Effect of CO2 on oxy-fuel combustion of coal-char particles in a fluidized bed: Modeling and comparison with the conventional mode of combustion

Abstract

A char combustion model is developed to study the effect of CO2 on the combustion of coarse char particles under oxy−fuel conditions in a fluidized bed (FB). It is a transient one−dimensional model, taking into account the heat and mass transfer from the bed to the particle and the heterogeneous combustion and gasification of char. The model shows good ability to predict the char temperature history measured in our previous work for different combinations of O2/CO2 and O2/N2 with various coal types. Simulations are carried out to establish the role of CO2 in oxy-fuel conversion at different O2 levels, particle sizes, and bed temperatures. The model is used to analyze the relative contribution of carbon in the char consumed by CO2 (gasification) and O2 (combustion), as well as the differences of the peak temperatures and the burnout times in O2/CO2 and O2/N2 for char particles in a commercial FB combustor. The results indicate that the conversion of coarse (mm size) char particles in an oxy-FB is controlled by the diffusion of O2 both in the O2/CO2 and O2/N2 case. The burn−out time decreases with the bed temperature also in both cases.. The lower O2 diffusion rate in CO2 compared to N2, is the main reason for the longer burnout time and lower peak temperature found using O2/CO2 at bed temperatures of 1073-1173 K. In that temperature window, the contribution of the CO2 -char gasification is limited, being notable only at high bed temperature in O2/CO2, e.g. 1223 K. In such high temperature conditions (rarely expected to be found in commercial coal FBC) the predicted burnout time of a lignite char-particle becomes shorter in O2/CO2 than in O2/N2.

Effects of CaO and CaCO3 on Heavy Metal Capture in Bottom Ash during Municipal Solid Waste Combustion under a CO2/O2 Atmosphere

“…The volatilizations of Pb and Cd are more sensitive to a change in the temperature, and the results are also in good agreement with those of previous studies. − The residue rates of Zn did not decrease as dramatically as those of Pb and Cd and still reached 60% at 1000 °C. It is well-known that CO 2 has a higher specific heat capacity than N 2 ; thus, to some extent, the high concentration of CO 2 would delay the ignition of the MSW and lower the combustion temperature, thus suppressing the volatilization of Pb, Cd, and Zn. , …”

The role of limestone during fluidized bed oxy-combustion of coal and biomass

“…The investment during the last few years has allowed demonstration-scale facilities to be commissioned in which the feasibility of oxy-firing has been proved [4][5][6][7]. Nevertheless, important research efforts are still ongoing concerning its efficiency, emissions and fuel-related issues [8][9][10][11][12].…”

Experimental study of a single char particle combustion characteristics in a fluidized bed under O2/H2O condition

“…Two-color pyrometry and thermocouples are two kinds of in-situ temperature diagnostic techniques, which were widely used to study fuel particle combustion [6,[13][14][15][16][17][18]22]. The thermocouple can accurately measure the particle temperature, but it restricts free movement of the fuel particle in the dense bed, even if the thermocouple is very small [23].…”

“…Also, the specific heat capacities of CO 2 and H 2 O are higher than that of N 2 , which is supposed to bring a higher thermal convection in h total . Finally, the oxidation of char in a fluidized bed can be considered as external diffusion control [15,18,20,31]. At 800 °C, the D O2 in N 2 , H 2 O and CO 2 are 1.96 cm 2 /s, 2.51 cm 2 /s and 1.59 cm 2 /s, respectively, and the higher D O2 means greater v oxi and T p .…”

“…Scala et al [15,16], Bu et al [17,18], and Saucedo et al [19] have studied char particle combustion characteristics in oxy-fuel fluidized bed reactors, either by experiment or simulation, and found that the gasification of char (C+CO 2 =2CO, +172 kJ/mol) is not negligible, which becomes more significant at high temperature. Roy and Bhattacharya [20] carried out combustion tests for oxy-fuel fluidized bed combustion, and found that the addition of steam increased the char particle temperature relative to the bed temperature, and the carbon conversion rate increased with increasing steam concentration.…”