Effect Mechanism Of Torrefaction Pretreatment On Gasification Products Of Maize Straw

Straw biomass is a clean and renewable energy source due to its abundance and wide distribution.Biomass gasification technology can convert the biomass into producer gas which can be used as clean fuels in boiler or engine for the heat and electricity supply.It is of great important to achieve the goal of"carbon neutrality"and"emission peak"in China.However,straw biomass has the major drawback of high content of water,high content of oxygen element,and low heating value,resulting in lower gasification efficiency and higher tar content,which makes the use of agricultural and forestry waste biomass"waste into treasure"face great challenges.Therefore,this thesis chooses the combination of torrefaction pretreatment and gasification technology to improve the quality of biomass raw materials and improve the gasification performance.（1）In this work,maize straw（MS）was gasified in a small-scale fixed-bed gasification reactor.It was investigated how gasification temperature and equivalent ratio affected the properties of producer gas,char,and tar.The results showed that with increasing gasification temperature and equivalent ratio,the yield of producer gas gradually increased,while the yield of biochar gradually decreased.In addition,the lower heating value（LHV）of the producer gas was increased by higher gasification temperature and lower equivalent ratio.The maximum gas content and LHV of the producer gas were obtained at a gasification temperature of 900oC and an equivalent ratio of 0.05.Furthermore,the contents of C,H,and volatiles decreased significantly with increasing gasification temperature and equivalent ratio,while the ash content increased remarkably.At last,the contents of phenols,alcohols,acids,and aldehydes in the liquid product decreased with increasing gasification temperature,while that of aromatics and esters increased significantly.As the increase of the equivalent ratio,the content of aromatics and phenols in the gasified liquid product decreased,while the content of esters increased.（2）Firstly,the maize straw was pre-treated by dry torrefaction in a programmed temperature controlled tube furnace,and the influence of temperature on its basic properties was studied.Then,a home-made small fixed-bed gasifier was used to study the effects of dry torrefaction temperature on the yield and properties of maize straw gasification producer gas,gasification char,and tar.The results showed that the content of oxygen element and volatiles gradually decreased with the increase in torrefaction temperature,while the content of carbon element and fixed carbon increased,resulting in an increase in calorific value from 17.26 MJ/kg to 25.50 MJ/kg.The mass yield of producer gas decreased with increasing torrefaction temperature,while the H₂ content increased significantly.The highest lower heating value（LHV）of the producer gas was 9.36 MJ/Nm³,which was obtained at a torrefaction temperature of 220oC.A higher torrefaction temperature resulted in a higher mass yield of biochar.In addition,the content of carbon elements and fixed carbon increased with increasing torrefaction temperature,while the volatile fraction and ash decreased,and the calorific value increased from 22.74 MJ/kg to 24.52 MJ/kg.A Higher torrefaction temperature resulted in a lower mass yield of tar.The tar was mainly composed of aromatics,phenols and aliphatic hydrocarbons,among which the relative content of aromatics and phenols is much higher than that of aliphatic hydrocarbons.With the increase of the torrefaction temperature,the content of aromatics first increased and then decreased,while the content of phenols first decreased and then increased,and the content of other aliphatic hydrocarbons did not obviously change.（3）Firstly,maize straw wet torrefaction was carried out in a miniature mechanically stirred high-pressure reactor,and the influence of temperature on its basic properties was studied.Then,a home-made small fixed-bed gasifier was used to study the effects of wet torrefaction pretreatment temperature on the yield and properties of maize straw gasification producer gas,gasification char,and tar.The results showed that as the temperature of wet torrefaction increased,the content of O element and volatile matter decreased significantly,the content of ash increased gradually but was lower than that of raw material,and the content of C element and fixed carbon increased significantly,which increased the calorific value from 17.26 MJ/kg to 27.24 MJ/kg.As the temperature of wet torrefaction increased,the mass yield of the producer gas gradually decreased,and the contents of CO,H₂,CH₄ and Cn Hm increased from 25.55%,13.85%,9.97%and 0.53%to 33.68%,14.65%,10.36%and 1.27%,respectively.The lower calorific value of the producer gas reached the highest value of 11.42 MJ/Nm³ at 180oC.The mass yield of char increased with the increase of wet torrefaction temperature,the volatile matter and ash gradually decreased,the content of C element and fixed carbon gradually increased,and the calorific value of char increased from22.74 MJ/kg to 28.09 MJ/kg.Higher torrefaction temperature significantly reduced the tar content.After wet torrefaction pretreatment,the aromatics content increased,while the phenols content decreased,and the aliphatic hydrocarbon content changed slightly,with aromatics dominating.