Research On The Reactivity Of Co-gasification Of Agricultural And Forestry Biomass Semi-coke And Coal

For the foreseeable future,coal will remain the predominant fuel in China.Coal gasification is one of the technologies for clean and efficient utilization of coal.However,the massive mining and consumption of coal decreases the quality and significantly reduced the gasification reactivity of coal.China also has a great mount of agricultural and forestry biomass,which has a high gasification reactivity as compared with coal.The biomass semi-char produced under the low-temperature pyrolysis condition overcomes many deficiencies of directly utilization of biomass.Co-gasification of coal and biomass semi-char takes full advantages of the high energy density of coal and high reactivity of biomass.Investigation on the characteristics and mechanism of co-gasification of coal and biomass semi-char is significance for developing of co-gasification technology,resolving the problem of the utilization of biomass and optimizing of the energy structure.In this work,two typical of Chinese high-silicon-aluminum Huainan coal and low-silicon-aluminum Ordos coal and three kinds of biomass?sawdust,rice husk,and wheat straw?with large differences in ash composition are used to study the co-gasification reactivity and reaction mechanism of coal char and biomass semi-char under CO₂ and H₂O atmospheres.The results obtained are summarized as follow:?1?The pyrolysis characteristics of sawdust,rice husk and wheat straw were investigated at temperature of 300?-600?in a fixed bed reactor.With the increase of pyrolysis temperature,the hydrogen and oxygen content in biomass was decreased,while the carbon content was increased.The H/C and O/C of the biomass feedstock and biomass semi-char at different pyrolysis temperatures were analyzed by means of the Van Krevelen diagram.It was found that the biomass semi-chars pyrolysised at temperature of 400?were similar to that of Huainan Coal and Ordos Coal.And 400?was selected as the pyrolysis temperature of biomass.?2?The co-gasification characteristics of Huainan coal char and Ordos coal char with three kinds of biomass semi-char were investigated in the PE-STA8000 TGA with the different heating rates in CO₂ atmosphere.Results indicated that high ash and silicon-aluminum content was the main reasons for the low gasification reactivity of Huainan coal char and rice husk semi-char,while sawdust semi-char,wheat straw semi-char and Ordos coal char have high reactivity due to their rich in alkali and alkaline earth metals.The order of the gasification reactivity of the investigated char sample was sawdust semi-char>wheat straw semi-char>Ordos coal char>rice husk semi-char>Huainan coal char.The co-gasification of Huainan coal char with sawdust semi-char and wheat straw semi-char showed an obviously promotion effect.Co-gasification of Huainan coal char and sawdust semi-char increased the reaction rate in the later stage of reaction,and the average activation energy of co-gasification was reduced about 50k J/mol compared with Huainan coal char gasification.The overall reactivity is improved for the co-gasification of Huainan coal char with wheat straw semi-char and the average activation energy of co-gasification was about 135k J/mol lower than that of Huainan coal char gasification.Adding wheat straw semi-char was more evident to improve the gasification reactivity of Huainan coal char.The average activation energy of co-gasification of Huainan coal char and rice husk char was similar to that of Huainan coal char.The addition of rice husk char could not improve the gasification reactivity of Huainan coal char.The improved the gasification reactivity was also observed for the co-gasification of Ordos coal char with sawdust semi-char and wheat straw semi-char,and the average activation energy of co-gasification was reduced by about 10k J/mol and 8k J/mol compared with that of Ordos coal char,respectively.The average activation energy of co-gasification of Ordos coal char and rice husk char was 25k J/mol above of Ordos coal char,the addition of rice husk char was also unfavourable to the gasification of Ordos coal char.?3?The co-gasification characteristics of Huainan coal char with sawdust semi-char and straw semi-char were investigated using the designed TGA in H₂O atmosphere at temperature of 900?-1200?.Results indicated that the promotion of co-gasification of coal and biomass char was observed when the carbon conversion achieved at 0.5 due to the developed pore structure and high content of K and Ca of biomass semi-char.However,with the increase of gasification temperature,this promoted effect was weakened,and even disappeared at the co-gasification temperature of 1100?.The chemical reaction controlled regime of Huainan coal char and Huainan coal char/straw semi-char was 900??1100?,while for the wheat straw semi-char,sawdust semi-char and Huainan coal char/sawdust semi-char was 900??1000?.For the chemical reaction controlled regime,the order of the activation energy of the used char samples were Huainan coal char>Huainan coal char/sawdust semi-char>Huainan coal char/wheat straw semi-char>wheat straw semi-char>sawdust semi-char.Compared with the Volumetric model and the Random pore reaction model,the Shrinking core model could more accurately describe the evolution of the pore structure during the co-gasification reaction of Huainan coal char with sawdust semi-char and wheat straw semi-char.