(8130 words) 1
Fixed and Capture Level Reduction operating modes for
2
carbon dioxide removal in a Natural Gas Combined Cycle power
3
plant
4
Stefania Moioli*, Laura A. Pellegrini 5
GASP – Group on Advanced Separation Processes and GAS Processing, Dipartimento di Chimica, Materiali e
6
Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Corresponding Author: stefania.moioli@polimi.it
Abstract
23
The characteristics (flowrate, composition, CO2 partial pressure) of the flue gas exiting of
24
a Natural Gas Combined Cycle (NGCC) power plant are different from the ones of coal-fired 25
units on which analyses in the literature are usually focusing, therefore for application of 26
Carbon Capture and Storage (CCS) to the removal of CO2 from this type of stream a detailed
27
study is needed. This work applies the method of flexible operation aimed at reducing the 28
economic disadvantages of a CCS plant, and considers the Capture Level Reduction (CLR) 29
mode for the carbon dioxide removal section of a NGCC power plant located in Italy, of which 30
type the number of units is constantly increasing. The CLR mode consists in emitting part of 31
the carbon dioxide at given times during the day by bypassing part of the rich solvent and thus 32
reducing the energy requirements for regeneration of the solvent and compression of the 33
removed CO2. For the study a proprietary code developed by the GASP group of Politecnico di
34
Milano, based on real energy prices and demand and taking into account different values of 35
possible carbon taxes, has been employed. The results show that the best load to the absorption 36
section strongly depends on the value of the considered carbon tax, in addition to the time-37
dependent request and price of electricity. A carbon tax of 5 €/tonCO2 generally do not favour
38
the operation of the CCS system, while higher values exert a more significant influence on the 39
revenues. Carbon taxes of 115 €/tonCO2 for the winter period and of 145 €/tonCO2 for the
40
summer period make the operation of the CCS section more economically advantageous than 41
emitting carbon dioxide to the atmosphere. In any case, also when applied to CO2 removal in
42
power plants fed by natural gas, the CLR mode results more advantageous than fixed or no 43
capture operation, thus making power plants companies increasing their efforts in reducing 44
greenhouse gas emissions. 45
Keywords: carbon dioxide removal; post-combustion; energy saving operation; flexibility;
47
carbon tax. 48
1. Introduction
49
The environmental impact of industrial processes is one key points in the production of 50
goods and sevices, with the emission of greenhouse gases to the atmosphere being one of the 51
main issues being considered nowadays. A worldwide growing demand of energy is occurring, 52
with more and more energy needed also in residential buildings for household services (Ma et 53
al., 2019). Considering that this sector consumes consumes about 20% of the total energy 54
demand and more than 35% of the global electricity production (Liang et al., 2019), causing 55
22% of the CO2 emissions, (Ma et al., 2019), effective carbon dioxide mitigation measures in
56
the activities related to this field are significant to suppress the critical global warming trends, 57
as stated by IPCC (IPCC, 2018). The main types of energy needed are related to primary energy 58
(coal and natural gas) and to secondary energy, i.e. the electrical power. In the field of electricity 59
generation, in particular, the electrical plant needs to be built or retrofitted in order to meet the 60
imposed acceptability criteria of energy supply and public acceptance. New renewable power 61
plants based on solar, wind or biomass are important but can still be employed as energy 62
supplier to the main generating units due to the limited energy supply potential and high costs 63
(Feretic and Tomsic, 2005). Other types of renewable sources as large hydropower plants can 64
act as main energy producer only in specific geographical areas. It follows that at the moment 65
and for the next few decades fossil sources still remain among the main sources for electricity 66
production, as also concluded by Yang et al. (Yang et al., 2008). 67
Compared to coal-fired units, natural gas-fuelled plants are increasing in Europe, US and 68
generally because they are relatively easier to construct and emit fewer pollutants (Zhang et al., 69
2016) including a lower amount of carbon dioxide. The cost of electricity generation is a 70
condition of major importance in the power plant industry and, at the same time, a drastic 71
reduction of CO2 emissions is needed so that CCS also in natural gas-fired power plant (NGCC)
is required. Indeed, though the mole fraction of carbon dioxide in the flue gas is lower, also 73
natural gas power plants produce and emit significant amounts of this acid gas to the atmosphere 74
and influence the global Earth temperature level. 75
In the last years, new solutions to favor the CO2 capture operation in power plants while
76
maintaining CO2 emissions reductions have been performed. Flexible plants allow to improve
77
the overall plant economics (Domenichini et al., 2013) because allowing a temporary increase 78
in the electrical output of the power plant during peak demands or when electricity prices are 79
high. Indeed, as described by Tait et al. (Tait et al., 2018) in reference to the application of 80
flexible operating modes in post-combustion capture (PCC) in coal-fired power plants, the PCC 81
process variables can be modified to adjust the CO2 removal level to a value which is optimal
82
for fuel cost, electricity selling price and carbon dioxide emission cost, increasing short-term 83
profitability. Moreover, the electricity available for transmission can be maximixed. 84
Several studies in the literature already analyzed the flexible operation of CCS power 85
stations, though mainly focusing on coal-fired units. Chalmers et al. (Chalmers and Gibbins, 86
2007), Gibbins and Crane (Gibbins and Crane, 2004) and Delarue et al. (Delarue et al., 2012) 87
focused on bypass and on solvent storage techniques. The variable capture level, optimized as 88
a function of the electricity price and CO2 emissions was considered by Errey et al. (Errey et
89
al., 2014) and Rubin et al. (Rubin et al., 2007), while the variable solvent regeneration was 90
taken into account by MacDowell and Shah (Mac Dowell and Shah, 2014) and Mechleri et al. 91
(Mechleri et al., 2017). Generally, the technical literature on flexible PCC in power plants is 92
focused on the purification of flue gases coming from coal-fired units. However, considering 93
the worldwide increasing trend of NGCC plants, separating CO2 also from flue gases of natural
94
gas power plants would allow a significant reduction of greenhouse gas emissions and studies 95
on flexible operation are needed (ENEL, 2018). Because of its characteristics, it may be 96
considered particularly suitable for the post-combustion CO2 removal system, because it is
97
located at the end of the power station. 98
Several methods for purification of gaseous streams, operated with different types of 99
equipment (Besagni et al., 2017a; Besagni et al., 2017b) and reviews on this topic (Yang et al., 100
2008) dealing also with LCA analyses (Cuéllar-Franca and Azapagic, 2015) and emerging CO2
101
removal systems (Abanades et al., 2015) including amino acid salt solutions (Zhang et al., 2018) 102
are reported. Granite and O’Brien (Granite and O'Brien, 2005) focused on methods for CO2
103
separations from flue and fuel gases based on electrochemical pumps, membranes and chemical 104
looping. Li et al. (Li et al., 2013) analyzed more than 1000 patents on absorption, adsorption 105
and membranes and concluded that there are no practical processes for removing carbon dioxide 106
from large CO2 sources with low added Cost of Electricity (COE). Mondal et al. (Mondal et al.,
107
2012) presented a comparative analysis of different technologies and considered emergin 108
technologies including enzymes, facilitated transport membranes and hydrate based separation. 109
However, most of these interesting new technologies are still under research, and when dealing 110
with industrial post-combustion CO2 removal chemical absorption by aqueous amine solutions
111
is generally preferred. 112
The present work deals with the application of flexible operation to a CO2 removal plant in
113
a NGCC power plant. This is the main novelty of the work, since previous literature on this 114
topic focused on applications of postcombustion CO2 removal to power plants, different
115
because of the raw material used for combustion (natural gas instead of coal), and so as for the 116
composition of the flue gas to be treated. As stated by Bui et al. (Bui et al., 2014), indeed, for a 117
global perspective and compatibility with zero emission energy all fossil fuels power generators 118
potentially need a CO2 removal technology, and studies should also focus on natural gas fueled
119
power plants. 120
The power output of fossil fuelled power plants varies according to changes in supply or in 121
demand within the electricity grid, in order to ensure the quality of the utility and the supply of 122
energy to the final user. On the whole year, the total request of electricity is characterized by 123
hourly peaks during the 24 h and also by daily peaks. The energy profile is different for each 124
month, with different levels of total amount of electricity needed. For instance, data of 2015 125
show that in the Italian territory the highest difference among months is during summer and 126
there are significant variations of prices, due to the great difference in the demand of electricity. 127
When the demand is high, the price is very high, with values higher than 110 Euro/MWh. 128
Similarly, when the demand is low, also the price is low, following the market trend. It follows 129
that the highest amount of revenues coming from the sales of electricity are obtained during the 130
peaks, when the demand of electricity is high and also the prices are very high. Therefore, 131
adding the energy consumption of the operation of a CO2 removal plant during these hours, and
132
then reducing the power sold, causes relevant economic disadvantages if compared to 133
performing the same in times of low electricity consumption. 134
The main objective of the work is the study of the flexible operation of the CCS system 135
applied to the Italian electricity production, for which no works can be found in the literature. 136
In particular, the paper focuses on an analysis of flexibility with Capture Level Reduction 137
(CLR) mode, recognized to be the most performing one for coal-fired power plants, for a plant 138
located in Italy, and performs a comparison with fixed CO2 removal operating mode and with
139
no capture operation. The aim is to find the advantages in terms of energy and economics and 140
to determine the performances for the CLR mode also in NGCC power plants. 141
2. Materials and methods
142
2.1. Modes of operation
143Several modes of operation, both fixed operation or flexible operation, are possible for the 144
running of the carbon dioxide removal plant. 145
When running in fixed mode, the operation is steady-state with no variations of captured 146
CO2 with time. In managing the plant, all variables are set to given values and the same
147
conditions are constantly maintained. 148
The removal of carbon dioxide may be high (usually 85%, 90% or 95%) or lower, if only 149
the available surplus of energy produced by the power plant and not sold to the final user is 150
employed for running the CO2 removal section. Indeed, with fixed operation at high carbon
151
dioxide % capture, the needed energy may be higher than the one available in the power system, 152
therefore two options are available: 153
1. a lower amount of electricity is sold to the final user, but this would cause the 154
unreliability of the power production and of the supply to the electricity grid; 155
2. the needed extra amount of power is taken from an external source, with additional 156
costs. 157
However, if operating with a lower CO2 removal, higher amounts of the acid gas would be
158
emitted to the atmosphere and a higher carbon tax would be paid. It follows that for fixed 159
operation a choice must be made taking into account the economics of the overall plant. 160
The operation in flexible mode makes the removal of carbon dioxide be higher or lower 161
than in the fixed configuration at low CO2 removal, with variations during the day (Moioli and
162
Pellegrini, 2019a). Its operation must be chosen on the basis of an economic optimization, with 163
the aim of minimizing losses of energy and money and at the same time excessive losses of 164
carbon dioxide to the atmosphere, which would cause high expenses due to the carbon tax. 165
Plant configurations for flexible CCS in the open literature (Chalmers et al., 2009a; 166
Chalmers et al., 2009b; Lucquiaud et al., 2014) are mainly of three types: 167
1. venting: not all the carbon dioxide is absorbed and part of it is emitted to the atmosphere 168
(Chalmers and Gibbins, 2007; Gibbins and Crane, 2004), mainly during the peak hours; 169
2. storing solvent: the plant reduces the energy penalty due to CCS by storing the solvent 170
to be regenerated and running the regeneration section in periods with low electricity 171
price or demand (Chalmers and Gibbins, 2007; Gibbins and Crane, 2004; Haines and 172
Davison, 2009; Moioli and Pellegrini, 2019c); 173
3. “varying the time for solvent regeneration”: the solvent is not stored but it can be more 174
or less regenerated, so the lean and rich loadings may vary during time (Mac Dowell 175
and Shah, 2014). 176
This paper focuses on the venting option, also called Capure Level Reduction (CLR) mode, 177
which is usually employed for energy and costs saving without incurring in relevant 178
modifications of the base scheme for the CCS plant, and so avoiding additional investment 179
costs. The CLR mode has already been studied in the literature applied to coal-fired power 180
plants and it has been considered in this work for evaluating its performances when applied to 181
the treatment of a NGCC flue gas stream. This operating mode is then also compared with the 182
fixed operation at different levels (from 60% to 100% of the base CO2 removal).
183
2.1.1. Fixed operation – 95%
184
The plant in fixed operation for the absorption of 95% of the carbon dioxide present in the 185
flue gas had been designed previously (Moioli and Pellegrini, 2018; Moioli and Pellegrini, 186
2019b) and has been considered for the work carried out in this paper. The design of the plant 187
had been performed in order to treat the very huge gas flowrate in suitable and realizable 188
columns while minimizing the energy consumption, which is located mainly at the reboiler of 189
the regenerating column and in the compression section. Therefore three packed absorption 190
columns with a diameter of 12.5 m each, similar to the size of columns considered and built by 191
Fluor (Fluor, 2017) and Shell (Shell, 2017) and acceptable in comparison with what reported in 192
the recent literature (Zhang et al., 2016) (also in the case of low packing height), have been 193
considered (Figure 1). The lean loading (0.224) and the flowrate (49.23 kmol/s) of the 30% wt. 194
MonoEthanolAmine (MEA) solvent have been chosen on the basis of a minimum energy 195
requirement analysis. 196
Because of the very high flowrate (Figure 1), the flue gas (HOTFLUEGAS) is divided into 197
three equal streams (FLUEGAS 1, FLUEGAS 2 and FLUEGAS 3), cooled in a heat exchanger 198
(P-COOLER 1, P-COOLER 2 and P-COOLER 3) and fed to the absorbers (ABSORBER 1, 199
ABSORBER 2 and ABSORBER 3) for flowing counter-currently to the 30% wt.
200
MonoEthanolAmine (MEA) solvent (LEANIN 1, LEANIN 2 and LEANIN 3). The rich aqueous 201
amine solution (RICHOUT 1, RICHOUT 2 and RICHOUT 3) exiting from the bottom of each 202
absorber is mixed in MIXRICH and the resulting stream RICH is fed to a pump for increasing 203
its pressure and to the heat exchanger ECOHEAT for increasing its temperature by heat 204
exchange with the lean solution (LEANOUT) from the bottom of the distillation column 205
(DESORBER). The rich solvent (RICHIN) is then fed to the top of the distillation column. The 206
pressure of the LEANOUT stream, after partial cooling in the lean-to-rich solution heat 207
exchanger, is lowered to 1 atm and its temperature is furtherly decreased by heat exchange with 208
cooling water in the COOLER. After this step and the make-up, the solvent (LEANIN) is split 209
into three identical streams (LEANIN 1, LEANIN 2 and LEANIN 3) and recycled to the top of 210
the absorbers. 211
The acid gas removed from the solution in the stripping column (CO2) is cooled to 212
condensate a major portion of the water vapor and is then sent to the CO2 intercooling
213
compression station (simplified in the scheme as CO2 COMPRESSOR). This section is
composed of several stages, with an additional unit for reduction of the water content at high 215
pressure, based on absorption by triethylene glycol (TEG). In the final part of the process the 216
stream is cooled and liquefied and a pump is used to increase the pressure of the carbon dioxide 217
stream up to the final pressure (150 bar) (Fout et al., 2015). 218
2.1.2. Fixed operation at partial load
219
In the fixed operation mode, the load has been set and maintained at 90%, 80%, 70% or 60% 220
of the one of the base case (95% CO2 removal). This mode of operation may result more or less
221
advantageous with respect to full load depending on the time of the year, as outlined in Section 222
3. 223
2.1.3. Flexible operation
224
The CO2 Capture Level Reduction mode (CLR) configuration involves a flexible CO2
225
capture that vents CO2 at partial load for specified time intervals. According to this method,
226
carbon dioxide is not totally absorbed as in the full capture base case (95% of the content in the 227
flue gas), but part of it is left in the treated gas and exits to the atmosphere. This is accomplished 228
by operating a bypass of the rich solution, which is fed without regeneration to the absorption 229
column. As a consequence, when bypass is operated, the energy requirement for CO2 stripping
230
and compression decreases as the solvent flowrate fed to the regeneration column lowers with 231
also a lower CO2 removal rate.
232
For a fixed design (the one of the base case), and a fixed lean loading as purity of the 233
regenerated solvent, lower steam at the reboiler is needed to treat a lower flowrate of rich 234
solution for regeneration, so more steam (coproduced in the power plant) is fed to a low pressure 235
turbine and the obtained power output of the overall plant is higher. 236
For a new power plant, the capital cost of flexibility should include the cost of oversizing 237
the low pressure turbine, generator, and power units to enable the increased power output when 238
the CO2 capture energy requirements are reduced. In a retrofit application to a power station
239
without CO2 removal plant, this option involves negligible capital cost if compared to a base
240
capture system because the base power plant has been already sized to have as input also the 241
steam flowrate that in the case of CO2 capture (added after construction for retrofitting) is used
242
for the reboiler and not fed to the turbine. 243
If compared to the base CO2 removal, the main disadvantage of this configuration is the
244
increase of CO2 emissions and the related costs. When the plant is operated at lower CO2 capture
245
levels, the overall emission rates are higher than the ones in the fixed-point operation, therefore 246
the CO2 emission penalty and the carbon tax may be relevant factors. The choice of the amount
247
of carbon dioxide to be absorbed must take into account these issues, as done in this study. 248
Figure 1 shows the scheme of the CLR mode. The difference between this configuration 249
and the base one is the presence of bypasses on the rich solvent streams exiting the absorbers. 250
When the load is lower than 100%, part of the rich streams (BYPASS 1, BYPASS 2 and BYPASS 251
3) is recirculated directly to the absorbers, after being mixed with the lean streams (LEANIN 1,
252
LEANIN 2 and LEANIN 3). With 100% load no rich solvent is recirculated, while, with 0%
253
load, all the rich solvent is recirculated and the desorber is totally bypassed. In this case, no 254
stripping steam would be required for regeneration and the column would not operate. 255
However, for a good operation of the overall plant, a non zero load is needed to prevent some 256
issues, as stripping drying out, and so, to promote a later fast load increase, a value of 30% has 257
been chosen as the lowest possible operation load, as suggested in the literature (Cohen et al., 258
2011). 259
261
Figure 1. PFD of the CO2 Capture Level Reduction (CLR) system.
262
2.2. Methodology
263The work has been carried out by performing an economic optimization with an in-house 264
tool developed by the GASP group of Politecnico di Milano on the basis of the methodology 265
outlined below and taking as input the result of simulations in ASPEN Plus®, in addition to the
266
data of electricity request and price. 267
The power that can be effectively sold on the electricity market is the difference between 268
the full power plant capacity and the energy required for the CO2 capture system.
269
( )
MAX
out out reb comp
W W W W (1)
where WoutMAX [MW] is the power station net capacity without capture system, 630 MW; Wreb
270
and Wcomp [MW] are respectively the reboiler and compression energy penalties, with the
271 ABSORBER 1 DESORBER P‐ COOLER 1 GASOUT 1 PUMP ECOHEAT RI C H PU M P LEA N IN CONDENSER CO 2 REBOILER LEANOUT LEA N EC O VALVE COOLER ABSORBER 2 P‐COOLER 2 ABSORBER 3 P‐COOLER 3 FL U EG A S 3 RICHOUT 2 RICHOUT 3 LE A N V A LV LEANIN 2 FL U EG A S 1 HOTFLUEGAS LEANCOLD RMEA RH2O LEANIN 3 GASOUT 2 GASOUT 3 CO2 for transportation and storage MIXRICH CO2 COMPRESSOR FLUEGAS 2 RICH LEANIN 1 BY P A S S 3 BY P A SS 2 BY P A S S 1 RICHOUT 1 LEANIN
reboiler energy penalty calculated as equivalent work considering that steam is withdrawn from 272
the turbine. 273
Wout is the power that can be sold to the market, but, as shown in Figure 2, the power required
274
by the final user varies hour per hour. Therefore, it may happen that the net power produced 275
Wout is higher than the one required.
276
The % CO2 removal value of fixed operation at low levels has been determined considering
277
the available energy resulting from the difference of the maximum energy produced by the 278
power plant and the power required by the final user. 279
In order to allow the operation of the plant for all the day with no taking of power from an 280
external source, the hourly peak of power sold to the final user has been considered for the 281
computation of the available energy, as shown in Figure 2. 282
283
a) b)
284
Figure 2. Maximum capacity of the plant and power required on the third Wednesday of a) January 2015 and
285
b) July 2015 (GME, 2017).
286 287
For January 2015 it results equal to 77.56 MW, higher than the overall power needed for 288
running the CO2 removal and compression system, while for July 2015 the difference between
289
the power produced and the power required by the final user is zero at noon and in the afternoon, 290 0 100 200 300 400 500 600 700 0 4 8 12 16 20 24 gr os s p o w er [M W ] hour January, 2015
power required by the final user max capacity 0 100 200 300 400 500 600 700 0 4 8 12 16 20 24 gr os s p o w er [M W ] hour July, 2015
power required by the final user 95% of power required by the final user 90% of power required by the final user max capacity
load % = 60 load % = 30
since it is the yearly peak of power demand and the peak is covered with all the power produced 291
by the plant. Therefore, the CO2 removal operation would cause an additional cost to the plant
292
for energy consumption, though allowing a removal of carbon dioxide and thus lower carbon 293
taxes. Alternatively, the power plant may provide a lower amount of electricity to the market 294
and use part of the produced power to run the CO2 capture section. For instance, as shown in
295
Figure 2, for a plant removing 95% of carbon dioxide, if only a 60% load is applied (i.e. the 296
plant works to remove only 60% of CO2 for which it has been designed), about 10% of the
297
produced power is used to run the CO2 capture section, so during peaks the power plant would
298
be able to provide to the final user only 90% of the requested power. The remaining power 299
(10%) needed should be bought by the user elsewhere. This value is equal to 5% if a load of 300
30% is applied, for which 95% of power produced can be sold. 301
For flexible operation, the conditions are chosen so to maximize the profit, which is the 302
objective function, in order to reduce the effect on power plant output loss due to CO2 capture
303
system by operating capture plant in flexible modes. 304
On hourly basis, the profit associated to the power station with CO2 capture system is:
305
2 2 , &
out energy CO CO Tax fuel Fuel b O M
PW C F C F C C (2)
where Wout [MW = MWh/h] is the net energy production in one hour exiting from the power
306
station; Cenergy [€/MWh] is the price of energy and CCO2Tax [€/tonCO2] is the carbon tax; FCO2
307
[tonCO2] is the amount of CO2 vented in an hour; Ffuel [kg/h] and Cfuel [€/kg] are the fuel
308
consumption and the fuel cost respectively; Cb,O&M [€/h] is the cost for base plant operation and
309
maintenance, excluding the cost of Wout, separately computed in Eq. (5) (this cost is composed
310
of a term related to the power plant operation and a term related to the CO2 removal section
311
operation, which is assumed to be present also during times of low or no % load because of the 312
operation of the auxiliary units). 313
The plant is assumed to be already in operation, so that start-up and shut-down costs have 314
been neglected, though taking into account the efficiency losses during transient CO2 capture
315
operation due to variations in % CO2 removal. Therefore, for the absorber and the stripper a
316
ramp of 5%/min (Cohen et al., 2012), assumed to be the same in either direction and the same 317
for absorber and regenerator (Cohen et al., 2011), has been considered. 318
The fuel consumption is constant because the power plant load is fixed to its base value resulting 319
therefore independent on the capture plant load. 320
The electricity price has been assumed to be known for each time interval (1h) over 24 h 321
operation period with values taken from Gestore dei Mercati Energetici (GME, 2017) on the 322
basis of historical electricity prices, given on an hourly basis and referring to the Italian market 323
in 2015. 324
The analysis has been performed considering a possible carbon tax from 5 to 200 €/tonCO2,
325
in order to take into account a wide range of values for this tax (CTC, 2017). 326
Two cases have been considered: 327
A. all the net power output of the plant is sold to the market, regardless the real market 328
needs (it is assumed in this case that the market is able to employ all the energy that is 329
produced by the power plant); 330
B. only the power really needed by the market on the basis of the hourly request (data taken 331
from GME (GME, 2017) is sold, or a lower amount in case the net power output from 332
the plant is lower than the market needs due to the CO2 removal and compression plant.
333
If additional power is produced and not sold to the market, this power is considered lost. 334
3. Results and Discussion
335
The analysis cosiders as case study the possible application of CO2 capture in Italy to a
336
power plant of 630 MW. January (generally the coldest month in this country) and July(the 337
month with the highest variation of demand and electricity prices (Moioli and Pellegrini, 2018)) 338
have been chosen as the months on which the analysis is focused, taking into account that a 339
different variation in the energy demand during the day strongly influences the flexibile 340
operation of the plant. 341
The study has been carried out considering the following cases: 342
- % CO2 removal when no flexible operation is applied (base case): 95%;
343
- modes of operation: no capture, fixed capture at 100%, 90%, 80%, 70% and 60% of the 344
base case (corresponding to 95% CO2 removal) and CLR flexible operation;
345
- power sold to the final user: all the power remaining after CO2 removal (case A) and
346
only the power required proportionally to the Italian request (case B); 347
- period of the analysis: hourly. 348
3.1. Flexible operation
349Figure 3 show the resulting optimal load obtained for the flexible operation of plants for 350
95% CO2 removal in January and in July considering that all the energy available is sold to the
351
market (case A). 352
a) b) 354
Figure 3. Optimum load profile in flexible system on a) 4th January 2015 and b) 23rd July 2015, considering
355
that all the energy available is sold to the market (case A) for different carbon taxes (5, 50 and 100 €/tonCO2).
356 357
A load of the plant equal to 100% means the plant is running to remove the total amount of 358
carbon dioxide for which it has been designed (i.e. for a plant designed for 95% CO2 removal,
359
the plant is being operated to remove 95% of carbon dioxide). Lower loads refer to fractions of 360
the total removal, meaning that part of the amine is recycled back to the absorber without being 361
regenerated. 362
The variations of load level almost follow the electricity price during the 24 h period: the 363
load is reduced at the time of high electricity price and is increased at the time of low electricity 364
price. With a carbon tax equal to 5 €/ton CO2, the capture system should be always switched
365
off because the capture operation is not justified from an economical point of view. Because of 366
its low value, the carbon tax has no effect on making the plant operators run the CO2 capture
367
system, considering that the loss in revenues from sales is higher than the payment of a tax for 368
all the carbon dioxide emitted to the atmosphere. The higher the value of the carbon tax, the 369
higher the average load of the CO2 capture system.
370
Moreover, considering the summer month, when the energy demand and the price for 371
electricity is very high, venting carbon dioxide into the atmosphere and paying a carbon tax as 372
high as 50 €/ton CO2 results more economically advantageous than operating CCS. This is
373 0 7 14 21 28 35 42 49 56 63 70 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 2 4 6 8 10 12 14 16 18 20 22 24 en er g y p ri ce [€ /M W h ] % lo ad hours
January 04th, 2015, base = 95% CO2 removal carbon tax [€/tonCO2] = 5 carbon tax [€/tonCO2] = 50 carbon tax [€/tonCO2] = 100
0 30 60 90 120 150 180 0% 10% 20% 30% 40% 50% 60% 0 2 4 6 8 10 12 14 16 18 20 22 24 en er g y p ri ce [€ /M W h ] % lo ad hours
July 23rd, 2015, base = 95% CO2 removal carbon tax [€/tonCO2] = 5 carbon tax [€/tonCO2] = 50 carbon tax [€/tonCO2] = 100 energy price [€/MWh]
mainly due to the high value of electricity, whose losses due to carbon capture operation heavily 374
affect the economics of the system. With a carbon tax of 100 €/ton CO2, on the contrary, flexible
375
absorption may be advisable. 376
An even higher carbon tax value, as 200 €/ton CO2, makes the carbon dioxide removal
377
system being operated at higher loads, both in winter and in summer (though not reported here 378
for reasons of limiting space). The price of electricity, however, makes the maximum % load 379
being different in summer and in winter, with the optimal one resulting 60% in July and 80% 380
in January. 381
The trend is different if the power sold to the user is proportional to the one required by the 382
market (case B). In this case, since the revenues are different, their value significantly 383
influences the profit function and hence the results of the optimization. It follows that the 384
optimal operation differs from the one reported in Figure 3, as shown in Figure 4 and in Figure 385 5. 386 387 a) b) 388
Figure 4. Optimum load profile in flexible system on a) 4th January 2015 and b) 23rd July 2015, considering
389
that only the energy required by the market is sold (case B) for different carbon taxes (5, 50 and 100 €/tonCO2).
390 391 0 7 14 21 28 35 42 49 56 63 70 88% 90% 92% 94% 96% 98% 100% 102% 0 2 4 6 8 10 12 14 16 18 20 22 24 en er g y p ri ce [€ /M W h ] % lo ad hours
January 04th, 2015, base = 95% CO2 removal carbon tax [€/tonCO2] = 5
carbon tax [€/tonCO2] = 50 carbon tax [€/tonCO2] = 100 energy price [€/MWh] 0 30 60 90 120 150 180 0% 20% 40% 60% 80% 100% 120% 0 2 4 6 8 10 12 14 16 18 20 22 24 en er g y p ri ce [€ /M W h ] % lo ad hours
July 23rd, 2015, base = 95% CO2 removal carbon tax [€/tonCO2] = 5 carbon tax [€/tonCO2] = 50 carbon tax [€/tonCO2] = 100 energy price [€/MWh]
a) b) 392
Figure 5. Optimum load profile in flexible system on a) 4th January 2015 and b) 23rd July 2015, considering
393
that only the energy required by the market is sold (case B) for a carbon tax of 100 €/tonCO2 and 200 €/tonCO2.
394 395
In particular, this effect is enhanced in winter. In July, because of peaks in the power 396
requirements and of the high price of energy, revenues from selling of electricity are high and 397
more similar to the ones of case A than revenues obtained in January. In January the power sold 398
to the final user is always lower than the one remaining after removal of carbon dioxide (as 399
shown in Figure 2a)) and revenues are lower. Therefore, in order to increase the overall profit, 400
losses of money due to the payment of the carbon tax for CO2 emissions must be avoided, so
401
the optimal load results equal to 100% during all day for most of the values of carbon tax. Only 402
if considering a carbon tax of 5 €/tonCO2, during the hour when the electricity price is the
403
highest of the day (from 17:00 to 18:00), a reduction of the load to 90% results more 404
advantageous. As for summer, on the other hand, 100% load is present only for times of low 405
electricity requirements and prices, i.e. during nights and early in the morning. 406
The influence of the carbon tax on the load is shown in Figure 6, where significant differences 407
result by comparing different hours of the day, mainly due to the combined effect of price of 408
electricity and market demand (Figure 6). Indeed, for the same day, the amount of electricity 409
sold, and consequently the demand from the market, exerts a high influence on the optimal 410 0 7 14 21 28 35 42 49 56 63 70 0% 20% 40% 60% 80% 100% 120% 0 2 4 6 8 10 12 14 16 18 20 22 24 en er g y p ri ce [€ /M W h ] % lo ad hours
January 04th, 2015, base = 95% CO2 removal carbon tax [€/tonCO2] = 200
carbon tax [€/tonCO2] = 100 energy price [€/MWh] 0 30 60 90 120 150 180 0% 20% 40% 60% 80% 100% 120% 0 2 4 6 8 10 12 14 16 18 20 22 24 en er g y p ri ce [€ /M W h ] % lo ad hours
July 23rd, 2015, base = 95% CO2 removal carbon tax [€/tonCO2] = 200 carbon tax [€/tonCO2] = 100 energy price [€/MWh]
operation of the CO2 removal plant. For this reason, though calculations have been performed
411
for both the cases, in the following only results for case B are presented. 412
413
a) b)
414
Figure 6. Variation of the optimum load profile as function of the carbon tax in flexible system at different
415
hours of the day (8,12,16 and 20) on a) 4th January 2015 and b) 23rd July 2015, considering that only the energy
416
required by the market is sold (case B).
417 418
The % variation in profit compared to no capture (for which the Cb,O&M for the CCS section is
419
not considered) is strongly influenced by the value of the carbon tax, the type of operation (fixed 420
or CLR) and the time of the year (Figure 7). In particular, there is a break-even point for a value 421
of the carbon tax which makes the power company choose to run in environmental safe mode, 422
by absorbing carbon dioxide. This value varies from winter (for CLR mode resulting equal to 423
115 €/tonCO2) to summer (for CLR mode being equal to 145 €/tonCO2). In addition, as
424
described in detail in Section 3.2, running the CO2 removal plant at different loads in fixed
425
mode provides different profits. The Carbon Level Reduction mode results however the most 426
advantageous one, because of its varying operation in response to hourly electricity prices and 427
demand. These two factors, together with the value of the carbon tax (Figure 8), affect the trend 428
of the daily profit. 429 430 0% 20% 40% 60% 80% 100% 120% 0 50 100 150 200 % lo ad
carbon tax [€/tonCO2]
January 04th, 2015, base = 95% CO2 removal
8 12 16 20 0% 20% 40% 60% 80% 100% 120% 0 50 100 150 200 % lo ad
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal
8 12 16 20
a) 431
b) 432
Figure 7. % variation in profit compared to no capture on a) 4th January 2015 and b) 23rd July 2015,
433
considering that only the energy required by the market is sold (case B).
434 435
a) b)
436
Figure 8. Variation of the daily profit as function of the carbon tax for a) 4th January 2015 and b) 23rd July
437
2015, considering that only the energy required by the market is sold (case B).
438 439 -300% -200% -100% 0% 100% 200% 300% 400% 500% 600% 700% 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 % v a ri at io n in p ro fi t co m p ar ed t o n o cap tu re
carbon tax [€/tonCO2]
January 04th, 2015, base = 95% CO2 removal
Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
-25% -20% -15% -10% -5% 0% 5% 10% 15% 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 % v ar ia ti o n in p ro fit co m p ar ed to no ca ptu re
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal
Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
-30 -20 -10 0 10 20 30 40 0 50 100 150 200 da il y pr of it [ k € /d a y]
carbon tax [€/tonCO2]
January 04th, 2015, base = 95% CO2 removal
Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
1 040 1 060 1 080 1 100 1 120 1 140 1 160 0 50 100 150 200 dai ly p ro fi t [k€ /day ]
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
For January, as the value of the carbon tax increases, the resulting profit decreases and, as the 440
load in fixed mode decreases, the profit decreases. Below 70% load economic losses with 441
negative profit result for high values of the carbon tax. A different trend results for July, for 442
which high profits are obtained because of the high amounts, and so high prices, of electricity 443
required by the market. So no value of profit below 0 occurs from 0 €/tonCO2 to 200 €/tonCO2,
444
though a decrease results for higher carbon taxes. Moreover, higher % loads are in this case less 445
advantageous, because they cause a reduction of the net power output sold to the market, and 446
the overall electricity demand is not satisfied. 447
The CLR mode provides a significant increase in the profits also in the case of application to a 448
NGCC power plant. A similar conclusion on the flexible operation had been obtained by 449
Husebye et al. (Husebye et al., 2011) for a coal-fired power plant, who underlined that profits 450
are also strictly related to the electricity price volatility. 451 452 a) b) 453 250 300 350 400 450 500 550 600 650 700 0 4 8 12 16 20 24 pow e r [M W ] hours
January 04th, 2015, base = 95% CO2 removal
carbon tax [€/tonCO2] = 5 carbon tax [€/tonCO2] = 100 carbon tax [€/tonCO2] = 200 power required by the user [MW]
300 350 400 450 500 550 600 650 700 0 4 8 12 16 20 24 po w e r [M W ] hours
July 23rd, 2015, base = 95% CO2 removal
carbon tax [€/tonCO2] = 5 carbon tax [€/tonCO2] = 100 carbon tax [€/tonCO2] = 200 power required by the user [MW]
c) 454
Figure 9. Variation of the power sold to the market during the day for a) 4th January 2015 and b) 23rd July
455
2015 and c) for different carbon taxes on 23rd July 2015, considering that only the energy required by the market
456
is sold (case B).
457 458
Figure 9 highlights the difference of the effect of consuming power for removal and 459
compression of carbon dioxide in winter and in summer. While, during winter, the all required 460
energy is sold to the market for any value of the carbon tax applied, in summer differences in 461
the power sold and in the revenues are relevant. For a carbon tax of 5 €/tonCO2, when no CO2
462
removal is applied, the profile of the provided power corresponds to the one of the power 463
requested by the market. For higher carbon taxes, on the other hand, the power sold to the 464
market is lowered at noon (for values higher than 70 €/tonCO2) and at 4 pm (for values higher
465
than 90 €/tonCO2) (Figure 9c)).
466
Differently from the fixed operation mode, in CLR mode the paid amount of carbon tax is 467
strongly dependent on the market, therefore a constant monotonic profile increasing with the 468
unitary carbon tax value does not result (Figure 10). The amount paid for no capture is much 469
higher (up to one order of magnitude), thus making the company always choose to operate the 470
CO2 removal system, at least partially. This result confirms the usefulness of policy decisions
471
in many countries to apply carbon tax. 472 300 350 400 450 500 550 600 650 700 0 50 100 150 200 po w e r [M W ]
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal 48 12 16 20
It is also in line with what previously found in the studies on coal-fired power plants by Gibbins 473
and Crane (Gibbins and Crane, 2004) and Chalmers et al. (Chalmers and Gibbins, 2007; 474
Chalmers et al., 2009a; Chalmers et al., 2009b; Lucquiaud et al., 2009; Lucquiaud et al., 2014), 475
concluding that removing carbon dioxide is economically valuable when the cost of the emitted 476
CO2 is significantly higher than the electricity price.
477 478 a) 479 b) 480
Figure 10. Variation of the carbon tax paid for emitting CO2 as function of the carbon tax for a) 4th January
481
2015 and b) 23rd July 2015, considering that only the energy required by the market is sold (case B).
482
3.2. Fixed operation
483Figure 11 shows % variation in profit compared to fixed capture at 100% load for 95% CO2
484
removal, considering that only the energy required by the market is sold (case B) (case A is not 485
reported here for reasons of space). For a 95% base CO2 removal plant run on July 23rd in case
486 0 50 100 150 200 250 300 350 400 0 50 100 150 200 car bon tax p aid [k €/ day ]
carbon tax [€/tonCO2]
January 04th, 2015, base = 95% CO2 removal
No Capture System Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
0 10 20 30 40 50 60 70 80 0 50 100 150 200 carb on tax paid [k€/d ay ]
carbon tax [€/tonCO2]
January 04th, 2015, base = 95% CO2 removal
Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
0 50 100 150 200 250 300 350 400 0 50 100 150 200 car bon tax p aid [k €/ day ]
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal
No Capture System Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
0 10 20 30 40 50 60 70 80 0 50 100 150 200 car bon tax p aid [k €/ day ]
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal
Fixed Capture System (100%) Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
B, operating CCS in fixed mode at reduced level results advantageous for any carbon tax and 487
any % of considered reductions. In particular, for lower loads, the advantage is higher. This is 488
because if more carbon dioxide is removed (higher loads), a lower amount of power is available 489
to the final user. In summer, with high peaks and high prices for electricity, paying carbon tax 490
for emitting carbon dioxide results more advantageous than performing a high absorption of it. 491
This advantage does not result in winter, for which a decrease in profit higher than 30% occurs 492
by simply operating at 90% load. Indeed, when the request for electricity is lower, more CO2 is
493
absorbed, the better is because a lower carbon tax is paid. So, among fixed operation modes, 494
the full 95% CO2 removal must be performed.
495
If choosing to operate in fixed mode, the load can be selected on the basis of the period of the 496
year, and on the requested energy. 497 498 a) 499 b) 500 -40% -35% -30% -25% -20% -15% -10% -5% 0% 5% 10% 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 % v a ri at io n in p ro fi t co m p ar ed t o f ixe d cap tu re ( 100 % )
carbon tax [€/tonCO2]
January 04th, 2015, base = 95% CO2 removal
Fixed Capture System (90%) Flexible System CLR (ramp 5%)
0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 % v ari ati o n in p ro fit co m p ar ed to fix e d ca ptu re (100% )
carbon tax [€/tonCO2]
July 23rd, 2015, base = 95% CO2 removal
Fixed Capture System (90%) Fixed Capture System (80%) Fixed Capture System (70%) Fixed Capture System (60%) Flexible System CLR (ramp 5%)
Figure 11. % variation in profit compared to fixed capture at 100% load for 95% CO2 removal on a) 4th
501
January 2015 and b) 23rd July 2015, considering that only the energy required by the market is sold (case B).
502 503
For both January and July, the flexible operation is also reported, showing that in any case it 504
provides economic advantages in comparison to the fixed operating mode. 505
4. Conclusions
506
In the efforts of increasing actions to reduce greenhouse gas emissions, carbon capture and 507
storage has gained increasing attention as a good and effective technology for decarbonizing 508
the electricity sector. 509
This paper has performed a techno-economic analysis for determining the best operating mode 510
of a post-combustion CO2 removal section in a NGCC power plant assumed to be located in
511
Italy, for which data of hourly electricity prices and demands are available. 512
Considering the maximization of the profit, carbon capture is applied when the losses of profit 513
due to the payment of the carbon tax are higher than the ones deriving from selling a lower 514
amount of electricity, in particular at high price. 515
Both the cases of all net power output being sold to the market and of only the requested power 516
output being sold to the market have been considered. Depending on the value of the carbon 517
tax, the best % load has been determined on a hourly basis. In addition, results of the daily 518
profit, the overall paid carbon tax and the power produced obtained for fixed 95% CO2 removal,
519
for lower fixed modes and for CLR operation mode have been deeply analyzed. 520
Results for case A show that in summer paying carbon taxes of 50 €/tonCO2 is more
521
economically advantageous than operating the CO2 removal plant, while the same value of
522
carbon tax favors the absorption operation in winter, when electricity prices are lower. 523
When considering also the electricity demand, different results are obtained. The power sold in 524
winter, and the price of electricity at which it is sold, make the system in CLR mode being 525
operated almost at full load for any value of the carbon tax, while in summer from 09 to 22 526
carbon dioxide is preferably vented to the atmosphere. 527
The study has also focused on the comparison of the fixed and flexible removal modes with a 528
no capture system, determining the breakeven point of the carbon tax which makes the CO2
529
removal operation advantageous (resulting for CLR mode in 115 €/tonCO2 for January 04th,
530
2015 and in 145 €/tonCO2 for July 23rd, 2015).
531
On the basis of the obtained results it can be concluded that, similarly to PCC in coal-fired units, 532
also the flexible CLR mode in NGCC plants allows to reduce the economic losses of operating 533
CCS and therefore this technique can be employed to favor CO2 removal in the field of
534
electricity generation. 535
As for the implications for theory and practice, the challenges of balancing variable 536
renewable energy supplies in a low carbon power producing system can be addressed by highly 537
flexible and low-carbon electricity generation with natural gas fired units equipped with a CO2
538
removal section for treatment of the flue gas stream. 539
The analysis carried out in this work and the obtained results can be used both at industrial 540
and academic level. As for the former, the developed methodology can be successfully 541
employed to assist the management and the operation of the CCS section of power stations in 542
order to favor the lowest economic an power production losses while being in compliance with 543
the environment. In addition, the study provides the academic institutions support for carrying 544
out further research on the application of technologies for treatment of flue gas streams, in 545
particular in the field of power production. 546
An interesting further development of the work may focus on the applications of the employed 547
methodology to treatment of power generating units bases on low carbon dioxide emissions 548
fuels as coal power plants co-fired with biomass. 549
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