Plasma gasification of municipal solid waste for waste-to-value processing
Introduction
Municipal solid waste (MSW) is generated in large quantities worldwide and its generation is expected to increase due to swift urbanisation and the resulting change in lifestyle. Globally, approximately 1.5 billion metric tonnes per year of MSW was produced in recent years and its generation is expected to increase to approximately 2.5 billion metric tonnes per year by 2025 [1]. The generation of MSW has also increased in New Zealand during recent years and has reached approximately 3.2 million tonnes per year, Fig. 1(a). Fig. 1 is the plotted MSW generation data, presented in the recent literature such as [1], and [2].
Proper waste management of such a huge amount of MSW is required to protect the environment from land and air pollution, and for the health and safety of humans and animals. The majority of MSW is landfilled in most countries. For example, in New Zealand, a majority of MSW that contains fractions of organic and inorganic materials (Fig. 1(b)) is sent to landfills [2]. Landfilling and associated recycling and collection strategies of MSW have been optimised over time; however, rapid accumulation of MSW, increasing cost of landfills, and landfill problems (e.g. toxins and leachate) are stressing city governments to find new sustainable techniques to cost-effectively dispose of MSW. Furthermore, landfilling can cause significant loss to the circular economy. For the circular economy, waste needs to be transformed into value (waste-to-resource (e.g. energy)) through effective waste treatment. Therefore, MSW needs to be ‘processed’ to value (i.e. waste-to-energy) for the circular economy, managing increasing demand of energy, and offset energy costs of waste-to-value processes.
Various techniques for MSW disposal and processing are available in the literature as an alternate of landfilling [3]. Waste can be disposed and processed using biological (e.g. composting, aerobic and anaerobic digestion) [4], hydrothermal (e.g. wet oxidation, thermal hydrolysis, liquefaction, and carbonisation) [5], and thermochemical (e.g. gasification, pyrolysis, and incineration) techniques [6,7]. These techniques can be used for waste disposal and waste-to-value processing [8]. Each technique has its pros and cons. Biological techniques convert MSW to biogas or compost, and are environmentally ‘safe’, but are expensive and time-consuming because they require a large area, are inefficient for hazardous waste, and biological degradation is a slow process [9,10]. Hydrothermal techniques reduce MSW volume, extract valuables from waste, and are relatively faster and environmentally ‘safe’, but have higher operational costs due to associated energy costs and are not operationally ‘safe’ as they have associated safety issues due to the required extreme conditions of temperature and pressure [3]. Thermochemical techniques convert MSW to charcoal, oil, syngas, or heat, and are also faster and environmentally ‘safe’, but have high operational costs [11].
Thermochemical techniques have been used for waste disposal and waste-to-value processing, as confirmed by several studies on incineration [[12], [13], [14]], steam gasification [8,[15], [16], [17]], and pyrolysis [[18], [19], [20], [21]]. However, limited studies on plasma gasification (an emerging thermochemical technique) have been conducted. This observation may be due to various challenges associated with plasma gasification such as its being a relatively new technology, requiring high capital & operational costs, is a highly energy-intensive process, has only a moderate technology & community readiness level, the requirement of proper waste sorting, the limited technology commercialisation success, and currently limited process understanding. These challenges are discussed in detail in Section 3. Plasma gasification has mainly been used for treating hazardous waste, and its use for waste-to-value processing is relatively new.
Plasma gasification can be a suitable technique for waste disposal and waste-to-value processing because it can extract recyclable commodities from landfill waste and can convert carbon-based waste materials into syngas and fuels. In other words, plasma gasification can help to achieve zero-waste accumulation, produce renewable fuels, and protect the environment.
This article reviews the current status of waste-to-value (municipal solid waste-to-syngas and other valuable products) processing using plasma gasification for the circular economy. Key findings and knowledge gaps were identified in ‘successful’ industrial application of plasma gasification for waste-to-value processing. After discussing challenges associated with the technology, a possible roadmap for ‘successful’ industrial application of plasma gasification was suggested in this study. Previously, various review articles on plasma gasification have been published such as Gomez et al. (2009) [22], Morrin et al. (2012) [23], Fabry et al. (2013) [24], Sanlisoy et al. (2017) [25], and Changming et al. (2018) [26]. However, it was difficult to find a study in the literature identifying challenges associated with plasma gasification for its ‘successful’ industrial application (novelty of the present study), especially for waste-to-value processing.
Section snippets
Introduction to plasma gasification
Plasma gasification has existed for many years since NASA advanced the process in the 1970s [27]. Plasma gasification is a thermal process in which waste is exposed to extreme thermal conditions (approximately 2000–14,000 °C) of plasma. Fig. 2 shows a schematic of plasma gasification of MSW. Plasma is the fourth state of matter, obtained by breaking atoms and molecules down to constituent ions and electrons after electrifying a gas [28]. Zhang et al. (2012) [6] showed a typical schematic of a
Key findings, knowledge gaps, and challenges associated with plasma gasification
After reviewing recent literature, it is evident that many studies claim promising capabilities of plasma gasification to convert MSW into value. There are, however, various challenges associated with the plasma gasification of MSW that need to be addressed and dealt with before successful industrial application can be achieved. The main challenges associated with plasma gasification for waste-to-value processing are displayed in Fig. 4.
Plasma gasification is a relatively expensive technology
Suggested road map for coping with plasma gasification challenges
Plasma gasification for waste-to-value processing for the circular economy has several challenges to overcome, discussed in Section 3, and can take many years to go from discovery to successful commercial use.
Conclusions
Plasma gasification can be a viable technology for waste-to-value processing for the circular economy; however, there are various challenges associated with plasma gasification which require attention for the successful, future commercialisation of plasma gasification. Reduction of initial capital costs of plasma gasification does not seem realistic; however, its operational costs can be targeted for reduction by generating revenue from synthesis gas and fuels produced from the process. Energy
Acknowledgement
The authors gratefully acknowledge Rotorua Lakes Council (RLC) New Zealand, Porirua City Council New Zealand, the University of Auckland New Zealand, and the American University of the Middle East Kuwait for their support.
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