Adding CO2 improves biofuel production

A NEW method of adding CO₂ into ethanol production has been discovered by researchers aiming to streamline biofuel production.

A team from the US’ Department of Energy's Lawrence Berkeley National Laboratory and Sandia National Laboratories working at the Joint BioEnergy Institute (JBEI) demonstrated that adding CO₂ gas during pretreatment of biofuel production neutralised the toxicity of the ionic liquids – the room-temperature molten salt solvent used to break down cellulosic plant material.

The team used CO₂, knowing that when the gas is absorbed by water, the resulting reaction increases its acidity. The ionic liquids used for pretreatment are typically highly alkaline, and must be washed away to not interfere with the enzymes and microbes used in the latter stages of biofuels production.

The enzymes are used to release the sugars from the slurry of cellulose and hemicellulose after pretreatment, a step called saccharification. The bacteria are then used to ferment the sugars into biofuel. CO₂ is produced as a by-product of fermentation, so can be recycled back into the feedstock for greater efficiency.

The process is easily reversible, allowing the ionic liquid to be recycled. Using CO₂ to control pH levels eliminates the need for separation and purification of ionic liquids after biomass pretreatment.

Blake Simmons, director of biological systems and engineering at Berkeley, said: “When the pH adjustment is reversible, it makes the overall process more efficient because you can repeat the pretreatment cycle several times. And it costs less because now you can do everything in one reactor instead of three.”

In the experiments, the team tested 15 types of ionic liquid at various concentrations. They determined that cholinium lysinate, which is formed by mixing the amino acids choline and lysine, was the most compatible with commercially available enzyme mixtures and fermentation mixtures.

They then conducted tests with various concentrations and pressures of CO₂. Applying just under 1m Pa of CO₂ to the system shifted the pH to a range that was optimal for the enzymes and microbes. This achieved 83% of the theoretical yield of ethanol from the glucose initially present in the biomass.

According to a preliminary economic analysis reported in the study, a CO₂-enhanced process could lower production costs by 50–65% compared with conventional ionic liquid-based pretreatment methods.

Seema Singh, director of biomass pretreatment at JBEI, said: “Pretreatment is the most expensive part of the biofuels production process. If you count the whole production cycle, pretreatment is second only to the cost of growing and obtaining the feedstock itself.”

The researchers said this process could soon be ready for implementation in ethanol production, but did not specify a timescale.

The next research step for the team is to adapt the method to the production of 'drop-in' advanced biofuels that can directly replace blendstock – materials blended in an oil refinery – for automotive, diesel and aviation fuels.

Energy & Environmental Science, DOI: 10.1039/C6EE00913A