Technical, Economic, and Environmental Review of Waste to Energy Technologies from Municipal Solid Waste

Global municipal solid waste production and electricity demand gradually increased as a result of urbanization, population increase


Introduction
Economic growth, industrialization, urbanization, and high standards of living have led to a rapid increase in demand for energy, thereby increasing the global municipal solid waste (MSW) production (Kaur et al., 2021).According to World Bank statistics, MSW produced worldwide reached 2.01 billion tonnes in 2016 and this is predicted to increase above 3.4 billion tonnes per year by 2050 (Kaza et al., 2021).Meanwhile, waste production in 582 © 2023, Program Studi Ilmu Lingkungan Sekolah Pascasarjana UNDIP Indonesia reaches 19.45 million tonnes per year with a composition of food waste, plastic, wood, metal, glass, cloth or fabric, leather, and rubber as shown in Figure 1.On the other hand energy demands still rely on fossil fuels which cause negative impacts on the environment, namely increasing greenhouse gas emissions.Therefore eco-friendly alternative energy resources are needed to provide for the global energy demand.The issue of MSW accumulation and the demand for alternative energy can be solved by utilizing energy from MSW.The process of utilizing energy from MSW is called Waste to Energy (WTE).Literature reviews related to WTE technology have been carried out by previous researchers.Beyene et al. (2018) discussed the current updates of WTE technology.Kaur et al. (2021) discuss the advantages and drawbacks of WTE technology.Giusti et al. (2009) discussed the effects of waste management procedures on human health.Roy et al. (2022) discussed the characteristics, methods, and waste-toenergy aspects of MSW management in Bangladesh.These reviews only address technical issues and are partly based on local perspectives, therefore it is important to conduct a comprehensive review related to the existing WTE technology, technical, economical, and environmental aspects of existing WTE technology.This article review aims to discuss the existing WTE technologies, the technical, the economic, and the environmental aspect of existing technologies.

Waste to Energy 2.1. Municipal solid waste (MSW)
Municipal solid waste (MSW) is all useless, unwanted, and discarded materials that result from people's daily activities that come from households, industries, schools, offices, shops, and others.The quantity, composition, and characteristics of MSW vary in each country depending on the rate of population growth, income, urbanization (Kaza et al., 2021), collection methods, and lifestyle (Rezaei et al., 2018).Table 1 shows the characterization of MSW.

Waste to Energy Technologies
The amount, composition, and characteristics of waste vary in each country depending on population growth rates, income, urbanization flows (Kaza et al., 2021), collection methods and lifestyle (Rezaei et al., 2018).Based on the composition of the waste, several alternative treatments can be carried out.Combustible materials with low to high calorific values such as plastic, paper, cloth, and wood are converted into energy with WTE technology.Noncombustible materials such as metals and glass are recycled if they are of economic value or disposed of in landfills.Some combustible materials such as paper, board, and plastic are also recycled.The heating value of combustible materials can be increased by an energy densification step.Materials that are dry or have a low water content are processed thermally, while materials with a high water content such as food waste, yard waste, and wood are processed biochemically or composting.Landfills are the last resort and are only used after the waste has been reduced, either by recycling or by converting it through WTE technology.Figure 2 shows a flowchart in determining the technology to be used for waste processing.
The WTE technologies used in each country vary depending on climatic conditions, population, generated waste types, and geographical conditions (Edjabou et al., 2015).WTE technologies can be classified into physical, thermal, biochemical, and bio-electrochemical technology.Through physical technology, MSW is converted to fuel, namely Refused derived fuel (RDF).Thermal technology includes incineration or combustion, gasification, and  (Azam et al., 2020) 583 pyrolysis (Tomić et al., 2017).During this process heat and syn-gas are generated.Anaerobic digestion and landfill are part of the biochemical conversion technology.In this process, organic matter is converted micro-biologically into biogas in an oxygen-free environment.Microbial fuel cells (MFC) and microbial electrolysis cells (MEC) are the newest MSW processing methods that utilize the role of microbes to produce electricity and hydrogen fuel (Beyene et al., 2018).Figure 3 shows the various technologies for processing MSW into energy and the resulting products.
The following are existing technologies to convert waste to energy.

Physical conversion
Physical conversion is the process by which MSW is physically/mechanically processed into energy to produce fuel/RDF.This process includes screening, sorting, separation, shredding, and drying.

•
Refuse Derived Fuel (RDF) RDF is a fuel made from combustible materials in MSW such as non-recyclable plastic, paper, cardboard, and other combustible materials.RDF is an alternative to landfill and includes an environmentally friendly method.MSW produced from commercial and domestic activities is chopped, dried, separated by different processes such as screening, air classification, and ballistic separation, and then packaged in pellet form to obtain a homogeneous material (Kaur et al., 2021).RDF can be utilized as fuel in cement plants, lime factories, and power plants as a substitute for conventional fuels such as coal.The characteristics and heating value of RDF are shown in table 2 below.

Biochemical conversion
Biochemical conversion is a methods in which organic materials are processed micro-biologically in an oxygen-free atmosphere to produce biogas.The main components of biogas are methane (CH4) and carbon dioxide (CO2).Anaerobic digestion and landfill are among the methods used to convert MSW into energy through biochemical processes.This process is carried out to treat MSW that has a high water content such as organic MSW and agricultural waste (Kaur et al., 2021).
Anaerobic digestion is a technique to decompose organic matter with the aid of anaerobic microorganisms under oxygen-free environment.In this process, sorting is carried out to separate metal, glass, and plastic from the organic materials in MSW so that the organic fraction of municipal solid waste (OFMSW) is obtained.OFMSW was then chopped, inserted, and kept in a bio-reactor under oxygen-free environment conditions and in the presence of acidogenesis and methanogenic microorganisms.The yield of methane produced depends on the operating conditions, MSW composition, reactor type, and residence time (Shah et al., 2021).Table 3 shows the characteristics of biogas produced from municipal solid waste (MSW).
• Landfill The landfill is the conventional and simplest biological method to obtain energy from MSW.The landfill produces biogas which can be used for heating purpose and electricity generation.The amount of biogas produced depends on MSW composition, MSW age, water content, and temperature (Bharathiraja et al., 2018).Table 4 shows the characteristics of biogas in landfills .

Thermal conversion
In thermal conversion, municipal solid waste (MSW) is converted in the form of heat or syn-gas to obtain the energy.This energy can be utilized to produce steam for electricity generation.Thermal conversion includes incineration, gasification, plasma gasification and pyrolysis.
• Incineration Incineration involves burning MSW at high temperatures (800-1000 ͦ C) in excess of oxygen.Incineration is common method in developing countries (Yong et al., 2019).Incineration can reduce MSW volume by as much as 80-90% (Y.Wang et al., 2018).Table 5 shows the characteristics of MSW incineration Gasification.
Gasification is a thermochemical method in which organic waste and carbon-containing waste materials are converted into syngas (Kaur et al., 2021).Gasification is a new technology in the WTE process that is widely used in developed countries (X.Y. Chen et al., 2015) and has an important role in energy production.The syngas consist of hydrogen, carbon monoxide, and methane as main components.The energy content of syngas is equivalent to onethird of the natural gas, which ranges from 4-50 MJ/Nm 3 .There are several types of gasifiers such as continuous fluidized bed (CFB), bubbling fluidized bed (BFB), fluidized bed (FB), and others, each of which has its advantages, disadvantages, and operating characteristics.Table 6 shows several types of gasifiers.
• Plasma Gasification Plasma gasification is a thermal conversion method to convert MSW into energy using an electric arc.Plasma is produced from the release of heat and light energy caused by the propagation of electricity through a non-conductive medium such as gas or air.Plasma gasification operated at 1400-2000 ͦ C under partial oxidation to produce high-quality of syngas (Prado et al., 2020).The ratio of reducing the amount of waste in gasification plasma is 300:1, while in incineration is 5:1.Plasma gasification is carried out at high temperatures so can ensure the disappearance of harmful compounds, toxic compounds, bacteria, and deadly viruses and closed system so that ash, dust, and toxic compounds are not released in the outside environment.The electrical energy produced from the gasification process is cheaper and more efficient than incineration (Kaur et al., 2021).
• Pyrolysis Pyrolysis is a new technology for WTE and is widely applied in developed countries (Meng et al., 2015).Pyrolysis can reduce MSW volume by 80-90%.Pyrolysis is an endothermic process in which heat is used to burn MSW in an oxygen-free environment.Pyrolysis produces three main products, namely pyro-oil in the form of a mixture of oil and water obtained from the condensation of steam, residue in the form of charcoal and ash which is rich in carbon content, and gas in the form of CO, CO2, and methane (Jamilatun et al., 2022).Several factors influence pyrolysis including the pretreatment process, the composition of raw material, heating rate, temperature, residence time, and type of reactor (Pitoyo et al., 2022).Rotary kiln is the most used technique for pyrolysis of MSW (Hasan et al., 2021).Table 8 shows the characteristics of the gas from the pyrolysis.

Bio-electrochemical conversion
Bio-electrochemical conversion includes microbial fuel cells (MFC) and microbial electrolysis cells (MEC).This technology is the newest WTE technology that utilizes the role of microbes to produce hydrogen fuel and electricity.
• Microbial fuel cells (MFC) Electrochemically active microorganisms (EAM) are used in MFC technology to produce electricity.MFCs involving both aerobic and anaerobic processes using bacteria as catalysts is a new approach to biohydrogen production.Various organic waste such as household waste, animal manure, and sewage sludge can be used as raw materials (Logroño et al., 2015).The use of organic waste makes MFC an eco-friendly technology that gives a dual purpose in waste management and bioelectricity generation (Xu et al., 2017).Table 9 shows electricity generation in different reactor designs and substrates.
• Microbial electrolysis cells (MEC) MEC is a smart and green technology to face the challenges of global warming and meet energy demands.MEC works by utilizing electrochemically energetic bacteria to convert MSW into H2 and chemicals (Kadier et al., 2017).Hydrogen production rate (HPR) in MEC is affected by the type of substrate, external voltage, electrode surface area, electrode spacing, membrane materials, and reactor design (Kadier et al., 2016).Compared to other nonconventional technologies, MEC has some advantages such as producing H2 at low energy inputs, no need for precious metals on the anode of MEC, high conversion efficiency to hydrogen, producing relatively pure hydrogen, and producing other valueadded products (Kadier et al., 2017).Table 10 shows the hydrogen production in MECs technologies from the literatures.H2) is a green fuel, a high calorific value fuel that has the highest energy density.Hydrogen (H2) has a calorific value of 120-142 MJ/kg.Figure 2 shows the potential for energy generation from H2 among different WTE technologies.Bioelectrochemical technology, namely MEC has the highest, followed by thermal conversion and biochemical conversion technology.
Bioelectrochemistry produces high purity of H2 (up to 90%) (Khan et al., 2017), so it has a high H2-potential for energy generation.Thermal conversion produces various gas compositions, namely CH4, CO2, CO, H2, and others with H2 content between 16-52%.Among the thermal conversion, incineration technologies have the lowest value because incineration is a combustion process that produces CO2 and H2O as the main gas composition (Thabit et al., 2022).Meanwhile, the biochemical conversion's gaseous product is mostly CH4, CO2, and a small amount of H2 (0-5%) in composition (X.Y. Chen et al., 2015) so it has a low H2-potential value.

Available Energy
Figure 3 shows available energy from waste which is the product of the lower heating value (LHV) of syngas and the volume of gas produced by the weight of waste in different WTE technologies.Available energy shows the potential for energy generation from waste.It can be seen from Fig. 3 that thermal conversion technology gives a greater value than biochemical conversion because thermal conversion produces a higher yield of syngas, which is 610-1240 m 3 /ton (M.He et al., 2010), compared to biochemical conversion, which is 30-142 m 3 /ton (Rahman et al., 2018).Plasma gasification produces the highest available energy value among thermal conversion technologies because plasma gasification has the highest LHV and syngas yield.The high LHV and yields of syngas provide greater available energy.

H2/CO ratio
Figure 4 shows the H2/CO ratio in various WTE technologies.H2 and CO are diatomic molecules that provide the building blocks of fuel science and technology.The ratio of H2/CO affects efficiency, combustion, and emissions.An increase in H2/CO will increase thermal efficiency, combustion temperature, and NOx emissions, and reduce HC and CO emissions (Sahoo et al., 2012).A high H2/CO ratio (>2) is required in the Fischer-Tropsch synthesis (Zaccariello & Mastellone, 2015).It can be seen from Fig. 4 that pyrolysis produces a higher H2/CO ratio than gasification.The high H2/CO ratio is caused by the water-gas shift reaction that converts CO to H2. Increasing the equivalent ratio (ER) in gasification, which is the ratio of actual oxygen to stoichiometric oxygen for complete combustion, will increase the oxidation of hydrogen to H2O thereby reducing the H2 content, and increase the oxidation of C and CO to CO2 which further reacts with C through the Boudard reaction to produce CO thereby reducing H2/CO ratio.Pyrolysis has an ER close to zero so it has a high H2/CO ratio.

Cold gas efficiency (CGE)
Figure 5 shows the cold gas efficiency (CGE) of the three thermal conversion technologies (gasification, plasma gasification, and pyrolysis).CGE is the ratio between the calorific value of the syngas produced and the calorific value of the feedstock.CGE is related to the heat of combustion from syngas and feeds waste.CGE is a function of LHV and the volume/mass flow rate of syngas and feeds waste.The higher the LHV and the volumetric rate of syngas, the higher the CGE.The high value of CGE results in great combustion efficiency.Plasma gasification (PG) has a high CGE value compared to other thermal conversion technologies because PG takes place at high temperatures, resulting in a large volumetric rate of syngas.Plasma gasification can convert the volume of waste into syngas and slag up to about 99% (Prado et al., 2020).3.1.5.Carbon conversion efficiency (CCE) Figure 6 shows the carbon conversion efficiency (CCE) in three thermal conversion technologies (gasification, plasma gasification, and pyrolysis).CCE is defined as the amount of carbon in the waste which is converted to carbon in the syngas in the form of CO, CO2, CH4, C2H6, C3H8, etc.The CCE indicates how much of the unconverted waste should be treated by another process.CCE also indicates the chemical efficiency of the process (Seo et al., 2018).
CCE is a function of carbon fraction and volumetric/mass flow rate of syngas and feeds waste.Plasma gasification (PG) provides the highest CCE value because the high temperature in PG produces a large volumetric rate of syngas, thereby increasing the conversion of carbon from waste to syngas.

Environmental Assessment
Figure 7 shows the emission factors for various WTE technologies.The emission factor shows how much CO2 is released to produce a certain amount of energy from waste.CO2 is the main component of greenhouse gas (GHG).It can be seen from the figure that incineration gives the highest emission factor between 0.6-1.1 tons/MWh, followed by gasification (0.2 tons/MWh), anaerobic digestion, and the landfill (0.12 tons/MWh), plasma gasification and pyrolysis (0.08 tons/MWh).), then MFC and MEC (close to zero).The high content of CO2 in incineration is because incineration is a combustion process that produces CO2 and H2O as the main components in the gas (Thabit et al., 2022).11 shows that the average investment cost for energy production technology from MSW is relatively higher than that of other renewable and non-renewable resources, especially for non-conventional WTE technology (Tangri, 2017).These make conventional WTE technologies such as AD, landfill, and composting preferred because of the risk of cost, investment capital, and lower operating costs, especially in developing countries.
Operational and maintenance costs related to WTE technology are shown in table 12. Operational costs include labor, overhead, insurance, depreciation, and utility costs.Operational and maintenance costs on non-conventional WTE technology are higher than on conventional technology.Operational and maintenance costs are influenced by several parameters including socioeconomic status, labor wages, high-efficiency targets, taxes, and insurance (Austin, 2013).

Table 1 .
Characteristics of municipal solid waste

Table 3 .
Characteristics of biogas from MSW anaerobic digestion

Table 4 .
Characteristics of biogas from landfills

Table 5 .
Electricity and emission generation from MSW incineration

Table 6 .
Comparison of several types of gasifiers

Table 7 .
Characteristics of gas from the plasma gasification process

Table 8 .
Characteristics of gas from the pyrolysis process

Table 9 .
Electricity generation from MFCs

Table 10 .
Hydrogen production in MECs

Table 11 .
Comparison of investment costs between WTE and non-WTE technologies

Table 12 .
Comparison of operational and maintenance costs on WTE technology The selection of waste to energy (WTE) technologies needs consideration of energy efficiency, financial, and environmental aspects.The results of the assessment of existing technology show that anaerobic digestion and landfill have a low-cost and low potential for energy generation.Incineration has a low potential for energy generation and high CO2 emissions and capital costs.Plasma gasification is superior in technical and environmental (high potential for energy generation, CGE, CCE, and H2/CO, and low CO2 emissions) and inferior in economical aspect (high capital and operating costs).MEC has a high H2potential for energy generation, low CO2 emissions, and the highest capital cost.Thus, plasma gasification is the best technology for converting waste into energy and the selection of WTE technologies is influenced by energy efficiency, economic, and environmental factors.://doi.org/10.3390/pr7100676Zaccariello, L., & Mastellone, M. L. (2015).Fluidized-bed gasification of plastic waste, wood, and their blends with coal.://doi.org/10.3390/en8088052Zhang, D., Huang, G., Xu, Y., & Gong, Q. (2015).Waste-toenergy in China: Key challenges and opportunities.Zhang, F., Ge, Z., Grimaud, J., Hurst, J., & He, Z. (2013).Longterm performance of liter-scale microbial fuel cells treating primary effluent installed in a municipal wastewater treatment facility.Environmental Science and Technology, 47(9), 4941-4948.https://doi.org/10.1021/es400631rZuo, Y., Cheng, S., & Logan, B. E. (2008).Ion exchange membrane cathodes for scalable microbial fuel cells.Environmental Science and Technology, 42(18), 6967-6972.https://doi.org/10.1021/es801055r httpshttps