The Research On Combustion Characteristics And NO_x Transportation Mechanisms Under Oxidant Staged Combustion Of Pulverized Coal

In the application field of fossil fuel energy,although the share of coal in Chinese energy structure was being kept between 69~78%since 1980s and the fraction dropped slightly in recent years,the coal consumption has increased from 1.4 billion ton in 2000year to 3.87 billion ton in 2014.Along with the abosulute increase of coal consumption,the requirement on the control of pollutants emission is more and more stringent.Recently,the concepts or policies,such as?Clean Coal Technology‘,?Clean Energy‘,?Near Zero Emission‘,and so on,were proposed and heightened indicating the technologies about low pollutants emission and low exhaust from coal combustion are still quite desirable.This paper will focus on how to efficiently reduce the pollutants emission and exhaust during the in-furnace pulverized coal combustion process by researching the characteristics of pulverized coal combustion and NO_x transportation mechanism in oxidant staged combustion.Oxidant staged combustion include air-staged,oxygen-enriched staged,and oxy-fuel staged combustion.In different applications,the combustion air is no longer pure air,thus the concept of?oxidant staged combustion?is adopted in terms of the essence of staged combustion(i.e.the oxygen is supplied in stages).This concept is more precise to depict the the combustion air staged combustion technologies used in field concerning the air-staged,oxygen-enriched staged,and oxy-fuel staged combustion of pulverized coal.In this paper,two sets of experiment facilities were utilized:fixed bed reaction platform and 20 kW down flame furnace(DFF).A series of experiments about pulverized coal combustion was sequentially arranged to gradually further the research of combustion characteristics(ignition characteristics,combustion duration,combustion rate,etc.)and pollutants(NO)transportation mechanism of the pulverized coal under various combustion conditions.In fixed bed reaction platform,the paper studied the pulverized coal combustion in terms of combustion atmosphere,oxygen concentration,combustion temperature,and particle diameter.On the basis of experimental data,the rules about ignition characteristics,combustion duration,fuel NO release process affected by the boundary conditions are analyzed and concluded.A cumulative percentage model(CPM)was built to describe the fuel NO release process along the pulverized coal combustion.With CPM as one of the important achievements in this paper,the basic research of NO release of pulverized coal combustion was brought into quantitative analysis.Furthermore,the oxidant staged combustion experiment was carried out in 20 kW DFF focusing on the effect of burner stoichiometric ratio(SR),burnout stream injection position,and oxygen concentration on combustion characteristics and NO release mechanism of pulverized coal.With the measurement of the concentration profiles of main physical and chemical parameters(such as flue gas temperature,CO,NO,etc.)along the furnace axis direction,the rules about enhanced char gasification and change of NO concentration profile were found and analized.Meanwhile,with the combination of CFD software,the rules found were integrated into numerical models(such as devolatilization model,particle surface reaction model,refined char gasification model,NO formation and reduction model,etc.).With these refined models,the oxidant staged combustion process of pulverized coal,under air-staged,oxygen-enriched staged,and oxy-fuel staged combustion,was successfully modeled with high consistence.The refined numerical model of pulverized coal combustion provide an important solution for the research of coal combustion.Basing on the comparison of NO profiles between experimental data and numerical result of oxidant staged combustion of pulverized coal in 20 kW DFF,two problems of current NO release mechanism were found:1,the discrepancy of NO concentration in reducing zone;2,the failure of prediction of NO profile under peak CO concentration condition.For solving these problems,author conceived the refinement of NO release mechanism and then embedded the refinement into the MFR numerical model.Through the MFR model,the necessity of CO prediction by refined char gasification and the importance of char nitrogen heterogenous conversion mechanism in NO prediction are proved.Basing on the research progress in 20 kW DFF,with both of the pre-certified chemical mechanism of pulverized coal combustion(including original NO numerical model)and the refined MFR model,the author predicted the operating result of a 50 kw down flame furnace which is a part of the intergrated pilot scale platform.The former mechsnism was used to quantitatively predict the CO profile and the final NO emission with high authenticity;the latter mechanism was used to qualitatively verify the adaptability of refined MFR model,and the characteristics of NO profile was captured.