Study On Cooperative Optimization Of Combustion Mechanisms Of Biomass And Coal Blends

With the constant adjustment of energy structures and the demand for energy conservation and emission reduction,many coal-fired power plants in China have carried out environmental protection transformation to adapt to the national development plan.The blend combustion utilization of biomass and coal not only effectively reduced nitrogen and sulfur pollutants and CO2 emissions,but also met the operation requirements of coal-fired power plants after easy transformation of original equipments,which saved the investment costs.However,the total emission of nitrogen and sulfur oxides was still huge.The strong slagging tendency of biomass was easy to cause ash slagging in the boiler or on the wall of the heat exchange tube during the co-firing process,which affected thermal efficiency and system safety.In recent years,the combustion technology of biomass-coal blending has received extensive attention.The volatile content of biomass was high,and the biomass volatiles were released rapidly during the blending combustion process.Under this circumstance,the homogeneous ignition of volatiles released much heat,which contributed to the ignition of coal char.At the same time,a large amount of free radicals such as CHi,NHi and H were formed in volatiles,which promoted the in-phase reduction of NOx.The less volatiles in coal separated out rapidly,and then formed coal char played an important role in the heterogeneous reduction reaction between coke and NO in the late combustion stage.Meanwhile,coal improved the burnout of biomass-coal blending.But there was a large difference between the slagging tendency of different coals.Therefore,it was necessary to judge the slagging strength according to the specific components,which could effectively forecast the slagging of biomass-coal blending combustion.In this paper,the experimental researches on additives,dynamic combustion characteristics and ash slagging characteristics of biomass-coal blending combustion were carried out.Firstly,combustion experiments of biomass-coal blending with kaolin,aluminium nitrate and ammonium sulfate were carried out on the tube furnace system.The effects of different proportions and types of additives on the release characteristics of NO and SO2 in straw-coal blending combustion were analyzed.It was found that adding 2%-5%kaolin improved the permeability of fuel and promoted combustion,resulting in the increase of NO emissions.But adding 10%kaolin reduced heating transfer and hindered the burnout of fuel.Kaolin could effectively reduce the release of SO2 by adsorption and co-melting with sulfate,and reduce the slagging.Both aluminium nitrate and ammonium sulfate could promote the combustion of the blending.The in-phase reduction of NO by NH3 decomposed from ammonium sulfate was directly affected by the addition amount of ammonium sulfate.Meanwhile,the ammonium sulfate promoted the eutectic reaction of minerals in fuel,which enhanced slagging.The aluminium nitrate could decompose O2 to increase the local oxygen concentration,which promoted the oxidation of volatile N and coke N.Secondly,the combustion experiments of 0-70/70-100?m Shenmu coal blended with 100-200?m straw with different proportions were carried out in the drop tube furnace system.It was found that with the increase of straw blending ratio,the ignition and burnout of blending were advanced,and the release of NO decreased.The volatiles of straw produced a large amount of CHi,H and NHi radicals.A high concentration of CO atmosphere was formed with the volatiles combustion,which inhibited the oxidation of volatile N and coke N,and promoted the in-phase reduction reaction of NO and volatiles.As a result,the releasing amount of NO was decreased.In the combustion process of 0-70?m Shenmu coal and straw,the release of volatiles increased.The residence time of volatiles in flue gas increased which delayed the ignition of coal char.The pores of char were abundant,which was beneficial to the heterogeneous reduction reaction of coke and NO.However,small particle coal was more likely to be stuck by molten straw ash and hindered its burnout characteristics,which released slightly more NO than that of large particle coal blending straw.Finally,the high temperature slagging experiments of straw ash and coal ash blending at the ratio of 10%/90%,30%/70%and 50%/50%under air,N2 and CO2 atmosphere were carried out.It was found that the increase of straw ash ratio increased the content of CaO and Fe2O3 in blends,which promoted the eutectic reaction and enhanced the slagging.The weak oxygen transfer reaction under N2 atmosphere caused the iron to exist in the form of Fe2O3.The Fe3+ was strong in polarity and difficult to melt with other minerals,so the slagging under N2 atmosphere was less.Under the CO2 atmosphere,the Fe2O3 was reduced to FeO.The Fe2+ was weak in polarity,and easy to react with CaO and SiO2 to form eutectics.The gasification erosion of CO2 formed a gully structure on the surface of slag,which promoted the condensation of gas phase minerals to enhance the ash viscosity.Therefore,the slagging under CO2 atmosphere was much more serious.