Comparison of MSW-to-Energy Processes

MSW-to-Energy is the use of thermochemical and biochemical technologies to recover energy, usually in the form of electricity, steam and other fuels, from urban wastes. The main categories of MSW-to-energy technologies are physical technologies, which process waste to make it more useful as fuel; thermal technologies, which can yield heat, fuel oil, or syngas from both organic and inorganic wastes; and biological technologies, in which bacterial fermentation is used to digest organic wastes to yield fuel. These new technologies can reduce the volume of the original waste by 90%, depending upon composition and use of outputs.

Components of MSW-to-Energy Systems

1. Front-end MSW preprocessing
2. Conversion unit (reactor or anaerobic digester)
3. Gas cleanup and residue treatment plant
4. Energy recovery plant (optional)
5. Emissions clean up

Incineration

• Combustion of raw MSW, moisture less than 50%
• Sufficient amount of oxygen is required to fully oxidize the fuel
• Combustion temperatures are in excess of 850^oC
• Waste is converted into CO2 and water concern about toxics (dioxin, furans)
• Any non-combustible materials (inorganic such as metals, glass) remain as a solid, known as bottom ash (used as feedstock in cement and brick manufacturing)
• Air pollution control system for fly ash, bottom ash, particulates etc.
• Needs high calorific value waste to keep combustion process going, otherwise requires high energy for maintaining high temperatures

Anaerobic Digestion

• Well-known biochemical technology for organic fraction of MSW and sewage sludge.
• Biological conversion of biodegradable organic materials in the absence of oxygen at mesophilic or thermophilic temperatures.
• Residue is stabilized organic matter that can be used as soil amendment
• Digestion is used primarily to reduce quantity of sludge for disposal / reuse
• Methane gas is generated which is used for heat and power generation.

Gasification

• Can be seen as between pyrolysis and combustion (incineration) as it involves partial oxidation.
• Exothermic process (some heat is required to initialize and sustain the gasification process).
• Oxygen is added but at low amounts not sufficient for full oxidation and full combustion.
• Temperatures are above 650^oC
• Main product is syngas, typically has net calorific value of 4 to 10 MJ/Nm³
• Other product is solid residue of non-combustible materials (ash) which contains low level of carbon

Pyrolysis

• Thermal degradation of organic materials through use of indirect, external source of heat
• Temperatures between 300 to 850^oC are maintained for several seconds in the absence of oxygen.
• Product is char, oil and syngas composed primarily of O₂, CO, CO₂, CH₄ and complex hydrocarbons.
• Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes
• Syngas typically has net calorific value (NCV) of 10 to 20 MJ/Nm

Plasma Gasification

• Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma)
• Temperatures are above 3000^oC
• Organic materials are converted to syngas composed of H2, CO
• Inorganic materials are converted to solid slag
• Syngas can be utilized for energy production or proportions can be condensed to produce oils and waxes

MSW-to-energy technologies can address a host of environmental issues, such as land use and pollution from landfills, and increasing reliance on fossil fuels. In many countries, the availability of landfill capacity has been steadily decreasing due to regulatory, planning and environmental permitting constraints. As a result, new approaches to waste management are rapidly being written into public and institutional policies at local, regional and national levels.

Salman Zafar

Salman Zafar is the CEO of BioEnergy Consult, and an international consultant, advisor and trainer with expertise in waste management, biomass energy, waste-to-energy, environment protection and resource conservation. His geographical areas of focus include Asia, Africa and the Middle East. Salman has successfully accomplished a wide range of projects in the areas of biogas technology, biomass energy, waste-to-energy, recycling and waste management. Salman has participated in numerous national and international conferences all over the world. He is a prolific environmental journalist, and has authored more than 300 articles in reputed journals, magazines and websites. In addition, he is proactively engaged in creating mass awareness on renewable energy, waste management and environmental sustainability through his blogs and portals. Salman can be reached at salman@bioenergyconsult.com or salman@cleantechloops.com.

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