Above. The Shinkoto Incineration Plant in central Tokyo. This facility processes 600,000 tonnes of waste a year using three lines.
As waste management becomes more of a problem, alternate forms of disposal of such wastes are being adopted. Waste incineration is one such process. The concept behind waste incineration is using the waste that would normally be put into a landfill as a fuel. This fuel could then be used to heat a boiler. The steam created in the boiler could then be used to power a turbine. The work from the turbine could then be used to then generate electricity. This could be very useful in an area where space is limited and large landfills cannot fit. Places like Japan, Denmark and Sweden have been leaders in using the energy generated from incineration for more than a century.
This illustration shows the waste incineration process. For each different type of incinerator the process might change slightly, but for the basic waste incinerator, this is how it works.
Above. Basic process of a waste incinerator.
The incoming waste is brought to the waste incineration plant and dumped into the holding area(1). The waste is then grabbed and dropped into a hopper(2). From the Hopper the waste is gradually fed into the incinerator(3). This incinerator runs at a range of temperatures depending on the type of trash being incinerated. The heat from the incineration of the waste is then used to heat up the working fluid (usually water) in the boiler(4). The steam from this process is then piped to a turbine generator to create electricity. The left over burnt waste and heaviest ash falls into a collection area(5). At this point an electromagnet can be used to pick up any left over metals that could then be recycled. The flue gases containing fine ash and other toxic vapors then pass through a scrubber reactor(6). This scrubber treats the flue gasses for acid pollutants such as SO2 and also dioxins. From the scrubber, the gases can then pass through a fine particulate removal system, which can further reduce the toxicity of the flue gasses(7). The flue gases are then released through the chimney stack(8).
The steam from the boiler can be used to power a turbine to produce electricity and/or be fed out of the facility to be used as district heating to homes and businesses. For recent industrial incinerators, every tonne of waste incinerated can produce 2 MWh of district heating and .67 MWh of electricity. This is energy created from something that would have otherwise been just left to decay in a landfill.
Waste is often processed before incineration by removing bulky materials, recyclable materials, and materials that cannot be combusted with heat generated by the incinerator along with other materials that can be harmful if combusted. This reduces the emissions of pollutants and other hazardous gases. The gases released due to incineration are cleaned and detoxified before being released into the air.
Waste incinerators can reduce the amount by volume of the incoming waste by 95%. Incineration helps reduce the volume of the waste but does not completely reduce the waste. This spent ash will still need to be disposed of. However, this is still beneficial by significantly reducing the amount of waste that needs to be land filled. The emissions produced by these incinerators are well within regulatory standards and allow for efficient energy production.
Incinerators are also very useful in destroying some highly toxic chemical waste and other hazardous waste. It is often imperative that medical waste be subjected to high temperatures, which destroy pathogens and other harmful toxins.
Types of Waste Incinerators
There are many different forms of waste incinerators:
- Moving Grate
- A moving grate is used to agitate the waste. The grates move to help burn the waste evenly, which ensures the waste to be as completely burned as it can be.
- Waste is loaded into a cylindrical kiln and rolled while the waste is combusted. This type of incinerator is the most widely used in industrial applications.
- Fluidized Bed
- A bed of sand is used while air is pumped from underneath the sand. Once the air breaks through the sand, the waste is introduced and the waste can now lay on the fluidized bed created by the air and the fuel can be introduced and incineration can begin.
The type of incinerator used is based on the type of waste that needs to be incinerated, the amount of waste needed to burn per hour, and the specific needs of the plant. As stated above the rotary-kiln is the most widely used do to the ability to very effectively burn many types of waste and the rotating nature of the kiln helps to evenly and fully burn all combustables.
One of the newer technologies for waste treatment is Gasification. Gasification is a thermo-chemical process in which biomass is heated, in an oxygen deficient atmosphere to produce a low-energy gas. This gas contains hydrogen, carbon monoxide and methane. This gas can then be used as a fuel in a turbine or combustion engine to generate electricity. Gasifiers that are fuelled by fossil sources like coal have been operating successfully for years. However, they are now starting to be developed to accept more mixed fuels, including wastes. New gas cleansing technology ensures that the resulting gas is suitable to be burnt in a variety of gas engines, with a very favourable emissions profile. Gasifiers operate at a smaller scale than incineration plant, and can also be provided in modular form to suit a range of different scales of operation. A number of British companies are leading in this emerging technology.
Pollution Produced by Incinerators
The pollutants produced by waste incinerators before flue cleaning are particulate matter, heavy metals, dioxins, furans, sulfur dioxide, and hydrochloric acid.
The major concern with operating an incinerator is that it releases Dioxin, a powerful carcinogen. Dioxin was the primary toxic component found in Agent Orange, a toxic herbicide that was sprayed from airplanes during the Vietnam War to kill the foliage that hid enemy trails from view. Many U.S. troops suffered post-war cases of cancer due to prolonged exposure to Agent Orange. A
specific form of Dioxin, 2,3,7,8-TCDD, has been officially labeled as “known to be a human carcinogen,” meaning that there is no safe level of Dioxin exposure which will not result in harmful effects to the human body. There is no threshold at which a less amount of Dioxin exposure is safe. Dioxin has been linked to causing breast cancer, birth defects, and to immune and reproductive system problems.
Other harmful materials released into the air can include lead, mercury,
cadmium, and acid gases. The amount of these materials can be can very significant before they are put through the flue gas cleaning system, which is used to reduce such pollutants.
Odor pollution is another pollutant that is a problem for older incinerators but not so much for newer style incinerators.
Overall, the pollutents produced by waste incineration processes are regulated and kept within an acceptable range.