low-grade coal and biomass co-combustion on fluidized bed: exergy analysis

Low-grade coal and biomass co-combustion

on fluidized bed: exergy analysis

Carmen Martına, Miguel A. Villamanana, Cesar R. Chamorroa,Juan Oterob, Andres Cabanillasb, Jose J. Segoviaa,*

aLaboratory of Thermodynamic and Calibration TERMOCAL, Department of Energy,

School of Engineering, University of Valladolid, E-47011 Valladolid, SpainbDepartment of Fossil Fuels, Research Centre for Energy,

Environment and Technology Ciemat, E-28040 Madrid, Spain

Received 31 July 2003

Abstract

The purpose of this work is to prove the technical feasibility of the bubbling fluidized bed co-combustion,

using biomass and low-grade coal mixtures and applying the exergy method. The pilot plant modelled is an

atmospheric bubbling fluidized bed combustion chamber with a nominal capacity of 1 MWth. We have applied

the mass balance, the energy balance and the exergy balance to the plant in nine experiments, which have been

performed at different operation conditions. The exergy analysis includes the calculation of the exergy

destruction and the exergetic efficiency of the plant for these experiments. An estimation of the irreversibility

cost is also evaluated.

q 2005 Elsevier Ltd. All rights reserved.

1. Introduction

Low coal rank or coal waste and biomass co-combustion in coal fired power plants is a good technique

that contributes to the reduction of greenhouse gases. It is also a solution to the waste disposal problem.

Co-firing biomass with coal has the capability to reduce both NOx and SOx levels from coal fired power

plants. It also reduces CO2 emissions, and soil and water pollution [1].

Energy 31 (2006) 330–344
www.elsevier.com/locate/energy
0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.energy.2005.01.008

* Corresponding author. Tel./fax: C34 983 423 420.

http://www.elsevier.com/locate/energy

Nomenclature

BL black lignite

cp specific heat capacity (J/kg K)

ctotal total cost per exergy flow input (V/year per kW)

Ctotal total cost (V/year)

Ca/S calcium/sulphur molar ratio

d.b. dry base_E exergy flow (MJ/h)_Ech chemical exergy flow (MJ/h)_Eph physical exergy flow (MJ/h)

Efconv carbon conversion (%)

Efret sulphur retention (%)

GD granulometrical distribution

H enthalpy

HHV higher heating value (kJ/kg)_I exergy destruction rate or irreversibility rate

k unit exergy cost

LHV lower heating value (kJ/kg)

mf fuel mass flow (kg/h)

P pine chips

p0 environment pressure

PA proximate analysis

Qloss heat loss to the environment (MJ/h)

Quseful heat rejected to the water (MJ/h)

RC refuse coal

Tbed bed temperature

T0 environment temperature (K)

UA ultimate analysis

vf fluidization velocity (m/s)

W work

x working hours

z cost of coal

DG0f standard molar Gibbs function of formation (J/mol)

30 standard molar chemical exergy (J/mol)

30el standard molar chemical exergy of an element (J/mol)

3ph specific physical exergy (J/kg)

h thermal or energetic efficiency

j exergetic or rational efficiency

C. Martın et al. / Energy 31 (2006) 330–344 331

In this context, the project deals with semi-industrial scale studies on the co-firing of different coal

wastes or low rank Spanish coal with biomass, in order to promote the clean, efficient use of these

resources. The biomass used in this study is made of pine chips, i.e. wood waste from forestry and wood

processing of the region where the pilot plant is located.

C. Martın et al. / Energy 31 (2006) 330–344332

This work is related to a previous one [2,3] which was involved in an EU-project ‘Combined

Combustion of Biomass/Sewage Sludge and Coals of High and Low Rank in Different Systems of Semi-

industrial and Industrial Scale’ the main results of which were summarized by Hein and Bemtgen [4].

The use of waste coal, with low heating value and high ash content, mixed with biomass is interesting

because of the complementary properties of the two fuels. Opposed to the typical characteristics of poor

coal, biomass has a low ash content and a high volatile content that is favourable, both from the technical

and environmental point of view, to a clean combustion of the coals that are being taken into

consideration. The goal of the project is to prove at semi-industrial scale the technical feasibility of the

fluidized bed as a clean technology for the combustion and determine the influence of co-firing of coal

waste and biomass on the combustion process, the gaseous emissions, the flue gas cleaning process, the

fireside fouling of the boiler and its influence on the heat efficiency and the composition of solid residues.

Co-combustion of coal and biomass has been quite extensively studied [1–9]. As it was presented by

Sami et al. [7] in a complete review of literature on coal co-firing with biomass fuels, biomass and coal

blend combustion is a promising combustion technology; however, significant development work is

required before large-scale implementation can be realized.

The main purpose of this paper is to present and discuss the most significant results achieved in the

bubbling fluidized bed combustion plant using exergy analysis. [10]

We do not try to undertake a deep study of the combustion process because this has been carried out

by other authors [11–13]; we have applied the exergy balance to the experimental installation and

calculated the exergetic efficiency of the process to quantify exergy destruction and parameters of the

installation that could improve the efficiency of the process.

2. Experimental section

2.1. Pilot plant

The pilot plant studied is an atmospheric bubbling fluidized bed combustion chamber. It has a

nominal capacity of 1 MWth. Fig. 1 shows the flowsheet scheme of the plant. The combustor vessel is

divided into three parts for easier maintenance. The lower part is a common windbox and a distributor

plate with a 1 m2 total surface. The distributor plate is formed by a staggered 608 array of 522 nozzles

supplied from the common windbox. The middle part is a vessel with 1.14 m internal diameter and 1.2 m

height. It is lined with approximately 2-inch. thick refractory materials, which are protected with

an abrasion-resistant paint. The third part is a vessel of 2.8 m height. The last 0.6 m of this vessel has an

increase diameter to minimize the elutriation of materials to the cyclone.

The air is provided by two fans which produce air flows at 500 N m3/h (secondary air) and

1500 N m3/h (primary and tertiary air). It is introduced into the combustor at three levels: the primary air

below the distributor plate, the secondary at the fuels feeding same location, and the tertiary about above

the bed. The fuels are introduced into the bed by means of a screw conveyor feeder.

Heat extraction is made by means of three heat exchangers: two of them located in the bed area and

the third one at the top of the freeboard to cool the flue gas. In Fig. 2, there is a detail of the heat

exchanger’s geometry. Each heat exchanger has 34 vertical section and 34 semicircular section and the

tube has a diameter of 2.54 cm.

BIOMASS

1

2

COAL

1

2

1 2

LIMESTONE

9

6

3

10

10

98

7

5

13

4

11

TEMPERATURE

PRESSURE

FLOW

12

O2 NO2

CO SO2

CO2 NO

1 HOPPER 2 FEEDING SCREW 3 COMBUSTOR

4 MULTICYCLONE 5 PRIMARY AIR 6 SECONDARY AIR

7 TERTIARY AIR 8 EXTRACTOR 9 ASH

10 HEAT EXCHANGER11 CONTROL 12 GAS SAMPLE COLLECTOR13 SAMPLES COLLECTED

Fig. 1. Scheme of the atmospheric bubbling fluidized bed pilot plant.


The pilot plant has a Hartmam and Braun distributed process control system, Contronic 3P. More

details of the pilot plant have been published previously [14].

2.2. Analytical procedures

For each fuel (black lignite, refuse coal or pine chips), proximate, ultimate, granulometrical

distribution, bed and fly ashes, and higher heating value have been determined by conventional methods.

14.47 cm

4.37 cm

Fig. 2. A detail of heat exchanger’s geometry.

Table 1

Proximate analysis (PA), ultimate analysis (UA), granulometrical distribution (GD), higher heating values, HHV and chemical

exergy, Ech for the fuels in dry base (d.b.)

Sample Black lignite Refuse coal Pine chips

PA (%d.b.)

Fixed carbon 30.50 9.12 21.41

Volatile 41.29 11.25 75.56

Ashes 28.84 79.63 3.03

Moisture 8.27 1.87 12.75

UA (%d.b.)

Carbon 48.76 12.67 52.03

Hydrogen 2.20 1.26 5.97

Nitrogen 0.90 0.53 0.41

Sulphur 9.86 1.48 0.07

Oxygen 9.44 4.43 40.08

Chloride !0.02 !0.02 !0.02

GD (wt%)

Mesh screen mm Partial Acumm. Partial Acumm. Partial Acumm.

C63 0.00 0.00 0.00 0.00 0.00 0.00

K63 C32 0.00 0.00 0.00 0.00 0.00 0.00

K32 C16 0.00 0.00 0.00 0.00 3.76 3.76

K16 C8 0.00 0.00 0.00 0.00 20.52 24.28

K8 C4 0.42 0.42 0.00 0.00 26.65 50.93

K4 C2 20.90 21.32 0.20 0.20 23.19 74.12

K2 C1 26.85 48.17 17.59 17.78 12.64 86.75

K1 C0.5 16.95 65.13 22.21 39.99 5.72 92.48

K0.5 C0.25 15.64 80.77 23.15 63.14 3.56 96.03

K0.25 C0.125 9.94 90.72 16.25 79.39 1.84 97.87

K0.125 C0.063 5.40 96.12 9.70 89.09 1.28 99.16

K0.063 3.88 100.00 10.91 100.00 0.84 100.00

HHV (kJ/kg) (db) 20,421 3936 20,630

Ech(kJ/kg) (db) 22,010 4220 21,469


The characteristics of the fuels employed in this study and their compositions are shown in Table 1.

Measurements of emissions were made continuously with a multicomponent infrared photometer,

Hartmam and Braun Uras 10P. It has five measuring channels: nitrogen oxide, nitrogen dioxide, sulphur

dioxide, carbon monoxide and carbon dioxide. The oxygen concentration was measured with a

zirconium oxide cell ETC 2000.

2.3. Combustion experiments

As fuels, two coals have been selected for study: black lignite and washing reject coal. The biomass

fuel for test runs was made of pine chips. This is the most common forest residue available in Spain,

often in mixtures with small amounts of other wood residues. It is also one of the most abundant residues

produced by Spanish wood industries.

We have analysed nine combustion experiments. The fuel used in them and their operation conditions

are shown in Table 2. The parameters that have been varied during experimentation are bed temperature,

Table 2

Operation conditions of the combustion experiments: number of the experiment, fuel used RC (refuse coal), P (pine chips), BL

(black lignite), percentage in weight of biomass, fuel mass flow (mF), bed temperature (Tbed), molar ratio of calcium and sulphur

(Ca/S) and fluidization velocity (vf)

Exp. Fuel %P mf (kg/h) Tbed (8C) Ca/S (mol) vf (m/s)

1 RCCP 58 294.3 850 5.99 0.63

2 RCCP 57 299.7 847 5.13 0.73

3 P 100 178.6 807 0.00 0.62

4 RCCP 57 272.7 805 2.66 0.62

5 BLCP 47 165.6 842 3.00 0.63

6 BLCP 47 165.6 840 2.99 0.71

7 BLCP 28 178.2 842 2.92 0.63

8 BLCP 28 178.2 850 2.92 0.73

9 BL 0 152.0 850 2.75 0.73


Ca/S molar ratio, biomass/coal ratio and fluidization velocity which have been achieved through the

modification of primary to tertiary air ratio.

3. Results

We have applied the mass balance, the energy balance and the exergy balance to the

atmospheric bubbling fluidized bed combustion chamber for the nine experiments referenced in

Table 2.

As inputs to the plant we consider the coal, the biomass, the limestone (which is added to avoid

sulphur oxides emissions) and the air. The outputs are the exhaust gases, the ash recovered in the cyclone

and in the bed and the heat transfer to the three heat exchangers. The difference between both represents

the heat loss to the environment.

The modelling of the experimental installation has been done using an EXCEL (MS) sheet where the

experimental data are introduced.

The uncertainties in the measured parameters are G2 8C for temperature; G0.1% (full-scale) for

pressure and G2% (full-scale) for mass flow.

3.1. Mass balance

Results of the mass balance are summarized in Table 3. This contains the composition of the exhaust

gas (dry base) per kJ input to the installation and normalized at 6% oxygen, these units allow a better

analysis of the emissions. In the last two columns, the efficiencies of carbon conversion (Efconv) and

sulphur retention (Efret) are shown.

Satisfactory combustion results have been achieved for black lignite and all mixtures coalCbiomass

studied, as shown in Table 3. All the experiments have achieved a carbon conversion related to the

residual heat value in the ash ranging 97.6–99.8%.

The addition of biomass to high sulphur black coal produces a decrease of SO2 emissions in

correspondence to the proportion of biomass, and, hence, the amount of sulphur, added. The increase of

Table 3

Flue gas composition expressed in dry base (mg/MJ) and 6% oxygen (mg/N m3)

Exp. Flue gas composition Efficiency

CO2 (%) Dry base (mg/MJ) 6% O2 (mg/N m3) Efconv (%) Efret (%)

SO2 CO NOx SO2 CO NOx

1 11.5 334 188 134 821 461 329 97.6 72.3

2 15.0 205 148 190 491 354 454 98.1 86.5

3 15.4 21 105 56 55 275 146 99.3 78.1

4 15.7 236 83 107 605 212 275 98.9 84.5

5 17.0 621 112 34 1864 337 101 98.7 91.0

6 16.9 538 130 39 1542 373 112 99.5 92.0

7 16.8 626 134 34 1898 406 104 98.4 92.8

8 17.7 687 113 33 1973 325 96 99.8 92.4

9 16.8 675 159 36 1963 461 105 99.4 94.1

Efconv, Efficiency of carbon conversion (%) and Efret, efficiency of sulphur retention (%).


fluidization velocity seems to cause a light reduction of SO2 emissions, both for refuse coal and black

lignite mixtures with biomass, increasing the sulphur retention rate. In mixtures of refuse coal and

biomass, the increase of molar Ca/S ratio, from about 2.5 to 5 did not cause any significant effect on the

sulphur retention rate which could be in connection with low sulphur content in fuel mixtures.

With all feed materials and mixtures, it was possible to reduce residual carbon content in bed ash to

!0.6%.

The NOx content in the combustion emissions remained at low levels in all cases. A slight increase in

the case of tip coal and biomass was observed. This fact is in connection to the higher content of nitrogen

in ash-free refuse coal when compared to ash-free black lignite.

3.2. Energy balance

The energy balance for a control region undergoing a steady-state reacting process may be written as

X_Q C

X_W Z

XP

_noutðHf CDHÞout K

XR

_ninðHf CDHÞin (1)

where the enthalpy of a compound is composed of Hf , associateD with the formation of the compound

from its elements at standard conditions, and DH, associated with a change of state at constant

composition. The equation can be rearranged to readX_Q C

X_W Z

XP

_noutHfout K

XR

_ninHfin C

XP

_noutDHout KX

R

_ninDHin (2)

The enthalpy flow reactants to the control region have the contributions of the fuels, the

limestone and the air. In addition to the combustion reactions, the reactions related to the

dissociation of the limestone and the formation of calcium sulphate must be taken into account, as


they are expressed below:

CaCO3/CaO CCO2

CaO CSO2 C1=2O2/CaSO4

The dissociation of CaCO3 is endothermic but formation of CaSO4 is exothermic through the

mass balance known. The net effect of limestone addition is that a part of the energy produced by

the combustion of the fuel is consumed to avoid the presence of SO2 in the flue gas.

As we have considered that the pressure and the temperature of the input matter flows are at standard

conditions, the term DHin for the reactants is zero. On the other hand, the enthalpy of combustion is the

lower heating value of the fuel with the minus sign.

For the products, the contributions are the flue gas and bed and cyclone ashes. We have taken into

account the temperature dependence of the heat capacity for the different components. The presence of

CaCO3, CaO, CaSO4, and CaS in the ash has been considered, so that the energy balance for the plant

can be written as:

X_Q þ

X_W ¼

Xfuel

_miðKLHVÞi þ _nCaCO3, H

fCaO þ H

fCO2

KHfCaCO3

� �

þ _nCaO, HfCaSO4

þ HfCaO KH

fSO2

� �þ

Xash

miðLHVÞash

þX

flue;gas

_mi

ðTout

T0

cpiðTÞdT þX

ash;bed

_mi

ðTout

T0

cpiðTÞdT

þX

ash;cyclone

_mi

ðTout

T0

cpiðTÞdT

(3)

The work input to the fans can be neglected:X

_W z0 (4)

Concerning the heat flow, the purpose of the installation is to recover heat to the water but there is also

a heat loss to the environment, so that both heat flows are negativeX

_Q Z _Quseful C _Qloss (5)

The heat transfer from the chamber to the water is calculated by measuring the flow matter and the

input and the output temperatures of the water for the three heat exchangers:

_Quseful ZKX3

iZ1

_miðhout;i Khin;iÞ (6)

In Table 4, we have summarized the more relevant results of the energy balance for the nine

experiments studied. The absolute value of useful heat flow transfers to the water (Quseful) (Eq. (6)); the

energy of the inputs to the chamber (Input) defined by

Table 4

Results of the energy balance: Heat transfer from the chamber to the water (Quseful), the energy of the input to the chamber

(Input), the energy output from the chamber (Output), the heat loss to the environment (Qloss) and the thermal efficiency of the

combustion chamber (h).

Exp. Quseful (MJ/h) Input (MJ/h) Output (MJ/h) Qloss (MJ/h) h (%)

1 1587.1 3256.5 2397.4 859.1 48.7

2 1596.4 3247.5 2428.8 818.7 49.2

3 1532.2 2907.2 2117.2 790.0 52.7

4 1563.2 2933.5 2208.0 725.5 53.3

5 1500.8 2762.8 2093.3 669.5 54.3

6 1524.2 2832.1 2131.3 700.8 53.8

7 1470.5 3080.0 2152.9 927.1 47.7

8 1603.6 3052.0 2332.9 719.1 52.5

9 1390.8 2524.6 1914.2 610.5 55.1


Input ¼Xfuel

_miðKLHVÞi � _nCaCO3, H

fCaO þ H

fCO2

KHfCaCO3

� �

þ _nCaO, HfCaSO4

þ HfCaO KH

fSO2

� �(7)

The energy of the outputs from the chamber (output) defined by

Output ZXash

miðLHVÞash CX

flue;gas

_mi

ðTout

T0

cpiðTÞdT CX

ash;bed

_mi

ðTout

T0

cpiðTÞdT

CX

ash;cyclone

_mi

ðTout

T0

cpiðTÞdT KQuseful (8)

where the heat loss to the environment (Qloss) is the difference between both and; the efficiency of the

combustion chamber (h) is calculated as the ratio between the absolute value of the heat transfer to the

water (Quseful) and the energy input to the chamber (input).

In Fig. 3, the energy balance is represented graphically as the distribution of the input energy (%).

It may be noticed that in the mixtures formed by refuse coalCbiomass (experiments 1, 2 and 4), the

energy input from biomass is not lesser than 86%.

The results show that the energetic efficiency of the process is some 50%, ranging from 47.7% for

experiment 7 to 55.1% for experiment 9, which is the highest efficiency achieved.

On the other hand, the heat loss to the environment represents 25% of the energy input, where 20% of

the outlet energy is found in the combustion gas.

3.3. Exergy balance

Finally, the experimental data of the pilot plant has been analysed using the exergy method which is

the main purpose of the present work.

The exergy balance for a control region undergoing a steady-state process may be written as

Fig. 3. Percentage distribution of input energy for the nine experiments studied.


_I Z _Einput K _Eoutput ZX

IN

_Ei KXOUT

_Ei (9)

where the exergy flow to or from the control region is associated with the inflow or outflow of matter,

heat transfer and work transfer. The difference between the two, the loss of exergy rate, is also called

exergy destruction rate or irreversibility rate (I).

The exergy associated to the flow of matter has two contributions the physical exergy (Eph) and the

chemical exergy (Ech) so that:

_Ei Z _Eph C _Ech (10)

The application of the exergy balance to the atmospheric bubbling fluidized bed combustion chamber

has been carried out as explained below. The exergy flow input to the combustion plant is

_Einput ZX

i

_Efuel;i C _Elimestone C _Eair (11)

where the physical exergy for these three streams is zero because it is considered that they enter into the

plant at environmental state (p0Z101.325 kPa, T0Z298.15 K).

The chemical exergy of the compounds has been calculated accurately [15,16] and the standard

chemical exergy of the fuel ðE0chÞ can be written as

E0ch Z DG0

f CX

nelE0ch;el (12)

where DG0f is the standard molar Gibbs function of formation, nel is the number of moles of the elements

in the compound under consideration and E0ch;el is the standard chemical exergy of the constituent

elements. These data were taken from Szargut [16] and are summarized in Table 5. In the calculation of

the Gibbs function, the entropy values are needed and they were estimated using the procedure described

in the literature [17]. The value obtained for the chemical exergy of the fuel is close to its higher heating

value, as it can be seen in Table 1.

For the chemical exergy of limestone, we have considered the useful work which can be obtained

from the reactions referred above in the energy balance, and the chemical exergy of the compounds were

taken from literature [16] and are summarized in Table 5.

Table 5

Values of the standard chemical exergy of different substances used in the calculations of the exergy balance (T0Z298.15 K;

p0Z101.325 kPa)

Fuel E0ch

a (kJ/mol) Flue gas E0ch

a (kJ/mol) Ash E0ch

a (kJ/mol)

C(s) 410.26 CO2 19.87 CaCO3 1.0

O2(g) 3.97 H2O(l) 1.1 CaO 110.2

H2(g) 236.09 H2O(g) 9.7 CaSO4 8.2

N2(g) 0.72 SO2 314.0

S(s) 609.6 CO 275.10

NO2 55.3

NO 88.9

a Taken from Ref. [16].


The exergy flow output from the combustion plant is:

_Eoutput Z _Euseful C _Eflue;gas C _Eash;bed C _Eash;cyclone (13)

where Euseful is the exergy of the heat flow transfer from the chamber to the water and it is calculated as:

_Euseful Z _Quseful 1 KT0=Tbed

� �(14)

We have used the bed temperature instead of the water temperature because the purpose of the plant is

not to produce steam water, only dissipated the heat rejected and quantified it in order to study the effects

of different fuels and fluidization conditions in the performance of the plant so that the irreversibility due

to heat transfer from fluidized bed to water is not included in the calculations.

The physical exergy of the streams has been calculated for the flue gases and for the ashes in the bed

and in the cyclone neglecting the effect of pressure drop. The thermal component of specific physical

exergy of the stream is

Eph Z

ðT1

T0

cpðTÞdT K

ðT1

T0

cpðTÞðT0=TÞdT (15)

where cp is the specific heat capacity, T0 is the environmental temperature and T1 is the temperature of

the stream considered. The values of the standard chemical exergy for the components of the ashes and

the flue gas were taken from the literature [16] and are summarized in Table 5.

To complete the exergy analysis of the plant, the exergetic efficiency or rational efficiency (j) is

calculated

j Z _Euseful= _Einput (16)

where both terms have been defined in Eqs. (11) and (14).

Table 6 summarizes the results of the exergy balance for the nine experiments studied: the exergy of

the inputs (Ein); the exergy of the outputs (Eout) of the chamber as defined in Eqs. (11) and (13),

respectively; the total exergy destruction or irreversibility rate (I) (Eq. (9)), the value of the exergy of the

heat flow transferred to the water (Euseful) (Eq. (14)), and the exergetic efficiency (j) (Eq. (16)).

The table also presents the values of the exergy of the flue gas and the exergy of the ashes and the

contributions of the physical exergy and chemical exergy to both streams are given separately.

Table 6

Results of the exergy balance for nine experiments studied

Exergy flow

(MJ/h)

Experiment number

1 2 3 4 5 6 7 8 9

Ein 3789.9 3784.4 3381.8 3425.2 3182.3 3240 3509.5 3486.5 2895.8

Eout 1660.8 1718.3 1498.3 1562.9 1545.9 1570.9 1584.7 1751.3 1493.2

I 2129.1 2066.1 1883.5 1862.3 1636.4 1669.1 1924.8 1735.2 1402.6

I/Ein (%) 56.2 54.6 55.7 54.4 51.4 51.5 54.8 49.8 48.4

Euseful 1165.8 1171.5 1109.3 1130.9 1099.5 1116.0 1077.3 1177.9 1021.6

jZEuseful/Ein (%) 30.8 31.0 32.8 33.0 34.6 34.4 30.7 33.8 35.3

Euseful/Eout (%) 70.2 68.2 74.0 72.4 71.1 71.0 68.0 67.3 68.4

Eflue gas 465.4 516.9 386.9 413.4 356.9 384.2 404.1 457.4 326.0

Eflue gas/Ein (%) 12.3 13.7 11.4 12.1 11.2 11.9 11.5 13.1 11.3

Ech flue gas 215.3 259.6 199.6 212.9 183.2 191.9 206.6 220.0 182.2

Eph flue gas 250.1 257.4 187.3 200.5 173.6 192.3 197.5 237.4 143.8

Eash 29.6 29.9 2.1 18.6 89.6 70.7 103.3 115.9 145.6

Eash/Ein (%) 0.8 0.8 0.1 0.5 2.8 2.2 2.9 3.3 5.0

Ech ash 20.9 22.1 1.8 13.5 68.1 51.9 76.2 84.9 110.8

Eph ash 8.7 7.8 0.4 5.1 21.5 18.8 27.1 31.0 34.8


For a better understanding of these results, those values are referred to the exergy input as a

percentage and they are represented graphically in Fig. 4.

The results obtained for the experiments studied show that a percentage ranging from 48.4

(experiment 9) to 56.2% (experiment 1) of the exergy input to the pilot plant is destroyed due to the

irreversibility of the process, they arise from the combustion process, the reactions of dissociation of the

limestone and also due to thermal interaction (Qloss) with the environment. Not all the exergy output is

useful exergy, an amount ranging from 67.3 (experiment 8) to 74.0% (experiment 3) of the exergy output

is transferred to the water (Euseful); the other exergy output to the plant is meanly the exergy of the flue

gas, ranging from 21.8% for experiment 9 to 30.1% for experiment 2. The exergy of the ash is only

significant in the mixtures containing black lignite, for example, in experiment 9 fed pure black lignite

reaches 9.7% of the exergy output.

Fig. 4. Percentage distribution of input exergy for the nine experiments.


The exergy losses in the flue gas and in the ash are taken into account in the definition of the exergetic

efficiency. It ranges from 30.8% for experiment 1 to 35.3% for experiment 9 which means that 70% of

the exergy input to the plant is destroyed or lost as shown in Fig. 4.

To decrease the exergy losses, the exit temperature of the exhaust gas may be reduced adding heat

exchangers into the plant. This allows recovering physical exergy of the combustion gas but not the

chemical exergy, which represents some 6% of the input of exergy. Also a better insulation of the plant

could reduce the heat loss to the environment which represents 12% of the input of exergy.

In order to apply the exergy method to optimise the plant we have analysed how the exergetic

efficiency depends on the operation parameters.

The comparison between experiments 1 and 2 or experiments 5 and 6 indicates that the fluidization

velocity does not affect the exergetic efficiency.

On the other hand, the experiments studied show that an increase in the Ca/S ratio produces a decrease

in the rational efficiency and the sulphur retention is not improved, so that high ratios are not convenient

from an environmental and an exergetic point of view.

The bed temperature directly affects the performance of the plant because of the definition of useful

exergy (Eq. (14)). Attending to the exergy analysis, the exergetic efficiency increases for higher bed

temperatures, although this conclusion is not clear in the experimental results because there are other

factors which operate simultaneously.

On the other hand, an advantage of combustion process at low temperatures, as occurs in these

experiments, is to avoid NOx emissions. This is one of the reasons to consider fluidized bed combustion

as a clean technology for coal.

3.4. Exergy costing

The analysis of the combustion chamber performance has been completed with the estimation of the

total cost of the exergy destruction (Ctotal) in V/year, for each experiment. We have calculated, for each

experiment, the cost of the fuel necessary to produce a useful exergy equal to the destructed exergy, in

the same conditions. It can be expressed as

Ctotal Z _Ixkz (17)

where x is the working hours of the plant; k is the unitary cost of exergy or the inverse of the exergetic

efficiency and z is the cost of coal. The values of total cost for the nine experiments, summarized in

Table 7, have been estimated assuming that the plant operates 7500 h/year and that the cost of coal is

0.664 ¢ V/thermia of higher heating value (1 thermiaZ0.8598 kW h), considering a 4550 kcal/kg

average higher heating coal and of 30.23 V/t coal national price [18].

To directly compare the results obtained in the irreversibility cost, the values of the total cost (Ctotal)

have been divided by the exergy flow input to the installation for the nine experiments (ctotal) and they

are summarized in Table 7.

A first conclusion is that the cost depends on the type of fuel used, the combustion of mixtures

containing pine chips and refuse coal is more expensive than the combustion of mixtures containing pine

chips and black lignite.

Table 7

Economical study: total irreversibility cost (Ctotal) and total irreversibility cost per input exergy (ctotal)

Exp. Ein (kW) I (kW) k j (%) Ctotal (V/year) ctotal (V/year per kW)

1 1053 591 3.25 30.8 82,243 21.7

2 1051 574 3.23 31.0 79,386 21.0

3 939 519 3.05 32.8 67,779 19.7

4 951 512 3.03 33.0 66,426 19.1

5 884 458 2.89 34.6 56,675 17.8

6 900 463 2.90 34.4 57,492 17.7

7 975 534 3.26 30.7 74,539 21.2

8 968 482 2.96 33.8 61,089 17.5

9 804 390 2.83 35.3 47,258 16.3


Only experiment 7 behaves in a different way, it is probably due to a bad performance of the

experiment because of values obtained for the efficiency. The other three experiments using black lignite

and pine give similar results.

On the contrary, in mixtures of refuse coal and pine chips, there is a clear relationship between

exergetic efficiency and cost. Comparing experiments 2 and 4, an increase of 31–33% (6.5%) in

the exergetic efficiency reduces the cost of 21–19.1 V/year per kW, which represents a decrease of 9%,

which is quite significant.

4. Conclusion

In this paper, we have applied the exergy analysis to nine experimental results of an atmospheric

bubbling fluidized bed combustion chamber. This plant has been used to study the behaviour of the

combustion of mixtures formed by waste coal and biomass. To calculate the energy and exergy balances,

the pressure and temperature of the different streams that enter or leave the plant, the bed temperature have

been measured and also the compositions of flue gas and ashes have been analysed for the mass balance.

The irreversibility and the exergetic efficiency have been quantified for the nine experiments, it ranges

from 31 to 35%. The exergy balance shows that the performance of the plant could be improved by

decreasing the exit temperature of the flue gas and with a better isolation of the combustor chamber.

Concerning the operation conditions, the fluidization velocity does not influence the exergetic

efficiency whereas an increase in the Ca/S ratio produces a decrease in the rational efficiency but the

sulphur retention is not improved continuously, so that, there is an optimum ratio.

It should be underlined that we have found no works concerning the application of the exergy method

to experimental data from biomass and coal co-firing for comparison in the literature.

Acknowledgements

Support for this work came from the Direccion General de Ensenanza Superior e Investigacion

Cientıfica Program FEDER. Spanish Ministery of Education, Project 1FD97-2221-C02-02 and Project

EET2001-4573-C04-03.


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