Crop Management Impacts Biofuel Quality: Influence of Switchgrass Harvest Time On Yield, Nitrogen and Ash of Fast Pyrolysis Products.

Although upgrading bio-oil from fast pyrolysis of biomass is an attractive pathway for biofuels production, nitrogen (N) and mineral matter carried over from the feedstock to the bio-oil represents a serious contaminant in the process.  Reducing the N and ash content of biomass feedstocks would improve process reliability and reduce production costs of pyrolytic biofuels.   This study investigated: 1) How does switchgrass harvest date influence the yield, N concentration ([N]), and ash concentration of biomass and fast pyrolysis products? and 2) Is there a predictive relationship between [N] of switchgrass biomass and [N] of fast pyrolysis products?  Switchgrass from five harvest dates and varying [N] from central Iowa were pyrolyzed using a free-fall reactor.  Harvestable biomass peaked in August (8.6 Mg ha^-1), dropping significantly by November (6.7 Mg ha^-1, P=0.0027).  Production of bio-oil per unit area mirrored that of harvested biomass at each harvest date; however, bio-oil yield per unit dry biomass increased from 46.6 % to 56.7 % during the season (P <0.0001).  Allowing switchgrass to senesce lowered biomass [N] dramatically, by as much as 68 % from June to November (P<0.0001).  Concurrently, bio-oil [N] declined from 0.51 % in June to 0.17 % by November (P<0.0001).  Significant reductions in ash concentration were also observed in biomass and char.  Finally, we show for the first time that the [N] of switchgrass biomass is a strong predictor of the [N] of bio-oil, char, and non-condensable gas with R² values of 0.89, 0.94, and 0.88, respectively. 

Keywords:

biomass crop, thermochemical conversion, biofuel contaminants, bioenergy, bio-oil, Panicum virgatum L.

---