Nitrogen in Biomass Char and Its Fate during Combustion: A Model Compound Approach

The emission of nitrogen oxide (NOx) from biomass combustion is still a concern, even though the nitrogen content of biomass in general is relatively small. To elucidate the pathways of fuel N conversion during combustion processes, many studies have focused on the pyrolysis of amino acids and/or model compounds. In contrast, the model compound char nitrogen and its fate during combustion have not been investigated. The partitioning of nitrogen between the volatiles and char and the type of nitrogen species released during char combustion are very important, because volatile N is better controlled by low NOx burners than the nitrogen retained in the char. In this study, low heating rate biomass model chars were prepared from a combination of either cellulose or glucose and five of the amino acids most commonly found in terrestrial biomass: L-proline, L-glutamine, L-histidine, L-asparagine, and L-tryptophan. The chars were characterized by X-ray photoelectron spectroscopy (XPS), and the partitioning of nitrogen was also calculated Furthermore, the impact of protein nitrogen on emissions of nitrogen species from combustion processes was investigated by thermogravimetric analysis-mass spectrometry (TGA-MS) combustion tests, which were used to calculate the char combustion kinetics and conversion of char nitrogen to different nitrogen containing species. Results indicate that there may be a correlation between the proportion of pyridinic N and pyrrolic N (and/or possibly some quaternary N) in the char and the N precursor. However, further correlations to the N species formed during the char combustion to the N functionalities in the chars were not obtained and merit further investigation. Finally, the char yields obtained and N partitioning of the model chars are comparable to those reported in the literature for biomass chars.