WASTE PLANT EMISSIONS
A plant burning waste produces four major types of product. First, there is the solid residue from the grate itself, normally termed slag or ash. Second, there is the chemical product resulting from flue-gas treatment systems. Third, there is a quantity of dust in the flue gases emerging from the plant boiler; this is normally captured with filters or an electrostatic precipitator. Finally, there is the flue gas itself.
Ash
The nature of the ash or slag emerging from the grate of a power-from-waste plant will depend on both the type of waste being burned and the combustion conditions. While its primary constituents will be solid, incombustible mineral material from the wastes, this residue will be contaminated with traces of a variety of metals. These traces may be in a toxic or harmless form. If the waste has not been carefully sorted beforehand, the slag may also contain larger slugs of valuable metal that can be recycled.
By careful control of the temperature in the furnace, it is possible to incorporate the trace metals into the mineral content of the ash and render them effectively harmless. This is a process called sintering. The effectiveness of the sintering process in rendering toxic metals harmless will be determined by measuring the amounts capable of being leached out by water. The ash may also contain some toxic organic compounds such as dioxins. Furnace conditions can minimize these too since a sufficiently high temperature will normally destroy such compounds. The effectiveness of this will again be determined by a leaching test.
If the ash or slag is too toxic for any other use it will have to be buried in a landfill. Modern facilities aim to render it sufficiently stable and benign that it can be used for road building or similar purposes. When they succeed, only a residual 1% of the original waste needs to be buried.
Fly Ash and Flue-gas Treatment Residues
Fine, solid particles called fly ash escape with the flue gas from a furnace. This fly ash will often contain high levels of toxic metals and must be captured. Capture is achieved either by using a fabric filter called a bag filter, or by employing a device called an electrostatic precipitator. Both should be capable of removing close to 100% of the dust from the flue gas. Once captured this dust must be safely buried in a landfill.
Other treatment systems are designed to remove gaseous components from the flue gas. This includes acidic compounds and harmful organic compounds. The processes used to remove these components, often similar to the treatment plants used for coal-fired power plants and described in detail in Chapter 3, result in by-products that also require disposal. Depending on the treatment process, the residue may be a solid or wet slurry. In the latter case, the slurry will normally be dried using the hot exhaust gases before disposal.
Flue Gas
Once treated, the flue gas from a waste combustion plant should be sufficiently clean to release into the atmosphere. However, the gas will usually need to be monitored to ensure that emission limits are being met.
Dioxins
One of the most potent environmental concerns during the last 20–30 years has related to the release of dioxins into the atmosphere. Dioxins are undesirable by- products of the manufacture of a variety of chemicals such as pesticides and disinfectants, but one particular compound called 2,3,7,8-tetrachlorodibenzo- p-dioxin has come to be identified as dioxin. This material was thought to be extremely toxic to humans, though more recent studies suggest earlier results were exaggerated. However, several dioxins are considered carcinogenic.
Dioxins can be found in urban waste but the principal danger is that the com- pounds are formed during waste combustion if the process is not carefully con- trolled. This is usually a matter of temperature. If the combustion temperature is too low some plastics and other materials can break down and then the components react to create dioxins. Once the combustion temperature is high enough, usually above 1000 ºC, then these compounds will normally not be produced. Some early waste incineration plants did not control the emissions sufficiently carefully and this led to instances of widespread contamination. Such instances have colored the perception of WTE plants ever since.
Dioxin emission levels are now closely regulated and emissions have fallen. In the United States, according to the Environmental Protection Agency, the total emissions of dioxins from large WTE facilities fell from over 8 kg (toxic equivalent) in 1987 to less than 14 g (toxic equivalent) by 2005. The European emission limit for dioxins is 0.1 ng/Nm3. Power-from-waste plants built in the first decade of the 21st century and beyond should be capable of reducing the emission level to one-tenth or even one-hundredth of this.
Heavy Metals
Heavy metals, particularly lead, cadmium and mercury have proved another source of concern. Less mercury is used today than in the past. This, combined with better filtration systems has reduced mercury emissions from power-from- waste plants in the USA from over 50 tonnes/year in 1990 to around 2 tonnes/ year in 2005. Coal-fired power plants release over 40 tonnes/year. Lead and cadmium emissions have fallen by 96% over the same period. In general the emissions of toxic metals from waste incineration plants should fall well below legal emission limits.
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