Environ. Sci. Technol. 2007, 41, 2001-2007
Minimizing Dioxin Emissions from Integrated MSW Thermal Treatment WAI HUNG CHEUNG, VINCI K. C. LEE, AND GORDON MCKAY* Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
The combustion of wastes has very significant benefits in reducing the volume of waste materials and producing energy. However, combustion processes produce emissions, which must be below the Best Practical Means (BPM) specified legislative limits. Several wastes, such as tires and meat meal, have been successfully combusted in cement kilns, up to 20% w/w, while retaining emission standards well below legislative limits. In the case of municipal solid waste (MSW) the introduction of large amounts of MSW into cement kilns is not practical because the additional kiln volume required is too great, the large amounts of ash generated will affect the cement clinker quality, and it would be difficult to sustain the required very high clinkering temperature of 1500 °C with large quantities of low calorific value MSW. A completely novel process, termed the CoCo process, has been developed, integrating MSW combustion in a synergistic fashion with the cement production. This process is based on combining the cement “front-end” calcination reaction and incorporating it with a high temperature, at 1200 °C, combustion process, providing a giant acid gas scrubber. A pilot plant was designed, constructed, and operated to demonstrate the benefits of the Co-Co process. The pilot plant achieved emissions minimization: dioxins were typically 0.5-1% of the European BPM limits, HCl, SOx, NOx, and particulates were 15, 10, 20, and 25% of BPM limits, respectively. Heavy metals were typically below 25% of BPM limit values.
1. Introduction The awareness of the release of highly toxic organic compounds, known as dioxins from the incineration of wastes is a relatively modern phenomenon (1, 2). The practice of incinerating organic liquid wastes from chemical plants and the release of emissions into the atmosphere became common practice in the 1950s and 1960s. Its extension to incinerating solid wastes, particularly municipal solid waste, also increased throughout the sixties and seventies. This particular practice had the advantage that these processes could recover energy from the burning waste, and there was a decrease of 80-90% by volume thus reducing the area required for landfilling wastes quite dramatically. It is well-established that combustion processes result in the formation and/or release of unwanted byproducts, such as SOx, NOx, TOC, HCl, HF, CO, and CO2, into the atmosphere. In addition, minor amounts of more toxic materials such as metals, dioxins, and furans are also released in the emissions. * Corresponding author phone: 852+2358-8412; fax: 852+23580054; e-mail:
[email protected]. 10.1021/es061989d CCC: $37.00 Published on Web 02/08/2007
2007 American Chemical Society
The present R&D project is based on developing synergy between MSW combustion and the Portland cement production process to minimize toxic releases in an effective and economically beneficial manner. In recent years it has been well-established that modern cement plants can utilize limited amounts of certain wastes in their kilns very effectively and sustain low emissions. Three clinker kilns in Spain (3), using waste tires and meat meal, had average dioxin emissions of 0.007 ng International toxic equivalent, I-TEQ/ normal cubic meters, Nm3 (“normal” is referred to the reference conditions of 0° temperature, 101.325 kPa pressure, dry, and 11% oxygen content condition). A survey of 16 cement kilns in Germany using secondary fuels had an average dioxin emission of 0.0016 ng World Health Organization toxic equivalent, WHO-TEQ/Nm3 (4) and several values from a survey on cement kilns in Thailand (5) also had an average value less than the European Best Practical Mean, BPM, value of 0.1 ng I-TEQ/Nm3 (6). Other survey data from Taiwan (7), France (8), England (9), and Italy (10) also confirmed the suitability of cement kilns for waste combustion. However, none of the secondary fuels in these studies included MSW, so there is no data integrating cement manufacturing and MSW combustion. The production and release of dioxin from waste combustion is of great public concern. There are many theories and counter theories relating to a dioxin formation mechanism. However, a few general basic facts have emerged: (i) The presence in some chemical form of the elements that make up dioxins, namely carbon, hydrogen, oxygen, and chlorines, has resulted in many researchers (11-16) proposing that the presence of carbon monoxide, unburnt carbon, or other unburnt combustion products (even soot particles), organic precursors, metal salts, and hydrogen chloride/chlorine are associated to the amount of dioxin formation. The dioxin formation takes place in a temperature window in the range 200 °C to 600 °C with a maximum rate of the formation reaction between 350 and 400 °C (17-21). Data from these references suggest that the rates are very slow in the ranges 200-250 °C and 550-600 °C. Obviously, in combustion systems, dioxins are formed during the flue gas cooling cycle via one of two mechanisms. The phenomenon known as “de-novo” synthesis (22) is based on reactions between organic carbon sources and inorganic chlorine in the presence of a metal catalyst, particularly copper chloride. An alternative theory (23) is based on dioxin formation from chloro-organic precursors rearranging on fly ash particles. (ii) The process conditions have a profound effect on the extent of dioxin formation (24-28) and from the earlier discussion, conditions that lead to incomplete combustion will favor dioxin formation, namely low combustion temperatures, short residence times in the combustion zone, poor turbulence in the combustion chamber, low oxygen levels leading to incomplete combustion, a slow flue gas cooling process in the critical temperature range.
2. Developing the Co-Co Process A conceptual MSW co-combustion system was designed to minimize dioxin emissions: “the Co-Co Process”. A joint project was undertaken by the Department of Chemical Engineering and Green Island Cement Company Limited to identify a set of process conditions that would lead to minimum formation of dioxin in the flue gas emissions from MSW combustion systems. The starting point was a review of Best Practical Means (BPM) (29) and these specify the following: a minimum gas temperature leaving the combusVOL. 41, NO. 6, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Co-combustion conceptual diagram.
FIGURE 1. Dioxin formation with temperature and residence time (30). tion chamber of 850 °C, a minimum residence time of 2 s in the combustion chamber, a minimum oxygen concentration in the exit gas of 6.5% v/v, and a dioxin concentration in the stack flue gas less than 0.1ng I-TEQ/Nm3 In modern conventional MSW incinerators a temperature of 850 °C can be sustained from moderately dry MSW alone; if the combustion exit temperature falls below 850 °C then supplementary fuel must be used. To elevate the combustor temperature above 850 °C will always require supplementary fuel, and this makes stand-alone incineration of MSW above 850 °C uneconomical. All well-operated modern MSW units are able to meet the BPM (29) dioxin emission levels by the use of end-of-pipe carbon adsorption and lime scrubbing. Consequently an optimum solution to minimize dioxins due to elevated temperature operation means integrating MSW combustion with another high-temperature process industry. A brief analysis identified the cement industry as a very suitable host. Many wastes have been successfully combusted in cement kilns (30), but they have the potential to flash vaporize and exit from the kiln without complete combustion; the low bulk density of MSW would mean very large rotary cement kilns and the variable MSW ash composition would mean less effective quality control on the final product cement clinker. Therefore, based on these factors, the key stage in our novel process was to develop a new front end calcination process for integrated cement manufacture. High-temperature MSW hot gases were generated at 1200 °C to heat and convert limestone to lime; 80% of the cement kiln raw material. Other literature data (17, 18, 27, 30, 31) indicate that two critical success factors to minimize dioxin formation are the combustion temperature and the combustion residence time. A plot correlating data from the references is shown in Figure 1. A value of 4 s is proposed for Co-Co and this theoretical dashed time line is shown between the other three times for plastic wastes for residence times of 2, 3, and 5 s (30), respectively. From Figure 1, for a 4 s residence time at 850 °C and no other treatment, the predictive dioxin concentration is 0.17 ng and at 1200 °C is 0.005 ng I-TEQ/Nm3. The possibility of utilizing this high-temperature reaction for the cement calcinations reaction was identified. Further synergy in this system was identified by using the lime to scrub out the dioxin precursor acid gas, HCl, which could be converted to harmless calcium chloride; this stage would also reduce SOx. With a huge >40:1 lime to HCl ratio greater than conventional dry acid gas scrubbers, in the cement process the HCl could be reduced by 80%. Finally, with rapid flue gas cooling (between 600 and 200 °C), active carbon injection (100 mg/Nm3) and baghouse solid/gas filtration (25), a final dioxin level
