Grate Furnaces

The grate furnace design is the most common in Sweden up to approximately 100 MW fuel, and the only one for capacities below 20 MW fuel (approximately 6 MW electricity)2. The ash has been through high temperatures, but not as high as in PF furnaces. The major part of the residues is bottom ash, and various fly ashes are a minor part.

In a PF furnace, the quantities of fly ash are so much larger than those of bottom ash that the composition of the fly ash corresponds very closely to that of the ashed fuel. With a grate furnace, however, the elements in the ash will be fractionated into several streams of residues. With a grate at more than 1,000°C, volatile elements and their compounds will concentrate in the fly ashes. If heavy metals are an issue, as in the regulations on recycling ash to forest soils, this may pose a problem with fly ash.

The bottom ash is a good road building material, and it is even better if the ash has been burned out and sintered. Even if its water absorption in laboratory tests is quite high, the road will be of good quality and will withstand freeze-thaw cycles. Because of their binding properties, the fly ashes are good building materials for roads. Tests have shown that up to 50% of the cement in stope mine filling may be replaced with these ashes.

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