Biohythane as a high potential fuel from anaerobic digestion of organic waste: A review

Recently, the bio-transformation of organic wastes to value-added products has gained huge interest due to problems related to waste disposal and fossil fuel depletion. Gaseous fuels such as biohydrogen and biomethane generation from biomass has been researched extensively and is well developed. Currently, biohythane, a gaseous mixure of biohydrogen and biomethane, with hydrogen composition of approximately 10–30% is gaining more attention due to its superior characteristics over other biofuels. The co-production of hydrogen and methane in two-stage process improves the total energy recovery up to 100% and 30% respectively compared to single-stage process. The optimal conditions for biohythane production are pH 5.5–6.5, 37 °C for 1–3 d during first stage, and pH 7–7.5, 37 °C for 10–15 d during second stage. Hythane with 10% and 20% (v v−1) hydrogen has achieved reduced NOx emission of 45% and 50% respectively in comparison with the methane fueled vehicles, whereas hythane with 30% (v v−1) hydrogen has achieved reduced CO2 emission of 69 g km−1. Although biohythane technology has high value as a vehicular fuel, its potential applications are yet to be explored. Enhanced biohythane quality and production rate can be obtained by selecting the most feasible substrates, controlling the microbial growth, using metabolically engineered strains and optimizing the bioreactor design. This review comprehensively discusses the mechanism of biohythane production; and its enhancement strategies such as pretreatment, co-digestion, optimization of physicochemical parameters and process integration to improvise the biohythane productivity. Finally, the techno-economic aspect for large scale biohythane production has also been addressed. In specific, this article discusses the latest achievements, their constraints and future prospects for lucrative biohythane generation.