| Combined with the current situation of China's energy structure of“more coal,less oil and less gas”,to meet the demand of carbon neutrality,on one hand,the technological transformation of the energy industry needs to develop renewable energy vigorously and transform it into the main energy source gradually in China;on the other hand,the withdrawal of coal power and deploy carbon capture devices on a large scale should be accelerated.Coal-fired power plants need to be reduced gradually,and even part of the generator set to retire ahead of schedule.However,considering the increasing demand for social power consumption and power grid peak shaving,heating and energy transformation costs,coal-fired unit stranded costs and coal enterprises and the obtain employment of personnel of the upstream enterprise economic and social problems,coal will still keep quite proportion for a long period of time in the future.Coal-fired power plant is one of the largest,concentrated CO2 emission sources,which is also the place suitable for large-scale centralized control of CO2 emissions.Oxy-fuel combustion has the advantages of low technical risk and easy scale up,and thus is considered as one of the carbon capture technologies most likely to be promoted on a large scale for coal-fired power stations.Fluidized bed oxy-fuel combustion has not only the technical advantages of oxy-fuel combustion,but also the advantages of fluidized bed combustion(such as flexible fuel types,stable furnace temperature control,low SOx/NOx emissions and so on).In the past two decades,oxy-fuel combustion technology has developed into different process routes,such as O2/CO2combustion,O2/H2O combustion and pressurized oxy-fuel combustion,etc.The study of combustion mechanism in different process routes are of great significance to the development and commercial application of oxy-fuel combustion technology.This work uses a combination of experimental and model researches to investigate the pressurized oxy-fuel combustion mechanism of a single coal particle in a fluidized bed reactor,and then quantitatively reveals the effects of different atmospheres and pressures on the different combustion processes(i.e.,drying,volatiles release,combustion,and char combustion).The main research contents and results are as follows:(1)Design and build a visualized pressurized fluidized bed(PFB)reactor to conduct combustion experiment work,and introduce"two-color pyremetry"and"thermocouple measurement"to measure the particle temperature and flame temperature in real time.It overcomes the shortcomings that it is difficult to accurately obtain the combustion informations such as particle temperature and combustion images in conventional combustion experiments,and obtain basic parameter information quantitatively in the combustion process.It also provides reliable data for the establishment of coal particle combustion model;(2)Use the visualized PFB reactor to study the effects of different combustion positions(i.e.,dense phase and dilute phase),atmosphere,coal type,and oxygen concentration on the ignition,devolatilization,and char combustion processes.The results showed that due to the high heat transfer coefficient in the dense phase of the fluidized bed,the ignition delay time of coal particle in the dense phase was much shorter than that in the dilute phase;with the wrapping and heating effect of the volatiles flame on the coal particle,the devolatilization time of coal particle in the dilute phase was significantly shorter than that in the dense phase;the peak temperature of char in the dilute phase was higher than that in the dense phase,and the corresponding burnout time of char particle in the dense phase was longer than that in the dilute phase,which were related to the better mass transfer in the dilute phase;(3)The combustion behavior(i.e.,drying,ignition,devolatilization,and char combustion)of single coal particle in O2/H2O atmosphere was experimentally studied in the visualized fluidized bed reactor.The effects of different coal ranks(i.e.,lignite,bituminous coal and anthracite),atmosphere(i.e.,O2/H2O,O2/N2 and O2/CO2),oxygen concentration,location(i.e.,dense phase and dilute phase),fluidization number,particle sizeand bed temperatureon combustion charateristics were investigated.The results showed that a high oxygen concentration accelerated the char oxidation and promoted the transition of coal ignition mode;at a low oxygen concentration,the volatiles flame in O2/H2O atmosphere was darker than that in O2/N2atmosphere;with the increase in oxygen concentration,thevolatiles flame temperature in O2/H2O atmosphere was enhanced dramatically compared to that in O2/N2 atmosphere,where the diffusion rate of oxygen in atmosphere became the dominant factor;the char has the shortest burnout time and lowest peak particle temperature in O2/H2O atmosphere compared to O2/N2and O2/CO2 atmospheres,because of the higher DO2 and char gasification rate in O2/H2O atmosphere;(4)An experimental study on the pressurized combustion characteristics of single coal particle in O2/N2 and O2/CO2 atmospheres under different operating parameters was conducted.The influence mechanism of parameters such as pressure on combustion characteristics of single coal particle was studied.The results showed that at the same oxygen concentration,the ignition delay time of coal particle increased with the increase in operating pressure;at the same pressure,the ignition delay time of the particle in O2/CO2 atmosphere was longer than that in O2/N2 atmosphere;with the increase of operating pressure,the flame temperature and char temperature increased but the increment gradually decreases,and the size of flame became narrower and longer;the burnout time of particle decreased dramatically as the increase of operating pressure,and the burnout time of particle in O2/CO2 atmosphere was significantly longer than that in O2/N2 atmosphere;with the increase of particle size,the peak temperature of char particle decreased slightly,and the burnout time increased significantly;(5)Considering the factors of heat and mass transfer and char gasification under fluidized condition,a millimeter-scale coal particle combustion model,including the drying,devolatilization and char combustion processes,was established.The reliability of the model was verified by experimental data,and the sensitivity of several key parameters in the model were analyzed.The results showed that the prediction error of the model for the devolatilization time was less than±20%,and the prediction errors for the burnout time and the peak temperature of char were less than±15%and±8%,respectively.The drying and devolatilization processes of coal particle were mainly controlled by the heat transfer coefficient between the fuel particle and the bed material,the thermal conductivity of coal particle and the size of coal particle.The increase of operating pressure led to the increase in heat transfer coefficient,which reduced the drying time and devolatilization time,and increases the peak temperature difference between inside and outside the particle.Compared with N2 atmosphere,CO2 atmosphere has a higher heat transfer coefficient,which also leads to the decrease of drying time and devolatilization time,as well as the increase of temperature difference between inside and outside particles,but the overall difference were very small(<5%).The influence of gasification reaction can not be ignored in pressurized oxy-fuel combustion.Especially under the conditions of high temperature,high pressure and high oxygen concentration,the proportion of gasification reaction in the char combustion were increased significantly because of the dramatic increase in the particle temperature. |
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