| With the increasingly severe problems of energy shortage and environmental pollution,the rational development and utilization of renewable energy has become a hot topic in current research.As the only one renewable energy that can be stored and transported on a large scale,biomass has attracted increasing attention due to its efficient conversion and clean utilization.Biomass gasification,as one of the thermochemical conversion technologies of biomass,can convert biomass into gaseous fuel and is one of the most effective ways to realize biomass energy conversion.The traditional biomass gasification technology uses steam,air or oxygen as gasification agent and has the problems of high energy consumption,high cost,low yield of synthesis gas(H2 and CO mainly)and high tar content which have become the bottleneck and main obstacles restricting the application of biomass gasification technology.Biomass chemical looping gasification is an application of chemical looping technology in the field of biomass energy utilization.In the fuel reactor,the utilization of the oxygen carrier to supply oxygen and heat for the gasification process to product synthesis gas can avoid the problems of dilution of synthesis gas by N2 in air,high cost in the pure oxygen production process and high energy consumption in steam gasification,and the oxygen carrier can catalyze tar cracking and increase the yield of synthesis gas;the oxygen carrier can be oxidized and regenerated in the air reactor,and the high-temperature flue gas can be used for combined cycle power generation.Chemical looping gasification technology is still in the experimental stage,and the integration of chemical looping gasification system and its coupling with chemical-power system are rarely investigated.In this study,with wood chips as raw material,combining biomass chemical looping gasification and polygeneration technology,a biomass chemical looping gasification for hydrogen-power-methanol polygeneration process is proposed.Aspen Plus software is used to perform process simulation,and its feasibility is analyzed from thermodynamic and economic aspects.Biomass chemical looping gasification polygeneration process mainly includes biomass chemical looping gasification unit(Fe2O3 as oxygen carrier and CaO as CO2 absorbent),synthesis gas purification unit(Rectisol),methanol synthesis unit(lowpressure gas-phase methanol synthesis),methanol refining unit(double effect triple column process)and power generation unit(Gas-steam combined cycle).The technical route is that the syngas of biomass gasification is purified and devoted to methanol synthesis,and part of the unreacted gas in the synthesis process is introduced to combined cycle for power generation to achieve the coupling of chemical process and power systems.Aspen Plus software is used to simulate the process of each unit,and the main unit is verified by the literature.The results showed that the simulation results are in good agreement with the literature.Based on a reliable model,the effect of operation parameters on system performances are investigated,including the ratios of steam to biomass(S/B),oxygen carrier to biomass(MCa/B,MFe/B),gasification pressure(PCLG),oxygen carrier regeneration temperature(TAR),methanol systhsis temperature(TMSR)and pressure(PMSR),and unreacted gas recycle ratio(Ru).The evaluation index includes synthesis gas yield(Gv),hydrogen to carbon ratio(H/C)of syngas,hydrogen efficiency(ηH),methanol efficiency(ηM),net electricity efficiency(ηnet),total energy efficiency(ηT),exergy efficiency(ηEx),CO2 emission rate(CER),and carbon utilization rate(CUR).The system parameters are optimized with the highest energy efficiency.The results showed that the mass ratio of steam and oxygen carrier to biomass has a greater impact on system performance.The optimal gasification pressure(PCLG)exists to optimize system performance and the effect of oxygen carrier regeneration temperature is small within a certain range.The best values of S/B,MFe/B,MCa/B,PCLG and TAR are 0.4,0.5,1.0,8 bar and 950℃,respectively.The optimal reaction temperature for methanol synthesis is 230℃,and the variation of reaction pressure at about 50 bar has the greatest effect on the system performance,and the unreacted gas circulation ratio has a smaller effect on the system performance after greater than 2.5.Under the optimal operating conditions,the H/C of the syngas is about 2.1,which is suitable for methanol synthesis.The methanol efficiency and total energy efficiency of the system are 51.89%and 64.15%,respectively.Exergy analysis is used to identify weak links in system energy utilization,and the results showed that the total system exergy destruction is 30.30 MW,which accounts for 32.89%of the input exergy.Biomass chemical looping gasification unit has the largest exergy destruction,of which the exergy destruction of fuel reactor is 11.43 MW,accounting for 37.72%of the total exergy destruction.Different CO2 emission allocation methods are used to evaluate the CO2 emission of polygeneration system products,and the results caused by different methods are quite different.Finally,economic analysis suggested that the methanol production cost of polygeneration process is 346 $/t,lower than that of traditional biomass gasification and close to that of natural gas.Besides,capital investment and biomass price have great influence on the methanol production cost. |
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