Energy from municipal solid waste

Energy from municipal solid waste

CHAPTER EIGHTEEN Energy from municipal solid waste Contents 18.1. Waste and its management strategies 18.2. Waste-to-energy concept 18.2.1 Traditiona...

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CHAPTER EIGHTEEN

Energy from municipal solid waste Contents 18.1. Waste and its management strategies 18.2. Waste-to-energy concept 18.2.1 Traditional steam cycle 18.2.2 Combustion of syngas in gas turbines or gas engines 18.2.3 Use of landfill gas 18.3. Waste-to-energy plant configuration 18.3.1 Boiler island 18.3.2 Power island and balance of plant 18.4. Project evaluation and economics 18.5. Sustainability challenges and environmental issues References

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18.1. Waste and its management strategies Since the beginning of their history, humans have generated solid waste and disposed of it in a variety of ways, such as in makeshift waste dumps or incinerations [1]. The industrial revolution has dramatically increased the production of goods and their consumption, which in turn resulted in growing waste discarded by the society, catalyzed by the ongoing urbanization. It therefore became necessary to provide landfills and incinerators for disposing waste, supported by special transportation, logistics and management services. The amount of waste has increased tremendously during the last years and consequently became an issue. Thanks to new materials like polyethylene or other plastics, glass, and resins, their complete natural breakdown may take too long and damage the environment. In response, some of the most advanced countries started to develop various measures to reduce waste by optimizing production and consumption patters, developing new natural or recyclable materials, packaging, etc. New approaches to sort waste, recycle materials, burn and recover heat from the waste decreased the amount of landfill, however, the scale is still too small. Almost any human activity generates waste, but its nature and composition differs according to the source. The municipal solid waste usually Sustainable Power Generation Copyright © 2019 Elsevier Inc. https://doi.org/10.1016/B978-0-12-817012-0.00032-3 All rights reserved.

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Figure 18.1 Municipal solid waste composition worldwide. Almost half of the waste comes from organics, with paper being second with around 17%, plastics have 10%, and glass amounts to 5%. Source: aggregated from [2,3].

incorporates residential, commercial, and municipal services waste, as well as some industrial waste if it is treated by the municipality [2,3]. Construction and demolition waste is treated separately in many cases as it allows for proper waste management and disposal. While process waste, as well as medical and agricultural, usually has specific ways of treatment due to its unique nature, municipal solid waste is a growing complicated compound mix and therefore needs different treatment. According to the studies [3], the average municipal solid waste composition worldwide is dominated by organic waste, followed by paper and plastics (see Fig. 18.1). However, this composition differs for various countries and location, e.g., urban and rural areas, and depends on the average income rate of the country. It should be noted that even at the European Union level there is no clear and established definition of the municipal waste, therefore the practices of its treatment in the EU, as well throughout the globe, differ. Unfortunately, the availability and quality of annual data are major problems for the waste sector. Solid waste and wastewater data are lacking for many countries, data quality is variable, definitions are not uniform, and interannual variability is often not well quantified [5]. Around the world, waste generation rates are rising. In 2012, the worlds’ cities generated 1.3 billion tonnes of solid waste per year, amounting to a footprint of 1.2 kilograms per person per day on average [6]. The current annual municipal solid waste generation is estimated at the level of 1.3–1.9 billion tonnes and is expected to increase to approximately 2.2 billion tons per year by 2025 [2,3]. Importantly, solid waste yearly generation rates range from less than 0.1 tons per capita in low income countries to more than 0.8 tons per capita in high-income industri-

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alized countries. Even though labor costs are lower in developing countries, waste management can constitute a larger percentage of municipal income because of higher equipment and fuel costs [5]. Available data shows the average split of MSW generation per region with the OECD countries being responsible for 44% of total world waste generation [3]. Waste disposal data are the most difficult to collect as many countries do not do this on the national level. Furthermore, in cases where data is available, the methodology of how disposal is calculated and the definitions used for each of the categories is often either not known or not consistent [3]. It has been estimated [1,3,5,7] that the recoded post-recycling solid waste in 2017 already amounts to over 1.4 billion tons per year, of which more than 70% are placed in landfills and dumpsites, 19% is recycled or recovered and 11–15% is led to energy recovery facilities. Moreover, only 20% of the landfilled solid waste is disposed of in sanitary landfills that reduce aqueous and gaseous emissions into the environment [1,7]. And this situation is also due to the fact that the number of people that lacks access even to the most elementary waste management services is estimated to at least 3.5 billion which may well increase to around 5.6 billion by 2050 if the businesses continue without any significant change in waste management practices [7]. The waste management sector follows a generally accepted hierarchy that responds to financial, environmental, social and management considerations and also encourages minimization of GHG emissions. In the order of preference, this hierarchy includes [3,8–10]: • Reduction of waste initiatives, seeking to reduce the quantity of waste at generation points by redesigning products or changing patterns of production and consumption. A reduction in waste generation has a two-fold benefit: reduction of waste and material during production and elimination of the emissions associated with the avoided waste management activities. • Reuse, recycling and materials recovery, to reduce quantities of disposed waste and return the materials to the economy. A value-based conception of waste, along with appropriate collection infrastructure, could prevent the loss of valuable waste and increase the timely reuse and recycling of used products [10]. • Incineration of waste with further energy recovery, which can reduce the volume of disposed waste by up to 90%, however, only for waste streams with very high amounts of packaging materials, paper, cardboard, plastics and horticultural waste. Recovering the energy value

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embedded in waste prior to final disposal is considered preferable to direct landfilling — assuming pollution control requirements and costs are adequately addressed. Typically, incineration without energy recovery (or non-autogenic combustion, the need to regularly add fuel) is not a preferred option due to costs and pollution. • Landfilling, which is a common final disposal for waste and ideally should be engineered and operated to protect the environment and public health. Landfill gas (LFG), produced from the anaerobic decomposition of organic matter, can be recovered and the methane (which is approx. 50% of LFG) burned at high temperatures with or without energy recovery to reduce GHG emissions. Proper landfilling is often lacking, especially in developing countries. • Controlled dumping, which is the least preferred option, when the waste at least undergoes registration and controlled placement/compaction. Controlled dumps usually have surface water monitoring, but no engineering measures and no landfill gas management systems. A wide range of mature technologies and established approaches are available nowadays to mitigate GHG emissions from waste. They include: • Landfilling with landfill gas recovery (for instance, for its combustion) to reduce the release of methane emissions directly to the atmosphere; • Post-consumer recycling to mitigate waste generation; • Composting of selected waste fractions to minimize GHG emissions generation; • Processes that further reduce GHG generation compared to landfilling. They are based on thermal treatment, including incineration and industrial co-combustion, mechanical-biological treatment with further landfilling of residuals, and anaerobic digestion. Therefore, the mitigation of GHG emissions from waste relies on multiple technologies whose application depends on local, regional and national drivers for both waste management and GHG mitigation [5]. While post-consumer waste is a relatively small contributor to global greenhouse gas emissions (< 5%) with total emissions of approximately 1300 million tons of CO2 -equivalent emissions in 2005. The largest source is landfill methane (CH4 ), followed by wastewater and nitrous oxide (N2 O). In addition, minor emissions of carbon dioxide CO2 result from incineration of waste containing fossil carbon C (from plastics or synthetic textiles) [5]. Existing waste-management approaches can provide effective mitigation of GHG emissions and provide public health, environmental protection,

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and sustainable development co-benefits. Collectively, these technologies can: • Directly reduce GHG emissions through landfill gas recovery, improved landfill practices, engineered wastewater management, • Avoid significant GHG generation through controlled composting of organic waste or state-of-the-art incineration. The latter can benefit from additional energy recovery. Today, an estimated 250 million tons of MSW are worldwide thermally treated annually to produce electricity and heat. This relevant global industry is usually called waste-to-energy (WTE) and is rapidly growing: an estimated 250 WTE facilities were built during the first 15 years of the 21st century, mostly in Europe and East Asia [1]. The great majority of these plants are based on the grate combustion technology of received or post-recycled MSW, and produce both electricity and heat. While there are some plants that implement gasification technology, the trend is towards very large facilities based on the principle of combustion on a moving bed [1]. Although various WTE technologies such as waste incineration, anaerobic digestion, gasification, and some others exist, mass incineration is still the most common [4]. Remark 18.1. In some literature, waste-to-energy is classified as part of biomass energy. In this book we consider biomass energy in a separate Chapter 19. Municipal solid waste is not the only type of waste that can be potentially utilized in utility-scale power generation. Many industrial processes have a by-product gas called synthetic gas, or syngas, which is a mixture of hydrogen, carbon monoxide, and other gases, e.g., carbon dioxide or nitrogen. Some gases may have much higher percentages of nonflammable gases, but can still be burnt, for instance, blast furnace gas, which is not directly attributed to syngas. Considered in some processes as waste products, they are either burnt in a flare or used as a fuel.

18.2. Waste-to-energy concept The dominant technology for energy recovery from municipal solid waste is based on a direct combustion of waste, when the heat is used to generate steam, which is fed into a steam turbine. This technology is similar to conventional steam power plant that burns solid fuels. However, the

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amount of energy recovered from the MSW combustion can vary significantly with the characteristics of MSW fed into the boiler (its composition, mass flow rate, heating value), combustion technology, configuration and features of the boiler, and the characteristics of the thermodynamic cycle [4].

18.2.1 Traditional steam cycle A WTE steam cycle is relatively simple and similar to a smaller size conventional thermal power plant driven by a steam turbine (see Fig. 18.2). A boiler burns fuel, represented by MSW, with the presence of oxygen (i.e., air). Its chemical energy of the combustion reaction is transferred to water that flows through the heat exchanger tubes inside so that it heats up and therefore evaporates. Further heating leads to the increased pressure of steam, which is then fed to the steam turbine, where it expands and transfers its energy to the turbine’s rotor. A steam turbine spins an electric generator that delivers electricity to the consumer. Cold steam from the steam turbine goes to the heat dissipation system, either in the form of condenser or district heating heat exchanger, where it gives its energy away to cooling or heating water, respectively. Finally, cold condensate is pumped back to the boiler. Similarly to a conventional steam power plant, WTE power plant cycle can be optimized to minimize its energy losses and therefore increase efficiency. As discussed in Chapter 7, major heat losses of a thermal power plant occur inside the condenser when the heat of the exhaust steam is

Figure 18.2 Simplified process diagram of a WTE plant. Similarly to a conventional thermal power plant, boiler burns waste to evaporate water, and the steam is used to spin the steam turbine and associated generator.

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taken away by cold water, leading to around 50% of energy waste. One of the easiest ways to recover this energy is to utilize this WTE plant for a cogeneration application, usually for local district heating needs. In this case the condenser can be used as a heat exchanger that transfers heat from the cold steam, released from the steam turbine, to the district heating fluid. Certain part of energy, however, cannot be transferred to the water inside the heat exchangers due to technological reasons and hence this energy “leaves” the boiler through the exhaust stack into the atmosphere. The amount of heat wasted to the atmosphere can be around 5–15% of the initial chemical energy of MSW and depends on the technology, fuel composition and plant operating conditions. Similar to a traditional coal-fired steam power plant, WTE plant’s thermodynamic cycle can also be optimized in many ways, including proper heat recovery throughout the cycle: • In a cogeneration application, steam may be extracted directly from the steam turbine and fed to the end user or to district heating heat exchangers. This may decrease the amount of steam that finally goes to condenser and reduce cooling water consumption. • Cold condensate that is pumped from condenser can first flow through the special heat exchanger called a preheater that uses the temperature of hot gases downstream the boiler. These gases still possess enough energy after they have heated and evaporated water. • Air required to burn waste is taken from the outside environment, and may be cold enough, which would require extra chemical energy to maintain reaction. Therefore, this air may also be preheated by the exhaust gases before entering the incinerator. The above mentioned ways are among the few that are always considered by designers of any type of a thermal power plant. As a WTE plant is usually smaller and located in highly populated areas, especially in developed countries, its design and operating parameters must be as competitive as possible to drive the costs down. This discussion will be conducted in detail in Section 18.4.

18.2.2 Combustion of syngas in gas turbines or gas engines While the combustion of solid waste is usually carried out in special boilers, which generate steam to implement the steam turbine cycle, the use of syngas is slightly different. Of course, syngas can also be burnt in boilers to produce steam, however, it is much more favorable to feed syngas to a gas turbine. While this approach seems obvious, it may lead to some issues

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regarding the combustion process: traditional gas turbines are designed to burn natural gas which contains more than 95% of methane and the process is optimized for lower emissions. Syngas has a very different composition which may not be easy to burn in such a combustion system, so that either it is mixed with natural gas for a more favorable composition, or the combustion chamber is optimized for this fuel. The process of a gas turbine based thermal power plant that burns syngas is the same as for traditional natural gas fired one (refer to Chapter 7) and may differ only in some special equipment for fuel treatment, control system and combustion chambers. Such cycles are implemented in industry where syngas (as a waste byproduct of the process) can be further utilized to generate electricity for the local needs. Moreover, using a gas turbine allows implementing a combined cycle process, that is, increasing efficiency beyond 40% of a steam thermal plant. Moreover, gas turbines offer higher flexibility of the process. If the amount of syngas is limited, the project does not require large capacities, or the composition does not allow using a gas turbine, another technology is applicable, namely, one can use reciprocating gas engines. Their capacity varies from a few kilowatts to a dozen of megawatts, and they can be combined as modules to rapidly increase the capacity of the whole facility.

18.2.3 Use of landfill gas Landfill gas that is the result of waste storage in landfills and contains approximately 50% of methane with roughly 50% of CO2 , and is contaminated with a small amount of other pollutants. In the worst case, it is freely released to the atmosphere from the landfills which are not properly managed, and causes significant damage due to its strong GHG nature. In order to prevent this, landfill gas is usually flared or, in simple words, wasted by a direct burning process. Though this combustion process has lower negative impact compared to its free release to the atmosphere and much lower GHG emissions levels compared to conventional coal combustion, it is still wasteful. In an ideal case, this gas should be gathered and fed to a power plant equipped with either reciprocating engines or gas turbines with the combustion chamber designed for specific fuel composition. Such power plants are similar to traditional simple cycle power plants driven by a heavy duty gas turbine, however, they are smaller in size and are usually built in modules. Moreover, they require special attention to the fuel composition. While gas engines are less sensitive to such fuels, traditional gas

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turbines may need fine-tuning of its fuel gas module, combustion chamber and control system to ensure reliable and efficient operation. Remark 18.2. Most commonly built WTE power plants burn municipal solid waste as base fuel and employ steam cycle based on boilers and steam turbines.

18.3. Waste-to-energy plant configuration The process of generating electricity in a conventional mass-burn conventional waste-to-energy power plant has the following major stages: 1. Waste is delivered by garbage trucks into a large receiving pit for pretreatment and further delivery to combustion chamber within the incinerator; 2. The incinerator or a special boiler is designed to efficiently use the chemical energy of the burning waste to evaporate water and supply superheated steam of certain parameters; 3. The high-pressure steam turns the blades of a turbine generator to produce electricity; 4. An air pollution control system removes various pollutants from the combustion gases before they are released through an exhaust stack to the atmosphere; 5. Ash and other waste products are collected from the boiler and the air pollution control system. Based on this process, the overall conventional WTE plant configuration can be arranged into the following major parts and equipment areas, as schematically represented in Fig. 18.3: • Boiler island, which comprises municipal solid waste or fuel management system, incineration system with a steam boiler, and emissions control and treatment equipment;

Figure 18.3 Equipment areas of a typical utility-scale waste-to-energy power plant. A typical WTE plant implies three major blocks: boiler island, turbine island, and balance of plant systems (mechanical and electrical).

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Power island with steam turbine, generator and necessary subsystems, supplied with them, and Mechanical and electrical balance of plant systems.

Remark 18.3. A WTE plant that operates on syngas as a product of waste gasification would have configuration and equipment areas similar to: • Simple cycle or combined cycle power plant, if this plant is based on gas turbine technology, either alone or in combination with the heat recovery equipment and steam turbine; • Gas engine based modular plant (usually container-based); • Conventional thermal gas-fired boiler and steam turbine, if syngas is burned within a traditional boiler. Evidently, a hybrid WTE plant would have a different configuration, which will be similar to a CCPP or a combination of a CCPP and a conventional WTE plant, depending on the design. While these plants are still limited, we will focus only on the conventional technology, yet paying specific attention to the sustainability questions which are similar to any type of WTE technology.

18.3.1 Boiler island Similarly to a conventional coal-fired power plant, the boiler island of a waste-to-energy plant unites those systems related to fuel (i.e., waste) handling, its pretreatment and combustion to generate steam, as shown in Fig. 18.4.

Figure 18.4 Simplified process diagram of a WTE plant. Schematic diagram of the process including boiler island configuration: waste reception and treatment, incineration in a boiler, emissions and process waste treatment system.

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18.3.1.1 Waste reception and treatment The waste is usually delivered to a power plant by special trucks and discharged into the reception bunker, which can hold enough waste for several days of operation. Ideally, this bunker should be closed and kept under lower pressure to avoid discharge of gaseous emissions from the waste inside and prevent the neighboring community from a harmful impact. This bunker has to be frequently cleaned thoroughly to avoid deep contamination according to strict environmental, health and safety procedures imposed by regulation. In many cases waste is kept in a bunker for a few days for fermentation which should reduce its humidity and increase its heating value. The waste in the bunker is mixed and delivered by overhead cranes with special grabs to the charging hoppers, which are a part of the waste feeding system. This system delivers waste to the combustion chamber of the boiler, either continuously or in batches, depending on the type of an incinerator implemented in a plant. Sometimes waste may undergo a milling process to ensure that it has relatively fine structure for combustion. Finally, the pretreated waste is fed to the incinerator or a combustion chamber of a special waste boiler.

18.3.1.2 Boiler There are three major types of waste boiler widely implemented in commercial operation which differ in combustion chambers: 1. Moving grates, which consists of a moving grate that transports waste through the combustion chamber. While on the grate, the waste is dried and burned at high temperature with continuous air supply from below the grate, while the ash and non-combustible parts leave the grate either downwards under normal gravity or at the end of the grate path. Properly designed grates transport and agitate the waste and evenly distribute air to make combustion process efficient (see Fig. 18.5). 2. Rotary kiln combustion chambers, which use a special inclined rotating cylinder transporting the waste through the furnace as illustrated in Fig. 18.6. This cylinder can be up to 5 m in diameter and up to 20 m long. Due to its rotation, the waste mixes and allows proper burning process. Rotary kiln combustion is sometimes used to burn hazardous materials like medical, biological or industrial waste. It can provide higher burning temperatures to achieve full destruction of viruses or microbes.

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Figure 18.5 Simplified schematic of a moving grate waste boiler. Waste is delivered to the bunker, pretreated and fed to the moving grate. Air is blown from below to maintain the incineration process. Hot gases follow the complicated hot gas path to transfer its heat to multiple heat exchanger tubes and evaporate water.

Figure 18.6 Simplified schematic of a rotary kiln waste boiler. Treated waste is delivered to the rotary kiln that mixes the waste and maintains combustion process. Hot gases follow the hot gas path to transfer its heat to multiple heat exchanger tubes and evaporate water.

3. Fluidized bed combustion chambers, which have the same logic as for coal: fine solid particles of fuel waste are mixed with the air coming from the bottom, fly out of the solid layer (like bubbles) and burn. While the technology seems quite efficient, it requires special pretreatment of waste and its consistency, which is very rarely achievable in practice. The schematic of a fluidized bed boiler within a WTE plant is presented in Fig. 18.7. Remark 18.4. There some other types of boiler and technology that may be utilized for waste, however, moving grate, rotary kiln and fluidized bed are the top three technologies used in commercially built and operated WTE plants.

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Figure 18.7 Simplified schematic of a fluidized bed boiler. Treated waste is delivered to the boiler combustion chamber to form a layer of particles. During combustion they start to “bubble” and burn, follow the hot gas path and, if unburnt, return back to the chamber.

For startup purposes and to maintain the furnace temperatures, an auxiliary firing system is implemented. This is usually done with fuel oil or natural gas and is similar to that in a conventional boiler or an HRSG. Like in a conventional fossil-fired boiler (as discussed in Chapter 8), hot gases of approximately 1000–1200◦ C from the combustion area go through the system of heat exchangers — water tubes, which absorb the heat of the flowing hot combustion gases and transfer it to water. Depending to the size and configuration, various designs of boilers may be exercised. The most established is the drum technology: evaporation of water takes place in a vessel called drum, which also serves as a water–steam separator to feed steam further to a downstream consumer. If the heat balance allows for higher energies, this steam may be further driven through a heat exchanger called superheater, which further increases is temperature and pressure, and, in turn, overall plant efficiency. Obviously, superheated steam is far better for further use in a steam turbine as it increases its output and, depending on the flow, allows for a multipressure design, though mostly without reheater due to relatively smaller size of plant compared to conventional coal-fired power plants.

18.3.1.3 Emissions and process waste treatment Combustion of municipal solid waste also releases waste in the form of solid ash, small particles that flow with the gases, and gaseous emissions. Therefore, any WTE facility must be equipped with modern treatment systems to comply with ecological regulations. These systems are similar to those in a coal-fired power plant and always include:

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Bottom ash handling; Fly ash treatment; Flue gas handling. Bottom ash is the noncombustible fraction of the waste that is burnt in the furnace. Sometimes referred to as grate ash, clinkers or slag, is usually generated at the rate of roughly 20–30% of the weight of the waste combusted, but only 5–10% by volume [1], however, it can vary on either side depending on the composition of waste. A bottom ash handling system has to allow for ash removal during the operation and its delivery to the intermediate storage exposed to the environment for its preliminary natural treatment, which can be simply in a pile on site with further mixture with cement or other material. In many cases bottom ash goes through magnets or eddy current separators to remove ferrous (steel and iron) and other metals (copper, brass, aluminum, nickel, etc.) for further recycling. Then, bottom ash can be utilized or landfilled, depending on the legislation of the country and the need of the local industry. It can be generally assumed that bottom ash generation rate is roughly 10% of the waste’s original volume. Contrary to bottom ash, fly ash particles are carried from the furnace by the flue gases. These are mostly residues of incomplete combustion of waste and therefore can be harmful if released to the atmosphere. Compared to coal combustion, where fly ash particle composition is rather limited due to coal’s nature, waste residues may contain heavy metals of antimony (Sb), arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni), thallium (Tl), and vanadium (V) [1]. This variety is due to inorganic nature of waste as a composition of many artificial products. However, fly ash particles can be removed with the same technology used in coal-fired power plants. These are (see Chapter 8 for detailed description): • Mechanical systems, both dry (gravitational, cyclone, and bag fabric filters) and wet; and • Electrostatic precipitators (ESP) that use static charge to catch particles from the flue gas flow. When captured, fly ash is collected and treated through multiple processes like chemical stabilization, solidification, or thermal treatment and stored in a fly ash silo. Due to high content of various chemical elements resulting from its nature, fly ash from waste combustion is difficult and too • • •

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expensive to reuse or separate valuable materials, and in most cases it goes to landfills after treatment. Gaseous pollutants may contain HCl mainly from the combustion of polyvinyl chloride (PVC, the world’s third-most widely produced synthetic plastic polymer, after polyethylene and polypropylene), SO2 from combustion of sulfurous compounds, HF from combustion of fluorine compounds, and nitrogen oxides NOx [1]. These gases are usually sent through a flue gas scrubber to remove poisonous acidic material right before the flue gases go through the fly ash filters. The NOx emissions control is carried out in a conventional way by injecting ammonia or urea in the hot gas path and, exposed to high temperature, the products of these reactions are neutralized up to 99%. This process is implemented, for instance, within SCR or SNCR systems (for more details refer to description presented in Chapter 8). Other poisonous substances like heavy metals (mercury or cadmium) can be removed from the hot exhaust gases by injecting activated carbon into the flow.

18.3.2 Power island and balance of plant Similar to any fossil-fired steam power plant, the power island of a conventional WTE power plant accommodates a steam turbine with its generator and related essential systems. Generally, WTE plants have lower capacity than large scale utilities operating on coal or natural gas, and steam turbines installed at WTE plants are therefore much smaller, with their sizes ranging from a few megawatts up to large units of several hundred megawatts with proven performance for such application. An efficient and reliable steam turbine is the key to harnessing energy from waste. They are usually of condensing type, i.e., employ a condenser to transfer energy from the cold steam and turn it into water. Depending on the capacity, smaller machines are usually single casing with one or two pressures and maybe with a few steam extractions for various plant and external needs. Electrical generators are rather standard for 50 or 60 Hz, depending on the grid. Major systems, supplied with the units, always include a lubricating and hydraulic oil system, various steam shut off and control valves, electrical subsystems for safe and reliable operation and control of the generator, etc.

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Like any thermal power plant, a WTE facility always has several essential balance of plant systems and equipment required for its overall operation. Mechanical BOP systems of a typical WTE facility include, among others: • A heat rejection system usually represented by an air cooled condenser or a mechanical draft small modular cooling tower to condense steam back to water; • A water treatment and recycling system to provide demineralized water for the water–steam cycle; • Grid water heaters (if any) to transfer heat of the steam to water which is fed to the grid; • Various pumps and valves to run the cycles and control the fluids. There are multiple other necessary systems which are present at any power plant facility, for instance, HVAC, compressed air, drainage, fire protection, etc. One can refer to Chapter 9 for details. Their design (and sometimes presence) depends on the configuration and size of a plant. Electrical balance of plant systems of a WTE facility incorporate, among others: • A power transfer system to supply electricity from the generator to the grid, which includes the main transformer, generator bus bars, control and protection system; • LV, MV, and HV electrical networks to feed internal systems; • Main and auxiliary transformers; • Generator circuit breakers to protect equipment and grid from anomalies; • A substation, usually based on a gas insulated switchgear (GIS), due to a smaller footprint and often location in the rural areas where space is limited and does not allow having an open switchyard; • Overall plant control, monitoring and protection systems for safe and reliable operation.

18.4. Project evaluation and economics The successful implementation of a WTE project rests primarily upon the following essential building blocks or key elements [11]: • A reason or need for the project because of a critical community solid waste disposal problem or crisis; • An implementing government agency or private project developer with political commitment willing and able to undertake the project;

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An adequate supply of solid waste for the project or means to assure waste stream control or attract sufficient quantities from other communities; • Markets for the recovered energy and recovered materials; • A project site that is environmentally, technically, socially, and politically acceptable. Even though WTE power plants are similar to conventional solid fuel fired steam power plants, the underlying investment decision-making process differs significantly. While a conventional power plant is based on the electricity and heat sales, a WTE plant is first aimed at burning waste and delivering heat and electricity, which are considered as an economically positive add-on to the waste-conquering problem and, of course, contribute to the investment calculations together with multiple incentives. Moreover, waste as fuel may have zero or even negative cost which would impact the investment analysis. So far, a WTE plant is one of the options to answer the growing urbanization population, consumption and consequently waste production, but usually for a developed country. Still, the potential of WTE implementation strongly depends on the specific economic, social, and political conditions of the country where the implementation strategy must be carried out [2]. WTE plants have capacities from several dozens to hundreds of megawatts (electrical capacity) with a bit low efficiency around 30% due to lower heating values of waste, however, with heat production it may increase to more than 85% [12]. WTE projects are capital intensive and require planning that frequently extends over a two to five year period [11]. The investor has to identify the right location within the community as the latter is the “producer” of solid waste. The community must be able to supply or divert enough solid waste to the proposed facility since solid waste serves as the feedstock for plant operations and energy production. The community must be able to guarantee both the quantity and the quality of its solid waste. The level of the wastes to be guaranteed will determine the ultimate size of the facility [11]. Normally, major reports include WTE into biomass data so that it may be difficult to allocate required data precisely. The capital costs of a WTE plant are comparable to biomass plants that operate of biogas or bio-liquid, and the data suggest the range of 450–2500 USD per kW of installed capacity [13] as shown in Table 18.1. WTE plant investment costs tend to be at the upper limit of the general biomass costs. This is mostly due to the fact that the primary objective •

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Table 18.1 CAPEX and OPEX components for a waste-to-energy plant. Technology CAPEXa OPEXa

WTE

LCOEa

$/kW

Fixed, $/kW-year

Variable, $/MWh

USD/kWh

up to 8000

100

5–15

0.04–0.08

Source: Based on [13,14], EIC and IIR databases and calculations. a WTE plant investment costs tend to be at the upper limit of the general biomass costs.

of these projects is not power generation, but waste disposal. Capital costs are often higher, as expensive technologies are used to ensure that local pollutant emissions are reduced to acceptable levels [13]. Importantly, WTE projects are often planned, carried out and financed by the community, which clearly puts them aside of the purely commercial thermal or renewable power plants. This is usually due to the fact that strong political and community support is vital for this type of project to become real. Moreover, the waste treatment policies can considerably change the feasibility study of a WTE plant project. There are multiple policy measures that can be granted to the WTE project [2]: • Various subsidies of a local government like renewable certificates, feedin tariffs or heat incentives; • Zero-waste policies that require decreasing landfills through any of the options, e.g., a WTE plant, while the landfill fees and taxes are kept high; • Carbon taxes; • Renewable targets that also cover WTE plants. It should be noted that WTE does not mean that it can easily lead to a zero waste future and other options can be omitted. WTE plants are an alternative which allows reprocessing bulk quantities of municipal solid or industrial waste and relieving landfills. But the WTE option has to be executed only together or after all other options are exhausted: reduction, reuse, recycling or recover.

18.5. Sustainability challenges and environmental issues In many scenarios and discussions regarding the global climate change or sustainability topics, the WTE technology is either neglected or only mentioned within the biomass energy technologies as having a very lim-

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ited share. However, WTE is one of the solutions to conquer global waste production (at least in the short to medium run) and one of the means to recover materials and energy from waste by thermal or biological treatment. Many countries use waste-to-energy plants to capture the energy stored in municipal solid waste. According to the EIA, the use of WTE plants in some countries is relatively high with about 70% in Japan, more than 50% in Norway, and more than 45% in Switzerland. This is partly due to the fact that these countries have very limited open space for landfills, and the energy demand is constantly growing. While reuse, recycling and recovery are the preferred solutions for waste treatment, thermal WTE technology has several important environmental advantages over direct landfilling of the MSW, which can be summarized as follows [1]: • Greenhouse gas emissions from WTE incineration contribute less to climate change than those from landfills. WTE mostly emit carbon as CO2 due to the combustion process (most of other pollutants are filtered out), whereas in landfills both methane and CO2 are generated and partly escape into the atmosphere. The global warming potential of the CO2 is 21 times lower than that of methane, explaining its lower contribution to climate change. Contrary to landfilling with some gas capture, which emits up to 2750 g/kWh of CO2 equivalent emissions, incineration has much lower levels of 1600 g/kWh [2]. • It saves land, as significantly more land is needed for a landfill than for a WTE plant. • It allows recycling of ferrous and nonferrous metals and granulates. • From biomass in the MSW, renewable energy that can replace fossil fuels is produced. It is generally assumed that 50% of the energy produced is biogenic due to biodegradable fraction and thus does not contribute to climate change. Despite several advantages, waste incineration is still treated with caution by the public, as some concerns still exist. Like any other technology, WTE does possess some disadvantages, especially in the sense of emissions and the impact to the environment: • Even though modern filtration and cleaning systems allow capturing most of the harmful substances, there some air emissions associated with waste combustion: metals (mercury, lead, and cadmium), organics (dioxins and furans), acid gases (SO2 , HCl), nitrogen oxides, and carbon monoxides [2].

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The ash from the furnace can still contain toxic chemicals and is usually disposed in landfills, which can potentially contaminate soil and ground water. • Like any other thermal technology, a conventional WTE plant consumes a lot of water for the water and steam cycle, cleaning, cooling, scrubbing, etc. The discharged water is contaminated with chloride and some soluble heavy metals (the exact composition depends on the nature of MSW). Instead of discharging this water, closed cleaning systems have to be adopted at site. Contrary to fossil combustion, where the fuel chemical composition is way simpler compared to municipal soil waste and therefore the emissions from fossils are more predictable and hence easier to treat, waste incineration emissions treatment is a challenge, which can be solved at the modern level of technology. The problem here is not technological but rather concerns the attitude of the operator: the cleaning systems will operate properly only if they are properly maintained, which is costly. And these costs are not negligible. This is a significant adoption barrier faced by the industry in the developing countries, along with the lack of trained personnel to successfully handle such a complex and daunting process [2]. •

References [1] Kalogirou E. Waste-to-energy technologies and global applications. CRC Press. ISBN 9781351977913, 2017. [2] World Energy Council. World energy resources 2016. London: World Energy Council. ISBN 9780946121588, 2016. [3] Hoornweg D, Bhada-Tata P. What a waste. A global review of solid waste management. Urban Development Series, vol. 15. World Bank; 2012. [4] Branchini L. Waste-to-energy: advanced cycles and new design concepts for efficient power plants. Springer International Publishing. ISBN 9783319136080, 2015. [5] Bogner J, Abdelrafie A, Diaz C, Faaij A, Gao Q, Hashimoto S, et al. Waste management, vol. Climate change 2007: mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom, New York, NY, USA: Cambridge University Press; 2007. [6] World Bank. Solid waste management. URL: http://www.worldbank.org/en/topic/ urbandevelopment/brief/solid-waste-management. [7] Waste atlas. URL: http://www.atlas.d-waste.com/. [8] United Nations Environment Programme (UNEP). Guidelines for national waste management strategies. Moving from challenges to opportunities. UNEP. ISBN 9789280733334, 2013. [9] Directive 2008/98/ec of the European Parliament and of the Council of 19 November 2008 on waste and repealing certain directives. Official Journal of the European Union 2008.

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[10] Van Ewijk S, Stegemann J. Limitations of the waste hierarchy for achieving absolute reductions in material throughput. Journal of Cleaner Production 2016;132:122–8. [11] Rogoff M, Screve F. Waste-to-energy: technologies and project implementation. Elsevier Science. ISBN 9781437778724, 2011. [12] Renewable energy sources and climate change mitigation. Special Report of the Intergovernmental Panel on Climate Change. Cambridge, UK, New York, NY: Cambridge University Press; 2011. [13] IRENA (International Renewable Energy Agency). Power generation costs in 2017. Abu Dhabi: IRENA; 2017. [14] Black & Veatch. Cost and performance data for power generation technologies; 2012.