Abstract
Background: Solid-state anaerobic digestion of agricultural stubble is attractive technology for energy and bioeconomy as well as it may lead to transitioning towards greenhouse gas neutrality; yet hydrolysis and syntropy affects the process and makes it economically nonviable. In this regard, present study investigates the effect of alkali and biochar addition for simultaneous increment of hydrolysis and syntropy for higher methane yield from pearl millet straw. Further, taguchi based design of experiment was coupled with grey relation analysis for multiple output evaluation and detailed techno-economic assessment was performed. Results: Study showed that 0.5 g/100g pearl millet straw of alkali and 10 g/L of biochar was the optimised dosing along with 20% total solid concentration and 4 as feedstock/inoculum ratio. Statistically, contribution of biochar and alkali was 48 and 21% respectively on the multiple output. The confirmation test revealed that hydrolysis rate constant, k for reactor having optimised conditions was 0.0521 d-1 while for control, it was 0.0595 d-1. Cumulative methane yield was also increased by 1.8-fold for optimised condition. Techno-economic assessment showed that capital cost and electrical efficiency of combined heat and power unit have dominant effect on the investment. Solid state anaerobic digestion of pearl millet straw with alkali and biochar showed US$ 25652 of net present value and showed to have payback time of 8.2 years with 11% of internal rate of return.Conclusion: The simultaneous increment of hydrolysis rate and syntrophic activity in optimized condition helped to achieve higher methane yield. Techno-economic assessment showed that shorter payback time and higher internal rate of return, making large scale project profitable and viable which may endorse sustainable bioeconomy with lower greenhouse gases.