Bioprocessing of Oat Hulls to Ethylene: Impact of Dilute HNO₃- or NaOH-Pretreatment on Process Efficiency and Sustainability

Abstract

Agricultural wastes are cost-effective, renewable, and abundant. Using bioethanol from oat hulls (o.h.) could be a promising technology to produce ethylene and valuable derivatives. This process follows the Principles of Green Chemistry, albeit facing certain challenges. We compared pretreatment options with either 4%-HNO 3 or 2%-NaOH for the complete recycling of o.h. to ethylene and coproducts. On a pilot-scale, 10 kg of o.h. was converted to 1.10−0.95 kg of bioethanol (93%-w); on a lab scale, bioethanol was converted to 0.55−0.39 kg of ethylene; under optimum pilot-scale conditions, this would be equivalent to ∼0.67−0.55 kg of ethylene. NaOH-delignification results in ∼20% loss in ethylene productivity. Residual Na ions in the pulp cause inhibition of both fermentation to ethanol and alumina-catalyzed dehydration to ethylene. HNO 3 -pretreatment excludes Na and reduces the content of fusel alcohols and acetaldehyde in bioethanol 10-fold. Compared to NaOH-delignification, HNO 3 -pretreatment gives ∼13 times more coproduct and ∼1.6 times less waste, which makes the o.h.-to-ethylene technology more sustainable and green. E-factor (wastes/products) is 1.5 and W-factor (water/products) is 1.8 if the pulp-washing step is considered. Both factors can be greatly improved by eliminating this step. The estimated production of ethylene from the local o.h. in Siberia is 43 KTPA (or 71 KTPA-ethanol).

Production of Cellulose Materials from Oat Agricultural Wastes: A Review of Recent Approaches

“…Selecting the pretreatment method is also important for deep transformation of cellulose. It is shown in [72] that the conditions of pretreatment of oat husks affect the productivity of ethylene. Thus, alkaline pretreatment of agricultural waste leads to a decrease in ethylene yield by 20% compared to a cellulose-containing sample obtained by the treatment with nitric acid.…”