Thermodynamic and Economic Analysis of Oxy-Fuel-Integrated Coal Partial Gasification Combined Cycle.

A novel partial gasification combined cycle (PGCC) system integrating coal partial gasification, oxy-fuel combustion, combined cycle, and CO2 separation is proposed. The coal-CO2 partial gasification technology is introduced in the coal gasification unit, and the oxy-fuel combustion technology is employed in the char combustion unit and gas turbine (GT) unit. The thermodynamic and economic analysis of the proposed system is carried out, showing that both energy and exergy efficiency have an increasing/decreasing tendency when the recycled flue gas (RFG) ratio of char combustion and GT increase. When the RFG ratios of char combustion and GT are 0.43 and 0.34, energy and exergy efficiencies reach maximum values of 48.18 and 45.11%, respectively. The energy efficiency of the PGCC-Oxy system is higher than that of the integrated gasification combined cycle (IGCC)-Oxy system by approximately 3%. It can be concluded from the economic analysis that the total investment on the PGCC-Oxy system is 3272.71 million RMB, and the internal rate of return (IRR) and payback time is 8.07% and 12.38 years, respectively.

Introduction

The emission of greenhouse gases is closely related to the climatic issues and attracted the world’s attention.[1−4] CO2 accounts for 63% of the total warming effects of all greenhouse gases.[5] It is reported that 41% of worldwide CO2 emission is caused by fossil fuels, especially coal, since 2010.[6] Over the past decades, coal played a critical role in the energy utilization industry because of its stable supply and low price. In China, coal accounted for 59% of the total energy consumption in 2018, while the share of renewable energy was 22.1%,[7] even though the renewable energy, including nuclear power and biomass, are developing rapidly and becoming increasingly important.[8] Coal will continue to play an important role in the future. The reduction of CO2 emission in the coal utilization process is an urgent issue and needs immediate attention.[9] There are mainly three types of CO2 capture technologies widely employed and researched and include precombustion, postcombustion, and oxy-fuel combustion.[10] In oxy-fuel combustion, fuel is combusted in a mixture of O2 and flue gas instead of air.[11] After oxy-fuel combustion, the flue gas is mainly composed of CO2 and H2O, so that CO2 separation is much more easily realized just after purification and compression.[12]

In recent years, many studies have been carried out on oxy-fuel combustion technology. A pressurized circulating fluidized-bed (CFB) oxy-fuel combustion power plant is proposed to increase the system efficiency and carbon capture rate.[9,13] Techno-economic analysis for the oxy-fuel combustion power plant was presented by Cormos and Hanak et al.[14,15] The oxy-fuel combustion technology is not only applied in the coal-fired power plant but also in the coal gasification industry. The energy and exergy efficiencies of biomass/coal co-gasification system were determined by Yan et al.[16] The integrated gasification combined cycle (IGCC) combined with oxy-fuel combustion was proposed by Kunze et al., which catches our attention.[17,18] In the referenced system, coal is completely converted into syngas at high temperatures and pressures, and the system efficiency is approximately 45%. However, the single conversion of coal results in waste of energy, and according to previous results,18 cascading utilization of coal has higher efficiency than single conversion. To improve the efficiency of coal utilization, a novel partial gasification combined cycle (PGCC) system based on cascading utilization of coal is proposed. The coal partial gasification technology aims not to overpursue high carbon conversion (60–85%) in the gasification process.[19,20] The high-activity part of coal is gasified and converted into syngas, while the low-activity part of coal is combusted for power generation. According to our previous results, the thermal efficiency of the PGCC system is higher than that of the IGCC and shows good feasibility.[21,22] Inspired by the IGCC-Oxy system above, it is worth studying the PGCC-Oxy system, in which O2 and flue gas are employed instead of air in char combustion.

In the conventional coal gasification process, O2 and steam are usually used as reactant gas,[23,24] and the addition of steam is beneficial for increasing the H2 content of syngas. The research on coal-O2/steam gasification has also been conducted by authors[25] and they found that the conversion of steam is relatively low, which means most of the steam is not involved in gasification reactions but just acts as a cooling agent. However, a lot of energy is spent for steam generation. Therefore, if there is no strict requirement for H2 content, O2 and CO2 can also be used as reactant gases in the coal gasification process, and CO2 acts as the cooling agent.[26] In this study, coal-O2/CO2 partial gasification and oxy-fuel combustion are combined in the PGCC-Oxy system to realize high efficiency and low carbon utilization of coal.

The PGCC-Oxy system is mainly composed of an air separation unit (ASU) unit, PGCC unit, as well as a carbon capture and storage (CCS) unit. The cryogenic distillation technology is used in the ASU,[27] where high-purity (>99.5%) O2 is generated. The CFB reactors are used as a gasifier and combustor due to their high fuel adaptability, low pollutant emissions, and low investment costs.[28,29] Coal is partially gasified with a mixture of CO2 and O2 to produce syngas. As a nonconventional means, CO2 rather than steam is used as a reactant gas;[30,31] thus, steam extraction from steam cycles is not required, which would increase the power output. Calcium is used in furnace desulfurization for sulfur removal.[32] The syngas generated from coal gasification is sent to the oxy-fuel gas turbine (GT) for power generation.[33] O2 and circulating flue gas are employed as the oxidizing agents instead of air. A high-temperature exhaust gas from GT is used for preheating the working fluid. The ungasified char is combusted with O2 and circulating flue gas, generating 900 °C flue gas; then the heat is transferred to the feedwater through different types of heating surfaces. Supercritical power generation technology is used in the steam cycles. The temperature and pressure of the main steam is 560 °C and 24.2 MPa, respectively. After heat recovery, H2O is separated from the flue gas. Finally, H2O-free gas is purified and pressurized to obtain high-purity CO2 is obtained.[8]

In this study, thermodynamic and economic analysis of the PGCC-Oxy system is carried out with a aim to resolve the critical issues of high efficiency and low carbon utilization of coal as well as provide techno-economic data for further commercial application.

PGCC-Oxy System

Process Description

The PGCC-Oxy system is mainly composed of an ASU, a dual-CFB gasifier and a combustor unit, combined cycles unit, and a CO2 separation unit. The diagram of the PGCC-Oxy system is shown in Figure . The Western Chinese coal is used, and the proximate and ultimate analysis is shown in Table . Coal and limestone are fed into the dense-phase area of the gasifier after crushing and then allowed to react with the reactant agents (O2 + CO2).[34] O2 is provided by the ASU, and CO2 is extracted from the CO2 separation unit. The reactant agents are preheated to ∼350 °C before entering the gasifier. Coal is partially gasified to produce syngas, which comprises CO, H2, CO2, and H2O. The ungasified char together with the circulating materials are transferred to the combustor and combusted with O2 and the recirculated flue gas. Then, the flue gas is generated after char combustion. The flue gas is split into two flows after the heat exchanger: one is recirculated into the combustor and the other is sent to the CO2 separation unit. Supercritical steam is generated in the heat recovery steam generation (HRSG) and expands in the steam turbines for power generation.

Figure 1

Diagram of the PGCC-Oxy system.

Table 1

Proximate and Ultimate Analysis of Western Chinese Coal

ultimate analysis		proximate analysis
wd(C)	61.14	wd(M)	7.55
wd(H)	3.18	wd(A)	20.94
wd(N)	1.23	wd(V)	25.12
wd(S)	0.56	wd(FC)	46.39
wd(O)	12.95	Qnet,ar/(MJ·kg–1)	21.93

The high-temperature syngas from the gasifier is first sent to the heat recovery and then to the syngas compressor. The pressurized syngas is combusted with O2 and the recirculating flue gas in the combustor of GT to produce the high-temperature and high-pressure exhausted gas that expands in GT for power generation. Similarly, the exhausted CO2-rich flue gas is split into two flows: one is recirculated to the combustor of GT and the other is sent to the CO2 separation unit. In the CO2 separation unit, CO2-rich gas flows into the acid condenser where H2O is removed.[8] Then, the H2O-free flue gas is pressurized to 11 MPa after the de-NO process and high-purity CO2 is obtained. The main parameters of the system are listed in Table . The Peng-Robinson with Boston-Mathias (PR-BM) model is adopted as the thermodynamic model.

Table 2

Primary Parameters of the Oxy-Fuel PGCC System

parameters	value
coal feed rate, kg/s	50
gasification temperature, °C	980
combustion temperature, °C	900
superheated steam	560 °C, 24.2 MPa
reheated steam	560 °C, 3.69 MPa
discharge temperature, °C	37
CO2 compression pressure, MPa	11

Air Separation Unit

As essential auxiliary equipment, the ASU, aims at providing high-purity O2 for gasification and combustion. It is one of the most energy-consuming units and has a great effect on system’s performance. Cryogenic technology similar to Zhang’s research is used,[27] and the diagram of the ASU is shown in Figure . The ASU is mainly composed of several heat exchangers, compressors, a high-pressure turbine (HP) column, and a low-pressure turbine (LP) column. The pressures in the HP and LP columns are 1 and 0.25 MPa, respectively. The air is first pressurized to 1 MPa and then passes through two heat exchangers. Finally, the air is separated in the HP column and LP column in turns, and high-purity (>99 wt %) O2 is generated.

Figure 2

Diagram of the ASU.

Dual-CFB Gasifier and Combustor

The diagram of dual-CFB gasifier and combustor is shown in Figure . Silica is employed as the bed and recirculated materials in the CFB gasifier and combustor. It is different from the entrained-flow gasifier in that the CFB gasifier has higher fuel adaptability, especially suitable for low-rank coal and other low heating fuels. In this scheme, coal is crushed into particles and mixed with limestone. Then, the solid mixture is continuously fed into the dense-phase region of the gasifier and allowed to react with O2 and CO2. The reactions of coal particles in the gasifier are very complicated and include devolatilization, char gasification, NO and SO transformation, and desulfurization,[35,36] as shown in the following reactions.As coal particles together with limestone are fed into the furnace where the gasification temperature rises to more than 950 °C, coal is decomposed into coal char and volatiles. Limestone is decomposed into CaO, and CaO reacts with H2S. Then, the char and volatiles react with O2 and CO2, and syngas is generated. O2 is from the ASU and CO2 is from the CO2 separation unit. The syngas mainly consists of CO, H2, CO2, and H2O, and it is different from the conventional coal-O2/H2O gasification in that the CO content is higher and the H2 content is lower. The syngas goes through the cyclone, and the solid material is separated. The separated solid material is sent through a loop-seal. In the meantime, there is a char outlet set in the dense phase of the gasifier, and the char would also flow into the combustor through the char outlet. Oxygen-enriched combustion technology is used in the char combustion process. Char is combusted with O2 and the recycled flue gas (RFG), and CO2-rich flue gas is generated. To ensure stable operation of the CFB combustor, the combustion temperature is maintained at around 900 °C.

Figure 3

Diagram of the dual-CFB gasifier and the combustor unit.

Devolatilization

Gas–solid heterogeneous reactions

Gas–gas homogeneous reactions

Sulfur transformation reactions

Nitrogen transformation reactions

Desulfurization

Steam Cycle Unit

The diagram of the steam cycle unit is shown in Figure , and the water and steam flows are shown with their temperatures and pressures. The heat from the char combustion is used for the steam generation, which is much higher than that of the flue gas from GT. Thus, supercritical power generation technology is used in the steam cycle unit and it is widely employed in the coal-fired power plants because of its higher power generation efficiency when compared to that in the subcritical power generation technology.[37] The feedwater is pumped at an increased pressure and goes through the deaerator, low-pressure heaters, and high-pressure heaters in order. There are three low-pressure heaters and three high-pressure heaters. Then preheated water flows through the heating surfaces that are distributed on the char combustor, and water is evaporated into supercritical steam. The temperature and pressure of the steam are 560 °C and 24.2 MPa, respectively. After doing work in the HP, the exhausted steam from the HP is sent back to the reheater where it is converted to subcritical steam and then expands in the intermedia pressure turbine (IP) and LP. The temperature and pressure of the reheated steam are 560 °C and 3.69 MPa, respectively. The exhausted steam from the LP is condensed in the condenser and mixed with the feedwater in the deaerator. Then, another water recirculation begins.

Figure 4

Diagram of the steam cycle unit.

CO2 Purification and the Compression Unit

The diagram of the carbon dioxide compression and purification unit (CPU) is shown in Figure , which is based on Xiong’ and Yan’s work.[13,38,39] The flue gas is sent to the flash reactor, where water is separated. The water-free gas is pressurized to 3 MPa in the multistage compressor and then NO is converted into HNO3 in the de-NO process. Since N2 is not involved in GT, NO emission from oxy-fuel combustion is much lower than that from air combustion.[40] The de-NO process is still needed[41] and the reactions in the de-NO process are shown below. After the de-NO process, the gas is sent to the heat exchanger where it is cooled and the separated products are heated. Finally, more than 92% CO2 of the flue gas is separated and CO2 of 95% purity is obtained.[42]

Figure 5

Diagram of the CPU.

Methodology

The methods of thermodynamic and economic analysis, including energy efficiency analysis, exergy efficiency analysis, and economic analysis, are presented in this section.

Thermodynamic Evaluation Criteria

Energy and exergy efficiencies are commonly used to evaluate the thermodynamic performance of the energy conversion system.[19,43,44] Energy analysis is based on the first law of thermodynamics, while exergy efficiency analysis is based on the second law of thermodynamics. The energy efficiency η (%) of the system is calculated as followswhere E (MW) is the electricity production, mcoal (kg/h) is the mass flow rate of coal, and LHcoal (MJ/kg) is the lower calorific value of coal. The exergy efficiency ε (%) of the system is calculated as followswhere EXelectricity (MW) is the exergy of electricity and is equal to E (MW) and EXcoal (MW) is the exergy of coal.

The specific exergy of the inlet coal is calculated by S–S equations, which are developed by Szargut and Styrylska[45,46]where C, H, O, N, and S represent the mass fractions of carbon, hydrogen, O2, nitrogen, and sulfur, respectively.

Economic Evaluation Criteria

The fixed capital investment (FCI), the internal rate of return (IRR), and the payback period are employed to describe the economic performance of the combined cycle system.

The FCI is calculated using the production capacity index method. In the case of the single equipment, it can be expressed by the following equationwhere C, F, and b are the capital investment of the equipment, the capital investment of the referenced equipment, and the production capacity index, respectively. When 0 < S/Sr, < 1, b = 1. When S/Sr, < 50 and the expansion of the proposed projects is achieved by increasing the size of the equipment, 0.6 < b < 0.7. When S/Sr, < 50 and the expansion of the proposed projects is achieved by increasing the number of equipments, 0.8 < b < 0.9. The price of each equipment is according to thermal power engineering limit design reference cost index.[47]

The IRR is commonly used to evaluate the feasibility of the engineering project. which is expressed as follows[19,48,49]where C represents the cash flow of the year t and n denotes the calculation year.

The net cash flow can be obtained as follows[50]where Cp, CM, and CF represent the annual income of products, material cost, and fuel cost, respectively. O&M denotes the ratio of the annual operation and management cost to FCI, α is the interest rate during the construction period, and the CRF represents the ratio of annual average investment.[51]The payback period is used here[52]where P is the payback period and n is the time.

Results and Discussion

Effects of RFG Ratio on the System Performance

Since there are pressure drops along the flue gas flow, the recirculation compressors are arranged to compensate for the pressure drops. The energy consumption of the flue gas recirculation processes would have a significant effect on energy efficiency. In contrast to the conventional oxy-fuel combustion system, there are two flue gas recirculation processes in this study: the char oxy-fuel combustion and gas oxy-fuel combustion. With some constant parameters, the energy consumptions of different processes are listed in Table and Figure .

Table 3

System Performance under Different RFG Ratios of Char Combustion

parameters	1	2	3	4	5	6
mass flow rate of coal, kg/h	180 000	180 000	180 000	180 000	180 000	180 000
O2/coal ratio	0.36	0.36	0.36	0.36	0.36	0.36
CO2/coal ratio	0.34	0.34	0.34	0.34	0.34	0.34
inlet O2 of char combustion, kg/s	6.5	8	16	24	32	40
inlet O2 of gas combustion, kg/s	46	46	46	46	46	46
RFG ratio of char combustion	0.31	0.35	0.43	0.46	0.48	0.50
mass fraction of O2 in char combustion, kg/kg	0.44	0.44	0.44	0.44	0.44	0.44
volume fraction of O2 in char combustion, m3/m3	36%	36%	36%	36%	36%	36%
RFG ratio of GT	0.34	0.34	0.34	0.34	0.34	0.34
Power Generation (MW)
ST	544.28	558.43	633.27	636.27	639.29	642.32
GT	129.51	129.51	129.51	129.51	129.51	129.51
gross power generation	673.80	687.94	762.79	765.79	768.80	771.83
Power Consumption (MW)
ASU	59.22	60.48	67.2	73.92	80.64	87.36
CPU	77.94	77.99	78.04	83.16	88.29	93.43
pumps	10.67	10.86	11.863	11.9	11.94	11.98
compressors	77.15	77.18	77.333	77.53	77.72	77.92
total	224.97	226.45	234.44	246.51	258.59	270.69
energy efficiency, %	40.93	42.09	48.18	47.36	46.53	45.70
exergy efficiency, %	38.32	39.40	45.11	44.34	43.56	42.79

Figure 6

Effects of RFG ratios of char combustion on the energy efficiency and exergy efficiency.

In the first part, the effects of the RFG ratio of char combustion on system performances are studied. The mass fraction of O2 in char combustion is maintained at 0.44. The power generation of ST increases from 544.28 to 642.32 MW, while the power generation of GT is almost constant when the RFG ratio of char combustion varies from 0.31 to 0.5. The O2 supply to char combustion needs to be increased to keep the mass fraction of O2 constant as the RFG ratio of char combustion increases. Thus, the energy consumption of the ASU increases from 59.22 to 87.36 MW and the increased amplitude is almost 47.51%. The energy consumption of the CPU increases from 77.94 to 93.43 MW and the increased amplitude is 19.87%, which is much smaller than that of the ASU. When the mass flow rate of O2 is lower than 16 kg/s, the char is in the anoxic condition and part of the carbon in char is converted into CO instead of CO2. When the mass flow rate of O2 is more than 16 kg/s, the carbon in the char is converted to CO2 completely, which leads to a significant increase in the CO2 content. Therefore, the power consumption of the CPU increases sharply from case 3 to case 4. The total energy consumption of the ASU and CPU is more than half of the total energy consumption. The total pressure drops of heat exchangers and recirculated pipes are approximately 0.004 MPa.[13] Therefore, when the RFG ratio of char combustion increases from 0.31 to 0.50, the energy consumption of compressors only increases from 77.15 to 77.92 MW. Similarly, the energy consumption of other processes differs slightly when the RFG ratio of char combustion varies. In view of energy and exergy efficiencies of the system, both of them have an increasing/decreasing tendency. Thus, when the RFG ratio of char combustion is 0.43, both energy and exergy efficiencies have maximum values of 48.18 and 45.11%, respectively.

The RFG ratio of char combustion is set as 0.43 according to the above results. The effects of the RFG ratio of GT on system performance are determined and are shown in Table and Figure . The recirculated flue gas needs to be pressurized before sending to GT, and higher combustion pressure results in larger pressure drops.[13] The power consumption of compressors increases from 67.38 to 123.38 MW when the RFG ratio of GT varies from 0.29 to 0.52 and the increased amplitude is around 83.54%. To maintain the mass fraction of O2 in GT, the mass flow rate of O2 increases with the increase in the RFG ratio of GT, resulting in the energy consumption of the ASU increasing from 55.44 to 105.84 MW. From the view of power generation, both power generation of ST and GT varies significantly, from 508.61 to 626.19 MW and from 116.63 to 218.61 MW, respectively, with the increase in the RFG ratio of GT. Combining with the above analysis, both the energy and exergy efficiencies have an increasing/decreasing tendency. When the RFG ratio of GT is 0.34, both the energy and exergy efficiencies reach the maximum values of 48.18 and 45.11%, respectively.

Table 4

System Performance under Different RFG Ratios of GT

parameters	1	2	3	4	5
mass flow rate of coal, kg/h	180 000	180 000	180 000	180 000	180 000
O2/coal ratio	0.36	0.36	0.36	0.36	0.36
CO2/coal ratio	0.34	0.34	0.34	0.34	0.34
inlet O2 of char combustion, kg/s	16	16	16	16	16
inlet O2 of GT, kg/s	32	46	68	80	92
RFG ratio of char combustion	0.43	0.43	0.43	0.43	0.43
mass fraction of O2 in char combustion, kg/kg	0.44	0.44	0.44	0.44	0.44
volume fraction of O2 in char combustion, m3/m3	0.36	0.36	0.36	0.36	0.36
mass fraction of O2 in char combustion, kg/kg	0.44	0.44	0.44	0.44	0.44
volume fraction of O2 in GT, m3/m3	0.36	0.36	0.36	0.36	0.36
RFG ratio of GT	0.29	0.34	0.40	0.46	0.52
Power Generation (MW)
ST	508.61	633.27	629.61	628.47	626.19
GT	116.63	129.52	173.04	193.44	218.61
gross power generation	625.24	762.78	802.65	821.91	844.80
Power Consumption (MW)
ASU	55.44	67.20	85.68	95.76	105.84
CPU	77.08	78.04	92.13	99.86	107.61
compressors	67.38	77.33	99.62	110.18	123.38
pumps	10.20	11.86	11.81	11.80	11.77
energy efficiency, %	37.86	48.18	46.82	45.99	45.25
exergy efficiency, %	35.44	45.11	43.84	43.06	42.37

Figure 7

Energy and exergy efficiencies of the system under different RFG ratios of GT.

Effects of O2/CO2 Ratio of Coal Gasification on the System Performance

In recent years, CO2 and O2 have been proposed as gasification agents instead of steam and O2 in the coal/biomass gasification, which would greatly improve the CO2 enrichment.[53] Moreover, the efficiency of gasification with CO2 and O2 is higher than that with air or O2-steam at the same O2 concentration.[54] Therefore, it is meaningful to study the effect of the O2/CO2 ratio on the system performance[26] and the results are listed in Table .

Table 5

Parameters under Different CO2/Coal Ratios

parameters	1	2	3	4
mass flow rate of coal, kg/h	180 000	180 000	180 000	180 000
gasification temperature, °C	982	926	887	859
O2/coal ratio	0.36	0.36	0.36	0.36
CO2/coal ratio	0.3	0.34	0.38	0.42
O2/CO2 ratio	1.06	0.95	0.86	0.78
inlet O2 of char combustion, kg/s	16	16	16	16
inlet O2 of GT, kg/s	46	46	46	46
mass fraction of O2 in char combustion, kg/kg	0.44	0.44	0.44	0.44
volume fraction of O2 in char combustion, m3/m3	0.36	0.36	0.36	0.36
RFG ratio of char combustion	0.43	0.43	0.43	0.43
RFG ratio of GT	0.34	0.34	0.34	0.34
Power Generation (MW)
GT	129.51	130.78	132.05	133.32
ST	633.27	633.43	633.52	633.58
gross power generation (MW)	762.79	764.21	765.57	766.90

The influence of O2/CO2 ratio on the gasification temperature, energy efficiency, and exergy efficiency is shown in Figure . The gasification temperature increases with increase in the O2/CO2 ratio because more O2 results in higher combustible part consumption thereby releasing more heat. Both the energy and exergy efficiencies increase from 48.11 to 48.18% and from 45.05 to 45.11%, respectively, when the O2/CO2 ratio varies from 0.78 to 1.06. The tendency of energy and exergy efficiencies differ indistinctively. The energy consumption in several units is shown in Figure . Since the O2 supply in the ASU is constant, the energy consumption is constant and it is the minimum among the three units, while the energy consumption of the CPU is slightly higher than that of compressors. The energy consumption of compressors decreases from 77.33 to 79.25 MW because of the decrease in the CO2 input. Since a multistage compressor is arranged in the CPU to pressurize the flue gas, an increase in the O2/CO2 ratio results in low power consumption of compressors. Therefore, the energy consumption of the CPU decreases from 81.02 to 78.04 MW.

Figure 8

Influence of O2/CO2 ratio on the gasification temperature, energy efficiency, and exergy efficiency.

Figure 9

Energy consumption of compressors and the CPU under different O2/CO2 ratios.

Comparison of the Proposed System with the Referenced System

The proposed system is compared with an IGCC-Oxy system,[17] and the mass flow rate of the main streams of the PGCC-Oxy system is shown in Figure . Since some data are not provided by authors of ref (17), only some necessary data are listed in Table . It can be seen that the energy efficiency of the PGCC-Oxy system is approximately 3% higher than that of the IGCC-Oxy system indicating that cascading utilization of coal is more advantageous than the single conversion of coal. To evaluate the quantity of pollutant emission of the flue gas, the content of SO2 and NO in the flue gas are listed in Table , showing that the emission of SO2 and NO can meet the national standard.

Figure 10

Mass flow rate of main streams in the PGCC-Oxy system.

Table 6

Optimization of System Performance

parameters	PGCC-Oxy	IGCC-Oxy
Input
mass flow rate of coal, kg/h	180 000
lower calorific value of coal, MJ/kg	21.93	25.17
total calorific value of coal, MW	1096.5
exergy of coal, MJ/kg	24.42
total exergy of coal, MW	1171.2
O2/coal ratio, kg/kg	0.36
CO2/coal ratio, kg/kg	0.34
RFG ratio of char combustion	0.43
mass fraction of O2 in char combustion, kg/kg	44
volume fraction of O2 in char combustion, m3/m3	36
RFG ratio of GT	0.34
mass fraction of O2 in GT, kg/kg	44
volume fraction of O2 in GT, m3/m3	36
Output
power generation of GT/MW	129.52
power generation of ST/MW	633.27
gross power generation of system/MW	762.78	599.30
consumption of compressors/MW	77.33
consumption of CPU/MW	78.04
consumption of pumps/MW	11.86
consumption of ASU/MW	67.2
total energy consumption/MW	234.43	110.34
captured CO2/kg/h	392108.4
net power generation/MW	528.35	488.96
energy efficiency/%	48.18	45.74
exergy efficiency/%	45.11

Table 7

SO2 and NO Emission in the Flue Gas

items	SO2/mg/Nm3	NOx/mg/Nm3
flue gas	97.3	88.5

Economic Analysis

Economic analysis is conducted to evaluate the economic performance of the PGCC-Oxy system. The basic data of economic analysis are listed in Table . The total investment cost of the PGCC-Oxy system is 3272.71 million RMB and is shown in Table . The investment cost per kW electricity is 6194.211RMB/kW, which is almost twice that of 660 MW-supercritical coal-fired power plant (3367 RMB/kW).[47] This is because there are two extra expensive facilities in the PGCC-Oxy system: one is the ASU, the cost of which is almost 634.71 million RMB since O2 instead of air is used as gasification and combustion reactants; the other is the CPU, the cost of which is almost 575.57 million RMB. The total investment cost is greatly increased by these two units. The dual-CFB reactors, HRSG, and CPU are the most expensive facilities in the system that account for more than 80% of the total investment cost. Based on the data in Tables and 9, the IRR and payback time are 8.07% and 12.38 years, respectively. Even though they are lower than that of PGCC without CCS,[22] it is still a promising technology that can be applied in the industry.

Table 8

Basic Economic Data

items	value	price	value
discount rate	8%	coal	0.57 RMB/kg
economic life of the project	30 years	water	0.047 RMB/kg
operating hours per year	5000 h	electricity	0.42 RMB/kW·h
O&M	4% of the fixed asset investment[55,56]	limestone	0.55 RMB/kg
construction years	3 years	tax rate	25%
interest rate	4.9%

Table 9

Economic Comparisons of the Combined Cycle System

items	PGCC-Oxy
Raw Material (Million kg/year)
coal	900
water	1551
limestone	232
electricity, kW·h/year	2.64 × 109
dual-CFB unita	907.70
ASUb	634.71
GTa	206
electric generatora	38
STa	192.56
ST-electric generatora	106.72
HRSGa	587.19
gas compressora	1.83
CPUb	575.57
pumpsa	5.78
fansa	4.55
heat exchangersa	12.1
total investment cost	3272.71
O&M cost	130.91
net power generation, MW	528.35
investment cost per kW, RMB/kW	6194.21
internal rate of return, %	8.07
payback time, year	12.38

The price of facilities is from the literature.[47]

The price of facilities is from the literature.[15]

Conclusions

In this work, thermodynamic and economic analysis of a novel energy conversion system based on coal partial gasification and oxy-fuel combustion is conducted. The conclusions are as follows.

When the RFG of char combustion and GT increase, both the energy efficiency and exergy efficiency have an increasing/decreasing tendency and they reach the maximum values when the RFG of char combustion and GT are 0.43 and 0.34, respectively.

When the CO2/O2 ratio increases from 0.78 to 1.06, the gasification temperature, energy efficiency, and exergy efficiency have an ever-increasing tendency.

When compared with the IGCC-Oxy system, the energy efficiency of the proposed system is approximately 3% higher.

The total investment cost of the PGCC-Oxy system is 3272.71 million RMB, and the IRR and payback time of the system are 8.07% and 12.38 years, respectively, which shows a promising prospect.