Biogas Integration for Cellulosic Ethanol Production

Author(s):

This post originally appeared in Biomass Magazine

As cellulosic ethanol production facilities have come online, anaerobic digesters have been co-located at the plants to recover biogas from soluble residues inherent in the ethanol production process. These soluble residues contain a source of energy that can be recovered using anaerobic digestion, and the captured biogas can help meet process energy needs. 

Recently in Iowa, DuPont held a grand opening for the world’s largest cellulosic ethanol facility. Once fully operational, the plant will produce 30 million gallons of cellulosic ethanol per year. The opening of the DuPont plant comes on the heels of Poet-DSM’s cellulosic ethanol facility opening in 2014. The Poet-DSM plant aims to produce 20 MMgy. 

The opening of these plants represents a tremendous milestone in our nation’s goal to produce domestic renewable fuel sources from nonfood feedstocks. These plants are also able to tout another major accomplishment—the production of cellulosic ethanol in a closed-loop system. Both plants use a biomass boiler and anaerobic digestion technology to produce process energy from the wastewater and solid lignin of the ethanol production process. Each plant is able to meet its energy needs through the integration of biomass co-firing and anaerobic digestion of wastewater. Since the cellulosic ethanol plants are located near corn ethanol plants, excess process energy from the cellulosic plants can offset some of the fossil fuel use at these facilities.  

Abengoa’s cellulosic ethanol plant in Kansas also integrated anaerobic digestion into the cellulosic ethanol production process. The anaerobically treated wastewater at Abengoa’s facility is estimated to offset $750,000 per year in energy purchases. Using anaerobic digestion to help treat the wastewater in the ethanol production process also provides an opportunity for waste reuse, which helps improve the overall cellulosic ethanol production footprint. 

Although there are numerous environmental benefits from the integration of anaerobic digestion into cellulosic ethanol production, the life-cycle energy balance and dramatic improvements in the carbon emission profile of the produced fuel can have a positive impact on the bottom line, too. This presents an opportunity for these companies to receive a price premium in markets such as California’s, which is putting pressure on fuel suppliers to drive down carbon emissions for fuel that is consumed in the state.

A lower-carbon profile of cellulosic ethanol is a requirement under the renewable fuel standard (RFS), which requires cellulosic fuels to have a 60 percent greenhouse gas (GHG) reduction compared to gasoline. The commercialization of anaerobic digestion technology that can be integrated or co-located with other renewable fuel production facilities can assist those producers with reaching carbon profile requirements under the RFS. Aggressive cellulosic fuel volumes are required by law under the RFS, and it is a promising sign that the first cellulosic ethanol production facilities to come online have integrated anaerobic digestion, at some scale, into the production process. 

These technology advancements are also good for consumers, who can be assured that through rigorous life-cycle accounting, they are using a fuel that offers significant GHG emission savings compared to fossil fuels. As more cellulosic fuel production facilities come online, I hope we continue to see the integration of anaerobic digestion to help meet process energy needs. The technologies work well together and offer multiple benefits for the environment and society.