A REVIEW OF THE KINETICS OF THE NITRIC-OXIDE CARBON REACTION

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 additi on or as natural inclusions (i.e. in coals). It is concluded that ther e generally exist three distinct reaction regimes, defined in terms of reaction temperatures. There is a near-ambient-temperature chemisorpt ion regime, in which steady gasification is not possible. In this regi me, both reversible and irreversible chemisorption processes are obser ved. A low-temperature gasification regime is observed at temperatures above similar to 500 K, but below similar to 1000 K. A high-temperatu re gasification regime is observed above this transition temperature. The low-temperature gasification regime is characterized by low activa tion energies, and the high-temperature regime by high activation ener gies. These two distinct gasification regimes however are not observed in all cases, and there is as yet no clear understanding of what dete rmines this. In the reaction of pure NO with carbons, the order of rea ction with respect to NO appears to be near unity in the gasification regimes and two in the low-temperature chemisorption regime. The appar ent order in gasification is strongly influenced by the presence of CO in the reactant gas stream, and in certain reaction configurations th e CO may be self-generated. In the presence of CO, the apparent order with respect to NO decreases, and there are several reports of apparen t fractional order. Other oxidizing gases (e.g. O-2, H2O) can also inf luence 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 can not be reliably predicted from the annealing behaviour of the carbons towards other oxidizing gases (i.e. O-2) Reaction rates of NO with car bon can also be influenced by the presence of mineral matter. There is no general agreement on the quantitative importance of mineral cataly sis in the reaction of carbons with NO. (C) 1997 Elsevier Science Ltd.