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Article from FILTRATION, 9(1), 2009 HOT GAS FILTRATION - A CASE STUDY HIGHLIGHTING SUCCESSFUL DIOXIN CONTROL Richard Williams (office@rwilliams.co.uk) INTRODUCTION The RODECS® is integrated with a high temperature ceramic air filter, using Brightcross CAN-1000B ceramic filter candles, and a sodium bicarbonate (NaHC03) dry sorbent injection system. Brightcross' have been producing CAN-1000 filter candles for ten years and they have been utilized successfully on 100's of air pollution control systems. These ceramic fibre filter candles are manufactured using an inorganic refractory binder for maximum high temperature strength at temperatures up to 1000°C. The CAN1000B one metre long (60 mm outer diameter) candles are also slightly tapered to enable increased reverse jet clean-down performance, aerodynamic flow around the elements and prevention of particulate build up (or 'bridging') between candles. Referring to Figure 1, during installation and commissioning of the RODECS® system (which includes the Air Pollution Control (APC) system), the filter candles were pre-conditioned. The process is designed to precoat the surface of the filter elements with an inert material of known properties (talc/magnesium silicate in this case). The precoating is undertaken in order to reduce and/or prevent the ingress of sub-micron particulate into the walls of the filter elements, which may otherwise cause 'blinding'. 'Blinding' increases the overall filter pressure drop to an unstable level, i.e. where a stable pressure drop can no longer be maintained by the reverse jet clean-down system, and will continue to increase thus reducing the required extraction and increasing absorbed power consumption of the fan as it struggles to maintain the extraction. The material being processed contained up to 40% by weight of polyvinyl chloride (PVC). Above 250°C, PVC releases nearly all of the chlorine in the form of HCI, leaving a hydrocarbon 'skeleton'. Therefore, due to such high levels of PVC contamination, processing of the material in the RODECS® resulted in extremely high pre-abatement HCl levels (9628 mg/m3). The high level of HCI is highlighted by comparison with the figures for three secondary aluminium smelters. Here, pre-abatement testing experience has shown that HCI levels are between 700 and 1000 mg/m3 Also, without optimizing the APC, dioxin/furan levels have been shown to be as high as 9.9 ng/m3. The main goal of the commissioning/optimization process was to enable all emissions to atmosphere, in particular dioxins, to meet those stipulated by the regulatory environmental body. In order to do so, consideration was given to the methods by which dioxins/furans could be controlled. METHODS FOR DIOXIN/FURAN CONTROL In order to control dioxin emissions, they first need to be destroyed in the incinerator itself - the emphasis being on good combustion conditions. Temperature, residence time and oxygen content are very closely connected to the quality of combustion and often summarized within the term '3 T's' i.e. Temperature, Time and'Turbulence plus Oxygen. The Waste Incineration Directive1 (WID) requires dioxin emissions to be <0.1 ng/m3, which is also the RODECS® guaranteed figure. WID stipulates combustion process requirements for temperature and residence time. However, it is important to note that it is possible to destroy all dioxins in a good combustion. A combustion process leading to low CO and TOC will also have very low dioxin emissions post combustion3. Figure 1: System installation. 
Therefore, a focus solely on temperature for dioxin destruction could be considered too simplistic an approach. As such, Leclerc et al.4,studying the effects of combustion and operating conditions on dioxin emissions, found that 'there are many variables that contribute to dioxin emissions and higher temperatures, by themselves, will not necessarily result in low dioxin emissions'. Others5 found that 'Temperature was not the most important parameter influencing the destruction of dioxins. On the contrary, the level of turbulent mixing (associated with the amount of CO) and residence time proved to be significant'. Although ignored by the WID but mentioned above, turbulence and oxygen are also important considera.tions. For example, a study using a pilot-scale combustor showed that controlling oxygen resulted in 'substantial prevention' of dioxin and furan formation6. As such, an oxygen probe is located post RODECS® which is useful in that oxygen content in the flue gas gives an indication of CO emissions. By monitoring, a better indication of the combustion efficiency can be gained which (as described above) is important information to have in order to gauge dioxin destruction. From the RODECS®, the exhaust gases pass to the APC equipment which is a ceramic filter with a sodium bicarbonate injection system. The de-novo synthesis mechanism is the main one by which dioxins form in the flue gases. The mechanism involves the formation of dioxins from organic and inorganic compounds (known as 'precursors') at temperatures between 200 and 400°C (known as the de-novo temperature 'window')7. The APC operates at temperatures above 400°C and by removing the precursors at such temperatures, the main mechanism by which dioxins are formed is prevented. For example, dioxins tend to form on carbon and fly ash particles which contain carbon, chlorine and trace metals as the gases cool8, and many studies have shown that high ash carbon content generally favours reactions leading to dioxin formation through de-novo synthesis4. Filtration Solutions PVC is found in the type of scrap being processed and is a strong chlorine source for the formation of copper chloride which is a catalyst for dioxins/furans9 formation. Ceramic elements act as a barrier filter, collecting this (along with the fly ash) as particulate. The injection of a 'secondary dust' in the form of the absorbent aids filtration as it enables the particles to agglomerate which in turn allows sub-micron particles to be more easily filtered. Ceramic filters are almost 100% efficient and allow particulate down to 0.1 µm to be filtered. ThUS, based on emission limits achieved in the UK, ceramic filtration systems have been listed as Best Available Technology (BAT) for particulate removal in a number of industrial process guidance notes issued by the Environment Agency in the UK including secondary aluminium processing, HCI removal and if operated at >400°C for dioxin/furan reduction. REMOVAL EFFECTIVENESS As described, during processing of contaminated scrap PVC will decompose to HCI. Although there is still some debate as to whether or not HCI levels can be linked to dioxin/furan emissions 10, it has been shown that major reduction of dioxin/furan formation can be brought about with upstream sorbent injection for HCI6. The APC system incorporates sodium bicarbonate injection in order to neutralise acid gases such as HCI, HF and S02. The action of sodium bicarbonate is a two stage process dependent on a minimum temperature of 100°C at which bicarbonate decomposes to carbonate and the diffusion of carbon dioxide produces a porous, high specific area sodium bicarbonate which is very efficient at absorbing acid gases. Below 100°C, efficiency is very poor. From 150-600°C, the efficiency increases even more. Operation at 400°C is therefore highly efficient when using this absorbent. Prior to determining the sodium bicarbonate additions, pre-abatement flue gas sampling was undertaken for HCI, HF and S02. Noting these figures and using the following reactions, the additions were calculated: NaHC03 + HCI -> NaCI+H20 + C02 2NaHC03 + S02 + 1/202 -> Na2S04 +2C02 + H20 NaHC03 + HF -> NaF + H20 + C02 Following completion of a set of preliminary emissions testing, the results in Table 1 were obtained.
REFERENCES
The FILTRATION journal relies entirely on contributions sent in by filtration and separation professionals from industry and academia. Illustrated and referenced papers should be up to 2500 words long and submitted via e-mail to ~Itrationsolutions@ntlworld.com. The content of papers is professionally reviewed prior to publication. Additional guidelines are available at www.filtrationsolutions.co.uk For further information please contact us on: office@rwilliams.co.uk
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