Basic Principles and Processes in the Operation of Incineration Technology.

Introduction

Incineration is the disposal of waste by controlled burning.Solid, liquid and gaseous wastes are converted into a small amount of ash and a large volume of exhaust gases. The gases are cleaned before being released into the atmosphere. Depending on the waste type, the ash can be used in construction or sent for landfill. Most incinerators recover and use the heat produced. Because of this, incinerators are often referred to as Energy from Waste (EfW) or Waste to Energy(W2E) plants. Incineration is sometimes called thermal treatment.

Incineration can be used for a wide variety of waste types including municipal solid waste (MSW) or domestic rubbish, industrial and chemical wastes, contaminated material from health care and residues from animal processing. Incinerators vary in size from small plants which process waste from a single factory or hospital to very large plants that handle the waste of whole cities. The basic principles are the same in all cases.

Combustion

When something burns it reacts with oxygen and is broken down into simpler molecules and heat is released. Three things are needed for combustion to happen – a fuel, a supply of oxygen and a high temperature.

Fuel: not all materials will burn so to get combustion a supply of a material that will burn is needed. The main fuel type in waste is organic material, i.e. material that contains carbon. This comes from both biological and petrochemical sources. Paper, timber, food waste and animal fats are examples of biological organic materials while plastics and solvents are petrochemical organic materials.

Oxygen: combustion is the reaction where a fuel combines with oxygen and is broken into simpler oxide molecules. When a fuel containing carbon is burnt the carbon is converted into carbon dioxide (CO2) with the release of heat. To burn properly there must be enough oxygen to react with all the fuel. The reaction can only occur where oxygen comes into direct contact with the fuel. For solid and liquids fuels this can only occur at the surface so it is best to break the fuel up into small particles or droplets.

Temperature: The reaction with oxygen will only occur at a high temperature. The temperature needed varies with the material and can be as high as 800oC. As the fuel heats up the moisture content is first boiled off. Next volatile gases are released and mix with oxygen in the air to burn. Finally the remaining solid carbon burns.

Combustion By-Products

If a fuel is burnt completely then all carbon in it is converted to carbon dioxide (CO2), all the hydrogen is converted to water (H2O), any sulphur is converted to sulphur dioxide (SO2) and any nitrogen into nitrous oxides (NOx). These will all be released as gases. If the fuel contains other elements, such as metals, it will produce a solid residue or ash. The amount of ash produced varies with the fuel composition but is usually less than 10% of the weight of the original fuel. Some of the ash will be in the form of very fine particles and will remain suspended in the exhaust gases. This is called fly ash.

Complete combustion can only take place if enough oxygen is supplied and the fuel and exhaust gases reach a high enough temperature for a sufficiently long time.

Incomplete Combustion

If the fuel is not burnt completely then large undesirable organic molecules can be created and released with the exhaust gases. In particular, if the fuel contains chlorine, incomplete combustion can lead to production of substances called dioxins and furans. These are a range of complicated organic molecules which are poisonous to varying degrees. Once released into the environment they get into the food chain, persist for long times and accumulate in the fatty tissue of animals. Exposure to relatively low levels of dioxins and furans can lead to a wide range of health issues such as liver disease, certain cancers, developmental and immune system problems.

Dioxins and furans can be produced by the incomplete combustion of any organic materials containing chlorine such as plastics or treated timber. To prevent their release very high temperatures must be reached and maintained. Burning of rubbish in bonfires is one of the main sources of dioxins and furans.

Incinerator Components and Operation

An incinerator is designed to achieve complete combustion of the waste with the maximum extraction of heat from the exhaust gases and extensive cleaning of the exhaust gases to ensure that only minimal amounts of harmful substances are released into the atmosphere.

Figure 1- Block Diagram of an Incineration Plant

The construction and operation of an incinerator can be broken down into five main stages:

  1. Waste Storage and Preparation. Sufficient waste to run the incinerator must be stored at the site. The way it is stored will depend on the waste type. Solid wastes are usually stored under negative pressure to minimize the release of odours. The waste may be sorted to remove any non-combustible materials such as metals. It may be mechanically processed to reduce the moisture content or to break it into evenly sized pieces for easier handling and burning. It is always mixed thoroughly to give a consistent fuel.
  2. The Combustion Chamber. This is the key part of the system where the burning actually takes place. There are several different designs depending on the waste type. A summary of the main types is given in table 1. The common features are:
  3. The supply and movement of the waste through the chamber is automated and adjustable.
  4. There is an adjustable forced supply of air. Insufficient air will lead to incomplete combustion while excess air makes it difficult to achieve the required temperatures. The air is usually drawn in from the storage area bringing any dust and odours with it to be destroyed in the combustion process ensuring that odours and dust do not escape from the building.
  5. The chamber is designed to achieve thorough mixing of the waste and the gases released with the supplied air to ensure complete combustion.
  6. There is a controllable supply of a secondary fuel such as natural gas. The amount of secondary fuel supplied is adjusted to ensure the required temperature is reached.
  7. The chamber is designed so that the exhaust gases are kept at the required temperature for a sufficiently long time.
  8. The chamber will usually have an ash collection and removal system.Ash from the combustion chamber is called Incinerator Bottom Ash. It is first cooled withwater and then sorted and graded and used as a building aggregate
  9. Energy Extraction. The hot exhaust or flue gases are passed through heat exchangers and the heat removed is used to raise steam. This can be used to supply a local heat demand or to drive turbines which will generate electricity. Ideally both are done and the system is called a Combined Heat and Power (CHP) plant.
  10. Flue Gas Cleaning is usually done in two stages.First,acid producing chemicals such as HCl and SO2and toxic heavy metals such as cadmium, mercury and lead are removed. This is usually done by wet scrubbing where a fine mist containing an alkaline solution, usually lime, and active carbon is sprayed through the exhaust gases to react with and capturethe harmful substances. Next fly ash is removed either by filters or electrostatic precipitators (ESP). NOx levels may be reduced byan additional step called Selective Catalytic Reaction or by the injection of ammonia into the combustion gases. These techniques are also used to clean the flue gases of coal fired power stations such as Moneypoint. The fly ash and residues from the scrubbing can contain high levels of toxic materials and are usually disposed of in a special landfill site.
  11. Flue Gas Monitoring. Finally a thorough and continuous analysis of the exhaust is made to ensure that amounts of harmful substances released are below the permitted levels. The substances monitored will depend on the waste type but include dioxins and furans, heavy metals, SO2, HCl, NOx and total organic carbon(TOC). The last is a measure of the amount of carbon not completely combusted.

Waste Type / Technology
Solid industrial or hazardous / Rotary Kiln
Municipal Solid / Moving Grate
Liquid or Gaseous / Static Hearth
Homogenous e.g. sludge / Fluidized Bed

Table 1- Combustion Chamber Types

Role of Incineration

EU guidelines on waste treatment rank the options in order of preference as reduce, reuse, recycle, recover energy, and finally dumping. Incineration is currently the main method of energy recovery. It is preferable to landfill as it uses the waste as a fuel reducing the amount of fossil fuels that need to be burned. Emission levels from incinerators are similar to those from coal burning power plants. Waste dumped in landfill decomposes over time with risk of the emission of pollutants, especially methane, to atmosphere and the leaching of contaminants into the water table. Methane is much more damaging (~25x) greenhouse gas than CO2. The EU has strict targets for the reduction of the amount of waste being dumped in landfills and incineration is generally seen as necessary in achieving these targets.

Advantages of Incineration

  • It is the only practical method of disposing of certain wastes such as unwanted chemicals and contaminated material which cannot go to landfill. If incineration is not available locally such material has to be exported.
  • Significantly reduces the quantity of material that must go to landfill and associated pollution risks.
  • Produces useful energy from waste, reducing fossil fuel consumption and resulting greenhouse gas (GHG) emission.
  • The costs, energy usage and GHG emissions can be lower than those in the collection, transport and processing involved in recycling.
  • Building incinerators in or close to urban areas reduces the cost of and emissions from waste transport and means the waste is treated where it is generated.

Disadvantages of Incineration

  • If incorrectly operated can lead to the release of harmful levels of pollutants. Even in proper operation small amounts of fine particles and pollutants are released. As the technology has matured emission levels have reduced dramatically – in Germany in 2000 dioxin emissions were ~1/1000thof the 1990 levels.
  • Incineration of MSW produces relatively large amounts of fly ash (approximately 4% of original waste weight) which must be dumped in secure landfills.
  • Incineration could reduce the incentive to recycle. The energy content of waste with all recyclable components removed is much less than unsegregated waste. Incineration plants could compete with recycling for some materials.
  • Incineration plants have a large initial capital cost and require long term contracts to be viable. This could hamper the deployment of future more efficient waste treatment technology.

Regulation of Incineration

In the EU incineration is governed by the Waste Incineration Directive (WID). This specifies the required temperatures and the maximum emission levels permitted for a wide range of pollutants. If more than 1% of the waste contains chlorine it is deemed hazardous waste and the exhaust gases must be raised to 1100oC for at least 2 seconds. For waste with lower levels of chlorine the requirement is 850oC for at least 2 seconds. The emission limits are based on continuous measurement of most of the pollutants but only six monthly measurements of dioxins, furans and heavy metals. In Ireland the EPA enforces the directive.

Status of Incineration in Europe

Energy extraction from municipal waste is a well established technology in Europe with all western European countries except Ireland having numerous plants. As shown in figure 2 the countries with the highest recycling rates also have high incineration rates.

Figure 2. MSW Treatment in Europe in 2007

In Ireland there arecurrently around 11 incinerators in operation at chemical and pharmaceutical plants burning waste produced on site. The EPA has granted licences for 3 commercial incinerators which will treat waste produced elsewhere. All three projects have generated major controversy. Construction on the plant in Duleek Co. Meath started in September 2009 and is due to be completed before the end of 2011. It will process 200,000 tonnes of MSW per year and export 11MW to the grid. The proposed plant at Ringaskiddy Co. Cork will have 2 separate incinerators. One will process 100,000 tonnes of industrial waste including hazardous materials while the other will treat 200,000 tonnes of MSW. The project is awaiting a final decision from An Bord Pleanala. The plannedincinerator at Poolbeg in Dublin will treat around 300,000 turns of MSW. The project has an EPA licence and planning permission but is waiting on government approval.

Figure 3: Distribution of MSW Incinerators in Europe in 2008