Experimental Study On Biomass Reburning And Advanced Reburning For NO_x Control

As the most important primary energy sources, coal combustion produces a large number of air pollutants. NO_x is one of the primary toxic pollutions formed in pulverized-coal combustion process, and it has drawn wide attention of the world because of its high hazard and hard treatment. Reburning is a low-cost NO_x control technology. In our country, the storage of biomass is abundance and the distribution is very wide. To combine the combustion of biomass with reburning and exploit the large-scale rational utilization of biomass is of momentous significance in establishing of the new field of energy and promoting of environmental protection.First of all, the background and present state of subject study were discussed. Then, the formation mechanisms and control technologies of NO_x were introduced. The formation approach of NO_x includes thermal-NO_x, fuel-NO_x and prompt-NO_x. In coal combustion, 75-90% of NO_x originates from fuel-NO_x, so the control of fuel-NO_x emissions is critical. The methods of control NO_x contain low NO_x burner technology and gas denitration. Reburning which is discussed in this paper is a three-step process, involving combustion of majority of the fuel under normal fuel lean conditions, followed by injection of a reburning fuel which is 10-30% of the total fuel to establish a fuel-rich zone in which NO_x formed in the primary combustion zone are reduced to molecular nitrogen, and finally injection of over-fire air to oxides unburned fuel exiting the reburning zone. Four different biomasses, wheat straw, cornstalk, peanut shell and wood chip, were evaluated as reburning fuels.Then, basic property and thermogrametric characteristics of biomass were studied. Through studies, biomass contains little nitrogen and virtually no sulfur, so biomass is green energy resource. Biomass contains high volatilization, low ash content, and also burning is easy, and when biomass is burning, the volatilization separates out fast in short time. So the biomass is the ideal reburning fuel.According to request of the research, the "muti-functional denitration test bed" is designed and built up. Using this test bed, a serial of experimental researches of NO_x reduction with biomass as reburning fuel was carried out. Experimental results showed bimass reburning was an effective NO_x reduction technology, NO reduction increased with decreasing biomass particle size. Experimental results also showed NO reductions as high as 50-70% could be achieved with about 15-25% of the heat input coming from the four reburn fuels. NO emissions can be reduced by more than 50% at the best experimental stoichiometric ratios of 0.6-0.8. NO reductions increased with increasing reburn zone residence time and initial NO concentration.In the end, experimental researches of NO_x reduction with advanced reburning (AR) which coupled biomass reburning with injection of N-agent (ammonia or urea solution) were carried out. The N-agent which can be added either to the end of rebuning zone or to the burnout out provided higher levels of NO control on the basis of biomass reburning. Besides the parameters of reburning on NO reduction, NH₃/NO molar ratio and N-agent injection location are also the important parameters in advanced reburning. With increasing NH₃/NO molar ratio a high NO reduction efficiency can be achieved. However, NH3 slip increased too with the increase of this ratio, and there was no additional NO reduction. Results showed that NO emissions can be reduced by about 80% at a NH₃/NO molar ratio of 0.6-0.8. Injection of alkaline promoter into the reburning zone with ammonia or urea can furthermore improve NO control. In experimental process, water-soluble Na₂CO₃ was added to N-agent and injected into the end of the reburning zone, and the presence of Na provided more active radicals (OH and H) to enhance NO reduction. At a reburn heat input of 20%, NO reductions of 86.1% were achieved with promoted AR. Ammonia or urea solution was injected into the end of reburning zone and the burnout zone at the same time, which was also researched. At a reburn heat input of 20%, the best NO reductions of 91.3% were achieved.