ARS | Publication request: Full-scale on-farm pretreatment of perennial grasses with dilute acid for fuel ethanol production

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 23, 2010
Publication Date: March 27, 2010
Citation: Digman, M.F., Shinners, K.J., Muck, R.E., Dien, B.S. 2010. Full-scale on-farm pretreatment of perennial grasses with dilute acid for fuel ethanol production. BioEnergy Research. 3:335-341.

Interpretive Summary: Switchgrass and other grasses are potential biomass sources for the production of cellulosic ethanol. However, pretreatment at the bioprocessing plant is a major barrier to development of this industry. Earlier laboratory research suggested that sulfuric acid could be used as a pretreatment at the farm, rather than at the bioprocessing plant, but practical methods of applying this acid are needed at the farm. Both switchgrass and reed canarygrass were ensiled in commercial, 9-foot diameter bag silos and each treated with sulfuric acid in two ways. The first method was to directly spray the acid on the grass on the conveyor into the bagging machine. The second was to first place the chopped grass in a cattle feed mixer, spray the acid on the grass and mix the grass and acid thoroughly before putting the crop in the bagging machine. Surprisingly, the direct approach of spraying acid on the grass on its way into the bagging machine produced more uniform mixing of the acid with the grass. These results suggest that farmers could pretreat grasses harvested for cellulosic ethanol with a strong acid much like they apply other silage additives, but with additional safety precautions.

Technical Abstract: Producers utilizing sulfuric acid to pretreat biomass on-farm will need a safe and cost-effective method for application that does not negatively impact the ensiling process or harvesting capacity. To that end, switchgrass (Panicum virgatum L.) and reed canarygrass (Phalaris arundinacea L.) were pretreated with sulfuric acid with two methods: during bagging (on-line) and thoroughly mixed in a commercial feed mixer (mixed) and both stored for 90 days. The two methods, applied at rates from 28 to 54 g/kg DM not only helped to preserve biomass substrates under on-farm conditions (anaerobic, ambient temperature and pressure) through inhibition of microbial activity but also enhanced conversion of cellulose to ethanol by SSF using Sacchromyces cerevisiae. Acid-pretreated substrate yielded 13 and 5 percentage points higher ethanol than fresh reed canarygrass and switchgrass, respectively. The on-line method of pretreatment out-yielded the mixed method both as a preservative and as an agent for enhanced cell wall degradation. This result was thought to be an outcome of more uniform acid application as indicated by the on-line method's more consistent pH profile and decreased fermentation products, as compared to the mixed method. Although significant levels of acetate and lactate were observed after storage in the biomass, Sacchromyces cerevisiae was not found to be inhibited in an SSF with a 10% solids loading.