Comparative assessment of the energy effects of biomass combustion
and co-

ﬁring in selected technologies

Stanis

ław Tokarski

, Krzysztof G

łod

, Marek

Scia

˛ _zko

, Jaros

ław Zuwała

TAURON Polska Energia S.A, Poland

Institute for Chemical Processing of Coal, Poland

a r t i c l e i n f o

Article history:
Received 15 December 2014
Received in revised form
18 May 2015
Accepted 14 June 2015
Available online 10 July 2015

Keywords:
Biomass co-

ﬁring

Biomass
Energy requirement
Utility plants
CFB boilers

a b s t r a c t

The rational use of renewable energy sources depends largely on the processing technology applied and
is a key element of sustainable development. The most widespread method of producing electricity from
renewable sources in power plants involves the co-

ﬁring of biomass with fossil fuels, utilizing the

existing infrastructure. In recent years, many existing units have been modernized to enable co-

ﬁring,

including dedicated systems for feeding biomass directly to the combustion chamber. Simultaneously,
work has also begun on building new units designed for the exclusive combustion of biomass, whereas
other units have been retro

ﬁtted to replace coal with biomass combustion.

To establish a comparative assessment of the energetic and environmental effectiveness of the con-

version of biomass into electricity, indicators are developed based on the unit consumption of energy,
chie

ﬂy electrical, that is used to prepare the fuel. These indicators reﬂect the effect the energy consumed

by different biomass conversion processes have on CO

emissions. The results of the calculations are

presented for technologies involving combustion and co-

ﬁring in both pulverized fuel and ﬂuidized bed

boilers. To determine the energy required from the biomass and coal portion of the mixture fed to the
boiler, a consistent methodology is developed analogous to the method of avoided fuel inputs used to
determine the indicators during the cogeneration of electricity and heat.

The input data for the simulations originated from industrial tests performed under different tech-

nological con

ﬁgurations.

1. Introduction

The generation of CO

free energy can be achieved by either

implementing CCS (carbon capture and storage) technologies or
partially (or completely) substituting fossil fuels with biomass
(commonly considered a CO

-neutral fuel). Apart from having a

positive environmental effect in terms of CO

direct emission re-

ductions, biomass combustion reduces the depletion of non-
renewable fuel resources.

The world CO

emissions statistics (2008) illustrate that 41% of

total CO

emissions originate from electricity and heat generation

industries. The share of the energy industry in total anthropogenic
GHG (greenhouse-gas) emissions volume reaches 83%, of which
94% is CO

[1]

thus, reducing these emissions in the energy supply

sector remains an important issue.

Currently, the Polish energy sector is facing a number of serious

challenges due to obligation to reducing CO

emission by 2020,

while maintaining a high level of energy security. The TAURON
group, second largest energy producer in Poland, joined R

Strategic Programme

“Development of a technology for highly

ﬁcient zero-emission coal-ﬁred power units integrated with CO

capture

”

[2]

developing post-combustion CO

capture

[3]

and oxy-

fuel combustion technology

[4,5]

, as well as biomass co-

ﬁring

[6]

ﬁnd economically and technically acceptable way to reduce CO

emissions.

This paper presents a newly developed methodology for

determining the energy- and environment-related indicators to
support the scienti

ﬁc debate concerning the real effectiveness of

the technological options available for the conversion of biomass
into electricity during combustion and co-

ﬁring. The methodology

used to determine the indicators for units that are used for the co-
ﬁring or combustion of biomass is consistent with analogous to the
method of reduced fuel inputs used to determine indicators for the
cogeneration of electricity and heat

[7]

* Corresponding author.

E-mail address:

kglod@ichpw.pl

(K. G

łod).

Contents lists available at

ScienceDirect

Energy

j o u r n a l h o me p a g e :

w w w . e l s e v i e r . c o m/ l o ca t e / e n e r g y

http://dx.doi.org/10.1016/j.energy.2015.06.044

0360-5442/

Energy 92 (2015) 24

e32

The Polish Draft Act on Renewable Sources of Energy

[8]

lists the

available renewable (non-fossil) energy sources, including wind
energy, solar energy, aerothermal energy, geothermal energy, hy-
drothermal energy, hydroenergy, energy from sea currents, tides
and waves, and energy obtained from biomass, biogas, agricultural
biogas and bioliquids. Each of these sources is subject to different
conditions for the exploitation of its full potential, in which a
notable factor is the average time over the course of a year during
which the source can be used. For example, the available time for
the use of solar energy by means of photovoltaic cells is approxi-
mately 700 h; for wind power plants, the maximum is approxi-
mately 2500 h; a biomass-

ﬁred power plant can operate at its rated,

nominal output for ca 7000 h; and the annual operational time of
biomass co-

ﬁring units is close to that of conventional, coal-ﬁred

units

[9]

To obtain a full picture, renewable sources of energy should also

be analysed in terms of their effect on the energy security of a
country. Based on the de

ﬁnition of energy security, as established in

the Polish Energy Law

[10]

e “Energy security is a state of the

economy which enables the satisfaction of the current and antici-
pated needs of customers for fuels and energy in a technically and
economically justi

ﬁed manner and with adherence to the re-

quirements of environmental protection

”, it can be concluded that

among the analysed types of renewable energy sources, only the
use of biomass, as a form of accumulated solar energy that is
relatively easy to store and transport, appears to satisfy all of the
conditions included in the de

ﬁnition.

As previously mentioned, biomass plays a fundamental role in

reducing effective carbon dioxide emissions in conventional elec-
tricity production by replacing coal

e fossil fuel; the combustion of

fossil fuels is a main source of anthropogenic CO

emissions. The

need to reduce global emissions of carbon dioxide, in the absence of
any other substitute fuels with a zero CO

emission index, estab-

lishes biomass as the only direct substitute for coal that can provide
the possibility of zero-emission combustion. These considerations
have driven the development of technologies for the co-

ﬁring and

combustion of biomass. In the present work, these technologies are
assessed in terms of their energy, economic and environmental
effectiveness.

Aspects of the assessment of biomass combustion and co-

ﬁring

on energy and ecological effects have already been subject to an-
alyses e.g.,

[11,12,13]

. The economic aspects of biomass co-

ﬁring

have been studied with regard to incentive systems

[14]

. The pro-

cess simulation package, ECLIPSE has been used here to analyse
three different biomass co-

ﬁring conﬁgurations and to perform

techno-economic assessment studies of each technology. The SI

(speci

ﬁc investment) and BESP (Break-Even Electricity Selling

Price) for each system were calculated and compared with the coal-
ﬁred plants. The sensitivity of the economics of these large power
plants to such factors as fuel cost, load factor and insurance, oper-
ational and maintenance costs for two discount cash

ﬂow rates was

investigated. The effect of applying the ROC (Renewable Obligations
Certi

ﬁcate) subvention to the economics of the power plants was

also assessed for a wide range of wood fuel costs. When retro

ﬁtting

coal-

ﬁred condensing power plants to co-ﬁre with biomass, two

methods were used to determine whether co-

ﬁring is an environ-

mentally friendly solution

[15]

to cover all signi

ﬁcant aspects of the

electricity production process that may in

ﬂuence the environment:

carbon footprint and energy evaluation. These environmental ac-
counting approaches were selected to determine the maximum
supply distance of biomass that allows the co-

ﬁring of coal and

biomass to be more environmentally ef

ﬁcient than pure coal

combustion. Furthermore, the geological origin of the coal com-
busted was considered because the environmental inputs for
feedstock varied. The results of the study showed that the addition
of approximately 20% biomass to the mass of the combustion
mixture decreases carbon-dioxide emissions by nearly 11

e25% and

total energy

ﬂow by 8e15%.

A previous study

[16]

evaluated the technical and economic

aspects of biomass co-

ﬁring electricity production with and without

CC (CO

capture) using different mixtures of coal and sawdust. The

effect of biomass co-

ﬁring on the performance of power plants was

evaluated in terms of energy ef

ﬁciency, auxiliary power consump-

tion, capital costs, operational

& maintenance costs, speciﬁc CO

emissions, electricity cost and CO

avoidance costs. Depending on

the feedstock composition, the biomass co-

ﬁring power plant

generated 750

e800 MW electricity in the case of carbon capture

and 980

e1027 MW electricity without capture. This indicated a

continuous decrease in both technical and economical perfor-
mances with increasing biomass content in the feedstock.

A techno-economic model for the estimation of economics of

co-

ﬁring was previously presented

[17]

using pilot plant test results

for biomass co-

ﬁring and general heat and mass balances. Addi-

tionally, a sensitivity analysis was performed with this developed
model to investigate the effects of different operating and logical
parameters on the economics of the biomass coal co-

ﬁring process.

In the process of co-

ﬁring of coal and biomass, directly deter-

mining the quantity of energy consumed by the auxiliary needs of
devices (e.g., blowers, ventilators, mills) that are associated directly
with the biomass is not technically possible. Therefore, an indirect
method must be used to determine the components related to the
ef

ﬁciency of the conversion of biomass and the actual reduction in

carbon dioxide emissions. In the present work, a detailed analysis
of this problem is undertaken using an example of a co-

ﬁring

process performed in a PF (pulverized fuel boiler) with the co-
ﬁring of sunﬂower husks and spent hops at three characteristic
loads(electric power output): 200 MW, 180 MW and 140 MW. The
identical procedure was applied for the co-

ﬁring of biomass in a

ﬂuidized bed boiler (CFB) at characteristic electric power output:
66 MW, 56 MW and 42 MW. In the case of biomass combustion in a
dedicated

ﬂuidized bed boiler, the process energy requirement

could be calculated directly from measurements taken for the
system as a whole.

In consequence, a de

ﬁnitive comparison of the following in-

dicators is possible: unit consumption of energy on driving process
devices, boiler net energy ef

ﬁciency, efﬁciency of electricity pro-

duction from biomass, and reduction in carbon dioxide emissions.

In this paper, the results of the calculations are reported for

technologies related to the combustion and co-

ﬁring of pulverized

fuel and in

ﬂuidized bed boilers. The results are based on data that

were

obtained

trial

tests

performed

under

different

Abbreviations

CCS

carbon capture and storage

GHG

greenhouse gas

carbon capture

pulverized fuel boiler

CFB

circulating

ﬂuidized bed boiler

hard coal

sun

ﬂower husks

brewer's spent grain

cereal waste

biomass mixture

ACC

alternative coal con

ﬁguration

COF

co-

ﬁring

REF

reference con

ﬁguration

S. Tokarski et al. / Energy 92 (2015) 24

e32

technological con

ﬁgurations and loads. The results enable a reliable

evaluation of the effectiveness of producing electricity from
renewable sources in the analyzed technological conditions.

2. Experimental

The study was conducted on the following four generating units

using industrial processes:

biomass co-combustion in a CFB boiler type CFB-260 (boiler

speci

ﬁcation: 260 Mg/h, temperature and pressure steam:

540

C and 13.7 MPa,

biomass co-ﬁring in a pulverized coal ﬁred boiler type OP-650k

(boiler speci

ﬁcation: stream steam 650 Mg/h, temperature and

pressure live steam: 540

C and 13.5 MPa),

100% biomass ﬁred ﬂuidized bed boiler type OFz-201 (boiler

speci

ﬁcation: stream steam 200 Mg/h, temperature and pres-

sure live steam: 510

C and 9.7 MPa), and

100% biomass ﬁred burning pulverized coal boiler type OP-120

(boiler speci

ﬁcation: stream steam 120 Mg/h, temperature and

pressure live steam: 500

C and 7.6 MPa).

The biomass co-

ﬁring in a ﬂuidized bed boiler CFB-260 was fed

by steam coal, sun

ﬂower husks and cereal waste. Tests were per-

formed for a baseline coal combustion and for the co-combustion of

coal with biomass for three characteristic boiler loads

e 100%, 80%

and 60%

e which correspond to the unit electric power outputs of

66 MW, 56 MW and 42 MW, respectively. Fuel analysis of coal,
biomass used in the co-

ﬁring trial tests shown in

Tables 1A, 1B, 1C

The biomass co-

ﬁring in pulverized coal ﬁred boiler OP-650k

was fed by steam coal and biomass in the form of brewer's spent
grain and sun

ﬂower husks. Tests were performed at baseline coal

combustion, and co-combustion tests of coal with biomass were
performed for three characteristic loads of 89%, 80% and 62%, which
correspond to the electric load of 200 MW, 180 MW and 140 MW.
Fuel analysis of coal, biomass used in the co-

ﬁring trial tests is

shown in

Table 2

. Mass fraction of Brewer's spent grain in the fuel

mixture was 2,8% for electric power output of 140 MW and 5,2% for
200 MW test. Mass fraction of sun

ﬂower husks in the fuel mixture

was 4,1%, 3,5% and 4,8% for electric power output of 140 MW,
180 MW and 200 MW test respectively.

The unit dedicated exclusively to biomass combustion in a

biomass boiler type

ﬂuidized OFz-201 uses a biomass mixture of

wood chips and sun

ﬂower husk pellets. Tests were performed on

the combustion of biomass for several characteristic loads

30 MW, 40 MW, 45 MW and 50 MW

e which correspond to 60%,

80%, 90% and 100% boiler load level, respectively. Fuel analysis of
coal, biomass used in the trial tests shown in

Table 3

Because of the requirements of the boiler manufacturer, tests

were performed on only one type of blend, with sun

ﬂower husks

forming 22% of the mass fraction and wood chips forming the
remaining 78%.

Technical solutions are available for the fuel delivery systems

(forest biomass and agro), i.e., blend the biomass before its delivery
to the boiler bunkers to burn a mixture during the administration of
the share, regardless of the current load of the boiler.

In the unit dedicated exclusively to biomass combustion in the

pulverized coal boiler OP-120, a biomass mixture of wood chips,
wood pellets and agro biomass was used. Three mixtures were
tested and were composed of wood chips, wood pellets and
biomass agro for three characteristic loads: 20 MW, 16 MW and
12 MW. At these loads, the steam boiler ef

ﬁciency (depending on

the parameters of the steam outlet) are in the range of
102.5

e106 Mg/h, 81e90 Mg/h and 71e74.6 Mg/h, respectively. Fuel

analysis of coal, biomass used in the trial tests shown in

Tables 4

and 5

To obtain stable parameters, the chemical composition of the

bed material and the particle size of the bed material during the co-

Table 1A
Fuel analysis of coal used in the co-

ﬁring trial tests (CFB-C).

Parameter

Coal

Coal sludge

Coal

Coal sludge

21.4

18.9

38.8

19.2

37.8

2.9

2.8

2.9

1.9

13.5

18.1

16,2

48,6

1.53

1.43

1.05

1.64

0.66

NHV

19 962

19 455

8 063

19 903

7 839

100

where:
Eel is the electric power outputs, MW
W

is the total moisture, %;

is the moisture, %;

is the ash, %;

is the total sulphur, %;

NHV is the is the net calori

ﬁc value, kJ/kg;

is the mass participation, %.

Table 1B
Fuel analysis of coal and biomass used in the co-

ﬁring trial tests (CFB-SU).

Parameter

Coal

Sun-

ﬂower husks

Coal

Sun-

ﬂower husks

Coal sludge

Coal

Sun-

ﬂower husks

Coal sludge

17.4

11.1

38.1

21.4

39.4

3.5

3.8

2.9

4.1

1.7

3.7

3.8

2.2

13.9

3.9

16.3

4.1

48.4

12.5

3.8

48.8

1.52

0.13

1.36

0.13

0.72

1.75

0.12

0.65

NHV

21 019

16 843

19 598

16 708

7 793

20 216

17 031

7 563

Table 1C
Fuel analysis of coal and biomass used in the co-

ﬁring trial tests (CFB-CE).

Parameter

Coal

Cereal waste

Coal

Cereal waste

Coal sludge

Coal

Cereal waste

Coal sludge

17.4

11.8

16.7

12.7

18.6

11.9

3.1

3.8

2.6

3.7

1.9

3.5

3.9

1.8

19.3

6.3

22.2

6.1

16.2

6.2

51.1

1.41

0.12

1.54

0.12

0.96

1.64

0.13

0.92

NHV

19 576

14 754

18 846

14 701

7 235

20 044

14 811

6 806

S. Tokarski et al. / Energy 92 (2015) 24

e32

ﬁring tests in the ﬂuidized bed boiler, the combustion of the fuel
mix started the day before testing. After each change in boiler load
to a mixture of a given type, the boiler was run for at least two
hours to obtain stable thermal parameters. The biomass boiler fed
two systems using the basic installation of biomass supply and the
installation of the modernized administration of the bed material.
Delivery systems for fuels (coal, coal sludge, and biomass) allow a
steady stream of fuel to be established, regardless of a change in
boiler load or a change to another fuel stream.

During the test, a series of changes in boiler ef

ﬁciency was

performed by varying the steady supply amount of coal, coal sludge
and limestone while maintaining the supply of biomass. This
regulation of the amount of fuel fed to the boiler reduced the boiler
load and increased the share of biomass fuel in the mixture. During
a series of tests at 60% load, the addition of coal sludge was halted.
Because of the need to meet the demand for heat when the tests
were conducted, the heat production on the exchanger resulted
from external weather conditions.

For a stable performance of the boiler during the tests of the co-

ﬁring pulverized coal ﬁred boiler, as in the case of the ﬂuidized bed
boiler, the mixture of fuel was ignited on the day before the test.
After each change in boiler load, the boiler was run for at least two
hours to obtain stable thermal parameters.

To ensure the administration of a particular type of biomass in a

given time, each biomass was fed into the boiler by two systems
through the primary biomass feeding installation and a simpli

ﬁed

installation. The share of biomass in the mixture was determined
by the coal and biomass weights averaged over the duration of the
tests. To obtain stable operating parameters in the pulverized boiler
during the combustion of only the biomass, the combustion of the
mixture of biomass was initiated on the night preceding the given

series. After each change in boiler load, the boiler was run for at
least two hours to obtain stable thermal parameters.

The biomass is fed to the boiler using a biomass feeding

installation, which allows any mixture of the three types of
biomass, i.e., two types of dry biomass (pellets of agricultural and/
or forestry biomass) and dried biomass in the form of chips.
Appropriate shares of different types of biomass in the mixture are
obtained by adjusting the steady stream of biomass in the form of
pellets with an appropriate ratio of agro and forest biomass.

The process was analyzed using measurement systems installed

on the plant. All resulting measurements and the analysis of the
composition of the samples are the average of the series of mea-
surements performed for a given boiler load and the composition of
the supplied fuel.

The energy ef

ﬁciency of the boiler including an uncertainty was

estimated regarding to proper procedures (according to standard
No. PN-EN 12952-15 Water-tube boilers and auxiliary installations.
Acceptance tests.). It is linked to a fractional uncertainty of indi-
vidual measurements.

3. Results and discussion

3.1. Process energy requirements for biomass co-

ﬁring

The consumption of electrical energy to drive the principal

boiler devices during co-

ﬁring that are associated with both coal

and biomass, which have different properties, depends partly on
the proportion of biomass in the mixture. For this reason, the dif-
ference in the consumption of energy for the needs of auxiliary
devices, as shown in

Table 6

, does not exhibit an unambiguous

trend.

To determine the process requirements associated with the

particular fuels present in the mixture (that is, separately for the
coal fuel and biomass), an indirect method must be used. For this
purpose, the consumption of energy must be analyzed by boiler
device for the case of combustion of coal fuel alone (the reference
test). These results must be compared with the results of the tests
of biomass co-

ﬁring using the proposed methodology described

below.

To determine the process energy consumption and the reduc-

tion in CO

emissions resulting from the production of electricity

when using biomass as a substitute for fossil fuel, three electricity
production scenarios were considered:

The reference conﬁguration with a unit used for coal-only

combustion. The indicators relating to the operation of the
unit, the boiler and auxiliary devices in this case were taken as
reference (baseline) values.

Co-ﬁring, in which a portion of the chemical energy of the fuel

supplied to the boiler comes from biomass.

ACC (alternative coal conﬁguration) determines the indicators

relating to the operation of the unit based on data obtained from
the analysis of the above two con

ﬁgurations, assuming that the

chemical energy from the biomass is replaced by energy from

Table 2
Fuel analysis of coal and biomass used in the co-

ﬁring trial tests.

Parameter

Coal

Brewer's spent grain

Coal

Brewer's spent grain

Test

PF-C

PF-HO

PF-SU

12.1

7.1

50.3

8.8

7.2

1.3

1.1

5.1

1.2

5.9

33.1

30.5

18.6

28.5

2.6

1.08

1.06

0.43

1.16

0.13

NHV

18 706

20 633

7 237

20 791

18 910

Table 3
Fuel analysis of biomass used in the trial tests (CFB-BM).

Parameter

Wood chips

Sun

ﬂower husks

Wood chips

Sun

ﬂower husks

40, 45, 30

50, 50opty

40.3

10.0

45.1

10.4

6.0

6.4

5.9

4.9

16.6

3.3

3.5

3.7

0.03

0.14

0.04

0.16

NHV

8 273

16 739

9 131

17 337

Test after optimization of its operation.

Table 4
Fuel analysis of biomass used in the trial tests (PF-BM).

Parameter

Wood chips

Wood pellets

Sun

ﬂower husks

37.7

9.2

10.6

5.7

4.9

5.7

2.5

1.6

2.9

0.03

0.06

0.12

NHV

10 973

16 988

16 555

S. Tokarski et al. / Energy 92 (2015) 24

e32

the coal (equivalent coal). If the coal used in the reference and
co-

ﬁring scenarios have identical physicochemical parameters,

then the indicators obtained for the alternative coal con

ﬁgura-

tion should be identical.

For a clear interpretation of the de

ﬁned component loads

associated with the conversion of the fuels used in the various
electricity production scenarios, refer to

Fig. 1

Based on the measured results

[18]

covering the tests that

involved the combustion of coal fuel only (without biomass),
henceforth called the reference tests, a graph was plotted (

Fig. 2

) for

the total load on the principal boiler devices (coal mills, mill ven-
tilators, primary and secondary air ventilators, waste gas ventilator)
depending on the rate of supply of coal to the boiler. This rela-
tionship is represented by three points for different boiler loads,
which were approximated by a second-degree polynomial. This

characteristic is a function of the fuel stream, despite the variability
in quality parameters. This relationship is supported by the fact that
the calori

ﬁc value of a given type of coal is a linear function of the

total percentage content of moisture and ash (

Fig. 3

). These pa-

rameters primarily affect the consumption of energy during the
grinding of coal and the quantity of waste gases produced; these
effects are re

ﬂected in the consumption of energy by process de-

vices. The assumption of this dependence is also supported by the
basic characteristics of the operation of the individual process de-
vices. Such a relationship is valid within the range of analysis and
for the analyzed group of devices. Previously performed analyses
also con

ﬁrmed this assumption

[19]

Based on the data shown in

Table 6

for the rate of supply of coal

fuel to the boiler in the mixture with biomass and the relationship
obtained (in the form of a second-degree polynomial), as shown in

Fig. 2

, the load on boiler process devices which is attributable to the

coal fuel, P

COF, can be determined. The results of the analysis are

Table 5
Mass participation of biomass

, % in fuel mixture used in the trial tests.

Type of biomass

PF-BM1 20

PF-BM2 20

PF-BM2 16

PF-BM4 16

PF-BM2 12

PF-BM4 12

Wood chips

Wood pellets

Sun

ﬂower husks

where:
PF-BM1 20 described in abbreviations: PF- Pulverized Fuel boiler, BM

e biomass mixture, 20 e characteristic electric power output, MW;

Table 6
Real electrical load on boiler process devices.

Parameter

PF-C 200

PF-C 180

PF-C 140

PF-HO 200

PF-HO 140

PF-SU 200

PF-SU 180

PF-SU 140

NHV

19 385

19 276

18 706

20 633

20 791

COF

97.1

88.1

71.9

89.6

65.0

87.6

79.8

62.6

chC

COF

522.8

471.9

373.9

513.8

374.4

506.2

460.9

361.7

NHV

7 237

18 910

COF

0.0

4.9

1.9

4.4

2.9

2.7

chB

COF

0.0

9.8

1.9

23.2

15.2

14.1

REF

2496.4

2358.8

2015.7

COF

2547.2

2016.5

2521.4

2424.1

2056.4

50.9

0.8

25.1

65.3

40.7

where:
PF-HO 200 described in abbreviations: PF- Pulverized Fuel boiler- type od boiler, HO

e brewer's spent grain -type of fuel used in the trial test:, 200 e characteristic electric

power output, MW;
NHV

/NHV

is the net calori

ﬁc value of the coal/biomass, kJ/kg;

COF/M

COF is the stream of coal/biomass supplied in the mixture for co-

ﬁring, Mg/h;

chC

COF/Q

chB

COF is the stream of chemical energy supplied in the coal/biomass, MW;

REF is the total electrical load on process devices during coal combustion, kW;

COF is the total electrical load on process devices during co-

ﬁring of coal and biomass, kW; and

T is the difference between the electrical loads on process devices during co-

ﬁring and in the reference test, kW.

Fig. 1. Conceptual scheme for determining component loads on process devices; the
symbols are detailed in the description of the methodology.

Fig. 2. Load on boiler devices as a function of the rate of supply of mass of coal fuel (for
boiler type OP-650k).

S. Tokarski et al. / Energy 92 (2015) 24

e32

given in

Table 7

; the unit load on process devices,

COF, is

computed using

(1)

COF

(1)

where

COF is the component electrical load on process devices

attributable to coal in the co-

ﬁring process, kW; M

COF is the

stream of coal supplied in the mixture for co-

ﬁring, Mg/h; (

Table 7

The load on the process devices attributable to biomass was

determined as the difference between the measured total load
during co-

ﬁring, P

COF, and the component load during co-

ﬁring

attributable to coal, P

COF (as determined above). The results of

the calculations of the formulae below are shown in

Table 8

COF

¼ P

COF

(2)

where

COF is the total electrical load on process devices during co-

ﬁring of coal and biomass, kW; P

COF is the component electrical

load on process devices attributable to coal in the co-

ﬁring process,

COF

(3)

where

COF is the unit load on process devices attributable to the

biomass in the co-

ﬁring process, kWh/Mg.

COF is the stream of biomass supplied in the mixture for co-

ﬁring, Mg/h; (

Table 8

As the load on the unit decreases, the unit electrical load on the

process devices attributable to biomass signi

ﬁcantly increases

during the co-

ﬁring process, and in both cases, a rising trend is

observed as the load on the unit is reduced. For the unit electrical
load attributable to coal during co-

ﬁring, an increase is noted in the

electrical load when the load on the unit decreases, but in this case,

the change is small (

Table 7

). This result indicates that a change in

the quantity of biomass supplied to the boiler, within the analyzed
range of loads, will cause signi

ﬁcant differences in the quantity of

process energy required by the boiler.

3.2. Consumption of equivalent fuel

The purpose of this analysis is to illustrate the effects of

replacing coal with biomass by determining the quantity of
“equivalent coal” that could replace the stream of biomass during
co-

ﬁring and to determine the process load for the alternative coal

scenario.

The equivalent quantity of conventional fuel that could replace

the stream of biomass can be found using Equation

(4)

EQU

$NHV

NHV

(4)

where

COF is the stream of biomass supplied during co-

ﬁring, Mg/h;

NHV

is the net calori

ﬁc value of the biomass, kJ/kg; and

NHV

is the calori

ﬁc value of coal during co-ﬁring, kJ/kg.

The total quantity of coal fuel used during the operation of the

boiler with coal-only combustion under the alternative coal
con

ﬁguration is given by

(5)

ACC

¼ M

COF

EQU

(5)

where

COF is the stream of coal supplied in the mixture for co-

ﬁring,

Mg/h; M

EQU is the stream of coal which could replace the biomass

(equivalent coal), Mg/h.

The electrical load on process devices in the alternative coal

scenario is found from the relationship shown in

Fig. 1

The results of the calculations are given in

Table 9

To assess the effect of the type (quality) of the fuel on the

electricity consumed to drive the principal boiler devices, re-
lationships were found based on the differences arising between
the different boiler operating con

ﬁgurations.

Fig. 3. Variation in calori

ﬁc value of coal as a function of total content of moisture and

ash.

Table 7
Load on boiler process devices attributable to coal fuel in the co-

ﬁring process.

Parameter

PF-HO 200

PF-HO 140

PF-SU 200

PF-SU 180

PF-SU 140

COF

2383.3

1830.1

2349.0

2196.5

1761.8

COF

26.6

28.2

26.8

27.5

28.1

where
P

COF is the component electrical load on process devices attributable to coal in the

co-

ﬁring process, kW; and

COF is the unit electrical load on process devices attributable to coal in the co-

ﬁring process, kWh/Mg.

Table 8
Component load on boiler process devices attributable to the biomass in the co-
ﬁring process.

Parameter

PF-HO 200

PF-HO 140

PF-SU 200

PF-SU 180

PF-SU 140

COF

163.9

186.4

172.4

227.6

294.6

COF

33.6

98.1

39.0

78.6

110.0

where:
P

COF is the component load on process devices attributable to the biomass in the

co-

ﬁring process, kW; and

COF is the unit load on process devices attributable to the biomass in the co-

ﬁring

process, kWh/Mg.

Table 9
Calculations of the equivalent coal stream and process electrical loads in the alter-
native coal con

ﬁguration.

Parameter

PF-HO 200

PF-HO 140

PF-SU 200

PF-SU 180

PF-SU 140

EQU

1.71

0.67

4.02

2.63

2.44

ACC

91.35

65.67

91.66

82.44

65.06

ACC

2411.2

1848.8

2416.1

2250.9

1831.8

where
M

EQU is the stream of coal which could replace the biomass (equivalent coal), Mg/

h;
M

ACC is the total stream of coal in the alternative coal scenario, Mg/h and

ACC is the load on process devices in the alternative coal scenario, kW.

S. Tokarski et al. / Energy 92 (2015) 24

e32

The component load on process devices attributable to the

“preparation” of the equivalent coal to replace the biomass was
found from the following equation:

EQU

¼ P

ACC

COF

(6)

where

ACC is the load on process devices in the alternative coal

scenario, kW.

COF is the component electrical load on process devices

attributable to coal in the co-

ﬁring process, kW.

The indicator of unit electrical load on the process devices per

unit mass of equivalent coal was determined from the following
equation:

EQU

(7)

EQU is the component process electrical load attributable to

the coal equivalent to biomass, kW; M

EQU is the stream of coal

which could replace the biomass (equivalent coal), Mg/h; The re-
sults of the calculations of the indicators for the coal equivalent to
biomass are given in

Table 10

The values obtained for the indices of process electricity con-

sumption per unit mass of equivalent coal are signi

ﬁcantly lower

than those obtained for biomass, but these values are also lower

than the values obtained for the coal combusted during co-

ﬁring.

The differences between the indicators found for the co-

ﬁred coal

and for the equivalent coal result from the idle load on the drives of
the process devices, which is attributable chie

ﬂy to work required

during the grinding of coal (co-

ﬁring). This contribution to the in-

dicators for co-

ﬁred coal decreases as the load on the unit

decreases.

3.3. Impact of process electricity consumption on the ef

ﬁciency of

the boiler and of electricity production

The principal indicators that determine the effectiveness of the

co-

ﬁring process and can be evaluated are the gross energy efﬁ-

ciency of the boiler and, in assessing the effect of the consumption
of energy for driving the auxiliary devices, the net ef

ﬁciency.

Because the type of fuel combusted did not affect the obtained

steam parameters (it was maintained on the same level) and
thereby mechanical and thermal ef

ﬁciency of the turbine, and thus

does not in

ﬂuence the efﬁciency of electricity production, the

effectiveness of the conversion of the chemical energy of the
biomass was evaluated only for the boiler. This conversion ef

ﬁ-

ciency was combined with a determination of the effect of the
biomass on the consumption of electricity to drive the boiler
devices.

The effect of the consumption of electricity by the principal

boiler devices on the (net) energy ef

ﬁciency of the boiler is given by

the following:

¼ h

mech

chF

$100

(8)

chF

is the stream of chemical energy of the fuel mixture sup-

plied for combustion [MW]; and
P

mech

is the mechanical energy required to drive the process

devices [MW].

Table 10
Comparison of loads between different operating con

ﬁgurations.

Parameter

PF-HO 200

PF-HO 140

PF-SU 200

PF-SU 180

PF-SU 140

EQU

27.9

18.7

67.1

54.4

70.0

EQU

16.3

28.0

16.7

20.7

28.7

where
P

EQU is the component process electrical load attributable to the coal equivalent to

biomass, kW; and

EQU is the unit electrical load on process devices attributable to the equivalent

coal in the equivalent coal scenario, kWh/Mg.

Table 11
Values obtained for the ef

ﬁciency of electricity production.

Parameter

PF-C 200

PF-C 180

PF-C 140

PF-HO 200

PF-HO 140

PF-SU 200

PF-SU 180

PF-SU 140

91.78

91.94

91.75

91.96

91.20

91.73

92.10

91.07

91.26

91.39

91.16

91.43

90.60

91.21

91.54

90.47

P el RES

43.7

101.8

19.6

39.6

56.2

where

is the gross energy ef

ﬁciency of the boiler, %;

is the net energy ef

ﬁciency of the boiler, %; and

P el RES

is the indicator of the unit load for the production of electricity from renewable energy sources, kW/MW

RES

Table 12
Values obtained for the ef

ﬁciency of the boiler during testing.

Parameter

CFB-C 66

CFB-C 56

CFB-C 42

CFB-SU 66

CFB-SU 56

CFB-SU 42

CFB-CE 66

CFB-CE 56

CFB-CE 42

COF

31.7

36.1

54.9

31.7

32.6

50.6

EQU

21.4

13.7

2.9

20.8

24.9

4.2

COF

21.6

5.4

14.3

22.8

5.7

13.6

92.08

92.58

91.63

91.35

91.85

91.62

91.88

92.17

91.37

91.31

91.65

90.91

90.60

90.93

90.90

91.10

91.23

P el RES

13.8

3.0

8.9

16.5

4.0

9.8

where

COF is the unit electrical load on process devices attributable to coal in the co-

ﬁring process, kWh/Mg.

EQU is the unit electrical load on process devices attributable to the equivalent coal in the equivalent coal scenario, kW/Mg;

COF is the unit load on process devices attributable to the biomass in the co-

ﬁring process, kWh/Mg;

is the gross energy ef

ﬁciency of the boiler,%;

is the net energy ef

ﬁciency of the boiler, %; and

P el RES

is the indicator of the unit load for the production of electricity from renewable energy sources kW/MW

RES

S. Tokarski et al. / Energy 92 (2015) 24

e32

The indicator of the quantity of process energy used for the

production of electricity from renewable sources is the indicator of
the unit electrical load on the process devices when electricity is
produced from such sources and is computed from the following
equation:

Pel RES

COF

el RES

(9)

where

COF is the component electrical load on process devices

attributable to biomass in the co-

ﬁring process, kW; E

el RES

is the

production of electricity from renewable energy sources, MW

RES

Calculation results are shown in

Table 11

3.4. Indicator values obtained for co-

ﬁring in a ﬂuidized bed boiler

and for biomass combustion in a dedicated system

The table below contains the

ﬁnal results calculated in accor-

dance with the proposed procedure for assigning the energy
consumed to drive the principal boiler devices during the process of
co-

ﬁring of coal fuel and biomass.
For units burning biomass only, the results are given for the

determined energy ef

ﬁciency of the boiler and for indicators of unit

consumption of process energy for the conversion of biomass.

Table 12

contains the results of an analysis performed for a

ﬂuidized bed boiler in which coal fuel (coal together with coal
slurry) is co-

ﬁred with sunﬂower husks and cereal waste at three

different values for the load on the unit (66 MW, 56 MW and
42 MW).

At the time of the test measurements, the unit was being used

for electricity and heat cogeneration, which caused a reduction in
electrical output.

Table 13

contains the results of an analysis performed for a

ﬂuidized bed boiler used for the combustion of BM (biomass) only.
The biomass was a mixture of wood chips and sun

ﬂower husks in a

ratio of 78:22. Measurements were taken for characteristic levels of
load on the unit: 30 MW, 40 MW, 45 MW and 50 MW. In addition,

indicators were calculated for the unit following the optimization
of its operation

eCFB-BM 50 opt.

Table 14

contains the results of calculations for a pulverized

fuel boiler used for the combustion of biomass only. The biomass
was a mixture of wood chips and sun

ﬂower husk pellets in various

proportions for different biomass types. Measurements were
taken for characteristic loads on the unit: 20 MW, 16 MW and
12 MW.

4. Conclusions

The indicators determined according to this methodology

enable a comparison and unequivocal assessment of the effec-
tiveness of the conversion of the chemical energy of biomass to
electrical energy. These indicators may be used as a basis for further
energetic, environmental and

ﬁnancial analyses.

From the results of the calculations performed for technologies

involving the combustion or co-

ﬁring of biomass in pulverized fuel

and

ﬂuidized bed boilers, the lowest process energy consumption

for the production of electricity from renewable energy sources is
obtained in the case of the co-

ﬁring of biomass in a ﬂuidized bed

boiler based on the data obtained in tests using different techno-
logical con

ﬁgurations. In the co-ﬁring of biomass in a ﬂuidized bed

boiler case, the energy ef

ﬁciency of the boiler is close to that ob-

tained by a

ﬂuidized bed boiler dedicated to the combustion of

biomass only. The highest indicators of process energy required per
unit of energy produced are recorded for the technology of co-

ﬁring

in a pulverized fuel boiler. This, however, is the technology for
which the greatest net energy ef

ﬁciency of the boiler was obtained

out of all of the technologies analysed. Another advantage of this
technology is the possibility of obtaining the greatest ef

ﬁciency of

electricity production because of the thermal parameters of the
steam supplied to the turbine.

In spite of their superior indicators, co-

ﬁring technologies are

subject to limitations in terms of the maximum quantity of biomass
that can be added. Therefore, to produce the identical quantity of
renewable electricity as in a dedicated unit, the co-

ﬁring process

has to be performed in several units.

Table 13
Values obtained for the ef

ﬁciency of the boiler during testing.

Parameter

CFB-BM 30

CFB-BM 40

CFB-BM 45

CFB-BM 50

CFB-BM 50 opt

90.61

90.30