Grate

Grate-type incinerators are suitable for large irregular-shaped wastes, which can be supported on a stationary or moving alloy grate that allows air to pass through from underneath into the waste. The primary furnace is followed by a secondary combustion chamber, where additional air and fuel are added, to ensure complete destruction of all toxic emissions. These types of incinerators are widely used for the incineration of solid mixed municipal waste (MSW), but can also be applied to sewage sludge, clinical waste, and commercial and industrial waste. They generally have limited application for hazardous waste incineration due to the high temperatures required in the chamber which could have adverse effects the structure of the grate.

Components of Grate Incinerators

Grate, furnace and the heat recovery stages of an example MSW incineration plant. All rights reserved.

Grate incinerators typically contain the following components:

• Waste feeder
• Incineration grate
• Bottom ash discharger
• Incineration chamber and boiler
• Incineration air feeding
• Auxiliary burners

Waste Feeder

The waste is discharged from the storage bunker into the feeding chute by an overhead crane, and then fed into the grate system by a hydraulic ramp or another conveying system. The filling hopper is used as a continuous waste supplier. It is filled in batches by the overhead crane. As the filling hopper surface is exposed to great stress, materials with high friction resistance are selected such as boilerplates or wear-resistant cast iron.

The junction between the lower end of the filling chute and the furnace consists of a dosing mechanism. The dosing mechanism may be driven either mechanically or hydraulically and its feeding rate is generally adjustable. Different construction methods have been developed for the various types of feeder systems:

• Chain grates/plate bands
• Feeder grates
• Variable taper feed chutes
• RAM feeders
• Hydraulic Ramp
• Feed screws

Incineration Grate

The roles of the incineration grate include loosening the materials to be incinerated and transporting the materials to be incinerated in the furnace. The grate requires good distribution of the incineration air into the furnace. A primary air blower forces incineration air through the small grate layer openings into the fuel layer. More air is usually added above the waste bed to complete combustion. The residence time of the wastes on the grates generally does not exceed more than 60 minutes.

Some fine material (referred to as siftings/riddling) may fall through the grate which gets recovered in the bottom ash remover or is recovered separately. It can be recycled to the grate for repeated incineration or removed for disposal. If the siftings are recirculated in the hopper, care is taken not to ignite the waste in the hopper.

Grate cooling is carried out to control metal temperatures and so improve grate life. The cooling medium can be air or water or other liquids such as oils. The heat absorbed by the liquid cooling medium can be used in the incineration process or supplied to an external process.

Bottom Ash Discharger

Example of a ram-type bottom ash discharger. All rights reserved.

The bottom ash discharger is sued to cool and discharge solid residue (bottom ash) that forces at the end of the grate. As well as this, the discharger serves as an air seal for the furnace, as it prevents flue-gas emissions and uncontrolled air entry into the furnace.

Water is commonly used to extract the bottom ash and bulky products via water-filled ram-types constructions and belt conveyors. The water used for cooling is separated form the bottom ash at the exit and can be recirculated to the ash discharger. The bottom ash can also be discharged in a dry environment by operating a ram-type discharger without water. In this case, the furnace is air sealed by piling up the bottom ash inlet section. Cooling of the bottom ash is then achieved by air without increasing the temperature of the discharger.

Incineration Chamber and Boiler

Example of an incineration chamber. All rights reserved.

The incineration chamber usually consists of a grate at the bottom, cooled and non-cooled walls on the furnace sides, and a ceiling or boiler surface heater at the top. The volatile gases produced from the municipal waste are driven off and burn above the grate with only a small proportion of the actual incineration taking place on the grate. The following characteristics influence the design of the incineration chamber:

• Shape and size of the incineration grate
• Sufficient residence time for the flue-gases in the hot furnace
• Partial cooling of flue-gases by the injection of secondary air

Different furnace designs with differing direction of the flue gas and the waste flow. All rights reserved.

The detailed design of a combustion chamber is usually linked to the grate type. Its precise design demands certain compromises as the process requirements change with the fuel characteristics. In general, three different designs can be distinguished based on the flow direction of the flue-gases in relation to the waste flow: unidirectional current, counter-current and medium current.

Incineration Air Feeding

Air is added at through various inlets in the combustion chamber which is usually described as primary or secondary. Tertiary air and recirculated flue gases are used as well but are less common. Primary air is blown into areas below the grate which cools the grate bar and caries oxygen into the incineration bed. The secondary air is blown into the chamber to ensure complete incineration of any waste left. And intensive mixing of flue gases.

The main roles of the incineration air:

• Provision of oxidant
• Cooling
• Avoidance of slag formation
• Mixing of flue gases