NO Release and Reactivity of Chars during Combustion:  The Effect of Devolatilization Temperature and Heating Rate

Abstract

The study has involved the investigation of the influence of pyrolysis heat treatment temperature (HTT) and heating rate on the reactivity and the release of char-N during temperature-programmed combustion (TPC) of a set of wire mesh reactor chars in a thermogravimetric analyzer−mass spectrometer system. The gas evolution profiles are bimodal and this indicates the presence of species of different reactivity. It was found that increasing pyrolysis temperature and heating rate both produced significant variation in the reactivity of the resultant chars with the former being more influential. This is apparent from the shift of peak positions and the change in the relative intensity of the low- and high-temperature peaks of CO, CO2, and NO evolution profiles. The differences in the CO2 and NO evolution profiles observed between the Gedling entrained flow reactor (EFR) and wire mesh reactor (WMR) chars of similar heat treatment temperatures may be explained by the different extents of pyrolysis experienced by the chars. However, chars produced in the WMR with pyrolysis temperatures up to 1200 °C show little variation in the char-N conversion to NO. This is believed to be due to the highly reactive nature of the chars which give rise to a high extent of reduction of the primary product NO formed during gasification leading to a low NO/char-N ratio. Heat treatment of the chars at lower heating rates and longer soak times to temperatures in the range 1100−1400 °C lead to reduced char reactivity and higher NO/char-N ratios under temperature-programmed combustion conditions. The results are consistent with the reduction of the primary oxidation product NO on the surface and in the pores of the char.

Advanced combustor design concept to control NOx and air toxics

“…It is interesting to note that Wang and Thomas (1996) found that higher rank coals (higher %C) had a higher NO x production, also Shimizu et al found for combustion under fluidized bed conditions that higher carbon contents lead to higher conversions of char-N to NO x . This may be in disagreement with the previous discussion.…”

“…This may be in disagreement with the previous discussion. However Wang and Thomas (1996) explain their results on the basis of higher reactivity (measured as the temperature for 50% char consumption) of low rank coals and not on carbon content itself. Likewise, Shimizu et al (1992) found a correlation between surface area (not %C) and NO x production.…”

“…In recent work by Wang et al (1996), chars were prepared from two bituminous coals (high-volatile and low-volatile) by subjecting the coals to various heating rates and final Illinois #6 Pittsburgh #8 Utah Black Thunder temperatures in a wire mesh reactor. The chars were subsequently burned in an Ar/20% O 2 mixture.…”

NOx EMISSIONS PRODUCED WITH COMBUSTION OF POWDER RIVER BASIN COAL IN A UTILITY BOILER

“…Coal-fired utility (tangentially fired) boilers using 1ow-NOX burners have successfully reduced NOX emissions to the 0.35 to 0.5 lb NOx/mm Btu range. Wang et al 1996 has commented that 1ow-NOX burners have successfully reduced thermal and prompt and gas-phase, fuel-derived NOX emissions but are not very successful in reducing the char-N component. Wang et al 1996 estimate that 60 to 90% of the NOX left after installation of Iow-NOX burners originates from the char-N component.…”

“…Wang et al 1996 has commented that 1ow-NOX burners have successfully reduced thermal and prompt and gas-phase, fuel-derived NOX emissions but are not very successful in reducing the char-N component. Wang et al 1996 estimate that 60 to 90% of the NOX left after installation of Iow-NOX burners originates from the char-N component. Cai et al 1993 say the same, estimating 60 to 95% of NO originates from cnar-N when using 1ow-NOX burners.…”