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Under certain conditions it is possible to generate other, potentially more harmful emissions. It is important this is avoided by appropriate choice of fuel, combustion or abatement equipment.
Although the levels of heavy metals in most virgin, untreated wood are extremely low, wood that has been treated with preservatives or coatings may potentially have high levels of heavy metals. The combustion of treated wood, therefore, is regulated under the Waste Incineration Directive. Wood used for outdoor applications, for example, might have been treated with CCA (Chromated Copper Arsenate) which if burned in unsuitable equipment, could allow the emission of arsenic into the atmosphere, as well as copper and chromium. CCA has now been replaced with less toxic alternatives within the EU.
Significant levels of heavy metals can, exceptionally, build up in some kinds of virgin, untreated timber if it has grown on land in which they are present in high concentrations. This could potentially be on land remediation sites, or where sewage sludge or other effluent has been spread on the land.
Treated wood can also contain halogenated compounds, such as those containing chlorine, and this too, must be burned in a WID compliant installation. Under certain conditions, with the simultaneous presence of carbon, oxygen and chlorine, it is possible to generate PCDD and PCDF (polychlorinated dibenzodioxins and polychlorinated dibenzofurans), known as dioxins and furans respectively. Significant generation of these, highly toxic, compounds requires very specific conditions, however, and good combustion conditions and equipment design can ensure very low emission levels. The most common mechanism requires carbon in a specific, particulate form (microcrystalline, not amorphous or highly ordered graphitic), molecular oxygen, and chlorine, within a very narrow temperature window of about 250-500ºC. Outside this range there is extremely low formation, and above about 600ºC the reverse destruction reaction increases extremely rapidly. The formation reaction is catalysed by the presence of some metal ions, in particular copper and also, to a lesser extent, iron. Because of the very specific reaction conditions necessary for formation, careful design can ensure very low levels. Such measures include maintaining low levels of particulates in the flue gases, in itself a product of good combustion and ensuring that any surfaces on which particulates can build up are maintained outside the critical temperature window. Rapid quenching of flue gas temperature also contributes to good design and practice, ensuring that emission levels are of the order of those from conventional oil boilers, and an order of magnitude below those of coal combustion.
The final class of emissions that can be encountered under certain conditions of fuel and equipment, are those known collectively as PAH (polycyclic aromatic hydrocarbons). Aromatic organic compounds are those containing rings of typically five or six carbon atoms, and PAHs are compounds of three or more of these rings. The range extends, though, from these small molecules up to the larger molecules that are found as part of the mixtures of chemicals known as creosote and tars. The presence of these, or any other such organic compounds in the flue gases, is indicative of incomplete combustion. The initial stage of combustion of organic material, pyrolysis, is one of thermal decomposition giving rise to the emission of a wide range of complex organic molecules which, when condensed, are known as pyrolysis oil. Provided the temperature, oxygen concentration and residence time are all sufficient, this stage is followed by combustion (oxidation) to carbon dioxide and water. If, however, one of these parameters is not sufficient to allow efficient combustion, PAHs and other unburned hydrocarbons can be emitted in the flue gases. This can arise because of poor quality fuel, especially with a moisture content above the design maximum for the particular combustion equipment, or because the controls for the boiler or stove are incorrectly set, such as insufficient secondary (or tertiary) air. As these gases cool within the flue sequential condensation of progressively smaller molecules takes place, leading to tar deposits, while smaller molecules remain as vapour until emitted into the atmosphere. These are all highly undesirable as a number of the PAHs are carcinogenic, and tar deposits within the flue can lead to corrosion and potentially a chimney fire. Once again, use of good quality fuel, of the specification appropriate for the combustion equipment, and well designed equipment operated according to the manufacturers’ instructions, can keep such emissions down to very low levels, similar to those of conventional liquid fuels, and an order of magnitude below those of coal.
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