اثرات یک مبدل کاتالیزگر بر روی انتشار PCDD/F،کلروفنول و PAH در اشتعال چوب های سوختی مسکونی Effects of a catalytic converter on PCDD/F, chlorophenol and PAH emissions in residential wood combustion

۱٫ Introduction A large number of residential wood combustion (RWC) appliances such as masonry heaters, boilers and stoves are used for heat production in many countries. RWC will increase even more in the future because biomass combustion is considered to be greenhouse gas neutral energy source. However, RWC is known to be an important source of pollutants such as fine particles (e.g. Boman et al., 2003; Sippula et al., 2007; Karvosenoja et al., 2008; Tissari et al., 2008, 2009), polycyclic aromatic hydrocarbons (PAHs) (e.g. McDonald et al., 2000; Hellén et al., 2008) and polychlorinated dibenzop-dioxins and polychlorinated dibenzofurans (PCDD/Fs) (e.g. Quaß et al., 2004). These pollutants impair local and regional air quality and cause adverse health effects (van den Berg, 1994; Boström et al., 2002; Pope et al., 2002; Kappos et al., 2004; Kettunen et al., 2007). PCDD/Fs constitute a group of persistent organic pollutants which can be unintentionally formed and released from combustion and other thermal processes when there is chlorine, oxygen and carbon available (Shaub et al., 1983; Anderson et al., 2002). Research of PCDD/F emissions started with municipal solid waste (MSW) incinerators where fuel is heterogeneous and contains chlorine and metals such as iron, copper, aluminum and tin. Although chlorine content of wood is much lower than that of MSW, all the requirements of PCDD/Fs formation are filled also in RWC. PCDD/F formation is a complex process which is studied widely but still not completely understood. Three routes have been presented to explain PCDD/F formation in combustion (Shaub et al., 1983; Dickson et al., 1992; Gullett et al., 1992; Tuppurainen et al., 1998, 2000). The homogeneous pathway covers formation from structurally related compounds such as chlorophenols (CPhs) at high temperatures (400–۸۰۰ C). The heterogeneous pathway is divided in two different routes: the de novo mechanism and the catalytic-assisted coupling of precursors, both at temperatures between 200 C and 400 C. These temperatures are typical in RWC appliances. PAHs are formed from incomplete combustion of pyrolysis gases or from light organic compounds (Tissari, 2008). Iron ore sintering plants and MSW incinerators are the most important PCDD/F emission sources in Europe (Quaß et al., 2004). PCDD/F emissions from solid fuel (wood and coal) combustion constitute more than 60% of all non-industrial sources (Quaß et al., 2004). Reductions of PCDD/F emissions in waste incineration and iron ore sintering increase more and more the importance of PCDD/F emissions from RWC (Tame et al., 2009). PAH emissions from residential combustion in Finland were estimated to constitute 64% of the total emissions of four genotoxic PAHs (benzo (a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene and indeno(1,2,3-cd)pyrene) in 2003 (Koskinen et al., 2005). Because of the health effects of RWC emissions and the influence of the emissions on the air quality there is a need for emission abatement. Improvements in combustion technique aim to decrease the formation of emissions. However, conventional combustion appliances with high emission factors are still widely used. Secondary reduction techniques such as fabric filters, electrostatic precipitators (ESPs), scrubbers and catalytic converters are used to reduce the emissions. However, because of the high price and technical issues, most of these techniques are not applicable for small-scale combustion appliances (<5 MW) (Hytönen and Jokiniemi, 2007). Catalytic converters offer low cost opportunity for emission reduction for combustion appliances with a conventional combustion technique. The reduction is based on the oxidation of pollutants from incomplete combustion such as CO into CO2 by means of a catalyst. Although catalytic converters are widely used in many countries e.g. the USA where there are emission factor regulations for catalytic wood stoves (US EPA, 1995), the research in this area is scarce. The effects of catalytic converters on the emissions are still not well known. Usually in catalytic converters noble metals are used as active catalysts. Supported palladium and platinum are among the most widely used catalysts (Carnö et al., 1997). It is known that transition-metal species, especially copper, catalyze PCDD/F formation via heterogeneous routes (Tuppurainen et al., 1998; Altarawneh et al., 2009). There are a few studies conducted where PCDD/Fs have been found to decompose by the catalytic effect of platinum and palladium (Ukisu and Miyadera, 2002, 2004). However, there are also studies where the amount of PCDD/Fs has been increased in consequence of platinum and palladium (Hart, 2004, 2008). Thus, the effect of a catalytic converter on the emissions of organic chlorocompounds such as CPhs and PCDD/Fs needs to be studied further. The main aim of this study was to find out if the catalytic converter increases the amount of PCDD/Fs and CPhs, while decreasing the PAH emissions. An additional aim was to find out the emission levels of PCDD/Fs, CPhs and PAHs in different combustion phases without the catalytic converter. Combustion measurements were performed in laboratory conditions using a sauna stove as a combustion appliance. PCDD/Fs, CPhs and PAHs were analyzed from flue gas samples which were collected during combustions with and without the catalytic converter. In addition, samples were taken and analyzed from different combustion phases.