Advanced Technologies (Biological and Thermochemical) for Waste-to-Energy Conversion

Abstract

This chapter reviews various technologies for waste to energy conversion. The advantages and disadvantages of each technology are discussed in this chapter. The effectiveness of a specific waste-to-energy process is determined based on the thermal potential of the waste, effectiveness of the system and the type of produced energy. The two main technologies for energy recovery from waste materials are thermochemical and biological conversions. Thermochemical conversion is defined as the decomposition of organic matters as a result of heating and/or oxidation of biomass and chemical reactions. It mainly includes combustion, gasification, pyrolysis and hydrothermal liquefaction. Combustion or incineration is an old and developed technology for the degradation of organics in the presence of oxygen. It can be performed as co-firing of biomass with coal for higher conversion efficiency and less fouling and corrosion problems compared to the combustion of biomass feedstock. Pyrolysis, defined as the decomposition of biomass in the absence of oxygen, produces syngas, liquid fuels and biochar. Hydrothermal liquefaction is another thermochemical process for the conversion of biomass to liquid fuels in the presence of water as the reaction medium. It involves a series of reactions, producing bio-crude oil, water-soluble products, solid residue and non-condensable gases as the main products. Another thermochemical process is gasification which involves the conversion of biomass into gaseous products using a gasifying agent. On the other hand, the biochemical conversion methods use microorganisms and enzymes in addition to heat and other chemicals to breakdown the biomass into gaseous or liquid fuels. Examples of biochemical conversions include anaerobic digestion, mechanical biological treatment and fermentation. Anaerobic digestion is the breakdown of biodegradable materials in the absence of oxygen. This process results in the reduction of volatile solids and the production of biogas that can be used to produce electricity and heat or cleaned and used as a natural gas substitute. Mechanical biological treatment is the integration of some of the waste management facilities such as materials recovery facilities (MRFs), composting and anaerobic digestion plants to recover recyclables and compost or digest the organic fraction of the waste through biological treatment. Fermentation involves the conversion of organic materials into bioethanol using various yeasts and microorganisms. Whereas, conversion of lignocellulosic biomass into ethanol is complex and expensive and there are still some challenges that limit its commercialization, e.g., the rigorous pre-treatments needed, the high cellulase costs and structural hindrances of lignin and hemicellulose, and high capital costs of such complex processes, etc. The biorefinery is a promising approach in which biomass such as lignocellulose could be converted into multiple high-valued biochemicals and biomaterials in addition to bioethanol.