A wide variety of conversion methods are available for realizing the potential of waste as an energy source, ranging from very simple systems for disposing of dry waste to more complex technologies capable of dealing with large amounts of industrial waste. These methods can be broadly divided into thermal and biological processes. Some of the popular technologies are summarized below:
- Gasification – Conversion of carbonaceous materials into synthesis gas by reacting waste at high temperatures with a controlled amount of oxygen and/or steam.
- Thermal depolymerization – process of reducing complex materials into light crude oil.
- Anaerobic digestion (AD) – Making use of microorganisms to break down biodegradable material in absence of oxygen.
- Mechanical biological treatment (MBT)– combination technique where recyclable elements are removed from a mixed waste stream and a biological process is used to extract energy from the elements. The types of biological processes utilized encompass anaerobic digestion, composting and bio-drying.
- Pyrolysis – Thermal degradation of organic materials through use of indirect, external source of heat. Product is char, bio-oil and syngas
- Plasma Gasification – Use of electricity passed through graphite or carbon electrodes, with steam and/or oxygen / air injection to produce electrically conducting gas (plasma). Organic materials are converted to syngas
Of the various modern energy conversion methods, pyrolysis and plasma gasification are attracting maximum attention these days, and these technologies have the potential to change the face of solid waste management in the coming years.
Present trends indicate a move away from single solutions such as mass burn or landfill towards the integration of more advanced WTE technologies, based on setting priorities for waste treatment methods. These include waste minimisation, recycling, materials recovery, composting, biogas production, energy recovery through RDFs, gasification and residual land filling.