Experimental Investigation Of Nitrogen Oxides Emission During Oxy-fuel Combustion Of Coal

The emission of carbon dioxide（CO₂）is one of the main reasons for globalwarming,and coal-fired power plants contribute a large part of the total CO₂emission.The attention on global warming from the international is so great that reducing CO₂emission is urgent.Oxy-combustion is a clean combustion technology which wouldreduce the CO₂emission to none approximately with some retrofits to coal-fired powerplants. In addition, it can reduce other pollutants emission, specially the nitrogen oxides（NO_x）.Therefore, the oxy-fuel combustion is a method which can solve global warmingand environmental pollution problems well with a bright future.In order to fully understand the mechanism of the NO_xemission decrease during theoxy-fuel combustion,three kinds of typical coal：LNC, NCP and XYC were selected asexperimental samples burned in the small-scale fluidized bed and drop tube furnace for aseries of studies.The results from the experiments of small-scale fluidized bed showed that theconversion ratio of coal N and char N to NO_xin O₂/CO₂atmosphere was lower than thatin O₂/N2atmosphere,and the char’s N conversion ratio which prepared in CO₂atmospherewas higher than that of the char prepared in N2atmosphere. The influence of increasingO₂concentration to N conversion and NO_xreduction during the combustion of coal andchar was complex：with the increase of O₂concentration Oxidation of coal N and char Nwas promoted,and the NO_xreduction was both promoted and suppressed. The emphasisof this multiple effect was different under different O₂concentration.While this effectbehaved differently during the combustion of coal and char.Higher initial NO_xconcentration would promote the NO_xreduction during the combustion of coal and charin both,while promoting the Char/NO_xreduction more intensive.The coal with more volatile matter and the char with more carbon reduced the NO_xmore intensive.The CO₂atmosphere promoted the char/NO heterogeneous reduction through three ways：promoting it directly,the catalytic affection of CO and changing the structure of char.The results from the experiments of drop tube furnace showed that conversion ratioof coal N and char N to NO_xduring oxy-fuel combustion was lower than that during aircombustion, while the NO_xreduction efficiency was higher than that during aircombustion. The NO_xconcentration under oxy-fuel combustion conditions was higherthan that under air combustion of different point in the drop tube furnace, and thecombustion of coal was more rapid in air combustion. The N conversion ratio of char washigher than that of coal. The N conversion of coal and char in drop tube furnace washigher than that in small-scall fluidized bed.The conversion ratio of coal N and char Nincreased and the NO_xreduction efficiency decreased with the increase of stoichiometric.The NO_xreduction of char was more sensitive to stoichiometric than that of coal. Ifincreasing initial NO_xconcentration of the recycled flue gas, the NO_xreduction would bepromoted both during the oxy-fuel combustion of coal and char.Compared to the droptube furnace, increasing the initial NO_xconcentration promoted the NO_xreduction duringchar combustion more beneficial under the small–scall fluidized bed conditions.