Energy and Sustainability Comparisons of Anaerobic Digestion and Thermal Technologies for Processing Animal Waste

Animal wastes from livestock farming of cattle, cow, hog, and poultry are increasingly being recognized as a valuable resource. The green energy derived from animal waste is considered to be carbon-neutral because animal feed is largely grown from photosynthesis of carbon dioxide. Electricity and heat thus generated are offsetting those generated from fossil fuels and are rewarded with greenhouse gas emission reduction credits. There are two well recognized groups of technologies: biochemical processes such as anaerobic digestion (AD), and thermochemical processes such as incineration, gasification and pyrolysis. This paper presents a theoretical comparison of the energy conversion efficiencies based on mathematical models of AD and Gasification processes. Other consideration of the environmental sustainabilities of the technologies in terms of emissions and final residue management are also presented. General guidelines are provided for the different types of livestock manure considered, and directions for further research are indicated.