(639r) Overcoming the Challenges of Using Corn Stover As Feedstock in Autothermal Pyrolysis"

Authors: 
Polin, J., Iowa State University
Whitmer, L., Iowa State University
Smith, R. G., Iowa State University
Brown, R. C., Iowa State University
Corn stover biomass is abundant across the Midwestern United States and is considered a potentially promising feedstock from which biorenewable fuels and chemicals could be produced. However, significant challenges to economically and efficiently convert the lignocellulose fractions into valuable products remain. Fast pyrolysis is an endothermic process capable of thermally deconstructing biomass into bio-oil vapors, non-condensable gases (NCG’s), and solid biochar at an elevated temperature (500⁰C). Autothermal operation differs from traditional pyrolysis with the addition of a small amount of air introduced in the reactor to generate sufficient exothermic energy to drive pyrolytic reactions and achieve a neutral energy balance. Pilot scale testing in a bubbling fluidized bed reactor resulted in low equivalence ratios (ER) during autothermal operation that ranged from 8-11% of the theoretical air requirements for stoichiometric combustion; these ER’s are almost half of those compared to autothermal gasifiers. Significant process intensification was reached when air-blown autothermal operation was achieved using 100% air as the fluidizing gas and a maximum biomass feed of 20 kg hr-1 was obtained. Bio-oil yields greater than 50 wt. % were achieved along with biochar and NCG’s yields around 20 and 30 wt. % respectively. Six bio-oil product streams were collected using a fractional recovery system consisting of three alternating pairs of condensers and electrostatic precipitators; this allowed bio-oil compounds across a range of molecular weights to be collected based on the compound’s dew point. Early technical challenges of processing corn stover involved the accumulation of biochar in the reactor and degradation of bio-oil heavy ends in the first stage fraction (SF1). These were overcome by adjusting the fluidization gas velocity to promote better char particle attrition and elutriation from the reactor as well implementing a new SF1 quench tower to facilitate the condensation and collection of viscous bio-oil products. Autothermal process intensification represents a significant advancement in pyrolysis technology and removes the heat transfer bottleneck traditionally associated with thermochemical conversion. Under these conditions, the reactor’s footprint was reduced by 40% while simultaneously increasing biomass throughput by a factor of four. This advancement is expected to significantly improve the scalability and process economics of corn stover pyrolysis systems.