Experimental Study On Pressurized Oxy-Fuel Combustion And Nitrogen Conversion Characteristics Of Coal/Biomass And System Simulation

Oxy-fuel combustion technology is widely recognized as one of the most technically feasible large-scale carbon capture technologies for coal/biomass fired power plants.However,conventional oxy-fuel combustion technology encounters several challenges,including the massive energy consumption for oxygen production,furnace negative pressure leakage which reduces the purity of captured CO₂,and an evident increase in energy consumption for compression and purification processes.Pressurized oxy-fuel combustion technology can overcome these disadvantages and is considered to be a more advantageous new generation of oxy-fuel combustion technology.Currently,pressurized oxy-fuel combustion power generation technology is still in its infancy,with research predominantly focused on coal and coke,and rarely involving biomass.There are considerable research gaps that require to be filled regarding the characteristics of coal/biomass mixed combustion under pressurized oxy-fuel conditions and nitrogen transformation.Based on these results,the present study conducts research centered around the characteristics of coal/biomass pressurized oxy-fuel combustion and nitrogen transformation as well as the simulation of pressurized fluidized bed oxy-fuel combustion systems,which can provide relevant references for the industrial application of pressurized oxy-fuel combustion technology.Firstly,the combustion characteristics of pulverized coal in a pressurized oxy-fuel atmosphere were investigated using a pressurized thermogravimetric analyzer,whereas the reaction mechanism of pressurized oxy-fuel combustion was analyzed from the perspective of apparent reaction kinetics.It was observed that the increasing of pressure from 0.1 MPa to 0.5 MPa had a significant promotional effect on combustion of pressurized coal,whereas the promotion effect weakened when the pressure is further increased to 1 MPa.Elevating the pressure and oxygen concentration a shift in the ignition mode of coal from heterogeneous to homogeneous ignition.Substituting CO₂ for N₂atmosphere increased the ignition temperature of the coal and decreased the combustion rate.The apparent activation energy of the coal combustion reaction increased with both pressure and oxygen concentration.The most probable mechanistic function for the pressurized oxy-fuel combustion reaction of pulverized coal was deduced by Malek method.The findings indicated that the mechanism functions of pressurized oxy-fuel combustion reaction varied as the reaction progressed.The rise of pressure and oxygen concentration both correspond to the higher chemical reaction order from F1 to F3.A fixed-bed pressurized oxy-fuel combustion system capable of rapid feeding under high temperature and pressure conditions was designed and constructed autonomously,which was used to systematically investigate the effects of fuel type,pressure,biomass blending ratio,and temperature on characteristics of coal/biomass oxygen-enriched combustion and nitrogen transformation under high heating rates.It was found that pressurization increases the combustion rate of both coal and biomass.When the pressure was increased from atmospheric to 0.5 MPa,the ignition time of coal and biomass was advanced,with the most noticeable improvement in combustion rate was observed under pressurization.When the pressure was beyond 0.5 MPa,the promoting effect of pressurization on coal ignition and combustion rate weakened.Compared to coal,biomass combustion was more significantly affected by pressure,aligning with the conclusions of pressurized thermogravimetric experiments,suggesting that the influence of pressure on the coal/biomass combustion characteristics is consistent at both low and high heating rates.Pressurization inhibited the emissions of NO and N₂O,and promote the emission of NO₂,while the total nitrogen oxides emissions demonstrated a significant decreasing trend with increasing pressure.The inhibitory effect of pressurization on biomass nitrogen oxides emissions was greater than that on coal.Increasing the biomass blending ratio inhibited the emissions of NO and N₂O in the combustion flue gas,and promoted the emissions of NO₂,while the total nitrogen oxides emissions demonstrated a decreasing trend with increasing blending ratio,and the total nitrogen oxides production reached lowest peak at a 60%blending ratio.In terms of the synergistic effect of co-firing,the experimental nitrogen oxides generations from mixed fuels were all lower than the theoretical values,indicating that the addition of biomass can suppress the emissions of nitrogen oxides.The raising of temperature promoted the emissions of NO and inhibited the emissions of NO₂ and N₂O in the pressurized oxy-fuel combustion flue gas,while the total nitrogen oxides emissions demonstrated a decreasing trend.As the temperature increased from 750℃to 900℃,the reduction rate in total nitrogen oxides emissions after blending 20wt%biomass was increased from 16.4%to 28.4%,compared to coal combustion alone,indicating that the higher temperature results in a more significant inhibitory effect of blending biomass on total nitrogen oxides emissions from coal.Finally,a circulating fluidized bed combustion power system with pressurized oxy-fuel combustion was further designed on the basis of a 660 MW supercritical circulating fluidized bed power plant,and a full-process simulation was carried out using Aspen Plus to analyze the performance and energy consumption of each subsystem under different pressure conditions.The ASU adopted an air-cooled dual-column air separation process with a full low-pressure external compression cycle,where the specific oxygen production energy consumption was 0.222 k Wh/kg.The CPU utilized a multistage compression with intercooling and dual flash tank process,achieving a final CO₂ capture rate of 98.38%.With the increase in pressure,the energy consumption of the ASU demonstrated a diminishing growth rate,and the energy consumption of CPU gradually decreased,while the volume flow of recirculated flue gas significantly decreased,and the energy consumption of the recycle fan reduced.When the pressure exceeded 0.5 MPa,the latent heat of water vapor in the flue gas could be utilized by the system,and the heat recovered from the tail-end flue gas demonstrated an increasing trend with higher pressures.The pressurized oxy-fuel combustion system achieves the highest net efficiency of electricity generation within the operating pressure range of 1.5～3 MPa.Considering that higher operating pressures greatly increase the equipment investment costs and operational complexities,a comprehensive comparison reveals that 1.5 MPa is the optimum operating pressure,with a net efficiency increase of 1.76%compared to the atmospheric pressure system.