Study On The Influence Of Graded Air Distribution On Slagging And Pollutant Emission Of Straw Particles Burning

Biomass is one of the most promising alternative energy sources for fossil fuels,it has the advantages of renewable,low-carbon emission,no harm to the environment or minimal harm,and resources are widely distributed,therefore it is suitable for local development and utilization.However,straw biomass usually has the characteristics of high alkali metal and ash content,and it is easy to produce the problems of high pollutant discharge,caking and slagging when burning.In this paper,a straw biomass combustion test-bed with the rated feed rate of 3kg/h,rated power of 48×10³k J/h and multi-layer secondary air distribution structure was developed.The emission of pollutants（NO_x,CO）from straw biomass combustion and bottom ash slagging were studied by means of primary and secondary air staged combustion and multi-layer secondary air distribution combustion.The ash content and slagging composition under different working conditions were analyzed to obtain better combustion operation conditions with low slagging rate and low pollutant emission.The experimental results are as follows:It was found that rice husk combustion had lower NO_xemission concentration than corn straw combustion under the working conditions of non-staged combustion and primary and secondary air staged combustion.For corn straw,Compared with the control group 1 and the primary and secondary air staged combustion,the concentration of NO_xwas low,with a minimum of about 140 mg/m³.The reduction rate of NO_xwas between 38%and 53%,and the NO_xreduction effect was very obvious.On the contrary,the concentration of CO in flue gas was high.Among the 4 kinds of secondary air multi-layer combustion conditions,The NOx emission concentration and slagging rate were low when the lower and upper secondary air accounts for 1/2 of the total secondary air,which was considered to be the best condition for corn straw combustion.Acid-base ratio,alkalinity index Al_c,Na content index and silicon ratio index G,with the actual combustion slagging situation can’t correctly predict the change of slagging tendency caused by the change of fuel zone T₁.T₁was introduced into silicon ratio index G,and the modified G_tcan well predict the slagging tendency of corn straw particles with temperature T₁in combustion zone.For rice husk,the overall level of NO_xreduction rate in multi-layer secondary air combustion was between 36%and 41%,with little change,and the NO_xreduction effect was not as obvious as that in corn straw combustion.There was no slagging during rice husk combustion.Therefore,NO_xand CO are selected as the optimization objects when choosing the optimal working condition of rice husk combustion,The optimal working condition was the secondary air condition when the ratio of primary and secondary air was（60%,40%）.There were a lot of Si O₂and KCl in the ash and slagging from corn straw burning,and the main components in the ash from rice husk burning were Si O₂and mullite.In the combustion test of the combustion test-bed designed in this paper,when using corn straw as fuel,the combustion efficiency can reach over 98%.Due to the high Si content in rice husk,the highest combustion efficiency can reach 97.78%when using rice husk as fuel.Corn straw and wood particles were mixed for combustion,with the increase of wood particles mixing ratio,NO_xat the outlet of the combustion test-bed shows a decreasing trend,and it tends to be stable under the combustion conditions of 50%and 60%wood particles mixing ratio.Corn straw and wood particles were mixed in the ratio of 1:1 had a good effect,and primary and secondary air staged combustion and multi-layer secondary air distribution combustion had a lower effect on reducing slagging in the mixed combustion condition of corn straw and wood particles.All in all,The lower,middle and upper secondary air accounts for1/3 of the total secondary air was the best working condition.