From O2/CO2 to O2/H2O combustion: The effect of large steam addition on anthracite ignition, burnout and NOx formation

Steam-moderated combustion has been proposed to supress flue gas recycling in oxy-fired units, but the influence of replacing CO2 by H2O has to be deeply studied. In this paper, oxy-fuel combustion of anthracite with large steam addition has been experimentally characterized, and main results are discussed as concerns the influence upon the ignition temperature, the burnout and the NOx formation. The tests have been carried out in an electrically-heated entrained flow reactor for a set of O-2/CO2 and O-2/H2O/CO2 atmospheres, with steam addition up to 40% vol. The results show that ignition temperature diminishes when steam is added in low rates (maximum decrease of 16 degrees C), but the trend is reversed for the higher steam concentrations (maximum increase of 18 degrees C). The effect of steam addition on coal burnout rates is more significant for the 21% vol. O-2 atmosphere, with a decrease of 2.2-5.3 percentage points, and almost negligible for the 35% vol. O-2. An outstanding reduction of NO specific emissions is detected when adding H2O, with decrements ranging 28-45% compared to the dry conditions. The transition from O-2/CO2 combustion to O-2/H2O combustion barely affects the anthracite conversion but significantly diminishes NOx formation rates.