Optical Diagnostics On Alkali Release Characteristics Of Coal/Biomass Pellets During Combustion Processes

Alkali metals will be released during the thermal conversion processes of high-alkali coal and biomass,causing fouling,slagging,and corrosion issues of facilities,and affecting the safety operation.Understanding the release patterns of alkali metals during the combustion of coal/biomass and developing the alkali release mechanisms are crucial steps in addressing alkalirelated issues and achieving large-scale utilization of high-alkali coal and biomass.However,current optical methods for the quantitative measurements of gas-phase alkali metals are not yet fully developed,and there is a lack of systematic research on the dynamic release characteristics of alkali metals during combustion processes.This work employed self-built single-pellet combustion systems and advanced optical diagnostic techniques to investigate the real-time release characteristics of elemental alkali metals and specific alkali metal species during combustion processes of individual coal and biomass pellets.The influence of reaction environments,fuel types,and other factors on the release characteristics of alkali metals was studied,and the mechanism of alkali release during combustion process was proposed,which provided fundamental data for the model development of large-scale coal/biomass combustion.Firstly,release characteristics of potassium(K)and zinc(Zn)during the combustion process of a zinc hyperaccumulator pellet,i.e.,Sedum alfredii,were studied based on a multi-point laserinduced breakdown spectroscopy(LIBS)method.The release behaviors of different chemical forms of K/Zn and the influence of additives were also investigated.The results indicated that time-resolved release curves of both K and Zn during combustion processes of Sedum alfredii pellets exhibited dual-peak profiles and could be divided into two stages: the devolatilization stage,and the char burnout and ash reaction stage.During the whole combustion process,61.5% portion of K and 24.2% of Zn were released into the gas phase,the major release of both K and Zn occurred during the char burnout and ash reaction stage,accounting for 77.6% and 69.8% of the total released K and Zn,respectively.Besides,K was mainly released in the form of water-soluble K,while Zn was mainly in the form of NH4Ac-soluble Zn.The inhibition effects of four types of additives on the release characteristics of K and Zn were different.All the four additives used in this work exhibited an inhibition efficiency higher than 55% on K release,while the inhibition effect on Zn release was relatively lower.Among them,Al2O3 showed the most significant inhibition efficiency on Zn release,reaching 37.0%.XRD results suggested that this may be due to the participation of Al2O3 and Zn through a transformation mechanism of Ca-Al-Zn,which converting volatile Zn to Ca3Al4 Zn O10 and retained in the ash.Next,in order to correlate the alkali release characteristics with the combustion characteristics of fuel pellets and investigate the influence of reaction environments on combustion and alkali release characteristics,pellet temperatures and elemental sodium(Na)release characteristics of burning Zhundong coal pellets were studied at the same time.These studies were carried out using a Hencken burner,which provided hot flue gases with varied ambient temperatures and oxygen contents.Based on the laser-induced phosphorescence(LIP)thermometry and LIBS method,surface temperatures and release profiles of elemental Na of burning Zhundong coal pellets under different flue gas conditions were quantitatively measured simultaneously.The results showed that the release curve of elemental Na and the profile of surface temperature exhibited similar tendency during Zhundong coal combustion processes.During the devolatilization stage,the release of Na showed a trend of first increasing and then decreasing.With increased ambient temperatures and oxygen contents,the peak of Na concentration increased,and the corresponding time occurred earlier.During the char combustion stage,with increasing oxygen contents,the peak of Na concentration increased significantly.This was mainly because the increase in oxygen concentration accelerated the diffusion rate of oxygen,thereby enhancing the reaction rate of char combustion,which then promoted the breakage of chemical bonds between alkali metals and char structure,and thus led to more Na release.In the ash reaction stage,Na were almost no longer released,but weak signals of Na release could still be observed under cases with high temperature and oxygen content.This phenomenon might be due to the increased possibility of the reaction between alkali metals with silicate and aluminate in the ash to form low-melting compounds with increasing temperature and oxygen content.The generated compounds may react with water vapor,leading to further release of Na.Subsequently,a multi-wavelength laser-induced photofragmentation method was developed,which can simultaneously measure alkali hydroxides,chlorides,and alkali atoms.By optimizing the system layout,quantitative measurements of alkali compounds(KOH and KCl)with a spatial resolution of 1 mm3 were achieved,and the calibration method of the multi-wavelength laserinduced photofragmentation system was investigated.Calibration curves of KOH/KCl were derived,which were then verified to be temperature-independent.Uncertainties of the calibration curves for KOH and KCl were approximately ±15%,and the uncertainty of atomic K measurement was about ±5%.The detection limit for KOH,KCl and atomic K is 120 ppb,140 ppb,and 6 ppb,respectively.Finally,based on the multi-wavelength laser-induced photofragmentation method,the dynamic release characteristics of multiple K species during combustion processes of pinewood and wheat straw were obtained,and the influence of reaction conditions on the K species during biomass combustion was studied.The results showed that during the combustion of the pinewood pellet,the release of K species was mainly in the form of KOH,while for wheat straw,it was mainly KCl.The increase of ambient temperature promoted the release of K species and shortened the reaction duration of each stage.The change in ambient oxygen content affected the form of K species release during the char burnout stage.When the ambient atmosphere changed from an oxidative atmosphere(combustion)to a reductive atmosphere(pyrolysis),the major K species released during the char burnout stage of pinewood changed from KOH to K atoms.With the capacity of spatial resolution of the multi-wavelength laser-induced photofragmentation method,release profiles of K species at different heights above the burning pinewood pellet were obtained.The results indicated that during the devolatilization stage,K may be released into the environment in the form of organic-K,followed by a gas-phase transformation of organic-K into KOH or KCl in the downstream region.By analyzing the K concentrations at different measurement heights,a potassium release and transformation mechanism of biomass combustion consisting of 11 reaction steps was developed.