Thermodynamic Analysis Of Biomass-based Series Polygeneration System Of Liquids-power

"Green"and"development"are the eternal themes of the progress of the Times.Accelerating energy transformation and developing clean energy utilization technologies are important breakthroughs for China to achieve the goal of"double carbon".As the world’s fourth largest energy source,biomass energy is not only a renewable resource,but also the only renewable carbon source that can be converted into liquid fuel by technical means.Using biomass energy to partially replace fossil energy can accelerate China’s energy transformation.Polygeneration technology is a kind of energy comprehensive cascade utilization technology with obvious advantages,which can organically coupling chemical production and power system,and realize high efficiency and clean carbon source utilization technology,and has extensive development potential.This study focuses on the analysis of thermodynamic performance of the polygeneration system.From the perspective of coupling and optimization of the polygeneration system,using Aspen Plus process simulation software equipped with Office Excel as the main technology,the energy utilization and environmental protection benefits of biomass based liquid fuel-power series polygeneration system are calculated and analyzed.By means of case study,the material conversion and energy transfer and utilization of cobalt based catalyst&partial cycle and iron based catalyst&partial cycle are analyzed in detail.The main results obtained are as follows:1.Subsystem simulation.Based on the theory of comprehensive cascade utilization of energy,this study built a biomass based liquid fuel-power series polygeneration system,which is composed of chemical production system and power system in series.Chemical production system:biomass gasification unit,conversion unit,purification unit,Fisher-Tropsch（FT）synthesis unit and oil refining unit.Power production system:gas turbine,steam turbine and waste heat boiler and other process units.The above subunits are simulated and calculated by Aspen Plus software.2.Polygeneration modeling calculation and optimization.Two systems integration schemes are designed in this study.One is the cobalt based&partial cycle scheme（Case1）in which the ratio of hydrogen to carbon is adjusted by the transformation unit to meet the needs of Fischer-Tropsch synthesis;the other is the iron based&partial cycle scheme（Case2）in which the crude syngas enters the purification unit directly to remove the acid gas without the transformation unit.In this study,the influence of catalyst type and cycle ratio on the system was analyzed,and thermodynamic performance of the system such as thermal efficiency and exergic efficiency was analyzed.Using carbon emission rate and carbon capture rate,the environmental benefits of the system are analyzed.The results show that:From the point of energy using:（1）Biomass-based series polygeneration system can realize comprehensive cascade utilization of energy;（2）Case1could achieve higher thermal efficiency and exergic efficiency（50.66%and 46.99%,respectively）,which were 6.64%and 6.62%higher than the maximum exergic efficiency（44.02%and 40.37%）of Case2.（3）In the two schemes,oil production,thermal efficiency and exergic efficiency all reached the maximum value when the cycle ratio was the maximum,that is,oil production,thermal efficiency and exergic efficiency were positively correlated with the cycle ratio r,and the increase of the cycle ratio was conducive to improving the thermal efficiency of the system.There is a negative correlation between energy generation and cycle ratio r.（4）The utilization rates of C and H of the two schemes are similar,but Case1 can achieve higher utilization rates of C and H,28.47%and 63.89%respectively,which are 2.12%and 6.46%higher than Case2 maximum cycle.The energy distribution of each subsystem of polygeneration was investigated and the main causes of energy loss were analyzed.The results show that the energy loss of the series polygeneration system mainly comes from the gasification unit and the combined cycle unit,and the energy loss rate of these units is more than 40%.The main reasons are as follows:（1）Gasification stage.A.A series of chemical reactions occur in the biomass particles in the gasifier.These chemical reactions have irreversible mass and heat transfer properties,resulting in energy loss;B.Heat loss in the process of chilling is due to the irreversibility of a series of physical processes as well as the irreversibility of mass and heat transfer.C.Low temperature solid ash and unburned carbon from the gasification process are not utilized,resulting in heat loss.（2）In IGCC stage,due to the large energy difference between chemical energy and electric energy,the conversion of chemical energy to electric energy causes a large energy loss.The energy loss of the water-gas conversion unit and the purification unit is less,because the conversion between chemical exergy only occurs in these units and the by-product steam reduces the energy loss.In the aspect of environmental protection benefit,the carbon capture rate and carbon emission rate of the system are analyzed.The results show that:（1）The carbon captured by Case2 mainly comes from the purification unit and the FT synthesis unit.The iron-based catalyst is CO₂ selective,and the carbon capture rate of Case2 will change with the change of cycle ratio,showing a linear increase trend.The carbon capture of Case1 mainly occurs in the purification unit,and the change of the cycle ratio will not affect the unit.Therefore,the carbon capture rate of the scheme has no direct relationship with the cycle ratio and will not change with it.（2）The carbon emission rate of the two schemes decreases with the increase of the cycle ratio.This is because the carbon emission of the system mainly occurs in the combined cycle power generation unit.The PSA tail gas and the dry gas of the oil processing and refining unit produces CO₂ through combustion reaction,so the carbon emission rate is directly related to the composition of the gas.With the increase of cycle ratio r,the total conversion rate of CO in FT synthesis section increased,leading to the decrease of C content in fuel gas entering the combined cycle power generation unit and the generation of less CO₂,resulting in a negative correlation between carbon emission rate and cycle ratio r.（3）The CO content of exhaust gas entering the gas turbine in Case2 is higher than that in the cobalt-based partial cycle scheme,so it will lead to a higher carbon emission rate,14.71%,5.86%higher than that in Case1.