Study On Pollutant Formation Mechanism And Ash Formation Characteristics During Oxy-fuel Co-combustion Of Biomass And Coal

More and more attentions are paid to the greenhouse effect caused by continuous increase of CO2 emission,so finding effective CO2 emission reduction measure has become the focus.Biomass utilization is considered as zero CO2 emission.Oxy-fuel combustion is considered as one of the most promising CO2 capture and storage(CCS)technology.Circulating fluidized bed(CFB)is considered as one of the best clean coal combustion technology because of plenty of advantages.Biomass/coal CFB oxy-fuel combustion technology is considered as one of the most promising CO2 negative emission technology,integrating the advantages of biomass energy utilization,CFB technology and CCS technology.In order to study on pollutant formation mechanism and ash formation characteristics,combustion experiments of biomass/anthracite,biomass/bituminous coal and biomass/lignite were conducted in a 50 kW oxy-fuel circulating fluidized bed combustor under O2/CO2 atmosphere respectively,with combustion temperature,combustion atmosphere,inlet oxygen concentration,biomass mixing ratio and excess oxygen coefficient considered.In addition,effects of minor component in recycle flue gas(RFG)on pollutant formation mechanism were investigated in O2/simulated RFG atmosphere and whether recirculated CO,NO and SO2 had synergistic effect on pollutant formation was verified.Compared with air atmosphere,NO,N2O,SO2 emission and fuel-N conversion ratio in 21%O2/79%CO2 atmosphere are lower during biomass/coal combustion.In O2/CO2 atmosphere,during biomass/coal combustion,NO and SO2 emissions increase with the increase of combustion temperature,while N2O emission decreases.Compared with coal combustion,fuel-N conversion ratio and SO2 emission are lower,indicating that blending biomass in coal can reduce pollutant emission.As biomass blending ratio increases,SO2 emission and fuel-N conversion ratio decrease.NO and SO2 emissions increase with inlet oxygen concentration increasing.With excess oxygen coefficient increasing,NO,N2O and SO2 emissions increase.In O2/simulated RFG atmosphere,pollutant formation and emission are not influenced by CO recirculation under the range of experiments.With the increase of NO concentration,N20 and NO emissions increase while CO emission decreases.NO reduction ratio is about 80%independent of recirculated NO concentration.As recirculated SO2 concentration goes up,CO,SO2 emissions and SO2 retention ratio increase,but NO emission decreases.Compared with CO,NO and SO2 single component recirculation respectively,CO/NO/SO2 recirculation simultaneously has a synergistic effect on pollutant formation,reducing NO and SO2 emissions more effectively.In O2/CO2 atmosphere,with combustion temperature,inlet oxygen concentration and biomass mixing ratio increasing,the median diameter of fly ash becomes smaller and the share of submicron particles in fly ash becomes more,and the content of K and Na in fly ash increase.Compared with air atmosphere,the median diameter and share of submicron particles of fly ash in 21%O2/79%CO2 atmosphere are bigger.The main minerals in fly ash are SiO2,CaSO4 and 3Al2O3-2SiO2,while K in the fly ash is mainly in the form of KAlSi2O6.Direct desulfurization and indirect desulfurization reaction occur simultaneously in O2/CO2 atmosphere while only indirect desulfurization reaction occurs in air combustion.