Incineration is the process of destroying waste materials by burning it within the temperature range of 9000C – 12000C. This is a controlled combustion process for burning solid combustible wastes to gases and residue containing little or no combustible material when properly carried out. It is a volume reduction process.
When incineration is operated correctly, it reduces the waste bulk by 90%. This reduction depends upon the recovery degree and composition of materials. This means that incineration however, does not replace the need for land filling but it reduced the amount to be thrown in it. The ash will be safe to use for filling low areas of land (such as erosion site).
It is recognized as a practical way of disposing hazardous waste materials such as biological medical waste.
In the past, incineration was conducted without separating materials thus causing harm to environment. This un-separated waste was not free from bulky and recyclable materials, even. This resulted in risk for plant workers health and environment. Most of such plants and incinerations never generate electricity.
Incineration comes with a number of benefits in specific areas like medical wastes and other life risking waste. In this process, toxins are destroyed when waste is treated with high temperature.
Though incineration is still widely used in many areas (developing countries especially) it is a waste management tool that is becoming controversial for several reasons.
Read Also : Traditional Solid Waste Disposal Methods
It destroys not only the raw materials, but also all the energy, water, and other natural resources used to produce it. Some energy can be reclaimed as electricity by using the combustion to create steam to drive an electric generator.
Incineration Process
Before waste products are subjected to incineration, it is good to have such wastes sorted in the first place. Thus readily combustible materials especially dry ones will burn faster and generate fine ash whereas wet bulky materials may require being pretreated or they take longer period and energy to burn. Some recommended steps for handling incinerators include the following:
The incinerator must be fully heated up before wastes are added, requiring about 30 min or longer, depending on ambient temperature, type of fuel, fuel moisture content, etc. however, most of the 14 small-scale units surveyed in Kenya (Taylor, 2003) were not being operated in this fashion, rather, safety boxes were loaded prior to lighting.
Firewood must have a low moisture content (<15%).
Temperature monitors when not used, there will be no indication that suitable temperature have been reached. (Grey or black smoke indicates poor combustion and low temperatures)
Manual operation requires the constant presence of an operator when burning waste Dry fuels must be added every 5 – 10 min.
Flame must not be extinguished during burnings
Grates must be regularly checked and raked to keep clear.
Fig.: Diagram of an Incineration Process.
Fig.: Incineration Process Flowchart (Beitou Refuse Incineration Plant)
Table: Description of Incineration Flow Chart
| The Process Includes | ||
| 1. | Garbage receiving and treatment procedures, including: entrance odometer, dumping platform, garbage bunker, garbage crane, ash bunker, ash crane, burning chamber, hearth system, etc. | |
| 2. | Combustion air procedures, including: primary and secondary air blower, air preheated, air reheater, etc. | |
| 3. | Bottom slag procedures, including: each bottom slag conveyor, ash ejector, vibration conveyor, etc. | |
| 4. | Fly ash procedures, including: fly ash conveyor, pneumatic conveying system, fly ash storage tank, fly ash stabiliser, etc. | |
| 5. | Waste metal procedures, including: magnetic selector, waste metal conveyor, waste metal bunker, waste metal compressor, waste metal crane, etc. | |
| 6. | Steam procedures, including: high pressure steam distributor, steam turbine, power generator, steam condenser, etc. | |
| 7. | Condensed water procedures, including: condensed water recycling equipment, such as condensed water tank, degassing & water feeding tank, etc. | |
| 8. | bag filter dust collector, attraction exhaust fan, etc. | |
| 9. | Wastewater procedures, including: inorganic wastewater treatment system, organic wastewater treatment system, sludge production system, etc. |
Incinerators and their Types
Incinerator can be understood more precisely as a furnace where waste is burnt. Modern incinerators are equipped with pollution improvement systems, which play their part in cleaning up the flue gas and such toxicants.
The following are the types of plants for burning waste;
1. Moving Grate
The incineration plant used for treating MSW is moving grate. This grate is capable for hauling waste from combustion chamber to give way for complete and effective combustion. A single such plant is capable for taking in thirty-five metric ton of waste every hour for treatment.
Moving grates are more precisely known as incinerators of municipal solid waste. This waste is poured in the grate with a help of crane from and opening or throat. From here, the waste has to move towards the ash pit.
Waste is further treated and water locks wash out ash from it. Air is then flown through the waste and this blown air works for cooling down the grate. Some of grates are cooled with help of water.
Air is blown through the boiler for another time but this time comparatively faster than before. This air helps in complete burning of the flue gases with the introduction of turmoil leading to better mixing and excess of oxygen. In some grates, the combustion air at fast speed is blown in separate chamber.
2. Fixed Grate
This was the fixed and much older version for grate. This kind generally is lined with the brick while lower or ash pit is made up of metal.
This grate generally has an opening at the top and for loading purpose; a side of the grate is left open. A number of fixed grate were first formed in houses, which today are replaced by waste compactors.
3. Rotary-kiln
Industries and municipalities generally use this sort of incinerator. This incinerator consists of two chambers i.e. primary and secondary chamber.
4. Fluidized Bed
In this sort of incineration, air is blown at high speed over a sand bed. The air gets going through the bed when a point come where sand granules separates and let air pass through them and here comes the part of mixing and churning.
Therefore, a fluidized bed comes in to being and fuel and waste are then can be introduced. The sand along with the pretreated fuel or waste is kept suspended and is pumped through the air currents.
The bed is thus mixed violently and is uptight while small inert particles are kept suspended in air in form of fluid like form. This let the volume of the waste, sand and fuel to be circulated throughout the furnace, completely.
Desirable Features of Incinerators
Incinerators are designed to efficiently and safely burn waste at specified rates and temperatures, with the residual ash containing no combustible material.
- Air and fuel are mixed in correct proportion
- Regulated combustion air
- Minimum exhaust gas residence time 1-2 seconds in the secondary chamber
- Proper residence time to obtain a complete burn out
- Provide for creating turbulence in combustion chamber
- High temperature and chemical resistant refractory lining
- Satisfy and exceed pollution control norms.
In summary, incineration is the combustion of waste in the presence of oxygen. After incineration, the wastes are converted to carbon dioxide, water vapour and ash.
Read Also : Criteria for Hygienic Storage of Solid Waste
This method may be used as a means of recovering energy to be used in heating or the supply of electricity. In addition to supplying energy, incineration technologies have the advantage of reducing the volume of the waste, rendering it harmless, reducing transportation costs and reducing the production of the greenhouse gas methane.