Modeling on Pore Variation of Coal Chars during CO₂Gasification Associated with Their Submicropores and Closed Pores

Abstract

When predicting the variation of pore structure during CO2 gasification of coal chars using the random pore model (RPM), it is impossible to calculate exactly the ψ parameter from the pore characteristics, which were obtained by means of N2 adsorption, such as BET surface area (denoted as N2 pore characteristics), of the char prior to gasification. The values of ψ, which were calculated from the pore characteristics of chars at various carbon conversions, should be fundamentally constant, unaffected by the conversion of the char. However, this investigation exhibited a drastic decrease of ψ at the initial stage of the gasification reaction. This phenomenon is the result of a significant increase of N2 pore characteristics, of which the starting chars are extremely small. This increase might be explained by the widening of submicropores which are undetectable through the N2 adsorption method or by the reopening of closed pores inaccessible even to helium molecules, followed by the formation of new micropores exceeding the detection limit of N2. Consequently, this study introduced the volume of submicropores and closed pores into the ψ equation as correction terms. The value of ψ at the reaction starting point was close to that at the intermediate stage of reaction, indicating that the accuracy for ψ estimation was elevated and that the submicropores and closed pores should be taken into account when using RPM.

Reactivity and Kinetic Analysis of Coal Chars Combustion under Oxy-fuel Conditions

“…Similar to the changes of YFB at 1573K, the average pore diameter of YFB increases with increasing carbon conversion at 1273K. This phenomenon is mainly related with the pore expansion and coalescence [15] . Fig.3 shows variation of pore volume with carbon conversion during O 2 /CO 2 combustion.…”

“…The correlations between surface area of YFB and carbon conversion is shown in Fig.4. It can be seen that the surface area of YFB increases at 1273K at the initial stage of reaction, this phenomenon is mainly caused by generation of a large number of new pores and micropore expansion [15] , and then decreases when the carbon conversion is higher than above 50 %, which is mainly caused by pore collapse.…”

“…Obviously, the random pore model can describe the variations of the surface area of coal char during O 2 /CO 2 combustion process. But some minor deviations still exists when the RPM predicts the experimental data especially at the middle stage of reaction, which is related with the hypothesis of structure parameter in the random pore model [6,15] . Higher temperature increases coal char combustion rate, and the pyrolysis temperature leads to a slight increase in the average pore diameter and surface area.…”

The effect of char properties on gasification reactivity

“…Although the model has received great successes in modeling gasification reactions of coal chars [5][6][7], it sometimes fails to describe the reactivity profiles of biomass chars, in which the reactivity increases with increasing conversion or exists a maximum in high conversion range. Two parameters were added in the random pore model to take inorganic element contents into account by Struis et al Modified random pore model has been recently used to investigate reactivity of biomass chars with fitting the experimental data [8][9][10].…”

Clean Coal Program Research Activities

“…They attributed this phenomenon to the closed micropores in fresh chars and the re-opening of these micropores in initial reaction. Morimoto et al (2006) calculated the ψ values at various conversions during CO 2 gasification and found that the ψ values decreased dramatically with conversion. They suggested that the calculation of structural parameter ψ should be modified.…”