Identification of effective factors to select energy recovery technologies from municipal solid waste using multi-criteria decision making (MCDM): A review of thermochemical technologies

Abstract

Thermochemical technologies used to burn municipal solid waste are one of the relatively new and appropriate methods for waste management. By burning waste, heat and gases have arisen which can be used as the energy source to generate electricity. Due to the high cost, pollution emissions and diversity of such power plants, comprehensive studies are required. Therefore, the purpose of this study is to investigate the thermal methods in the field of waste management and to optimum for the most suitable methods. The selected thermochemical technologies include incineration, gasification, plasma, and pyrolysis. Data gathering was done through a questionnaire completed by experts. The decision-making models based on the Analytical Hierarchical Process (AHP) and the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) methods are proposed to select the best thermochemical technology by the technical, economic, and environmental criteria. Ultimately, the environmental criteria (0.355), economic criteria (0.338) and, technical criteria (0.307) were introduced as the most effective criterion in choosing thermal methods respectively. The results of both AHP and TOPSIS models showed that the plasma method is the optimal thermochemical technology in terms of three existing criteria. Incinerator, gasification, and pyrolysis systems were ranked next.

Introduction

Nowadays, there are various methods for processing and disposal of Municipal Solid Waste (MSW). One of these methods is burning wastes that can eliminate toxins and hazardous substances from wastes by combustion and oxidation. Thermochemical systems generate energy by heating MSW. This energy can be in heat or fuel forms. The most important thermochemical technologies considered in this study are direct combustion, pyrolysis, gasification, and plasma.

Indirect combustion methods, MSW is burned in the combustion chamber directly. The most important technology in this group is incinerators. Incineration is a specific process for burning combustible materials and converting into gas and ash (residual material) [1]. The main components of the incinerator system are the feeding system, exhaust gas system, and residue disposal system. Most modern incinerators use the continuous feeding systems, Moving grates with the primary initial combustion chamber from heat-resistant materials. Typical Incinerators types include: Mass burn, Refuse Derived Fuel (RDF) and industrial incinerator [2].

Pyrolysis is a thermal process, performed by heat at the absence of oxygen. It converts carbonaceous materials into gas fuels that are an appropriate alternative for natural gas. This process can be fed continuously and non-continuously as the incineration process and produce coal, bio-oil, and gas [3]. This process is carried out at a temperature above 800 °C and generally in a Fluidized Bed Reactor (FBR). Cellulose molecules in MSW are separated from each other instead of burning due to a lack of oxygen. The separated molecules are mainly methane, carbon dioxide, hydrogen, carbon monoxide, and water [4].

The gasification process is a modified pyrolysis process with a small amount of oxygen. Oxidation provides the necessary heat to maintain the process in the gasification system [5]. This process occurs at temperatures above 700 °C, which low combustion of organic carbon materials such as combustible materials, organic materials, and plastics from entire waste, to produce combustible gas fuels such as carbon monoxide, hydrogen, and some hydrocarbons which are mainly methane [6].

Plasma is a process that converts organic matter into synthetic gas, electricity, and slag using plasma. A plasma torch powered by an electric arc is used to ionize the gas and catalyze organic matter into synthetic gas (Syngas composed of H2, CO) Inorganic materials are converted to solid waste (slag) [7].

Rahman et al. chose an appropriate waste to energy conversion technology for Dhaka city. Waste disposal is an important issue that leads to pollution of soil, water, and air. Also scarcity of land and its rising prices, especially in Dhaka, capital of Bangladesh has created new problems for the development of landfill sites. Therefore, the AHP method was used to select the most suitable method for conversion of household wastes into energy in the Mirpur-Dhaka campus. Three options including anaerobic digestion, pyrolysis, and Plasma Gasification (PG), and nine criteria were selected for comparison technical, environmental, and economic aspects. This study result showed that PG was the optimal technology for waste conversion into energy in the study area [8].

Li et al. studied the Application of thermal plasma technology for the treatment of solid wastes in China. By social and economic development, China has had a rapid increase in solid waste production and the increasing burden of managing this material. Nowadays, waste materials in China are mainly managed by combining landfill, waste incineration, and composting. Furthermore to different processing methods, the thermal plasma method is suggested. This method converts solid waste into Syngas and ash, which is used to generate heat and electricity. Current laboratory researches and industrial development of plasma method for processing solid wastes in China and discussed challenges [9].

Moya et al. examined the waste conversion technologies into energy from MSW of Quito in Ecuador. In this study, the potential of energy generation from MSW in Quito was estimated in biochemical and thermochemical processes using general models. Results showed that this city waste contains 63.3% of disintegrating waste and 30.7% of indivisible waste. The electricity generation potential from biochemical and thermochemical processes was 0.78 and 0.07 MW per tonne of MSW, respectively. Finally, it became clear that the MSW of Quito has a high potential to produce biogas and thermal energy [10].

Radwan et al. explored ways and opportunities for MSW mainly on food, paper, and plastic waste. This study has proposed an inverse method to reduce and minimize municipal waste. This approach can reduce separation and transportation costs. The study also shows that waste incineration is a safe method for waste management and that the ash from this process can be used in the manufacture of concrete waste management in Saudi Arabia [11].

Shakorfow conducted a study on biomass incineration, combustion, pyrolysis, and gasification methods. This study discussed biomass and gasification of biomass in connection with its important characteristics. To generate heat and electricity with the least damage to the environment, all methods of incineration, pyrolysis, combustion, and gasification of biomass were tested. This study aimed to discuss the process with emphasis on gasification, which is an effective and economical process for hydrogen production. It was found that biomass is a good option for the gasification process, although not used sufficiently. The gasification process is more practical and economical compared with incineration, pyrolysis, and combustion. Hydrogen production and environmental protection are also its primary goals [12].

According to the literature review there is no model and technique to determine the best thermochemical technology, considering multiple factors affecting the choice of this type of waste management system. On the other hand, In terms of managing these systems, the economic factor is the only factor influencing the choice. However the decision for making a suitable choice is a multi-dimensional action, because several other important factors need to be considered to choose the best options. Therefore, a tool for managing decision-making and policy-making seems necessary. In this study, two common methods in the multi-criteria decision making (MCDM) model, AHP, and The Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS), were used. The purpose of this study is to determine effective criteria and ranking thermochemical systems in waste management for producing energy from MSW.