Integrated Performance Evaluation Of Hydrogen Production From Biomass Staged-gasification Based On Exergy Theory

With the rapid development of economy,the contradiction between the growth of energy demand and the deplection of fossil energy is becoming increasingly serious.It is highly significant to solve the energy and environment problems by producing hydrogen as an alternative to fossil fuels from renewable biomass.According to the different utilization of pyrolysis gas and char,three systems of biomass staged-gasification for hydrogen are carried out,namely,staged-gasification with char gasification and pyrolysis gas combustion（Sys 1）,staged-gasification with char gasification and pyrolysis gas reforming（Sys 2）,and staged-gasification with char combustion and pyrolysis gas reforming（Sys 3）.Three systems,using wheat straw and corn straw as the feedsocks,are modelled and simulated based on Aspen Plus.To compare and improve the energy,resource and environment performances of the systems,a thermodynamic exergy model and an environmental exergy model are established based on the exergy theory and life cycle assessment method.In addition,the comprehensive performance comparisons of hydrogen production process between biomass staged-gasification and natural gas refoming（NGS）are carried out.The main results are shown as follows.Firstly,exergy analyses of the hydrogen production systems indicate that:（1）The largest exergy loss occurs in pyrolysis and combustion units,and the exergy loss of each unit and system is mainly caused by internal irreversibility.（2）For no matter combustion unit or gasification and reforming processes,the irreversiblility of pyrolysis gas as reactant is less than that of char.As for the system,it is conducive to generate hydrogen by making full use of the hydrogen element（H）in biomass instead of H in water.（3）The addition of CaO bed material in the reforming process of Sys 2（Sys 2-Ca）can improve the system performance,and the hydrogen yield（YH₂）and exergy efficiency（ψ）are increased by 0.5 mol/kg and 1.3percentage points,respectively.The optimum gasification temperature（T_G）and mass ratio of steam to biomass（S/B）are 700℃and 0.4 for both Sys 2-Ca and Sys 3 using wheat straw as the feedstock,and the maximum values of YH₂ andψfor both systems can nearly reach 36 mol/kg and 59%,respectively.Sys 1achieves the optimum under the conditions of T_G 700℃and S/B 0.5,the corresponding YH₂ andψare respectively 34.9 mol/kg and 57%,which are lower than those of Sys 2-Ca and Sys 3.Sys 1 and Sys 2 are advantageous when the actual convertion efficiency of tar in Sys 3 is less than 90%,as the system layouts of Sys 1 and Sys 2 are conductive to tar conversion.Sys 2-Ca for hydrogen using corn straw reaches its optimum at the T_G of 700℃and S/B of 0.8,however,the thermodynamic performances of Sys 2-Ca using corn straw（YH₂=34.9 mol/kg,ψ=55.1%）are worse than those using wheat straw.（4）The thermodynamic performances of NGS（YH₂=152.9 mol/kg,ψ=75.4%）are significantly better than that of biomass systems.However,being considered the effect of improve energy density of raw material,the energy enhancement rateηof NGS（40.2%）is lower than that of biomass systems（45.0%-49.7%）.Secondly,based on the exergy life cycle assessment,the results are shown as follows:（1）The life cycle environmental impacts of Sys 2-Ca and Sys 3 using wheat straw are almost at the same level.The resource utilization efficiency（γ）,renewability performance（I_r）and environmental performance（I_e）are approximately 45%,1.8 and 10.5,respectively,which indicate that both Sys 2-Ca and Sys 3 are renewable and environmentally friendly systems.Sys 3 has slight adavantages compared with Sys 2-Ca when the actural conversion efficiency of tar is not taken into account.By comparision,theγof Sys 2-Ca using corn straw（41.9%）is lower than that using wheat straw,but the I_r（2.58）and I_e（16.9）of corn straw system are better.（2）Theγof NGS（69.0%）is greater than that of biomass systems,however,the environmental performences of NGS are obviously inferior to the biomass systems,as it is non-renewable（I_r=0.69）,and has negative impact on environment（I_e=-4.2）.Thus,NGS is a not sustainable process.（3）The analysis of influence factors on the environmental performance for Sys 2-Ca using wheat straw as the feedstock indicates that,the largest contribution to total cumulative exergy consumption is the usage of renewable biomass（75.2%）,while the non-renewable resources are relatively small.The major environmental type is global warming（GWP）,which takes up to 35%of total environment impact,followed by eutrophication（EP）,photochemical ozone formation（POF）,and thermal pollution（TP）,which are more than 15%.The proportions of human toxicity（HTP）,solid waste（SW）and acidification（AP）are relatively small.In addition,the main loads of environmental exergy are generated during biomass cultivation and production stages.Finally,the sensitivity analysis and data quality assessment indicate that:（1）The cumulative exergy consumption and total environmental impact of Sys 2-Ca using wheat straw are highly sensitive to tar conversion efficiency,the sensitivities of which respectively reach 47.6%and 38.3%,and they are 22.9%and 24.4%to combustion efficiency,respectively.In addition,the calculation method of environmental impact distribution coefficient of straw and the consumption of fuel and electricity during biomass cultivation process have an influence on the indicator results of environment impact,while the influence of CaO cycle characteristics on the indicator results is small.（2）The relative uncertainties of the total environmental impact and cumulative exergy consumption of different systems are below 6%and 4.5%,respectively.Meanwhile,the relative uncertainties of the comprehensive indicators of different systems are within 8.1%,indicating that the evaluation results are reliable.