Research On The Characteristics Of Oxy-Coal Combustion Process In A Pressurized Fluidized Bed

Pressurized oxy-fuel combustion（POFC）is considered as one of the most promising carbon capture technologies in terms of high carbon capture efficiency and low cost.However,although a large number of energy efficiency analyses have shown that an increase in combustion pressure can further increase the net power generation efficiency by 3%～5%,there are few experimental studies of pressurized oxy-coal combustion,and most of them were conducted in a pressurized thermogravimetric analyzer or pressurized tube furnace with the batch coal feeding.Almost no report about the design and development work of the POFC system can be found.The changes of combustion pressure and atmosphere make great impacts on the reaction,heat/mass transfer and flow structure,which raises new challenges for the design and operation of the POFC system.The lack of design and operation experiences of the POFC system hinders the development of the POFC technology.The pressurized devolatilization experiments and char combustion experiments were conducted in a tube furnace and a small fluidized bed reactor,respectively.A combustion model including reaction kinetics,mass and heat transfer,and energy balance was developed and validated by the experimental results.Then,a 30 k W POFC system was developed and the effects of pressure increase on the system operation characteristics and pollutant emissions were investigated.Finally,a 1 MW POFC system was designed and the process simulation on it was established.The main research content and conclusions obtained from the work are shown as follows:The pressurized devolatilization experiment and pressurized char combustion experiments were conducted in a tube furnace and a small fluidized bed reactor,respectively.The effects of pressure and atmosphere on the char produce rate,char characteristics,reactivity,burnout time and ash characteristics were investigated.The experimental results showed that the char produce rate was increased with an increase in pressure under the nitrogen atmosphere,while it was decreased under the CO₂ atmosphere.The pressure increase can reduce the burnout time of the char under both air and O₂/CO₂ atmosphere.The pressure increase had no significant influence on the ash composition,but it decreased the particle sizes of the ash.A comprehensive char particle combustion model was developed and validated by the experimental results.The simulation results showed that an increase in pressure would reduce the burnout time of char particles（diameter is millimeter level）only when the combustion was kinetically controlled or combined kinetic-diffusion controlled.Once the combustion became diffusion-controlled,a further increase in pressure would have an insiginificant effect on the char burnout time.The effects of pressure on the ignition time was closely related with the ignition mechanism.For the fuel which has very low volatiles,it was heated by the heat transfered from gas and bed materials and hence the ignition time was reduced by the pressure increase.Besides,it was found that the pressure increase significantly enhanced the char-CO₂gasification under O₂/CO₂ atmosphere,and the total carbon consumpted by gasification increased with the pressure.A 30 kW POFC system was designed and developed,and the pressure increase and atmosphere switch processes was fast and smooth.The CO₂ concentration in the dry flue gases exceeded 90%under all the tested pressures,which meets the requirements of the following carbon capture processes.The effects of combustion pressure,atmosphere and other key operating parameters on the system operation characteristics were carefully investigated.The experimental results showed that the pressure increase can reduce the ratio of unburnt carbon in the fly ash and increase the combustion efficiency.The elevating pressure was beneficial to decrease the NO_x emissions,while the N₂O emissions were increased.The pressure increase had no significant influences on the SO₂ emissions but it can increase the desulfurization efficiency of the calcium-based sorbent.A 1 MW POFC system was designed and a process simulation on it was established.The unburnt carbon ratio,oxygen/CO₂ concentration and heat load along the height of the reactor was predicted.The effects of the secondary flow input height and ratio of the primary and secondary flows on the reacotors were carefully discussed,and the optimized design parameters were proposed.