Combustion of fossil fuels is used today worldwide for energy production, but it results in significant releases of CO2 in the atmosphere. A net decrease of CO2 emissions can be achieved by the thermal use of regenerative energy sources such as biomass and biomass by products. Especially Co-firing biomass waste and coal, offers a number of technical, economical and environmental benefits in addition to reduced net CO2 emissions, such as conservation of fossil fuel resources, minimization of waste disposal and reduction of the dependence on fuel imports.
Although co-combustion of waste wood with coals is a promising technique, it entails some environmental risks due to the chemical nature of waste wood. Waste wood is usually impregnated with a number of substances, which can result in the release of toxic compounds like dioxins, furans and heavy metals. Since the early 1950s pentachlorophenol (PCP) and lindane have been used as wood preservatives, because of their fungicide and insecticide properties. Organic substances and especially PCP are known precursors of PCDD/F during combustion processes. Furthermore, unchlorinated PCDD/Fs have been observed in the lignin structure, which could result in the production of chlorinated compounds through simple chlorination reactions. The concentration of chlorinated organic substances, emitted during co-combustion of coal-waste wood depends on the type of solid fuel, the type of the combustor and the operating conditions. In addition to toxic organic compounds, waste wood may contain a number of metal elements like Zn, Mn, Cd, Ni, which upon combustion at high temperatures will be released in the gas phase. Heavy metal emissions during Co-combustion depend on a number of factors such as the element properties (volatility, absorbance on the surface of solid particles), the combustor characteristics and the presence of high HCL concentrations in the gas stream which affects metal volatility.
The objectives of this work were to investigate the operation of an industrial scale combustor during co-combustion of waste wood and low grade lignite, to determine the PCDD/F and heavy metal emissions, to compare them with emissions from co-combustion of natural wood, and to correlate gas emissions with the fuel mixture properties.