A review of the kinetics of the nitric oxide-carbon reaction

Abstract

The literature on the kinetics of the NO-carbon reaction is reviewed. Data are examined both from studies in which catalysis played no role and from studies in which catalysts were present as a result of addition or as natural inclusions (i.e. in coals). It is concluded that there generally exist three distinct reaction regimes, defined in terms of reaction temperatures. There is a near-ambient-temperature chemisorption regime, in which steady gasification is not possible. In this regime, both reversible and irreversible chemisorption processes are observed. A low-temperature gasification regime is observed at temperatures above ∼ 500 K, but below ∼ 1000 K. A high-temperature gasification regime is observed above this transition temperature. The low-temperature gasification regime is characterized by low activation energies, and the high-temperature regime by high activation energies. These two distinct gasification regimes however are not observed in all cases, and there is as yet no clear understanding of what determines this. In the reaction of pure NO with carbons, the order of reaction with respect to NO appears to be near unity in the gasification regimes and two in the low-temperature chemisorption regime. The apparent order in gasification is strongly influenced by the presence of CO in the reactant gas stream, and in certain reaction configurations the CO may be self-generated. In the presence of CO, the apparent order with respect to NO decreases, and there are several reports of apparent fractional order. Other oxidizing gases (e.g. O 2, H 2O) can also influence the kinetics and course of the NO-carbon reaction. The thermal history of the carbon can have a significant effect on its reactivity in NO. It appears however that the annealing of carbons towards NO cannot be reliably predicted from the annealing behaviour of the carbons towards other oxidizing gases (i.e. O 2). Reaction rates of NO with carbon can also be influenced by the presence of mineral matter. There is no general agreement on the quantitative importance of mineral catalysis in the reaction of carbons with NO.