Energy and materials recovery from post-recycling wastes: WTE

One of the most misunderstood technologies in some parts of the world and widely adopted technologies in others is the recovery of energy and materials by the controlled combustion of post-recycling wastes. This technology is commonly called waste-to-energy, or simply WTE. After all possible efforts for recycling or composting wastes, there remains a large post-recycling fraction that is either landfilled or used as fuel in WTE power plants that also recover metals and minerals. Several nations, e.g., Switzerland, Japan, Sweden, Belgium, Denmark, and Germany, have succeeded in phasing out landfilling by processing all theãir post-recycling municipal solid wastes (MSW) in WTE power plants. This paper reviews the evolution and importance of WTE in the twenty-first century, with special focus on the world’s largest economies: the EU, US, and China.


Introduction
The discovery of fire and the invention of the wheel are crucial events in the story of humanity. The use of fire to heat and feed humans is a million years old. At about 2300 BC, controlled fire brought about the Bronze Age and, a thousand years later, the Iron Age. The Industrial Revolution of the eighteenth century was made possible by the steam engine. At present, most of the electricity, cement, and metals used by humanity are produced by controlled combustion.
Over the years, the residues of human activities were landfilled, mostly in waste dumps and, as of the twentieth century, in regulated landfills and more recently in sanitary landfills equipped to treat the liquid effluent and capture some of the landfill biogas. Controlled combustion of municipal solid wastes (MSW) started in Nottingham, England in 1874, in New York City, in 1885, and in Hamburg, after the outbreak of cholera, in 1896. Denmark was also a pioneer in the use of waste to energy (WTE), and presently, one third of the district heating is provided by the steam of WTE plants. The history of "incineration" in Denmark [1] is representative of other northern European countries.
The invention of an inclined moving grate to convey wastes through the furnace (1927) was by Josef Martin, founder of Martin GmbH in Munich [2]. The objective of the early furnaces was to incinerate wastes and reduce them to ash. Therefore, they were called "incinerators". Regrettably, this term is still used in the European Union and elsewhere, although modern WTE power plants are as different from the incinerators of the past, as today's automobiles differ from the 1910 Model T Ford.

WTE emissions to the atmosphere
The technology of combustion with energy recovery advanced very much in the second half of the twentieth century, but one major environmental problem was not recognized until the ninety eighties: the emission of volatile metals and toxic dioxins. Of course, inadequate understanding of emissions to the atmosphere is a common problem for all high temperatures industrial processes. For example, in the sixties the author developed the Noranda process [3] for smelting and converting copper which, since then, has avoided the emission of millions of tons of sulfur to the atmosphere. In the case of WTE plants, the emission problem was amplified by the fact that urban wastes contain heavy metals and chlorine compounds that result in the formation of dioxins and furans. The 1970 Clean Air Act in the US [4], and similar measures in other countries, led to the solution of this major emission problem of WTE plants: their air pollution control system was modified to include activated carbon injection and baghouse filters, so that volatile metal and dioxin molecules attach to the carbon particles that are then captured on fabric filters, in the form of "fly ash". As a result, dioxin emissions of WTE plants were reduced by a factor of 1000 and volatile metals by a factor of 100. For example, a 2012 study by Dwyer and Themelis [5] showed that the annual dioxin emissions of all US WTE plants, combusting about 26 million tons of urban wastes, were only 3.4 g. There have been other studies by Columbia University and others of the WTE industry of France, Korea, and China with similar results. Figure 1 shows the generally accepted order of preference of various methods for managing wastes. The first, and obvious, way is to reduce consumption and the generation of waste. This is easier said than done because consumption depends on economic level and culture. For example, US citizens, on average, produce about 0.8 tons of municipal wastes per capita, while the German average, at GDP per capita equal to 85% of the US, is only 0.6 tons per capita. The next two means of managing wastes, recycling and composting, can broadly be called "Recycling". The international experience is that after all possible recycling and composting efforts, there remains a post-recycling fraction, which is the subject of this paper. As shown in Fig. 1, there are two principal ways for managing "post-recycling" MSW: combustion with energy recovery (commonly called "waste to energy" or WTE) or various types of landfilling, ranging from the best, sanitary landfills, where some of the methane generated is captured and the leachate is treated, to the worst, waste dumps which at some locations are set to fire to create landfill space.

Comparison of US WTE and landfill toxic dioxin emissions
When WTE projects are proposed by municipal authorities, opponents claim that they will be a significant source of toxic emissions; as noted earlier, this was true before the implementation of the modern Air Pollution Control (APC) systems. The 2002 study "Landfill Fires" of TriData Corporation [6], sponsored by the US Federal Emergency Management Agency reported an average of 8400 unintended dump and landfill fires in a year. According to the US EPA "Inventory of dioxin emissions" [7], the average landfill fire burns 225,000 kg of wastes, and the activity level of toxic dioxin emissions is 700 nanograms per kilogram burned. On this basis, Dwyer and Themelis [5] estimated that the 2010 dioxin emissions from US landfill fires were 1750 g of toxic dioxins, i.e., about 500 times of the amount emitted by all US WTE plants (3.4 g). Toxic dioxins also exist in the leachate of sanitary landfills but are managed by the leachate treatment system; of course, this does not apply to ordinary landfills and waste dumps [8]. During landfill fires, volatile metals in landfilled MSW are also emitted in the absence of any emission control.

Practical limits to recycling
The hierarchy of waste management indicates that every possible effort should be made to achieve a high rate of recycling. However, this depends on three factors: 1. The municipality must provide public information and collection of marketable recyclables. 2. Citizens and businesses must separate recyclables at the source. 3. There must be markets and facilities for processing the collected recyclables (e.g., paper mills, metal smelters), and the cost of collection and processing must not be economically prohibitive relative to their economic value.
At a hypothetical efficiency of 70% for each of these three factors, the expected rate of recycling would be 34% (0.7×0.7×0.7). Figure 2 [9] shows that the recycling plus composting rate in the US leveled off after 2011, to about 32% in 2013. In the period of 2014 [8] to 2018 [9], recycling increased from 55.5 million metric tons to 62.7 million metric tons, and composting increased from 26.4 million metric tons to 32.7 million tons. The total "recycling" of 95.4 million tons was less than one third of the total US MSW in 2018.

The case of Milan, Italy
The city of Milan (Milano) in Italy is a good example of intensive recycling and composting. A study by the author showed that the city collects five separate streams: (1) paper, (2) glass, (3) metals plus some types of plastics, (4) compostable organics, and (5) all other (i.e., post-recycling) wastes that are combusted in WTE power plants.
The city provides each building with five large bins and each apartment in a building with a ten-liter plastic canister where citizens store, for a day or two, their food wastes in biodegradable bags they purchase at supermarkets [10]. Periodically, the city inspects the waste bins in buildings and levies heavy fines for non-compliance, e.g., for disposing paper in the wrong bin. To reduce transport costs, city collection trucks are equipped with different compartments. Table 1 shows that the very intensive waste management system of Milan results in a recycling plus composting rate of 40%. Milan has achieved the goal of "zero landfilling" because all of its post-recycling waste goes to WTE power plants.

The case of San Francisco
Now and then, there are reports of cities that are aiming to be "zero waste". The city of San Francisco in California has in place a very intensive recycling and composting program. Despite the success of this program, in 2018 San Francisco landfilled 429,000 short tons of post-recycling waste [11]. This corresponded to landfilling of 0.49 short tons of waste per capita, which is lower than the US average of 0.63 tons, but it is still a long way from "zero waste" or "zero landfilling".

Global generation and disposition of MSW
Two widely cited studies by the World Bank reported on the generation and disposition of global MSW in 2012 [12] and in 2018 [13]. One of the authors is Perinaz Bhada-Tate, whose Columbia thesis was on the potential for a WTE plant in Mumbai [14]. The 2018 World Bank report [13] estimated the global generation of MSW to be two billion tons and expected it to increase to 3.4 billion tons by 2050. Table 2 shows how the global MSW was disposed in 2018, using the percent distribution reported in the World Bank report [13].
We will now examine how the above World Bank numbers compare with other information. In 1999, the EU [15] directed member states to phase out landfilling. The landfilling rate in the EU decreased (Fig. 3, [16]) from 64% of the urban waste in 1995 (156 million tons) to 25.3% (62 million tons) in 2015. During the same period, the EU recycling plus composting rate increased to 46.3% (112 million tons), and the WTE capacity doubled to 28.4% (61 million tons). Figure 4 shows that some E.U. members are still highly dependent on landfilling. Globally, landfilling remains the dominant means of managing post-recycling wastes. However, as shown at the top of Fig. 4, several nations, e.g., Switzerland, Japan, Sweden, Belgium, Denmark, and Germany, have succeeded in phasing out landfilling by processing all their post-recycling MSW in WTE power plants.
In 2018, the US recycled 69 million short tons of MSW, composted 42.4 million, and combusted 34.6 million tons with energy recovery (US EPA, [9]). These numbers are supported by other non-government reports. However, the landfilled tonnage reported by US EPA (146 million short tons, [9]) is grossly understated, according to two studies [17,18] that reported landfilling of about 260 million tons annually. Also, the 2021 analysis by Themelis and Bourtsalas [19] of data provided by 1164 MSW landfills showed that 316 million metric tons were landfilled in 2018; most likely, some of this waste was inert material that should have been disposed in construction and demolition landfills. However, even using the 260 million estimate of Powell [18], the per capita landfilling of MSW is one of the highest in the world.
In 2020, Japan generated 41 million tons of urban wastes, of which 8.3 million were recycled and the rest combusted in WTE facilities [20]. To date, most recycling in China, India and other Asian and African countries is done by informal recyclers and is not officially recorded.

Global WTE capacity
A detailed compilation [21] of the world's WTE plants in 2013 showed a global capacity of 228 million tons of wastes. Since then, the Chinese WTE capacity has increased by nearly 100 million tons (Fig. 5; Themelis and Ma, [22]). Therefore, the 2021 global WTE capacity is estimated to be over 330 million tons. EcoProg has reported [23] a higher 2021 WTE capacity of 456 million tons. Both numbers are much higher than the 2018 World Bank estimate of 220 million tons; the World Bank report [12] also underestimated the 2012 global WTE capacity at 121 million tons; both World Bank reports [12,23] use the improper term "incineration" for WTE and do not mention the major advantages of WTE over landfilling: reduction of methane emissions, production of electricity and land conservation. At the estimated WTE capacity of 330 million tons and average production of 0.3 MWh/ton MSW, the annual production of

The WTE technologies
There are many thermal processes for recovering energy from post-recycling urban wastes; the total installed capacity of all but two technologies is less than half a million tons of MSW. Some of these processes are called "plasma" or "gasification", and all require external energy and wastes of higher heating value than post-recycling MSW (6-12 MJ/kg). There have been several unsuccessful attempts using typical MSW; the costliest was by Air Products and Chemicals at Teesside, UK, where a US$900 million plant consisting of two thermal plasma lines was built and later shut down [24]. The reasons for this failure were not technical but economic: much lower than expected production of electricity. Columbia University [25] studied the same thermal plasma process at its pilot stage in the US "Alter NRG process"; it was found to be technically viable, but the calculated production of electricity, per ton of US MSW, was 0.617 MWh/ton, i.e., only 15% higher than the current US WTE average (0.55 MWh/ton).
As stated earlier, globally, the dominant WTE technology is combustion of as-received MSW on an inclined moving grate (Fig. 6). This technology was developed by Martin GmbH of Germany, later also by Hitachi Zosen Inova [26] and, in this century, by China Everbright and other companies in China. The installed global capacity of the moving grate technology is estimated at over 250 million tons. Both the Martin and the HZI (VonRoll) technologies are described in detail in the book "Recovery of Materials and Energy Recovery from Urban Wastes, edited by Themelis and Bourtsalas [27].
The second large WTE technology is the combustion of shredded MSW in a circulating fluid bed [22]. It was first used in Austria and in this century developed in China, where its total capacity is estimated at about 30 million tons; this technology is also described in detail in the book "Recovery of Materials and Energy from Urban Wastes" [27]. The moving grate and the circulating fluid bed (CFB) technologies grew at the same rate in China until 2010 but, since then, there has been hyperbolic growth of the moving grate (Fig. 7, [22]).

Twenty-first century growth of WTE in China
The enormous growth of the WTE in China has been due to the Chinese government including WTE in the national plan for renewable energy and instituting the following policies:    Themelis and Ma [22] provided a list and processing capacities of all WTE companies in China. The building of hundreds of new WTE plants in China resulted in intense competition within the WTE industry and in assembly line fabrication [22] of moving grates and other equipment for WTE power plants. In contrast to the first WTE plants in China, the twenty-first century WTE facilities are visually appealing (Fig. 8) and are designed to meet the EU and US environmental standards.
An important "fallout" of the rapid growth of the Chinese WTE industry is that the capital cost per ton of capacity (CAPEX) in China decreased substantially. Since the repayment of CAPEX is the major cost item per ton of MSW processed (i.e., the "gate fee"), Chinese progress has made WTE technology cost-competitive with sanitary landfilling. Therefore, cities in the developing world may skip the sanitary landfill stage and move directly from landfilling, or waste dumping, to WTE power plants. This is already happening in India, Ethiopia, Serbia, Turkey, Vietnam, and other nations.

WTE and landfilling a) Electricity production of WTE facilities
The average electricity provided to the grid by the US WTE plants is reported [29] to be 0.55 MWh/metric ton MSW combusted, i.e., 16.5 million MWh. Since the 2021 production of electricity in the US was 4100 million MWh [30], the WTE contributed 0.4% of the national total; this amount could be increased ninefold if all of the post-recycling MSW of the US went to WTE plants instead of landfills, and much more if the turbine exhaust of WTE plants is used for district heating. The corresponding number for China is estimated to be 0.5% (35 million MWh) of the national total (7000 million MWh; [30]). In some countries, the exhaust steam from the WTE turbine is used for district or industrial heating or for water desalination. b) Greenhouse gas emissions According to the EPA Inventory of GHG emissions [31], 2504 million MWh were produced by fuel com- bustion, and 1495 million tons of carbon dioxide were emitted; therefore, 1.8 MWh were produced per ton of CO 2 emitted. In comparison, the US MSW contains about 30% carbon, of which one third is fossil-based. Since 0.55 MWh of electricity are provided to the grid per ton of MSW combusted, the WTE production of electricity is 1.5 MWh/per ton of CO 2 emitted. There are ongoing efforts for carbon capture and sequestration from the atmosphere, where the carbon dioxide concentration is only 0.05%. The stack gas of WTE plants contains 12%-15% CO 2 and could be a source of carbon, as proposed in several studies of this subject [32,33].
The Landfill Methane Outreach Program of the EPA [34] compiles the operating data of all methane-capturing landfills. The analysis by Themelis and Bourtsalas [19] of 2018 data for 396 LMOP operating landfills showed that they received 210 million short tons of wastes and that 5.06 million short tons of methane were captured, i.e., an average capture of 0.024-ton CH 4 /ton of US landfilling capacity. On the basis of the anaerobic reaction of biodegradables in US MSW, the average rate of methane generation was estimated [19] at 0.05 ton of CH 4 per ton of annual deposition of waste.
The total methane emissions from all 1164 operating landfills in the US was estimated [19] to be 11.9 million metric tons of CH 4 . At the CO 2 /CH 4 equivalence of 25 for the 100-year horizon (Intergovernmental Panel on Climate Change, [35]), this number corresponds to CO 2 -equivalent emissions of 270 million metric tons, i.e., 5.1% of the US energy-related carbon dioxide emissions. However, IPCC [35] has estimated that for a 20-year horizon, the CO 2 /CH 4 equivalence is 72 [35]. In view of the multi-billion climate disasters of recent years, this number is more appropriate in comparing the GHG emission effect of the landfilling and the WTE routes for managing post-recycling MSW.  (Table 1), the use of land by humans for landfilling is somewhere between 70 and 140 million square meters. For comparison, the surface area of the island of Manhattan in New York City is 58 million square meters.

d) Reduction of toxic emissions
As reported earlier in this paper, toxic emissions from unintended landfill fires are hundreds of times greater than those from WTE plants.

Conclusions
The technology of producing electricity and heat by the controlled combustion of post-recycling wastes, commonly called waste-to-energy or simply WTE, has made enormous progress in the last thirty years. It is now the only way of managing the post-recycling fraction of MSW by entire nations that have phased out the traditional route of landfilling. The WTE power plants also recover metals and minerals used in construction. Several nations, e.g., Switzerland, Japan, Sweden, Belgium, Denmark, and Germany, have succeeded in phasing out landfilling by processing all their post-recycling MSW in WTE power plants.
WTE has several environmental and economic advantages over landfilling, including the production of electricity, recovery of metals, reduction of greenhouse gases and toxic emissions, and conservation of land. In the first part of this century, China became the front runner in the use of WTE. The Chinese WTE capacity in 2020 was higher than that of the EU, US, and Japan combined. An important "fallout" of the massive growth of the Chinese WTE industry in the twenty-first century is that the capital investment per ton of capacity decreased substantially. Since capital repayment is the major cost item per ton of plant capacity, cities in the developing world may skip the sanitary landfill stage and move directly from landfilling, or waste dumping, to WTE power plants.
Most people know about recycling and its benefits but are not aware that over 330 million tons of the post-recycling waste goes to WTE power plants. However, the tonnage of post-recycling waste that is landfilled globally is still four times the amount combusted with energy recovery.
There are environmentalists who continue to oppose the processing of post-recycling MSW in WTE power plants, thus prolonging landfilling. Also, billions of dollars are spent on the exploration of habitat in other planets while, each year, we collectively transform a Manhattan-size piece of the Earth's surface to bury our urban wastes. Data availability References have been provided to most data used in this review paper. The remaining data have been obtained by earlier studies by the author. Any questions on these data should be directed to corresponding author so that he may provide additional information.

Declarations
Conflict of interest Nickolas J. Themelis is the Editorial Board member of Waste Disposal & Sustainable Energy, and is the Guest Editor of the special issue "Thermal processing of post-recycling urban wastes (WTE)".
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