Study On The Pressurized Oxy-fuel Combustion Characteristics Of Coal Pellet Under Concentrating Photothermal Heating

Oxy-fuel combustion technology is considered to be one of the most potential CO₂capture technologies.And pressurized oxy-fuel combustion technology is considered to be a more efficient and cleaner second-generation oxy-fuel technology due to its advantages of high power generation efficiency and low pollutant emissions.Due to the difficulty in setting up an experimental facility for achieving high temperature and high pressure conditions,there are relatively few experimental studies on the combustion characteristics of coal pellet in the range of high heating rate and wide pressure.A pressurized concentrating photothermal apparatus with rapid heating rate was independently established,which could realize real-time measurement of multi-parameters such as particle size,temperature,combustion dynamic image,gas emission information,etc.during pyrolysis and combustion process.The pressurized pyrolysis and pressurized oxy-fuel combustion characteristics of Hongshaquan（HSQ）and Wucaiwan（WCW）coals had been researched in detail,so as to have a deeper understanding and grasp of the reaction process of pressurized oxy-fuel combustion,and provided an important theoretical basis for the design of pressurized oxy-fuel combustion boilers.Pyrolysis is the first step of the combustion process.First,the chars were prepared under different pressures（0.1～1.5 MPa）and pyrolysis atmosphere（N₂、CO₂）on the pressurized concentrating photothermal apparatus.The effect of pressure and pyrolysis atmosphere on the structural characteristics and combustion reactivity of char was studied.It was found that with the increase of pressure,the char yield under N₂ atmosphere increased,but the char yield under CO₂ atmosphere decreased.The specific surface area of HSQ char prepared under CO₂atmosphere was 6.46 and 9.03 times that of char prepared under N₂ atmosphere at 0.1 MPa and 1.5 MPa,respectively.With the increase of pressure,the chemical structure of char prepared under two kinds of atmospheres tends to be more stable,so the combustion reactivity of char prepared under high pressure decreased.However,at the same pressure,the degree of aromatization of HSQ char prepared under CO₂ atmosphere was higher than that prepared under N₂ atmosphere.Under higher pressure,the gasification reaction of CO₂ with char was strengthened,and the consumption of amorphous carbon increased,which made the difference under two kinds of atmospheres more obvious.However,due to the dominant effect of the difference in physical pore structure,the HSQ char prepared under CO₂atmosphere had better combustion reactivity.The combustion reactivity of char was affected by the physical structure and chemical structure of the char.Subsequently,the effect of pressure（0.1～2.5 MPa）,oxygen concentration（21%～40%）and atmosphere conditions on the ignition characteristics and gas release characteristics of oxy-fuel combustion of coal pellet were studied.Three different ignition modes were observed under different experimental conditions,namely the heterogeneous ignition of char（mode 1）,the homogeneous ignition of volatile（mode 2）,and the homogeneous and heterogeneous ignition of coal pellet（mode 3）.With the increase of pressure and oxygen concentration,the ignition mode of coal pellet presented an evolution trend of mode 1→mode2→mode 3.The pressure value at which the ignition mode converted decreased as the oxygen concentration increased.Under O₂/CO₂=30%/70%and O₂/N₂=30%/70%atmosphere conditions,as the pressure increased,the ignition time and burnout time of coal pellet decreased,and the ignition mode presented a transition of mode 1→mode 2→mode 3,and the maximum expansion ratio of the pellet gradually increased.But when the pressure was greater than 1 MPa,the ignition time was basically unchanged,meanwhile,the ignition mode remained as mode 3,the change of ignition mode and ignition time with pressure was consistent.Compared with O₂/N₂ atmosphere conditions,the pellet burned faster due to the consumption of carbon by the gasification reaction under O₂/CO₂ atmosphere conditions.Regardless of O₂/CO₂ atmosphere and O₂/N₂ atmosphere,increased pressure could greatly reduced the NO emission in the flue gas,but the NO emission was lower under O₂/CO₂atmosphere.Finally,the influence of gas flow rate on the oxy-fuel combustion characteristics of coal pellet was studied under different pressures（0～1 MPa）.It was found that the effect of gas flow rate on the ignition mechanism of coal pellet was mainly reflected in low pressure conditions,and the ignition mechanism of pellet under high pressure conditions was mainly affected by oxygen concentration.Under low pressure conditions,with the increase of gas flow rate,the ignition of pellet would advance,while under high pressure conditions,the ignition time of pellet would increase with the increase of gas flow rate.When the oxygen concentration was low,the increase of the gas flow rate would increase the incomplete oxidation of char and increase the production of CO,which promoted the reduction of NO and CO/char.For high oxygen concentration,the increase of the flow rate would correspondingly reduced the residence time of flue gas,which suppressed the the reduction of NO and CO/char...