The NO and N2O formation mechanism during devolatilization and char combustion under fluidized-bed conditions

Abstract

In an exclusively electrically heated, laboratory-scale, fluidized-bed compustor (MM-FBC) made of quartz glass, the formation rates of NO, N 2 O, and HCN are studied by using a high performance FT-IR spectrometer in combination with a long-path, low-volume gas cell. The emission characteristics of single, spherical fuel particles (5–15 mm in diameter) of bituminous coal, subbituminous coal, and beech wood are studied. The bed temperature is varied between 600 and 900°C. The fluidizing velocity is 0.68 m/s and the oxygen partial pressure is varied between 0.05 and 21 kPa. The bed material is silica sand with a mean diameter of 225 μm. Besides changing combustion conditions and fuel type and diameter, an iodine addition technique is applied to study the relative importance of the homogeneous and the heterogeneous chemistry during devolatilization and char combustion. Iodine addition suppresses the radical concentrations to equilibrium level and has proven as a very effective method for kinetic measurements. With a semitheoretical model, the measurement results are discussed and reaction paths are evaluated. The formation characteristics during devolatilization can be explained mainly by the homogeneous HCN oxidation and NO and N 2 O formation. NO and N 2 O formation during char combustion consists of two different paths. NO is mainly heterogeneously formed by char-nitrogen oxidation. But a volatile species, which has been identified as HCN, is simultaneously released in low concentrations. This species is homogeneously oxidized to NCO which further reacts with the heterogeneously produced NO to form N 2 O.