Recently, to overcome climate change caused by global warming, the issue of effective alternative fuel sources has come to the fore for purposes of decreasing the use of fossil fuels and creating forms of economical new energy through technical development. However, demand for fossil fuels is expected to increase continuously. The emission of carbon dioxide caused by the use of fossil fuels can be reduced by efficiency improvement and the application of carbon capture and storage (CCS) to high-efficiency coal power generation technology such as integrated gasification combined cycle (IGCC) and clean energy through coal gasification and conversion at the same time. Coal gasification transforms the coal, the fossil fuel with the highest emission of carbon dioxide per unit of energy production, into pure and highly efficient raw materials. A number of gasifiers with a variety of abilities are in operation or under construction worldwide. Among these, it has been reviewed that the recovery of carbon dioxide is circulated in the gasifier for gasification. Although the study is still being conducted, researchers are tracing their methods to ensure whether results are accurate at the laboratory scales.
Examples of gasification reactions, which dominate the overall reaction, are CO2 gasification and steam gasification. The slowest reactions in gasification, which govern the overall conversion rate, are the heterogeneous char gasification where mass transport limitations play a more important role.
Catalytic reactions have a lower rate limiting the free energy of activation more than the corresponding un-catalyzed reaction, resulting in a higher reaction rate at the same temperature.
In this study, low rank coals were selected as Samwha coal and Drayton coal including lignite and sub-bituminous. The coals were crushed and sieved through a 74 μm (200 mesh) screen. The samples were mixed as 7 wt. % catalysts (calcium carbonate, sodium carbonate, potassium carbonate, and dolomite) to enhance gasification reactivity. The concentration of CO2 varied in the range of 10 % to 90 %. The samples were analyzed in a thermogravimetric analyzer (TGA) and gas chromatograph (GC) connected to fixed bed reactor with CO2. The effect of catalysts and carbon dioxide of low rank coal on CO2 gasification was studied to evaluate the reactivity of the char. We compared the shrinking core model (SCM), volumetric reaction model (VRM) and modified volumetric reaction model (MVRM) of the gas-solid reaction models.
Regardless of the rank of coal, the influence of catalyst increased the enhancement of carbon dioxide. The reaction rate of all the chars showed a rapid increase in the initial stage of the reaction, followed by a decrease in the middle stage. The char without catalyst did not show any significant change in the reaction rate. The reaction rate of the char with the catalyst was 5 times faster than that of raw char. The char gasification is fast in the following order: Na2CO3, K2CO3 > CaCO3, dolomite > non-catalyst.
The carbon conversion rates of K2CO3 and Na2CO3 reached a high peak symmetrically in the range of 0.3 and 0.5. On the other hand, the carbon conversion rate of CaCO3 and dolomite followed a left-sided peak, smaller than that of the previous, in the range of 0.1 and 0.3. This tendency to peak and to shift was present at all CO2 concentrations. The same tendency has been found in other studies.
Due to the change in the total reaction surface area, the reaction rate initially increased and then the reaction rate of CaCO3 and dolomite decreased gradually from the time when the reaction rate has passed the peak. That is, the surface area that reacts with the reactant inside the microstructure of the char is increased at the initial stage of the reaction, and the total surface area is reduced by the gasification of the carbon.
In general, the reaction rate of char gasification increases with an increase in the CO2 concentration. The gasification rate increased with the rise in the partial pressure of CO2 due to increases in the diffusion of CO2 molecules and adsorption on the surface of the char. while gasification rate reached a saturated value at the concentration of 70%. The reverse reaction (CO disproportionation) occurred on the alkali metal species at a lower temperature and a higher CO2 concentration, which inhibited the CO2 gasification rate. It is considered that the optimum CO2 concentration, which produced the highest carbon conversion rate, is 70% CO2. It is very likely that gasification with CO2 at high concentration could become a problem in the case of low-temperature CO2 gasification.
The kinetic parameters for the three different chars obtained at isothermal combustion derived from the three different conversion models, where R2 is the correlation coefficient. Most of the correlation coefficients are above 0.95, which indicates that a fairly good linearity of all correlations has been achieved.
The correlation coefficient values, by linear regression, of SCM are higher than that of VRM at low concentration. While the correlation coefficients values of VRM are higher than that of SCM at high concentration. The correlation coefficient values of MVRM are the highest of all the three models at all concentration. The linearity of the kinetic models was high in the order of: MVRM > SCM > VRM.