Formation of HNCO, HCN, and NH 3 from the pyrolysis of bark and nitrogen-containing model compounds

Abstract

Bark pellets have been pyrolyzed in a fluidized bed reactor at temperatures between 700 and 1000 °C. Identified nitrogen-containing species were hydrogen cyanide (HCN), ammonia (NH 3), and isocyanic acid (HNCO). Quantification of HCN and to some extent of NH 3 was unreliable at 700 and 800 °C due to low concentrations. HNCO could not be quantified with any accuracy at any temperature for bark, due to the low concentrations found. Since most of the nitrogen in biomass is bound in proteins, various protein-rich model compounds were pyrolyzed with the aim of finding features that are protein-specific, making conclusions regarding the model compounds applicable for biomass fuels in general. The model compounds used were a whey protein isolate, soya beans, yellow peas, and shea nut meal. The split between HCN and NH 3 depends on the compound and temperature. It was found that the HCN/NH 3 ratio is very sensitive to temperature and increases with increasing temperature for all compounds, including bark. Comparing the ratio for the different compounds at a fixed temperature, the ratio was found to decrease with decreasing release of volatile nitrogen. The temperature dependence implies that heating rate and thereby particle size affect the split between HCN and NH 3. For whey, soya beans, and yellow peas, HNCO was also quantified. It is suggested that most HCN and HNCO are produced from cracking of cyclic amides formed as primary pyrolysis products. The dependence of the HNCO/HCN ratio on the compound is fairly small, but the temperature dependence of the ratio is substantial, decreasing with increasing temperature. The release of nitrogen-containing species does not seem to be greatly affected by the other constituents of the fuel, and proteins appear to be suitable model compounds for the nitrogen in biomass.