farming. Agric Ecosyst Environ2001;87(1):51e65.

[60] Danish Energy Agency. Technology data for energy plants.Generation of electricity and district heating, energy storageand energy carrier generation and conversion. Copenhagen,Denmark; 2012. pp. 5e212.

[61] Centre for Biomass Technology. Straw for energyproduction. Centre for Biomass Technology; 2004. p. 34.

[62] Centre for Biomass Technology. Halmasker, kemisksammensætning; 1998.

[63] Lund H, Mathiesen B, Christensen P, Schmidt J. Energysystem analysis of marginal electricity supply inconsequential LCA. Int J Life Cycle Assess2010;15(3):260e71.

[64] Nielsen P. Heat and power production from straw[Produktion af kraftvarme fra halm]; 1999e2004.

[65] Ecoinvent Centre. Ecoinvent data v2.2. Dubendorf,Switzerland: Swiss Centre for Life Cycle Inventories; 2010.

[66] Gabrielle B, Gagnaire N. Life-cycle assessment of straw usein bio-ethanol production: a case study based onbiophysical modelling. Biomass Bioenergy2008;32(5):431e41.

[67] Wilhelm WW, Johnson JMF, Hatfield JL, Voorhees WB,Linden DR. Crop and soil productivity response to cornresidue removal. This paper is a joint contribution of theUSDA-ARS and the Agricultural Research Division of theUniversity of Nebraska. Published as Journal Ser. no. 13949Agron J 2004;96(1):1e17.

[68] Davidson EA, Janssens IA. Temperature sensitivity of soilcarbon decomposition and feedbacks to climate change.Nature 2006;440(7081):165e73.

[69] Mathiesen BV, Munster M, Fruergaard T. Uncertaintiesrelated to the identification of the marginal energy

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4 133

technology in consequential life cycle assessments. J CleanProd 2009;17(15):1331e8.

[70] Ekvall T, Weidema B. System boundaries and input data inconsequential life cycle inventory analysis. Int J Life CycleAssess 2004;9(3):161e71.

[71] Weidema BP, Ekvall T, Heijungs R. Guidelines forapplication of deepened and broadened LCA. Int J Life CycleAssess e Co-ord Action Innov Life-Cycle Anal Sustain(Calcas) 2009:1e49.

[72] Weidema BP. Market information in life cycle assessment.Copenhagen: Danish Environmental Protection Agency;2003. pp. 1e147.

[73] McLenaghen RD, Cameron KC, Lampkin NH, Daly ML, Deo B.Nitrate leaching from ploughed pasture and theeffectiveness of winter catch crops in reducing leachinglosses. N Z J Agric Res 1996;39(3):413e20.

[74] Kirchmann H, Johnston AEJ, Bergstr€om LF. Possibilities forreducing nitrate leaching from agricultural land. AMBIO e JHum Environ 2002;31(5):404e8.

[75] Martinez J, Guiraud G. Alysimeter study of the effects of aryegrass catch crop, during a winter wheat/maize rotation,on nitrate leaching and on the following crop. J Soil Sci1990;41(1):5e16.

[76] Eriksen J, Thorup-Kristensen K. The effect of catch crops onsulphate leaching and availability of S in the succeedingcrop on sandy loam soil in Denmark. Agric Ecosyst Environ2002;90(3):247e54.

[77] Whitmore AP, Schr€oder JJ. Intercropping reduces nitrateleaching from under field crops without loss of yield: amodelling study. Eur J Agron 2007;27(1):81e8.

[78] Brentrup F, Kusters J, Kuhlmann H, Lammel J.Environmental impact assessment of agriculturalproduction systems using the life cycle assessmentmethodology: I. Theoretical concept of a LCA methodtailored to crop production. Eur J Agron2004;20(3):247e64.

[79] Pehnt M. Dynamic life cycle assessment (LCA) of renewableenergy technologies. Renew Energy 2006;31(1):55e71.

[80] European Commission. Integrated pollution prevention andcontrol: reference document on best available techniquesfor large combustion plants. Edificio Expo, Inca Garcilaso s/n, E-41092 Seville e Spain: Joint Research Centre, Institutefor Prospective Technological Studies; 2006. pp. 1e578.

[81] Kaminski J. Technologies and costs of SO2-emissionsreduction for the energy sector. Appl Energy2003;75(3e4):165e72.

[82] Graus WHJ, Worrell E. Effects of SO2 and NOx control onenergy-efficiency power generation. Energy Policy2007;35(7):3898e908.

[83] Østergaard PA. Reviewing optimisation criteria for energysystems analyses of renewable energy integration. Energy2009;34(9):1236e45.

[84] Dalgaard T, Olesen JE, Petersen SO, Petersen BM,Jørgensen U, Kristensen T, et al. Developments ingreenhouse gas emissions and net energy use in Danishagriculture e how to achieve substantial CO2 reductions?Environ Pollut 2011;159(11):3193e203.

[85] Nielsen S, Sorknæs P, Østergaard PA. Electricity marketauction settings in a future Danish electricity system with ahigh penetration of renewable energy sources e acomparison of marginal pricing and pay-as-bid. Energy2011;36(7):4434e44.

[86] Lim S, Lee KT. Leading global energy and environmentaltransformation: unified ASEAN biomass-based bio-energysystem incorporating the clean development mechanism.Biomass Bioenergy 2011;35(7):2479e90.

[87] Iqbal MT. Simulation of a small wind fuel cell hybrid energysystem. Renew Energy 2003;28(4):511e22.

[88] Jurado F, Cano A, Carpio J. Modelling of combined cyclepower plants using biomass. Renew Energy2003;28(5):743e53.

[89] Ro K, Rahman S. Control of grid-connected fuel cell plantsfor enhancement of power system stability. Renew Energy2003;28(3):397e407.

[90] Ummels BC, Gibescu M, Pelgrum E, Kling WL, Brand AJ.Impacts of wind power on thermal generation unitcommitment and dispatch. IEEE Trans Energy Convers2007;22(1):44e51.

[91] Farrell AE, Plevin RJ, Turner BT, Jones AD, O'Hare M,Kammen DM. Ethanol can contribute to energy andenvironmental goals. Science 2006;311(5760):506e8.

[92] Claus F, Brian M, Henrik L, Frede H, David C, Poul Ø, et al.Biomasse og det intelligente energisystem; 2012.

[93] Kim S, Dale BE. Life cycle assessment of various croppingsystems utilized for producing biofuels: bioethanol andbiodiesel. Biomass Bioenergy 2005;29(6):426e39.

[94] Rinaldi R, Schuth F. Acid hydrolysis of cellulose as the entrypoint into biorefinery schemes. ChemSusChem2009;2(12):1096e107.

[95] Pimentel D. Ethanol fuels: energy balance, economics, andenvironmental impacts are negative. Nat Resour Res2003;12(2):127e34.

[96] Parajuli R. Life cycle assessment of wheat straw as a fuelinput for district heat production. Aalborg University; 2013.pp. 1e128.

[97] Ea Energy Analyses. Analysis of the market for bio energylocally and internationally. Final report, 29 September 2010ed. Frederiksholms Kanal 4, Copenhagen K: Ea EnergyAnalyses; 2010. pp. 1e90.

[98] Natarajan E, Nordin A, Rao AN. Overview of combustion andgasification of rice husk in fluidized bed reactors. BiomassBioenergy 1998;14(5e6):533e46.

[99] Nguyen TLT, Hermansen JE, Nielsen RG. Environmentalassessment of gasification technology for biomassconversion to energy in comparison with other alternatives:the case of wheat straw. J Clean Prod 2013;53(0):138e48.

[100] Parajuli R, Dalgaard T, Jørgensen U, Adamsen AP, KnudsenMT, Birkved M, et al. Biorefining in the prevailing energyand materials crisis: a review of sustainable pathways forbiorefinery value chains and sustainability assessmentmethodologies. Renew Sustain Energy Rev [unpublishedresults].

[101] Blengini GA, Brizio E, Cibrario M, Genon G. LCA of bioenergychains in Piedmont (Italy): a case study to support publicdecision makers towards sustainability. Resour ConservRecycl 2011;57(0):36e47.

[102] Concawe, EUCAR, EC. Well-to-wheels analysis of futureautomotive fuels and powertrains in the European context;2007. pp. 1e88.

[103] Searcy E, FlynnPC. Processingof straw/corn stover: comparisonof life cycle emissions. Int J Green Energy 2008;5(6):423e37.

[104] Cherubini F, Bird ND, Cowie A, Jungmeier G,Schlamadinger B, Woess-Gallasch S. Energy- andgreenhouse gas-based LCA of biofuel and bioenergysystems: key issues, ranges and recommendations. ResourConserv Recycl 2009;53(8):434e47.

[105] Kaltschmitt M, Reinhardt GA, Stelzer T. Life cycle analysisof biofuels under different environmental aspects. BiomassBioenergy 1997;12(2):121e34.

[106] Botha T, von Blottnitz H. A comparison of theenvironmental benefits of bagasse-derived electricity andfuel ethanol on a life-cycle basis. Energy Policy2006;34(17):2654e61.

[107] EPRI. Renewable energy technology characterizations.Washington, D.C. 20585: Electric Power Research Institute;1997. pp. 1e283.

b i om a s s a n d b i o e n e r g y 6 8 ( 2 0 1 4 ) 1 1 5e1 3 4134

[108] Mann M, Spath P. Life cycle assessment of a biomassgasification combined cycle system. National RenewableEnergy Laboratory; 1997. pp. 1e30.

[109] Marland G, Turhollow AF. CO2 emissions from theproduction and combustion of fuel ethanol from corn.Energy 1991;16(11e12):1307e16.

[110] Pimentel D, Patzek TW. Ethanol production using corn,switchgrass, and wood; biodiesel production using soybeanand sunflower. Nat Resour Res 2005;14(1):65e76.

[111] Uihlein A, Ehrenberger S, Schebek L. Utilisation options ofrenewable resources: a life cycle assessment of selectedproducts. J Clean Prod 2008;16(12):1306e20.

[112] Dalgaard T, Jørgensen U, Kristensen I, Gylling M, Dejgaard J,Dubgaard A, et al. Concepts for a multi-criteriasustainability assessment of a new more biobased economyin rural production landscapes. In: International FarmingSystems Association Conference. Aarhus, Denmark; 2012.p. Workshop 5.1. 9 p.

[113] Yuankun H. On food safety problem in new stages ofagricultural development. Rural Econ China2001;275(3):16e57.

[114] Dorado MP, Lin SKC, Koutinas A, Du C, Wang R, Webb C.Cereal-based biorefinery development: utilisation of wheatmilling by-products for the production of succinic acid. JBiotechnol 2009;143(1):51e9.

[115] Cherubini F, Ulgiati S. Crop residues as raw materials forbiorefinery systems e a LCA case study. Appl Energy2010;87(1):47e57.

[116] Cherubini F, Jungmeier G. LCA of a biorefinery conceptproducing bioethanol, bioenergy, and chemicals fromswitchgrass. Int J Life Cycle Assess 2010;15(1):53e66.

[117] Gylling M, Jørgensen U, Bentsen NS, Kristensen IT,Dalgaard T, Felby C, et al. The þ10 million tonnes study:increasing the sustainable production of biomass forbiorefineries. Fødevareøkonomisk Institut, KøbenhavnsUniversitet; 2013. pp. 1e32.