Experiments On Transformation Characteristics Of Nitrogen During Coal/biomass Co-combustion Under O₂/CO₂ Atmosphere

Oxy-fuel combustion technology uses CO₂ instead of N₂ mixing with O₂ to participate in combustion.It is considered to be an effective large-scale CO₂ capture technology.There may be synergy between coal and biomass in co-pyrolysis and co-combustion.But there is little work involved nitrogen distributions in the co-pyrolysis of coal and biomass under CO₂ atmosphere,and NO_x formation characteristics during co-combustion in O₂/CO₂ atmosphere.In this paper,the nitrogen transformation characteristics during coal/biomass co-pyrolysis under CO₂ atmosphere was studied.And the NOx formation characteristics during coal/biomass co-combustion under O₂/CO₂ atmosphere was investigated based on the research of co-pyrolysis process.The research results can enrich the theory of nitrogen transformation and migration mechanism,and provide a theoretical basis for better control of NO_x formation in co-combustion.Firstly,experiments on nitrogen transformation characteristics of coal/biomass co-pyrolysis under CO₂ atmosphere were carried out in a tube furnace reactor.A typical coal?Pingliang bituminous coal?and a biomass?wheat straw?were used as fuel,and the pyrolysis temperature was 700-900°C.The influence of CO₂ atmosphere was investigated through analysing the nitrogen content of char,tar,HCN and NH₃.Moreover,the effects of synergy in co-pyrolysis on nitrogen distributions was studied.The results show that CO₂ reacts with the fuel at 800-900°C.Gasification can promote the release of nitrogen into the volatiles,resulting in increasing the NH₃yield of the two fuels and the HCN yield of wheat straw,and promoting the formation of N₂ by about 10%.The synergistic effect during the Pingliang coal/wheat straw co-pyrolysis reduces the yield of HCN and NH₃.It is speculated that due to the different reaction extent between lignin,cellulose or hemicellulose in wheat straw with Pingliang coal at different temperatures,the nitrogen content of char is decreased at 700°C and increased at 800-900°C.The gasification reaction occurs when the mixture is pyrolyzed at high temperature under CO₂ atmosphere,which reduces the nitrogen content of char and forms more NH₃ and N₂.Then experiments on nitrogen transformation characteristics of coal/biomass co-combustion under O₂/CO₂ atmosphere were carried out in a tube furnace reactor and a fluidized bed,and the NO and N₂O concentration during combustion were measured and analyzed.Pingliang bituminous coal and wheat straw were used as fuel in experiments.The effects of O₂/CO₂atmosphere,oxygen concentration and blending ratio on NO_x formation were investigated.O₂/CO₂ atmosphere can promote gasification reaction of fuel to produce a large amount of CO,which promote NO reduction reaction and reduce the NO emission by about 10%.The formation of N₂O under O₂/CO₂ atmosphere is directly affected by the formation of HCN.CO₂ can promote the formation of HCN in wheat straw and inhibit the formation of HCN in Pingliang coal,resulting in that the N₂O yield of wheat straw under O₂/CO₂ atmosphere is higher than that under O₂/Ar atmosphere.And the N₂O yield of Pingliang coal performs oppositely.Increasing oxygen concentration can increase the concentration of-O and-OH radicals,causing more formation of NO and N₂O.NO_x emission during co-combustion of Pingliang coal and wheat straw is reduced,which may due to the promotion of NO heterogeneous reduction by biomass ash and CO.