1. Combustion

Immediately after being charged into the fluidised bed the feed material is rapidly dried, heated and thermally decomposed. [GLOSS]Volatile matter[/GLOSS] is liberated and a [GLOSS]char[/GLOSS] subsists. Depending on local flow conditions, volatiles appear in the emulsion or the bubble phase and are partly oxidised. Their combustion in the bed remains incomplete and is continued in the freeboard zone. Adequate temperature, residence time and turbulence are essential to convert them completely. Improper burning conditions result in [GLOSS]incomplete combustion[/GLOSS].

2. Products of Incomplete Combustion

The products of incomplete combustion include carbon monoxide, simple aromatics, polycyclic aromatic hydrocarbons (PAHs) and a number of oxygenated and heterocyclic compounds. Especially of concern are the polychlorinated benzenes (PCBzs), phenols (PCPs), biphenyls (PCBs), dibenzo-dioxins (PCDDs) and dibenzo-furans (PCDFs). The inherent characteristics of fluidised bed combustion enable PCB emissions to be low even before gas clean up. Control of the products of incomplete combustion is achieved with thorough mixing in the bed to obtain good combustion.

The char generally remains in the bed, to be combusted to extinction (fly ash). Some particles, however, are entrained during this shrinking-core process.

Sulphur oxides

In-bed desulphurisation is easily possible by the addition of limestone (CaCO₃) or dolomite (CaCO₃-MgCO₃) to the feed. Moreover, the ash has a sizeable neutralising power, which is most active at 850 °C, the optimal operating temperature for most fluid bed combustors. Sulphur oxides (SO₂ and SO₂) emission is not a major concern in refuse firing, since municipal solid waste is quite lean in
sulphur.

Nitrogen oxides

Thermal nitrogen oxide generation is minimal in fluid bed incinerators, because of the low operating temperatures.

It is important to mention that there is no overall minimum for CO, SOx and NOx; only a certain optimum for the sum of the emissions can be reached. For example, the increasing amount of excess air will decrease the CO content of flue gas but the NOx will be increased.

Hydrogen chloride

During co-combustion of waste as fuel, a considerable amount of HCl maybe produced, since the high Cl content of the fuel. It can be partially neutralised in the flues following the combustion chamber. For thermodynamic and kinetic reasons the removal rate is optimal at medium (450 °C) and at low (<<200 °C) temperatures.

Fly-ash, heavy metals, clinker

In a stoker incinerator part of the potential fly-ash is sintered together and reports to the [GLOSS]clinker[/GLOSS], thrown off at the end of the grate. In fluid bed units all fine ash is entrained, which definitely increases the dust load of the flue gas. The burnout is comparable in both cases. Heavy metals evaporate to a somewhat lesser extent in a fluid bed plant, because of the lower operating temperature. The clinker discharged from a fluid bed is extremely clean because of the churning action of the sand.

Keywords

Fluidised bed combustion, emissions, incomplete combustion.

Source:

[1] Technical Brief from Residua & Warmer Bulletin: Fluidised bed combustion. http://www.residua.com/wrftbfbc.html