Methodology for Selecting an Ideal Thermal Gasification Technique for Municipal Solid Waste Using Multi-Criteria Decision Analysis  
[PERSON]  
1 Department of Mechanical Engineering, King Saud University, Riyadh 11421, Saudi Arabia; [EMAIL_ADDRESS] (A.S.); [EMAIL_ADDRESS] (H.S.A.) [PERSON]  
2 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]  
2 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]  
1 Department of Mechanical Engineering, King Saud University, Riyadh 11421, Saudi Arabia; [EMAIL_ADDRESS] (A.S.); [EMAIL_ADDRESS] (H.S.A.) [PERSON]  
2 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]  
2 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]  
1 Department of Mechanical Engineering, King Saud University, Riyadh 11421, Saudi Arabia; [EMAIL_ADDRESS] (A.S.); [EMAIL_ADDRESS] (H.S.A.) [PERSON]  
2 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]  
2 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]  
1 Department of Industrial Engineering, King Saud University, Riyadh 11421, Saudi Arabia; 2  
[PERSON]
environmental hazards, excessive energy costs resulting from conventional sources, and increasing demand. Presently, about 70% of the waste generated in urban regions all over the world is landfilled, about 19% is recycled, and about 11% is used to generate energy [1]. Conversion of waste to energy helps to simultaneously solve the requirement to discard waste as well as the need for energy [2]. Incineration by complete burning of waste is an effective method for reducing the volume of MSW and for producing energy for different purposes. Approximately 60% of the weight and 90% of the volume is reduced by this method [3]. One of the disadvantages of incineration is the gaseous emissions which include global warming-causing gases like carbon dioxide and methane and other harmful emissions like nitrous oxide, sulfur dioxide, and other nitrogen oxides [4]. Combustion of MSW using a fluidized bed has the potential to provide cooling and electricity under specific geographical and operating conditions [5]. Thermal gasification of MSW is a more complex process compared to incineration, wherein the waste is converted to simpler molecules, and hence the production of gases like dioxins and furans is prevented [6]. Thermal gasification is the process of converting solid fuels into combustible gaseous products by heating the fuel to temperatures of 750 to 1200 \({}^{\circ}\)C in an endothermic and controlled atmosphere, which is usually sub-stoichiometric. The process takes place in four phases, namely, drying, pyrolysis, combustion, and reduction. During the moisture removal process, the moisture in the fuel is evaporated. During the pyrolysis process, the volatiles, light hydrocarbons, and tar are separated from the fuel. During the combustion process, carbon and hydrogen in the fuel are changed to carbon dioxide, methane, and hydrogen. In the reduction phase, the carbon dioxide is reduced to carbon monoxide. The gas is called a syngas or producer gas, and it contains the following constituents: carbon monoxide, hydrogen, methane, carbon dioxide, and nitrogen [7]. Fixed-bed gasifiers are widely employed in commercial applications. In fixed-bed gasifiers, the feedstock is placed in a stationary bed. Fixed-bed gasifiers can be divided into three categories, updraft, downdraft, and cross-draft gasifiers, depending on which direction the feedstock and the gas flow. Apart from fixed-bed gasifiers, entrained flow gasifiers, fluidized bed gasifiers, circulating fluidized bed gasifiers, and dual fluidized bed gasifiers are other types of gasifiers applied to gasify MSW. The fluid used for gasification is air, oxygen, or steam, and each of these methods has its advantages and disadvantages.