Oxy-biomass Ignition in Air and Relevant Oxy-combustion Atmospheres for Safe Primary Recycle and Oxy-burner Development

Resultsfor ignition behaviour of pulverised biomass fuels in a 20 litre (L) spherical combustion chamber are presented and discussed. Four types of biomass currently used in UK utility pulverised fuel boilers have been tested for ignition behaviour in air, so at 21%v/v O2, and also, to assess relative performance under oxy-fuel combustion conditions, in a 21%v/v O2, balance carbon dioxide (CO2) balance mixture (21Oxy) and a 25%v/v O2 mixture (25Oxy) respectively. Peak pressures (Pmax) during constant volume ignition and combustion with 2500J and 5000J igniters were measured and recorded. The pressure ratios (P/R), defined as the ratio of the maximum pressure (Pmax) to the pressure at the start of ignition (P0) for each test are reported. A P/R above a threshold of 2.5 is taken as an indication of positive ignition. All four biomass types ignited nearly as readily in 25Oxy as in air at a range of fuel concentrations. Ignition was much less readily achieved in 21Oxy for all fuel concentrations and peak pressures were also generally lower. Results were more erratic with 2500J igniters compared to 5000J igniters, suggesting a relatively stronger ignition source is required with these biomass samples than with pulverised coals previously tested; this is tentatively attributed to larger particle sizes and higher moisture contents. Implications for pulverised fuel oxy-fuel combustion applications are: 1) a primary recycle (PR) stream with 21%v/v O2 would give improved pulverised fuel (PF) milling safety when compared to air firing but reduced ignitability in the burners; 2) a 25%v/v O2 primary stream would approach air behaviour in mills and burners. These preliminary results suggest that approximately 25%v/v O2 may give air-like performance in oxy-fuel pulverised coal plants using oxy-biomass.