Co-firing coal with waste wood is the most promising option to contribute to the thermal recycling of solid wastes. However, the utilisation of this fuel mixture in the existing combustion systems may have serious effects on fuel preparation, combustion behaviour, gaseous emissions and ash composition. Co-combustion tests were performed in the industrial boiler of PINDOS SA (North-western Greece), using Greek lignite from Ptolemais reserve, uncontaminated waste wood, MDF residues and power poles. The results showed that co-combustion is technically feasible provided that agglomeration problems could be confronted. Moreover, it can meet strict environmental standards in relation to toxic gas and heavy metals emissions.
Wood is one of the most valuable materials, widely used for its remarkable properties (high strength, low specific weight, good insulation properties, availability). The extensive use of wood and wood-based products results in high quantities of waste wood and wood residues. In contrast to oil and natural gas reserves, wood and wood wastes are available in all continents. In Greece, it is estimated that 4.3 x 106 tons (dry matter) are annually produced from forestlands, forest wood residues and timber processing. The available waste wood quantities are estimated to be 1.070.000 m3/yr (~640.000 tons/yr), which correspond to 288.000 toe, in case of thermal recycling. This significant amount of wood wastes comes from sawmills and wood processing companies, demolished buildings and other structures, utility poles, panel industry, pallets and packaging wood, furniture, city parks and city trees.
The need for a practical, economic and environmentally sound solution for old timber and used wood-based products arises by the requirements for resource conservation and by recent regulations. Waste wood and wood-based products can be recycled through various processes, such as
(a) energy production,
(b) conversion into other wood products and
(c) chemical treatment.
One should note that wood combustion has a "neutral" balance with respect to CO2 emissions. However, sorting and cleaning of wood wastes such as demolished wood, power poles and railroad sleepers increases the cost of recycling.
The implementation of techniques, which improve wood quality, often causes contamination of waste wood with dangerous substances, such as preservatives, glues, colours and other chemicals, which constitute a potential threat to the environment. Waste wood containing these substances must be reused and/or disposed off by taking precautions to alleviate deterioration of the environmental quality. Lack of appropriate grading rules and engineering design guidelines for contaminated waste wood has limited, thus far, the efficient utilisation of these materials as energy sources.
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 renewable energy sources such as biomass and biomass by-products. Especially co-firing of 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, minimisation of waste disposal and reduction of fuel imports. Thus, co-firing coal with waste wood is the most promising option to contribute to the thermal recycling of significant quantities of solid wastes. However, the utilisation of this fuel mixture in the existing combustion systems may have serious effects on fuel preparation, combustion behaviour, gaseous emissions and ash composition.
Despite the fact that co-combustion of waste wood with coal 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 dioxins and furans 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, adsorbance 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:
(a) to investigate the operation of an
industrial scale combustor during co-combustion of waste wood and a low grade
(b) to determine the CO, SO2, NO, PCDD/F and heavy metal emissions,
(c) to compare them with emissions from co-combustion of natural wood, and
(d) to correlate gas emissions with the fuel mixture properties.
Co-combustion tests were performed in the industrial boiler of PINDOS SA, located in North-western Greece, which comprises a moving grate system and a multi-fuel burner. Lignite, from Ptolemais reserve (Kozani Perfecture), uncontaminated wood (process by-product), MDF residues and power poles were used in the co-combustion tests. Prior to testing, a theoretical estimation of ash deposition tendency was obtained, using appropriate indices. Following the above tests, a demonstration of co-combustion was performed.
Oxygen concentration and CO, SO2 and NO emissions were continuously monitored, during the co-combustion tests. Gaseous samples were collected from the flue gas duct after the multi-cyclone unit and before the air draft fan, in order to investigate the presence of PCDD/F, particulates and heavy metals in the flue gases. The sampling for PCDD/F was performed isokinetically according to VDI guideline 3499. The EPA Method 29 was implemented for the isokinetic sampling of particulates and heavy metals. The operation data of the boiler were recorded and ash samples from the combustion chamber and the multi-cyclone unit were collected and analysed for unburned carbon and heavy metals. PCDD/F measurements were performed in a HRGC/HRMS system (HP5890/MAT 95 Finnigan) and the analysis of metals in an ICP-AES spectrophotometer. Finally, material and energy balances were set up to evaluate the overall environmental impacts of the process.
The technical feasibility of coal-wood wastes co-combustion was proven by the experimental results, since agglomeration problems could be confronted. The obtained results showed that in most cases emissions (CO, SO2, NO, PCDD/F and heavy metals) were lower than the legislative limits without using costly waste gas scrubbing. Finally the extensive use of waste wood and lignite blends at PINDOS SA moving grate did not result in additional maintenance costs.
The Authors wish to thank EU for the financial support of this work through the THERMIE project Low emission co-combustion of different waste wood types and lignite derived products in industrial power plants.