It’s common knowledge that the global accumulation of waste is a long-term environmental threat for future generations. We haven’t found the perfect way to dispose of all of our waste. There are, however, existing technologies that allow us to manage our waste more effectively. One of the best examples of these is responsible solid waste incineration.
This article will discuss the process of incinerating solid waste, how solid waste incinerators work, and how today’s solid waste incineration processes can turn its byproducts into energy.
What is Waste Incineration?
Waste incineration is simply the burning of garbage. The incineration process, often described in the industry as thermal treatment, uses special incinerators that burn waste materials to ash, heat, and flue gas (i.e., gas exiting from a flue, such as a chimney, to the surrounding air).
The ash, heat, and gas can be dissipated into the surrounding environment without energy or material recovery. This was especially true in older facilities in the US, and in other parts of the world without adequate environmental regulation. These older facilities run the risk of producing hazardous substances due to the unsatisfactory level of combustion process control and gas cleaning as well as untreated disposal.
Conversely, in more modern facilities, the resulting byproducts can be recovered and used for other purposes, recycling them in effect. For example, the heat produced from the burning process can be utilized for generating electricity and solid wastes, such as fly ash can be used as the material for making bricks, shingles, or tiles.
Incinerating waste is widely practiced, and actually popular, in nations such as Singapore, the Netherlands, and Japan where there is a scarcity of land. Other European countries such as France, Germany, and Luxembourg also use incineration to dispose of municipal waste.
Waste Incineration in the United States
The first incinerator in the US was constructed in 1885 in Governor’s Island, New York. This and later incinerators were seen as an effective way of solving the nation’s trash problem and hundreds of facilities became operational by the end of the mid-20th century.
During the Industrial Age and until the late 1960s, there was virtually no concern about the negative environmental and health impact of the air and water pollution from these facilities. Since then, however, people have become more aware of being proactive in protecting the environment.
In the US, the Clean Air Act of 1970 (CAA) established strict standards to limit pollution. To continue operating, incineration facilities needed to conform to strict standards that prohibit the uncontrolled burning of trash and that limited pollution.
This led to the closure of many facilities as well as the construction of new CA-compliant facilities in the 80s.
The CAA requirements are regularly updated. For example, new more stringent requirements for Maximum Achievable Control Technology (MACT) were adopted in the 1990s as the EPA recognized the hazards of mercury, dioxins, and other hazardous pollutants from incineration.
Waste to Energy
A more recent development is facilities that recover the heat from incineration and use that energy to generate electricity. These are referred to as “waste to energy” facilities or WTE in short. The terminology refers to the energy recovery process that makes some modern incinerators an electricity-generating utility as well as a waste disposal service.
Benefits of Waste Incineration
Proper and responsible waste incineration provides various benefits:
Some progressive countries use modern waste treatment and incineration facilities to convert heat used in the burning of trash to electric power. The governments of Sweden and Denmark have turned many of their incinerators into energy generators, making them world experts in recycling energy. Waste incineration produced 13.7% of Sweden’s domestic heat consumption and 4.8% of electrical consumption in Denmark.
The incinerator bottom ash can be used as an aggregate in creating lightweight blocks, pavement concrete, bulk fill, and more. Environment conscious entities are using novel technologies to create bricks, tiles, shingles, and other construction materials from ash.
Incineration can decrease the solid mass of the original waste, which is already compacted by garbage trucks, to a further 80 to 85%. It can also reduce the volume of trash up to 95%. How compacted the mass depends on the composition of the garbage materials.
At present, there are 72 incinerators operating in the US handling a percentage of the nation’s garbage, with the rest of the garbage being composted, recycled, or disposed of in landfills. The reduction of solid waste by incineration drastically reduces the amount of trash that ends up in a landfill.
Incineration can also be used to treat hazardous waste (such as materials contaminated with hazardous chemicals) or medical waste (such as hospital waste contaminated with blood or other potentially infectious materials). The high heat of incineration can destroy these hazards. Hazardous and medical wastes can only go to special incinerators that are permitted to treat these types of wastes.
Waste Incineration Process
All wastes that are intended to be incinerated undergo a process called “mass burn.” Each facility has different ways to do a mass burn, but each follows a general process outlined below:
Waste sorting and shredding
The waste is first sorted out, both mechanically and manually. Machines and personnel remove oversized items, recyclable items, and metals. The remaining trash is then shredded.
Waste drying and batching
In some facilities, the waste material is processed or dried so that only around 30% moisture remains. After this, the waste is divided into more manageable batches. The volume of each batch is carefully computed and monitored so it can be burned at the lowest cost and in the shortest time possible.
The remaining garbage undergoes the combustion processes. For specifics on how waste is burned, check out the earlier section titled “Parts of an incinerator”
The heat from the combustion process is used to generate steam. The steam is then used to provide energy to run generators that produce electrical power.
The cooled gas is subjected to thorough cleaning in the facility’s flue gas cleaning system. Here, it is treated with filters, precipitators, and scrubbers to ensure that most pollutants are removed before discharge.
Finally, the treated gas is discharged to the atmosphere. Ideally, the gas coming out from the chimney should be transparent with no visible smoke since the discharged gas should be particle-free.
Residuals, or the solids that remain after combusting, are dumped in an ash pit ready to be disposed of in a landfill.
Parts of a Solid Waste Incinerator
While many facilities may differ in processes and technologies, waste incinerators usually have standard parts.
The most common type of waste incinerators is called a moving grate incinerator, often referred to as a Municipal Solid Waste Incinerator (MSWI). Rubbish is dumped into a moving grate that goes through the different chambers of an incinerator. The constantly moving grate enables a fast, efficient, and complete movement and processing of waste products. A properly maintained moving grate incinerator can handle 35 metric tons of waste per hour at 8,000 hours per year.
Understanding how a typical moving grate incinerator works is best illustrated by understanding how each of its major parts works:
A waste crane picks up massive loads of garbage from a sorted mound.
The throat is a large long tunnel that leads to the primary combustion chamber.
Primary Combustion Chamber
Waste from the throat goes to the main combustion chamber where the material is burned. Most often, this chamber is already hot thanks to the high ambient temperature that is constantly controlled and retained.
The secondary chamber is often called the “afterburner.” Facilities in Europe, Australia, Canada, and the US are required by law to have an afterburner. This chamber helps reduce or prevent harmful particulates from forming by burning them off.
Many countries have laws that require all flue gas remain in the secondary chamber for at least 2 seconds at a temperature of 850 degrees Celsius to break down toxic organic substances.
Using superheaters, heat from the flue gasses can be used to convert water to steam. The superheated steam can then be used to drive turbines to generate electric power in WTE facilities. The flue gas is now at 200 degrees Celsius at this point.
Flue Gas Cleaning System
Before exiting the facility, the flue gas goes through the cleaning system to purge acids, heavy metals, and other toxic particulates.
Once the flue gas is treated, it exists through the flue stack, commonly called a chimney. Laws require a stack height of at least 3 meters for localized incinerators. Bigger ones, however, have multi-story chimneys, especially those that handle the massive trash produced by large cities. Also, chimneys may be built higher or lower than recommended due to various atmospheric conditions.
The flame coming from the burners ignites the garbage. The majority of the incinerators are equipped with low nitrous oxide burners or modulated gas flow burners. The intensity of the flames is carefully controlled.
Fuel tanks are used to store fuel. Fuel tanks must be carefully insulated for safety.
After incineration, the remnant ash is then collected in an ash pit for disposal. Some entities purchase the ash for their own use.
Incineration Waste Disposal
Waste from solid waste incineration includes flue gas, heat, and ash.
Harmful particles and substances are filtered through the flue gas cleaning system. At a prescribed time, the filters and scrubbers of the systems are replaced or cleaned. Facility operators always keep a close watch of the system. That’s because clogged systems make the gas treatment less efficient. This material may be combined with ash for recycling or disposal or sent to landfill.
The heat from combustion converts water to steam, which is then sent into turbines to produce electrical power. This electricity is used to power the other electrical systems of the facility. In some countries in Europe, excess electricity is released into the grid to power nearby communities.
Proper disposal of waste incineration ash constitutes its own special process. The ash is run through a baghouse filtering system that captures particulates. Minute amounts of ash that escape the system, called fly ash particles, are captured via funnels called hoppers.
All resulting ash is dumped into an ash pit. Water is poured on the ash pit to prevent ash dust from escaping. The moist residue is then transported into a building where it is loaded to leak-proof disposal trucks.
Finally, the trucks transport the remaining ash to a designated landfill. That landfill must be certified and designed to prevent groundwater contamination; ash particles are so tiny that it might seep into the groundwater beneath the bedrock.
Alternatively, incinerator ash may be recycled to manufacture, bricks, tile, or other items.
As you can see, there is a lot more to solid waste incineration than simply burning garbage. There are complex processes involved. Whether a community resorts to waste incineration or dumping their garbage in a landfill depends on a variety of factors including resources, amount of land, and eco-vulnerabilities of that community.
When done and managed right, waste incineration is a cleaner, more efficient, and one of the most efficient modern waste management systems.
Still, whatever waste management process is used, in the end, producing less waste is the best and most effective way to reduce the environmental, health, and social impacts of garbage.