Distribution of PCDD/F and PCB at Different Positions of Circulating Fluidized Bed Municipal Solid Waste Incinerators

. Because of a high moisture content and a low heating value of Chinese municipal solid waste (MSW), circulating fluidized bed (CFB) incinerators have been widely adopted in China for incinerating MSW since 1998. In this study, two typical CFB incineration plants (A and B) were investigated for contents and fingerprints of PCDD/F and PCB at different positions downstream their post-combustion zone, aiming to draw a full picture of formation and distribution of these organochlorinated pollutants. Both flue gas and ashes were sampled at five different positions of Plant A, from high-temperature superheater to outlet of baghouse filter, representing a huge range of flue gas temperatures; for Plant B, five ash samples were collected at different positions of the waste heat boiler (from high-temperature superheater to lower economizer). A continuous increase in contents of PCDD/F and PCB in flue gas was observed from superheater to inlet of air pollution control system (APCS) in Plant A, with the most significant rise noticed at air preheater. The load of PCDD/F and dioxin-like PCB in ashes also amplified steadily along the cooling path of flue gas in both plants. Changes in formation pathways are discussed based on homologue and isomer distribution patterns.


Introduction
Currently, in China, the majority of municipal solid waste incineration plants use either grate or fluidized bed incineration systems, which account for over 95% of all waste incineration furnaces [1,2]. The grate incineration system is typically used in economically developed large cities due to its large capacity, high cost, and high heat value requirements for waste [2][3][4]. The fluidized bed incineration system is mostly domestically designed and constructed with relatively lower capacity and cost, and the ability to add some coal as auxiliary fuel [3,5]. Municipal solid waste in China is characterized by high moisture content and low heat value, and the fluidized bed waste incineration furnace with added coal as auxiliary fuel has better adaptability, making it widely used in small and medium-sized cities and the central and western regions of China [6,7]. In recent years, the municipal solid waste incineration rate in China has been increasing year by year, which has exacerbated the problem of secondary pollution caused by the emission of persistent organic pollutants such as dioxins, as well as the disposal problem of solid residues such as bottom ash, grate ash, residues from flue gas purification systems, and fly ash. Among them, the dioxin emissions and fly ash disposal problems in fluidized bed incineration systems are particularly severe [8].
Dioxins can be detected in both the flue gas and fly ash generated by waste incineration furnaces. The concentration and distribution characteristics of dioxins are affected by many factors, including the composition of the waste, the type of incineration furnace, the flue gas purification system and the operating conditions [9]. Understanding the distribution of dioxins in the flue gas and fly ash at different stages of the combustion process can help establish a comprehensive understanding of the entire process of dioxin generation and emission in waste incineration, and provide a basis for further adopting relevant measures to control the emission of dioxins in the flue gas. Mariani et al. [10] measured the concentration of PCDD/F in the flue gas at different stages of the grate incineration system and found that the concentration of PCDD/F in the flue gas increased sharply after the boiler. Wang et al. [11,12] investigated the distribution of PCDD/F in the ash of different equipment in waste incineration systems. The highest PCDD/F content was found in the economizer ash, which was due to the higher operating temperature (about 370℃) of the economizer that promoted the generation of PCDD/F significantly [12]. There is a significant correlation between the content of PCDD/F and PCB in various types of ash and the operating temperature of the corresponding equipment [12].
Previous studies on dioxin emissions from waste incineration plants have mostly focused on the final emissions after bag filter systems, with little attention paid to the entire process of dioxin generation and emissions in the post-combustion zone, and even less on the distribution of dioxins in fly ash from different heat transfer surfaces in the energy recovery system. Based on this, this paper sampled the flue gas and fly ash at different stages after combustion from two typical fluidized bed waste incineration plants in China to discuss the concentration and distribution characteristics of PCDD/F and PCB in the flue gas and fly ash. The purpose of this chapter is to establish a comprehensive understanding of the entire process of dioxin generation and emissions in waste incineration plants, and to provide guidance for the targeted implementation of dioxin reduction measures [13,14].

2
Methods and materials Figure 1 shows the process flow diagram of the two typical circulating fluidized bed municipal solid waste incineration systems studied in this paper. Municipal solid waste enters the furnace after sorting and crushing. The suspended fluidized bed in the furnace is supported by upward airflow, which enhances combustion and promotes mixing of solid materials. The flue gas from combustion reaches the cyclone separator, where the solid materials carried with it are sent back to the fluidized bed for further combustion. The flue gas continues to the heat recovery system (consisting of the high-temperature superheater, low-temperature superheater, convective tube bundle, economizer, and air preheater), then flows through the semi-dry scrubber, activated carbon injection system, bag filter system, and other flue gas purification equipment, and is finally discharged into the atmosphere through a chimney. The two circulating fluidized bed incineration systems studied in this chapter are referred to as Furnace A and Furnace B, and their corresponding sampling locations are shown in Figure 1. Furnace A was put into operation in 2005, with a rated waste treatment capacity of 400 t/d, an actual incineration rate of 300 t/d, a flue gas flow rate of 125,000 Nm 3 /h, and a coal blending rate of about 20 wt.%. During the normal operation of the incinerator, flue gas samples were taken in accordance with the sampling method detailed in Section 2.2. Five sampling points were sampled simultaneously, and two samples were collected in sequence for each sampling point, with a sampling duration of 2 hours for each set of samples. The sampling positions and actual temperatures were as follows: A-S1, high-temperature superheater outlet (540-560℃); A-S2, economizer inlet (490-510℃); A-S3, economizer outlet (260-280℃); A-S4, flue gas purification system inlet (170-180℃); A-S5, after bag filter (about 130℃). The collected samples were divided into the flue-gas gas phase and fly ash solid phase. The fly ash samples collected from the first four sampling points were designated as FA1-FA4. As the sampling point of A-S5 is located after the bag filter, little fly ash was collected, and thus the baghouse re-feed ash was collected as FA5. Furnace B was put into operation in 1998, with a rated waste treatment capacity of 300 t/d and a coal blending rate of 10-15 wt.%. During the shutdown period of the incinerator, fly ash samples from different heat exchange equipment in the heat recovery system were collected according. The sampling positions and corresponding temperatures during normal incinerator operation were as follows: B-S1, hightemperature superheater (820-640℃); B-S2, lowtemperature superheater (640-540℃); B-S3, convective tube bundle (540-510℃); B-S4, upper economizer (510-310℃); B-S5, lower economizer (310-270℃).
The gas-phase and solid-phase samples collected from the five sampling points of Furnace A were pre-treated and analyzed separately for PCDD/F and PCB. Due to two sets of online sampling, two gas phase samples and two solid phase samples were collected at each point, and the average of the two sets of samples was taken for analysis. Fly ash samples collected from the five sampling points of the heat recovery system in Furnace B were also analyzed for PCDD/F and PCB.   before economizer -high-temperature homogeneous routes; economizer to inlet of APCS -low-temperature heterogenous pathways. High-temperature homogeneous synthesis: 500-800 ℃; low-temperature heterogenous formation: < 500 ℃. In CFB MSW incineration system, formation of PCDD/F is mainly attributed to heterogeneous catalytic pathways.
The variations of the total amount and toxicity equivalents (I-TEQ) of 17 toxic PCDD/F isomers in the flue gas at different stages of Furnace A are shown in Figure 3. Similar to the trend of total PCDD/F, both the concentration and toxicity equivalents of 2,3,7,8-PCDD/F increased gradually during the cooling process of the flue gas and were mostly removed by the flue gas purification system. The maximum range of increase in concentration and toxicity equivalents of 2,3,7,8-PCDD/F was between the economizer outlet (A-S3) and the inlet of the flue gas purification system (A-S4), which is mainly attributed to the low-temperature catalytic synthesis of PCDD/F (especially PCDF) by fly ash in the air preheater segment.

Distribution of PCB in flue gas at different stages
The total concentrations of 209 PCBs in the flue gas at different stages of post-combustion zone of Furnace A are shown in Figure 4, while the variations of the total quantities and toxicity equivalents (WHO-TEQ) of 12 dioxin-like PCBs (dl-PCBs) are shown in Figure 5. The PCB concentrations in the flue gas at different stages are generally one order of magnitude higher than the corresponding PCDD/F concentrations. PCB concentrations showed an overall increasing trend during flue gas cooling (from A-S1 to A-S4), but there was no obvious increase between A-S2 and A-S3. Compared with the total amounts of PCBs, the variation trend of dl-PCBs and their toxicity equivalents is more similar to that of PCDD/F, indicating that dl-PCBs not only have similar biological toxicity to dioxins, but also have similar formation mechanisms to PCDD/F. The removal efficiency of the flue gas purification system for PCB toxicity equivalents was 98.6%, which is similar to the removal efficiency of PCDD/F (98.8%) [15].

Conclusion
A continuous rising trend was observed in the PCDD/F-, PCB-and TEQ-concentrations in the effluent downstream the gas cooling process, until the inlet of APCS. The most significant rise for PCDD, PCDF and I-TEQ were noticed at economiser (PCDD) and air preheater (PCDF and I-TEQ). The removal efficiencies of the toxicity equivalent of PCDD/F and PCB were 98.8% and 98.6%, respectively. A rather high linear correlation was observed between dl-PCB and PCDD/F (R 2 = 0.99, p < 0.001), higher than that between PCB and PCDD/F (R 2 = 0.92, p < 0.01), suggesting the closer formation pathways of dl-PCB to that of PCDD/F. This study may help establish an overall understanding of each step in the formation and emission of dioxin-like POPs throughout the waste incinerators. The conclusions of this paper are expected to provide reference and guidance for future research on the formation, inhibition and degradation of dioxins.