Effect of fuel composition on the conversion of fuel-N to nitrogen oxides in the combustion of small single particles

Abstract

Older fuels produce more N₂O in fluidized bed combustion than younger fuels. In this study the effect of fuel composition on the formation of nitrogen oxides was examined with solid fuels and model compounds at 1073 K, the temperature typical of fluidized bed combustion. Differences in the release of HCN and NH₃ were investigated under pyrolysis conditions, and the formation of nitrogen oxides (NO, N₂O and NO₂) was determined in combustion. All experiments were performed in an entrained flow reactor. HCN and NH₃ were determined by ion selective electrodes, and HCN was also measured by standard method. Nitrogen oxides were detected and measured with an Ff-IR spectrometer. The experiments showed fuel oxygen to play an important role in the formation of NH₃ from fuel-based HCN during pyrolysis stage and, therefore also in the formation of N₂O. As the fuel-O/fuel-N ratio increased, as it does with younger fuels, conversion of HCN to NH₃ was enhanced and the ratio of NH/HCN in the pyrolysis gases increased. In model compound studies, phenolic OH-groups in particular were found to increase the conversion of HCN to NH₃. The importance of the fuel-0/fuel-N ratio has also been demonstrated in studies carried out in pilot scale combustors. It is proposed that, with older fuels, HCN remains inactive during pyrolysis stage and is able to reach the combustion zone, where it readily can oxidize to N₂O. In low rank fuels, where the oxygen concentration is higher, there are more oxygen-containing radicals available to convert fuel-based HCN to NH₃. Moreover, in lower rank fuels the aromaticity in the fuel is lower and the tars lighter, which facilitates the release of nitrogen. Thereby, more HCN is released during early pyrolysis, and reactions between HCN and oxygen-containing radicals, mostly OH, to produce NH₃ become more probable. It is particularly this fast volatile-N which has been found to evolve as NH₃ during pyrolysis. In the combustion zone NH₃ reacts mostly to NO. The formation of NO₂ was insignificant in the combustion conditions studied in this work.