Release And Transformation Of Alkali Metal Potassium During Torrefied Biomass Combustion

Energy has always been an important material basis for the development of human civilization.At present,the main body of energy consumption in China is still the fossil fuel led by coal.In order to achieve the magnificent target of"carbon peak and carbon neutrality",the development of renewable energy is imperative,biomass has a broad application prospect as a green carbon resource.Torrefaction,as a means of biomass pretreatment,can reduce water content and inorganic salts of biomass,improve calorific value and energy quality,and improve the economy of transportation and storage,which is conducive to the large-scale thermochemical application of biomass.However,torrefaction will enrich the content of alkali metals in biomass,and the alkali metals will be released into the gas phase,which will lead to soot accumulation,blockage and corrosion of equipment.In order to solve the above problems,in this paper,biomass was torrefied and pretreated at different temperatures,and the comprehensive combustion characteristics of torrefied biomass,chemical analysis of biomass potassium migration/transformation during torrefying and combustion,and real-time release characteristics and kinetics of potassium from torrefied biomass were studied.Firstly,corn straw and rice husk were torrefied in a tubular furnace at different temperatures to study the fuel and combustion characteristics of torrefied biomass.The results showed that with the increase of torrefying temperature,biomass quality and energy yield decreased obviously,high calorific value increased gradually,and gasification capacity improved.Low torrefying temperature can promote the combustion of biomass and improve the combustion performance,but too high torrefying temperature will make the combustion performance worse.In stages of devolatilization,biomass activation energy increased first and then decreased with torrefying temperature.In the char combustion,the activation energy increases with the increase of torrefying temperature.In the aspect of off-line chemical analysis,chemical extraction combined with ICP-AES and XRD were used to study the release of potassium and the change of its occurrence form during biomass torrefaction and combustion at different temperatures.The results showed that the alkali potassium was not released during the torrefaction,and the proportion of water-soluble potassium decreased and the proportion of ammonium acetate-soluble potassium increased with the higher torrefying temperature.When the combustion temperature is 650℃,the release of potassium mainly comes from the release of part of organic potassium volatilization.In the range of 700～800℃,the main release mechanism of K is the vaporization of KCl.When the combustion temperature continues to rise,the increase of potassium release is related to the decomposition and release of K₂CO₃ and K₂SO₄.In the ash of torrefied biomass samples,the proportion of insoluble K was the highest,followed by water-soluble K,and the proportion of ammonium acetate-soluble K and hydrochloric acid-soluble K was relatively small.In the aspect of alkali metal online measurement,multi-point LIBS measurement system and two-color experimental instrument were mainly used to quantitatively study the real-time release of alkali metal potassium during the combustion of torrefied biomass,and the kinetics simulation of alkali metal release was used to analyze its relationship with combustion kinetics.The results showed that the peak values of water-soluble K and total K released in devolatilization with high torrefied temperature were lower.In char combustion,with the prolongation of combustion time,the unreleased water-soluble potassium in devolatilization is also released in large quantities in this stage.The peak value of total potassium release in devolatilization of corn straw torrefied at low temperature was larger.The release of alkali potassium in rice husk is mainly in stages of devolatilization,and the release resistance of the original rice husk is larger,while the release amount of potassium in torrefied rice husk is much higher than that of the original rice husk because the structure of torrefied rice husk is more loose.The release of water-soluble potassium plays a leading role in the total potassium release rate during corn stalk combustion.High temperature torrefaction promoted the release ratio of potassium in char combustion and low temperature torrefaction promoted the release ratio of potassium in devolatilization.In the devolatilization,the vapor potassium is greatly affected by volatilization and gas flow,and the kinetic parameters of combustion are strongly correlated with the kinetic parameters of alkali potassium release.In char combustion,the release of potassium is mainly KOH evaporation,and the correlation between the two kinetic parameters is weak.