Among the endothermic gasification reactions of hydrocarbons, the speed of carbon dioxide gasification reaction is the slowest at practical operat­ing temperatures. Most advanced gasification technologies produce carbon dioxide as a component in their syngas products. The gasification using CO2 has not been popularly attempted, due to its poorer thermal efficiency and inferior energetics compared to steam gasification. However, due to the growing concerns of greenhouse gas emissions as well as the roles of carbon dioxide as a major greenhouse gas, various technologies includ­ing the capture of CO2, its reduction, utilization in carbon gasification, and conversion into other petrochemicals are actively pursued and developed. Gasification of biomass or coal coupled with CO2 management is also an environmentally prudent option.

Complete combustion of biomass or fossil fuels generates carbon dioxide. Because carbon dioxide is chemically very stable, its reactivity is limited. Therefore, the conversion of carbon dioxide into far more reactive carbon monoxide is one of the technological options, whereas the direct conversion of carbon dioxide into hydrocarbons is another. The two types of reactions are categorized under the reduction of carbon dioxide, and finding ener­getically prudent pathways for CO2 reduction is a challenge in modern fuel chemistry. The first group of chemical reactions includes the Boudouard reaction and the reverse water gas shift reaction:

C(s) + CO2(g) = 2CO(g)

CO2(s) + H2(g) = CO(g) + H2O(g)

As can be seen from Table 5.9, the temperature for Kp > 1 for the forward reactions as written to proceed for the Boudouard reaction and the reverse water gas shift (RWGS) reaction are 697°C and 814°C, respectively. Also, the RWGS reaction requires hydrogen as a reactant, which generally makes the process conversion costly.

Lee et al. [34] studied the kinetics of carbon dioxide gasification of various coal char samples for a temperature range between 800°C and 1,050°C using a unified intrinsic kinetic model and compared with the literature values obtained for various carbon, coal, and char samples. The Arrhenius acti­vation energy values obtained for the carbon dioxide gasification for these samples are shown in Table 5.10 [34].

Obtained from independent investigations by various investigators on diverse carbonaceous materials, the activation energy values for the kinetic rate equations for carbon dioxide gasification are around 60 kcal/mol or 250 kJ/mol. This high activation energy is also indicative of the nature of chemical reaction which requires a high temperature reaction to attain a

practically significant reaction rate. If we check the (E/RT) value for carbon dioxide gasification at 1,000°C, then the value becomes

E = 60,000

RT 1.987 * 1273

This (E/RT) value is within the range of the values for most industrially prac­ticed petrochemical reactions, often used as a rule of thumb. The study [34] also established that the kinetic rate of the noncatalytic carbon dioxide gas­ification of coal char at practical operating conditions, such as 900°C and 250 psi, is substantial. The rate was found to be about two to four times slower than that of steam gasification at the same T and P conditions.