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Full-scale co-firing trial tests of sawdust and bio-wastein pulverized coal-fired 230 t/h steam boiler

Jaroslaw Zuwala*, Marek Sciazko

Institute for Chemical Processing of Coal, Zamkowa St. 1, 41-803 Zabrze, Poland

a r t i c l e i n f o

Article history:

Received 25 January 2008

Received in revised form

13 February 2008

Accepted 11 March 2010

Available online 1 April 2010

Keywords:

Biomass

Co-firing

Slagging and fouling

* Corresponding author. Tel.: þ48 32 271 00 4E-mail address: zuwala@ichpw.zabrze.plURL: http://www.ichpw.zabrze.pl

0961-9534/$ e see front matter ª 2010 Elsevdoi:10.1016/j.biombioe.2010.03.003

a b s t r a c t

Co-firing trial tests of sawdust and bio-waste coming from cereal production with hard coal

were carried out at Skawina Power Plant in Poland (1532 MW in fuel, currently belonging to

CEZ Group). Skawina Power Plant is a tangentially-fired pulverized coal unit with nine

boilers (4 boilers of 210 t/h and five boilers of 230 t/h live steam respectively) that produces

590 MW electricity and 618 MW of heat (district heating and process steam).

The paper presents an analysis of energy and ecological effects of sawdust and bio-waste

co-firing in the existing pulverized hard coal boiler. The mixture of coal and biomass was

blended in the coal yard, and fed into the boiler through the coal mills. During the tests,

combustion of mixtures composed of hard coal and sawdust (with mass share of 9.5%) and

hard coal e bio-waste (6.6% mass basis) were examined. The co-firing tests were

successful. Based on the analysis of the test results, the influence of biomass co-firing on

specific components of energy balance (e.g. stack losses and boiler thermal efficiency) was

discussed, in comparison to combustion of coal alone. The emission indices during coal

combustion were calculated and compared to the emission indices for biomass co-firing. It

was proved that co-firing of both biomass sorts leads to a decrease of CO and SO2 emis-

sions. Due to the possibility of considering the part of the energy generated during biomass

co-firing as renewable energy, the procedure for biomass based renewable energy share

determination is presented and illustrated with an example.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction At present, the greatest interest is directed towards

The use of biomass for the production of renewable energy in

the power industry is a technology which in spite of its

increasing implementation until the present day still requires

extensive promotion to develop intensively. All new opera-

tional and research experience in this field, especially with

regard to the co-firing of biomass and hard coal in thermal

power generation and cogeneration plants, promote actions

being taken towards optimizing the process (thermal cycle)

and emission parameters.

1; fax: þ48 32 271 08 09.(J. Zuwala).

ier Ltd. All rights reserved

processes that allow the co-firing of biomass and coal in utility

boilers. This proved to be the fastest track towards the use of

biomass in large capacity power units, simultaneously

guaranteeing the high conversion efficiency of the chemical

energy contained in the combusted fuel.

The present paper presents the selected results of biomass

co-firing process and emission research done for pulverized

OP-230 type coal boiler operating in one of the Polish system

power plants. Boilers of this capacity, being a base of 200 MW

power units very common in Poland are representative for the

.

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 41166

most of the Polish power generating sector. The entire scope

of the presented research was executed by the Institute for

Chemical Processing of Coal in Zabrze, Poland.

Due to the fact that the legal regulations in force allow (in

accordance with European RES Directive [1]) a certain share of

the energy produced in the biomass co-firing process to be

classified as renewable energy, the paper also gives the

methodology of balancing energy generated in the co-firing

process and originating from renewable energy sources. The

algorithm is illustrated by an example calculation.

Fig. 2 e Skawina Power Plant location and other CEZ plants

in the close neighborhood.

2. Co-firing experience at the Skawina PowerPlant

2.1. Description of Skawina power plant

The bituminous coal-fired combined heat and power plant

Skawina has an installed capacity of 590 MW electric and

618 MW thermal, supplying electricity to the local distribution

and trading companies and district heat mainly to the city of

Krakow and the city of Skawina. It produces about 2.8 TWh of

electricity per annum. Within its combined production of

power and heat, the plant supplies heat and hot water to the

westernpart of Krakowand the townof Skawina (Figs. 1 and 2).

Skawina is located 21 km southwest of Krakow, the third

largest city and cultural capital of Poland. Polish abundant

reserves of hard coal provide a secure source of fuel. Skawina

is strategically located on the electric grid in the greater Kra-

kow area, and is critical to maintaining the balance of the

electric system. One of Krakow’s principal providers of heat,

supplying over 20% of the city’s thermal requirements, Ska-

wina’s thermal output is necessary to ensure improved reli-

ability of the district heating system.

In Poland, the obligation for energy utilities to purchase

electricity produced from RES (renewable energy sources) (the

“quota system”) was introduced in December 2000. The Ordi-

nance of the Ministry of Economy of 14th August 2008 [2]

confirms quotas of electricity sold to the final consumers con-

nected to the grid on the territory of the Republic of Poland

during the period of 2010e2017 as follows: 10.4% for 2010–2012,

Fig. 1 e Skawina Power Plant.

10.9% for 2013, 11.4% for 2014, 11.9% for 2015, 12.4% for 2016 and

12.9% for 2017. This system of quotas also allows electricity

produced from co-firing of biomass or (biogas) and fossil fuels

taking into account the proportion of chemical energy of used

renewable energy fuel. The same regulation of the Minister of

Economy covers the obligatory requirements for the certifica-

tionandcontrol systemsallowing for theaccounting of a proper

share of electricity produced during the co-firing process, as

“green electricity”.

Co-firing tests of sawdust and bio-waste with hard coal

have been carried out at Skawina Power Plant.

2.2. Fuel analysis

Biomass sorts used in the co-firing tests were:

� sawdust originating fromwood (a mixture of deciduous and

coniferous tree wood with particle sizes of ca. 5 mm and

upper calorific values as received amounting to 13 MJ kg�1),

and

� bio-waste from cereal production and processing (pressing

of barley and rye grains as well as chicory and sugar beet

roots, with particle sizes of ca. 10 mm and upper calorific

values as received amounting to 8 MJ kg�1). The scope of the

processing provided by the supplier exclusively includes

mechanical and thermal treatment that does not cause any

mechanical fouling or chemical contamination of the fuel.

The processing is executed in such a way that such bio-

waste is considered as biomass according to the defini-

tions covered by [2,3].

Having investigated the biomass market and logistic

capability rates, a (maximum) mass share of biomass in the

fuel blend was established at the level of 6.6% (for bio-waste

co-firing) and 9.5% (for sawdust co-firing). To increase the

transparency and clarity of renewable energy settlement

procedures the above-mentioned types of biomass are not co-

fired simultaneously.

Tables 1 and 3 present the fuel analysis of biomass, coal

and fuel blends for co-firing tests. For the purposes of coal and

biomass co-firing tests, two blends were prepared: fuel blend

Table 1 e Fuel analysis of coal, bio-waste and blend “A”used in the co-firing trial tests.

Parameter Components Blend“A”

Coal 1 Coal 2 Coal 3 Coalslurry

Bio-waste

Proximate analysis

Total moisture

Wtr, %

7.10 12.60 6.90 24.40 73.50 17.41

Moisture, Wa, % 3.20 4.40 2.10 4.60 6.40 4.03

Ash, Aa, % 36.60 31.0 8.00 49.50 3.70 30.20

Total sulphur,

Sta, %

0.72 0.74 0.62 1.28 0.32 0.80

Incombustible

sulphur, SAa , %

0.26 0.31 0.15 0.46 0.03 0.29

Combustible

sulphur, SCa, %

0.46 0.43 0.47 0.82 0.29 0.51

Higher heating

value, Qsa, kJ/kg

18,748 20,136 31,271 13,204 20,757 20,358

Lower heating

value, Qia, kJ/kg

17,959 19,270 30,159 12,550 19,446 19,477

Lower heating

value, Qir, kJ/kg

(as received)

17,137 17,407 28,560 9438 3755 16,623

Table 3 e Fuel analysis of coal, sawdust and blend “B”used in the co-firing trial tests.

Parameter Components Blend“B”

Coal 1 Coal 2 Sawdust

Total moisture Wtr, % 12.20 12.60 56.20 16.54

Moisture, Wa, % 4.10 4.50 8.60 4.68

Ash, Aa, % 28.10 22.60 16.90 24.93

Total sulphur, Sta, % 0.74 0.90 0.04 0.73

Incombustible sulphur,

SAa , %

0.30 0.31 0.04 0.28

Combustible sulphur,

SCa, %

0.44 0.59 0.00 0.46

Higher heating value,

Qsa, kJ/kg

21,487 23,341 16,356 21,707

Lower heating value,

Qia, kJ/kg

20,587 22 371 15,189 20,755

Lower heating value,

Qir, kJ/kg (as received)

18,642 20,266 6007 18,061

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 4 1167

of symbol “A”, containing coal and bio-waste, and as fuel

blend “B”, composed of coal andwood sawdust. Tables 2 and 4

give the relationships between the mass shares of coal,

sawdust and bio-waste.

Table 5 presents the ash analysis and the ash-fusion

temperatures of bio-waste and sawdust used in the co-firing

tests. These parameters have been later used for the evalua-

tion of the influence of biomass co-firing on boiler furnace

slagging and fouling by means of fusibility correlations [4].

2.3. Blending method

Coal is delivered to Skawina Power Plant by rail transport. The

coal is unloaded into the storage yard. Coal is transferred from

the coal yard to the loading hopper by a bulldozer and from

there to the power plant coal bins by belt conveyors. There

also is a possibility of a direct unloading of the coal from

lorries onto the conveyors. The sawdust and the bio-waste

wood waste used in the tests were transported to the power

plant by trucks, the capacity of which ranged between 40 and

120 m3 (about 13e40 tonnes each). The space in the coal yard

was limited, so the biomass was stockpiled just outside the

main coal yard.

Table 2 eMass shares of individual components in blend“A” .

Specification Mass,Mg

Mass share [%]

Component Sizegrade

Asreceived

Calculated as drymass

Bio-waste e 38 6.64 1.92

Coal 1 Fine coal 240 41.96 44.07

Coal 2 Fine coal 100 13.99 20.30

Coal 3 Pea coal 80 16.43 17.50

Coal slurry e 120 20.98 16.20

For the co-firing tests, coal and biomass were mixed by

a bulldozer in the coal yard. Because the blending of coal and

sawdust were done in the coal yard, some minor fluctuations

in the blending ratio were expected.

2.4. Description of the tests

The co-firing tests at Skawina Power Plant were carried out

during February and March 2006. Week-long continuous

testing was conducted. Before that test program, some very

simplified preliminary tests were carried out, mainly based on

injecting small proportions of sawdust mixed with coal. The

main objectives of the tests were:

- to mix both biofuels separately with two different coal

blends,

- to test different proportions of sawdust and bio-waste and

to find the maximum mass share of biomass with which it

was technically possible to co-fire at Skawina Power Plant,

and

- to check the effect of co-firing on the ESP (electrostatic

precipitator) operation.

During the co-firing tests, the following process values

were monitored:

- fuel behavior on the coal conveyors, in the hoppers and coal

mills,

- combustion behavior (ignition and flame stability),

Table 4 e The mass share of individual components inblend “B” mass.

Specification Weight,Mg

Weight [%]

Component Sizegrade

Asreceived

Calculated as drymass

Sawdust - 30 9.51 4.77

Coal 1 fine coal 165 52.30 55.24

Coal 2 fine coal 120.5 38.19 39.99

Table 5 e The ash analysis and the ash-fusiontemperatures of sawdust and bio-waste used in theco-firing tests.

Coal Blend “A”(6.6% bio-waste)

Blend B 9.5%sawdust

Calculated chemical constitution of coal ash and fuel blends ash

(weight percentage)

SiO2 53.91 52.89 53.81

Al2O3 29.67 28.30 29.45

Fe2O3 6.80 6.64 6.77

CaO 2.45 3.53 2.46

MgO 2.06 2.20 2.07

Na2O 0.70 0.75 0.70

K2O 2.97 3.21 2.99

P2O5 0.09 0.24 0.38

SO3 0.00 0.19 0.00

Mn3O4 0.00 0.06 0.00

TiO2 1.35 1.30 1.34

BaO 0.00 0.00 0.00

SrO 0.00 0.00 0.00

Mass share of

co-firing ash in the

total ash amount, %

0 6.438 0.758

Table 6 e Boiler energy balance elements for bio-wasteand coal for blend “A” co-firing tests.

Specification Unit Blend “A”without bio-waste

Blend A

Boiler steam output t/h 212.80 217.60

Boiler thermal power MW 148.90 152.18

Boiler thermal load % 92.52 94.61

Average fuel flux t/h 25.50 31.06

Chemical energy flux MW 163.22 167.95

Steam energy flux MW 204.93 209.27

Heat losses

Stack MW 12.01 11.38

% 7.36 6.78

Ignition (unburnt CO

chemical enthalpy)

MW 0.01 0.02

% 0.01 0.01

Unburned carbon in

slag / bottom ash

MW 0.08 0.51

% 0.05 0.30

Unburned carbon

in fly ash

MW 1.32 2.81

% 0.81 1.67

In slag/bottom ash MW 0.07 0.13

% 0.04 0.08

In fly ash MW 0.14 0.25

% 0.09 0.15

To environment

(radiation and convection)

MW 0.66 0.67

% 0.41 0.40

Boiler efficiency (gross) % 91.23 90.61

Table 7 e Boiler energy balance elements for sawdust forblend “B” co-firing tests.

Specification Unit Blend Bwithoutsawdust

Blend“B”

Boiler steam output t/h 208.30 201.70

Boiler thermal power MW 145.79 141.51

Boiler thermal load % 90.56 87.69

Average fuel flux t/h 25.89 24.38

Chemical energy flux MW 158.24 154.09

Steam energy flux MW 199.93 193.98

Heat losses

Stack MW 10.36 10.37

% 6.55 6.73

Ignition (unburnt

CO chemical enthalpy)

MW 0.02 0.01

% 0.01 0.01

Unburned carbon in

slag / bottom ash

MW 0.05 0.13

% 0.03 0.09

Unburned carbon

in fly ash

MW 1.20 1.25

% 0.76 0.81

In slag/bottom ash MW 0.06 0.07

% 0.04 0.04

In fly ash MW 0.11 0.12

% 0.07 0.08

To environment

(radiation and convection)

MW 0.65 0.64

% 0.41 0.42

Boiler efficiency (gross) % 92.14 91.83

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 41168

- slagging and fouling (process data and visual inspections),

- heat transfer in the furnace (process data and furnace exit

temperatures)

- emissions (SO2, NOx, dust),

- burn-out (UBC e unburned carbon in fly ash and quality of

bottom ash), and

- quality of by-products.

The duration of the test period was seven days and during

that time the sawdust and bio-waste were mixed with several

types of Polish coal. Two different mass shares of biomass

were tested during the test.

2.5. Test results e process and emission parameters ofco-firing

The main objective of the research was to conduct a full

measurement of the process parameters to evaluate the

energy and emission operational indices of OP-230 type boiler

while co-firing blends of coal and biomass.

During the tests, the blending process was carried out in

the coal yard. The fuel blend was then transported to the

power plant bins by belt conveyors. The behavior of the fuel

blend in the coal mills (being primarily a major concern while

co-firing biomass using the direct technology) was carefully

monitored through the tests. Specifically, special attention

was paid to the mill power consumption, grinding results and

mill outlet temperatures.

Grinding coal and biomass mixture in the ball mills had

aminor effect on the coal fineness results: the amount of bigger

particles increased and the amount of smaller parts decreased.

Both caused a negative effect on the burn-out efficiency of coal.

Based on the measurement, the energy balance for the

relevant fuel combinations was developed and the energy

indices were calculated.

Tables 6 and 7 compare the basic energy parameters of the

co-firing process, for bio-waste and sawdust respectively.

Differences in the individual parameters of energy balance

were studied in order to determine the impact of biomass co-

firing on the individual thermal and calorific parameters

comprising the energy balance of the boiler. The comparative

Table 8 e Relative energy characteristics of coal and bio-waste co-firing.

Item Specification Unit Parameter value Relativeefficiency

[%]Blend Awithoutbio-waste

Blend“A”

1 Boiler thermal

efficiency (gross)

% 91.23 90.61 99.3

2 Stack loss % 7.36 6.78 92.1

3 Loss of ignition % 0.01 0.01 100.0

4 Unburned carbon

in slag / bottom ash

% 0.05 0.30 600.0

5 Unburned carbon

in fly ash

% 0.81 1.67 206.2

6 Loss in slag /

bottom ash

% 0.04 0.08 200.0

7 Loss in fly ash % 0.09 0.15 166.7

Table 10 e The relative emission characteristics of coaland bio-waste co-firing.

Item Specification Unit Parameter value Relativeemission

[%]Blend Awithoutbio-waste

Blend“A”

1 SO2 mg/GJ 361.8 238.4 65.9

2 NO2 mg/GJ 94.3 65.5 69.5

3 CO mg/GJ 5.7 4.3 75.5

4 Dust mg/GJ 16.9 12.6 74.5

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 4 1169

analysis involved boiler thermal efficiency as well as the

determined components of heat losses.

After determination of the parameters with coal firing and

during biomass co-firing, the results can be presented in the

form of the relative efficiency and are given in Tables 8 and 9.

The values and the results obtained in the course of the

measurements allow a comparative analysis of the influences

on the process efficiency and the emission levels of firing basic

fuel and biomass co-firing to be conducted. Based on the

conducted research on coal and biomass co-firing in OP-230

type boilers, it may be concluded that, based on the experi-

ment results co-firing of coal blends with mass share of 6.6%

of bio-waste or 9.5% of sawdust does not cause instability of

operation of the boiler itself or of fuel feeding systems. The

existing boiler fuel feeding systems as well as the coal milling

installations guarantee the achievement of a homogeneous

coalebiomass blend, which is fundamental for the correct and

undisturbed operation of pulverized coal burners.

The achieved boiler thermal efficiency in the case of

sawdust co-firing coal is practically unchanged (the difference

amounts to ca. 0.3 of a percentage point in comparison to the

Table 9 e Relative energy characteristics of coal andsawdust co-firing.

Item Specification Unit Parameter value Relativeefficiency

[%]Blend Bwithoutsawdust

Blend“B”

1 Boiler thermal

efficiency (gross)

% 92.14 91.83 99.7

2 Stack loss % 6.55 6.73 102.7

3 Loss of ignition % 0.01 0.01 100.0

4 Unburned carbon

in slag / bottom ash

% 0.03 0.09 300.0

5 Unburned carbon

in fly ash

% 0.76 0.81 106.6

6 Loss in slag /

bottom ash

% 0.04 0.04 100.0

7 Loss in fly ash % 0.07 0.08 114.3

coal firing tests). Measured thermal losses for both boilers

were at a similar level, except for the unburned carbon loss,

which was 3 times higher. This is caused by the fact that the

amount of bigger particles was increased and the amount of

smaller particles was reduced. Both have a negative effect on

the burn-out efficiency of the coal.

The achieved boiler thermal efficiency in the case of bio-

waste co-firing is reduced by ca. 0.62 of a percentage point,

while stack heat loss was maintained at a similar level as in

the case of the reference coal firing. The remaining losses

related to incomplete fuel combustion and the physical

enthalpy of ash and slag were increased, which may again

indicate the influence of non-homogeneity of the received fuel

mixture.

The steam temperature and the flue gas temperatures

before and after LUVOweremonitored during the tests. All the

values were in the usual range of 150e160 �C.Measured levels of pollutants such as CO, SO2, NO2 and

dust were used to determine the emission efficiency charac-

teristics during coal and biomass co-firing. The obtained

values were subsequently recalculated to give the pollutant

emission indices. Relative emission efficiencies for these

parameters are presented in Tables 10 and 11.

Additionally, the pollutant emissions levels were

compared with the permissible levels set by the Polish stan-

dards acc. to Journal of Laws. No. 163, item 1584 as shown in

Table 12 and in Figs. 3 and 4.

The addition of biomass to coal caused a reduction of

emissions of all analyzed pollutants, except for the emission

of dust, which increased during co-firing of bio-waste. The

addition of sawdust to coal resulted in the reduction of dust

emissions (ca. 40%), the reduction of SO2 (c. 11%) and CO

emissions (ca. 11%). During co-firing NO2 emission level

Table 11 e The relative emission characteristics of coaland sawdust co-firing.

Item Specification Unit Parameter value Relativeemission

[%]Blend Bwithoutsawdust

Blend“B”

1 SO2 mg/GJ 316.1 314.4 99.5

2 NO2 mg/GJ 81.2 81.2 100.0

3 CO mg/GJ 3.4 3.8 110.3

4 Dust mg/GJ 10.7 7.1 67.1

Table 12 e The comparison of calculated emission indices with the permitted limits.

Item Emission Unit Parameter valueb Permissible levelsa

Blend A without bio-waste Blend “A” Blend B without sawdust Blend “B”

1 CO mg/m3 1964.0 1833.5 1988.1 1759.5 2350

2 SO2 mg/m3 511.7 503.8 510.7 454.5 600

3 Dust mg/m3 91.7 96.8 67.0 40.0 350

a According to Journal of Laws, No. 163, item 1584 [5].

b Calculated as 6% O2.

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 41170

remained practically unchanged when compared to the firing

of the reference coals.

The level of pollutant emissions to the environment while

firing coal and co-firing coal with biomass meets the

requirements determined by the Ordinance of the Minister of

the Environment [3].

2.6. Test results e evaluation of the influence of biomassco-combustion on boiler furnace slagging and fouling

Boiler surfaces slagging hazard depends strongly on the

properties of ash, which can be described by characteristic ash

fusion temperatures: initial deformation temperature, soft-

ening temperature, hemispherical temperature and flow

temperature. Ashes over softening temperature can be

strongly adhesive, which results in slagging. For precise

prediction of ash fusibility, various correlations between

fusion temperatures and standardized (given in the form of

oxides) chemical composition of ashes have been determined.

The most popular parameters used for slagging hazard

determination are [4]:

� Ratio of basic (B) compounds to acidic compounds (A) in

a direct form cm:

cm ¼ Fe2O3 þ CaOþMgOþNa2Oþ K2Oþ P2O5

SiO2 þAl2O3 þ TiO2(1)

or in a simplified form:

Rðb=aÞ ¼ Fe2O3 þ CaOþMgOSiO2

(2)

0

500

1000

1500

2000

2500

CO SO2 Dust

Co

ncen

tratio

n in

th

e flu

e g

as, m

g•m

-3

permissible levelcoalcoal + bio-waste

Fig. 3 e The comparison of calculated emission indices

during coal combustion and coal and bio-waste co-firing

with the permitted limits.

� Fouling index Fu

Fu ¼ cmðNa2Oþ K2OÞ (3)

� Slag viscosity index e SR ratio

SR ¼ SiO2

SiO2 þ Fe2O3 þ CaOþMgO(4)

Although application of fusibility correlations does not

ensure entirely reliable evaluation of slagging and fouling

inclination in other than laboratory conditions, it can be

useful by comparing the effects of firing various fuels in the

same boiler [4].

Based on Table 5, the chemical constitution of coal and

biomass blend ashes were determined and the obtained

values (1)e(4) were used for the fusibility correlations to

determine the slagging and corrosion hazard resulting from

co-firing of sawdust and bio-waste. The results are given in

Table 6 and in Fig. 5.

Obtained values of cm and R(b/a) are similar to coal alone

firing (Fig. 5). According to [4], R(b/a) value around 0.15 gives

information about low slagging hazard. This can also be

confirmed by the rather high ash melting temperatures of

both biofuels (Table 5).

The significant influence of biomass co-firing on the values

of Fu is also shown in Fig. 5. It is especially noticeable for

sawdust co-firing due to its higher ash content compared to

bio-waste co-firing. Values of fouling index over 0.6 are

representative for fuels of significant fouling character. Co-

0

500

1000

1500

2000

2500

CO SO2 Dust

Co

nc

en

tra

tio

n in

th

e flu

e g

as

, m

g•m

-3

permissible levelcoalcoal + sawdust

Fig. 4 e The comparison of calculated emission indices

during coal combustion and coal and sawdust co-firing

with the permitted limits.

0

1

2

3

4

5

6

7

8

9

10

cm R(b/a) Fu

0

10

20

30

40

50

60

70

80

90

Sawdust Bio-waste Coal Sawdust co-firing Bio-waste co-firing

Sawdust Bio-waste Coal Sawdust co-firing Bio-waste co-firing

SR

Fig. 5 e The comparison of the calculated indices for determination of the slagging hazard possibilities.

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 4 1171

firing of sawdust and bio-waste can then bear a strong fouling

risk.

Some sources describe the boiler surfaces corrosion hazard

as higher when a co-firing plant applies NOx reducing tech-

nologies. In the case of Skawina plant, a pulverized coal based

reburning system and OFA systems are applied to reduce NOx,

which means the corrosion hazard and its possible diagnostic

tools should be the subject of further research.

3. Co-firing based renewable energy sharedetermination method

Due to the significant difference in the price of electrical

energy generated from renewable sources and fossil fuel

plants, the method of qualifying both kinds of energy gener-

ated in a co-firing process required unambiguous and

coherent procedures to be set up. The lack of such procedures

could create a risk of unjustified cases of qualifying energy

generated from conventional fuels (coal) as renewable energy.

In addition to the unjustified increases of income, such an

occurrence would make the sales of real renewable energy

sources difficult (an increase in competitiveness).

Consequently, resulting from the above situation, the

generation of renewable energy could statistically grow, while

in reality it would be conventional energy. In addition to the

lack of expected advantageous social and environmental

effects, such a situation could lead to an increase in the prices

of electricity.

Energy sources where conventional fuel (hard coal) and

renewable fuels (sawdust and bio-waste) are fired are not

entirely renewable energy sources. According to the stipula-

tions [5], only a portion of the electrical energy and heat

generated in generating units (determined as sets of

Table 13 e Production data from the plant’s 24-houroperation period.

Coal LHV [kJ/kg] 19,543

Biomass LHV [kJ/kg] 6200

Heavy fuel oil LHV [kJ/kg] 42,500

Hard coal consumption [Mg] 3945

Biomass consumption [Mg] 68

Heavy fuel oil consumption [Mg] 1

Coal derived chemical energy GJ 77,097

Biomass derived chemical energy GJ 422

Heavy fuel oil derived chemical energy GJ 43

Total chemical energy of all the fuels GJ 77,561

Electricity sales (net production) MWh 5691

Renewable electricity sales (net

production)

MWh 30.93

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 41172

equipment, designated to produce electricity or heat,

described by means of commercial and technical data) jointly

co-firing biomass or biogas and other kinds of conventional

fuels in the combustion process, are qualified as renewable

energy.

Due to the use of the existing header systemof connections

(interconnecting pipelines) between the combustion units in

the plant, all boilers are to be treated as one combustion unit.

In order to break down the entire amount of electricity

generated in the course of co-firing into renewable and

conventional energy portions, the formula presented in the

ordinance on obligatory purchase shall be applied directly [5]:

EO ¼Pn

i¼1 MBiWBiPni¼1 MBiWBi þ

Pmj¼1 MKjWKj

E (5)

where:

EO e biomass derived power (electricity or heat), MWh

or GJ,

E e total plant output (electric or thermal) , MWh or GJ,

MBi e mass of biomass combusted in the system, t,

Mkj e mass of fossil fuel combusted in the system, t

WBi e calorific value of combusted biomass, MJ$t�1,

WKj e calorific value of combusted fossil fuel, MJ$t�1,

n e number of biomass types combusted in the system;

m e number of fossil fuel types combusted in the produc-

tion unit.

Due to the above-mentioned collector system of connec-

tions of all the power boilers for renewable energy balancing

purposes, the existing thermal diagramof the power plant has

been simplified in such a way that all boilers and the turboset

have been replaced by one equivalent unit (Fig. 6).

Assuming the formula (5) for the existing plant set-up, the

formula can be adjusted as follows:

EO ¼ MBWdB

MBWdB þMwWdw þMMWdME (6)

where:

EO e biomass derived power (electricity or heat), MWh

or GJ,

E e total plant power (electricity or heat), MWh or GJ,

Fig. 6 e Simplified plant diagram for calc

MB e mass of biomass co-fired in the plant weighed with

belt conveyor scales, t;

MW e mass of coal co-fired in the plant weighed with belt

conveyor scales, t;

MM e mass of heavy fuel oil co-fired in the plant, t;

WdB e lower heating value of co-fired biomass, MJ t�1,

Wdw e lower heating value of co-fired coal, MJ t�1,

WdM e lower heating value of co-fired heavy fuel oil, MJ t�1,

The quantity of renewable energy produced in the power

plant during 24 h has been determined using formula (6) and

the data collected during those 24 h, when a fuel blend con-

sisting of coal and the biomass was co-fired (presented in

Table 13). The calculation algorithm is represented graphically

in Fig. 7.

4. Conclusions

The paper presents the results of coal and biomass co-firing

tests in a pulverized coal steam boiler of 230 t/h live steam

production, operating in Skawina Power Plant in Poland.

Process and emissionmeasurements aswell as the balance

calculations led to the conclusion that coal and biomass co-

firing consisting of bio-waste in the mass share of 6.6% or

ulating the renewable energy share.

Fig. 7 e Graphic presentation of the renewable energy share calculation algorithm.

b i om a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 4 1173

sawdust in the mass share of 9.5% do not negatively affect the

energy efficiency or emission levels of the tested boiler.

The determined emission levels of toxic pollutants result-

ing from coal firing and biomass co-firing meet all require-

ments of the emission standards as specified in the Regulation

of Ministry of Environment [3].

Fig. 8 e Direct injection of biomass to the coal ducts inside

the boiler house.

During the calculations it was found that a combination

of biomass and coal firing can result in worse fouling

conditions than coal firing alone. Therefore, the thickness of

ash deposits increase, which can also increase the slagging

intensity.

Basing on the trial tests results, the Skawina Power Plant

decided to co-fire biomass on the commercial scale. Presently,

after almost two years of direct co-firing of biomass, plant

owners decided to introduce a direct biomass injection to the

fuel feeding system omitting the coal mills which turned out

to be the serious bottleneck for increasing biomass share in

the fuel blend. The pilot installation was constructed (Fig. 8)

and is under the operational testing at the moment [5].

In all the analyzed cases, an advantageous influence of

biomass co-firing, consisting of reduced emissions of

pollutants into the atmosphere, was observed. Due to the

fact that the research covered only emission levels related

to the firing process, it is considered advisable to extend

further research work to the analysis of emission levels

related to all processes related to biomass processing, i.e.:

transportation and preparatory processes. It seems that

a good tool to execute this kind of analysis could also be the

so-called cumulative analysis of the full product life cycle,

popular in Europe [6].

r e f e r e n c e s

[1] Directive 2009/28/EC of the European Parliament and of theCouncil of 23 April 2009 on the promotion of the use of energyfrom renewable sources and amending and subsequentlyrepealing Directives 2001/77/EC and 2003/30/EC.

[2] Regulation of the Minister of Economy of 14th August 2008 onthe detailed range of obligations for obtaining and presentingfor redemption of certificates of origin, for making substitutefees as well as for purchasing of energy and heat producedfrom renewable energy sources and confirmation of dataconcerning the quantity of electricity produced fromrenewable energy sources (Journal of Laws, No. 156, item. 969).

b i om a s s an d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 1 6 5e1 1 7 41174

[3] Regulation of the Minister of Environment of 20th December2005 (Journal of Laws, No. 260, item. 2181).

[4] Pronobis M. Evaluation of the influence of biomass co-combustion on boiler furnace slagging by means of fusibilitycorrelations. Biomass Bioenergy 2005;28:375e83.

[5] Chaberka M, Zygmanski W, Sciazko M, Zuwala J. Thedevelopment of biomass co-firing concepts on the example of

Skawina Power Plant. Chapter in the monograph. In: Taler J,editor. Modern technologies and machinery for powerproduction. Cracow University of Technology, ISBN 978-83-7242-439-6; 2007 (in Polish).

[6] Christiansen HF. LCA of procurement and conversion ofbiomass and fossil fuels e used for energy production inDenmark; 1997.