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| Fig. 1: Data from Taiwanese incineration plants averaged electricity generation per tonne from 2000 - 2009 [4] (Image Source: R. Bendekgey) |
There are multiple ways to utilize waste as energy, each with potential benefits and drawbacks. A conventional method is to burn municipal waste to run a steam cycle power plant. This has the benefit of reducing the amount of solid waste taking up space in landfills, but waste proves to be a dirty fuel, releasing heavy metals such as Lead, Mercury and Cadmium, as well as trace organic compounds including polychlorinated dioxins and furans. [1] Another way to convert waste to energy is to anaerobically digest organic matter, which produces methane, or biogas. This often happens in landfills, as food is buried without access to oxygen, but the methane produced is not utilized and escapes into the atmosphere, acting as a very potent greenhouse gas. This methane can be extracted from landfills and utilized instead of allowing it to be discharged into the atmosphere. Another option is to isolate this organic waste and intentionally anaerobically digest it to create and extract methane. This therefore only works for organic waste such as food waste or human waste at water treatment plants.
Anaerobic digestion of organic waste produces significantly more methane in a shorter timeframe than landfill decomposition, though energy output depends heavily on the waste's composition. Anaerobically digesting organic waste has been estimated to produce two to four times more methane per tonne of municipal solid waste in just three weeks than the amount of methane produced by a tonne of waste in a landfill over six to seven years. [2] Estimates of 150 kg of methane produced per tonne of anaerobically digested methane suggest that the amount of methane produced by 1 tonne of municipal solid waste in a landfill is from 38-75 kg. [2] However, energy output can vary based on type of waste; a study in San Francisco in 2007 showing 0.000435 m3 of methane produced through aerobic digestion per 1 g of volatile solids from food waste from restaurants contrasts with a study in Beijing in 2015 showing 0.392 m3 of methane produced through aerobic digestion per 1 g of volatile solids from food waste and straw. [2]
Incineration generates more energy per tonne of waste compared to digestion to biogas due to the fraction of the municipal solid waste that can be utilized. For energy content, an Irish Study from 2004 found that incineration of one tonne of municipal solid waste can export 564 kWh, while biogas utilization in combination heat and power plant from 1 tonne of municipal solid waste results in exports 151 kWh of electricity. [3] This takes into account the fraction of organic matter in 1 tonne of municipal solid waste. This data agrees with yearly data from Taiwanese waste incineration plants, which over the course of 9 years had a high energy content of 508 kWh/tonne, a low of 419 kWh/tonne, and an average of 475 kWh/tonne as seen in Fig. 1. [4]
Consideration of energy output and environmental impact are necessary for planning and implementation of waste to energy systems, including minimizing landfill size, emissions, or maximizing energy. Incineration therefore produces the most energy per tonne, as all of the waste can be turned into energy with correspondingly less mass in landfills. However, this method creates more emissions than biogas combined cycle utilization, with 124 kg CO2 equivalent emissions, while the digested biogas results in -1132 kg CO2 equivalent emissions, in comparison to letting landfill gas escape into the atmosphere. [3] In addition to more carbon emissions, incineration releases aforementioned toxic pollutants such as heavy metals and dioxins from burnings plastics. Isolating organic matter and digesting into biogas creates far more biogas in a far shorter amount of time, but the landfill will be releasing gas regardless, and collection and utilization will lower emissions and provide energy.
© Rose Bendekgey. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. A. Ruth, "Energy from Municipal Solid Waste: A Comparison with Coal Combustion Technology," Prog. Energy Combust. Sci. 24, 545 (1998).
[2] A. Kumar and S. R. Samadder, "A Review on Technological Options of Waste to Energy for Effective Management of Municipal Solid Waste," Waste Manag. 69, 407 (2017).
[3] J. D. Murphy and E. McKeogh, "Technical, Economic and Environmental Analysis of Energy Production from Municipal Solid Waste," Renew. Energy 29, 1043 (2004).
[4] W.-T. Tsai and K.-C. Kuo, "An Analysis of Power Generation from Municipal Solid Waste (MSW) Incineration Plants in Taiwan," Energy 35, 4824 (2010).