| Hydrogen,as a kind of secondary energy which is beneficial to the sustainable development of human beings,has the advantages of abundant sources,friendly environment,high energy density and good storage,so its demand is increasing.China has the energy structure characteristic of "more coal,less gas and less oil",so the technology of producing hydrogen from coal will become an important research direction of hydrogen energy development in China.However,at present,the traditional process of hydrogen production from fossil fuel has the problems of high-power consumption and low recovery rate of H2 separation,and the emission of CO2 also hinders its further development.Three reactors chemical looping hydrogen generation(TRCL)is a promising hydrogen production process because it does not need H2 purification and can capture CO2 directly.TRCL is composed of fuel reactor(FR),steam reactor(SR)and air reactor(AR),which are respectively carried out fuel oxidation reaction,hydrogen production reaction and exothermic reaction of oxygen carrier(OC)regeneration through the circulation of OC between the reactors.At present,TRCL’s research on system integration mainly focuses on the influence of the selection of integration process and the optimization of operational variables on the overall energy efficiency and exergy efficiency of the system.However,in the energy conversion process,there is insufficient research on the energy grade promotion mechanism of heat input,the matching relationship between quantity and quality of energy,the flow direction of exergy and the specific exergy destruction process of each equipment are not clear.Through the concept of energy grade,this paper studied the thermodynamic characteristics of energy conversion in methane-TRCL,syngas-TRCL and water-gas shift(WGS)systems,and established the mathematical relations between the energy grade improvement ratio of input heat,the utilization potential or driving force of chemical reaction,the possible chemical conversion rate of input heat,exergy efficiency and the product ratio after input heat transformation.At the same time,the effects of input heat energy grade,quantity of heat,hydrogen content in syngas and steam content on the thermodynamic performance of the system are studied.The results show that methane-TRCL is suitable for a large amount of low-temperature heat,syngas-TRCL is suitable for the case where the heat input value is greater than 310.52 kJ/mol,and WGS has poor thermodynamic performance and applicability under variable conditions.The integrated system of coal gasification and TRCL was established by using Aspen plus software,and the thermodynamic standard data of related substances were calculated,and the thermodynamic evaluation index was determined.The simulation unit of the integrated system consists of four parts:coal gasification,TRCL,heat exchange and turbine,and product compression.With cold gas efficiency,carbon conversion rate,hydrogen production efficiency,net electrical efficiency,equivalent efficiency,equipment exergic efficiency,system exergy efficiency,objective exergy efficiency,energy quality factor and energy quality factor ratio as evaluation index,determine the best operating condition of the oxygen coal mass ratio,steam coal quality ratio,Fe2O3 feed temperature,OC circulation,steam input in SR and AR reaction temperature.The final system hydrogen production was 372.15 kg/h,hydrogen production efficiency was 42.55%,exergy efficiency was 52.98%,and energy quality factor ratio was 0.944.Finally,in the best state,exergy balance and equipment efficiency of 20 devices or processes in the system were calculated.In addition,the changes of exergic flow direction,specific causes of exergy destruction and energy quality factor in chemical processes and physical processes were analyzed in detail,including coal gasification reaction,fuel reaction in FR,steam reaction in SR,air reaction in AR,chilling,purification,cyclic reflution,heat transfer,gas turbine,steam turbine and compression. |
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