Effects of CaO and burnout on the kinetics of no reduction by beulah zap char

Abstract

The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOx from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL), its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed-bed reactor at temperatures from 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. They also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22–26 kcal/mol): in the high temperature regime, they are all higher, but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. Using a series of six NDL chars, the effect of char burnout level on the reaction of NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area (determined by CO2 uptake at 573 K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in the reduction process. The correlation of rate with CaO surface area is quantitative and also holds for the three char types (NDL, NDW, and NCa) in the low-temperature regime. It does not hold for the three char types in the high-temperature regime.