Better together: Gas turbine cogeneration improves energy recovery from WTE plants

Waste-to-energy is the most expensive type of power generation today. However, its energy productivity - and thereby economics - can be doubled by taking advantage of both the electricity and heat produced. Moreover, this thermal efficiency can be increased by combining natural gas turbines with WTE plants. by Nickolas Themelis If there were a social ladder of fuels, hydrogen would be at the top followed by methane, fuel oil, coal and wood chips. Municipal solid waste (MSW) would be in the lower middle class and food waste at the bottom. Originally employed to get rid of wastes, first-generation incinerators were not designed to recover energy. But the increasing cost of fossil fuels and landfilling, coupled with a growing awareness of environmental impacts, prompted a gradual transformation. Modern waste-to-energy (WTE) plants are the most expensive power generators in the world, but they use a renewable fuel that brings in substantial revenues. The global WTE industry processed an estimated 143 million tonnes of MSW in 2004 (Table 1), which is about 10% of the wastes buried in large landfills. The European Union is the biggest user of this technology, followed by Japan and the US. TABLE 1. Global waste-to-energy, 2004. source: biocycle1,2 Country/region Estimated amount combustedin WTE plants(million tonnes) EU-25 48.8 Japan 40.0 US 26.3 Taiwan 7.0 Singapore 4.0 China 3.0 Switzerland and Norway 3.8 South Korea 1.0 All other 9 Total 143 The push to optimize the efficiency of thermal conversion has resulted in significant technological advances. These will become increasingly important as the quantity of wastes being generated increases, fossil fuel prices rise, and global warming continues. Global generation of municipal solid waste Economic development in both developed and developing nations has invariably been accompanied by increased consumption of materials and a corresponding increase in the generation of MSW. For example, the 2004 survey of MSW waste management by the Earth Engineering Center (EEC) of Columbia University and BioCycle1 found that the amount of MSW generated in the US rose from 336 million tonnes in 2002 to 353 million tonnes in 2004 (Table 2) - that is, at a rate of 2.5% per year. In this two-year period, recycling increased by 10 million tonnes and landfilling by nearly the same amount. Most of the recycling is carried out in coastal states and most of the WTE facilities are on the East Coast (Figure 1). TABLE 2. MSW generation and disposal in the US, 2002 and 2004. source: eec and biocycle1,2 MSW generated Recycled or composted Waste-to-energy Landfilled 2002 Amount (million metric tonnes) 335.8 89.6 25.8 215.3 Percentage (%) 100 26.7 7.7 65.6 2004 Amount (million metric tonnes) 352.6 100.4 26.3 226.0 Percentage (%) 100 28.5 7.4 64.1 FIGURE 1. Disposal o MSW by region. source: biocycle1 Click here to enlarge image The generation of non-recycled MSW per person in the US in 2004 remained at 2.3 kg/day - the highest in the world.1 In comparison, generation of non-recycled MSW in the EU is 0.9-1.6 kg/day and 0.7-1.5 kg/day in the urban areas of Asia, according CEWEP and WTERT data. My estimate of the global generation of MSW that ends up either in WTE plants or urban landfills is 1500 million tonnes. The estimated global use of WTE is 143 million tonnes (Table 1). Energy generation by WTE plants in the US There are 88 WTE power plants operating in 27 states of the US, fuelled by 26.3 million tonnes of MSW. The dominant technology is combustion of as-received fuel (stoker-type grate systems) (Table 3). TABLE 3. WTE plants in the US. source: integrated waste services association Technology Number of plants Capacity (US tons/day) Capacity (US tons/year) As-received fuel 64 71,354 22.1 Refuse-derived fuel (RDF) 15 20,020 6.3 Modular 9 1342 0.4 Total 88 92,716 28.8 According to the US Department of Energy,2 WTE facilities provided 13.5 billion kWh in 2002 to the electricity grid (Table 4). In addition, these plants generated an estimated 1.3 billion kWh of thermal energy. Average energy generation was therefore 563 kWh per tonne of MSW combusted. TABLE 4. Generation of renewable energy in the US, 2002 (excluding hydro power). source: doe2 Energy source Amount generated (billion kWh) % of renewable energy Geothermal 13.52 28.0 Waste-to-energy a 13.50 28.0 Landfill gas a 6.65 13.8v Wood/biomass 8.37 17.4 Solar thermal 0.87 1.8 Solar photovoltaic 0.01 0.0 Wind 5.3 11.0 Total 48.22 100 a www.eia.doe.gov/cneaf/solar.renewables/page/mswaste/msw.html WTE plants produced more energy in 2002 than all other renewable sources of electricity apart from geothermal and hydro power (the latter is not included in Table 4). For comparison, wind power provided 5.3 billion kWh in 2002 and solar energy only 0.87 billion kWh. It is interesting to note that, although the tonnage of MSW landfilled in the US is eight times greater than that combusted in WTE facilities, the electricity obtained from landfill gas is one half that of the WTE energy. Energy generation by WTE plants in Europe In 2003, there were 409 WTE facilities in Europe (Figure 2), about two-thirds of the world total. As shown in Table 2, the EU is the largest user of WTE followed by Japan and the US. A recent study3 of 97 WTE plants in the EU found that these plants processed a total of 24 million tonnes of MSW annually (2004 data) and generated or cogenerated a mix of electricity and thermal energy. The weighted mean calorific value (lower heating value) was 10 MJ/kg, which is 2774 kWh of chemical energy per tonne of MSW combusted. FIGURE 2. European WTE facilities. source: cewep Click here to enlarge image null The WTE plant at Glostrup serves local municipalities by providing electricity and on-site district heating. Danish WTE plants commonly achieve high energy efficiencies. photo: rambøll Click here to enlarge image In contrast to the US, where little use is made of the so-called ‘waste’ steam, European plants produce more thermal than electrical energy (Table 5). TABLE 5. Results of analysis of 97 plants by CEWEPa MSW processed 24.1 million tonnes Weighted mean thermal energy in MSW 2774 kWh/tonne Net electricity to grid 302 kWh/tonne Net thermal energy to district heating, 878 kWh/tonne Total energy use - assuming equivalence of thermal and electric energy 1180 Overall thermal efficiency (1180/2774) - assuming equivalence of thermal and electric energy 42.5% a From data provided by Dr Ella Stengler Table 5 assumes that electrical and thermal energies are equivalent. However, the Integrated Pollution and Prevention Control (IPPC) regime in the EU provides weighting factors based on the fact that different amounts of fuel are used to produce electricity and steam. The IPCC (Best Available Technology Reference Document) BREF for waste incineration4 specifies that 1 kWh of electricity is equivalent to 2.4 kWh of heat. On this basis, the overall thermal efficiency of the 97 EU plants would be 57.8% (i.e. considerably higher than that shown in Table 5). By the same token, the BREF efficiency of the US plants, which produce an average of 515 kWh of electricity, would be about 44%. WTE district heating - the neglected resource In her presentation at the 2006 North American WTE Conference (NAWTEC 14), Bettina Kamuk of Rambøll, Denmark, called WTE district heating ‘the neglected resource’. The truth of this statement is evident when you compare the 44% BREF thermal energy efficiency of US WTE facilities with the 57.8% BREF thermal efficiency of the 97 EU WTEs. The Zabalgarbi WTE plant near Bilbao, Spain, uses a gas turbine generator to improve its energy efficiency. photo: martin gmbh Click here to enlarge image According to Kamuk, the energy efficiencies obtained in Danish WTE plants are much higher than those found in the CEWEP study of 97 WTE facilities (Table 5). On average, Danish WTEs provide about 450 kWh of electrical energy and 1970 kWh of thermal energy per tonne of MSW combusted. Denmark has a population of 5.4 million and has 31 WTE facilities distributed nationwide (Figure 3). FIGURE 3. Distribution of Danish WTE facilities. source: rambøll Click here to enlarge image The conventional wisdom of ‘economies of scale’ is defied in Denmark because its relatively small WTE plants are sited as close as possible to the communities they serve in order to allow the ‘waste’ steam to be used for district heating. This also reduces the waste transport distance. Kamuk reported that Danish WTE plants derive US$60/tonne of MSW from the sale of thermal energy and US$40/tonne from the sale of electricity. The electricity and thermal energy revenues allow citizens to pay considerably lower disposal fees to WTE facilities compared with the US. Combination of gas turbine and WTE power plant Another way to increase the energy efficiency of WTE plants is to combine them with a gas turbine generator. This has been done at the Zabalgarbi WTE facility near Bilbao in Spain (Figure 4) and at several other WTE facilities including that in Sakai in Japan (Figure 5). FIGURE 4. Combined gas turbine and WTE power plant at Zabalgarbi, Spain. source: cnim - constructions industrielles de la mediterranee Click here to enlarge image null FIGURE 5. Combined gas turbine-WTE power plant at Sakai, Japan. source: tohru nishioka, environmental division, sakai city, japan Click here to enlarge image The basic concept of the gas turbine-WTE combination is the use of the exhaust gases of the turbine to superheat the steam generated in the WTE boiler. This has two benefits: It allows the WTE boiler to operate at a lower steam temperature, thus reducing the tube temperature and corrosion rate. By superheating the WTE steam to a temperature higher than is possible in the corrosive environment of WTE combustion gases, more energy is recovered from the WTE power plant in the steam turbine. At the Sakai City WTE facility, the energy in the natural gas input to the gas turbine is used very efficiently to: generate electricity in the turbine superheat the steam from the WTE boiler pre-heat the water feed to the boiler control the temperature of the cleaned exhaust gases so as to prevent the formation of a steam plume in the stack gas. Current and potential generation of energy by WTE plants worldwide If we assume that the 97 WTE plants in the CEWEP study (Table 5) are representative of the rest of the European WTE plants, the generation of WTE plants in Europe is 15.9 billion kWh of electricity and 46.2 billion kWh of thermal energy. A waste-to-energy plant in Germany processes 221,000 tonnes of waste per year to generate electricity, heat and steam. Cogeneration will be an obvious way to improve WTE economics. photo: kreis weseler abfallgesellschaft mbh &co. kg Click here to enlarge image As shown above, WTE plants in the US generate another 13.5 billion kWh of electricity. Japan is a big user of WTE, but most of its WTE facilities are relatively small, do not provide much district heating and use electricity for vitrifying the WTE ash. Therefore, on the average, they generate an estimated 250 kWh per tonne for a total of 10 billion kWh of electricity. The rest of the global WTE plants generate an estimated 7 billion kWh of electricity. This brings the global energy generation to about 46 billion kWh of electricity and the same amount of thermal energy. To appreciate the contribution of the global WTE industry to the conservation of fossil fuels, it is worth pointing out that the energy it generates reduces the use of coal by about 35 million tonnes. This environmental benefit would be increased five-fold if the thermal efficiency of the plants in the CEWEP study was achieved globally. It would increase much more if the MSW now going to landfills was combusted in thermally efficient WTE facilities. If the contribution of landfill non-captured methane to greenhouse gas emissions is also considered, it is clear that a gradual move from landfill to WTE is one of the low-hanging fruits in reducing global warming. Notes >1. Simmons, P., Goldstein, N., Kaufman, S.M., Themelis, N.J and Thompson, J. ‘The State of Garbage in America’, BioCycle, 47(4), p. 26 (April 2006). www.jgpress.com/archive.html Kaufman, S.M., Goldstein, N., Millrath, K. and Themelis, N.J. ‘The State of Garbage in America’, BioCycle, 45(1), p. 31 (January 2004). www.jgpress.com/archive.html 2. Energy Information Administration, Annual Energy Outlook 2002. DOE/EIA-0383(2002). US Department of Energy, 2001. www.gcrio.org/OnLnDoc/pdf/aeo2002.pdf 3. Study by Dr-Ing Dieter O. Reimann (Bamberg, Germany), commissioned by the Confederation of European Waste-to-Energy Plants (CEWEP) (www.cewep.com) 4. European IPPC Bureau. Integrated Pollution Prevention and Control. Reference Document on the Best Available Techniques for Waste Incineration. July 2005. http://eippcb.jrc.es/pages/FActivities.htm Professor Nickolas Themelis is Director, Earth Engineering Center, Columbia University, and chairman of the Waste-to-Energy Research and Technology Council (WTERT).Fax: +1 212 854 5213e-mail: njt1@columbia.eduweb: www.columbia.edu/cu/wtert To comment on this article or to see related features from our archive, go to www.waste-management-world.com and click the ‘Forum’ tab. Energy recovery from landfills Approximately 1.4 billion tonnes of MSW worldwide are disposed in landfills deep enough to generate biogas (approximately 50% methane and 50% CO2). The annual generation of methane is estimated at 62 million tonnes, of which less than 10% is recovered in controlled landfills equipped to capture biogas. An estimated 2.4 million tonnes of methane are captured in the US; 70% is used to generate 6.65 billion kWh of electricity and the rest is flared. The global 57 million tonnes of non-captured landfill methane correspond to greenhouse emissions of 1.3 billion tonnes of CO2 or 4% of the total greenhouse gas anthropogenic emissions.