Environ. Sci. Technol. 2006, 40, 6411-6417
Investigation of Novel Incineration Technology for Hospital Waste Y A N G S H E N G L I U , * ,† L A N L A N M A , † YUSHAN LIU,‡ AND GUOXING KONG§ Department of Environmental Engineering, Peking University, the Key Lab of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China, Taiyuan Chenfeng Environmental Engineering Co. Ltd., Taiyuan, 518034, China, and Zhenzhou Tianchen Environmental Protection Co. Ltd., Zhenzhou, 030006, China
Conventional incineration systems for hospital waste (HW) emit large amounts of particulate matter (PM) and heavy metals, as well as dioxins, due to the large excess air ratio. Additionally, the final process residuess bottom and fly ashes containing high levels of heavy metals and dioxinssalso constitute a serious environmental problem. These issues faced by HW incineration processes are very similar to those confronted by conventional municipal solid waste (MSW) incinerators. In our previous work, we developed a novel technology integrating drying, pyrolysis, gasification, combustion, and ash vitrification (DPGCV) in one step, which successfully solved these issues in MSW incineration. In this study, many experiments are carried out to investigate the feasibility of employing the DPGCV technology to solve the issues faced by HW incineration processes, although there was no MSW incinerator used as a HW incinerator till now. Experiments were conducted in an industrial HW incineration plant with a capacity of 24 tons per day (TPD), located in Zhenzhou, Henan Province. Results illustrated that this DPGCV technology successfully solved these issues as confronted by the conventional HW incinerators and achieved the expected results for HW incineration as it did for MSW incineration. The outstanding performance of this DPGCV technology is due to the fact that the primary chamber acted as both gasifier for organic matter and vitrifying reactor for ashes, and the secondary chamber acted as a gas combustor.
Introduction Hospital waste (HW) may contain human organs, bandages, needles, syringes, test tubes, blood tubes, tissue cell culture, and other plastic materials. Incineration has been the most widely used treatment of hospital waste (1). In Western Europe more than 600 HW incineration plants have been in operation (2). The incinerators commonly employed in HW incineration include low-cost rough incinerators (LCI), twostage excess-air incinerators (TSI), rotary kiln incinerators (RKI), and air-starved pyrolytic incinerators (ASPI). The LCI has been principally employed in developing countries. The more recent versions of simple brick incin* Corresponding author fax: 86-10-6275 9804; e-mail: liu_ysh@ 263.net. † Peking University. ‡ Taiyuan Chenfeng Environmental Engineering Co. Ltd. § Zhenzhou Tianchen Environmental Protection Co. Ltd. 10.1021/es060190z CCC: $33.50 Published on Web 09/19/2006
2006 American Chemical Society
erators equipped with two chambers and a short chimney were built on-site. The excess air ratio for the LCI reaches up to 200% (3). These incinerators have widespread deficiencies in their design, construction, siting, operation, and management, resulting in high emission levels of dioxins as much as 4000 ng TEQ/Nm3 (3). Therefore, use of LCI simply converted the HW pollution into serious air pollution. The TSI generally has two chambers (4). The waste from the charging system is delivered into the primary chamber and incinerated at 850 °C under excess air condition (with an excess air ratio of 150-200%). Ash is removed from the bottom of the primary chamber mechanically or manually. The secondary chamber is used to re-burn the flue gas produced in the primary chamber at a temperature over 1100 °C under excess air condition. Auxiliary fuel is necessarily supplied for the high-temperature combustion. A case study by Kerdsuwan (4) demonstrated that PM levels of stack emission in this system was as high as 388 mg m-3, even though it was equipped with a wet Venturi scrubber. The RKI is designed with a primary chamber, where the waste is heated and volatilized, and a secondary chamber, where combustion of the volatile fraction is completed. Both chambers are operated under excess air condition. RKI was the common mass-burning systems required 50-100% of excess air over the stoichiometric value (5), resulting in a large amount of flue gas to be scrubbed. Due to the turbulent motion of the waste in the primary chamber, particulate entrainment in the flue gas is more intensive for RKI than for air-starved incinerator designs. An experimental campaign conducted by Sbrilli et al. showed that the stack emission of dioxins from the RKI system installed in Southern Italy never met the EU legal limit of 0.1 ng TEQ/Nm3 (6). In short, the TSI and the RKI were the conventional massburning systems with large excess-air ratios. Despite the employment of advanced and complicated air pollution control systems (APCS), harmful emissions of acidic gases (SOx, HCl, HF, NOx, etc.) (6) and volatile organic compounds (VOCs) especially polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polychlorinated dibenzop-dioxins/-furans (PCDD/Fs) (7, 8), and PM, as well as heavy metals (1), were emitted; additionally, the large volume of final process residues constitutes another major problem (9, 10). Compared with municipal solid waste incineration, HW incineration had the higher emission factor of dioxins (15.025 mg t-1 of waste or 145 ,000 ng TEQ t-1 of waste) (11), because HW had many components (e.g., organic polymers, tissues, and infectious waste) with a high content of chlorine leading to the formation of dioxins (12, 13). Most of these dioxins were concentrated in the fly and bottom ashes. Due to the large excess-air ratio and intensive PM entrainment, the conventional mass-burning systems (e.g., TSI and RKI) produced large amounts of flue gas containing toxic organics, heavy metals, PM, and acidic gases, and left a great deal of fly and bottom ashes which were in the proportion of 1030% of the original amount of HW. Additionally, the auxiliary fuel consumption led to increased operation costs. To address these issues more effectively and provide for energy efficient, environmentally and economically sound solutions, an advanced incineration technology integrated with pyrolysis and gasification processes, namely air-starved pyrolytic incinerators (ASPI), received much attention in recent years (14-18). The ASPI combines pyrolysis, gasification, and combustion in an integrated or modular approach. HW is first gasified at 600-800 °C in the primary chamber (or pyrolytic chamber), and then the pyrolysis gases are VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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directed to the secondary chamber with an afterburner operated at 1100 °C, and afterward to a heat exchanger where they are cooled down to 250-300 °C. Finally, the stack gases pass the APCS (15). Compared with TSI and RKI, ASPI produces much lower emissions of PM and dioxins. According to Quass et al. (15), the ASPI equipped with dry stack gas cleaning or dry/wet stack gas cleaning emitted dioxins of 0.08 ng TEQ Nm-3 and 0.02-0.06 ng TEQ Nm-3, respectively. However, the RKI equipped with dry/wet stack gas cleaning emitted dioxins of 2-5 ng TEQ Nm-3. Grochowalski (9) investigated the concentrations of dioxins in stack gases emitted from 13 modern HW incineration plants in Poland which were equipped with ASPI, and found that dioxins emissions were between 0.015 and 0.42 ng TEQ Nm-3 with an average of 0.12 ng TEQ Nm-3. Kerdsuwan (4) reported on two TSI systems imported from Australia operated by the Bangkok Metropolitan Administration of Thailand (10 TPD for each); PM levels of stack emissions for this system were as high as 388 mg m-3; however, the emission levels of dioxins were not available. However, the ASPI system still discharges a large amount of bottom and fly ashes. In comparison with TSI and RKI, the toxic components in ASPI were overwhelmingly transferred from the flue gas into the ashes. Bottom ash generally contained high levels of heavy metals with potential leachability (19, 20), and even high levels of dioxins (6, 21). Particularly, fly ash must be detoxified and stabilized because it contained significant concentrations of heavy metals, such as lead (Pb), chromium (Cr), copper (Cu), and zinc (Zn), as well as organic pollutants such as PAHs and dioxins (6, 10, 20). Several technologies to treat these ashes have been proposed. Solidification, stabilization, vitrification, classification by granulometric particle size, and aging or weathering were some of the currently available methods (22, 23). Except for vitrification, it is difficult to apply these techniques to fly ash because of the high concentrations of chlorine compounds existing in the forms of dioxins and alkali chlorides. Due to the hindrance of alkali chlorides to hydration of cement, dioxins were difficult to destroy or stabilize by cementation or chemical treatment (24). Vitrification was the most promising solution of the various available technologies for fly ash treatment (23). When fly ash was heated to >1300 °C (23), or >1100-1200 °C together with the proper amount of glass-forming additives (25), relatively inert glasses were formed; dioxins were completely destroyed, and heavy metals were also stabilized through incorporation into the glass matrix. Therefore, the bottom and fly ashes produced in the TSI, RKI, or ASPI systems were unstable and unfriendly to the environment without further processing to improve their characteristics. These further treatment processes were efficient to stabilize and/or detoxify these ashes, but the energy requirement made them expensive, especially the separate melting process for fly ash vitrification. All these issues (i.e., large excess air ratio and a great deal of toxic residues) faced by HW incineration processes are very similar to those confronted by conventional municipal solid waste (MSW) incinerators. In our previous work (26), the DPGCV technology was developed which successfully solved these similar issues in MSW incineration. In this study, many experiments are carried out to investigate the feasibility of employing this DPGCV technology to solve the issues present in the HW incineration processes, although there is no MSW incinerator was used as a HW incinerator till now. The process flow chart of HW incineration unit and the illustration for the primary chamber are presented in Figures 1 and 2, respectively, of the Supporting Information. 6412
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To demonstrate the feasibility of this technology, emissions of air pollutants including particulate matter (PM), CO, SO2, NOx, HCl, HF, heavy metals (Hg, Cd, As, Pb, and Cr), and dioxins were measured for comparison with their corresponding legal limits in the United States (USEPA), Europe (EU), and China (China Pollution Control Standard for Hazardous Waste Incineration (CPCS)). Loss on ignition (LoI) of bottom ash was measured to investigate the combustion completeness. The leachability of heavy metals in bottom ash was also measured according to the Identification Standard for Hazardous Waste (ISHW), i.e., GB5085.31996 (27), to examine immobility of heavy metals in the vitrified slag. Dioxins levels in bottom ash were determined for comparison with those from the air-starved pyrolytic incineration technology. Additionally, the results from this study were compared with those from our previous work to examine the applicability of DPGCV technology application in the incineration of HW.
Experimental Section Experimental Setup. As described in our previous work (26), the DPGCV technology comprised four separate parts: (a) primary chamber, a rotating cylindrical reactor (clockwise) with an inner rotating eccentric grate (anticlockwise), where pyrolysis, gasification, and combustion of HW and vitrification of residues occur, (b) secondary combustion chamber, a cylindrical reactor with a fuel nozzle, where the yielded flue gases in primary chamber were burnt out to completely destroy toxic organic compounds, (c) waste heat boiler, and (d) air pollution control system (APCS). The process flow chart is illustrated in Figure 1 of the Supporting Information. For a newly installed HW incineration unit, the ignition process was needed. At first, split firewood and/or anthracite were heaped on the grate, and then 25 L of gasoline was sprayed onto the heaped pyrophorus. The flame-thrower fixed on the top of the primary chamber ignited. Simultaneously, the auxiliary fuel was injected into the secondary chamber and ignited by another flame-thrower fixed on the top of the secondary chamber. After about 20 min, HW was added. It took about 30 min to keep the combustion temperature over 1200 °C in the secondary chamber, and about 4 h to make various parameters stable in the primary chamber. The high temperature in the secondary chamber could be maintained by virtue of the heat emitted from the combustibles oxidation without auxiliary fuel. HW was fed using a pair of rollers from the hopper via the drying zone into the pyrolysis zone for thermolysis at 600700 °C whereby the required heat was supplied by the upstream flowing fuel gas coming from char gasification zone where part of char was gasified with air at about 800-900 °C. The incombustibles were conveyed into the melting zone for vitrification with air at about 1200 °C, where the rest char was completely gasified and the produced combustibles were burnt out ensuring the melting temperature. The primary air was pre-heated to ∼400 °C within an economizer by hightemperature flue gas leaving the secondary chamber, and then injected beneath the rotating grate into the primary chamber. The primary air level was approximately 50% of the stoichiometric requirements for combustion. The upgoing air was further heated by the hot slag bed on the rotating grate ensuring high melting temperature in the vitrification zone; simultaneously, the hot slag bed was cooled. This “cooled” hot slag bed was located between the vitrification zone and the grate, which efficiently isolated the grate from the high-temperature zone, preventing the grate from directly contacting the melting slag. An illustrative diagram for the primary chamber and the schematic process are shown in Figure 2a and b of the Supporting Information. As shown in Figure 2a, a pair of rollers was running continuously and crossly, crushing the HW when its width
between two rollers was over 20 cm. The cylinder rotated clockwise at a speed of 10 revolutions per hour (rph), driven by an electric engine through the gear wheel fixed on the cylinder’s bottom, so HW from the hopper could be evenly distributed into the primary chamber. The eccentric grate with three cones rotated counterclockwise at a speed of 5 rph, driven by another electromotor through the gears in the back surface of the lowest cones of the grate. The reverse rotation between the cylinder and the eccentric grate enhanced the gas turbulence within the HW bed. With continuous rotation of the grate, the vitrified residues were discharged. The primary chamber was dimensioned to enable sufficient residence time to allow for almost complete volatilization of organic matter contained in the HW while the (heavy) metals would ideally be retained in the char. The residence times for solid and flue gas were 2.0-2.5 h and 2.0 s, respectively. The pyrolysis gas left the primary chamber together with the gases produced in the gasification and vitrification processes, and entered the secondary chamber for combustion at 1200-1300 °C with secondary supplied air to destroy toxic organic compounds (combustion gas residence time was 2.0 s). After heating the primary air (at ambient temperature) in the economizer, the high-temperature flue gas entered a waste heat boiler to raise steam. In succession, the flue gas was then scrubbed through the APCS and emitted into the atmosphere. The APCS consisted of three basic components: spray-dryer system, baghouse, and activated carbon filter. In the spray-dry system, alkaline droplets (
