Study On Exergy Characteristics Of Microwave Assisated Biomass Gasification Process

Energy production is very unbalanced in China,and fossil energy is still the main part of the total energy production.However,the problem of energy shortage and environmental pollution caused by the use of fossil energy are becoming more and more serious.As a new type of renewable energy,biomass energy has huge reserves and broad prospects for utilization.As a thermochemical conversion technology of biomass,microwave-assisted gasification technology of biomass can efficiently convert biomass energy which is difficult to be directly used into natural gas,hydrogen and other gas fuels with high quality.As an energy evaluation index,exergy can comprehensively consider the amount and quality of energy,so it is important to study the exergy characteristics of microwave-assisted biomass gasification process for improving the utilization rate of biomass energy.In this paper,a numerical model of microwave-assisted biomass gasification is established to simulate the process of microwave-assisted biomass gasification,and the exergy characteristics and exergy losses of microwave-assisted biomass gasification process are analyzed.Finally,the effects of different inlet velocity,equivalence ratio and initial bed temperature on the gasification characteristics,exergy characteristics and exergy losses are explored.Local chemical exergy and local physical exergy are mainly distributed in the dense region with large solid volume fraction,and their distribution is consistent with that of solid volume fraction.The local kinetic exergy is mainly distributed in the region with larger velocity,but its value is smaller than that of local chemical exergy and local physical exergy,which can be ignored.The local exergy loss of heat transfer is mainly distributed in the entrance area with higher temperature gradient.The local exergy loss of chemical reaction is mainly distributed in the area with larger solid volume fraction and close to the entrance.The local exergy loss of mass transfer is mainly distributed in the entrance area and the area around the bubble with larger component concentration gradient.The local exergy loss of flow is mainly distributed in the area with larger velocity gradient.The local exergy loss ratio of chemical reaction and local exergy loss ratio of heat transfer in the entrance area are not much different,which are between 40%-60%.The local exergy loss ratio of chemical reaction in the solid area is larger and the local exergy loss ratio of heat transfer in the gas area is larger.Properly increasing the inlet velocity can effectively increase the volume fraction of combustible gas in gas production,heating value,gas yield and gasification efficiency.At the same time,it can increase the gasification temperature and the reaction rate of pyrolysis reaction and carbon combustion reaction,so as to reduce the local chemical exergy,improve the local physical exergy of the gas phase,and then increase the physical exergy and the chemical exergy of the gas production.However,increasing the inlet velocity will increase the local exergy loss of chemical reaction and heat transfer,and then increase the overall exergy loss.Increasing the equivalence ratio will reduce the volume fraction of the combustible gas in the gas production and heating value,but it will increase the gas yield and the gasification efficiency.When the equivalence ratio is 0.1-0.3,the local chemical exergy and the local chemical exergy of gas phase will decrease.When the equivalence ratio is less than 0.1,the local exergy loss of chemical reaction is large,and the increase of equivalence ratio has little effect.When the equivalence ratio is 0.1-0.3,increasing the equivalence ratio reduces the local exergy loss of chemical reaction.When the equivalence ratio is greater than 0.3,increasing the equivalence ratio will slightly increase the local exergy loss of chemical reaction.When the equivalence ratio is less than 0.3,increasing the equivalence ratio will reduce the total exergy loss.When the equivalence ratio is greater than 0.3,increasing the equivalence ratio has no significant effect on the total exergy loss.Increasing the initial bed temperature can effectively increase the gasification temperature,and then increase the gasification reaction rate,finally increase the volume fraction of biomass combustible gas,improve the heating value,gas yield and gasification efficiency of the gas production.At the same time,it will reduce the local chemical exergy,thus greatly improve outlet exergy of the gas production.Increasing the initial bed temperature from 500 oC to 700 oC,the export physical exergy and chemical exergy increased by 103.45% and 60.83 %,respectively.Increasing the initial bed temperature will also increase the local exergy loss of heat transfer and chemical reaction,which will increase the overall exergy loss.