Formation of HNCO, HCN, and NH3 from the pyrolysis of bark and nitrogen-containing model compounds
Artikel i vetenskaplig tidskrift, 2004
Bark pellets have been pyrolyzed in a ﬂuidized bed reactor at temperatures between 700 and 1000C. Identiﬁed
nitrogen-containing species were hydrogen cyanide (HCN), ammonia (NH3), and isocyanic acid (HNCO). Quantiﬁcation of HCN and to some extent of NH3 was unreliable at 700 and 800C 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 applica-ble 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 NH3 depends on the compound and temperature. It was found that the HCN/NH3 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 NH3. 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.