Synthesis of different biofuels from livestock waste materials and their potential as sustainable feedstocks – A review

Edible crops are widely used as biomass feedstocks for the production of liquid biofuels (bioethanol and biodiesel). However, the use of edible crops as feedstocks for the biofuels causes global food supply issues and ethical dilemma. Livestock waste materials, specifically animal manures, contain all the available feedstocks (carbohydrate, lipid, and protein) for the production of diverse biofuels. In addition, the massive production of livestock manures offers their potential use as plentiful feedstocks for biofuels. Nevertheless, they are used for only limited applications such as soil amendment and combustion. In this review, the suitability of livestock manures to be used as different biofuels is comprehensively discussed comparing to the current technical development for bioenergy synthesis processes. The economic feasibility of manure derived biofuels is compared with that from the other biomass resources. Lastly, the challenges of current technical developments and the future research directions for biofuel production from the livestock manures are provided.