Impact of temperature and fuel-nitrogen content on fuel-staged combustion with coal pyrolysis gas

At an experimental facility of the Stuttgart University, the process of fuel splitting and staging is investigated with a view to lower NO_x emissions. Raw coal is introduced into either an electrically heated, entrained flow reactor or a fluidized bed reactor where it is split into pyrolysis gas and residual char. In a following staged combustion, the remaining char or raw coal is used as primary fuel and the pyrolysis gas as reburn fuel. The test facility allows a systematic study to evaluate the effect of different parameters on NO_x reduction independently of each other. Pyrolysis experiments, combustion studies, and calculations have been performed to explain the effective NO_x reduction with coal pyrolysis gas. To compare the results and show the advantage of this process, investigations with fuel staging were carried out. With gaseous reburn fuels and residence times higher than 1.5 s, minimum NO_x emissions below 200 mg/m³ at 6% O₂ could be achieved independently of the coal type (200 mg/m³ is approximately equivalent to 100 ppmv). Reburning with coal pyrolysis gas increases the optimum air/fuel ratio from 0.75 to 0.9, yielding the same or even lower NO_x emissions. The main parameters are stoichiometry and residence time in the fuel-rich secondary zone. The results also show an increase of the reduction efficiency with increasing reburning temperatures in the reburn zone. The tar components in the reburn fuel and the fuel-N content are very important parameters in achieving excellent results when using coal pyrolysis gas. These appear to have a positive effect on the reduction of NO_x emissions. To explain this effect, the model of the perfectly stirred reactor (PSR) was employed. The modeling results agree well with the experimental investigations.