Integrated Optimization Of Exergy,Advanced Exergy And Life Cycle Environmental Impact

Coal to synthetic natural gas(coal-to-SNG)as one of the vital technical ways for clean coal technologies,can mitigate the problem of natural gas shortage in China,ensure energy security,stabilize fuel prices and diversify energy options.Although synthetic natural gas as clean fuel,its utilization will bring about significant environmental benefit,the process of producing synthetic natural gas inevitably causes higher resource and energy consumption and pollutant emissions.The exergy analysis can evaluate the resource and energy utilization efficiencies,explore the losses of energy quality in energy conversion systems,and identify inefficient sub-processes.Besides,the combination of exergy and environmental impact has advantages in improving and solving environmental problems,which can provide the researched system with more insights into energy conservation and emission reduction.This study takes a typical coal-to-SNG process as a case study.Based on exergy balance,the exergy efficiency,exergy destruction and exergy loss of each sub-process are qualified,and inefficient sub-processes and corresponding improvement potential are determined.By further decomposing exergy destruction into endogenous and exogenous exergy destruction or avoidable and unavoidable exergy destruction,the interactions between system components and the actual improvement potential are explored.On this basis,by establishing the relationship between exergy destruction and environmental impact,the environmental benefits of the entire coal-to-SNG life cycle brought by improving SNG production process is predicted and the feasible emission reduction strategies are further designed.This study results show that the exergy efficiency of this coal-to-SNG process is58.35%.It consumes coal resource of 1511.69t/h(3.02kg/8)~3SNG),water resource of2230t/h(4.46kg/8)~3 SNG),results in exergy destruction and loss of 3974.12MW(28.61MJ/8)~3 SNG).uue to 86.27%of exergy destruction is endogenous exergy destruction,the interactions between system components are weak and the improvements should mainly focus on internal irreversibility of components.The avoidable exergy destruction accounts for 61.88%of total exergy destruction,indicating that the researched system still has great improvement potential and improvement space.In traditional exergy analysis,coal gasification unit,methanation unit and rectisol unit should be improved first.However,advanced exergy analysis shows that improvement priorities should be given to coal gasification unit,methanation unit and water gas shift unit.Through the"cradle to gate"life cycle environmental impact assessment,the environmental impact caused by the coal transportation stage is minimal due to the shorter transport distance.Although coal mining stage contributes outstandingly to ozone layer depletion,its contribution to other environmental impacts is 5%-10%.Therefore,the performance of synthetic natural gas production process should be improved intensively.Combined with the development status and trends of the coal-to-SNG industry,this industry still has a substantial improvement potential to reduce resource and energy consumption,pollutant emissions and environmental impacts.Therefore,a scenario analysis of coal-to-SNG technology improvement is considered by combining advanced exergy analysis with environmental impact assessment.With the recent-,medium-and long-term technological development of SNG production,the acidification potential,ozone layer depletion potential and global warming potential of the entire coal-to-SNG life cycle will be decreased continually,rather than the improvement potential of abiotic resource depletion potential,eutrophication potential and human toxicity potential will flatten out.It is consequent to realize the technical optimization of each unit and the entire system,but the potentials for energy conservation,emission reduction and environmental impact reduction brought by this improvement(about 15-25%)are relatively limited.Therefore,it is further to adopt goal-oriented measures to achieve higher energy conservation and emission reduction potential.