Novel Assumption about the Mechanism of Catalytic Gasification: On the Basis of the Same Catalytic Effect of Alkali between C–CO₂ and Fe–CO₂ Reactions

Abstract

Catalytic gasification with alkali addition is one of the novel gasification technologies, and traditional catalytic gasification mechanisms are based on the alkali–C interaction. With the study of the catalytic performance of alkali metals in Fe–CO2 and C–CO2 reactions, on the contrary, the catalytic effect is attributed to CO2 gas activation on the alkali surface. On the basis of the same catalytic effect of alkali catalysts in C–CO2 and Fe–CO2 reactions, a new catalytic gasification mechanism is proposed in the paper. The catalytic mechanism is as follows: first, the activation and ionization of CO2 are improved on the alkali surface, and CO2 converts to CO and O* on the alkali surface. O* attacks the carbon matrix and Fe, and a higher activity of O* can be obtained by activation of CO2, thus speeding the CO2 reaction with the carbon matrix and Fe. The catalytic effect of alkali catalysts during C–CO2 and Fe–CO2 reactions originates from the activation of CO2 on the alkali surface. This proposed mechanism of catalytic gasification gives new insight for the process of catalytic gasification.

Potassium-Induced Phenomena and Their Effects on the Intrinsic Reactivity of Biomass-Derived Char during Steam Gasification

“…During gasification, char undergoes structural changes, such as generation of new pores, enlargement and coalescence of existing ones, particle fragmentation, ,, etc., all of which influence the specific surface area, porosity, and pore size distribution ,, and affect the accessibility of the oxidizing agents to the inner particle during the char conversion. , When it comes to minerals, alkali and alkaline earth metals (AAEMs) are of importance. − ,,, For biomass-based feedstocks, among the inherent AAEM species, potassium (K) is of greatest interest , followed by calcium (Ca), which is typically found as a carbonate or an oxide. − The importance of potassium lies in the fact that it enhances the gasification rate by forming active potassium-oxygen complexes. − The role of Ca is however unclear with literature reporting (a) enhancement of pore structure development for coal chars during gasification , and (b) inhibition of potassium deactivation ,, and hence gasification rate promotion, especially in the presence of steam, plausibly due to the formation of Ca-K active compounds. , Nevertheless, the catalytic activity of Ca is manifested at early stages of char conversion ( X < 0.4), whereas K enhances the reaction rate at the later stages, − with K being indisputably more active toward char gasification than Ca. , Other elements influencing the catalytic char gasification reactivity are iron (Fe), sodium (Na), and magnesium (Mg), but their role in the overall conversion is limited because of their low abundance and reduced (or even inhibiting) activity compared to potassium. , …”

“… 29 − 33 The importance of potassium lies in the fact that it enhances the gasification rate by forming active potassium-oxygen complexes. 34 − 38 The role of Ca is however unclear with literature reporting (a) enhancement of pore structure development for coal chars during gasification 39 , 40 and (b) inhibition of potassium deactivation 39 , 41 , 42 and hence gasification rate promotion, especially in the presence of steam, 41 plausibly due to the formation of Ca-K active compounds. 39 , 42 Nevertheless, the catalytic activity of Ca is manifested at early stages of char conversion ( X < 0.4), whereas K enhances the reaction rate at the later stages, 43 − 45 with K being indisputably more active toward char gasification than Ca.…”