NO x and N2O Formation Mechanisms—A Detailed Chemical Kinetic Modeling Study on a Single Fuel Particle in a Laboratory-Scale Fluidized Bed

Abstract

NO x (i.e., NO and NO2) and N2O are known as harmful pollutants. In fluidized bed combustion these are formed from the nitrogen in the fuel. To develop effective primary measures reducing the emissions, more knowledge on the mechanism of formation and destruction ongoing in fluidized beds has to be obtained. In this work, a detailed chemistry model is combined with a two-phase model for a stationary fluidized bed to calculate the emissions of a single fuel particle in a laboratory-scale stationary fluidized bed. The single particle model consists of a simple model for the H2O release during drying, a model for the volatiles composition, and a model for the nitrogen chemistry during char combustion. The detailed reaction mechanism consists of a homogeneous part, heterogeneously catalyzed reactions on the bed material, and radical recombination reactions on the solids’ surface. The results confirm that devolatilization and char combustion are of nearly equal importance for NO and N2O formation. During devolatilization, NO is formed from HCN and NH3, while N2O is formed almost exclusively from HCN. During char combustion, NO is mostly formed by heterogeneous oxidation of char nitrogen, while N2O is formed from homogeneous oxidation of HCN. On the other hand, there is also a back coupling of NO on the homogeneous burnout of the carbon containing species, by sensitizing the oxidation of CH4.

N2O decomposition over the circulating ashes from coal-fired CFB boilers

“…The slope of each line represents the reaction order v. The correlation shows, for all kinds of tested bed materials, the reaction order is close to unity, i.e., v = 1.0. The result is the same as that for the homogeneous thermal decomposition, which was found to be quite close to a first-order reaction [1,[18][19][20].…”

Experimental investigations of the conversion of fuel-N, volatile-N and char-N to NOx and N2O during single coal particle fluidized bed combustion

“…However, most of NOx is formed during char combustion for both two types of coal. This is because most of NOx is formed by heterogeneous oxidation of char-N during char combustion, while N 2 O is formed by the homogeneous reaction of HCN [23].…”

Thermal valorization of footwear leather wastes in bubbling fluidized bed combustion

“…It is well known that the ratio of HCN and NH 3 concentrations formed during fuel pyrolysis is very important for further fuel-N conversion, as these are the main precursors of nitrogen oxides. During devolatilization, NO is formed from HCN and NH 3 , while N 2 O is formed almost exclusively from HCN (Kilpinen and Hupa, 1991;Pels, 1995;Lö effler et al, 2001). Generally, in coal combustion, NH 3 is considered to be the most important volatile N-containing species in pyrolysis since measurements showed that NH 3 is usually present in higher concentration than HCN in the combustion chamber (Å mand, 1991).…”