Oxy-steam combustion: The effect of coal rank and steam concentration on combustion characteristics

This paper addresses the experimental investigation of coal combustion characteristics (ignition, burnout and NO formation) under oxy-steam combustion conditions. Two coals are selected in order to compare the effect of the rank: bituminous and sub-bituminous ones. The experiments have been conducted in an electrically-heated entrained flow reactor for a set of O-2/N-2, O-2/CO2 and O-2/H2O/CO2 atmospheres, with O-2 concentrations up to 35% and H2O concentrations up to 40%. Regarding ignition, 10% H2O reduces ignition temperature (max. 16-19 K) but the trend is reversed when supplying additional steam to 25% and 40%. This behaviour is similar for both coals, with slight larger variations in the case of the low rank coal. Burnout degree of the sub-bituminous coal is barely affected by the steam concentration since all observed conversions are very high. Larger increments (up to 6.1 percentage points) are obtained for the bituminous coal, with a maximum burnout degree for the 25/35% H2O/O-2 atmosphere. A very different effect of steam on NO formation is found depending on the coal rank. Significant reduction rates are observed for the bituminous coal in comparison to the dry O-2/CO2 atmospheres, with a maximum diminution of 24% when 40% H2O replaces CO2. On the contrary, the higher volatile content in the sub-bituminous coal leads to NO increments up to 9%. For all the combustion characteristics studied, the increase of O-2 concentrations attenuates the effects caused by the steam addition.