What is Thermal Treatment?

Thermal Treatment is a process by which heat is applied to waste in order to sanitise it and reduce its bulk, prior to final disposal. While thermal treatment plants can operate without generating energy, Irish waste management policy requires thermal treatment facilities to incorporate energy recovery capacity; the energy generated can, in turn, be used for example to power the plant and for district heating systems.

Thermal treatment can involve a number of processes most of which are commonly referred to as incineration. According to the most recent EU Directive on Incineration of Waste 2000:��Incineration plant "means any stationary or mobile technical unit and equipment dedicated to the thermal treatment of wastes with or without energy recovery. This includes the incineration by oxidation of waste as well as the other thermal processes such as pyrolysis, gasification or plasma processes in so far as the substances resulting from the treatment are subsequently incinerated"

EU Directive on Incineration of Waste (Approved by EU Parliament and Council November 2000).

Purpose of Thermal Treatment

The primary function of thermal treatment is to convert the waste to a stable and usable end product and reduce the amount that requires final disposal in landfills. Thermal treatment processes recover the energy in municipal solid waste (MSW) and convert it to electricity and/or useful heat, gas etc. Because a high proportion of MSW can produce methane on landfilling, recovering energy from the waste will help reduce the 'greenhouse effect'.

Thermal Treatment in Europe

When Ireland was failing to address the waste issue our European neighbours were developing and implementing sustainable integrated waste management practices. Thermal treatment is used as an integral part of integrated waste management throughout Europe because it is a safe, clean technology superior to landfill and is compatible with high levels of recycling. Countries with high recycling levels also have high levels of thermal treatment. The graph bellows shows domestic waste management in EU countries from 1999-2000 indicating the percentages recycled, incinerated and landfilled.

Types of Thermal Treatment

While the terms thermal treatment and incineration are often considered synonymous, it is important to understand that in the context of the Dublin Waste Management Plan the phrase thermal treatment refers to a number of processes, of which incineration in one such process. Other thermal treatment processes include Gasification and Pyrolysis.

Incineration

Incineration is the most common technology used for converting municipal solid waste into energy. Pre-sorted (segregated) waste or unsorted waste is the fuel in a combustion process that uses excess air to ensure complete combustion. Waste is delivered to the plant, where it is stored in a large enclosed bunker that serves as waste buffer capacity. The bunker area is kept under negative pressure thus preventing odours from escaping the building. Overhead cranes feed the waste to the incineration unit, where it is converted into energy. This usually takes place on a moving grate in the bottom of the combustion chamber.

Pyrolysis

Pyrolysis is a thermal pre-treatment method, which transforms pre-shredded waste into a gas and a residue consisting partly of inert materials, and partly of a char. The process takes place in an externally heated reactor in which the pyrolysis gas is formed. The gas may be used partly to heat the reactor and partly for generating stream in a boiler. The char may be used as a low-grade fuel in a large power plant, if available or it may be disposed of in a landfill.

Gasification

This process is similar to pyrolysis in some ways. With gasification, the pre-treated waste is fed into an externally heated reactor where the carbonaceous material in the waste stream reacts with a gasifying agent (e.g. air, oxygen or steam) at temperatures of 800-1100°C or higher. Chemical reactions form a combustible gas (with traces of tar) which is stored or burned to generate heat. The process is very inefficient in terms of energy recovery and therefore has a higher cost of processing.

The likely technology for the project will be conventional incineration, with the energy from the waste being converted into electricity for the national grid.

How Do Incinerators Work?

Click on image to enlarge.

Incineration - The Facts

• Old badly managed incinerators are being closed down in Europe where they do not comply with the new standards for monitoring and operating. However, new facilities are being provided in their place. Incineration capacity across Europe is increasing.
• Modern incinerators reduce waste to between 6-13% of the original volume. Of this, approximately 5-10% is bottom ash and is inert and only 1-3% of the original volume is fly ash requiring disposal in a special facility.
• Uncontrolled burning of waste is one of the biggest threats to the Irish environment today because it involves burning waste at temperature levels which create dioxins. Modern well-managed incinerators burn waste at much higher temperatures at which dioxins are destroyed. For example, a modern municipal incinerator treating 1 million tonnes of waste in controlled conditions, releases just 0.54 grams of dioxins to the atmosphere. A 2001 EPA report estimates that 60,000 tonnes of waste burned in the back yard produced 18 grams of dioxins.
• There is a broad spectrum of chemicals emitted from incinerators but even in rural situations the overall contribution is usually less than 1% of existing background levels - including dioxins and furans.
• Even if 1 million tonnes of municipal waste was to be incinerated in Ireland this would contribute less than 2% of the dioxins emitted to the air (EPA 2001).
• The EPA, FSAI and WHO have all indicated that properly managed well run incinerators do not impact on the environment or on human health.
• There are 11 incinerators operating in Ireland. Studies carried out by the Food Safety Authority of Ireland (FSAI) show no increase in dioxins in dairy products produced in the vicinity of the plants.
• Levels of dioxins in mothers breast milk and in dairy products (key indicators) have decreased over the past 20 years despite the increased use of incineration in Europe.
• Legislation controlling emissions from incinerators is among the strictest environmental legislation in the world.
• Incineration will not compete with recycling as infrastructure like thermal treatment plants and landfill will be sized to take the appropriate non-recyclable percentage of the waste stream.

Some Frequently Asked Questions about Incineration

What residues do incinerators produce?

Incinerators produce two different residues: Bottom Ash and Fly Ash. Bottom ash is between 5-10% of the original volume of waste but up to 20-25% of the original weight. It consists of part of the waste stream that cannot be burnt, i.e. glassy elements, grit, metals, inert matter, etc. The metal fraction of the bottom ash can be separated out using magnets and the remaining fraction is a stable aggregate which can be used in the construction of roads, etc. Fly ash, however, is hazardous and is usually 1-3% of the original volume of waste. This material requires special handling in an appropriate facility in Ireland or abroad. It is the by-product of the gas cleaning processes.

Do Incinerators Produce Electricity?

One tonne of Municipal Solid Waste in a modern incinerator produces 650Kwh of electricity. In a plant handling 400,000 tonnes of waste enough electricity is generated to service 20,000 homes. If heat is recovered from the plant and provided to local communities through a district heating system, the energy efficiency of the plant is between 75-80%. An average conventional power plant has an efficiency of 35%.

Do emissions from modern incinerators harm health and the environment?

Properly managed and monitored Municipal Waste Incinerators do not impact on the environment, health or food quality. This is because incineration of waste is strictly controlled and the gases emitted are cleaned and scrubbed to ensure that any emissions are extremely low.

However, incinerators do emit a broad spectrum of chemicals to the environment - albeit in extremely small quantities. Many opponents of incineration argue that because incinerators emit these chemicals they should not be built. However, the reality is that chemicals like dioxins already exist in our environment and come from very familiar sources like smoking, traffic, illegal burning of waste - even home heating systems. What determines whether they do us harm is the amount or DOSE we are exposed to; for example, common chemicals like salt can be toxic to the human body if taken in large enough quantities. Even if we incinerated 1 million tonnes of municipal waste in Ireland, this would contribute less than 2% of the dioxins emitted to air (EPA, 2001). Most dioxins will continue to come from uncontrolled burning of waste in back gardens, bonfires and accidental fires.

What are dioxins?

Dioxins are the unwanted by-product of low temperature uncontrolled burning.

At a certain dosage, 17 dioxins have been identified as posing a risk to human health. Most of our exposure (over 90%) to dioxins comes through the food chain. They persist in body fat and remain in the body for a very long period of time. There are many sources of dioxins to the Irish environment including smoking, transport, home heating and some types of industrial processes, but back-yard burning of waste is the single greatest source of dioxins to the Irish environment.

"If dioxins are so toxic, and incinerators produce them, surely building incinerators will increase the levels of dioxins in the Irish environment?"

Properly managing our waste will reduce the impact waste has on our environment - including dioxin emissions. Currently most dioxins come from uncontrolled backyard burning of waste. Most of this waste should be composted or recycled with the remainder going to disposal in properly managed incinerators or landfill sites. If all of the waste that is currently being burned was handled in this way, the levels of dioxins in Ireland would decrease dramatically. This is because backyard burning usually occurs at a temperature of about 200-400°C which is the temperature at which dioxins are formed; incinerators burn waste at over 850°C, the temperature at which dioxins are destroyed. Ensuring that waste management has the least possible impact on the environment will involve providing all of the infrastructure necessary to maximise recycling, recover energy from the waste that cannot be recycled and keep landfill to a minimum.