Environ. Sci. Technol. 2007, 41, 7509-7515
Municipal Solid Waste Fueled Power Generation in China: A Case Study of Waste-to-Energy in Changchun City H E F A C H E N G , * ,† Y A N G U O Z H A N G , ‡ AIHONG MENG,‡ AND QINGHAI LI‡ Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, and Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, People’s Republic of China 100084
With rapid economic growth and massive urbanization in China, many cities face the problem of municipal solid waste (MSW) disposal. With the lack of space for new landfills, waste-to-energy incineration is playing an increasingly important role in waste management. Incineration of MSW from Chinese cities presents some unique challenges because of its low calorific value (3000-6700 kJ/kg) and high water content (∼50%). This study reports a novel wasteto-energy incineration technology based on co-firing of MSW with coal in a grate-circulating fluidized bed (CFB) incinerator, which was implemented in the Changchun MSW power plant. In 2006, two 260 ton/day incinerators incinerated 137 325 tons, or approximately one/sixth of the MSW generated in Changchun, saving more than 0.2 million m3 landfill space. A total of 46.2 million kWh electricity was generated (38 473 tons lignite was also burned as supplementary fuel), with an overall fuel-to-electricity efficiency of 14.6%. Emission of air pollutants including particulate matters, acidic gases, heavy metals, and dioxins was low and met the emission standards for incinerators. As compared to imported incineration systems, this new technology has much lower capital and operating costs and is expected to play a role in meeting China’s demands for MSW disposal and alternative energy.
Introduction With the sustained and rapid economic growth in the last three decades, China is undergoing massive urbanization. The total population increased from 962.6 million in 1978 to 1375.6 million by the end of 2005. Meanwhile, the country’s urbanization rate increased from 17.4 to 41.8%. The country’s rapid economic growth also presents some unprecedented environmental challenges. Municipal solid waste (MSW) is one of the major problems that affect China’s environmental quality and the sustainable development of its cities. Increased waste generation results from both the increasing population and the improved lifestyle of the people. The country produces more than 150 million tons of MSW each year, and MSW generation is increasing at an annual rate of 8-10%. * Corresponding author phone: (+1) 650 723-1478; fax: (+1) 650 725-3162; e-mail:
[email protected]. † Stanford University. ‡ Tsinghua University. 10.1021/es071416g CCC: $37.00 Published on Web 10/04/2007
2007 American Chemical Society
MSW refers to the materials discarded in urban areas for which municipalities are usually held responsible for collection, transport, and final disposal. In China, the physical components of MSW typically include food waste, paper, textiles, rubber, plastic, glass, metals, wood, and inert materials (e.g., stones, ceramics, ashes, etc.). Small amounts of industrial wastes and construction wastes occasionally may also end up in MSW. Waste sorting is not implemented in China, and these components are not separated. On the other hand, some of the MSW components, such as metals and paper, are highly recycled because of their value. The informal recycling sector, comprised of street pickers, dump pickers, and itinerant buyers, is involved in waste scavenging and recycling activities in China, as in other developing countries (1-3). Because of the lifestyle differences and current trash disposal practice, MSW in China shows some distinct compositional characteristics. Comparison of the average compositions of MSW in selected cities in China with those in other countries is shown in Table S1 of the Supporting Information. Food waste makes up the largest fraction (∼50%) of MSW in most cities in China, while the contents of paper and metals are very low because of their high recycling levels. This is similar to the situations in many developing countries. In contrast, wood and yard trimmings constitute the largest component of MSW generated (47%), and paper is the second-largest component (34%) in the U.S. MSW in developed countries typically has a high content of paper, while that of food waste is relatively low. The number of MSW management facilities increased from 12 in 1979 to a total of 479 by the end of 2005 in China, but they could only dispose of 52% of the total MSW generated (0.49 million tons/day) (4). Landfill, composting, and incineration disposed of approximately 85, 5, and 10% of the wastes, respectively (4). More than half of the landfills in China do not have leachate collection and treatment systems, causing serious surface water and groundwater contamination in many sites (4, 5). Contamination of water and soil from poorly managed municipal landfills poses growing health and ecological threats in China. On the other hand, construction of new lined landfills is restricted by the land space available in metropolitan areas. MSW composting and compost application were widely promoted in China in 1990s. However, due to the lack of waste sorting prior to disposal and appropriate materials separation processes, the contents of noncompostables such as glass and plastic are high, while the nutrient contents are relatively low (0.5-1.1% N, 0.30.7% P, and 0.3-0.6% K) in the compost. Elevated levels of heavy metals (e.g., Hg, Pb, and Cr) and pathogens are also found in the compost products (5). As a result, most MSW composting facilities have ceased to operate because of technical difficulties and low market demand for compost products. Incineration is an alternative in MSW disposal with the primary benefit of substantial reduction of the waste’s weight (up to 75%) and volume (up to 90%). Organic wastes are broken down, and bacteria and viruses are also destroyed during incineration. MSW incineration in developing countries is generally limited by several factors, including significant capital and operating costs, potential environmental impacts, and technical difficulties of operating and maintaining an incinerator and its pollution control equipment (3). One distinct characteristic of MSW in developing countries is its high moisture level (typically around 50%), which is much higher than that (20-30%) in the MSW in the U.S. and European countries. Because of the traditions of materials recovery and extensive picking in China, the calorific values in the VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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MSW streams are rather low. The high moisture content further lowers the MSW’s energy content. Results of proximate and ultimate analyses for MSW generated in selected cities in China are shown in Table S2 of the Supporting Information. The MSW’s calorific values (3000-6700 kJ/kg) are typically less than half of those (8400-17 000 kJ/kg) of the MSW in developed countries, which are mainly composed of sorted organic wastes (6, 7). As a result, supplementary fuels (e.g., coal, natural gas, or oils) are often necessary for incineration of such low energy content wastes. Besides waste reduction, incineration can also generate revenues from energy production, known as waste-to-energy, which partially offsets the cost of incineration. The heat released from combustion of the MSW can be collected through steam generation, which is subsequently used for heating or power generation. MSW power plants are designed to dispose of MSW and to produce electricity as a byproduct of the incinerator operation. MSW is a source of biomass: food waste and yard trimmings are examples of biomass trash, while materials that are made out of glass, plastic, and metals are not (8). The U.S. Environmental Protection Agency (U.S. EPA) considers MSW to be a renewable energy resource because the waste would otherwise be sent to landfills (9). The Energy Information Administration of the U.S. Department of Energy now includes MSW in renewable energy only to the extent that the energy content of the MSW source stream is biogenic (8). Currently, most MSW power plants in China rely on equipment imported from North America and Western Europe, which costs 0.6-0.7 million Yuan Ren Min Bi (RMB)/ daily ton treatment capacity. Such imported equipment is very expensive relative to the economic levels of most cities in China. Furthermore, imported equipment, which is designed for incinerating MSW in developed countries, does not perform well in China and other developing countries, primarily because of the high moisture content and the low calorific value of the wastes (3, 10). Diesel is often added as a supplementary fuel to support combustion of the low energy content MSW, which substantially increases the incinerator’s operating cost. In addition, personnel training, maintenance, and repair of imported incineration equipment are also expensive. There is a significant demand for reliable yet relatively inexpensive MSW incinerators in China. Particularly, wasteto-energy incineration adds the benefits of recovering energy from the wastes. Tsinghua University has recently developed a grate-circulating fluidized bed (CFB) combined combustion technology for waste-to-energy incineration. This incineration system incorporates four patented technologies (1114) and is well-suited for disposal of non-sorted high moisture content and low energy content MSW. In 2000, the first fullscale waste-to-energy incineration facility (2 × 150 ton/day incinerators) based on domestic technologies and equipment was constructed by Tsinghua University in Chaolai Agricultural Park, Beijing (15). Another MSW incineration facility designed by Tsinghua University, which consists of two 260 ton/day incinerators for electricity generation, was constructed in Changchun, Jilin and began full operation in October 2005. A total of 137 325 tons MSW was incinerated in the facility in 2006, generating 46.2 million kWh electricity, which is enough to power more than 30 000 local homes.
Changchun MSW Power Plant Setup Changchun is the capital city of the Jilin Province, China, with a total urban population of 3.15 million and 21 336 Yuan RMB gross domestic product (GDP) per capita in 2005. MSW is generated at a rate of approximately 3000 tons per day. In 2003, Jilin Xinxiang Corp. Ltd. invested a total of 160 million Yuan RMB to construct the first waste-to-energy facility in 7510
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Changchun. Two 260 ton/day grate-CFB incinerators were designed and installed by a joint team from Tsinghua University and Beijing Boiler Works. The facility construction was completed in June 2004, and the waste-to-energy system began full operation in October 2005. Currently, 520 tons/ day, or one/sixth of the MSW generated in Changchun, is incinerated in the facility. Figure 1a shows the schematics of the grate-CFB combustion system of the Changchun MSW power plant. It consists mainly of an integrated feeder and dryer, combustion chamber, steam generation system, and pollution control system. Trash bags unloaded from trucks were first torn apart. The ferrous metals were removed by a magnetic separator, while the heavy and bulky materials, such as bricks, were removed by air classification. The waste was then stored in the garbage tank for several hours to up to 2 days before being lifted by a clamshell crane and dropped into the feeder. Leachate from the MSW in the garbage tank was collected and treated by injection into the combustion chamber. Combustion of MSW with a high moisture content could cause sharp pressure fluctuation in the combustion chamber due to water flash vaporization and the quick-release of volatile components of the waste. This was avoided by the grate design of the integrated feeder and dryer (Figure 1b); MSW on the grate was dried by the hot flue gas drawn from the furnace-exit level passed through the MSW layer and by the heat radiated from the combustion chamber as well. Local pyrolysis/combustion also occurred during the feeding processes, which led to further water removal. As a result of the combination of these three drying effects, the moisture content of the waste could be significantly reduced. The dried waste then fell into the dense phase zone of the combustion chamber without causing much disturbance to the combustion. The use of a grate allowed the waste to be fed and dried simultaneously. Consequently, the incinerator could adapt to MSW with a wide range of moisture contents (up to 55%). The inside of the combustion chamber was covered with firebricks to minimize heat loss and to prevent corrosion. The combination of grate and CFB also allowed wastes with relatively large sizes (up to 50 cm) to be incinerated directly without shredding. The combustion chamber was equipped with an auxiliary burner fired with diesel fuel, which was only used to start up the combustion process (340-400 kg of light diesel per startup). MSW combustion was generally not self-sustaining because of the waste’s low heating value, and pulverized coal, fed by a screw conveyor into the combustion chamber, had to be added as a supplementary fuel. The supplementary fuel made combustion of the MSW possible and helped to ensure that the temperature in the combustion chamber never dropped below 800 °C and that the pollutant emissions complied with legal requirements. The walls of the furnace chamber were lined with vertical tubes containing water. Heat transfer from the hot combustion gases in the furnace boiled the water in the tubes, producing high temperature (450 °C) and high pressure (3.82 MPa) steam. The steam flowed from the incinerator to a 3000 rpm condensing steam turbine that drove a 6 MW electric generator; the thermal energy in the steam was converted to mechanical energy and then to electricity. After the steam exited the turbine, it was condensed, and the water was pumped back to the incinerator. A series of heat recovery sections, known as superheater, reheater, economizer, and air heater, was located downstream of the furnace chamber, which served to extract additional heat from the flue gas to improve overall energy conversion efficiency. After the exchange of heat, the temperature of the flue gas was brought to a value close to but not lower than 200 °C, to avoid steam condensation that would boost the corrosive action of the flue gas.
FIGURE 1. Schematic diagram of the incineration facility (a) and integrated feeder and dryer (b).
TABLE 1. Key Parameters of Grate-CFB Incinerators and Power Generation System item
value
incinerator capacity steam output steam parameters boiler efficiency auxiliary fuel (coal) consumption residence time of flue gas in combustion chamber moisture content in original MSW MSW calorific value condensing steam turbine electric generator
2260 ton MSW/day 218 ton/h 3.82 MPa, 450 °C g79% e20% of total fuel mass g2 s e55% g3100 kJ/kg 16 MW, 3000 rpm, 3.43 MPa, 435 °C 16 MW, 6300 V, 3000 rpm
The coarse dust was separated from the flue gas by two cyclone units with a gas-solid separation efficiency of 9899%, which were constructed with fire-resistant and wearresistant materials. Thereafter, acidic gases (HCl, Cl2, and SO2) in the flue gas were removed through a dry scrubber, where a slurry of hydrated lime was sprayed into the hot flue gas to absorb the pollutants. The heat of the flue gas was used to evaporate all the water droplets, leaving a nonsaturated flue gas to exit the scrubber. Activated carbon could also be added, if necessary, for the removal of dioxins and some heavy metals (e.g., mercury). The resulting dry material, including fly ash, was collected in a downstream particulate control device (a fabric filter). The cyclone collector and fabric filter were designed to catch approximately equal volumes of dust; this configuration reduced the cost while maintaining a high overall dust removal efficiency. The bottom ash drained from the furnace bottom and the fly ash collected in the particulate matter control devices were conveyed to a storage silo. The purified flue gas was exhausted to the atmosphere through a stack.
Table 1 summarizes the key parameters of the grate-CFB incinerator and the power generation system of the Changchun MSW power plant. Full-scale testing of the waste-toenergy facility was conducted between August and November 2005 to examine the operating characteristics of the grateCFB incinerators. Emissions of air pollutants were also measured to check the performance of the air pollution control equipments. The sampling and analysis of particulate matters, CO, O2, SO2, NOx, HCl, Hg, Cd, Pb, and dioxins, in the flue gas at the inlet of the stack were carried out according to standard methods specified by the State Environmental Protection Administration of China (16), following procedures similar to those used by Liu and Liu (10). Loss on ignition of the bottom ash was measured at 600 ( 25 °C for 3 h to determine the completeness of combustion.
Results and Discussion Incinerator Performance. Under typical operating conditions, MSW with a water content of ∼50% was fed at a rate of 180 kg/min to each grate-CFB incinerator. The thickness VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Mass Balance of Two 260 Ton/Day Grate-CFB Incinerators for MSW Incineration (in 24 h)a fuel (ton/day)
fly ash (ton/day)
date
MSW
lignite
bottom ash (ton/day)
from cyclone
from fabric filter
solid residue to fuel ratio (%)
Aug 5, 2005 Aug 19, 2005 Nov 14, 2005 Nov 15, 2005 Nov 16, 2005 av
406 415 491 502 478 458
143 133 121 135 115 129
81 78 78 77 78 78
32 31 27 22 26 28
37 42 43 31 22 35
27 28 24 20 21 24
a
Hydrated lime and activated carbon added for pollution control end up as fly ash, and their masses are included in the solid residue.
TABLE 3. Fuel (MSW and Coal) Consumption of Changchun MSW Power Plant and Electricity Generation in 2005 and 2006 operation period
July to Sept, 2005
Oct to Dec, 2005
Jan to June, 2006
July to Dec, 2006
total
MSW incinerated (ton) lignite burned (ton) coal equivalent (29 300 kJ/kg) consumed (ton) coal equivalent to MSW fuel ratio total fuel (coal + MSW) energy (million kWh) electricity generated (million kWh) fuel-to-electricity generation efficiency (%) electricity supplied to public grid (million kWh) electricity consumed within facility (million kWh) ratio of net electricity generation (%)
28831 10471 4789 0.17 72.85 9.85 13.5 7.40 2.45 75.1
35013 11431 5228 0.15 83.69 11.52 13.8 8.96 2.56 77.8
70380 18341 8388 0.12 150.97 23.49 15.6 18.63 4.86 79.3
66945 20132 9208 0.14 153.60 22.68 14.8 18.01 4.67 79.4
201169 60375 27613 0.14 461.10 67.53 14.6 53.00 14.54 78.5
of the MSW layer (bulk density: ∼350 kg/m3) on the drying grate was 0.35 m, and the residence time was 11.5 min. Thermal radiation from the furnace to the MSW layer was 640 kW. Assuming that 90% of the heat was absorbed by the water in the MSW, this could cause evaporation of 7.6% of water in the MSW. Another 0.8% reduction in the moisture content of the MSW could be brought by drying air (∼150 °C) passing through the MSW layer. The heat generated due to local pyrolysis and combustion of MSW on the drying grate raised the temperature of MSW to 200-500 °C at the exit end of the integrated feeder and dryer. Measurements show that the moisture content in the MSW was typically reduced to less than 10% before entering the combustion chamber (data not shown). Therefore, the combined effect of heat radiation, drying, pyrolysis, and combustion during feeding significantly reduced the moisture content of the MSW, making combustion of MSW with a high moisture content possible. In the CFB furnace, the solid fuels (MSW and coal) were suspended by upward-blowing jets of air during the combustion process. The combustion furnace and flue gas temperatures were controlled in the range of 800-900 °C, with a flue gas residence time of no less than 2 s. Destruction of dioxins and other organochlorines was rapid at this temperature range (17). Incomplete combustion and dioxin formation would occur at lower temperatures (18, 19), while NOx began to form at higher temperatures. Mass balance of the grate-CFB incinerators was characterized from August to November 2005. Table 2 shows the rates of fuel consumption and solid residue production of the facility. Loss on ignition of the bottom ash ranged from 0.3 to 1% in many random samples, which is indicative of near complete fuel combustion. On average, the ratio of solid residue to fuel mass was 24%, with a slightly greater production of bottom ash than fly ash. One reason for the high solid residue to fuel ratio was that poor quality lignite, having low heat values of 11 700-15 500 kJ/kg and ash contents of >35%, was used as the supplementary fuel. Another factor is that MSW in Changchun contained a significant fraction (11.3%) of non-combustibles, as shown in Table S1 of the Supporting Information. Incineration led to an overall reduction of MSW and coal mass by 76%. 7512
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Comparison of the sizes of the bed materials and the solid residues from the grate-CFB incinerator with those from a conventional coal-fired CFB boiler is shown in Table S3 of the Supporting Information. The bed materials, bottom ash, and fly ash all had larger size ranges in the grate-CFB incinerators as compared to those of the coal-fired CFB boiler. This could be attributed to the heterogeneous nature of the MSW and the fact that the wastes were not shredded before incineration. The bottom ash and fly ash are excellent civil engineering materials for the construction of buildings and roads, such as cement admixtures, concrete admixtures, walling material, road material, and construction backfill (20, 21). After the separation of ferrous materials, the bottom ash and fly ash from the waste-to-energy facility were utilized for the production of commercial floor tiles. Overall, MSW incineration at the Changchun MSW power plant could save more than 0.2 million m3 landfill space each year. Table 3 summarizes the MSW incinerated and coal consumed at the waste-to-energy facility in the second half of 2005 and in 2006 as well as the electricity generation. The two grate-CFB incinerators incinerated 201 169 tons MSW from July 2005 to December 2006. A total of 60 375 tons lignite was also consumed as supplementary fuel during this period. Coal apparently constituted 23.1% of the total fuel mass because the energy content (mean calorific value: 13 400 kJ/kg) of the burned lignite is less than half of that of standard coal equivalent (29 300 kJ/kg). When converted to the coal equivalent, the coal equivalent to MSW fuel ratio was only 0.14. A total of 67.53 million kWh electricity was generated from the combustion of the MSW and lignite from July 2005 to December 2006. The overall fuel-to-electricity efficiency was approximately 14.6%, which is less than half of the average efficiency (33-35%) of modern coal-fired power plants. This value is comparable to the actual plant efficiencies (low 20% range) of biomass fueled power plants (22). There are several reasons for the low efficiency of the waste-toenergy facility. The capacity (6 MW) of the MSW power plant is rather low as compared to coal-fired power plants that are typically in the 100-1500 MW range. A small capacity plant such as this one tends to be lower in efficiency because of economic trade-offs (i.e., efficiency-enhancing equipment cannot pay for itself in this case). The high moisture content (∼50%) in the MSW and increased excess air with feeding of
a Emission standards given in ref 16. b Analysis methods used for these pollutants are the standard ones set by the State Environmental Protection Administration of China. c Values for lead, cadmium, mercury, and their compounds include both solid and gaseous phases. d Below the respective method detection limit. e According to ref 33. f Dioxin concentrations are corrected to dry gas, 11% O2, 0.1 MPa, and 273 K conditions; TEQ is defined as TEQ ) Σ(concentration of dioxin congeners × TEF), where TEF is the toxic equivalent concentration, defined as the relative potency of different dioxin congeners as compared to TCDD (2,3,7,8tetrachlorodibenzo-p-dioxin). g According to ref 34.
gravimetry, GB/T 16157-1996 Ringelmann smoke chart, GB 5468-91 nondispersive infrared spectrometry, HJ/T 44-1999 formaldehyde absorbing-pararosaniline spectrophotometry, GB/T15262-94 ultraviolet spectrophotometry, HJ/T 42-1999 mercuric thiocyanate spectrosphotometry, HJ/T 27-1999 gold amalgamation-cold vapor atomic absorption spectrometrye flame atomic absorption spectrophotometrye flame atomic absorption spectrophotometrye gas chromatography/mass spectrometryg 80 Ringelmann category 1 (20%) 150 260 400 75 0.2 0.1 1.6 1.0 25.4
