Article pubs.acs.org/EF
Simulation Optimization of a New Ammonia-Based Carbon Capture Process Coupled with Low-Temperature Waste Heat Utilization Yu Zhang,†,‡ Jianmin Gao,*,† Mingyue He,§ Dongdong Feng,*,†,‡ Qian Du,† and Shaohua Wu† †
School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Heilongjiang 150001, People’s Republic of China ‡ Faculty of Engineering, University of Nottingham, Nottingham NG7 2TU, United Kingdom § Harbin Boiler Company, Limited, 309 Sandadongli Road, Harbin, Heilongjiang 150046, People’s Republic of China ABSTRACT: Although ammonia-based CO2 capture has attracted global research attention, several inherent issues with this technology remain to be resolved. To address these problems, a new design for carbon capture using ammonia is proposed on the basis of anti-solvent crystallization, also known as precipitation crystallization. The crystallization of a low carbonized absorbent was found to be enhanced in the crystallizer using an anti-solvent process, which can maintain a high absorption rate and simultaneously prevent crystallization from occurring in the absorption tower. Energy consumption for sorbent regeneration is reduced by regenerating the crystal product rather than the rich solution. Energy-cascade utilization is an effective way to improve the use of energy. In this work, steam was used to drive a heat pump that extracts energy from discharged flue gas from a wet flue gas desulfurization system in a power plant to enable the recovery of low-temperature residual energy; this energy can be used in the crystal regeneration process, thereby further reducing the energy required for regeneration. Aspen Plus (version 8.4) software was adopted to simulate the flue gas condensation, heat-pump circulation, and steam drive subsystems. The simulation results showed that 10 heat pumps (6.04 MW) can meet the regeneration energy requirement of the CO2 capture process in a 300 MW coal-fired unit and recycle 40.6 MW of low-temperature heat. The extraction steam requirement is low, which reduces the impact on power generation. CO2 reaction with monoethanolamine solvent.19 Simulations conducted for a 300 MW coal-fired power plant showed that an ammonia-based capture system, which requires a capital investment representing 32.1% of the power plant capital investment, can consume 16% of plant power output.20 Existing techniques for ammonia sorbent regeneration need to be improved significantly to improve the capture efficiency and further reduce the energy penalty.21 To address these problems, a new ammonia capture technology based on an anti-solvent crystallization process has been proposed,22,23 with ethanol being the anti-solvent used and NH4HCO3 being the major crystal product formed from the crystallization process. While more details can be found in previous works,22,23 the new process concept involves the use of low rather than high concentrations of ammonia for CO2 absorption as a means to reduce the amount of ammonia released while maintaining a relatively high average absorption rate, the use of an anti-solvent crystallization process to speed up the crystallization of carbonized ammonia, which is normally a very slow process in typical traditional aqueous conditions, and the regeneration of ammonia sorbent via thermal decomposition of the crystalline products formed in the antisolvent crystallization process, which leads to a significantly lower energy requirement compared to the sorbent regeneration process with a rich aqueous solvent. In this work, the regeneration energy is further reduced using the recoverable
1. INTRODUCTION Carbon capture and storage (CCS) has been identified as a critical greenhouse gas reduction solution by the International Energy Agency (IEA) and Intergovernmental Panel on Climate Change (IPCC).1−6 Combustion of fossil fuels is the major source of global CO2 emissions,7,8 with coal-fired power generation accounting for about 30% of the total emissions.9,10 Although chemical absorption with aqueous amine solvent is the state-of-the-art technology for separating CO2 from lowpressure flue gases,11,12 the successful development of more efficient and cost-effective carbon capture technologies plays a decisive role in determining the overall economic performance of CCS because carbon capture accounts for over 70% of the cost of the whole total CCS chain.13,14 Alternative capture technologies have been under intensive investigation for some time, and aqueous ammonia-based absorption capture technology has been considered as being one of the most promising post-combustion CO2 capture technologies. In comparison to aqueous amine-based capture systems, ammonia-based systems have been found to have lower energy penalty, high loading capacity, virtually no solvent degradation issues, and the ability to remove multiple other acidic gases in addition to CO2.15−17 Many technical or operational issues, however, still remain to be adequately addressed, such as the formation of solid ammonium bicarbonate in the absorption tower and the need to further improve the absorption kinetics to reduce the energy requirement in regenerating the ammonia solvent. As revealed in the study by Wang et al.,18 the rate constant of the reaction of CO2 with ammonia, which involves the formation of carbonic acid, is an order of magnitude lower than that of the © XXXX American Chemical Society
Received: December 16, 2016 Revised: March 15, 2017 Published: March 17, 2017 A
DOI: 10.1021/acs.energyfuels.6b03361 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Technical flowsheet for improving use of low-temperature waste heat.
Figure 2. Aspen Plus flowsheet of the simulated system.
low-grade heat contained after the wet flue gas desulfurization (FGD) system. The temperature of the flue gas exiting the boiler is usually at ca. 140 °C, and after a typical wet FGD process, it is reduced typically to 50−60 °C. This sensible heat of the flue gas is mainly transferred to the desulfurization slurry to increase its temperature and to the latent heat of the water vapor in the wet-saturated flue gas. Extracting this latter portion of heat for the regeneration of the crystal product is the focus of this paper. The crystalline product of anti-solvent crystallization-based carbon capture technology can decompose completely at a temperature of 80 °C. The use of heat-pump technology can convert low-temperature waste heat into 80 °C high-grade energy that can meet the requirements for crystal product regeneration, thereby achieving the effective use of low-grade heat. The steam used to preheat the boiler backwater is extracted to drive a compression-type heat pump that can extract energy from the circulating slurry desulfurization system for crystal
product regeneration. The steam discharged from the heat pump is then condensed as boiler circulating water. This approach not only addresses the problem of environmental pollution by low-temperature waste heat but also makes effective use of recoverable very low-grade energy to reduce the energy penalty of carbon capture. Other benefits may also include potential water savings as a result of the condensation of water vapor in the wet flue gas after FGD. To gain a full understanding of this low-grade energy recovery system and assess to what extent this system can offset the energy requirements of CO2 capture, the heat balance of the entire process was comprehensively analyzed by Aspen Plus simulation. The condensing temperature of the heat pump was selected according to the regeneration temperature. The heating capacity of the heat pump, compressor power, absorption heat, coefficient of performance (COP), and other performance parameters were assessed by the simulation. Power outputs for different steam extraction scenarios were also calculated. B
DOI: 10.1021/acs.energyfuels.6b03361 Energy Fuels XXXX, XXX, XXX−XXX
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2. SYSTEM CONFIGURATION The heat pump serves as the link between the source of low-grade heat contained in the circulating slurry of the wet FGD system and the upgraded heat that is used in the regeneration process of the capture system. The heat-pump compressor is driven by a steam turbine. Figure 1 shows the technical flow diagram of the system process, while Figure 2 illustrates the Aspen Plus flowsheet of the simulated system. The system can be divided into three subsystems, including the flue gas condensation, heat-pump cycling, and steam drive. The heat absorption by the refrigerant in the heat-pump evaporator reduces the temperature of the FGD slurry, and this gives rise to the enhanced heat transfer from the flue gas to the limestone slurry in the FGD. As a result of heat recovery by the heat pump, the new considerably lower equilibrium temperature reached by the system leads to condensation of more water vapor present in the flue gas, leading to the release of more sensible/latent heat contained in the water vapor. After heat absorption in the evaporator, the liquid refrigerant is then converted into its vapor, which is subsequently adiabatically compressed for isothermal condensation in an integrated downstream condenser, and the heat released is then used to meet the energy requirement of the regeneration process of the capture system. After cooling, the formed liquid refrigerant in the condenser will be recirculated back into the heat recovery system enclosed in the FGD unit (the refrigerant evaporator) to complete the full cycle of the heat-recovering process, The heat-pump compressor is driven by the power plant extraction.
Figure 3. Effect of reducing the temperature of wet-saturated flue gas at different initial temperatures on output heat.
3. ASPEN PLUS SIMULATION OF FLUE GAS CONDENSATION The flue gas condensation system was used to simulate the amount of recoverable heat under different conditions of the wet FGD system based on a 300 MW coal-fired power plant. The effects of the temperature and flow rate of the wetsaturated flue gas were analyzed. 3.1. Process Definition. Assuming that the only lowtemperature heat source is the balanced wet-saturated flue gas exiting the wet desulfurization absorption tower, the specified components were assumed to be N2, O2, H2O, CO2, and a small amount of SO2. The only specified material flow is the inlet of this wet-saturated flue gas. Considering a 300 MW wet desulfurization unit as the example, the wet-saturated flue gas had a temperature of 56 °C, pressure of 1 bar, and flow rate of 1 219 480 Nm3/h. The mole fractions of each component were 70% N2, 5.8% O2, 17% H2O, 7% CO2, and 0.2% SO2. The specified physical property method used was the electrolyte balance method ELECNRTL in Aspen Plus. The flue gas condensation subsystem was assumed to be a process of water condensation releasing heat. The software has one heat-exchange module; therefore, the heater module was selected. Output heat flow can be added to the heater module to facilitate the calculation of condensation heat transfer. 3.2. Simulation Results and Analysis. Figure 3 shows the effect of the extent of temperature reduction on condensation as a result of cooling wet-saturated flue gas from different initial temperatures of 50−58 °C. The output heat (heat absorbed from the wet flue gas system unit time) of the flue gas condensation process increased with the temperature reduction for different temperatures of the wet-saturated flue gas, but the trend gradually slowed as the temperature reduction increased. Wet-saturated flue gas at an initial temperature of 56 °C and reduced by 10 °C yielded 50.9 MW heat; wet-saturated flue gas initially at 50 °C and reduced by 10 °C gave 39.6 MW heat. Figure 4 shows the analogous effect on the liquid ratio (the molar ratio of liquid water to vapor water). This shows the same trend as Figure 3. The higher the temperature, the higher the saturated humidity. The process of cooling wet flue gas
Figure 4. Effect of reducing the temperature of wet-saturated flue gas at different initial temperatures on the liquid ratio after condensation.
represents a change from one saturated state to another: the greater the temperature reduction, the greater the saturation humidity difference will be and the more condensation will take place. Figure 5 shows changes in output heat for different flue gas flow conditions when cooling the gas by 10 °C from different
Figure 5. Effect of the wet-saturated flue gas flow rate at different initial temperatures on output heat.
initial temperatures. The output heat of condensation varies linearly with the flue gas flow rate. At a constant gas saturation (temperature and humidity), as the flow rate increased, more water condensed with a decreasing temperature, thereby increasing the amount of heat gained. Analogous data for the liquid ratio of the wet flue gas are shown in Figure 6. The flue C
DOI: 10.1021/acs.energyfuels.6b03361 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7 shows the influence of changes in the working fluid flow rate on the production heat, compressor power,
gas flow rate had no effect on the liquid−gas ratio once the gas had condensed.
Figure 6. Effect of the wet-saturated flue gas flow rate at different initial temperatures on the liquid ratio after condensation.
Figure 7. Influence of change of the working fluid flow rate on the performance parameters of the heat pump.
4. ASPEN PLUS SIMULATION OF THE HEAT-PUMP CYCLE 4.1. Process Definition. The flowsheet of the heat-pump cycle subsystem is shown in Figure 2, and the module selection is shown in Table 1. The heat cycle process of the heat pump is
absorption heat, and COP. Once the heat pump working parameters are determined, changing its working medium will not change the COP; therefore, this parameter remained constant. Production heat increased as the flow rate of the working medium increased. Compression work also increased as a result of the constant COP. A single heat pump will produce more heat; the working medium requirement will be greater; and the pump volume will be larger.
Table 1. Selection of Heat-Pump Cycle Subsystem Modules module
selected user model
function
COMPRESS CONDENSE VALVE EVAPORAT
compr heater valve heater
compressor condenser expansion valve evaporator
5. ASPEN PLUS SIMULATION OF STEAM DRIVE 5.1. Process Definition. This section discusses simulation of the steam expansion process in the turbine. The physical properties of the steam cycle typically used in Aspen Plus are SREAM-TA and STEAMNBS.24,25 Some research considers SREAM-TA to be a relatively good physical property method; therefore, we selected this method. The turbine model of the compr module was selected. The isentropic efficiency of the steam turbine was set at 0.85, and the mechanical efficiency was assumed to be 0.85. The exhaust steam after working was returned to the condenser as circulating water or as a lowtemperature heat source. 5.2. Selection of Steam Extraction Points. In the steam cycle process of a power plant, there are several positions from which steam can be extracted, as shown in Figure 8: the high-, medium-, and low-pressure (entrance) sections of the steam turbine (HP, IP, and LP, respectively) and the points for preheating the water supply (A, B, C, D, E, F, G, and H). The parameters for the extraction points were selected from Zhang,26 as shown in Table 3. From the simulation results of Zhang, we can conclude that, if steam is extracted from the LP turbine, the impact on the power plant is small. Steam extraction points D, E, F, G, and H were therefore selected to investigate the compression work that can be obtained by extraction of different proportions of steam. The maximum
based on the ideal steam compression cycle. The heat-pump condensation temperature was set at 85 °C; the cycle temperature difference (the difference between the condensation and evaporation temperatures) was 45 °C. The degree of supercooling of the condenser was set at 5 °C under a constant pressure. The compressor model of the compr module was selected; isentropic compression was adopted; isentropic efficiency was chosen as 0.85; and the mechanical efficiency of the compressor was selected as 0.85. The degree of superheating of the evaporator was set at 12 °C under a constant pressure to prevent the compressor from hydraulic phenomena. Condensation and evaporation are constant pressure processes. A thermodynamic calculation process was mainly adopted for the heat-pump cycle. The commonly used non-random twoliquid (NRTL) thermodynamic model was selected as the physical property method. 4.2. Simulation Results and Analysis. The evaporation temperature of the heat-pump cycle was set to 40 °C, and the condensation temperature to 85 °C. R245fa was selected as the working medium, and its flow rate was set at 37.5 kg/s. The simulation results are presented in Table 2. Table 2. Simulation Results temperature of compressor exhaust, T2′ (°C)
condensation pressure, P2 (MPa)
evaporation heat, P1 (MPa)
produced heat, Q (kW)
compressor power, W (kW)
absorption heat, q (kW)
heating coefficient, COP
85.1149
0.8928
0.2504
6040
1387
4862
4.357
D
DOI: 10.1021/acs.energyfuels.6b03361 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 8. Sketch of steam extraction points in the steam cycle.
Table 3. Steam Condition Parameters for Each Steam Extraction Point extraction steam point
pressure (MPa)
temperature (°C)
flow rate (kmol/h)
HP A B IP C LP D E F G H
11.6 5.872 3.574 3.11 1.623 0.788 0.789 0.335 0.131 0.0709 0.0200
483.5 381.4 315.6 535.0 437.2 340.4 340.4 243.1 151.7 101.3 60.1
908192 63294 75695 769202 34112 670228 64861 35717 19765 33676 27047
amount of usable steam that can be extracted was selected as 0.8. 5.3. Simulation Results and Analysis. The variation in steam output power with the extraction ratio at each steam extraction point is shown in Figure 9. The power capability of steam from point D is much higher than that from other points: small extraction ratios are able to meet the technical demand. When the value of the extracted steam is taken into account, the closer a steam extraction point is to the LP section of condenser, the more energy loss can be recovered, the better the efficiency of power generation, and better optimization of generation efficiency of the plant power can be achieved. The greatest impact on the power point is achieved using steam extraction point D; steam from the LP section has a relatively lower impact on the power plant and can also meet the process requirements. Figure 9b shows a partially enlarged section of Figure 9a. Extracting the maximum steam from point H generates output work of 13.0 MW; to meet the compression work requirement for a single heat pump, 9% steam extraction is needed. Similarly, extracting the maximum amount of steam from point G gives an output of 34.5 MW; the compression work of a single heat pump requires only 3.5% of this output. Extracting 60% of the steam from point F or 25% from point E yield work outputs of 19.8 and 21.1 MW, respectively; 4.5% steam extraction from point F or 2% from point E are needed to meet the compression work requirements of a single heat pump. Several heat pumps are required to meet the demands for regeneration energy consumption in carbon capture technology using the proposed low-temperature waste heat quality improvement system. Preference for LP steam can maximize the energy-saving potential of the transformation process and optimize the generating efficiency of the power plant.
Figure 9. Variation in steam output power with the extraction ratio for each steam extraction point.
6. INTEGRATION OF THE THERMODYNAMIC SYSTEM 6.1. Determination of the Parameters of Flue Gas. The temperature of flue gas after passing through the wet desulfurization unit, slurry washing, and purification in a 300 MW coal-fired power plant is 56.7 °C. To meet the regeneration energy consumption requirements of a carbon capture process based on anti-solvent crystallization, the temperature of the wet-saturated flue gas needs to be reduced by 10 °C; i.e., the temperature of the flue gas ahead of the decarburization unit is 46.7 °C, and that after the absorption process is 42.3 °C. 6.2. System Integration Process. A thermal system can be built using the power plant as the main body coupled with a low-temperature residual energy utilization system based on the carbon dioxide capture process to achieve energy-saving and emission-reduction targets. The main components of the thermodynamic system include the thermal power plant steam−water system, combustion system, carbon dioxide capture system, and low-temperature energy-upgrading system. Figure 10 shows the integrated thermodynamic system. Considering a 300 MW coal-fired power station, the carbon dioxide capture devices are installed downstream of the desulfurization equipment. The discharge flue gas is first treated for dust removal, followed by desulfurization and then carbon capture. The wet-saturated flue gas after desulfurization slurry washing and purification is used as a low-temperature heat source to provide heat by a heat pump. The flue gas is then sent to the carbon dioxide capture device for decarburization. E
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60.5 MW can be obtained if extracting 12% of the steam from point IP or 27% of the steam from point E. Using the heating method proposed here, only 17% of the steam needs to be extracted from point E but 42% of the steam can be extracted from point F or 33% from point G, although steam from points F and G does not satisfy the requirements of the direct heating method.
7. CONCLUSION According to the principle of energy-cascaded utilization, heatpump technology was adopted. The circulating slurry of the wet desulfurization unit was used as a heat-transfer medium, and the latent heat of the purified wet-saturated flue gas was recycled to improve the quality of the low-temperature waste heat system. Simulation of such a low-temperature waste heat system was built using Aspen Plus, and steam from the steam cycle was used to drive the compressor of the heat-pump system. The effects of different temperature reductions on the heat output were simulated: wet-saturated flue gas at a temperature of 56 °C that was reduced by 10 °C can yield 50.9 MW latent heat. R245fa was selected as the working fluid of the heat-pump system. The heat produced by a single heat pump was 6040 kW; the compressor power was 1387 kW; and the heat absorption was 4862 kW, at a condensation temperature of 85 °C. Three steam extraction schemes are proposed, where E, F, or G can be selected as steam extraction points. Integration of this thermal system has the effect of considerable energy savings: 48.6 MW of low-temperature residual energy can be recovered.
Figure 10. Thermodynamic system integration.
6.3. Integration Results and Energy Consumption Analysis. Three steam extraction schemes are proposed: E, F, or G can be selected as steam extraction points. Steam from these three extraction points in the LP cylinder are used for the heat-pump system; steam for traditional heating must be extracted from E. Considering the extraction location, the heatpump method has a considerable advantage. The results of thermodynamic system integration are shown in Table 4. The Table 4. Results of Thermal System Integration steam extraction position extraction steam temperature extraction steam pressure extraction steam flow reducing temperature of wet-saturated flue gas from desulfurization number of heat pumps absorption heat compression power
unit
E
F
G
°C MPa kmol/h °C
243.1 0.335 6072
151.7 0.131 8302 10
101.3 0.0709 11114
MW MW
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
10 48.62 13.87
ORCID
Jianmin Gao: 0000-0001-6385-583X Notes
steam temperature from the E point was 243.1 °C; the flow rate was 35 717 kmol/h; and 6072 kmol/h steam needed to be extracted. The steam temperature from the F point was 151.7 °C; the flow rate was 19 765 kmol/h; and 8302 kmol/h steam needed to be extracted. The steam temperature from the G point was 101.3 °C; the flow rate was 33 676 kmol/h; and 11 114 kmol/h steam needed to be extracted. A comparison between heating based on low-temperature residual energy utilization and the traditional direct heating method is shown in Figure 11. The carbon capture technology based on anti-solvent crystallization and power plant steam cycle have been integrated by Zhang:27 the results show that
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant 51576056), the National Science and Technology Supporting Program (2014BAA02B03 and 2014BAA07B03), and the National Natural Science Foundation Innovation Research Group: Heat Transfer and Flow Control (Grant 51421063) is gratefully acknowledged.
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REFERENCES
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