|
|
| Fig. 1: Landfill MSW Components. [5] (Source: Wikimedia Commons) |
As the world's human population continues to increase, waste production and energy demand continue to increase too. Finding ways to meet these energy demands, and to handle said waste becomes more and more of a pressing issue. More often than not, these issues are thought of and combatted separately, but proponents suggest plasma gasification can help handle the waste while producing energy thereby impacting the two with one process or at least impacting the waste without taking much energy compared. This report aims to analyze the benefits and drawbacks of plasma gasification and to understand how it may play a part in the both the fight to minimize landfill waste and the energy crisis.
Plasma gasification is a special type of gasification that uses plasma as the heating and reacting mechanism. Generally, gasification works by heating up carbonaceous waste (wastes with carbon such as paper, plastics, word, etc.) to upwards of 700°C in a chamber or series of chambers. At these temperatures, the molecular bonds start breaking down. The products of this molecular breakdown then react with air and steam which are introduced into the chamber in a very controlled manner so as to control the products and to prevent combustion. This results in the generation and release of certain fuels primarily carbon monoxide and hydrogen gas which may be used in a generator or other chemical process for getting electricity. [1]
In plasma gasification, plasma torches are used to heat the waste to the necessary temperatures. These torches can have temperatures of 6000°C to 10000°C which leads to full molecular bond dissociation in contrast to partial dissociation for lower temperature gasification methods. [2] This more complete breakdown suggests more waste can be broken down to the atomic level which allows for the more complete extraction of fuel from the waste. This fuel a mixture of carbon monoxide, hydrogen gas, and hydrocarbons is generally referred to a syngas (a portmanteau of synthetic gas). The solid that is left over from this process, accounting for about 10%-20% of the initial waste, is a glass-like mixture of components often referred to as slag or plasmastone and has commercial uses in construction with potential for use in oil clean up and agriculture. [3]
It should be noted that the high temperatures found in plasma gasification also serve to break down toxic molecules to their atomic constituents thereby rendering the byproducts safer. As such, even more waste particularly toxic waste can be processed in this manner compared to conventional gasification processes at lower temperatures. [4]
To understand the potential for plasma gasification, the status quo for waste must be analyzed. In 2018, 292.4 million tons of municipal solid waste (MSW) was generated in the United States and that number is increasing. For context, municipal solid waste consists of waste collected at the municipal waste disposal site and includes most residential, industrial, institutional, and commercial waste. Of this waste, about 50% is sent to landfills, another 32.1% gets recycled or composted, and 11.8% gets combusted for energy. This does not include construction and demolition which generated for 600.3 millions tons of waste of which another 143.8 million tons of waste went to the landfill. In total, about 290 million tons of waste got sent to landfills in the United States in 2018 alone. [5] Considering the world as a whole, about 2 billion tons of waste was generated according to the world bank in 2018 of which, about 30% was incinerated, composted, or recycled. The rest was sent to landfill or dumped in the open. [6]
This creates a space/volume and climate change problem. In 2016, it was found that landfills in the United States are responsible for about 17% of all methane one of the most potent greenhouses gases released in the United States. [7] Some of this methane, which is about half of what is commonly referred to as landfill gas (the other half being CO2), is able to be captured and used as fuel for generating electricity but most goes unused. This is particularly true in developing countries where waste management can be less structured. Furthermore, states are beginning to look to minimize the amount of land used for landfill as land used for landfill comes into competition with agricultural, residential, and industrial needs. To that end, anything that can reduce the amount of waste generated and reuse the waste already buried in landfills is being considered. [8]
As mentioned, plasma gasification may be one such method of addressing this waste. The primary target for such gasification processes are carbonaceous wastes although due to the nature of plasma gasification, it can certainly handle more than just these. Exact numbers on what percentage of landfill waste this would be are challenging to find but of the landfilled MSW in 2018, about 20% of the landfilled waste would not be expected to be good candidates for gasification. This would be the metals, glass, and other shown in Fig. 1. [5] This suggests that up to 80% of the landfilled MSW may be good for gasification which would result in a significant reduction in the amount of landfilled MSW. Getting this usable waste from the landfill pipelines would require sorting but could use similar sorting efforts to current recycling and composting efforts.
The numbers for energy use, generation, and economics of scale are perhaps even more difficult to find but what can be found is a bit less clear cut. An independent consultancy report by Scientific Certification Systems (SCS) in 2010 compared the lifecycle greenhouse gas emissions of a plasma gasification process with the emissions from incineration facilities, landfills with energy/methane capture facilities, and the natural gas combustion process. The report found the plasma gasification process generated the least greenhouse gas emissions when producing the same amount of energy with about 31 million tons less of CO2 equivalent/MWh when compared to landfills with energy recovery and about 50,000 tons of CO2 equivalent/MWh less when compared to natural gas combustion. [9] This suggests that it is significantly better for the environment than trying to capture energy from landfill gas.
On the other hand, a report found that for a plasma gasification plant capable gasifying 10 tons per day, the operation costs are high enough compared to revenue that there is a negative total profit per ton of waste gasified. At about 100 tons per day, however, plasma gasification starts yielding a positive profit from the energy produced and sold, and becomes more attractive. [10] This suggests that, for smaller scale plasma gasification plants, subsidies, or some form of governmental support would likely be needed.
Based on the problem and the capabilities of plasma gasification, it seems to be a viable option for helping to reduce the amount of waste that gets sent to landfill. Although the economics are not attractive at smaller scales, the carbon footprint is minimal and is a solid alternative to the landfill carbon footprint and economics become more favorable at larger scales. That said, the application of plasma to gasification is fairly new and further analysis should be done to fully understand the implications of large-scale incorporation despite the fairly favorable current outlook.
© Adura Jibodu. 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] "Review of Technologies for Gasification of Biomass and Wastes," E4tech, June 2009.
[2] D. J. Roddy and C. Manson-Whitton, "Biomass Gasification and Pyrolysis," in Comprehensive Renewable Energy, ed. by A. Sayigh (Elsevier, 2012), Vol. 5, p. 133.
[3] J. Zhang, B. Liu and S. Zhang, "A Review of Glass Ceramic Foams Prepared from Solid Wastes: Processing, Heavy-Metal Solidification and Volatilization, Applications," Sci. Total Environ. 781, 146727 (2021).
[4] V. Zhovtyansky and V. Valinčius, "Efficiency of Plasma Gasification Technologies for Hazardous Waste Treatment," in Gasification for Low-Grade Feedstock, ed. by Y. Yun (IntechOpen, 2018).
[5] "Advancing Sustainable Materials Management: 2018 Fact Sheet," Environmental Protection Agency, December 2020.
[6] S. Kaza et al., "What a Waste 2.0," World Bank, 2018.
[7] "U.S. Methane Emissions Reduction Action Plan," White House Office of Domestic Climate Change Policy, November 2021.
[8] S. Leão, I. Bishop, and D. Evans, "Spatial-Temporal Model for Demand and Allocation of Waste Landfills in Growing Urban Regions," Comput. Environ. Urban Syst. 28, 353 (2004).
[9] N. Bîrsan, "Plasma Gasification The Waste-to-Energy Solution for the Future," Problemele Energeticii Regionale 3, 107 (2014).
[10] Y. Byun et al., "Thermal Plasma Gasification of Municipal Solid Waste (MSW)," in Gasification for Practical Applications, ed. by Y. Yun (IntechOpen, 2012).
.gif){kind=link}