Thermogravimetric Experiment And Tangential Boiler Simulation Of Co-combustion Of Semi-coke And Bituminous Coal Under Oxygen-enriched Atmosphere

Semi-coke is blended with bituminous coal in oxygen-enriched atmospheres to be efficiently used as an energy source.The purpose of this paper is to deeply understand the co-combustion characteristics of semi-coke and bituminous coal under oxygen-enriched atmosphere and the combustion status in power plant boilers by means of experiments and simulations,so as to provide guidance for engineering applications.Combustion characteristics and kinetic behavior of blends under oxygen-enriched atmosphere are studied with thermogravimetric analysis.When the temperature is lower than 750?,the pyrolysis characteristics of the fuel are exactly the same in N2 and CO2 atmospheres.However,when the temperature is higher than 750?,the reaction characteristics of the fuel changes dramatically in CO2 atmosphere.Semi-coke gasifies more easily than bituminous coal in CO2 atmosphere at high temperature.The ignition temperature of each sample in 21%O2/79%CO2 atmosphere is about 15? higher than that in air.The burnout behavior of semi-coke is significantly improved when N2 is replaced by CO2 at the same oxygen concentration.Increasing oxygen concentration can improve ignition and burnout behaviors further.Differential thermogravimetry(DTG)curves of semi-coke show quite different behaviors in air and 21%O2/79%CO2 atmospheres.There are two distinct peaks in air,while in 21%O2/79%CO2.there is only one.Activation energies of blends are obtained by Starink method at four heating rates.These activation energy values are,then,used to determine the most probabilistic mechanism function of each stage by Malek method.The results show that the activation energy usually decreases with the increase of the conversion degree and the proportion of bituminous coal.The average activation energy in air atmosphere is slightly smaller than that in 21%O2/79%CO2 atmospheres.It is worth noting that the activation energies of the blends increase with the increase of oxygen concentrations.With the increase of temperature and the reaction proceeding,the reaction mechanism of the sample also changes correspondingly.When the conversion degree reaches 0.55,it seems that the most probable mechanism function is more prone to change.In addition,the Fluent was used to simulate the co-combustion of semi-coke and bituminous coal in a tangential furnace under air and oxygen-enriched atmospheres.The combustion model of pulverized coal is based on the finite rate/eddy dissipation model and char multiple-surface-reactions model.The Euler-Lagrange coupling method is used to analyze and calculate pulverized coal particles.Bituminous coal has good ignition characteristics in the furnace,and the temperature distribution is reasonable and symmetrical.With the increase of oxygen concentration,tangential high temperature distribution will appear in the strong wind area.However,the reasonable temperature field of semi-coke can not be obtained in the furnace.When the proportion of semi-coke in the co-combustion material reaches 40%,the temperature field is still reasonable and uniform,which can meet the power requirements of the boiler.When the proportion of semi-coke is further increased to 60%,the temperature in the furnace decreases obviously and the combustion characteristics deteriorate.Through the analysis of combustion field under different oxygen concentration conditions,it is considered that the effect of 30%O2/70%CO2 atmosphere on combustion is similar to that of traditional air on combustion.In 30%O2/70%CO2 atmosphere,the burnout rate of each sample is significantly improved compared to air atmosphere.The emission of NOx at the outlet of the furnace in air atmosphere is very close to that in 30%O2/70%CO2 atmosphere.Because of its low volatility and high ash content,semi-coke is difficult to ignite and burn out,so it can not be used as a single fuel for pulverized coal boilers.According to the results of this study,the combustion characteristics of semi-coke can be improved by co-combustion with bituminous coal in the O2/CO2 atmosphere to meet the combustion standard of pulverized coal boilers.