Reduction of NO over Wheat Straw Char

Abstract

The kinetics of NO reduction over wheat straw char in the presence and absence of CO was investigated in a laboratory scale fixed bed quartz reactor. Experiments were performed with char from both the raw straw and from washed straw. The wash procedure removed most of the content of potassium and other catalytic species in the straw. The temperature was in the range 600−900 °C, the NO concentration was in the range 50−1000 ppmv, and CO was in the range 0−5 vol %. The char from raw straw was more active than the washed char by up to a factor of 2.5, but both chars were found to be very active compared to the activity of coal chars reported in the literature. At temperatures above about 850 °C the difference in reactivity decreased between the two straw chars, probably because the catalytic species in the char from raw straw was vaporized and/or reacted to form catalytically inactive potassium silicates. The reaction order for NO was about 0.7 independent of temperature for both chars. The rate expressions for NO reduction over chars from raw and washed straw respectively are the following: −rNO = 2.06 × 105 × exp(−16180/T) × mol kg-1 s-1, and −rNO = 9.53 × 104 × exp(−15950/T) × mol kg-1 s-1. The addition of CO resulted in an immediate increase in the rate of NO reduction, but the rate decreased over time and could in some cases stabilize at a level similar to that when CO was absent. This effect is not understood in detail. A reaction mechanism for reduction of NO over char is proposed and discussed.