Pilot-scale operation and economic assessment of a two-stage, straw—manure fermentation system

Abstract

A two-stage fermentation system was evaluated. The system consisted of a high-rate fermenter (to produce methane from beef-cattle manure) and a low-rate fermenter (to produce methane from straw and slowly degrading material in the effluent from the high-rate fermenter). Three trials were completed. Results from these trials showed that methane yields of 0.25 m 3 CH 4/kg volatile solids (VS) straw can be achieved in less than 100 days of fermentation at 35°C, and that injection of anhydrous ammonia does not significantly increase the methane yield. Using only the effluent from the straw fermenter as make-up liquid in the system inhibited methane production. This inhibition was attributed to salt accumulation. Based on the assumptions used to design the system (40 Mg total solids (TS) per day) and in the economic analysis, a straw cost of slightly over $15/Mg TS was the break-even cost below which it was economically advantageous to add straw to manure for fermentation to methane. It was economically advantageous to compact the straw to densities between 150 and 300 kg/m 3 to reduce the straw fermenter volume and cost. Economies of scale were apparent, especially at the smaller plant sizes. The discounted cash flow rate of return (DCFRR) increased rapidly up to a plant size of about 20 Mg TS/day, then increased linearly as plant size continued to increase. The plant sizes necessary to achieve a DCFRR of 15% were 6.4, 13, and 43 Mg/day for straw costs of $0, $15, and $30/Mg, respectively. Income tax bracket had only minimal effect on the DCFRR.