| China's coal-based energy structure determines the basic energy status of coal.The main utilization of coal energy is combustion in the power generation,and its combustion power generation accounts for more than 65%of the total power generation.At the same time,the efficient utilization of char generated by coal pyrolysis through combustion power generation is a key link in the utilization of low-rank coal.Nitrogen oxides(NOx)produced during combustion is difficult to obtain effective control strategy due to its chemical instability and complex chemical reaction processes.High concentrations of NO,cause a series of air pollution problems,including photochemical smog,acid rain and greenhouse effects,and are also one of the main components of smog.And NOx can cause lung disease in humans and is a very harmful gas.It should be noted that in China,70%of NO,comes from coal combustion.Therefore,it is imperative to effectively control NOx emissions during the combustion of coal and char.The oxy-fuel(O2/CO2)combustion technology of coal can reduce the NOx emissions and enrich the concentration of carbon dioxide(CO2)in flue gas to more than 90%,which is easy to separate and recover.Based on the research of the preheating combustion technology,this paper will further study the combustion characteristics and NOx emission characteristics of coal/char in an oxy-fuel atmosphere.And we apply this technology to achieving MILD(Moderate and Intense Low-oxygen Dilution)combustion of solid fuel,providing a theoretical basis for engineering applications.The main research work and conclusions are as follows:(1)A stable preheating combustion process can be achieved in coal/char combustion under an air(O2/N2)atmosphere.During the preheating process,a portion of the fuel-carbon is converted to a combustible carbon-based gas,and another portion of the fuel-carbon reacting with the oxygen is exothermic to maintain the temperature in the circulating fluidized bed.The high-temperature preheating coal gas component mainly includes N2,CO2,CO,H2 and CH4.The conversion ratio of fuel-nitrogen in the preheating process exceeds 50%,which indicates that the migration conversion path of fuel-nitrogen in both the preheating process and the combustion process is important for NOx emissions.The strong reducing atmosphere within the circulating fluidized bed makes less fuel-nitrogen to flow into the down-fired combustion chamber,contributing to achieving low-NOx emissions.NOx emissions can be continuously reduced by changing the structure of the secondary air nozzle and keeping the position of the tertiary air distribution away from the secondary air nozzle position while maintaining high combustion efficiency.The NOx emissions from bituminous coal and char can be as low as 151 mg/m3(@6%O2)and 140 mg/m3(@6%O?)while maintaining high combustion efficiency(above 96%).(2)The oxygen concentration in an oxygen-enriched air(O2/N2)atmosphere has a consistent effect on the coal or char preheating characteristics.Under the premise of ensuring that the volumetric flow rate of the distribution gas at all levels is constant,increasing the oxygen concentration of the primary gas can increase the temperature in the circulating fluidized bed and the reaction residence time of the fuel particles,and finally enhance the reducing atmosphere in the circulating fluidized bed.At the same time,the reduction ratio of fuel-nitrogen to nitrogen during the preheating process increases.Compared with the air combustion conditions,when the local oxygen concentration in the circulating fluidized bed increases,the NO emission is greatly reduced.However,as the oxygen concentration in the down-fired combustion chamber increases,the NO emissions increase.When the oxygen concentration in the circulating fluidized bed and the down-fired combustion chamber increased at the same time,the NO emission is still lower than the air condition.The variation trend in the combustion efficiency is consistent with the variation trend in temperature.When the oxygen ratio of the secondary gas increases and the position of the tertiary gas distribution moved downward or the single-layer distribution of the tertiary gas is converted into a double-layer distribution,the NO emission will decrease.(3)The MILD combustion of coal/char can be realized through preheating with a circulating fluidized bed in an oxy-fuel(O2/CO2)atmosphere.This gas-solid mixing mode indicates that the essence of solid fuel MILD combustion is that the particulate combustion reactions are dispersed throughout the low-oxygen space in the reactor,where it is easier to establish the MILD combustion of char under an O2/CO2 atmosphere.Under the MILD combustion condition,the obvious flame front disappears,the temperature distribution is uniform,and the high combustion efficiency can be maintained,thus we achieve clean and efficient combustion of coal/char.When char achieves a MILD combustion,NO emissions can be reduced by half compared to flame combustion.When coal achieves a MILD combustion,the higher concentrations of CO,H2 and CH4 in the preheated coal gas promote the reduction atmosphere near the burner exit of the combustor,so that the effect of MILD combustion on NO reduction becomes weaker,the NO emission is almost unchanged.Under the solid fuel MILD combustion condition,the reduction zone can be expanded by reasonably arranging the tertiary gas position,and the NO emission can be further reduced.However,this approach comes at the cost of reducing fuel combustion efficiency.(4)The effects of oxygen-staging gas distribution on NO emissions during coal/char oxy-fuel preheating MILD combustion are the same.As the oxygen concentration of the primary gas increases,the preheating temperature increases,and the conversion ratio of each component in the fuel increases,wherein the conversion rate of nitrogen element is higher than 0.40.In order to obtain the migration and conversion path of the fuel-nitrogen during the preheating process,the gas in the return feeder is switched from O2/CO2 to O2/N2.As the oxygen concentration of the primary gas increases,the reduction ratio of fuel-nitrogen to nitrogen increases to more than 30.0%.The CO and H2 concentrations increase with oxygen concentration increasing in the primary gas,which is beneficial to the fuel stable combustion in the down-fired combustion chamber.In addition,increasing the oxygen concentrations of the primary and secondary gas and reducing the oxygen concentration of the tertiary gas can reduce NO emissions.The NOx emissions from bituminous coal and char can be as low as 30.2 mg/MJ and 26.8 mg/MJ.These results indicate that the oxy-fuel preheating MILD combustion technology with a reasonable oxygen-staging gas distribution can achieve a clean and efficient combustion of solid fuels. |
PDF Full Text Request