Bioresource Technology 148 (2013) 53–60

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Bioresource Technology

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Techno-economical study of biogas production improved by steamexplosion pretreatment

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.08.111

⇑ Corresponding author. Tel.: +98 3113915623; fax: +98 3113912677.E-mail address: karimi@cc.iut.ac.ir (K. Karimi).

Marzieh Shafiei a, Maryam M. Kabir b, Hamid Zilouei a, Ilona Sárvári Horváth b, Keikhosro Karimi a,⇑a Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iranb School of Engineering, University of Borås, 50190 Borås, Sweden

h i g h l i g h t s

� The economy of steam explosionpretreatment for biogas productionwas studied.� Aspen plus and Aspen Process

Economic Analyzer were used for theevaluation.� Total energy efficiency of the process

is calculated to 80%.� Production cost of methane from

wheat straw and paper tube was0.36–0.48 €/m3.� The production cost of methane

decreased by 36% using thepretreatment.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 June 2013Received in revised form 16 August 2013Accepted 19 August 2013Available online 27 August 2013

Keywords:BiogasPretreatmentSimulationSteam explosionTechno-economical analysis

a b s t r a c t

Economic feasibility of steam explosion pretreatment for improvement of biogas production from wheatstraw and paper tube residuals was investigated. The process was simulated by Aspen plus�, and the eco-nomical feasibility of five different plant capacities was studied by Aspen Process Economic Analyzer.Total project investment of a plant using paper tube residuals or wheat straw was 63.9 or 61.8 millionEuros, respectively. The manufacturing cost of raw biogas for these two feedstocks was calculated to0.36 or 0.48 €/m3 of methane, respectively. Applying steam explosion pretreatment resulted in 13%higher total capital investment while significantly improved the economy of the biogas plant anddecreased the manufacturing cost of methane by 36%. The sensitivity analysis showed that 5% improve-ment in the methane yield and 20% decrease in the raw material price resulted in 5.5% and 8% decrease inthe manufacturing cost of methane, respectively.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Currently, one of the most important fundamentals for sustain-able development is production of fuels from biomass, as an alter-native to fossil fuels. Bioethanol and biogas are the two commonbiofuels (Börjesson and Mattiasson, 2008; Klass, 1998). While bio-ethanol is currently produced from sugar or starch-containingmaterials, biogas is produced from industrial or municipal wastes.

The use of sugar or starch based materials for fuel production hasled to a ‘‘food vs. fuel’’ conflict. Therefore, several research and pilotscale studies were performed using lignocellulosic biomass insteadof food driven biofuels. Lignocellulosic materials have the advan-tage of being available in very large quantities at relatively lowprices (Saddler, 1993; Taherzadeh and Karimi, 2008a).

The main drawback of using lignocellulosic materials is their ri-gid structure which is resistant to enzymatic and microbial attacks.Therefore, a pretreatment step is required to open up the structureand reduce the crystallinity of lignocelluloses (Taherzadeh andKarimi, 2008a). Among the pretreatment methods, steam explo-sion, with or without addition of a catalyst. This is one of the most

54 M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60

thoroughly investigated methods, and it has been reported to suc-cessfully increase the methane yield of different lignocellulosicmaterials (Dereix et al., 2006; Dererie et al., 2011; Take et al.,2006). Steam explosion typically operates within a temperaturerange of 160–260 �C for a few seconds to several minutes (Taher-zadeh and Karimi, 2008a; Vivekanand et al., 2013).This methodproduces low amounts of waste and consumes moderate amountsof energy (Taherzadeh and Karimi, 2008a).

Biogas production from lignocelluloses has several advantagescompared to bioethanol production. Compared to ethanol, theoverall energy efficiency is much higher in biogas production(Murphy and Power, 2009). Furthermore, conversion of lignocellu-losic pentoses to ethanol is one of the obstacles in the process,since the ordinary industrial yeast species are unable to assimilatepentoses (Karimi and Zamani, 2013). Therefore, one of the besteconomical solutions is the assimilation of pentoses in a separatebiogas production process after bioethanol production (Shafieiet al., 2011). However, all suggested processes have their own dif-ficulties (Taherzadeh and Karimi, 2008b), hence the best solutioncould be to produce biogas as the only product from lignocellulosicbiomass. The advantage of this single step process is the simulta-neous hydrolysis and biogas production, which can be appliedprincipally for all types of fermentable materials. The producedbiogas can be then utilized in generation of power and heat, andit can also be upgraded to gaseous vehicle fuels (Murphy andPower, 2009; Taherzadeh and Karimi, 2008b).

Techno-economical analysis is a useful tool to examine theprofitability and performance of the proposed process by combin-ing process simulation with economic calculation. The applicationof techno-economical analysis allows comparing alternative pro-cesses and optimizing the process parameters such as capacity ofthe plant and composition of the raw materials. Ekman et al.(2012) and Sassner et al. (2008) proved that steam treatment ofstraw, spruce (a softwood), salix (a hardwood), and corn stover iseconomically feasible for ethanol production. On the other hand,technical and economical analyses for biogas production in theNetherland from different wastes showed that biogas productionis not economically feasible unless subsidies are provided fromthe government (Gebrezgabher et al., 2010). Nevertheless, to ourknowledge, there is no reference detected in the literature on tech-no-economical processes evaluating lignocellulosic biomass forbiogas production.

The current study evaluates the economic feasibility of steamexplosion pretreatment applied for improvement of biogas produc-tion from wheat straw and paper tube residuals. The process wassimulated using Aspen Plus� simulation software (Aspen Tech,USA) based on experimental data provided by previous studies(Bauer et al., 2009; Teghammar et al., 2010). A process includinga continuous pretreatment system combined with solid statedigestion was examined. Economical evaluations were conductedby Aspen PEA (Process Economic Analyzer). The effects of plant

HPSteam

PRaw MaterialsFeedstock Handling

2

LP Steam 1

Water

Pretreatment1

3

W

4

LP Steam 2

5

Fig. 1. Block flow diagram of the process for biogas production from l

capacity and price of raw materials and byproducts on the econ-omy of the process were also investigated.

2. Process development

Production of biogas from easily digestible materials is a well-developed technology, and the solid state technology for utilizingthe energy crops such as corn is already used in many countries,e.g., Germany (Yliopisto, 2009). However, an efficient process forbiogas production from cellulose containing materials which areresistant to biodegradation is still under development. An over-view of a biogas production plant from paper tube residuals andwheat straw using steam explosion pretreatment is presented inFig. 1. The process includes feedstock handling, steam explosionpretreatment, and anaerobic digestion. In order to provide steamand electricity, it is assumed that the plant is located close to anexisting power plant, and the byproducts, i.e., steam and solid res-idues as a fuel, considered as valuable by-products in the mainplant.

2.1. Plant size and feedstocks

Lignocellulosic wastes, e.g., straw, wood chips, paper residues,and grass as well as forest and agricultural residues, can supplyas the most abundant raw materials (Lin and Tanaka, 2006). Theaverage of 5 year production of barely, oat and wheat in Swedenis 1.5, 0.7 and 2.3 million tons, respectively (FAO, 2013). Theamount of produced straw is 1 to 1.2-fold of the weight of thesecereal grains. This amount is cultivated in the estimated area of914,000 hectares.

Choosing the correct capacity for a plant depends on severalparameters including availability of raw materials, technical bottlenecks, transportation costs for both raw materials and products, aswell as market demands for the products. In recent studies, thecapacity of plants for production of biogas (Murphy and Power,2009) and bioethanol (Aden and Foust, 2009; Aden et al., 2002;Ekman et al., 2012; Lohrasbi et al., 2010; Shafiei et al., 2011) wasselected to treat raw materials within the range of 100,000–600,000 tons annually. An example of using lower capacity is foundin a study evaluating the production of biogas in the Netherlandsusing 70,000 ton/year of farm originated wastes. However, thisstudy showed that the process is not economically feasible unlesssubsidies are provided from the government (Gebrezgabher et al.,2010). Therefore, a plant with a capacity of 200,000 ton/year ofraw material (dry basis) was chosen as a base case for the presentinvestigations, which is similar to the one used in a previous studyevaluating bioethanol production from wood in Sweden (Shafieiet al., 2011). Furthermore, different capacities ranging from50,000 to 600,000 tons/year were also evaluated. It was assumedthat the plant would be in full operation for 8,000 h/year (Shafieiet al., 2011).

retreated MaterialsAnaerobic Digestion 8

7

Biogas

Solid Residue

ater Recirculation

Nutrients

6

ignocellulosic raw materials using steam explosion pretreatment.

M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60 55

An important aspect in the choice for the capacity of a plant isthe availability and transportation cost of raw materials. The re-quired amount of straw for the base case is only 4% of the totalamount of produced straw in Sweden. In the case of paper tuberesiduals, the required amount for the biogas plant is less than halfof the total amount of collected paper from households in Sweden,which is 383,000 tons (Weine, 2012). However, in Sweden, besidesrecycled paper from households, recycling companies collect paperwith different qualities from industries too (STENA, 2013). Thisrecycled paper can also serve as another source for further biogasproduction.

In this study, the raw materials were considered for a plant witha single feedstock, and the manufacturing cost was evaluated;however, running a plant with only one type of raw material ispractically questionable. Therefore, the design of the plant shouldbe flexible enough, so that primarily a raw material with higherprofitability is used and the more expensive raw material will onlybe used when the other one is not sufficient.

The composition of raw materials was defined based on previ-ous experimental data (Bauer et al., 2009; Teghammar et al.,2010). Moisture content of paper tube residuals and wheat strawwas 5% and 15%, respectively. Major components of paper tuberesiduals were 23% lignin, 60% cellulose, and 17% hemicellulose.Wheat straw contained 50% cellulose, 27% hemicellulose, and23% lignin.

2.2. Feedstock handling and pretreatment unit

After transportation of the raw materials to the plant, they areweighed and unloaded from truck trailers. The received materialsare reduced in size as a preliminary treatment, and then dumpedinto storage piles. Conveyor belts are used for transporting thematerials to a continuous pretreatment system. The pretreatmentprocess modifies the structure of the cellulose, lignin, and hemicel-lulose. Thus, the carbohydrate bonds can be easily exposed by thehydrolytic bacteria during the following anaerobic digestion pro-cess (Taherzadeh and Karimi, 2008a). The pretreatment methodis based on a steam explosion process developed by Aden et al.(2002). This process was used for pretreatment of corn stover forimprovement of ethanol production. In this pretreatment unit,the feedstock is conveyed to a pre-steamer. The pre-steamer is ahorizontal vessel which is fed with variable frequency drive setof screw conveyors. Low pressure steam is injected to the pre-stea-mer which increases the temperature to 100 �C and removes thenon-condensable gases. Screw conveyors are used for dischargingthe preheated materials to a flash vessel as well as for dischargingthe material from the flash vessel to a continuous pretreatmentreactor. This reactor is a high pressure horizontal extruder usingsteam as a motive force. High pressure steam is used to increasethe pressure (Aden et al., 2002). Since continuous process is usedfor the pretreatment, the retention time for heating-up to the de-sired temperature would be reduced, two minutes residence timewas therefore assumed for the treatment.

2.3. Biogas production

The pretreated materials are subjected to the biogas productionunit where solid state digestion is used for biogas production basedon the technology presented by DRANCO (Germany). This technol-ogy is a well-established and reliable technology used for treat-ment of solid organic wastes. Each digester has 3150 m3 volume,and the retention time of the materials in the reactor is 2–4 days.The overall residence time before the materials are fully digestedis about 20 days (Baere, 2010). The digesters are continuous verti-cal plug flow reactors with cone bottom shape. The semi-digestedmaterials are extracted from the middle of the digester by screw

pumps, and mixed with the pretreated materials at ratio of about5:1 and then returned to the top of the digester. Gravity is the driv-ing force for movement of the solids toward the bottom of the di-gester. The fully digested materials are extracted from the bottomof the digester and sent to the dewatering unit (Baere, 2010).

2.4. Process simulation and economic calculations

Mass and energy balances were calculated for the process(Figs. 1 and 2) using the Aspen Plus� program. This software waschosen because of its high ability to handle solid state materials.This feature together with its broad property data base makes thissoftware the best tool for simulation, design, and optimization ofthe processes (Aspen-Tech, 2013). The results of rigorous evalua-tions, using detailed equipment models, are presented in Tables1 and 2. A data bank prepared by NREL (National Renewable En-ergy Laboratory, USA) (Wooley and Putsche, 1996) for the calcula-tion of physical and thermodynamic properties of biomasscomponents was introduced to Aspen Plus�.

In the next step, Aspen PEA was used for the economical evalu-ations. Key features in Aspen PEA include interactive equipmentsizing to determine operating costs and investment analysis aswell as automatic generation of block and process flow diagrams.It also has interfaces to all major process simulation tools, includ-ing Aspen Plus (Aspen-Tech, 2013). Based on the results from sim-ulation, the sizes of equipments were calculated using Aspen Plus�

or Aspen PEA, and then the costs of all major equipments wereestimated using Aspen PEA. The main equipments were crusher,flash vessels, conveyor belts, biogas digesters, belt press, pumps,vessels, and heat exchangers as well as auxiliary equipmentsincluding piping, insulation, electrical equipment, and instrumen-tation. The costs for pretreatment vessels were calculated accord-ing to an NREL report (Aden et al., 2002). The main assumptionsfor all process scenarios are as follows:

� The plant capacity is 200,000 tons/year of raw materials (basedon dry weight).� The plant is located in Sweden.� Cost indexes at 2013 are used for equipments, chemicals, and

labor costs estimations.� Straight line depreciation for 7 years with zero salvage value

was used for calculation of the plant depreciation.� The project is one hundred percent equity financed.� In the project investment, 15% contingency was considered.� The construction period is 64 weeks followed by 40 weeks

start-up period.� Working capital is based on 5% of the total investment cost.� The desired internal rate of return in the process is 10%.� Project life is 15 years.� Operating supplies and laboratory charges are respectively 18%

and 15% of the operating labor cost.� Taxation rate value is 30%.

These values were chosen based on the data available in the lit-erature (Peters and Timmerhaus, 1991; Turton et al., 2009). Theprices of the raw materials and products are presented in Table 3.The cost for production of methane was calculated based on the to-tal capital investment of the project and the annual operatingcosts. A discounted cash flow analysis was used to evaluate thepayback period of the plant.

2.5. Sensitivity analysis

The plant capacity affects the manufacturing cost as well as thetotal capital investment. Therefore, plant capacities ranging from50,000 to 600,000 tons/year of dry raw material were evaluated

Fig. 2. Simplified process flow diagram for biogas production from lignocellulosic raw materials improved by a steam explosion pretreatment step.

Table 1Stream components in the process presented on Fig. 1 using paper tube residuals as feedstock.

Component mass fraction 1 2 3 4 5 6 7 8

Water 0.050 1.000 1.000 1.000 0.996 0.713 0.095 0.701Hexoses – – – – – 0.013 – –Pentoses – – – – – 0.024 – –Hexosan 0.570 – – – – 0.151 – 0.028Pentosan 0.162 – – – – 0.024 – 0.004Lignin 0.218 – – – – 0.061 – 0.224Methane – – – – – – 0.242 –CO2 – – – – – – 0.663 –Biomass – – – – – 0.011 – 0.041Othera – – – – 0.004 0.003 – 0.002Total (kton/year) 210.5 108.0 59.0 145.0 149.5 737.7 172.7 200.3

a Other components are mainly furfural hydroxymethyl furfural, and solubilized lignin.

Table 2Stream components in the process presented on Fig. 1 using wheat straw as feedstock.

Component mass fraction 1 2 3 4 5 6 7 8

Water 0.150 1.000 1.000 1.000 0.993 0.715 0.095 0.702Hexoses – – – – – 0.010 – –Pentoses – – – – – 0.037 – –Hexosan 0.423 – – – – 0.125 – 0.023Pentosan 0.229 – – – – 0.038 – 0.007Lignin 0.198 – – – – 0.061 – 0.225Methane – – – – – – 0.242 –CO2 – – – – – – 0.663 –Biomass – – – – – 0.011 – 0.041Othera – – – – 0.007 0.003 – 0.002Total (kton/year) 235.3 81.7 61.4 125.0 128.8 743.5 172.5 202.1

a Other components are mainly furfural, hydroxymethyl furfural, and solubilized lignin.

56 M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60

using Aspen PEA program and compared with the base case(200,000 tons/year). In order to investigate the effect of pretreat-ment on the economy of the process, simulation and economicevaluation for a plant without a pretreatment unit were also con-ducted. Quality and type of feedstock as well as transportationcosts of feedstock may affect the manufacturing cost of the prod-uct. Thus, a sensitivity analysis was performed to investigate theeffect of feedstock price on the manufacturing cost. Changes in

the manufacturing cost with respect to other required materialsand to the byproduct’s prices were also evaluated.

3. Results and discussion

The economic feasibility of biogas production from wheat strawand paper tube residuals improved by steam explosion pretreat-

Table 3Prices used in the economical evaluation for the raw materials andfor the products.

Materials Price (€/kg)

Raw materialsFresh water 0.0001a

Wheat straw 0.08b

Paper tube 0.05b

High pressure steam 0.008c

Low pressure steam 0.004c

Nutrients/pH control materials 0.6d

ProductsSteam 0.003e

Solid fuel 0.02f

a Data from Turton et al. (2009) corrected by cost index of2013.

b Data from Odhner et al. (2012).c Data from Peters and Timmerhaus(1991) corrected by cost

index of 2013.d Data from Lohrasbi et al. (2010).e Estimated based on the data for low and high pressure

steam.f Estimated based on the data from Shafiei et al. (2011).

M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60 57

ment was studied. For this purpose, the process was simulatedusing Aspen Plus� and the economical evaluation was performedby Aspen PEA.

3.1. Pretreatment process

The block flow diagram and process flow diagram of the steamexplosion process for production of biogas are presented in Figs 1and 2, respectively. Based on the simulation results (Fig. 2) andcapacities suggested by Aden et al. (2002), two identical units forunwrapping and preparation of the raw materials are required.The amounts of material to feed each unit were calculated to26.3 and 29.4 ton/h for paper tube residuals and wheat straw,respectively. Each of these units has bale unwrap and shredderfeed conveyor, truck scale, and truck unloading forklift, bale mov-ing forklift, shredder, and magnetic separator. Pretreatment tem-peratures of 180 �C and 190 �C are considered for pretreatment ofwheat straw and paper tube residuals, respectively (Bauer et al.,2009; Teghammar et al., 2010). After pretreatment, 7–8% of thehexosan and 50% of the pentosan are hydrolyzed to their corre-sponding monomers (Tables 1 and 2). The heat loss from the reac-tor was not accounted in the energy balance calculations (Adenet al., 2002). Based on the simulation results, three identical unitsfor pretreatment consisting of pre-hydrolysis equipments, screwreactor cyclone, conveyor belts and expansion tanks are required.Each of these units has the capacity of 37.3 ton/h.

After completion of the pretreatment, the material is conveyedto an expansion tank. The tank is a cyclone with atmospheric pres-sure in which over 90% of the steam used for heating is recoveredas a vapor. Beside major changes in the structure of cellulose, aportion of holocellulose is converted to glucose and xylose mono-mers and then degrades to hydroxymethylfurfural (HMF) and fur-fural. About 70% of the furans are separated from the pretreatedsolid and moved to the vapor phase (Aden et al., 2002). This steamis a low pressure steam contains over 98% water; however, pres-ence of furans decreases its value compared to pure low pressuresteam. The pretreated materials contain less than 0.05% of furans.

Based on the simulation results, the moisture content of the rawmaterial as well as the amount of water added to the feedstockprior to the pretreatment are key factors that affect the amountof steam required for the pretreatment. The amount of water andsteam used in the pretreatment process affects the size of thepretreatment equipment as well as the size of the expansion tank.Therefore, in the simulation, the amounts of water and low

pressure steam are calculated in such a way that the preheatedmaterials would contain 30% total solids in all cases. For paper tuberesiduals with 5% moisture content, 108 and 59 kton/year waterand low pressure steam were required, whereas these amountsfor wheat straw (containing 15% moisture) were 81.7 and61.4 kton/year, respectively (Tables 1 and 2). The pretreated mate-rial contained 70% moisture when they were fed to the digesters.

3.2. Biogas production

The biogas digester was also simulated by Aspen plus� and theamount of produced methane was calculated by the program.Totally, fifteen digesters with 3150 m3 capacity containing allauxiliary equipments as well as feed pumps and screw pumpsare required. The input flow rate to each digester is 6.1 ton/h. Apackage for supplementation of nutrients and control of pH wasconsidered in the biogas production unit. The solid residue fromthe digesters is dewatered and contains 30% dry matter. Thisresidue mainly consists of lignin (75% of dry weight) whichremains intact during the digestion process and is considered asa valuable byproduct for production of heat.

The most important advantage of solid state biogas productionis less digester volume required. Another advantage of this tech-nology is the production of no or limited amount of excess waste-water (Baere, 2010). The results from simulation have shown thatthe process can run with 100% water recycling. The water insidethe solid residue (70%) contains excess soluble materials whichwill be purged from the process. Furthermore, the furans areflashed out from the process in the pretreatment unit.

3.3. Total project investment and manufacturing costs

Five different plant sizes were analyzed for the total projectinvestment as well as for the manufacturing cost of the methanefrom the pretreated materials. Total project investment for a biogasproduction plant with 200,000 tons/year of dry paper tube residu-als and wheat straw as feedstocks was calculated to be 63.9 and61.8 million Euros, respectively, and the corresponding manufac-turing cost of methane was 0.36 and 0.48 €/m3of methane (or0.18 and 0.23 €/m3 of biogas). Investments for feedstock handling,pretreatment, and biogas production units were contributed to 5%,13%, and 82% of the total project investment, respectively. Addi-tionally, 0.10–0.21 and 0.10 €/m3 are suggested to be able to coverthe costs for upgrading and pressurizing the raw biogas, respec-tively (Yliopisto, 2009). Therefore, the manufacturing cost of pres-surized methane from paper tube residuals and wheat straw wouldbe 0.56–0.67 and 0.68–0.79 €/m3, respectively. These results arecomparable with the results of other studies producing biogas fromeasily degradable raw materials. The price for upgraded and pressur-ized biogas from organic waste was reported to be 0.55–0.65 €/m3,while the price for biogas production using energy crops was0.53–0.74 €/m3 (Mårtensson, 2007). The cost of biogas from energycrops were estimated to be 0.68–0.85 €/m3 including addition of0.16–0.27 €/m3 for the gas distribution.

In Table 4, comparison of some biogas production processes andtheir yearly profits are presented. Yearly profit was defined accord-ing to the following equation:

Yearly profit ¼Methane sale €

year

� �� Total manufacturing cost €

year

� �

Total capital investmentð€Þ ð1Þ

The yearly profit were 0.27 and 0.14 (€/€.year) for paper tuberesiduals and wheat straw in the current work (Table 4). In anotherstudy, the price for the production of biomethane utilizing wheatgrains and other crops in Ireland were estimated to be0.83–0.90 €/m3; however, this manufacturing price was not lowenough to be economically feasible (Murphy and Power, 2009).

Table 4Comparison of results obtained during the current work with from similar studies.

Raw materials Pretreatment Biogasproduction

Total capital investment(M€)

Capacity ofraw material

Production cost Yearly profit (€/€.year)

Base case Paper tube Steamexplosion

Solid statefermentation

63.9 200 ktpya 0.56–0.67b,c 0.27e

Wheat straw 61.8 0.68–0.79c,d 0.14e

Murphy andPower(2009)

Wheat, sugar beet Yes Solid statefermentation

205.6 988 ktpya 0.9c Not profitablef

Wheat Yes 220.3 714 ktpya 0.84c

Tricase andLombardi(2009)

Animal sewage + co-substrates

No Conventional 0.237 5000 m3

1000 m343 kW electricity + 1295 GJ/yearheat 38,868 €/year total cost

0.13

Gebrezgabheret al. (2010)

Animalmanure + energymaize + other

No Conventional 6.7 (for all additionalunits to an existing plant)

70 ktpya Electricity sales: 900,000 €/year. Not profitable (0.2 ifsubsidies are provided)

a ktpy: kilo ton per year.b The production cost of raw biogas is 0.18 €/m3 (or 0.36 €/m3 of methane).c Upgraded and pressurized methane.d The production cost of raw biogas is 0.23 €/m3 (or 0.48 €/m3 of methane).e The calculation are based on selling raw biogas at the price of 0.57 €/m3 of methane.f The project was not profitable with selling the pressurized methane at price of 1 €/m3.

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

Man

ufac

turi

ng c

ost

of m

etha

ne (€

/m3 )

20

40

60

80

100

120

0 50 100 150 200 250 300Tota

l pro

ject

cap

ital

inve

stm

ent

(mill

ion

€)

Percentage of original plant size(Base case is 200,000 ton/year raw material on dry basis)

Fig. 3. Variation of the total project investment as well as manufacturing cost ofmethane with the project capacity when paper tube residuals (j) and wheat straw(N) are pretreated by steam explosion prior to anaerobic digestion.

58 M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60

The yearly profit for the process presented by Tricase and Lombardi(2009) was 0.13 (€/€.year), while this value was 0.2 (€/€.year) onlyif governmental support is available for the process presented byGebrezgabher et al. (2010).

The results of changes in manufacturing and total projectinvestment’s cost for different biogas plant sizes using paper tuberesiduals or wheat straw as feedstock are presented in Fig. 3.Although the total project investment for both of these substrateswere in the same order, the manufacturing costs of methane fromwheat straw was higher than that from paper tube residuals, dueto the higher price of the raw material in the case of wheat straw.However, there might be limitations in availability of paper tuberesiduals; thus, construction of a flexible plant with ability to useseveral types of raw materials is inevitable. On the other hand,the price of the feedstock for plants with 2 and 3-folds capacitiescompared with the base case was increased by 10% and 20%,respectively, because of the increase in the transportation cost.As a result of this increase, the manufacturing cost of methanefor plants with 3-fold capacity using wheat straw as a raw materialshowed a slight increase. The total project investment required fora plant does not increase linearly with the plant size, but is be-lieved to increase according to the ‘‘six-tenth’’ rule (Turton et al.,2009). Total project investments for plants with different capaci-ties were calculated by Aspen PEA (Fig. 3). Generally, a plant withlarger size requires less investment per each unit of product.Therefore, larger plants are more economically feasible unlesstechnical or logistical problems occur. However, there are somelimitations that should be considered here, e.g., technical bottlenecks, availability of raw materials, transportation costs for bothraw materials and products, and market demands for the products.

In the construction of the plant, 42% of the fixed capital invest-ment (FCI) is attributed to the purchased equipment cost. In biogasproduction plants, large size for digesters are required in order toprovide sufficient retention time for the materials, and beside thecosts for the equipments, the civil costs of the digesters is also high(10% of FCI). Moreover, since a biogas production plant using ligno-cellulosic materials with the pretreatment is not a proven technol-ogy yet, 15% of FCI was chosen for the contingencies andunpredicted charges. The rest of the FCI is attributed for piping,instrumentation, electrical works, painting, contract fee, and gen-eral and administrator over heads.

The total project investment for a plant without a pretreatmentunit was 56.4 million Euros. The average annual production ofmethane from these feedstocks and in the base case (i.e.,200,000 tons/year capacity) was 70.5 million Nm3; however, if no

pretreatment is used, this amount reduces to 36.8 million Nm3/year. Although the total operating costs for pretreatment andsteam requirement are reduced from the methane manufacturingcost, it will still increase to 0.56 €/m3 when pretreatment is notintroduced. Thus, the results showed that while the applicationof a pretreatment unit requires 13% increase in the total capitalinvestment, it reduces the manufacturing cost of the producedmethane by 35.7%.

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Met

hane

pro

duc�

on c

ost (

€/m

3 )

Raw material price (€/kg)

Fig. 5. Effect of raw material price on manufacturing cost of methane from papertube residuals (triangles) and wheat straw (squares). For each raw material the basecase is presented by empty points.

-0.060 -0.040 -0.020 0.000 0.020 0.040 0.060

Fresh water (0; 0.1; 1 /ton)

HP Steam (5; 8; 10 /ton)

Methane yield(95%; 90%; 80%)

LP Steam 2 (5;3;0 /ton)

Solid residue (4;2;0 /ton)

M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60 59

3.3.1. Cost distributionTotal manufacturing cost from paper tube residuals and wheat

straw was 24.1 and 32.4 million €, respectively. These numberswere calculated by considering the credit of low pressure steamand solid residue as byproducts of the pretreatment. The respectivevalues for the credit of by products were �1.20 and �1.54 million€. Fig. 4 shows the breakdown of the methane manufacturing cost.The most significant fraction of this cost is attributed to the rawmaterials, mainly to the cost of lignocellulosic substrate. Utilitycost, mainly due to the consumption of high pressure steam, isthe second highest cost within the manufacturing cost for theprocess.

3.3.2. Sensitivity analysisEffect of feedstock price on the manufacturing cost of methane

was evaluated and is presented in Fig. 5. Furthermore, effect ofprices for fresh water and high pressure steam (HPS) as well aslow pressure steam (LPS) and solid residue as products was alsoinvestigated (Fig. 6). Moreover, effect of methane production yieldon the manufacturing cost of methane is presented in Fig. 6. Theresults of sensitivity analysis showed that manufacturing costwas highly sensitive to the price of the cellulosic raw material.For instance, 20% decrease in the price of these raw materials re-sulted in 7–8% reduction in the manufacturing cost of methane,whereas the price of other raw materials used in the process doesnot highly affect the manufacturing cost of the produced methane.This is in line of previous studies (Murphy and Power, 2009). Fur-thermore, the yield of methane production is a critical parameteraffecting the economy of the process, whereas the value of the

46%

1%

3% 1%

15% 7% 4%

2%

21%

Raw material Operating labor Maintenance and repairs

Operating Charges Utility Local taxes

Insurance Plant overhead General expenses

Dir

ectM

anuf

actu

ring

Cos

t

(a)

58%

1%

2% 1%

12%

5%

4% 1%

16%

Dir

ct M

anuf

actu

ring

Cos

t

(b)

Fig. 4. Breakdown of manufacturing cost of methane from paper tube residuals (a)and wheat straw (b). (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.).

Variation in maufacturing cost of methane ( /m3) from base case depending on the price of each material.

Fig. 6. Effect of prices for fresh water, high pressure steam (HPS), low pressuresteam (LPS), and solid residue price as well as the methane production yield on themanufacturing cost of methane from pretreated paper tube residuals (colored bars)and wheat straw (empty bars). Methane price for base case is 0.36 €/Nm3 ofmethane for paper tube residuals and 0.48 €/Nm3 of methane for wheat straw. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

additional byproducts has a minor effect on the profitability. Whenthe yield improved from 90% to 95%, the manufacturing cost de-creased by 5.5%. Yield of methane production is directly relatedto the amount of cellulose and hemicellulose in the raw materialsas well as the process operation conditions. Further developmentfor purification and utilization of furfural and HMF as valuablebyproducts of the process can improve the profitability, and as aresult providing a clean steam free from the chemical contamina-tions, which can be reused in the process.

By selling the raw methane for 0.42 and 0.53 €/m3 from papertube residuals and wheat straw, respectively, the payback periodof the plant would be 5 years. Accordingly, when the selling priceincreases to 0.45 and 0.57 €/m3, the payback period would be3 years.

3.3.3. Energy balanceEnergy efficiency was calculated by dividing energy outputs

with the energy inputs of the process (Sassner et al., 2008). Theenergy is produced in forms of methane, solid residues, and lowpressure steam. The energy input includes steam and electricityrequirements of the process as well as the energy available in theraw materials. The energy balance of the plant is presented inTable 5. Total energy input of the base case is 153.9 ± 0.4 MW.Total energy output is 123.1 ± 0.5 MW in the form of lignin, lowpressure steam, and methane (Table 5). Therefore, total energyefficiency of the process is 80%. In another study, it was shown that

Table 5Energy requirements for the process utilizing 200,000 tons/year raw material.

kg/s Combustion heat (J/kmol) Mol. weight MJ/kg MW

Paper tube Wheat straw Paper tube Wheat straw

Input materials and energy for the processCellulose 4.17 3.46 2.81E + 09 162 17.3 72.3 60.0Xylan 1.18 1.87 2.35E + 09 132 17.8 21.0 33.3Lignin 1.60 1.62 3.27E + 09 122 26.7 42.8 43.4HP Steam 5.03 4.34 11.0 9.5LP Steam 1 2.05 2.13 4.2 4.4Electricitya 3.0 3.0Total 154.3 153.5Output materials and energy for the processLignin 1.57 1.59 3.27E + 09 123 26.7 41.6 42.1LP steam 2 4.53 3.91 9.4 8.1Methane 1.45 1.45 8.00E + 08 16 50.1 72.6 72.5Total 123.6 122.7

a Efficiency of electricity production was assumed to be 33%.

60 M. Shafiei et al. / Bioresource Technology 148 (2013) 53–60

the energy efficiency of a biorefinery for production of bioethanoland biogas from spruce wood was 79% (Shafiei et al., 2011). Fur-thermore, it was shown that the overall energy efficiency of biogasproduction is much higher than that of ethanol production (Mur-phy and Power, 2009). The total required energy for the processesutilizing paper tube residues and wheat straw were 18.2 and16.9 MW, respectively, which were 25% and 23% of the fuel energyof the produced methane (72.6 and 72.5 MW) (Table 5). Therefore,it can be concluded that methane production by the process eval-uated in this study can be a promising and economically feasiblealternative for renewable energy production.

4. Conclusion

This study shows that steam explosion pretreatment of wheatstraw and paper tube residuals can significantly improve the econ-omy of biogas production plants. The sensitivity analysis showsthat the methane yield and the raw material price were the mostimportant parameters affecting the manufacturing cost.

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