Novel Incineration Technology Integrated with Drying, Pyrolysis

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Environ. Sci. Technol. 2005, 39, 3855-3863

Novel Incineration Technology Integrated with Drying, Pyrolysis, Gasification, and Combustion of MSW and Ashes Vitrification Y A N G S H E N G L I U * ,† A N D Y U S H A N L I U ‡ Department of Environmental Engineering, Peking University, the Key Lab of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China, and Shen Han’s Solid Waste Treatment Equipment Company Limited, Shenzhen, 518034, China

The conventional mass burn systems for municipal solid waste (MSW) emit large amount of acidic gases and dioxins as well as heavy metals due to the large excess air ratio. Additionally, the final process residues, bottom ash with potential leachability of heavy metals and fly ash with high level of heavy metals and dioxins, also constitute a major environmental problem. To deal with these issues more effectively, a novel MSW incineration technology was developed in this study. MSW drying, pyrolysis, gasification, incineration, and ash vitrification were achieved as a spectrum of combustion by the same equipment (primary chamber) in one step. In practice, the primary chamber of this technology actually acted as both gasifier for organic matter and vitrifying reactor for ashes, and the combustion process was mainly completed in the secondary chamber. Experiments were carried out to examine its characteristics in an industrial MSW incineration plant, located in Taiyuan, with a capability of 100 tons per day (TPD). Results showed that (1) the pyrolysis, gasification, and vitrification processes in the primary chamber presented good behaviors resulting in effluent gases with high contents of combustibles (e.g., CO and CH4) and bottom ash with a low loss-on-ignition (L.o.I), low leachability of heavy metals, and low toxicity of cyanide and fluoride. The vitrified bottom ash was benign to its environment and required no further processing for its potential applications. (2) Low stack emissions of dioxins (0.076 ng of TEQ m-3), heavy metals (ranging from 0.013 to 0.033 mg m-3), and other air pollutants were achieved. This new technology could effectively dispose Chinese MSW with a low calorific value and high water content; additionally, it also had a low capital and operating costs compared with the imported systems.

Introduction Within the waste management hierarchy, incineration with energy recovery was a desired and viable option often used in industrialized nations (1). Incineration was a most effective form of managing the disposal of MSW, which reduces potential environmental risks and potentially converts MSW * Corresponding author phone: 86-10-62759804; fax: 86-1062751184; e-mail: [email protected]. † Peking University. ‡ Shen Han’s Solid Waste Treatment Equipment Company Limited. 10.1021/es040408m CCC: $30.25 Published on Web 04/15/2005

 2005 American Chemical Society

into recoverable energy (2). Comparing incineration to other disposal options, advantages would become evident in specific applications, especially as added prohibitions and increasingly burdensome costs were placed on land disposal. The conventional mass burn systems required 40-100% excess air over the stoichiometric value (3), resulting in a large amount of flue gas to be scrubbed. Despite that the advanced and complicated Air Pollution Control System (APCS) was employed, harmful emissions (4) of acidic gases (SOx, HCl, HF, NOx etc.) (5) and volatile organic compounds (VOCs) (especially polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polychlorinated dibenzop-dioxine/-furans (PCDD/Fs) (6, 7)) as well as heavy metals (8) were emitted; additionally, the final process residues also constitute a major problem (9, 10). The conventional mass burn system left a large amount of fly and bottom ash, which were produced in the proportion of 10∼30% of the original amount of waste. The extreme heterogeneity of the bottom ash (11, 12) combined with the potential leachability of heavy metals meant that some form of processing (e.g., weathering (13) and washing (14)) was likely to be required to improve the characteristics of the bottom ash prior to utilization in most civil engineering applications. Particularly, fly ash must be detoxified or decontaminated because it contained significant concentrations of heavy metals (10), such as lead (Pb), chromium (Cr), copper (Cu), and zinc (Zn), as well as organic pollutants such as PAHs and dioxins. Several technologies to treat these ashes were proposed. Solidification, stabilization, vitrification, classification by the granulometric size particles, and aging or weathering were some of the currently available methods (12, 15). Except for vitrification, it was 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. Because of the hindrance of alkali chlorides to the hydration of cement, dioxins were difficult to destroy or stabilize by cementation or chemical treatment (16). Vitrification was the most promising solution of the various available technologies for fly ash treatment (15). When fly ash was heated to >1300 °C (15), or >1100∼1200 °C together with the proper amount of glass-forming additives (17), 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 conventional mass burn 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 were energy requiring and thus expensive, especially the separate melting process for fly ash vitrification. To address these issues more effectively and to provide for energy efficient and environmentally and economically sound solutions, a new MSW processing technology, pyrolysis and gasification of MSW, has received much attention recent years (18, 19). Pyrolysis is theoretically a zero-air indirect heat process. However, in practical applications, it is an air-starved process in that combustion is occurring with air levels less than stoichiometric requirements for combustion (2). In pyrolysis, waste organic compounds are distilled or vaporized to form combustible gas. Heat for the pyrolysis process can be provided by the partial combustion of the pyrolysis gas and by the combustion of elemental carbon. The unoxidized portion of the combustible gas may be used as fuel in an VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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external combustion chamber (e.g., secondary combustion chamber), with the resulting energy recovered by conventional waste-heat-boiler technology. MSW pyrolysis and in particular gasification is obviously very attractive to raise the fuel gas caloric value and to reduce and avoid corrosion and emissions by retaining alkali and heavy metals (except mercury and cadmium), sulfur, and chlorine within the process residues, to prevent large PCDD/F formation, and to reduce thermal NOx formation due to lower temperatures and reducing conditions (20). Recently, advanced thermal waste treatment technologies combining pyrolysis, gasification, and incineration in an integrated or modular approach are growing from long lasting research, technology, and developmental activities into demonstration and even commercialization (1, 21). The majority of these technologies aim primarily on improving environmental compliance mainly by effectively destructing air pollutants and vitrification of the residues using high combustion or gasification temperatures. The very recent advanced technology, OxiTherm process by Babcock borsig Power GmbH, combines drying, pyrolysis, gasification, and combustion of MSW in a single step on a water-cooled grate in a compact designed shaft furnace with high-temperature air combustion of the yielded gas and melting of incombustibles (1). Oxygen is injected underneath the loose pile of waste at supersonic velocity, ensuring combustion and vitrification at about 1500 °C. However, no information is currently available in open literature on its further development and realization.

Status Quo of MSW Incineration in China In China, MSW has been increasing at an annual rate of 8∼10% in recent years. In 2000, municipal residents discarded about 140∼160 million tons of MSW. Although 345 MSW management facilities (including 298 landfills, 24 composting factories, and 23 incinerations) were constructed up to 2000, these were far from being sufficient. The total capacity for all the previously mentioned management facilities was 49.4 million tons annually; therefore, only 35∼40% of MSW was treated or disposed. Locating waste landfills in China has become more and more difficult owing to limited land space. As an alternative, incineration has the advantage of obviously reducing waste volume and mass. However, MSW incineration in China is not profitable currently. First, the imported equipments are very costly, and all the projects being developed or under construction have utilized foreign government credit in China. Facility investment has reached up to 700 000 Yuan RMB/daily ton of capacity, which is at least 4 times higher than for the landfill. Second, Chinese waste is of low calorific value and high water content (ranging from 35 to 60%), as compared to those from developed countries whose main components are sorted organic wastes. In China, the average calorific value of municipal waste is 1000∼1500 kcal kg-1 in the eastern region and large cities and 500∼800 kcal kg-1 in certain middle-sized and small cities. Therefore, the MSW incineration has to be conducted with fuel (e.g., coal, natural gas, or oils) as auxiliary energy, which increases the operation cost. Among the 16 MSW incineration plants investigated (22), there were 11 plants with a high concentration of air pollutant emissions, especially for CO, particulate matters, and dioxins, which significantly exceeded their respective legal limits in the China Pollution Control Standard for MSW Incineration (CPCS) (23). Therefore, the current incineration processes are not the best suitable options for MSW in China. In this study, we developed a novel incineration technology, which combined MSW drying, pyrolysis, gasification, and combustion processes as well as incombustible vitrification processes in a single step on an eccentric rotating grate 3856

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FIGURE 1. Process flowchart of the MSW incineration unit. in a compact designed cylindrical furnace (i.e., primary chamber, see Figures 1 and 2). The yielded gas was further burned out in the secondary chamber at around 900∼1100 °C. Many experiments were carried out to examine its characteristics. Emissions of air pollutants including particulate matters (PM), CO, SO2, NOx, HCl, and HF; heavy metals consisting of Hg, Cd, As, Pb, and Cr; and dioxins were measured for comparison with their corresponding legal limits in CPCS. Loss on ignition (L.o.I) of bottom ash was measured to investigate the combustion completeness. The leachability of heavy metals in bottom ash was also measured according to Identification Standard for Hazardous Waste (ISHW) (i.e., GB5085.3-1996 (24)) to examine immobility of heavy metals in the vitrified slag.

Experimental Procedures Experimental Setup. The novel process developed in this study comprised four separate parts: (a) primary chamber, a rotating cylindrical reactor with an inner rotating eccentric grate, where pyrolysis, gasification, and combustion of MSW and vitrification of residues occur; (b) a secondary combustion chamber, a cylindrical reactor with a fuel nozzle, where the yielded flue gases in the primary chamber were burnt out to completely destroy toxic organic compounds contained in the fuel gas; (c) waste heat boiler; and (d) air pollution control system (APCS). The process flowchart was illustrated in Figure 1. For a newly installed MSW incineration unit, the ignition process was needed. At first, split firewood and/or anthracite was heaped on the grate, and then 25 L of gasoline was sprayed onto the heaped pyrophorus. The flame-thrower was 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, MSW was added. It took about 1 h to keep the combustion temperature higher than 850 °C in the secondary chamber and about 8 h to make various parameters stable in the primary chamber. MSW was fed using a pair of rollers from the hopper via the drying zone into the pyrolysis zone for thermolysis at 600∼700 °C, whereby the required heat was supplied by the upstreamflowing fuel gas coming from the char gasification zone where part of the 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 of the char was completely gasified and the produced combustibles were burnt out ensuring the melting temperature. The primary air was preheated to around 400 °C within an economizer with high-temperature flue gas leaving the secondary chamber and then injected beneath the rotating

TABLE 1. Typical Proximate and Ultimate Analyses of MSW in Taiyuan analytical parameter

FIGURE 2. (A) Illustration for primary chamber structure. 1: Hopper; 2: electrometer and roller; 3: primary chamber cylinder; 4: refractory materials; 5: water cooler; 6: electrometer (for rotating grate); 7: rotating grate; 8: gear wheel for cylinder; 9: primary air supply; 10: bottom ash outlet; and 11: electrometer. For a 100 TPD incineration system, the specification of the primary chamber is as follows: cylinder height, 6 m; cylinder diameter, 3.0 m (inner) and 3.8 m (outer); hopper length, 4.5 m; water cooler height, 3 m; grate height, 2 m. (B) The schematic processes in the primary chamber. 1: Drying zone; 2: pyrolysis zone; 3: gasification zone; 4: vitrification zone; and 5: slag zone. grate into primary chamber. The primary air level was approximately close to 75% of the stoichiometric requirements for combustion. The upgoing air was further heated by a hot slag bed on the rotating grate ensuring a high melting temperature in 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 was shown in Figure 2a,b. As seen in Figure 2a, a pair of rollers was running continuously and crossly, which could crush MSW when its width between two rollers was over 20 cm. The fine MSW is beneficial for complete combustion. The cylinder rotated clockwise at a speed of 10 revolutions per hour (rph), which was driven by an electromotor through the gear wheel fixed

value

Proximate Analysis moisture ash fixed carbon and volatile matter

40.5% 18.8% 40.7%

Ultimate Analysis carbon hydrogen oxygen nitrogen chlorine sulfur moisture ash (inerts) caloric value (kcal/kg)

23.18% 2.37% 13.82% 0.67% 0.37% 0.29% 40.5% 18.8% 1500

on the cylinder’s bottom, so MSW from the hopper could be evenly distributed into the primary chamber. The eccentric grate with three cones was rotated counterclockwise at a speed of 5 rph, which was 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 MSW bed. With the grate rotation, the residues were discharged. The primary chamber was dimensioned to enable sufficient residence time allowing for almost complete volatilization of the organic matter contained in the MSW while the (heavy) metals would ideally be retained in the char. The residence times for solid and glue 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 900∼1100 °C with secondary supplied air to destroy toxic organic compounds. After heating the primary air (at ambient temperature) in the economizer, the hightemperature flue gas entered the waste heat boiler to raise steam. In succession, the flue gas was then scrubbed through APCS and emitted into the atmosphere. The APCS consisted of three basic components: spray-dryer system, bag house, and activated carbon absorption column. In the spray-dry system, lime slurry droplets (less 30 µm in diameter) were sprayed into the flue gas through a rotary atomizer to remove the acid gases produced by sulfur, chlorine, and fluorine in the MSW. Then, the flue gas entered the bag house, and the PM was removed. Afterward, the flue gas was further purified by the activated carbon absorption column, where particulates and heavy metal emissions were further controlled. The fly ash collected from the secondary chamber, boiler, and APCS was mixed with fresh MSW and then fed from the hopper into the primary chamber. The chemicals such as CaO, CaCl2, and CaF2 contained in the APCS fly ash and the great deal of glass pieces originating from the fresh MSW (accounting for 2∼3 wt % of MSW) were beneficial to reduce the vitrification temperature of ash in the melting zone. Under the action of the previous materials, the vitrification temperature was reduced to about 1000∼1100 °C, which was far below the normal temperature, 1300 °C (5, 25). At present, a 100 TPD MSW incineration plant has operated for 15 months in Taiyuan of Shanxi Province, and another 200 TPD plant is in construction in Ji’nan of the Shandong Province. From November 20-25 in 2003, experiments were carried out in the MSW incineration plant with a capability of 100 TPD located at Taiyuan to examine the characteristics of this newly developed technology. The typical proximate and ultimate analyses of MSW in Taiyuan were shown in Table 1. VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Sampling train for HCl, HF, NOx, SO2, or heavy metals. 1: Sampling probe; 2: thermometer; 3: filter holder; 4: impingers; 5: silica gel; 6: flow meter; and 7: connection to vacuum pump. Methods of Sampling and Analysis. Dioxins in Flue Gas. The outlet of the stack was sampled for PCCD/Fs in accordance with U.S. EPA Method 23, using a U.S. EPA Modified Method 5 sampling train. The sampling period lasted for 8 h. The XAD-2 resin was spiked prior to sampling with isotopically labeled PCDD/F surrogate standards (Shimadzu Techno-Research Inc., Japan). The filter resin and impinger solutions were extracted with organic solvents, and the extract was purified by chemical and solid-phase chromatographic techniques. Particulate and gas-phase extracts were pooled (26). Measurement of PCDDs and PCDFs was performed using a HP5890 high-resolution gas chromatograph and a VG70-S high-resolution electron impact mass spectrometer. The detection limit for T4∼P5CDD/F was 0.0008 ng m-3, for H6∼H7CDD/F was 0.002 ng m-3, and for O8CDD/F was 0.004 ng m-3. The I-TEQ was calculated using NATO (1989) international toxic equivalent factors (I-TEQs). All samples were analyzed in triplicate. PM, CO, O2, SO2, NOx, HCl, and HF. The sampling and analysis of PM, CO, O2, SO2, NOx, HCl, and HF in the combustion gas were carried out according to standard methods described in CPCS (23). Their concentrations were measured in the outlets of the primary chamber and secondary combustion chamber and the inlet of the stack. The sampling train for SO2, NOx, HCl, and HF was shown in Figure 3. The impingers were submerged in an ice bath to enhance condensation. The absorption solutions for HCl, SO2, NOx, and HF were aqueous solutions of sodium hydroxide, ammonium sulfamate, dilute sulfuric acid/ peroxide, and sodium hydroxide, respectively. All the chemicals mentioned previously were of analytical purity. The other information related with sampling and analysis was listed in Table 2. The sampling train and analysis apparatus for CO or O2 was presented in Figure 4. The CO tester and O2 analyzer were corrected using the standard gas with determinate concentration before the experiment process. The concentration range for CO monitoring was 0∼50 000 mg m-3; its accuracy and relative standard deviation (RSD) was (3 and 0.36%, respectively, and the detection limit was 10 mg m-3. The analysis of O2 was performed by an oxygen analyzer (OX-1A/HL-201, China); the concentration range for O2 monitoring was 0∼25% (v %), and its accuracy and RSD was (0.7 and 1.0%, respectively. PM was collected using glass fiber filters (0.3 µm pore size). After that, the particulate was completely recovered, 3858

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dried, and weighed. This method could be used to determine concentrations ranging from 0.05 to 100 mg m-3; the inaccuracy of this method was about (10% for concentrations above 0.05 mg m-3. Heavy Metals in Stack Emissions. Metal concentrations in the combustion gas, either in particulate matter or in the gas phase, were estimated according to standard procedures (23). After particulate matter collection, gaseous emissions passed through an aqueous acidic solution of hydrogen peroxide (analyzed for all metals) and an aqueous acidic solution of potassium permanganate (analyzed for mercury). The recovered samples were digested to be analyzed. Cd, Cr, Pb, Zn, Cu, and Ni were quantified by atomic absorption spectroscopy (AAS) using direct aspiration. As and Hg were also quantified by AAS, using, respectively, graphite furnace and cold vapor generation techniques. The standardization and calibration (of sampling train and spectrometer) were made according to standard procedures as well as the preparation of field blanks (23). The detection limits for Cd, Cr, Pb, Zn, Cu, Ni, As, and Hg were, respectively, 3, 50, 100, 5, 20, 40, 0.1, and 1 µg L-1. The blank concentrations were always lower than the detection limits. To check the accuracy of the results, recovery assays were carried out spiking subsamples with known quantities of the analyte (23). The recoveries were between 95 and 105%. Each average was based on at least three analyses, repeated until it reached a relative standard deviation lower than 5%. The calculations were based on the measuring of combustion gas flow rates and respective pollutant concentrations. The pollutant concentrations were compared with domestic and international legal limits. For that, they were converted to dry standard conditions (273 K and 101.3 kPa) referred to 11% O2 in combustion gas, being designated by Cstd@11%O2 and Vstd@11%O2 (23). Leachability of Heavy Metals of Bottom Ash. Heavy metal leachability was evaluated by a toxic characteristic leaching procedure (TCLP) according to the Chinese standard acetic acid leaching test (24). The sampled bottom ash was sieved and that passing #4 mesh was used for the test. This test consisted of contacting a given amount of solid material with distilled water (liquid/solid weight ratios ) 6) and keeping the pH of the leaching medium at 5 ( 0.2 units throughout the test by the addition of a 0.5 N acetic acid solution; the maximum amount of acetic acid solution to be added was 4 mL g-1 solid sample. After 24 h, the solid-liquid separation was carried out through the vacuum filtration on a 0.45 mm membrane filter, and the resulting leachate was diluted with distilled water to a volume that was 20 times the weight (g) of the solid material tested. Finally, the leachate was analyzed for concentrations of heavy metals including Hg, Pb, As, Cd, Cu, Zn, Ni, Be, Cr, and Cr6+. Additionally, concentrations of cyanide and fluoride were also evaluated by ISHW to further investigate the toxicity of bottom ash. L.o.I of Bottom Ash. Bottom ash was preliminarily dried in an oven at 105 °C until its weight kept constant. L.o.I. was then determined by heating W0 g of dried sample (W0 ≈ 10 g) at 650 °C for at least 3 h until it kept a constant weight (for example, W g). Therefore, L.o.I could be calculated by the following equation:

L.o.I ) (W0 - W)/W0 × 100%

(1)

Results and Discussion Mass Balance. The stoichiometric air requirement for complete combustion (per 100 kg of MSW) at zero excess air was calculated as shown in Table 3. The ultimate analyses of MSW were illustrated in Table 1. Therefore, for a 100 TPD MSW incineration system, the stoichiometric air requirement for complete combustion was 18 236 m3 h-1. The practical air supply was 21 883 m3 h-1

TABLE 2. Detailed Information for Sampling and Analysis of HCl, SO2, NOx, and HF pollutants

sampling duration (min)

analysis methodsa

detection limit (mg/Nm3)

accuracy (%)

RSDb (%)

20 20 30 60

colorimetry using mercuric thiocyanate iodine titration method ultraviolet spectrophotometric method ion selective electrode method

10 10 10 0.2

2.0 5.0 3.8 2.9

1.5 2.5 3.3 1.5

HCl SO2 NOxc HF

a The analysis methods in this column are the standard ones for these pollutants enacted by the Chinese EPA, which are also included in CPCS. RSD: relative standard deviation (i.e., reproducibility of the analysis method). c Concentrations of NOx were not tested until the absorption solution was stored for at least 6 h at room temperature. NOx absorbed in the solution could be completely oxidized into NO3- during this period. b

FIGURE 4. Sampling and analysis for CO and O2. 1: Sampling probe; 2: filter holder; 3: dewetting (glass wool); 4: CO nondispersive infrared tester or O2 analyzer; 5: flow meter; and 6: connection to vacuum pump.

TABLE 3. Calculation of Air Requirement at Zero Excess Air

element

weight (kg)

carbon 25.18 hydrogen 6.65a oxygen 48.04b nitrogen 0.67 sulfur 0.29 chlorine 0.37 ash 18.8 total 100

mol of mol of oxygen atomic element required combustion weight (kmol) (kmol) production 12.011 1.008 16.000 14.008 32.006 35.453

2.096 6.597 3.003 0.048 0.009 0.010 11.763

2.096 3.293 -1.501 0.000 0.009 0.000 0.010c 3.908

CO2 H2O N2 SO2 HCl

a ,bIncluded

hydrogen and oxygen from 38.5% moisture. c An assumed value for partial burning of metals and other inorganics.

with an excess air of 20%. The primary air supply was 13 585 m3 h-1, approximately close to 75% of the stoichiometric requirement for complete combustion. The secondary air supply was 8298 m3 h-1, which ensured that the toxic compounds contained in the flue gas were completely destroyed under excessive oxygen condition. The mass balance for the MSW incineration system located at Taiyuan with capability of 100 TPD was illustrated in Figure 5. The air-starved process in the primary chamber led to the low PM carry-over in effluent gas and the small volume of fly ash production (only 1.7% of the amount of fresh MSW). The air pollutant levels in different points (labeled in Figure 5) were monitored and shown in Table 4. Air Pollutants. The air pollutant levels in the outlets of the primary and secondary chambers and the inlet of the stack were measured (see Table 4). The legal limits for each component in the European Union (EU), U.S. EPA, and Chinese EPA were also listed in Table 4. Because of the air-starved process in the primary chamber, the oxygen concentration in the flue gas was only 2.5%; however, concentrations of CO and CH4 reached up to 4.8 and 5.5%, respectively, resulting from pyrolysis and gasification processes. In the secondary chamber, the flue gas with rich combustibles (e.g., CO and CH4) was incinerated

completely with high-temperature air containing rich oxygen; therefore, the CO level in the outlet was only 40 mg m-3. The levels of air pollutants (including CO, NOx, HCl, SO2, and PM) emitted from the stack met their respective emission limits set by the Chinese EPA, EU, and the U.S. EPA. The HF emission level met the Chinese legal requirement but exceeded the EU required emission limit. However, the U.S. EPA did not set the HF emission limit. As shown in Table 4, the NOx emission level was far below the legal limit in the Chinese EPA or the U.S. EPA, although no any additional DeNOx measures were employed. As compared with the conventional mass burning system, lower NOx emission was one of the major advantages for the newly developed incineration system. Additional complicated measures were needed in the mass burning system to reduce the NOx emission below 70 mg m-3 (dry, 11% O2); for example, a catalytic reaction system (CRS) was used to remove NOx (27). In CRS, the catalyst TiO2/V2O5/WO3 was employed, and a substoichiometric feed ratio (NH3/NOx) was used to minimize the slip of unconverted ammonia. However, unconverted ammonia was responsible together with SO3 and H2O for the formation and deposition of ammonium bisulfate, which caused serious problems of corrosion and a pressure drop in the cold equipment downstream of the CRS (27). The primary air supply was only 75% of the stoichiometric requirement for complete combustion, so the gas turbulence could not carry over large amounts of PM. The PM level in the primary chamber outlet was as low as 2500 mg m-3, which was only 1/3∼1/4 of those emitted from the conventional MSW incineration systems in China (5). PM levels in stack emissions met the legal limit of EU or the U.S. EPA. Controlling PM will control emissions of most toxic metals and toxic organic compounds adsorbed onto the PM. The excess air level for the new developed technology was only 20%, which was far below the recommended excess air levels for conventional mass burn systems (see Table 5). The lower excess air level resulted in the lower flow-rate of the flue gas as compared with the conventional mass burn system, which was beneficial to reduce the equipment investment of APCS. The excess air level had a significant effect on the NOx formation. When the air supply in the primary chamber increased, the excess air ratio would be enlarged, and the air-starved process would gradually become similar to the conventional mass burning process. As can be seen in Figure 6, the NOx concentration in the flue gas from the primary chamber increased with an increase of the excess air ratio. When the excess air ratio was 2.2, the NOx concentration reached the maximal value, 120 mg m-3 (dry, 11% O2). Therefore, the conventional mass burning system needed to implement the additional measures to reduce the NOx emission below 70 mg m-3 (dry, 11% O2) (27). Dioxin Emissions. Table 6 showed the dioxin emissions results. Since 2,3,7,8-TCDD was the most toxic congener, it was assigned by convention a toxicity rating of 1.0 (called a toxic equivalent factor or TEF). The TEFs for the other 2,3,7,8positional congeners were determined by the ratio of the VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Mass balance for a 100 TPD MSW incineration system. 1: Primary chamber; 2: secondary chamber; 3: waste heat boiler; 4: lime-milk spray tower; 5: bag house; 6: I.D. fan; and 7: stack.

TABLE 4. Air Pollutant Levels in Outlets of Primary and Secondary Chambers and the Inlet of Stack componentsa

primary chamber outlet

secondary chamber outlet

stack inlet

Chinese EPA limits

CO (mg/m3) O2 (%) NOx (mg/m3) SO2 (mg/m3) HCl (mg/m3) HF (mg/m3) PM (mg/m3)

4.8d 2.5 30 100 170 21.5 2500

40 8 50 105 180 23.2 2300

35 8 40 12 30 4.5 30

80 6∼12% 500 300 70 7.0 80

a

U.S. EPA limitsb

89/369/EEC limitsc 100

357.14 55 178.57 50

300 50 2 30

b

The CH4 concentration in the primary chamber outlet was measured to be 5.5%. 40 CFR Part 62, Federal Plan Requirements for Small Municipal Waste Combustion Units Constructed on or Before August 30, 1999. Subpart JJJ Emission limits for existing Small Municipal Waste Combustion Units with a capacity less than or equal to 250 TPD. The limit values in the U.S. EPA regulation (at 7% oxygen) were recalculated at the standard percentage oxygen concentration, 11%. c Council directive on the prevention of air pollution from new municipal waste incineration plants (89/369/EEC). d Unit in %.

TABLE 5. Recommended Excess Air Levels for Mass Burn Systems mass burn grate system

recommended excess air (%)

ref

Martin Von Roll Bartolemeis Detroit Stoker Volund Westinghouse

90 80 85∼110 80 90 50

28 2 29 30 2 2

FIGURE 6. Effect of excess air ratio on NOx concentration in flue gas from primary chamber. toxicity of each individual congener to that of 2,3,7,8-TCDD. The toxicity of any mixture of PCDDs and PCDFs could then be expressed by multiplying the concentration percentage of each individual 2,3,7,8-positional congener present in the mixture by its respective TEF. The result for each congener 3860

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was called the toxic equivalent (TEQ). The unit of TEQ was identical to that in which the concentrations of the individual congeners were expressed. The TEQ of the mixture was obtained by the addition of individual TEQs (31). On the basis of the experiments (Table 6), the emission indicatrix of 0.076 ng of TEQ m-3 was below the advanced international standards of 0.1 ng of TEQ/m3 (31). The total emission of dioxins and furans (mass basis) was 17.13 ng m-3, which was at least a factor of 5 below the U.S. EPA legal limit of 89.29 ng m-3 at 11% O2 (conversion from 125 ng m-3 at 7% O2). At present, the legal emission limit for dioxins in China was 1.0 ng of TEQ m-3 (23). Some researchers (32) measured emissions of dioxins from the imported mass burning system installed in Shenzhen and reported that the average emission level of dioxins was 0.17 ng m-3 (at 11% O2), which was far below the Chinese legal limit (1.0 ng m-3) but much higher than the international legal one (0.1 ng m-3). Therefore, as compared with this imported mass burning system, the newly developed technology could more effectively control the emission of dioxins; in other words, this new technology was more suitable for the domestic MSW incineration. Stack Emissions of Heavy Metals. It was estimated that in Europe, 3.0, 1.7, 0.7, and 7.0% of the emissions of Cd, Cu, Pb, and Zn, respectively, originated from refuse incineration (33). Morseli (34) indicated that Cr and Mn were mainly retained in the slag (70%), while Pb, Zn, and As were equally partitioned between fly ash and slag. Nevertheless, the metals mentioned previously only had a small fraction (less than 5%) in flue gas. Because of the relatively high vapor pressure, Hg was predominant as the gas phase in flue gas. Vogg (35) indicated that more than 66% mercury in the flue gas would penetrate the APCS and be released into the atmosphere in

TABLE 6. PCDD/Fs Levels in Emitted Flue Gas levelsa

PCDDs

PCDFs

a

TABLE 7. Stack Emissions of Heavy Metals I-TEQ (ng of TEQ/m3)

PCDD/Fs

(ng/m3)

I-TEF

2,3,7,8-T4CDD T4CDDs 1,2,3,7,8-P5CDD P5CDDs 1,2,3,4,7,8-H6CDD 1,2,3,6,7,8-H6CDD 1,2,3, 7,8,9-H6CDD H6CDDs 1,2,3,4,6,7,8-H7CDD H7CDDs O8CDD total PCDDs 2,3,7,8-T4CDF T4CDFs 1,2,3,7,8-P5CDF 2,3,4,7,8-P5CDF P5CDFs 1,2,3,4,7,8-H6CDF 1,2,3,6,7,8-H6CDF 1,2,3, 7,8,9-H6CDF 2,3,4,6,7,8- H6CDF H6CDDs 1,2,3,4,6,7,8-H7CDF 1,2,3,4,7,8,9-H7CDD H7CDFs O8CDF total PCDFs PCDDs + PCDFs EU legal limit, I-TEQ U.S. EPA legal limit (ng m-3)

N.D. 0.18 0.0071 0.66 0.089 0.1 0.12 1.6 0.36 0.69 0.17 3.98 0.051 1.2 0.16 0 3.4 0.062 0.063 0.018 0.070 5.3 0.17 0.17 2.3 0.19 13.15 17.13

1.0

0

0.5

0.00355

0.1 0.1 0.1

0.0089 0.010 0.012

0.01

0.0036

0.001 0.1

0.00017 0.03827 0.0051

0.05 0.5

0.0082 0

0.1 0.1 0.1 0.1

0.0062 0.0063 0.0018 0.0070

0.01 0.01

0.0017 0.0017

0.001

0.00019 0.03819 0.07646 0.1

PCDD/Fs levels at 11% O2.

b

125b

Total mass basis.

a typical MSW incinerator. As far as the mercury was concerned, the removal efficiency was typically less than 30% with the corresponding emission level of 0.20 mg m-3 (36). Stack emissions of heavy metals such as Hg, Cr, Cd, and Pb showed a consistent relationship with PM (37), so linear or greater reductions in emissions could be expected as PM emissions were reduced, which is consistent with the more effective removal of fine particles. The quantity of PM that was carried over by the products of combustion rising from the bed of burning solid waste depended upon the velocity of the gases leaving the primary chamber. The velocity mainly depended upon the amount and distribution of primary air. The new developed technology in this study had a low excess air ratio (i.e., a low flue gas velocity) and a low PM carry-over (see Table 4) as compared to the conventional mass burn systems, so the stack emissions of heavy metals could be more effective to be controlled. Similar results were demonstrated by diagnostic tests of the Quebec WTE facility (38). As shown in Table 7, concentrations of Hg, Cd, and Pb in stack emissions were far below their respective U.S. EPA legal limit. Additionally, concentrations of Hg + Cd and Ni + As were at least a factor 4 below their respective EU legal limit of 0.2 and 0.1 mg m-3. Therefore, the stack emissions of heavy metals were effectively controlled in this new developed technology. Bottom Ash. Leachability of Heavy Metals in Bottom Ash. Fly ash was collected from the secondary chamber, wasteheat boiler, and APCS and then again fed into the primary chamber for vitrification in the melting zone. No fly ash was discharged in this system, so there was no extra energy needed for vitrifying these hazardous wastes, which was superior to the conventional mass burning system. The results in Table 8 indicated that the vitrification process that occurred in the melting zone of the primary

heavy metals

emission levels (mg/m3)

Chinese legal limits, (mg/m3)

U.S. EPA limitsa (mg/m3)

Hg Cd As Pb

0.033 N.A.b 0.018 0.013

0.2 0.1 1.0 1.6

0.057 0.071

Cr Zn Ni Cu

0.021 0.026 0.010 0.010

4.0c

1.14

89/369/EEC limits (mg/m3) Cd + Hg ) 0.2 Ni + As ) 0.1 Pb + Cr + Cu + Mn ) 5.0

a Emission limits in U.S. EPA (at 7% O ) were converted into the 2 standard limits at 11% O2. b N.A.: not available. The detection limit for Cd is 3 × 10-6 mg/m3. c For the total emissions of Cr + Sn + Sb + Cu + Mn.

TABLE 8. Leachability of Heavy Metals in Bottom Ash (mg/L) heavy detection Chinese EPA U.S. EPAa Japanesea limits limits metals leachability limits limits Hg Pb As Cd Cu Zn Ni Be Cr6+ Cr a

1.98×10-4 N.A.b N.A. N.A. 0.23 0.38 0.12 N.A. N.A. 0.045

Cited from ref 39.

10-4 0.04 0.04 0.03 0.04 0.03 0.10 0.002 0.004 0.004 b

0.05 3.0 1.5 0.3 50 50 10 100 1.5 10

0.2 5.0 5.0 1.0

0.005 0.3 0.3 0.3

5.0

N.A.: not available, under detection limit.

chamber was very effective in reducing the toxicity of bottom and fly ashes. Concentrations of Hg, Pb, As, Cd, and Cr in the leaching solution met Chinese EPA, U.S. EPA, and Japanese TCLP requirements. Concentrations of Cu, Zn, and Ni in the leaching solution were higher than those of other heavy metals but also met the Chinese EPA TCLP requirements. Particularly, we could find that the vitrification process could also control the leachability of Cr6+, which was much more toxic even than Cr. Although the leaching of heavy metals was limited well below the environmental regulation, the leachability of cyanide and fluoride was also evaluated to assess the potential toxicity of bottom ash for the possible applications in the environment. The levels of cyanide and fluoride in leachate were 0.0041 and 0.09 mg L-1, respectively, which were far below their respective legal limits, 1.0 mg L-1 for cyanide and 50 mg L-1 for fluoride. The dioxins contained in fly ash were completely destroyed during the vitrification process (40, 41). Therefore, the vitrified bottom ash could be safely used as potential materials in the environment. L.o.I. Three parallel samples were weighed and measured to evaluate the results of L.o.I, which were calculated according to eq 1. The average value of L.o.I for these three samples was 3.0%, which was well below the legal limit, 5.0%, so the fixed carbon contained in the MSW was completely incinerated and converted into combustion gases such as CO2 and combustibles. Capital and Operating Cost Comparisons. Capital Cost Comparison. The capital costs for the 100 TPD incineration system (without power generation system) in Taiyuan and the 200 TPD incineration system (with power generation system) in Ji’nan were about ¥ 9.5 million RMB, and ¥ 19 million RMB, respectively (hereafter, ¥ is the symbol for Chinese Yuan). Therefore, the facility with a capacity of 100 TPD was about ¥ 95 000 RMB/daily ton of capacity, while the VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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facility with a capacity of 200 TPD was ¥ 190 000 RMB/daily ton of capacity when the capital charges were expressed in 2000 RMB. There have been several imported incineration facilities in China since the 1980s. The Qingshuihe MSW incineration plant with a capacity of 1000 TPD in Shenzhen was the first waste-to-energy plant in China, which was imported from Canada in the 1990s. The capital cost for this plant was ¥ 7.2 billion RMB. From 1998 to 2004, the Shenzhen municipality imported three MSW incineration systems with capacities of 300∼500 TPD from Canada, and the facilities had costs from ¥ 600 000 RMB/daily ton of capacity to ¥ 800 000 RMB/daily ton of capacity. In December 2003, two waste-to-energy plants with capacities of 800 TPD and 450 TPD were installed in the Nanshan and Yantian Districts of Shenzhen. The facilities were imported from Belgium. The total capital cost for these two plants was over ¥ 700 million RMB, and the facility cost was nearly ¥ 600 000 RMB/daily ton of capacity. In conclusion, the facility costs for the imported incineration systems ranged from 600 000 RMB/daily ton of capacity to 800 000 RMB/daily ton of capacity, which was at least 3∼4 times higher than the newly developed incineration system in this research. Operating Cost Comparison. The operating costs for the Nanshan and Yantian incineration systems were between ¥ 210 and ¥ 240 RMB ton-1; if the maintenance costs were added, the operating and maintenance costs ranged from ¥ 310 to ¥ 340 RMB ton-1, so the local government had to supply a great deal of money to keep these plants running. The sky-high operating and maintenance costs have become a great burden on the local government. Similarly, the other imported incineration systems also had the costly operating and maintenance costs, varying between ¥ 280 and ¥ 350 RMB ton-1. After operation for about 15 months, the operating cost for the newly developed incineration systems was about ¥ 52 RMB ton-1, and the maintenance cost was about ¥ 20 RMB ton-1, so the total cost for operating and maintenance was about ¥ 72 RMB ton-1. In comparison, the operating and maintenance costs for the imported systems were at least 4 times higher than those for the new system developed in this research.

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Received for review April 12, 2004. Revised manuscript received February 18, 2005. Accepted March 14, 2005. ES040408M

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