Evaporation Of Potassium And Entrained Effect Of Volatiles On Potassium During Agro-stalks Combustion

Biomass is an available renewable energy resource with the advantage of carbon neutrality.With the increasing fossil energy resource shortages and environmental pollution,biomass shows good utilization prospects.Biomass combustion is the main method for high-efficiency and large-scale utilization of biomass.However,the release of potassium(K)compounds during the combustion of biomass will cause fouling,corrosion,slagging,and particulate emissions.These issues have seriously inhibited the utilization of biomass.This paper will focus on the investigation of the characteristics of K release during the combustion of biomass(mainly including the evaporation of K and the entrained of volatiles on the release of K),and provides theoretical support for solving the above problems caused by the release of K.Firstly,the effect of entrained flow of volatiles on the release of K during the combustion of biomass was investigated.The pure corn stover and the mixture of corn stover and potassium chloride(KCl)were burned in a tube furnace(TF)between 500 and650 °C.The ashes residues were analyzed by X-Ray Fluorescence analyzer(XRF).Entrained ratio of KCl was calculated according to mass conservation of ashes and that of elements of ashes.Results showed that 2% to 5% amount of KCl in the solid phase was entrained into the gas phase by volatiles.The entrained ratio varied with temperatures,and the maximum value occurred at 550 °C.Secondly,in order to describe the release rate of KCl from biomass ash at ash-heating stage during biomass combustion,the ashes of pure corn stover and wheat straw and their mixtures with KCl were heated in a simultaneous thermal analyzer(STA)and tube furnace(TF)at different temperatures.The evaporation rate of KCl was calculated via analyzing the curves of thermogravimetric(TG)and differential thermogravimetric(DTG).The main conclusions are following:(1)The release rate of KCl at 925 °C is about 6 times higher than that at 800 °C.(2)The difference of KCl release rate between corn stalk ash and wheat stalk ash is negligible.(3)Lower combustion temperature can significantly inhibit the evaporation of KCl in biomass combustion.On the basis of the above data,a distributed-molten model was established to describe the evaporation of KCl from biomass ash at ash-heating stage.In this model,the molten liquid of KCl is evenly distributed in ash,and the evaporation is the main method for release of KCl.The evaporation rate of KCl was calculated by a code.The model was validated using various experiments with comparison of earlier model(collected-molten model).The following conclusions are drawn:(1)Good agreement is obtained between the predictions from the model and the results from various experiments(relative error <15%).(2)The distributed-molten model is applicable in broader scope than collected-molten model especially for evaporation of KCl at low concentration,high temperature and large volume of ash.(3)The physical mechanism of the distributed-molten model is clear,and the model can be applied to the analysis and calculation of the release of other inorganic components during biomass combustion.Lastly,in order to compare models for K release during biomass combustion,codes of several typical evaporation and kinetic models were written to calculate the release rate of K.Calculation results were analyzed in comparison with the thermogravimetric analysis of ashes from agro-stalks.And the following conclusions are drawn:(1)Predictions from evaporation model and Zhang / Cao’s kinetic models agree better with K release in respect of temperature than the others.(2)The maximum release rate of K in Peters’ model occurs at much lower temperature,while that in Fatehi’s model occurs at much higher temperature than experimental results.(3)The kinetic models and experiments indicate that the effects of K content in straw and heating rate are greater than that of final temperature on the release rate of K at 900-950 °C.