Novel two-stage fluidized bed-plasma gasification integrated with SOFC and chemical looping combustion for the high efficiency power generation from MSW: A thermodynamic investigation

Abstract A novel municipal solid waste (MSW)-based power generation system was proposed in this study, which consists of a bubbling fluidized-bed (BFB)-plasma gasification unit, a high-temperature solid oxide fuel cell (SOFC), a chemical looping combustion (CLC) unit and a heat recovery unit. Process simulation was conducted using Aspen Plus™ and validated by literature data. The energetic and exergetic assessment of the proposed system showed that the net electrical efficiency and exergy efficiency reached 40.9% and 36.1%, respectively with 99.3% of carbon dioxide being captured. It was found that the largest exergy destruction took place in the BFB-Plasma gasification unit (476.5 kW) and accounted for 33.6% of the total exergy destruction, followed by the SOFC (219.1 kW) and then CLC (208.6 kW). Moreover, the effects of key variables, such as steam to fuel ratio (STFR), fuel utilization factor (Uf), current density and air reactor operating temperature, etc., on system performance were carried out and revealed that the system efficiency could be optimized under STFR = 0.5, Uf = 0.8 and air reactor operating temperature of 1000 °C. Furthermore, the proposed process demonstrated more than 14% improvement in net electrical efficiency in comparison with other MSW incineration and/or gasification to power processes.

However, to the best of our knowledge, no attempt has been made so far to integrate SOFC and 82 CLC with BFB-plasma gasification for highly efficient power production as well as CO2 capture. 83 Therefore, this work is set out to study the feasibility of such a novel process and to gain insights of 84 its thermodynamic performance. reactor, respectively, and burned. Then, the flue gas from the chemical looping system is processed 97 in HRSG to recovery heat. The detailed configuration of the proposed process is illustrated in Fig.2. 98 The detailed description of each subsystem is presented in following sections.     105 The pre-treated MSW is crushed into 10 to 25 mm and fed into the fluidized-bed gasifier together 106 with oxygen and steam. The amount of oxygen and steam is controlled to maintain autothermal state 107 with the operating temperature in the range of 650 to 800 ºC and to achieve a higher carbon conversion [8,28]. A higher gasification temperature is beneficial for the promotion of syngas yield 109 but is also associated with a higher mineral melting possiblity that leads to the agglomeration and 110 defluidization of the gasifier, which subsequently causes the blocakge accident. In this study, the 111 oxygen equivalence ratio (ER) and steam to fuel mass ratio (STFR) are adopted to quantify the feeding 112 rate of the gasification agent. The ER and STFR parameters can be calculated as follows:

BFB-plasma gasification
Oxygen needed for the fluidized-bed gasifiction is supplied from a cryogenic air separation unit 116 (ASU), while steam is extracted from the HRSG. In the gasifier, carbon, oxygen and steam are 117 contacted and reacted intensively to convert the solid into syngas. The detailed chemcial reactions 118 in the gasifer can be referred in [29] .The crude gas from the gasifier mainly contains CO,CO2,CH4,

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H2O and H2 in conjunction with a certain amount of tar and char. Besides, ash and inorganic material 120 can also be brought out with the raw syngas. Then, the crude gas is sent to the readily-controllable 121 plasma converter where complex organics are exposed to the ultra violet light induced by a carbon 122 plasma electrode and cracked into CO and H2 at the uniform temperature of 1200 ºC. At the same 123 time, particulate materials in the raw gas enters to the centrifugal designed plasma converter where 124 they are converted into molten slag. The outlet syngas exits the plasma converter and is cooled in 125 the heat exchangers (HE1 and HE2) followed by a gas cleaning unit, in which the contaminates and 126 sulphide are removed by a ceramic filter and a sorbent bed respectively [9,22]. The clean syngas is 127 heated up and fed to the SOFC subsystem. Table 1 illustrates the ultimate and proximate analysis of 128 the selected minicipal solid waste employed in this study. The main operating conditions of the two 129 stage fluidized-bed plasma gasification subsystem are shown in Table 2.
Where the subscript 'react' represents the reacted molar flow rate of the gas species in the SOFC 169 cell.

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The current density (i, A/cm 2 ) is obtained by the total current (I) divides by the active surface 171 area (Aa).
The inverter efficiency for DC to AC conversion is assumed to be 95% [35]. Thus, the actual power 174 output from SOFC is expressed by: The main operating conditions and assumptions for the CLC subsystem is presented in Table 4.

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In order to control temperature in the air reactor, excessive air cooling approach is employed as 193 cooling agent to avoid agglomeration of oxygen carriers.  198 The effluent gases from the FR and AR are at high temperature and pressure states and they are 199 directly sent to the CO2 gas turbine and air gas turbine for the additional power generation. Then, 200 the gases from the two turbines are forwarded to HRSG unit to recovery heat for steam generation.

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The different pressure steam streams produced from the HRSG are led to steam turbines for power 202 generation. After the heat recovery in HRSG, the stream initially from FR is cooled to 30 ºC and water 203 is separated from this stream. The CO2 rich stream is then directed to a four-stage intercooled 204 compressor to the pressure of 120 bar which is ready for the pipeline transportation. Table 5 presents 205 the main specifications adopted in this subsystem.    Table 1), 262 respectively.  (Fig. 3a), the current simulation is closer to experimental results. Besides, the current 269 simulation of the gas compostion at the outlet of the plasma coverter is totally consistent with the 270 results of the experiment (Fig.3b). As the outlet syngas from the converter is fed to the downstream   Table 6, a good 280 agreement is achieved between our simulation data and reported value. The deviation is found to be 281 in the range of 0 to 5.8%, which indicates the SOFC model developed in this study is reliable.  Table 7 288 that the simulation value is nearly identical to the experimental data and the relative difference is 289 very small (<10%) which shows the simulation methodology of CLC is appropriate/acceptable.

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The proposed process was simulated according to the basic operating conditions shown in Table   295 1 to Table 5. At the conditions of STFR =0.5, fuel utilization of 0.8, current density of 2200 A/m 2 and 296 operating temperature of AR of 1000 ºC, the simulation results, such as temperature, pressure, mass 297 flow and molar composition for the key state points (see Fig.2), are listed in Table 8.
To improve power generation efficiency of the HRSG & CC subsystem, pinch analysis was 299 conducted by adjusting the steam flow rates of high pressure, medium pressure and low pressure to 300 construct the hot and cold composite curves with a minimum approach temperature of 10 ºC.  After the pinch analysis, the energy and exergy performances of the proposed process are 311 computed and presented in Table 9. The net electricity generated in this process is 815.7 kW with a net electrical efficiency of 40.9%. The total exergy fed into the process is 2223.9 kW resulting in the 313 exergy efficiency of 36.7%. It can also be noticed from this table that the electricity generated by 314 SOFC shares largest proportion of the total gross electricity, accounting for 42%. The air gas turbine 315 contributes to 552.3kW electricity due to the expansion of large amount of depleted air.

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While from the electricity consumption perspective, the air compression unit takes up largest 317 share of 325.9 kW because of ample air requirements in SOFC unit as the air utilization factor of 18.2% 318 (see Table 3). The exergy destruction and exergy efficiency distributions of the key components in the 332 proposed process are presented in Fig. 5(a) and Fig. 5(b), respectively. The exergy destruction for a 333 unit is defined as the difference between inputs exergy and output exergy, while exergy efficiency for a unit is defined in literature [39]. It can be noticed from Fig. 5(a)  for 68.2, 54.9, 47.6, 43.9, 34.7, 34.2 kW exergy destruction, respectively.

345
As indicated by Fig. 5(b), the HRSG1 has the highest exergy efficiency of 98.2% due to the small  362 In this study, the influences of four key operating parameters, i.e., the steam to fuel ratio, fuel 363 utilization factor, current density and operating temperature, on both energy and exergy efficiencies 364 are examined. Fig. 6 shows the effect of STFR on system efficiency. It can be seen in Fig. 6

398
The effect of changing the air reactor temperature of the CLC on the energy and exergy efficiencies 399 is presented in Fig.9. Based on Fig.9  it is reasonable to conclude that the proposed process is thermodynamically more performing and 450 can realize low-to-zero CO2 emission.