Mechanism Study On Ash Deposition And Its Inhibition On Heating Surfaces Of Biomass Combustion

Biomass direct combustion power generation technology is one of the main ways to apply biomass energy efficiently.During the biomass combustion process,alkali metals(K)and chlorine are released into the gas phase,resulting in serious deposition on the heat transfer surface of the boiler.And,further,serious corrosion problems will happen,especially in the presence of Cl.At present,the research on deposition mechanism is still very rough,and the deposits samples derived from boiler are often roughly divided into inner and outer layers for further testing and research,which makes the specific dynamic process of deposition unclear.This paper conducts a detailed and in-depth study on the deposition process in the biomass circulating fluidized bed(CFB)boiler.By analyzing and testing the deposition samples under different deposition times,the dynamic process of the deposition is completely pieced together.In order to collect deposits samples with different deposition time,a set of temperature-controlled deposition collection system was designed and built.During the deposition collection process,the ash deposition probe is inserted into the boiler.There is a deposition collection ring on the probe as deposition collection section.It is convenient to collect multiple deposits samples without affecting the normal operation of boiler.After sampling,the deposits samples do not need to be removed from the collection ring,and is directly tested to retain the original morphology of the deposits.The dynamic process of high alkali and high chlorine biomass ash deposition was studied in a 50 MW biomass CFB boiler in Guangdong province.The deposits samples collected by temperature-controlled deposition collection system were tested by SEM/EDS and XRD.The dynamic process of deposition can be divided into three stages:(1)the initial stage of deposition: thermophoresis deposition occurs in the submicron particles in fly ash.(2)KCl condensation stage: after the initial deposition stage,the deposition surface becomes relatively rough,resulting in an increase of condensation rate of gaseous KCl,which forms a dense KCl layer on the surface of the deposition layer.(3)capture of fly ash particles: the KCl layer at a low melting point is partially melted at a high temperature.And its viscosity increases,which can capture fly ash particles in flue gas more efficiently.Similarly,the dynamic process of high alkali and low chlorine biomass ash deposition was studied in a 12 MW biomass CFB boiler in Jiangsu province.The temperature-controlled deposition collection system was used to sample the deposits with different depositon time to study the process of biomass ash deposition in low chlorine environment.The dynamic process of deposition can also be divided into three stages :(1)the initial deposition stage,(2)the KOH condensation,and(3)stable growth of the deposits.The initial stage of deposition is basically the same as that of high-alkali and high-chlorine biomass,which is formed by thermophoretic deposition of submicron particles in fly ash.After the initial deposition stage,the deposition surface becomes relatively rough,and the condensation rate of KOH in the gas phase increases,so it begins to deposit,and further reacts with H2 O,SO2 and O2 in the flue gas to form K2SO4.The surface roughness of the deposition continued to rise with the growth of the deposition,and the inertial impact of fly ash began to become one of the main factors of the deposition.Meanwhile,as the deposition thickens,the surface temperature rises and the condensation rate of KOH decreases.The third part is the mechanism of additives inhibiting deposition.Through the preliminary experiment in the tube furnace,the additives of kaolin,mica,Mg(OH)2 and industrial sulfur were tested.And finally,kaolin and sulfur were selected to be added to CFB boiler in the power plant to study their mechanism of inhibiting deposition.The effect of kaolin was relatively better,and a good inhibition effect was produced.After the addition of kaolin,much Ca SO4 and a small amount of Ca Cl2 were found in the deposition,while the content of KCl was reduced.Factsage was used to simulate the Ca CO3+ KCl+ kaolin system,and it was found that a large amount of KCa Cl3 was produced in the system,which is the complex salts of KCl and Ca Cl2.Ca SO4 in the deposits was probably formed by the sulfurization of Ca Cl2.The fourth part is the study of alkali metal deposition in other atypical locations of CFB boiler.The wind cap deposition and the material return inclined tube deposition in CBF boiler were studied respectively.It was found that the more severe the erosion of flue gas was,the more dependent the deposition was on KCl.The KCl content in each deposition stage of wind cap deposits was over 55%,and the phenomenon of gradual decrease of KCl content on the deposition surface would not occur with the deposition growth.And the melting of KCl was a necessary condition for deposition growth.For the deposition in material return inclined tube,the eutectic melting of the alkali metal Na Cl and Ca Cl2 was found.The biomass ash deposition process was modeled by COMSOL Multiphysics.Physical processes such as fluid drag force,thermophoresis force,particle inertia rebound or adhesion after impact and KCl condensation are considered.Finally,the change of the thickness and surface temperature of the deposits is calculated,which is basically consistent with the experimental results.The simulation results of the 10-day deposition process show that the deposition rate gradually slows down with time going on.Finally,a scheme for inhibition of alkali metal deposition and corrosion in CFB boiler burning high alkali and high chlorine biomass was proposed.By adjusting the arrangement of the water cooling wall,the flue gas temperature was further decreased to inhibit the condensation of gaseous potassium chloride and the melting of KCl in the deposition.At the same time,according to the experimental results of deposition inhibition by kaolin,we proposed to add 0.9% kaolin(mass ratio of kaolin to biomass fuel)to decrease KCl content in deposits.In the actual industrial operation,before combusting high chlorine biomass,low-chlorine biomass fuel can be used for 2 days to form deposition layer with no KCl.In this way,KCl will not contact with the heat exchange surface directly,thus preventing corrosion.