| The structure of Chinese energy consumption has long been dominated by coal,and the total national consumption of coal accounted for 56%in 2021.The production of anthracite was about 362 million tons,accounting for 8.8%of Chinese coal production.Anthracite belongs to high-rank coal,with high carbon content,high calorific value,low ash and volatile matter(Vdaf<10%).The characteristics of high coalification and low volatile matter have caused the contradiction of high efficiency combustion and low NOx emission in the utilization of anthracite,whose original NOx emission ranges from 600 to 1300 mg/m3(@6%O2).Meanwhile,the power station boilers which burn anthracite have failed to achieve ultra-low NOx emission(below 50 mg/m3,@6%O2).Therefore,it is a significant requirement for the development of efficient combustion and ultra-low NOx emission technology for anthracite.The essential reason for high NOx emission of anthracite is that its combustion temperature is higher than 1400℃,which inevitably generates a large amount of thermal NOx.In this dissertation,an innovative technology of preheating combustion is adopted to change the conventional combustion of anthracite into preheating combustion,in which the anthracite is preheated to improve the properties and then generates preheated fuel for combustion.The combustion temperature of preheated fuel is controlled below 1200℃.And combining with air-staged combustion,the inherent collaboration between efficient combustion and low NOx emission is achieved.This dissertation focuses on the combination of experimental research and theoretical analysis based on a pulverized coal preheating combustion test platform of 30 kW and a thermogravimetric mass spectrometry test device.The mechanism of material migration and transformation of anthracite during the preheating,the combustion characteristics of preheated fuel,the quantitative evaluation method of combustion characteristics for preheated fuel,and ultra-low NOx emission were systematically carried out.And the preheating technology of anthracite was applied on a 0.4 MW preheating-gasification test platform.The main conclusions are as follows:1)After preheating,anthracite is converted into three phases:preheated coal gas,preheated char and preheated tar.The main components of preheated coal gas are CO,H2,CH4,CO2 and N2.The coal gas precipitates from the inside of particles during the preheating process,which results in more pore channels of preheated char.And the BET surface area of preheated char is enlarged by 10～20 times compared with the raw coal.Meanwhile,the preheated tar was mainly derived from the cracking of large aromatic ring structures in the pulverized coal,and the yield of preheated tar was lower than 3.51 g/kg.Moreover,the increase of preheating temperature and extending reaction time could enhance the conversion from fuel-nitrogen to N2.The maximum conversion rate from fuel-nitrogen to N2 was 56%during the preheating.2)A novel quantitative evaluation mechanism for the combustion characteristics of preheated fuel was proposed by using the method of Coats-Redfern to obtain the apparent activation energy of anthracite and preheated char,and combining with the combustion heat of preheated coal gas and the sensible heat of preheated char.The higher δ value represents the better preheating combustion characteristics.Meanwhile,the δ value of preheated fuel has a positive correlation with the combustion efficiency.It is verified that the δ value is suitable for the quantitative evaluation on the combustion characteristics of preheated fuel.3)Enhanced preheating and staged combustion are the main methods of reducing NOx emissions during preheating combustion of anthracite,which consisted of strengthening the conversion of fuel-nitrogen to N2 by increasing the preheating temperature and extending the reaction time,changing the nozzle structure to control the mixing and reaction rate of preheated fuel and secondary air,and adjusting the reduction and oxidation atmosphere by deep-air-staging in the combustor.4)Ultra-low NOx emissions during the preheating combustion for anthracite was achieved.The NOx emission concentration was 40.77 mg/m3(@6%O2)and the conversion rate from fuel-nitrogen to NOx was 1.26%.Meanwhile,the model of preheating combustion based on Aspen Plus was established,and the error of NOx emission was 7.71%.The results of simulation could provide a theoretical guidance for the prediction and optimization of NOx emission for preheating combustion.5)The preheating-gasification engineering test successfully increased the preheating temperature to more than 1000℃,and the structural characteristics of gasified char were further improved for anthracite.After deep gasification,the conversion rate of nitrogen increased by 16.71%-25.19%for anthracite.This dissertation provides a theoretical foundation for the development and application of efficient and clean combustion technology for anthracite,and promotes the development ultra-low NOx combustion of anthracite. |
PDF Full Text Request