The combustion of polymer pellets in a bubbling fluidised bed

Wastes burned in incinerators usually contain polymers, whose combustion can be associated with noxious emissions, unless the conditions are properly selected. This paper investigates how polymers burn in a fluidised bed; in fact, the combustion of a number of polymers, including several types of polyethylene, polystyrene and a polyamide, was studied in a laboratory-size, bubbling, fluidised bed, filled with quartz sand, with no external heating. Pellets of a polymer were mostly thrown into such a bed of sand, fluidised and maintained hot by a fuel-lean mixture of propane, methane or hydrogen in air, which burned soon after entering the bed. In addition, polymers were also used as the only fuel, i.e., added to a hot bed fluidised by only air. Visual observations of burning polymer pellets up to ∼240 mg were made, as well as video records obtained and the flue gas composition monitored, when the combustor was run at 800–1000 °C with 1.1–2.0 times more O2 than required for complete combustion. It is clear that a polymer burns as if its volatile content were 100%. The polymer pellet first melts at a rate controlled by heat transfer. However, the melt and the gaseous products of thermal decomposition are dispersed, albeit sometimes slowly, in a fluidised bed. Although the high U/Umf of above 10 caused some back-mixing of the gas leaving the bed and the combustion efficiency was high (assessed from O2 consumption and CO2 production), long streaks or plumes of fuel-rich gases (from each polymer pellet) did reach the freeboard, i.e., these plumes burned as transient diffusion flames at a rate controlled by mixing. By increasing the temperature and the residence times of gas in the bed and freeboard, the observed emissions of CO and hydrocarbons could be considerably reduced. The concentrations of NO were low, except when the polymer contained chemically-combined nitrogen, as in a polyamide. It is concluded that bubbling fluidised beds can be good for incinerating polymers, possibly together with other wastes.