Types of Incineration Based on Tehnological Features      <pdf version>

Martin system according to which the furnace has a grate that moves to and fro, able to push the burning waste back to the loading zone. The burning waste is mixed there with freshly loaded one, which enables thorough drying. Since the grates are usually large, the incineration residue is rapidly cooled and automatically falls into the slag receiver in the next cycle.

Düsseldorf system: the grate is composed of sloping rolls, or drums, arranged in a cascade-like way. Waste is slowly turned and shifted, falling from one drum onto another. Gasses resulting from the combustion process move in the direction opposite to the movement of the waste in order to dry them; light gasses are ignited. Additionally the air blows from below the grate to cool it and to regulate the combustion process. The system allows to modify the air intake for each drum separately.

Von Roll system depends on several grate blocks arranged in a cascade. Waste loaded on the feeder is at first pushed on the drying grate and then on the combustion grate; slag and ash fall upon a post-combustion grate. The last grate is equipped with a poker that rakes the slug and allow light ash to separate, so that it can be post-combusted.

Fluidized bed furnaces are used for destroying sediments and solid or liquid waste of the same or similar composition, size and energetic value. Due to the fast progressing corrosion of the furnace and frequent blockage of the air intake, installations of this type are not as popular as other systems for thermal neutralisation of waste. However, they serve to destroy some sorts of industrial waste or to incinerate RDF, that is fuel produced from pressed homogenous communal waste.

The furnaces have the shape of a steel vertical cylinder covered with heat-resisting material. They are equipped with a bed of hot sand remaining in a state of constant fluidization. The air that ensures swelling and fluidization of the sand bed up to 100% of the primary volume is pumped, after preheating, under pressure of 0.2-0.3 atmosphere through a nozzle at the bottom of the furnace. The waste is mixed into the sand or it is loaded from the upper part of the furnace. The temperature of the bed is limited by the temperature of the slag melting point and it is no higher than 1000 °C.

Rotary furnaces are used mainly for destroying industrial and toxic waste. Liquid waste, gas products of organic processes, sludge, paste and solid combustible substances of a low ignition temperature are burnt most often in this type of furnaces. Due to high operating costs the installations are hardly used for municipal waste neutralisation. Typical examples of this method are rotary furnaces used in cement kiln.

The rotary furnace is a cylindrical container covered with heat-resisting material. It rotates around a slightly inclined horizontal axis with the speed of 1-5 revolutions per minute. Solid waste is loaded at the higher end of the cylinder and it slides down inside along the container. Liquid waste and the additional fuel are injected into the main chamber or into the secondary ones. Flue gas can remain in the chamber for not longer than 2 seconds with the 10-250% of excess air. Usually additional burners are used to ignite and support organic substance combustion. Gasses arising during the process travel from the first chamber to another, a post-combustion chamber. There, in a high temperature and with a considerable amount of excess air, post-combustion of organic substance contained in the gas takes place. The usual temperature in the rotary furnaces is 1300 °C but it may reach even to 1600 °C.

Shelf furnaces are suitable to incinerate waste with a large content of water. They are of cylindrical shape, covered with heat-resisting material. Inside the furnace there are horizontal shelves in amount of 4 to 14. Each shelf is provided with a horizontal arm pushing the waste from the loading zone to the place from where the waste is dumped on the following shelf and then to the point at which ash is unloaded. The method ensures drying of the waste, its combustion and, finally, cooling of the ash. The combustion products and air are moving in the direction opposite, or counter-current, to the movement of the waste.

The shelf furnaces are more efficient when working continuously. Their start-up requires a long time - up to 24 hours. The temperature in the combustion chamber cannot exceed 1000 °C, that is the temperature of the slag melting point, thus a large amount of excess air is necessary.

Lately a new incineration method, called Thermoselect, has been popularised. We can distinguish here a few stages of waste treatment:

· tenfold condensation of not segregated waste by pressing;

· degassing of the condensed waste in a channel, in the temperature of approx. 600 °C;
· gasification in oxygen, in a high-temperature reactor, in the temperature of approx. 1200 °C;
· obtaining of gas, metals and mineral residue in the process of phase separation, in the temperature of 2000 °C;

· abrupt cooling of high-temperature gas in order to decrease probability of synthesis of organic compounds;

· purification of synthesis gas (acid, base and by means of active carbon filter).

Gasification is the conversion of a solid or liquid feedstock into a gas by partial oxidation under the application of heat. Partial oxidation is achieved by restricting the supply of oxidant, which is normally air. Whilst processes vary considerably, typically the gas is formed above 750°C. For organic-based feedstocks, such as most wastes, the resultant gas is typically a mixture of carbon monoxide, carbon dioxide, hydrogen, methane, water, nitrogen and small amounts of higher hydrocarbons. The gas has a relatively low calorific value, typically 4 to 10 MJ/Nm3 (the CV of natural gas is about 39 MJ/Nm3). This gas, sometimes called producer gas, can be used as a fuel in boilers, internal combustion engines or gas turbines.

Although air is usually used as the oxidant, oxygen-enriched air or oxygen can also be used. When not using air, the resulting gas, often called synthesis gas, will have a higher calorific value (typically 10 to 15 MJ/Nm3) than that formed using air due to the absence of nitrogen.
For most waste feedstocks, the gas will contain tars and particulate matter which may need to be removed or treated before the gas is suitable for combustion. The degree of this contamination will depend on the gasification technology used.

Gasification is not a new technology, although its application to waste feedstocks is still being developed. Coal gasification has been used since the early 1800s to produce town gas and the first four-stroke engine was run on producer gas in 1876.

The pyrolysis process is based on indirect heating with no access of air. Decomposition and degassing takes place in a chamber heated to the temperature of 450 - 750 °C. In an additional, or secondary, chamber supplied with air intake, gasses are post-combusted. The pyrolysis products are oil and water condensates and carbon-containing solid residue. The process is used for reduction of waste volume and obtaining of high energy value products, like gas and coke. The gas produced by pyrolysis contains steam, carbon dioxide, carbon oxide, hydrogen, methane, aliphatic hydrocarbons (C2 to C4) and primary tar. The primary tar may contain dioxins and furans. The low-temperature oven gas is composed of toxic compounds like HCl, HF, H2S, HCN and NH as well as dust with a high content of heavy metals.

The method is supposed to be more expensive than the incineration method as it requires initial preparation of waste and thorough purification of the flue gas. The residue produced in this technology is toxic and cannot be stored together with municipal waste. The residue amounts to 35% of waste input, 15% of which is carbon and several percent are organic gasses. The content of heavy metals in the residue is higher than in slag from municipal waste incinerator.

Plasma is often equated to a bolt of lightning. An arc of plasma is formed when electricity flows through a gas. Theoretically, the plasma arc transforms waste into three forms:

· Organic compounds are broken down into their respective elements, which may recombine into harmless gasses.
· Metals are melted and may be recovered, and
· Other inorganic substances are vitrified into a glassy, non-leachable, non-toxic substance.
· Gases are combusted in a secondary furnace.

Proponents of plasma technology also claim that the process differs from incineration because of the lack of a "flame" and the pyrolytic conditions of the furnace. Plasma arc furnaces are being promoted as environmentally-sound solutions for disposing medical and hazardous waste - including low-level radioactive material, contaminated sludges and incinerator ash.

Pawel Gluszynski, Western Pyromania Moves East, Waste Prevention Association, Greenpeace, 1995.Incineration in Asia: http://userpages.umbc.edu/~vbushn1/asia.html