Combustion Characteristics of Biomass Residues and Biowastes: Fate of Fuel Nitrogen

Biomass residues and wastes will have an important role in the future energy mix, Residues from agriculture or industrial processes are usually cheap, widespread, and continuously produced, and their heating value is comparable to that of wood. However, many of these materials are rich in proteins and, thus, nitrogen. Therefore, to be able to exploit these materials in an efficient way, fundamental research on their nitrogen chemistry is essential. In the work presented here, the release of gaseous compounds from five biomass residues from different processes was tested in a bench-scale single-particle reactor. The tested fuels were dry distiller's grains and solubles (DDGS), palm kernel cake (PKC), rapeseed cake (RC), fermented sewage sludge (FSS), and chicken manure (CM). All of these materials have sufficiently high heating values and are either already used or projected to be used soon in industrial-scale boilers. The setup allowed us to study the total C and N release and the nitrogen partitioning between volatiles and char. Release profiles of CO, CO2, and NO for each fuel were measured at 800, 900, and 1000 degrees C and 3 and 10 vol % O-2. DDGS, PKC, and RC showed similar release profiles, while FSS and CM reacted faster because of a lower content of fixed C and faster char oxidation because of the catalytic effect of the (high) inorganic content. The maximum amounts of carbon dioxide released from these fuels, 80-100 g of CO2/MJ, are similar to the levels for coals. The value of total N released as NO was minimum for FSS, 0.3 g of N/100 g dry basis (db)., equal to around 0.5-0.6 g of N/100 g db for DDGS. PKC, and CM, and maximum for RC, equal to 0.8 g of N/100 g db. The total fuel N to NO conversion showed a trend similar to what was reported in past studies: the fuel with the lowest amount of fuel N, PKC in this study, released the most of initial N as NO, while the fuel with the highest fuel N content, CM, presented the lowest conversion. From the current analysis, it appears that under mild conditions, at 800 degrees C, all of the fuels, except FSS, released carbon and nitrogen proportionally. FSS released most of its nitrogen during devolatilization at all temperatures, while for the other fuels, the amount of volatile N increased significantly with the temperature. The data presented here seem to indicate that the large amount of fuel N contained in the tested fuels could resort in enhanced thermal De-NOx reactions, which, with the correct operational conditions, could keep NO emissions below legal limits even without additional gas-cleaning equipment.