1
A comparative exergoeconomic evaluation of biomass post-firing and
2
co-firing combined power plants
3
Hassan Athari a ([email protected]), Saeed Soltani b 1([email protected]), Marc
4
A. Rosen c ([email protected]), Seyed Mohammad Seyed Mahmoudi b
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([email protected]), Tatiana
6
a
Department of Mechanical Engineering, University of Ataturk, 25240 Erzurum, Turkey
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b
Faculty of Mechanical Engineering, University of Tabriz, 16471 Tabriz, Iran
8
c
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street
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North, Oshawa, Ontario, L1H 7K4, Canada
10
d
Morosuk d ([email protected])
Institute for Energy Engineering, Technische Universität Berlin, Marchstr 18, 10587 Berlin, Germany
11
12
Abstract
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This paper reports the results of energy, exergy and exergoeconomic analyses of biomass
14
gasification-based power plants. Three configurations are considered: externally fired
15
biomass combined cycle, biomass integrated co-firing combined cycle and biomass
16
integrated post-firing combined cycle. The latter configuration is found to be more
17
economically effective (on a large scale). The energy efficiency for the biomass integrated
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post-firing combined cycle plant is about 3% and 6% higher than the biomass integrated co-
19
firing combined cycle and externally fired combined cycle plants, respectively. The
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parametric studies demonstrate that the energy and exergy efficiencies can be increased (for
1
Corresponding author: Tel: +98 41 33358695; Fax: +98 41 33354153
E–mail address: [email protected] (Saeed Soltani)
1
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the three configurations) by raising the compressor pressure ratio; however, higher pressure
22
ratios lead to lower values of the total product cost only for the biomass integrated post-firing
23
plant. Increasing the gas turbine inlet temperature leads to a slight decrease in the unit
24
product cost for the externally fired biomass combined cycle and the biomass integrated co-
25
firing combined cycle.
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Keywords: Energy analysis; Exergy analysis; Exergoeconomic analysis; Biomass
27
gasification; Combined cycle power plant
28
1. Introduction
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Non-renewable resource depletion is increasing due to the world’s continuously growing rate
30
of energy use. The increase of energy consumption provides, from an economic perspective,
31
a platform for societal development [1], yet it also constitutes a potential ecological threat
32
that can affect present, and especially future, generations. Environmental risks of energy use
33
can be classified dually as follows: depletion of non-renewable natural resources and
34
environmental discharge of harmful wastes. Increasing energy use also increases greenhouse
35
gas emissions. Harm from waste releases to the environment can be expressed in terms of the
36
impact on the depletion of nonrenewable natural resources, under the presumption that the
37
losses should be compensated or prevented. To reduce non-renewable resource depletion, the
38
use of renewable energy should increase, and the substitution of non-renewable fuels by
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biomass provides one option [2]. Many energy polices, ranging from global to national and
40
regional, have been put in place to foster the use of biomass fuels. Much research is ongoing
41
on technologies for using biomass energy and converting it to higher-value fuels, as well as
42
on its renewability and CO2 emission mitigation potential. The conversion of biomass to
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biofuels can be achieved primarily via biochemical and thermochemical processes.
2
44
The most common processes for biomass chemical upgrading are pyrolysis and gasification
45
[3]. Pyrolysis is less advanced and focuses on bio-oil production, while gasification mainly
46
yields a flexible gas stream, for which the primary gases are H2, CO, CH4, CxHy, CO2 and N2
47
[3, 4], for energy and chemical applications. Combustion of biomass is common in its
48
utilization, especially in the combined production of heat and power. Most of such systems
49
produce steam and electricity, and use Rankine cycles with backpressure turbines [4].
50
As discussed above, because of the significant role of biomass gasification as a supplier of
51
clean energy, many energy applications exist for using the products of biomass gasification,
52
the most common being gas engines, gas turbines and combined cycles [5, 6]. Despite
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significant recent advances, research is needed to enhance thermodynamic and economic
54
performance, e.g., exergoeconomic analyses [7, 8, 9]. Many factors affect the performance
55
and economic characteristics of co-fired combined cycles [7]. The externally fired combined
56
cycle (EFCC) has some positive features (for example, it can utilize biomass only as a fuel
57
and does not require filters), and some negative ones (for example, its fuel has a relative low
58
calorific value and, as a result, the cycle has a low energy efficiency) [10, 11]. In the co-firing
59
combined cycle, natural gas and biomass are combusted simultaneously. The process yields
60
various fuel fractions and is reasonably efficient [12, 13]. The co-firing combined cycle
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mitigates some of the potential challenges of combusting when low calorific fuels rather than
62
natural gas in turbines. But de-rating can occur for commercial turbines in this instance,
63
because low calorific value gases often necessitate off-design operation [14, 15, 16].
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A review of the research on the cycles that potentially can be used for biomass combustion
65
shows that new configurations are needed. To aid in the development of these technologies,
66
all alternatives should be evaluated thermodynamically and economically. In line with this
67
need, exergoeconomic analysis is one of the most comprehensive approaches and therefore is
68
used here.
3
69
In this paper the three biomass-fired combined cycles: externally fired combined cycle
70
(EFCC), biomass integrated co-fired combined cycle (BICFCC) and post-firing combined
71
cycle (BIPFCC), are proposed and evaluated using energy and exergy analyses and
72
exergoeconomics. The objective of the investigation is to enhance understanding of the cost
73
formation process within each cycle configuration. The BIPFCC cycle is of potential interest
74
for repowering gas turbine plants; it has a high energy efficiency (40%) and low discharge
75
temperature (440–480
76
exergoeconomic parameters, are conducted for the three configurations.
◦
C) [12]. Parametric studies, considering thermodynamic and
77
78
2. Descriptions of examined systems
79
Descriptions of the three biomass-fired combined cycles examined here and their operations
80
follow:
81

The externally fired combined cycle (Fig. 1) includes steam and gas turbine cycles
82
[10]. Wood fuel is input to the gasifier. Biogas from the gasifier and heated air from
83
the gas turbine enter the combustion chamber. Exhaust gases from the combustion
84
chamber heat the compressed air in an air preheater and then produce steam for the
85
steam cycle in a heat recovery steam generator (HRSG).
86

The biomass integrated co-fired combined cycle (Fig. 2) also includes steam and gas
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turbine cycles and uses dual fuels: natural gas and biomass (wood) [13]. Air exiting
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the compressor passes through the pre-heater to the combustion chamber. Natural gas
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fuel is input to the combustion chamber. The biogas from a downdraft gasifier is
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conveyed to the post-combustion unit where it mixes with the combustion gases from
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the gas turbine. Combustion occurs with the oxygen from the combustion chamber
4
92
gases, and the exhaust gases from the post combustion unit preheat the air flow and
93
pass through a HRSG, in which steam for the steam cycle is produced.

94
The biomass integrated post-firing combined cycle (Fig. 3) also includes steam and gas
95
turbine cycles and uses dual fuels: natural gas and biomass (wood). The biogas from a
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downdraft gasifier flows to the post-combustion unit, where it is combusted using the
97
oxygen content of the combustion gases exiting the gas turbine. The difference
98
between the two plants is that the BIPFCC has no Air Preheater.
99
100
2. Analyses
101
2.1. Assumptions and simplifications
102
For the analyses of the EFCC, BIPFCC and BICFCC cycles, we assume steady-state
103
operation at adiabatic conditions. Additional assumptions are listed in Table 1.
104
2.2. Energy and exergy analyses
105
The performances of the cycles are assessed considering mass, energy and exergy balances
106
[20, 21, 22, 23] for the overall cycles and their components, and also considering the
107
chemical equilibrium principles for gasification [10, 24]. Analyses of the EFCC and BICFCC
108
have been reported by authors in [10, 13], and the same approach has been applied here for
109
the BIPFCC.
110
Energy and exergy efficiencies of the cycles are determined as follows:
111
η
112
ε=

W
net,cycle
(1)
 fuel LHVfuel
m
Wnet,cycle
(2)
E F,cycle
5
113
The gasification results are validated by comparing them with results from other experimental
114
[25] and theoretical [24] studies. The comparison (Table 2) demonstrates good agreement. In
115
addition, it is noted that present model is advantageous to the Zainal equilibrium model [24]
116
in some important aspects. For instance, the sum of carbon monoxide and hydrogen in biogas
117
more accurately matches the experimental results [25]. This difference with the experimental
118
results is due to simplifications considering the chemical equilibrium state. The models for
119
the systems developed here are founded on thermodynamic formulations. These models are
120
incorporated into Engineering Equation Solver (EES) [26]. Note that there is a “variable
121
information” window in EES software, which permits the specification of minimum and
122
maximum values as well as guess values. These are often based on thermodynamic equations.
123
This allows constraints to be set for some values and appropriate results to be attained.
124
125
2.3. Exergoeconomic analyses
126
An exergoeconomic analysis helps assess techno-economic performance on an exergy basis,
127
and determines the cost per unit exergy of the products of a system [27]. A cost rate is
128
determined with exergy costing for each exergy transfer in the form of either matter or power
129
or heat transfer, and these cost rates are related as follows [27]:
130
 c E 
n
j 1
j
j
 Z   c j E j out
in
m
(3a)
j 1
131
  cE Alternatively, using the
where c is the cost per unit exergy of each stream and C
.
132
concept of “exergy of product” and “exergy of fuel”, we can write
133
cF E F  Z  cP E P
134
The above equation indicates that the cost rates associated with all input exergy streams to a
135
component as well as the cost rates associated with its the capital investment and operation
(3b)
6
136
and maintenance ( Z  Z CI  Z OM ) (shown on the left side of the equation) must be accounted
137
for in the cost rate of the exiting product exergy streams (shown on the right side). The
138
calculation procedure is described elsewhere [27], and uses the F-rule and P-rule. The cost
139
data for each component of the systems (Zk) are drawn from appropriate sources [18] and
140
modified to a reference year (2011) via the Marshall and Swift equipment cost index [28]:
141
Reference year cost = (Original cost)(Reference year cost index)/(Original year cost index)
142
To assess the cost rate of each stream, Eq. (3a) is applied to each component, following the
143
approach described in [18]. Definitions of the fuel exergy and product exergy are given in
144
Tables 3, 4 and 5, along with cost balances and auxiliary equations. Several parameters
145
which significantly affect the exergoeconomic performance of the system are determined for
146
each component: average unit costs of fuel cF and product cP, cost rate of the exergy
147
 , relative cost difference r and exergoeconomic factor ƒ [27]. The
destruction rate C
D
148
relative cost difference rk is defined for the kth component as:
149
rk 
150
and the exergoeconomic factor is defined f k as:
151
152
c p,k  c f,k
fk 
(4)
c f,k
Z k
Z k  c f,k E D,k  E L,k 
The economic objective function is the total product cost, determined as:
nf
nk
153
(5)
c p,total 
 Z k   cF i E F i
i 1
i 1
np

i 1
(6)
E Pi
7
154
Several parameters (pressure, temperature, mass flow rate) for each main stream in the
155
EFCC, BICFCC and BIPFCC plants at selected locations are listed in Tables 6a, 6b and 6c,
156
respectively.
157
158
3. Results and discussion
159
Parametric analysis is used to demonstrate the influence of various operating parameters on
160
the performance of the cycles. For consistency, all three cycle configurations supposed to
161
have 10 MW capacities. For the BICFCC the amount of natural gas is 0.01 kmol/s. Fig. 4
162
shows the variations in the energy efficiencies of the cycles with pressure ratio. There, the
163
BIPFCC plant efficiency is observed to be about 3% and 6% point higher than the
164
corresponding efficiencies of the BICFCC and EFCC plants, respectively. As pressure ratio
165
changes, all efficiencies are observed to be maximized at particular values of the gas turbine
166
inlet temperature. Nonetheless, the BIPFCC and BICFCC plants have the lowest and highest
167
optimum pressure ratios, respectively, for a fixed gas turbine inlet temperature (TIT = 1400
168
K).
169
A similar result is observed for the exergy efficiencies of the three cycles in Fig. 5, where the
170
exergy efficiency of the BIPFCC plant is seen to be about 6% and 10% points higher than
171
those for the BICFCC and EFCC plants, respectively. This observation is attributable to the
172
facts that 1) the EFCC plant, as opposed to the BICFCC and BIPFCC plants, combusts only
173
biomass, and 2) biomass fired systems have lower efficiencies than fossil fuel systems. The
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BIPFCC plant uses Less biomass than the BICFCC (see Tables 5b and 5c). The BICFCC is
175
comprised of an air preheater, causing more biomass to be combusted.
8
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Figs. 6 and 7 show the variations in the energy and exergy efficiencies of the cycles with
177
various gas turbine inlet temperatures (TITs) for a compressor pressure ratio of 9. Both
178
efficiencies are observed in these figures to increase with TIT.
179
Fig. 8 shows the variation of mass of air per mass of steam for the three cycles, as the
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pressure ratio changes, for a fixed value of TIT (1400 K). As rp increases, the mass of air per
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mass of steam decreases for BIPFCC plant. But the pressure ratio has little influence on the
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ratio of mass of air per mass of steam for the EFCC and BICFCC plants. This is due to the
183
fact that the compressor delivery temperature for the BIPFCC is not directly affected by the
184
constant HRSG inlet gas temperature; this is not the case for the BICFCC and EFCC plants.
185
Clearly, the air preheaters in the BICFCC and EFCC plants mitigate the influence of rp on
186
this ratio. This quantity is highest for BIPFCC plant, followed by the BICFCC and EFCC
187
plants. However, increasing TIT decreases the mass of air per mass of steam for the EFCC
188
and BICFCC plants and increases this ratio for the BIPFCC plant (see Fig. 9).
189
Fig. 10 shows the variation with pressure ratio of the total product cost for the three cycles,
190
for a fixed value of TIT (1400 K). For better understanding of the effect of biomass cost on
191
the total unit product cost, two biomass costs are considered. As rp increases, the total product
192
cost rises slightly for the EFCC and BICFCC plants, but decreases for the BIPFCC plant.
193
Regarding Fig. 8, when rp is raised the air mass flow rate decreases. Furthermore, costly
194
components are mainly observed to be in the gas turbine cycle and a lower air mass flow rate
195
leads to lower component costs in the gas turbine cycle. This observation explains the
196
decrease in total unit product cost with increasing rp. Note that the highest total unit product
197
cost is exhibited by the BIPFCC plant, followed by the BICFCC and EFCC plants,
198
respectively. Increasing the biomass cost influences, in order, the EFCC plant the most,
199
followed by the BICFCC and BIPFCC plants. But increasing rp increases the total cost
200
difference for the BIPFCC plant, even with different biomass costs. While the EFCC plant
9
201
uses only biomass fuel, this higher difference between total product costs is reasonable. For
202
other plants, combusting natural gas instead of biomass reduces this difference. In the
203
BIPFCC plant, the ratio of biomass flow rate to the natural gas flow rate is very small (see
204
table 7), justifying the lower total product cost for different biomass costs.
205
Fig. 11 shows the variation of total product cost for the three cycles, as the TIT changes, for a
206
fixed value of rp (9). As TIT increases, the total product cost decreases slightly for the EFCC
207
and BICFCC plants. However, increasing TIT increases the product cost for the BIPFCC
208
plant. This can be attributed to the increase of air flow rate in the BIPFCC plant with TIT (see
209
Fig. 9) which leads to costly gas turbine cycle components and consequently a higher total
210
unit product cost.
211
The variation of the exergoeconomic factor with rp, for a fixed value of TIT (1400 K) for the
212
BICFCC, BIPFCC and EFCC plants is shown in Fig. 12. As rp increases, we see that the
213
exergoeconomic factor increases for the all cycles, suggesting that increasing rp raises the
214
capital investment costs relative to the exergy destruction costs. Regarding, The EFCC is
215
seen in Fig. 12 to have the highest exergoeconomic factor, followed by the BICFCC and
216
BIPFCC plants. In rank order, the capital investment costs are highest for the EFCC,
217
BICFCC and BIPFCC plants. The higher values for the EFCC and BICFCC plants are mainly
218
due to their large air preheaters. However, increasing TIT decreases the exergoeconomic
219
factor for the BIPFCC plant and has little influence on that factor for the EFCC and BICFCC
220
plants (Fig. 13). That figure also indicates that changes in rp are more significant than
221
changes in TIT in making the cycle more economically attractive.
222
Exergy efficiencies for the three configurations and their components are shown in Fig 14 for
223
the maximum energy efficiency condition. The gas turbine exergy efficiency is the highest
224
for the three configurations, and the BIPFCC plant exhibits the highest gas turbine exergy
225
efficiency. Since chemical reaction occurs in the post combustor, the combustor and the
10
226
gasifier, the associated irreversibilities are high. The BICFCC combustion chamber has the
227
highest exergy efficiency and the BIPFCC plant the lowest exergy efficiency. In the post
228
combustion chamber, the BICFCC has the highest exergy efficiency. The gasifier exergy
229
efficiency is the same for the three configurations because the same conditions are assigned
230
to them. Similarly, the steam turbine, pump and HRSG exergy efficiencies are same for three
231
configurations. The air preheater exergy efficiency is highest for EFCC plant. The exergy
232
efficiency differences of the compressor, for the three configurations, are minor; the highest
233
value is observed for the BIPFCC plant.
234
Selected performance parameters are given in Table 7 for the three configurations of a 10
235
MW plant operating at full load and for a thermodynamic optimized condition. The energy
236
and exergy efficiencies of the BIPFCC plant are 3.19% and 5.71% points higher than for the
237
BICFCC also 5.7% and 9.73% higher than EFCC plant. The total product cost for the
238
BIPFCC plant is about three times greater than that for EFCC plant and about two times
239
greater than BICFCC plant because of higher price of natural gas in comparison with biomass
240
fuels and also high consumption of natural gas in BIPFCC plant. The steam turbine product
241
cost and gas turbine product cost for the BIPFCC plant is about four times greater than that
242
for EFCC plant and steam turbine product cost in BIPFCC is about two times greater than
243
BICFCC plant and gas turbine product cost in BIPFCC is about 1.5 times higher than
244
BICFCC. The total purchase equipment cost for EFCC plant is higher than BIPFCC and
245
BICFCC plants as the price of air preheater, HRSG, gasifier and steam turbine for the case of
246
EFCC plant are comparatively higher, as indicated in Table 7. The high price of gasifier in
247
the EFCC plant is due to the higher biomass (the sole fuel) flow rate in this configuration.
248
The higher cost of the HRSG is attributable to the higher steam flow rate and lower LMTD,
249
both of which lead to a larger size for this component. For the EFCC plant, the heat
250
exchanged in the air preheater is comparatively higher and the LMTD is comparatively lower
11
251
than BICFCC, and the cost of air preheater in this configuration is higher. The higher steam
252
flow in the EFCC plant causes a higher steam turbine cost in this configuration. Little
253
difference is observed for the costs of gas turbine in the three configurations. However, the
254
cost of compressor in BIPFCC is the lowest. Referring to Table 7, although the exergy loss in
255
the EFCC plant is higher than that in the BICFCC and BIPFCC plants, the cost of exergy loss
256
in the BIPFCC plant is the highest because of a higher value of fuel unit cost for the BIPFCC
257
plant.
258
Relative cost difference is the highest for EFCC plant and decreases in BICFCC and BIPFCC
259
plants. The highest value of exergoeconomic factor in EFCC plants shows that the capital
260
investment cost is higher than exergy destruction cost in this configuration and this value for
261
BICFCC and BIPFCC plants, respectively. Also, the capital investment cost is higher than the
262
exergy destruction cost for the BICFCC and lower for the BIPFCC plant.
263
The relative cost difference
264
lower than the corresponding values for the EFCC and BICFCC plants. This can be explained
265
by noting that the capital investment and operating and maintenance costs for the BIPFCC
266
plant are lower than the corresponding values for the EFCC and BICFCC plants. However,
267
the exergy destruction cost associated with the BIPFCC plant is higher than that associated
268
with the EFCC and BICFCC plants.
269
Tables 8, 9 and 10 show the results of exergoeconomic analyses of the EFCC, BIPFCC and
270
BICFCC cycles when they are optimized at the conditions specified in Table 7. In all three
271
plants, the highest exergoeconomic factor is for the steam turbine, which exhibits the highest
272
component cost relative to the exergy destruction cost, while the lowest exergoeconomic
273
cycles is for the combustor, which exhibits the highest exergy destruction cost relative to the
274
component cost in this component. Among the three cycles, the EFCC combustor has better
r
and the exergoeconomic factor f for the BIPFCC plant are
12
275
conditions from a thermoeconomic point of view because it exhibits the lowest relative cost
276
difference. However, the better condition for the combustor from a thermodynamic point of
277
view is observed for the BICFCC plant. The air preheater has the highest exergoeconomic
278
factor in the BIPFCC plant, where it is observed to have the highest component cost relative
279
to exergy destruction cost. The best condition for this component from a thermoeconomic
280
point of view is in the EFCC plant, while the best thermodynamic condition for the air
281
preheater is observed in the BICFCC plant. The post combustion chamber exhibits the best
282
thermodynamic and thermoeconomic conditions in the BIPFCC plant, where the post
283
combustion chamber cost relative to exergy destruction cost for this component is higher than
284
for the BICFCC plant.
285
Overall, the BIPFCC plant is more effective than the EFCC and BICFCC plants, from the
286
perspectives of thermodynamics and economics, but the usage of natural gas in this
287
component is high, which is a negative aspect. However, the exclusive usage of biomass,
288
which is a renewable and relatively environmentally benign fuel, by the EFCC plant provides
289
a significant advantage.
290
4. Conclusions
291
The following three systems are successfully proposed and analyzed from the viewpoints of
292
energy, exergy and exergoeconomics: externally biomass fired combined cycle, biomass
293
integrated co-firing combined cycle and biomass integrated post-firing combined cycle. The
294
main conclusions drawn from the present work follow:
295
296

The energy efficiency of the BIPFCC plant is approximately 3% higher than that of
the BICFCC plant and 6% higher than that of the EFCC plant.
13
297

The mass of air per mass of steam is highest for the BIPFCC plant, followed by the
298
BICFCC and EFCC plants, but increasing the pressure ratio reduces this value for the
299
BIPFCC plant and increases it slightly for the BICFCC and EFCC plants.
300

The lowest exergoeconomic factor of the three cycles is observed for the combustor,
301
which exhibits the highest exergy destruction cost relative to the component cost,
302
while the highest exergoeconomic factor is observed for the steam turbine, which
303
exhibits the highest component cost relative to the exergy destruction cost.
304

The energy and exergy efficiencies of the three biomass fired configurations are
305
maximized at particular values of compressor pressure ratio, but increasing the
306
pressure ratio decreases the total product cost for the BIPFCC plant and increases it
307
slightly for EFCC and BICFCC plants.
308

309
310
slightly for EFCC and BICFCC plants.

311
312
Increasing the TIT raises the total product cost for the BIPFCC plant and decreases it
The total unit product cost is the highest for the BIPFCC plant and second highest for
the BICFCC plant.

The exergy efficiencies for the three configurations and their components are
313
determined for the maximum energy efficiency condition. The gas turbine exergy
314
efficiency is the highest for three configurations, and the BIPFCC plant exhibits the
315
highest gas turbine exergy efficiency. The BICFCC combustion chamber has the
316
highest exergy efficiency while the lowest exergy efficiency is for the BIPFCC plant.
317
The post combustion chamber of the BICFCC exhibits the highest exergy efficiency,
318
while the air preheater exergy efficiency is highest for the EFCC plant.
14

319
The total product cost for the BIPFCC plant is higher than that for the BICFCC and
320
EFCC plants, but the BIPFCC plant has a lower relative cost difference and
321
exergoeconomic factor, indicating that this configuration is more cost effective.

322
323
Although the exergy loss in BIPFCC plant is lower relative to the BICFCC and EFCC
plants, its cost is higher.

324
As rp increases, the exergoeconomic factor increases for all cycles, suggesting that
325
increasing rp raises the capital investment costs relative to the exergy destruction
326
costs.

327
328
Increasing TIT decreases the exergoeconomic factor for the BIPFCC plant but has
little influence on the EFCC and BICFCC plants.
329
It is expected that the results will prove beneficial for designers and engineers of such
330
systems.
331
Nomenclature
332
AP
Air pre-heater
333
BICFCC
Biomass integrated co-fired combined cycle
334
BIPFCC
Biomass integrated post-firing combined cycle
335
C
Cost rate ($/h)
336
c
Cost per exergy unit ($/GJ)
337
E
Exergy rate (kW)
338
EFCC
Externally fired combined cycle
339
f
Exergoeconomic factor (-)
15
340
G
Gasifier
341
GT
Gas turbine
342
HRSG
Heat recovery steam generator
343
LHV
Lower heating value (kJ/kg)
344
P
Pump
345
Pi
Pressure at state i; partial pressure for species i (kPa)
346
PCC
Post combustion chamber
347
r
Relative cost difference (-)
348
rp
Pressure ratio (-)
349
Ti
Temperature at state i (K)
350
TIT
Gas turbine inlet temperature (K)
351

W
Power (kW)
352
x
Steam quality (-)
353
Z
Investment cost of components ($)
354
Z
Investment cost rate of components ($/ h)
355
Greek Letters
356
η
Energy efficiency (-)
357
ηis,C
Isentropic efficiency of compressor (-)
16
358
η is,GT
Isentropic efficiency of gas turbine (-)
359
η is,ST
Isentropic efficiency of steam turbine (-)
360

Exergy efficiency (-)
361
Subscripts
362
C
Compressor
363
CC
Combustion chamber
364
COND
Condenser
365
F
Fuel
366
GT
Gas turbine
367
in
Input
368
i
Index for thermodynamic state point
369
is
Isentropic
370
o
Reference environment state
371
P
Product
372
ST
Steam turbine
373
Superscripts
374
CI
Capital investment
375
OM
Operation and maintenance
17
376
References
377
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378
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380
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381
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382
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427
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432
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446
447
448
449
450
451
452
453
454
455
456
21
457
Table captions:
458
Table 1
459
Assumptions invoked to simplify the analyses
460
Table 2
461
Comparison of gasification constituent breakdown (in %) for model and experimental
462
approaches, considering gasification at 800 oC of wood with 20% moisture.
463
Table 3
464
Fuel and product exergy definitions, cost balances and auxiliary equations for EFCC.
465
Table 4
466
Fuel and product exergy definitions, cost balances and auxiliary equations for BICFCC.
467
Table 5
468
Fuel and product exergy definitions, cost balances and auxiliary equations for BIPFCC.
469
Table 6a
470
Parameter values for key streams in the EFCC plant*.
471
Table 6b
472
Parameter values for key streams in the BICFCC plant*.
473
Table 6c
474
Parameter values for key streams in the BIPFCC plant*.
475
Table 7
476
Performance parameters of three 10 MW biomass fired plants at optimum operating
477
conditions.
478
Table 8
479
Exergoeconomic factors for BICFCC cycle components
480
Table 9
481
Exergoeconomic factors for BIPFCC cycle components
22
482
Table 10
483
Exergoeconomic factors for EFCC cycle components
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
23
506
Figure captions:
507
Fig. 1. Externally fired combined cycle (EFCC).
508
Fig. 2. Biomass integrated co-fired combined cycle (BICFCC).
509
Fig. 3. Biomass integrated post-firing combined cycle (BIPFCC).
510
Fig. 4. Variation of energy efficiencies of three cycles with pressure ratio (TIT = 1400 K).
511
Fig. 5. Variation of exergy efficiencies of three cycles with pressure ratio (TIT = 1400 K).
512
Fig. 6. Variation of energy efficiencies of three cycles with gas turbine inlet temperature (r p =
513
9).
514
Fig. 7. Variation of exergy efficiencies of three cycles with gas turbine inlet temperature (r p =
515
9).
516
Fig. 8. Variation of mass of air per mass of steam for three cycles with rp (TIT = 1400 K).
517
Fig. 9. Variation of mass of air per mass of steam for three cycles with TIT (rp = 9).
518
Fig. 10. Variation of total product cost of three cycles with pressure ratio and different
519
biomass costs (TIT = 1400 K).
520
Fig. 11. Variation of total product cost of three cycles with TIT (rp = 9).
521
Fig. 12. Variation of exergoeconomic factor for three cycles with rp (TIT = 1400 K).
522
Fig. 13. Variation of exergoeconomic factor for three cycles with TIT (rp = 9).
523
Fig. 14. Exergy efficiency of EFCC, BICFCC and BIPFCC components at maximum energy
524
efficiency condition (TIT = 1400 K, THRSG,IN = 940 K).
525
24