Improved Performance in Fluidised Bed Combustion by the Use of Manganese Ore as Active Bed Material

Insufficient mixing between fuel and air during combustion results in carbon monoxide and unburnt hydrocarbons in the flue gases. In order to minimize these emissions commercial combustion plants are operated with an excess of air added to the furnace. This increases the heat loss associated with the flue gas and thus decreases the overall efficiency of the plant. If combustion is carried out in a fluidised bed, the mixing between fuel and oxygen could be facilitated by using an active bed material with the ability to absorb and release chemically bonded oxygen, depending on the local oxygen concentration. Such active oxygen carriers would also facilitate conversion of relatively stable fuel components such as methane inside the bed, where ignition may be hampered by thermal inertia. This technology, Oxygen Carrier Aided Combustion (OCAC), which is a spin-off technology from Chemical-Looping Combustion (CLC), has recently been proposed. In this study, the potential of using manganese ores as active bed material has been investigated. Combustion of methane in a fluidised bed was carried out with two mixtures of manganese ore and silica sand (50/50 wt.%) in a laboratory circulating fluidised bed combustor. The performance of the manganese ores as active bed materials was compared with the performance of only silica sand in the same experimental unit. The main conclusion drawn from these experiments is that it was possible to significantly reduce the exhaust concentration of carbon monoxide by replacing 50 wt.% of the bed inventory of silica sand with manganese ore. Both ores released gaseous oxygen in inert atmosphere and oxygen was present in the exhaust gas at all times during combustion. The use of the active bed materials thus increased the fuel conversion for a given air-to-fuel ratio.