Clean energy conversion from municipal solid waste and climate change mitigation in Thailand: Waste management and thermodynamic evaluation

Energy for Sustainable Development 15 (2011) 355–364

Contents lists available at ScienceDirect

Energy for Sustainable Development

Clean energy conversion from municipal solid waste and climate change mitigation in Thailand: Waste management and thermodynamic evaluation Seksan Udomsri ⁎, Miroslav P. Petrov, Andrew R. Martin, Torsten H. Fransson Division of Heat and Power Technology, Department of Energy Technology, Royal Institute of Technology (KTH), Brinellvägen 68, 10044 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 14 July 2010 Revised 13 July 2011 Accepted 13 July 2011 Available online 1 September 2011 Keywords: MSW management Energy recovery Electrical efficiency CO2 emissions Climate change mitigation

a b s t r a c t Enhanced energy security and renewable energy development are currently high on the public agenda in Southeast Asia, which features large populations and expansive economies. Biomass and Municipal Solid Waste (MSW) have widely been accepted as important locally-available renewable energy sources and represent one of the largest renewable energy sources worldwide. This article presents an evaluation of the potential of MSW incineration for climate change mitigation and promotion of biomass-based electricity production in a more sustainable direction in Thailand. The energy recovery potential of MSW is analyzed by investigating various types of incineration technologies. Both conventional technologies and more advanced hybrid dual-fuel cycles (which combine MSW and natural gas fuels) are considered in analyses covering cycle performance and CO2 emissions. Results show that MSW incineration has the ability to lessen environmental impact associated with waste disposal, and it can contribute positively towards expanding biomass-based energy production in Thailand. Hybrid cycles can be proposed to improve system performance and overall electrical efficiency of conventional incineration. The hybrid cycle featuring parallel interconnection is somewhat more attractive in terms of efficiency improvement: electrical efficiency increases by 4% and CO2 emission levels are reduced by 5–10% as compared to the reference incineration case. The reduction of greenhouse gas emissions is even more attractive when methane gas emitted fro m existing landfill sites is to be compared. © 2011 Elsevier Inc. All rights reserved.

Introduction and objectives Resource constraints and sustained high fossil fuel prices have created a new phenomenon in the world market. Many countries, particularly those under development that rely on imported fossil fuels, already have strained resources and suffer as a result. The energy sector is one of the most sensitive areas in Thailand since almost half of the total energy supply is imported (DEDE, 2007a). Fossil fuels currently dominate electricity production in Thailand. Natural gas accounts for the majority of country's total electricity generation (68.4%), followed by lignite and coal (22.4%) and oil (2.8%). Renewable resources (mostly hydropower) comprise less than 10% of electricity production (DEDE, 2007b). It is imperative that developing countries like Thailand employ sustainable energy solutions in order to secure a high standard of welfare for future generations. Exploring underutilized fuels like Municipal Solid Waste (MSW) is vital towards contributing to a sustainable energy supply while minimizing greenhouse gas emissions currently produced by open dumping and landfilling.

⁎ Corresponding author. Tel.: + 46 8 790 6140; fax: + 46 8 20 41 61. E-mail address: [email protected] (S. Udomsri). 0973-0826/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.esd.2011.07.007

Rapid expansion of industry, urbanization and increasing population, especially in large cities like Bangkok, has dramatically increased the amount of MSW generated in Thailand. Although the proper handling of MSW is high on the public agenda, issues related to sound MSW management – including recycling programs, waste reduction, and disposal – have not been addressed adequately. Open dumping and landfilling are currently the most popular forms of MSW disposal, although the majority of facilities do not employ modern techniques like landfill gas recovery. Improper waste management causes severe environmental impact, including groundwater contamination, air quality deterioration, and greenhouse gas emissions (uncontrolled methane gas emitted from anaerobic decomposition). While the need for a complete sustainable energy solution is apparent, solid waste management is also an essential objective, so it makes sense to explore ways in which the two can be joined. Thailand's current solid waste management strategy focuses on bulk collection and mass disposal. Recently, the Thai government has attempted to implement an “integrated waste management system” that includes (i) source reduction, (ii) waste diversion e.g. material recovery, composting, and incineration and (iii) final disposal (Kaosol, 2009). Several technological methods can be employed to manage MSW in a sustainable way before landfill, such as incineration with or without energy recovery; composting of organic wastes for the production of a fertilizer; anaerobic digestion for biogas production;

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or recovery and recycling of usable materials (Kaosol, 2009). This article concentrates on an evaluation of the potential of MSW incineration in Bangkok Metropolitan Area (BMA) as it is believed that incineration of MSW using modern environmental controls represents one of the most logical paths for waste management and energy recovery in Thailand (Udomsri et al., 2008). In the BMA, there have previously been plans for implementing four new MSW incinerators in the areas of On-nuch, Tharaeng and Nongkhaem. However, anti-incineration campaigns caused the suspension of implementing these four new incineration plants (Greenpeace Thailand, 2002). The main objective of this work is to present an overview of MSW management and to investigate the opportunities and potential for new MSW power plants with a focus on the Bangkok region. MSW incineration with energy recovery is one of the best solutions to reduce volume and weight of refuse among other waste management technologies. The energy recovery potential of MSW is analyzed by investigating various types of integrated incineration technologies. In particular hybrid dual-fuel cycles, which integrate MSW and highquality fuels like natural gas in an innovative fashion, are considered to improve the efficiency of MSW conversion to electricity if compared to conventional MSW incineration. Current situation of MSW and electric power in Thailand MSW characterization

16

5 4

12

3 8 2 4 0

1 2000 2001 2002 2003 2004 2005 2006 2007 Total MSW generation

0

MSW generation in BMA

Fig. 1. MSW generation in the BMA and Thailand during 2000–2007.

MSW generation in the BMA (million tons/year)

MSW generation in Thailand (million tons/year)

Currently, Thailand produces nearly 22 million tons of wastes annually; MSW constitutes 67% while the remainder is comprised primarily of non-hazardous industrial waste (Thailand Environment Monitor, 2003). Bangkok Metropolitan Area (BMA), the capital city with 5.7 million citizens and 3.0 million temporary daily visitors (Srisuk et al., 2002), produces up to 27% of this total amount, i.e. 3–4 million tons MSW annually (PCD, 2003; Thailand Environment Monitor, 2003). The most widespread form of waste disposal is landfilling, and in the BMA nearly 100% of MSW is collected and handled in this fashion (Muttamara et al., 2002; Thailand Environment Monitor, 2003). Improper waste management causes severe environmental impact, which has been pointed out by Udomsri et al. (2005). Percapita waste production varies across Thailand, according to relative income levels: 0.4–0.6 kg/day in rural areas, 1.3–1.5 kg/day in the BMA, and up to 2.2 kg/day in tourist areas like Phuket (Thailand Environment Monitor, 2003). Fig. 1 presents the trend and amount of MSW generation in the BMA and Thailand during 2000–2007 (Kaosol, 2009; PCD, 2006). Waste generation in the BMA has declined slightly after 2002 because of the encouragement of recycling activities (Chiemchaisri et al., 2007). MSW is produced by a combination of residential or domestic waste and other sources, determined by the relative proportion of industrial, commercial, or tourism activity in the area.

The major components of this waste stream – food scraps, paper packaging, lawn clippings etc. –- are very important biomass contributors. The percentage of consumer packaging wastes increases relative to the population's degree of wealth and urbanization. A summary of average MSW composition in the BMA and some other major cities of Thailand is presented in Table 1 (PCD, 2006). As can be seen, around 43% of MSW generated in the BMA derives from residential sector, consisting mostly of food waste. Plastics and glass are significant fractions as recycling programs that are practically non-existent. A high percentage of plastics is a significant factor for increasing the calorific value of waste, making it a more attractive fuel. The energy content (LHV) of MSW varies throughout the country from 6 to 12 MJ/kg; however in this study we used 9–10 MJ/kg for MSW in Bangkok area (Suksankraisorn et al., 2004; PCD, 2006; Suksankraisorn et al., 2010). The moisture content of the waste varies throughout the country in the same pattern with the energy content. MSW management Thailand's current solid waste management strategy focuses on bulk collection and mass disposal. Recently, the Thai government has attempted to implement an “integrated waste management system” that includes waste sorting, composting, and incineration (Greenpeace, 2002). Among other national MSW management strategies, this program is in final stages of implementation (PCD, 2003). Table 2 presents an overview of MSW management practices in the 13 largest municipalities/cities in Thailand (Thailand Environment Monitor, 2003). The collection efforts of MSW within the large cities (Muang municipalities) are more variable and typically have more efficient collection systems than smaller towns (Tambon municipalities). In the BMA, tremendous progress has been made in recent years for the collection of household solid waste. In the 1990s the fleet of waste collection vehicles was expanded and improvements were made in management at district offices. As a result in the BMA, nearly 100% of MSW is now collected and trucked to one of the three transfer stations, namely On-nuch, Nongkhaem and Tharaeng, see Table 3 (Thailand Environment Monitor, 2003). Two private companies have contracts with the BMA to operate the waste transfer sites and to transport waste to landfills. Nowadays a two-step waste processing scheme is employed. • Primary collection: collection at households is operated by the BMA. Approximately 2200 vehicles have been employed to collect approximately 8800–9000 tons of waste per day. • Secondary transport: at the transfer station, BMA moves the waste into large hauling trucks, which are weighed. Private sector firms then haul the waste to landfills located 10 to 110 km away. Small-scale recycling shops are also located near these disposal sites, where materials gathered by collection crews or others can be sold. In Thailand, more than 1000 disposal sites have been found, however around 104 disposal sites have been constructed under appropriate standards and are located in the 76 provincial capitals (Thailand Environment Monitor, 2003). As a result in the large cities, 57% of these municipalities have engineered or sanitary landfills, while 30% have open dumping and the rest employs controlled dumps. In contrast, only 4% of MSW generated in Tambon municipalities have landfills, while the rest relies on open dumps (Thailand Environment Monitor, 2003). MSW treatment and processing MSW disposal in Thailand has still not met sanitary purpose such as open dumping, and open burning. The most common methods used for MSW in Thailand are composting, incineration, sanitary landfill

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Table 1 MSW composition by average values of waste from the BMA and some major cities of Thailand. MSW composition Provinces

Food

Paper

Plastics

Glass

Metal

Rubber/leather

Textile

Wood

Ceramic

Others

Bangkok Nonthaburi Chonburi Chiang Mai Phitsanulok Nakorn- Ratchasima Phuket Song-Kla

42.68 45.36 39.99 29.26 45.30 41.56 44.13 37.80

12.09 8.61 14.06 12.17 10.87 11.13 14.74 9.95

10.88 10.85 15.95 18.33 16.91 17.29 15.08 13.87

6.63 6.88 9.10 6.75 7.12 6.12 9.67 10.26

3.54 4.34 3.55 3.86 4.16 3.10 3.44 3.70

2.57 2.40 3.17 3.09 1.92 2.42 2.28 3.99

4.68 3.53 3.54 4.01 1.77 1.92 2.07 2.83

6.90 8.17 4.60 14.32 8.61 11.85 5.26 5.99

3.93 5.00 3.03 4.44 1.53 1.80 1.39 3.29

6.11 4.87 3.01 3.48 1.82 2.82 1.95 8.33

Generation rate kg/capita/day 1.54 1.08 1.50 1.06 0.80 0.85 2.17 1.22

Biological treatment Biological treatment is one of the best methods used to handle organic materials. The organic materials can be composted to produce fertilizers, or anaerobic digestion can be used to produce biogas and a liquid fertilizer in anaerobic digestion tanks. Biogas is a by-product of this process, which can be fired in gas engine or gas turbine. Biogas, which is a production of biological degradation, is essentially a mixture of methane and carbon dioxide. Presently, there are three existing anaerobic digestion plants in operation in Thailand (Tamrongluk, 2005); (i) Rayong municipality, capacity of MSW 70 tons/day for providing 625 kW electricity generation; (ii) Samutprakarn-Rachathewa, MSW anaerobic digestion pilot plant with capacity of 10 tons/day; and (iii) Chonburi, MSW central treatment plant with capacity of 300 tons/ day for providing 950 kW electricity generation. Although biological treatment is one of the best waste disposal methods, it is however suited for wet and organic waste only.

management for many industrialized nations. However, it is not popular in developing countries like Thailand. There are two main incineration plants which are in operation in Thailand presently and they are located in the main tourist areas. The first plant is in the area of Samui Island, Surat Thani province since 1997 and has a capacity of burning 140 tons MSW per day (International POPs Elimination Project, 2006). The second plant has operated on Phuket Island since 1998 with a capacity of 250 tons MSW per day to provide 2.5 MW of electricity (Incinerator Phuket Plant, 2006). These two plants have generated some local opposition even though they employ modern environmental controls. Since 1999 Greenpeace Southeast Asia together with several NGO's have lobbied against new incineration plants and claimed that they have negative environmental and social impact. Anti-incineration campaigns caused the suspension of implementing four new MSW incinerators in the BMA with a capacity of 1350 tons/day each, in the areas of On-nuch, Tharaeng and Nongkhaem (Greenpeace Thailand, 2002). In this study, waste incineration with energy recovery is considered and analyzed with the aim to reduce volume and weight of refuse. Positive environmental benefits can be achieved in parallel (i.e. reduction of greenhouse gas emissions via minimizing open dumping and expansion of a biomass-based energy production method).

MSW incineration Electricity production in combination with energy recovery from flue gases in thermal treatment plants is an integral part of MSW

Landfills Landfilling is one of the most popular forms of final waste disposal and is being employed in Thailand, especially in the BMA, although

and open dumping. Open dumping represents the majority of MSW disposal (65%), followed by biological treatment like composting etc. (10%), while incineration and landfills account for 5% each. Other methods like recycling etc. share around 15% (Kaosol, 2009; Nguyen Ngoc and Schnitzer, 2009).

Table 2 Current waste management from the 13 largest municipalities/cities in each region of Thailand. City

Land area (km2)

Sub-districts

Northern region Chiang Mai Phitsanulok Lumpang

40 18 22

Northeastern region Khon Kaen Nakorn Ratchasima Ubon Ratchathani

Solid waste management Collection

Disposal

14 1 4

75% privatized Municipal-operated 100% privatized

Private engineered landfill Municipal engineered landfill Private engineered landfill

46 38 29

1 24 4

Municipal-operated Municipal-operated Municipal-operated

Municipal controlled dump Open dump (army site) Open dump (army site)

Central region Rayong Kanchanaburi Nonthaburi Pattaya

17 9 39 53

4 5 5 4

Municipal-operated Municipal-operated Municipal-operated 90% privatized

Municipal controlled dump Municipal open dump Provincial open dump Municipal sanitary landfill

Southern region Hat Yai Surat Thani Phuket

21 99 12

– 6 –

Municipal-operated Municipal-operated 50% privatized

Municipal open dump Municipal open dump Private incinerator; Provincial engineered landfill

Municipal-operated Municipal-operated

Engineered landfill Sanitary Landfill

Bangkok area Kampang-saen Rachathewa

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Table 3 MSW disposal sites and transfer stations in the BMA. Transfer station

Waste transferred (% of total MSW/day)

Hauling distance (km)

Final landfill site

On-nuch Tharaeng Nongkhaem

40.5 26.9 32.6

10 110 80

Rachathewa Kampangsaen Kampangsaen

the majority of facilities do not employ modern techniques like landfill gas recovery. Sanitary landfill is a common landfilling method and usually located relatively far from the source. Sanitary landfill is a disposal process that employs environmental control equipment to minimize health hazards including water pollution. Although landfilling is the main MSW treatment method in the BMA, only two landfill gas power plants have been built over the last ten years. The installed capacity of these power plants is 435 kW and 1 MW, respectively. Internal combustion engine has been used to produce electricity (Tamrongluk, 2005). In landfills, anaerobic decomposition of waste generates landfill gas, which contains approximately 50% methane. As reported by IEA, methane emissions from MSW in modern landfills contribute around 50–100 kg/ton waste, of which 50% can be collected and 50% is released to the atmosphere (IEA, 2007a). The potential of landfill gas recovery in Thailand has been estimated and modeled by the World Bank Thailand using the USEPA LandGEM (United State Environmental Protection Agency) model together with a survey of waste disposal sites (Thailand Environment Monitor, 2003). In the Bangkok area, it has been found that the potential for landfill gas recovery is 22.4 and 27.6 million m 3 per year at the Kampangsaen landfill for low and high estimate average values, respectively. At the Rachathewa landfill, these figures are 27.2 and 30.3 million m 3 per year for the low and high estimate averages, respectively. Although methane generated from MSW in modern landfills can be captured and used for energy recovery, around 50% of this gas is still released to atmosphere (IEA, 2007a). Because of stringent environmental regulations and heavy pressure from local residents, landfilling is probably no longer a good solution for MSW disposal in the future. Open dumping Open dumping dominates waste management in Thailand because of unavailable waste collection organization, although it is not the right solution, nor any sustainable way for handling MSW. In the large cities or Muang municipalities, around 30% and 13% of MSW are disposed by open dumping and controlled dumps, respectively. In Tambon municipalities 94% of generated MSW still relies on open dump or controlled dump (Thailand Environment Monitor, 2003). Open dumping is an unsafe and contaminating method for waste disposal. Waste is simply disposed of and left to decay without proper control. Waste disposal by open dumping is prohibited in most countries. Controlled dump is similarly the same as open dumping but it employs the basic requirement of waste management practices such as correct placement of the waste in thin layers and compaction and cover.

been presented by Chiemchaisri et al. (2007). There are 95 landfills and 330 dumpsites in operation in Thailand currently. Total emission of methane has been calculated using the assumption of 425 sites (95 landfills; 330 open dumps) and presented in Table 4 (Chiemchaisri et al., 2007). This study found that waste disposal in 425 disposal sites (landfill and open dumping) generated methane emissions around 116 thousand tons and would increase threefold to 339 thousand tons per year by 2020 (Chiemchaisri et al., 2007). Table 4 shows that about haft of the total methane emissions are generated in the BMA. Proper disposal of these wastes is essential if public health and the environment are to be protected. With regard to specific environmental impact, the effect of MSW disposal by landfilling in the BMA (Khlong Canal On-nuch) has been investigated by Muttamara et al. (2002), who found that the levels of Biochemical Oxygen Demand (BOD) in leachate and Suspended Solids (SS) in nearby water bodies greatly exceeded the upper limit allowed by standards of 20 and 60 mg/L, respectively. Dissolved oxygen was found to be very low, about 0.88 to 1.90 mg/L. Khlong Canal water also contains high levels of Manganese (Mn, up to 1.4 mg/L) compared with the standard value of 0.3 mg/L. Furthermore, the ambient air in the area contained high levels of methane and carbon dioxide: 13.1 mg/m 3 and 1760 mg/m 3 respectively, from nighttime measurements (Muttamara et al., 2002). Current energy situation Considering the energy situation in Thailand, total energy demand was 64,686 ktoe in 2007, of which 82.1% derived from fossil fuel energy, and renewable energy accounted for the remainder. The total energy supply was 110,115 ktoe with a net import of 47.1% (DEDE, 2007a). Of the total imported energy, crude oil accounted for a major fraction with 39,846 thousand tons, followed by natural gas (8869 ktoe), coal with 8816 ktoe and petroleum products (825 ktoe). Domestic natural gas is the main energy source, followed by lignite, crude oil and condensates (liquefied light hydrocarbons). For the case of electricity, natural gas is the main energy source for electricity generation (68.4%), followed by lignite and coal (22.4%). Smaller fractions were attributed to power generation via hydro power, fuel oil and diesel (DEDE, 2007b). Figs. 2a and b show the domestic supply of primary energy (DEDE, 2007a) and the country's total electricity generation in 2007, respectively (DEDE, 2007b). In 2007, the total peak demand reached 22.3 GW while the yearly electricity consumption was just over 133 TWh (DEDE, 2007b). The BMA features almost 40% of the total electricity consumption (around 52 TWh). The total installed capacity was 28.8 GW in 2007. Public utilities, represented by the Electricity Generating Authority of Thailand (EGAT), own power plants comprising nearly 57% of installed capacity, while private producers account for the remainder (DEDE, 2007a). Since electricity market in Thailand is regulated, EGAT is a state-owned utility responsible for electricity generation, transmission and sales to two Table 4 Solid waste disposal sites and total methane emissions in different regions of Thailand (Chiemchaisri et al., 2007). Region

Number of sites

Amount of waste (tons/day)

Methane emissions (thousand tons/ year)

Landfill

Open dumping

Landfill

Open dumping

Landfill

Open dumping

22 8 17 30 16 2 95

70 23 56 100 81 – 330

854 229 477 1162 455 9000 12,177

950 904 1214 2263 2373 – 7704

12.03 5.10 10.86 19.09 6.30 54.83 108.21

1.44 0.77 1.90 1.62 1.46 – 7.49

Environmental impact from improper waste disposal Difficulties for MSW management in large communities like Bangkok and major regional cities have become evident in recent years. The generated amount of MSW in the domestic sector tends to increase every year. Because the overall waste collection service has not fully covered all service areas, uncollected waste together with improper disposal have unavoidably created health hazards and environmental contamination. An investigation of MSW management and emission inventory including current disposal technologies has

Northern Eastern Southern North-eastern Central BMA Total

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around 380–400 °C and 40 bar respectively because of higher corrosion risks in the MSW-fired boiler due to halogen acidic compounds and alkali salts. As a result, the net electrical efficiency for conventional MSW incinerator is fairly low (around 22–24%). In case of producing electricity only with a simple condensing steam turbine, the system is able to recover less than 30% of the available energy in the waste. Improving system performance is a great challenge.

MSW incinerator and gas turbine hybrid cycle

Fig. 2. Supply of primary energy in Thailand in 2007 (a); country's total electricity generation in 2007 (b).

distributing authorities which are Metropolitan Electricity Authority (MEA) and Provincial Electricity Authority (PEA). MEA is the main responsible for electricity distribution in the BMA. MSW incineration technologies Waste incineration with energy recovery is considered and analyzed in this section. Energy recovery from flue gases in thermal treatment plants is an integral part of MSW management. Energy from waste can be used in different applications depending on the final user i.e. purely electricity production, combined heat and power, or combined heat, cooling and power. For simplicity and considering only electricity, the analysis of this study considers only electrical power production, i.e. condensing mode of operation. Conventional MSW incineration Incineration is one of MSW treatment technologies that can significantly reduce the volume of MSW and destroy contaminated material. Incineration process involves the combustion of organic material to produce energy in the form of heat. Although there are a number of incinerator technologies in use today, mass-burn combustion over a moving grate with generation of superheated steam remains dominant. In general, the energy conversion from conventional grate combustion involves a steam Rankine cycle. Maintaining sufficiently low tube temperatures is required in order to avoid hot corrosion and ash melting problems, particularly in the superheater (Otoma et al., 1997; Petrov and Hunyadi, 2002). In Waste-to-Energy (WTE) plants, superheat temperatures and pressures are limited to

Efforts are being made to improve overall electrical efficiency of MSW incineration. One promising technique is a combination of a waste incineration plant and a combined cycle power plant. Hybrid dual-fuel cycle (hybrid cycle) is a promising solution with a simple concept to increase the electrical efficiency of MSW incineration while avoiding corrosion problems, by employing external steam superheat. A relatively straightforward method towards improving system performance involves integrating the steam production from the MSW incinerator with the steam bottoming cycle of a conventional gas turbine (GT) fueled with clean-burning natural gas (NG) (Korobitsyn et al., 1999; Consonni, 2000; Petrov and Hunyadi, 2002). As the exhaust gases from the GT are not corrosive, a heat recovery steam generator (HRSG) can raise the superheat temperature of MSW-generated steam typically up to 500–550 °C. Within this study, four different types of combined hybrid dual-fuel power plant configurations have been selected and modeled (see Figs. 3a and d). Fig. 3a represents a fully-fired (series connected) hybrid dual-fuel cycle with steam superheat partially by gas turbine exhaust (Case 1). All steam is produced and partially superheated in the MSW incinerator. The heat in the flue gas discharged after the GT is providing additional superheat of the steam to higher temperatures and is then utilized as a secondary air for the MSW combustor — this implies that there is no need for air preheating. The second proposed hybrid dual-fuel cycle as shown in Fig. 3b represents a parallel connection between MSW incinerator and gas turbine (Case 2). Again, all steam is produced and partially superheated in the MSW incinerator, then it is additionally superheated by the GT flue gases, after which the feedwater in the steam cycle can be preheated before entering the MSW boiler by utilizing the remaining heat in the GT exhaust. A key advantage of all parallel configurations is the lesser degree of integration and the consequent possibility to arrange for individual operation of the two sub-units, if necessary. Fig. 3c illustrates a fully-fired hybrid dual-fuel cycle where all steam is generated in the MSW incinerator but the superheat is provided entirely by GT exhaust heat, after which the GT exhaust is sent to the MSW incinerator as secondary air (Case 3). The advantage with this case compared to any other hybrid configuration lies in its much simpler construction without the need of superheat in the MSW incinerator. This makes the system more attractive in terms of maintenance and investment costs. However, one of the major disadvantages of any fully-fired (series) configuration is that the two exhaust gas flows are mixed and the NG-derived gases cannot be cooled down to 90 °C as in Cases 2 and 4. The flue gas treatment system would need to process larger amount of diluted gas volume — this also impacts the investment cost. Fig. 3d demonstrates the most complicated hybrid dual-fuel cycle considered in this study. Steam is generated and superheated both in the MSW incinerator and in the HRSG, at optimized pressure levels. The MSW provides partial superheat, after which the MSW-produced steam is further superheated in the HRSG by GT flue gases. The two superheated steam lines are supplied to a common steam turbine at their relevant pressure levels. Finally, the remaining low-temperature heat from gas turbine exhaust after the HRSG is used for feedwater preheating in the steam cycle.

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Fig. 3. (a) Fully-fired hybrid dual-fuel cycle with steam superheat partially by gas turbine exhaust (Case 1). (b) Parallel-powered hybrid dual-fuel cycle with steam superheat partially by gas turbine exhaust (Case 2). (c) Fully-fired hybrid dual-fuel cycle with steam superheat entirely by gas turbine exhaust (Case 3). (d) Parallel-powered hybrid dual-fuel cycle with steam superheat and parallel steam generation in the HRSG (Case 4).

Cycle performance analysis: assumptions and definitions The results were obtained by computer simulation via Aspen Utilities (Aspen Technology) at steady state conditions and full-load operation. All configurations are simulated with constant energy input from MSW incineration of 405,000 tons of MSW annually. In the present study a conventional MSW incineration/steam cycle has been first modeled and used as a reference case. MSW is combusted in a conventional grate boiler with flue gas recirculation for NOx control. A combustion air preheater and regenerative feedwater preheaters are employed for enhanced heat recovery. A steam circuit with comparatively high complexity featuring three feedwater preheaters has been chosen. The simulated MSW-fired basic steam cycle has been optimized for the highest possible electrical efficiency at given parameters. Pressure drop losses and internal energy consumption (pumps and fans within the steam cycle) have been taken into Table 5 Key parameters of the reference MSW-fired steam power cycle. Parameter

Value

Incinerator/Boiler type Superheat temperature Evaporation pressure Turbine inlet pressure Condenser parameters Deaerator parameters Feedwater preheaters Combustion air preheating Exhaust gas temperature O2 in exhaust gas Gross electric output MSW fuel input MSW energy content (LHV) Net electrical efficiency

Moving grate 380 °C 36 bar 33 bar 0.056 bar/35 °C 3.6 bar/140 °C 2LP + deaerator + 1HP 180 °C, by exhaust gas 200 °C after air-heater 6.5 vol.% 34 MW 405,000 t/a 9 MJ/kg 24% (LHV)

account for electrical efficiency calculations. Table 5 shows the major parameters and performance of the reference MSW incineration case, estimated for handling 405,000 tons of MSW annually, at 7500 h of full-load operation per year. The results for hybrid dual-fueled cycles are also obtained by computer simulation at steady-state conditions and full-load operation. All configurations are simulated in cold-condensing mode with constant fuel input from MSW incineration (405,000 tons of MSW annually) while the natural gas input is varied in order to compare the effect of fuel input ratio on electrical efficiency. The steam leaving the MSW incineration is superheated in a HRSG where exhaust gases from the gas turbine are used as a heat source. The steam pressure in the MSW-fired boiler is adjusted in all hybrid cycles in order to optimize the heat recovery from gas turbine exhaust. In general, superheated steam temperatures are in the range of 440–530 °C depending on the size of gas turbine. As a result, sufficient superheat can be supplied at low NG ratios, while additional steam must be generated by the gas turbine exhaust at high NG ratios. Table 6 shows the major parameters and performance of the selected gas turbine.

Table 6 General input parameters of the Gas Turbine. Parameter

Value

Standard performance parameters: Ambient conditions Electrical power output (varying size) Pressure ratio Gas turbine exhaust gas temperature Electrical efficiency Energy value (LHV) for natural gas Natural gas density Exhaust gas temperature (to stack) Electrical efficiency of gas turbine combined cycle

25 °C, 105 Pa 40–70 MW 17 545 °C 34% (LHV) 49.1 MJ/kg 0.8 kg/m3 90 °C 51%

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bottoming cycle only (MSW-fired steam boiler in this case) (Petrov and Hunyadi, 2002). In both cases, the overall performance of the hybrid cycle is compared to the reference cases of a separate steam cycle and an individual gas turbine combined cycle at the same conditions. The equation can be presented as follows: (1) additional or incremental efficiency for the topping cycle; and (2) electrical efficiency attributed to the MSW fuel (Consonni, 2000; Petrov and Hunyadi, 2002). ηadded

Fig. 4. Cycle electrical efficiency as a function of fuel energy input ratio for four hybrid dual-fuel cycles. The reference line represents an average electrical efficiency of two separate single-fuel units, one MSW incineration steam cycle and one NG-fired gas turbine combined cycle, at a varying ratio of natural gas to total fuel input.

While advanced methods for integration of otherwise conventional technologies such as combined hybrid dual-fuel power plants provide significant efficiency improvement, a proper basis of comparison must be established given the fact that two very different fuels are employed. The electrical efficiency improvement by hybrid dual-fuel cycles can be evaluated in a number of ways. The electrical efficiency for any type of hybrid dual-fuel cycle can basically be expressed via the relationship of the net useful energy output to fuel energy input, while an added capacity or incremental efficiency can be defined for the topping cycle only (gas turbine in this case) or for the

capacity

=

PGT + ΔPST : mNG LHVNG

ð1Þ

Here, PGT is the power output from the gas turbine, while ΔPST is the added output from the MSW-fired steam cycle after integration with the gas turbine. The additional efficiency is of great importance in evaluating the thermodynamic improvement especially for the steam cycle when a gas turbine is added as topping cycle. The electrical efficiency attributed to the MSW-fired steam cycle in a hybrid configuration can also be defined by the extra power generated by the steam turbine after hybrid integration, related to the MSW fuel energy input. ηMSW =

PTotal −Pnet;cc : mMSW LHVMSW

ð2Þ

Here, PTotal is the total net power output from the hybrid dual-fuel cycle, while Pnet,cc is the power output of an individual gas turbine combined cycle at the same scale and conditions, which represents the reference case for best possible efficiency of topping fuel utilization (natural gas in this case).

Table 7 Simulation results and performance comparison for the modeled hybrid cycle configurations. Cycle type

PGT

PST

Super heat

Fuel flow MSW

Fuel flow NG

NG to total fuel

Cycle el. Efficiency

MSW el. efficiency

Difference to refer.

GT share of total output

MW

MW

°C

kg/s

kg/s

Energy ratio

%(LHV)

%(LHV)

Effic. line

Case 1 0.20 0.25 0.30 0.35 0.40 0.44 0.48

12 18 24 33 42 54 63

44.2 51.5 55.0 62.0 65.0 69.0 69.0

400 440 460 490 510 530 530

15 15 15 15 15 15 15

0.72 1.14 1.48 1.99 2.49 3.17 3.68

0.21 0.29 0.35 0.42 0.47 0.54 0.57

33.00 36.41 38.07 40.86 41.55 42.35 41.82

28.30 30.47 31.34 33.53 33.54 33.60 29.97

2.10 3.14 3.39 4.14 3.18 2.20 1.10

Case 2 0.20 0.25 0.30 0.35 0.40 0.44 0.48

12 19 25 34 46 57 70

48.0 52.0 56.8 62.5 69.8 72.5 76.3

420 450 473 498 525 530 530

15 15 15 15 15 15 15

0.79 1.19 1.53 2.04 2.72 3.34 4.08

0.22 0.30 0.36 0.43 0.50 0.55 0.60

34.45 36.67 38.90 41.02 43.11 43.29 43.63

29.82 30.71 32.43 34.41 35.67 33.91 33.00

3.22 3.29 3.94 4.02 4.17 2.85 1.81

Case 3 0.20 0.25 0.30 0.35 0.40 0.44 0.48

12 18 24 33 43 56 66

46.5 52.0 55.5 61.5 65.2 71.0 72.0

400 440 460 495 510 530 530

15 15 15 15 15 15 15

0.79 1.14 1.48 1.99 2.55 3.29 3.85

0.22 0.30 0.35 0.42 0.48 0.54 0.58

33.59 36.68 38.31 40.65 41.57 42.85 42.57

28.71 31.00 31.88 33.31 33.38 33.82 31.60

2.41 3.40 3.61 3.95 3.17 2.52 1.17

Case 4 0.20 0.25 0.30 0.35 0.40 0.44 0.48

12 18 25 34 47 58 72

48.0 52.8 57.0 62.6 69.0 73.0 78.8

405 440 460 498 520 530 530

15 15 15 15 15 15 15

0.79 1.14 1.53 2.04 2.78 3.40 4.19

0.23 0.29 0.36 0.43 0.50 0.55 0.60

34.12 37.10 39.00 41.06 42.74 43.39 44.24

29.49 31.60 32.64 34.12 35.01 34.71 34.55

2.87 3.95 4.04 4.20 3.62 2.85 2.17

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Modeling results Cycle efficiencies Electrical efficiencies for the chosen hybrid cycle configurations (Cases 1–4) are detailed in Fig. 4 and Table 7. The straight line represents the average efficiency of two separate single-fuel units, one reference steam cycle and one reference gas turbine combined cycle, at a varying ratio of natural gas to total fuel input, i.e. 24% with 0% NG (pure MSW-fired steam cycle) and 51% with 100% NG (straightforward NG-fired combined cycle). The basic parameters for the MSWfired steam power cycle used in all hybrid dual-fuel configurations are the same as reported in Table 5. Simulation parameters, calculation results and efficiency improvements for all cases of hybrid dual-fuel configurations are presented in Table 7. Power outputs from the MSW-fired steam cycle in all hybrid configurations have been substantially improved. The electrical efficiency attributed to the MSW energy conversion is further evaluated and presented in Fig. 5 below. The results presented above show that the hybrid dual-fuel cycle of any configuration provides significantly higher electrical efficiencies than a composite of individual single-fuel power plants performing at the same energy input ratio and same conditions. The cycle electrical efficiency improvement can grow up to around 4%-point for each hybrid configuration, at a certain optimum fuel input ratio. Optimum values for the highest electrical efficiency of combined hybrid dual-fuel cycles in Cases 1 and 3, with exhaust gas from gas turbine mixed with primary air for MSW incinerator, are located at 0.30–0.50 NG to total fuel energy input ratio. For Cases 2 and 4 the optimum values lie in a slightly broader range, 0.22–0.55 of NG share. Cases 2 and 4 exhibit high electrical efficiencies even at low NG-toMSW ratios. Cases 2 and 4 can also operate at higher NG-to-MSW ratios and still provide 2.5%-point efficiency increase over the reference case. The efficiency of MSW energy conversion in the overall hybrid cycle increases up to 10%-points when the typical efficiency of single-fuel gas turbine combined cycle is to be considered. In return, the efficiency of only natural gas utilization could increase up to 60% if the efficiency of MSW energy conversion is assumed at the reference value of conventional MSW incineration. Potential for electricity production from MSW The optimum values for the highest electrical efficiency of incineration technology with energy recovery and combined hybrid dual-fuel cycles are capable of providing peak capacity of up to 35 and

Fig. 5. Electrical efficiency attributed only to the MSW fuel, as a function of fuel energy input ratio. Efficiency is calculated by assuming reference NG utilization with typical efficiency of a straightforward gas turbine combined cycle at the same scale and conditions.

115 MW corresponding to 0.3 and 0.9 TWh annually, respectively. These systems can reduce the amount of waste by up to 0.4 million tons per year. The hybrid power plant provides enhanced efficiency of energy conversion and inherent flexibility while keeping the complexity and therefore the investment costs in the same range as those for two individual single-fuel units. Conceivably, four conventional incineration units as previously planned or four new hybrid power plants could be constructed in the BMA, thus providing nearly 1.2 to 3.5 TWh/year of electricity respectively, while handling up to 1.6 million tons of MSW each year. Considering total MSW available in the BMA, the overall potential can increase to nearly 2 or 7 TWh of electricity production in conventional incineration or hybrid cycles, respectively. Thus, electrical power generation via conventional incineration or hybrid power plants can cover from 4% to 14% of Bangkok's electricity consumption, respectively. In parallel, recycling of non-organic materials combined with biological treatment of the organic waste e.g. biogas utilization from anaerobic digestion, composting etc. could probably be employed to handle the MSW. These methods are suitable for wet organic waste with high moisture content and low fraction of inert materials. The potential for landfill gas recovery from two landfill sites in the BMA has been estimated at 50 to 58 million m 3/year (Thailand Environment Monitor, 2003). The potential for landfill gas recovery and electricity generation from landfill gas has been found to be up to 73–85 GWh for the BMA region. Greenhouse gas emissions Although it is obvious that combined hybrid dual-fuel cycles provide attractive electrical efficiency improvements over conventional MSW incineration, an environmental assessment for greenhouse gas emission in terms of methane and CO2 has also been conducted. The MSW-only power plant is found to emit 440 kg fossilCO2/MWh (Udomsri et al., 2010) at the calculated efficiency level of around 24%, where MSW is assumed to be 2/3 carbon neutral. Fig. 6 shows the CO2 emissions of fossil origin from the four proposed hybrid cycles as a function of NG to total energy input ratio. The reference line represents average CO2 emissions of two separate single-fuel units at the same conditions. As shown in Fig. 6, hybrid cycles are capable of reducing CO2 levels by 5–10% in comparison with the reference incineration case, even for small NG-to-MSW fuel ratios. MSW incineration has the ability to lessen environmental impact associated with waste disposal. The MSW-only power plant emits approximately 440 kg fossil-CO2/MWh, while the hybrid cycles generate around 380–395 kg CO2/MWh. The fossil CO2 emitted by the MSW fuel in the proposed hybrid configuration cases is in the range of 318–376 kg/MWh that is about

Fig. 6. Net fossil CO2 emissions from four different proposed hybrid cycles as a function of NG to total energy input ratio. Reference line represents average CO2 emissions of two separate single- fuel units, MSW incineration steam cycle and NG-fired gas turbine combined cycle at a varying ratio of natural gas to total fuel input.

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Table 8 Emission reduction for conventional incineration, hybrid cycles and methane avoided from landfills. Incineration technologies

Landfilling without gas recovery Conventional incineration

Hybrid cycle

440 2

390 7

CO2 emission reduction; compared with existing plants⁎ 140 (thousand tons CO2/year)

800

CO2 production (kg CO2/MWh) Power production from 3,200,000 tons MSW/year (TWh)

Methane CO2equivalent Emissions production (kg/ton MSW) Emissions avoided from 405,000 tons MSW (thousand tons/year) Total emissions avoided from 3,200,000 tons MSW (thousand tons/year)

50–100 1150–2300 20–40 466–932 160–320 3680–7360

⁎ Emission factor for electric power is 506 kg CO2/MWh.

14–28% less than for the MSW-only power plant. Hybrid dual-fuel cycles offer a promising solution with a simplified concept to increase the electrical efficiency of energy recovery from MSW incineration while avoiding corrosion problems, by employing external superheat of the MSW-generated steam. Hybrid cycles can also provide significant CO2 emission reductions in comparison with individual single-fuel power plants. This makes MSW even more attractive by virtue of enhanced waste utilization for energy recovery. The fossil CO2 emission reduction from hybrid cycles has also been evaluated and compared with current technology: the CO2 emissions from existing electricity production in Thailand. The emission factor for Thai electric power is in the range of 506–583 kg CO2/MWh [UNFCCC, 2009; IEA, 2007b]. However, the emission factor of 506 kg CO2/MWh has been used in this study [UNFCCC, 2009]. The results show that the hybrid dual-fuel cycle still provides the highest CO2 reduction in comparison with the current situation, and a reduction of more than 800,000 tons could be obtained (3.2 million tons of waste annually in the BMA). This figure is even more attractive when the methane emitted from existing landfill sites is to be compared. Uncontrolled methane emissions from MSW in modern landfills contribute 50–100 kg/ton, which is equivalent to 1150–2300 kg CO2/ton (IEA, 2007a). Although it is believed that 50% to 75% of methane generated from landfill can be collected, the present situation shows that very little landfill gas is being recovered in the BMA. In terms of uncontrolled methane emissions, landfilling without gas recovery (as now practiced in the BMA region of Thailand) produces and releases to the atmosphere up to around 320,000 tons of CH 4 annually, the equivalent of more than 7.4 million tons of CO2 annually (3.2 million tons of waste). Although methane generated from MSW in modern landfills can be captured and used for energy recovery, up to 50% of this gas still escapes to the atmosphere (IEA, 2007a). Therefore, even if landfill gas recovery was employed at full scale and the gas utilized properly, the BMA would still emit up to around 160,000 tons of CH4 annually, the equivalent of 3.7 million tons CO2. Table 8 shows emission reduction for conventional incineration, hybrid cycles and methane avoided from landfills. Diverting the MSW flow from a landfill to an incineration plant instead, would immediately cut down to zero all methane emissions related to MSW decay. The electricity production from one MSW incineration plant can lead to a reduction of around 55–110 tons/day methane emissions, equivalent to roughly 1280–2550 tons CO2/day. Conceivably, if four hybrid incineration plants could be constructed in the Bangkok area, their operation would result in the saving of a total of nearly 1,860,000–3,720,000 tons of CO2-equivalent per year, compared to landfills with present situation.

note that there is no single method for MSW disposal that can handle all waste materials in environmentally sustainable way. MSW incineration is one reasonable and sustainable MSW management method in terms of weight and volume reduction coupled to energy recovery. It can reduce large amounts of waste and greenhouse gas emissions, and simultaneously enhance energy recovery. It is however important to stress that modern pollution control and flue gas scrubbing technologies must be employed in order to avoid harmful pollutants like dioxins. It is believed that incineration of MSW using modern environmental controls, in combination with recycling programs and biogas utilization from organic wastes, represents the most logical path that Thailand and other countries in Southeast Asia should strive towards via promotion of economically efficient and environmentally sound practices. Part of the society shares an accepted opinion that waste incineration is a toxic process and that its promotion as renewable source will have an adverse effect. Energy recovery from the steam in MSW power plant is of great importance and hybrid cycles can be proposed to improve the overall electrical efficiency of conventional incineration. The simulation results from this study show that all hybrid dual-fuel configurations provide higher electrical efficiency than the average efficiency of two individual single-fuel units at the same NG and MSW fuel energy input. The maximum improvement of electrical efficiency is found to be up to 4%-points for all hybrid cycles, when compared to the reference cases, leading also to greenhouse gas emission reductions. Hybrid dual-fuel cycles can provide significant CO2 reductions in comparison with separate thermal power plants fired with MSW or natural gas respectively. The integrated hybrid cycle is capable of reducing CO2 emission levels by 5–10%, even for small ratios of natural gas to MSW fuel. MSW incineration is even more attractive when methane emissions generated from the existing landfill sites are to be compared. The incineration system can handle more than 1.6 million tons of MSW each year that leads to a reduction of CO2 equivalent of up to 3.7 million tons per year for the BMA region of Thailand. Acknowledgments The authors wish to convey their sincere appreciation to Swedish International Development Cooperation Agency (SIDA), Department for Research Cooperation, SAREC for financial support of this project (Contract No. SWE-2007-047). The authors wish also to acknowledge the World Bank Thailand, for providing key information (i.e. Thailand Environmental Monitor Report, Waste management report, etc.). References

Conclusions MSW incineration can play significant role for not only greenhouse gas emission reduction but also improved waste management in Thailand, and it can contribute positively towards expanding biomass-based electricity production. It is however important to

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