Biochemical methane potential for wheat-based fuel ethanol and beef feedlot integration

Abstract

Assessing the economic and environmental benefits of constructing a biorefinery consisting of an ethanol plant, an anaerobic digester and a beef feedlot depends on understanding the methane potential of substrates available within the system. Fuel ethanol is produced from wheat in Western Canada and literature values for the methane potential of wheat-based ethanol byproducts is scarce. This study consisted of conducting biochemical methane potential (BMP) assays at thermophilic temperatures on ethanol byproducts typically produced downstream of distillation in wheat-based ethanol facilities. One experiment focused on the methane potential of the byproducts alone and two more experiments focused on the effect of amending the highest potential byproducts with feedlot manure at 1:1 and 2:1 volatile solids ratios of byproduct to manure. Methane yields for whole stillage, thin stillage and wet cake were 585 ± 32, 547 ± 76 and 495 ± 45 ml/g VS added, respectively. Reliable methane production rate constants for these byproducts could only be determined from the third experiment, but were 0.106, 0.090 and 0.105 day-1, respectively. When feedlot manure was added to the ethanol byproducts, methane yield results were proportionally equal to the ratio of byproduct to manure, except in the case of thin stillage. The combination of thin stillage and feedlot manure yielded 125% and 119% of expected results. Overall, feedlot manure stabilized and increased methane production rate constants for wheat-based ethanol byproducts. It was concluded from the results that whole stillage should be used in anaerobic digesters when manure is not available near ethanol facilities. When an ethanol plant, feedlot and digester biorefinery is being considered, a mixture of feedlot manure and thin stillage should be digested to achieve proportionally higher methane yields from each substrate.