Experiment Of Nitrogen Transformation On Coal/Biomass Combustion In Fluidized Bed Under O₂/CO₂ Atmosphere

Controlling and reducing CO₂emissions is a very important development direction for clean and efficient coal utilization.The pressurized oxy-fuel combustion technology of coal and biomass is one of the most promising way to achieve carbon dioxide zero emission and can realize large-scale and efficient CO₂capture.However,the conversion and transformation characteristics of nitrogen from fuel to pyrolysis products in CO₂atmosphere and the formation mechanism of nitrogen oxides in oxy-fuel combustion need to be further researched.This paper is aimed at researching the transformation and conversion mechanism of fuel nitrogen in the oxy-fuel combustion process of PL coal and wheat straw.So the pyrolysis experiment of coal and biomass in CO₂atmosphere and the oxy-fuel combustion experiment in O₂/CO₂atmosphere were carried out on a small-scale fluidized bed as well as on a pressurized fixed bed.The formation characteristic of nitrogen oxides is analyzed on the basis of pyrolysis experiment results,which can provide theoretical support and basic data for further nitrogen oxides emission control.Firstly,pyrolysis experiments of PL coal and wheat straw under CO₂and Ar atmosphere at 750～850℃were carried out in a small-scale fluidized bed.In addition,the mixing ratios of coal/biomass co-pyrolysis experiments were set as 10%,30%and 50%.The pyrolysis experiment results indicated that CO₂can effectively suppress the formation of NH₃.The yield of NH₃in the pyrolysis process of PL coal and wheat straw at 850℃were reduced by 84.49%and 86.71%,respectively.For HCN,the HCN yield of PL coal is suppressed and the HCN yield of wheat straw is promoted.The yield of HCN in the pyrolysis process of PL coal and wheat straw at850℃were reduced and increased by 49.23%and 50.09%,respectively.Furthermore,CO₂can suppress the overall nitrogen conversion rate of NH₃+HCN for both coal and biomass,which causes more fuel-N to convert into tar-N and N₂-N.For the blends of PL coal and wheat straw,the co-pyrolysis synergistic effect of coal/biomass influences the selectivity of HCN and NH₃.The co-pyrolysis synergistic effect favors the formation of HCN but has an opposite i MPact on the transformation from fuel-N to NH₃.The synergistic effect promotes the overall nitrogen conversion rate of NH₃+HCN under a CO₂atmosphere,and the overall nitrogen conversion rate of NH₃+HCN is suppressed under argon atmosphere,conversely.In addition,the synergistic effect suppressed the yield of environmentally harmless N₂,and more fuel-N was retained in the char.Secondly,oxy-fuel combustion experiments of PL coal and wheat straw under O₂/CO₂atmosphere were carried out in a small-scale fluidized bed at 750～850℃.In addition,the mixing ratios of coal/biomass oxy-fuel co-combustion experiments were set as 10%,30%and 50%.The oxy-fuel combustion experiment results indicated that NO is the most important nitrogen oxide of PL coal and wheat straw.The main source of NO from PL coal is the coke combustion stage,and the main source of NO from wheat straw is the volatile devolatilization stage.The amount of NO generated during the volatile devolatilization stage and the coke combustion stage decrease with the increasing combustion temperature.O₂/CO₂atmosphere promotes the conversion of fuel nitrogen to NO.In the stage of volatile devolatilization,CO₂promotes NO production.In the coke combustion stage,CO₂promotes the conversion of PL coal to NO,however,inhibits the production of NO from wheat straw.In the co-combustion experiment of PL coal and wheat straw,with the increase of oxygen concentration from 11%to 31%,the NO formation showed a decrease trend,the yield of NO in the oxy-fuel combustion process of PL coal and wheat straw at 850℃under O₂/Ar and O₂/CO₂atmosphere were reduced by 25.87%and 24.97%,respectively.There is a synergistic effect in the co-combustion process of PL coal and wheat straw.The synergistic effect inhibits the conversion of fuel nitrogen to NO and the NO formation decreases with the increase of the mixing ratio.Finally,pressurized oxy-fuel combustion experiments of PL coal and wheat straw under O₂/CO₂atmosphere at 750～850℃were carried out in pressurized fixed combustion bed.In addition,the mixing ratios of coal/biomass oxy-fuel co-combustion experiments were set as 10%,30%and 50%and the pressure were set as 0.1 MPa to 3.1 MPa.The experimental results of pressurized fixed bed oxy-fuel combustion indicated that,with the temperature increasing from 700℃to 1100℃,the yield of NO reduce by 80.96%.The increase in oxygen concentration has an inhibition effect on the NO.With the combustion pressure increasing from 0.1 MPa to3.1 MPa,the NO formation of PL coal and wheat straw decreased significantly.The synergistic effect exits,which inhibits the conversion from fuel nitrogen to NO.The NO formation decreases with the mixing ratio increase.