Effects Of Ca And Fe On Coal Nitrogen Release In Oxy-fuel Combustion

Coal as our most important conventional energy sources, while giving off heat in the combustion process, also releases a large number of pollutant emissions to the environment. These pollutants include NOx, SOx, fine particulate matter, trace metals and others. Studies have shown that coal-firing generated NO and NO2which are precursors of acid rain, and participate in the formation of photochemical smog pollution, and NO2is also a greenhouse gas. Nitrogen oxides will generate serious harm to human health and ecological environment. The understanding of NOx Formation Mechanism and Emission was an important foundation of the hazard assessment of nitrogen oxides, regulations and the emission control, to carry out research in this area has important scientific and practical significance.Oxy-fuel combustion technology, insteading of traditional air combustion, is a new coal firing technology which can reduce the emission of CO2. In oxy-fuel combustion, O2which was seperated from air, and recycled flue gas took the place of air during traditional air combustion. It caused higher CO2concentration in flue gas, the cost of CO2capture would decrease as well. In oxy-fuel combustion, nitrogen was low, which would decrease the formation of thermal NOx and prompt NOx, so that the emission of NOx in whole would drop greatly. The general conclusion is that the emission of NOx in oxy-fuel combustion would decrease to1/3to1/2of that in air combustion, the main reason is related to volatile matter and char. There are lots of mineral matter, like Ca and Fe, which have great effect of coal pyrolysis and combustion. The mineral matter also have different effects on nitrogen species formation. The reason of NOx emission decrease in oxy-fuel combustion was analysed in this thesis, the nitrogen species(HCN, NH3, NO and char N)formation in CO2atmosphere was studied, the char combustion and the reaction of char in NO were studied too. The effects of Ca and Fe on nitrogen species formation were also analysed. The main contents were as follows:With the methods of demineralization, mineral matter addition both of physically and with solution, the effects of Ca on nitrogen species distribution were studied. In conventional air combustion, N2is the dominant gas. The concentration of CO2would be higher than90%in oxy-fuel combustion with recycled flue gas. The pyrolysis atmospheres during experiments were N2and CO2, in order to study the effects of different atmospheres on nitrogen species formation. N2with high concentration would change nitrogen balance during pyrolysis, there is also reaction of CO2and precursors of NOx, which means that the two atmospheres have great effects on nitrogen species formation during pyrolysis. At the same time, Ar, which is inert, was choose as one pyrolysis atmosphere to analysis and compare. The results show that with three different Ca-based additives, i.e.,(CH3COO)2Ca, Ca(OH)2and CaCl2, char yield decreases with the increasing amount of Ca additives in all cases. The effect of Ca on HCN and NH3formation during pyrolysis depends on both the form of Ca and the amount of addition. HCN and NH3formation decreases with Ca addition in Ar, but increases in N2. There is no NH3detected during pyrolysis in CO2. In addition, NO concentration detected decreases sharply with increasing the amount of Ca addition in the atmosphere of CO2.The effects of FeCl3addition on nitrogen distribution under different pyrolysis environments were studied by the experiments in horizontal tube furnace. Two mixing way were introduced, one was physically mixing, the other was mixing with a saturated aqueous solution of FeCl3. The results indicate that at the temperature of1000℃, FeCl3addition with DEM decreased HCN in pyrolysis atmosphere of Ar. In N2, FeCl3increased the release of HCN due to high concentration of N2. In atmosphere of CO2, the formation of HCN was much less than that in other environments, because of the effect of FeCl3on devolatilization and char conversion. In Ar or N2environments, NH3decreased sharply with FeCl3addition respectively. But there was no NH3measured under CO2.The effects of Fe on NO formation under the reactions in air and oxy-fuel combustion were studied with the method of physically addition. The particle size of char generated at800℃in CO2is larger than that in N2. However, at1300℃, it is smaller in CO2atmosphere due to particle breaking by gasification of char and CO2. The Fe addition increases the NO conversion ratio, and the effect of Fe rises steeply with the process going until it becomes stable in the end. The results also indicate that the release of NO increases more significantly with the Fe addition in oxy-fuel environment.Finally, the effect of Fe2O3on reaction of CO2-char and NO has been studied by experiments of devolatilization in high temperature drop tube furnace and the reaction of char and NO in horizontal tube furnace. It was found that the addition of Fe2O3promotes the reaction of Ar-char and NO. With Fe2O3addition, the peak of NO release dropped16%during the reaction of Ar-char and NO, from that point on, the effect of Fe2O3became stable. The reaction of char and NO was not a quick one, so the effect of Fe2O3was also lasting and effective. There was a strong connection between the release of NO and CO.