Pyrolysis and combustion of peat and wood as single particles and as a layer

Abstract

Studies were made of the reactivity of peat and peat-bound nitrogen during pyrolysis and combustion. Nitrogen oxides react with atmospheric humidity in the same way as sulphur oxides, forming corresponding acids. Scandinavian peats can contain up to 3% nitrogen and their sulphur content seldom exceeds 0.5%. The data were completed with results from birch chips in order to compare peat with wood. Conditions in the two laboratory-scale combustion reactors, which were continuously weighed, resembled mostly conditions in grate-based boilers, but critical factors such as gas composition, gas flow-rate and temperature could be much more precisely controlled than in full-scale boilers. No work on the tendency of peat nitrogen to form nitrogen oxides under similar conditions has been published previously. Pyrolysis experiments with three peat types indicated that the concentration of nitrogen in the solid residue increased between 573 and 773 K. The burning rate was sensitive to changes in oxygen content at low temperatures and the evolution profiles of NO and CO 2 were very similar during flame combustion in the reactor for single particle combustion, which simulated a single fuel particle moving on a grate. In layer combustion, which simulated the moving volume unit of a fuel layer on a grate, the burning rate was also sensitive to the temperature and oxygen content in the primary air blown through the fuel layer. In contrast to peat the mass loss rate of burning, room-dried birch could not be raised by increasing the primary air flow-rate in the range 0.1–0.3 m/s. The degree of conversion of fuel nitrogen to NO was about five times higher in single particle combustion (20–37% with peat), where much excess air was available, than in two-stage layer combustion (4–6% with peat), where the amount of excess air was effectively minimized. The degree of conversion with birch was about double that with peat but, owing to the low nitrogen content in birch, the concentration of nitrogen oxides was lower in the flue gases in birch combustion. The results will assist in the development of grate-based boilers. In the experiments where two-stage air feeding was used the conversion range of fuel nitrogen to nitrogen oxides with sod peat reached 4–6%, corresponding to a concentration of 100–150×10–4% nitrogen oxides per 10% CO 2 in flue gases, which is roughly half of the concentration in full-scale grate combustion of sod peat.