PCDD/F and PCBz Emissions during Start-up and Normal Operation of a Hazardous Waste Incinerator in China

The PCDD/F emissions from incinerator start-up are a major contributor to the total amount of such emissions, as has been reported in studies of numerous municipal solid waste incinerators.

by Xiaoqing Lin, Qunxing Huang, Tong Chen, Xiaodong Li, Shengyong Lu, Hailong Wu, Jianhua Yan, Miaosheng Zhou, Hua Wang

However, very few studies have examined the start-up process at hazardous waste incinerators (HWIs). In this work we studied the emissions of PCDD/Fs and other pollutants, such as PCBz, at the stack during start-up and normal operations in a HWI. We found that the PCDD/F emissions during start-up  were greater than during normal operations, and were comparable to the annual emissions during the normal combustion regime. The emissions of PCDD/Fs were highest during start-up when the temperature was around 500°C, reaching 59.5 ng/Nm3 (5.49 ng I-TEQ/Nm3) when no APCDs were applied. The emission values of PCDD/Fs during start-up with the application of APCDs and during normal operations were very low, which indicates the importance of APCDs when starting up a HWI, as well as before feeding waste. The chlorination degree, the ratio of PCDFs/PCDDs and the congener profiles were also discussed during start-up and normal operations, with the results suggesting different formation mechanisms of PCDD/Fs. PCBz emissions are two or three orders of magnitude higher than those of PCDD/Fs, and 1,2,4,5-TeCBz was the best correlated PCBz used as a PCDD/F indicator in real HWI flue gas.
In waste treatment, incineration has multiple advantages, such as volume reduction, energy recovery, pathogen elimination and chem ical-toxicity destruction (Dempsey, 1993). Hazardous waste (HW) further refers to industrial hazardous waste, medical ha zardous waste and household hazardous waste. With increasing hazardous waste production in China, from 8.3 million tonnes in 2000 (National Bureau of Statistics of China, 2001) to 14.3 million tonnes in 2009 (National Bureau of Statistics of China, 2010), more attention should be paid to the safe disposal of HW. However, stack emissions from hazardous waste incinerators (HWIs) are a serious concern, not to mention the harmful effect on workers at the incinerator plant and vicinity residents (Aramunt et al., 2005; Ferré-Huguet et al., 2005; Mari et al., 2007).
Polychlorinated dibenzo -p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are unintentional by-products from combustion sources (Lee et al., 2003; Chang et al., 2006; Wang et al., 2009; Hites, 2011). Three mechanisms are involved in the formation of PCDD/F: (1) gas-phase formation of PCDD/Fs from chlorinated aromatics, such as chlorophenols at high temperature (600–1200°C); the formation of PCDDs is favoured over PCDFs (Sidhu et al., 1995; Khachatryan et al., 2003; Qu et al., 2009); (2) de novo synthesis of PCDD/Fs from carbonaceous matter (Stieglitz et al. , 1989; Hell et al. , 2001) and polycyclic aromatic hydrocarbons (PAH) (Iino et al. , 1999) at temperatures between 250 and 400°C (Hagenmaier et al., 1987; Yamamoto et al. , 1989); (3) PCDD/F formation from condensation reactions of precursors such as chlorobenzenes (PCBz) and chlorophenols (PCP) (Mulholland and Ryu, 2001; Ghorishi and Altwicker, 1996) at temperatures between 250 and 650°C (Nakahata and Mulholland, 2000). The last two mechanisms of heterogeneous reactions are endorsed as major pathways (Hell et al., 2001), and dust or soot deposits on walls and boiler tubes are responsible (Blumenstock et al., 2000; Aurell and Marklund, 2009; Lin et al. , 2010) in particular in the presence of catalytic compounds such as copper compound (Chang et al., 2006; Shao et al. , 2010; Lin et al., 2012; Tuan et al., 2012).
PCDD/Fs emission from hazardous waste (including hospital waste) incinerators is the main source of PCDD/Fs   emission of waste incineration, which accounts for approximately 80% of total PCDD/Fs emission (610.5 g I- TEQ) of waste incineration in China (NIP, 2007). As previous study, PCDD/Fs concentrations in stack gas of many HWIs were higher than the national standard of 0.5 ng I-TEQ/Nm3 in 2006 (Liu et al. , 2006). After investigating PCDD/Fs emission from 14 domestic-made medical waste incinerators in China, we found that only two incinerators met the European Union standards of 0.1 ng I-TEQ/Nm3, and even another two plants exhibited emission levels above 10.0 ng I-TEQ/Nm3 (Gao et al., 2009). Therefore, it is necessary to pay more attention to PCDD/Fs emission from the HWIs. Large amounts of PCDD/Fs would be produced during transient combustion conditions from incinerators, even lasting long time after combustion conditions became well controlled (Zimmermann et al. , 2001). Huge emissions of PCDD/Fs have been perceived in several municipal solid waste incinerators (MSWIs) after transient combustion conditions of start-up, shutdown or malfunctioning (Neuer- Etscheidt et al., 2006; Tejima et al., 2007; Wang et al., 2007a, b; Chen et al., 2008) and the temporary stack emission of PCDD/Fs resulting from start-up is between 0.8 and 3 times that of a whole year’s normal operation. The formation of PCDD/Fs during start-up has been recognized as an important source of the PCDD/F emissions from modern MSWIs (Blumenstock et al. , 2000). However, few studies have allowed estimating the importance of start-up for PCDD/F emissions in HWIs, since such HWIs suffer considerably from thermal shock during periodic shutting down. This study compares the formation of PCDD/Fs and PCBz during start-up with that during normal operation conditions in a hazardous waste incine rator (HWI) in China. The PCDD/F contribution during start-up and normal operation is discussed and the correlation among representative PCBzs as surrogates for PCDD/Fs is evaluated.

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published: Aerosol and Air Research, 5|2014
Keywords: Hazardous Waste, China