Methane fermentation of energy crops: Maximum conversion kinetics and in situ biogas purification

Abstract

A seven-year study addressed both the limitations on the rate and efficiency of biomass conversion, and the enrichment of biogas methane content. Energy crops examined included sorghum ( Sorghum bicolor), napiergrass ( Pennisetum purpureum), corn ( Zea mays), and a sorghum/α-cellulose mix. High solids digestion work (25–30% effluent total solids (TS)) showed the need to address ammonia toxicity and trace nutrient limitations. Trace nutrient supplementation and control of the feedstock C/N ratio enabled stable operation of digesters at volatile solids (VS) loading rates up to 24 grams per kilogram reactor wet mass per day (gVS kg −1 day −1), with mean methane production rates of 7.5 L kg −1 day −1. Acid-extractable metal concentrations were used as an indicator of bioavailable metals. Initial work with low solids digestion (8–10% TS) resulted in efficient VS conversion but low methane production rates. Subsequent work using trace nutrient supplementation enabled stable operation of intermittently-fed (three times per week) digesters at loading rates up to 12 gVS kg −1 day −1, resulting in methane production rates up to 3.3 L kg −1 day −1. Continuous feeding of corn at rates up to 18 gVS kg −1 day t-1resulted in a mean methane production rate of 5.4 L kg−1 day-' with a 67% VS conversion efficiency. The maximum methane production rates for both the high solids and low solids systems are among the highest observed for biomass conversion. An in situ technique to enrich digester offgas was developed to take advantage of the differing solubilities of C0 2 and CH 4, in which dissolved C0 2 was removed from the reactor in a recycled leachate stream and gas-stripped in an external stripper. Such a system easily enriched the remaining digester offgas to over 90% methane, and contents in excess of 98% were achieved. Quantitative evaluation of system variables defined the effects of leachate recycle rates, leachate alkalinity, and pH on the resulting offgas methane contents.