NOx emission characteristics of fluidized bed combustion in atmospheres rich in oxygen and water vapor for high-nitrogen fuel

The present study examines the influence of water vapor in combustion atmosphere on NOx emission from oxygen-enriched combustion of air-dried distilled spirit lees and its char in a laboratory fluidized bed of 760 mm high and 68 mm in inner diameter. Steam was added into the fluidizing gas to vary the vapor content in the combustion atmosphere. At a combustion temperature of 950 degrees C and a steam-to-fuel mass ratio (S/F) of 0.5, the presence of water vapor reduced the NOx concentration in the flue gas for low O-2 contents (<= 30 vol.%) in the combustion agent but increased the NOx emission for high O2 contents (>= 40 vol.%). The possible causes were clarified for this shift from suppression to promotion of fuel-N conversion into NOx with raising O-2 concentration in the combustion agent. Varying temperature from 850 degrees C to 1150 degrees C resulted in a peak conversion of fuel-N to NOx in the temperature range of 950-1050 degrees C for all the tested O-2 concentrations. Increasing the O-2 concentration tended to lower the critical temperature corresponding to such a peak fuel-N conversion. Testing results also suggested that the presence of excessive water vapor in the combustion atmosphere would inhibit the release of fuel-N in the devolatilization stage and promote the formation of some reducing gases. In addition to the anticipated diluting effect, the steam addition also shortened the reaction time for homogeneous and heterogeneous NOx reduction by the reducing gases and char. It is postulated that the formation of OH radicals at high O-2 content and high temperature could cause the observed increase in the NOx emission. (C) 2014 Elsevier Ltd. All rights reserved.