Principles of Biomass Pyrolysis with Emphasis on the Formation of the Nitrogen-Containing Gases HNCO, HCN and NH3

Doctoral thesis, 2003

This thesis investigates some aspects of the pyrolysis and combustion of biomass. The emphasis of the work is on the formation of nitrogen-containing species under pyrolysis. Since the low nitrogen concentrations in biomass make identification of nitrogen-containing gases from biomass difficult, proteins have been used as model compounds for the nitrogen in wood. Apart from HCN and NH3, HNCO has also been observed to be a pyrolysis product from wood bark as well as from all protein model compounds used. The temperature was found to have a marked impact on the distribution between the nitrogen-containing species. The temperature influence on the selectivity between HNCO and HCN was similar for the proteins and for cyclic amides, which are believed to be intermediate pyrolysis products from proteins. The results in this work indicate that the proteins in biomass pyrolyse without interaction with the other constituents of biomass. Hence, experiments with model compound proteins with amino acid compositions similar to that found in the biomass of interest will give the same pyrolysis products as the biomass itself. The advantage of model compounds over biomass is that the pyrolysis products are easily quantified with high accuracy. The effects of particle size and reactor temperature on the devolatilisation and char combustion of biomass pellets in a fluidised bed have also been investigated, as have the effects of particle size, form of biomass, final temperature and heating rate on the shrinkage and fragmentation behaviour of biomass particles under pyrolysis. The findings are expected to contribute to the fundamental understanding of, and lead to improved modelling of, the processes involved in pyrolysis and combustion in general, and of nitrogen oxide reduction in particular.