Adding CO2 Facilitates Greener Biofuel Production Process

The recipe for making ethanol has a new addition, a secret ingredient in the form of carbon dioxide. This recent addition is the next major leap towards streamlining the process of biofuel production.

Scientists at the Sandia National Laboratories (Sandia) and the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed this new innovation at the Joint BioEnergy Institute (JBEI). The researchers added carbon dioxide gas at the deconstruction stage of the production process. They were then able to prove that CO₂ was able to neutralize the toxic ionic liquids. The researchers used salt solvents that were molten at room temperature as ionic solvents to break cellulosic plant material down. In addition, the solvent can be recycled for use again, making for a reversible process.

The addition of CO₂ in the biofuel production process has a major advantage. Extracting and purifying the liquid after the pretreatment of biomass and before fermentation and saccharification, two important steps in the production, can be avoided when CO₂ is used to control pH.

Reducing the production cost is the main challenge in expanding the biofuels market. The article published in the July 2016 issue of the online journal Energy and Environmental Sciences, throws light on this obstacle.

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.

Seema Singh, Director of Biomass Pretreatment, JBEI

Adding CO₂ can lead to a reduction of 50 – 65 percent of productions costs, compared to the standard methods that involve ionic liquid pretreatment, according to a preliminary economic evaluation report.

The simple and easy way to integrate the method into the industrial processes is the major highlight of using CO₂ in neutralizing ionic liquid. In addition, when compared to other pH modification methods and standard industrial gases, using CO₂ is a less toxic option.

The application and removal of pressurized gas in an industrial setting are things we’ve been doing for more than a century. This technique fits into an already robust and reliable industrial system.

Blake Simmons, Vice President for Deconstruction, JBEI

As the ionic liquids used in JBEI during the pretreatment have high levels of alkali, they need to be washed away in order to prevent any interference with microbes and enzymes that are used in the production phases that follow. In the process of saccharification the sugars in the slurry of hemicelluloses and cellulose are let out after pretreatment. In the process called fermentation the bacteria converts the sugar to churn the biofuel out.

The authors of the study noted that carbon dioxide is generated by the microbes during the process of fermentation. Using the gas for pretreatment thus gives way for green energy source.

Incorporating gaseous CO₂ in this process means there’s no need for a neutralization step, and the pH can be switched on a dime by the addition or release of CO_2. 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.

Blake Simmons, Vice President for Deconstruction, JBEI

Creating a ‘Silver Shotgun’

A lot of effort has been taken in the past by researchers to minimize the amount of energy and handling required for the various stages of the process. Though few other studies concentrate on designing bacteria and enzymes that can endure the ionic liquids, this particular research concentrates on reducing the damaged caused to microbes and enzymes by neutralizing the ionic liquids.

We are continuing to simplify biofuels production, and there are many ways to do this. Our mission at JBEI is to conduct innovative science to develop multiple solutions for bioenergy development that industry can pick from, depending upon which works best for their business model. We’re not creating a silver bullet. We’re creating a silver shotgun.

Blake Simmons, Vice President for Deconstruction, JBEI

The researchers’ use of carbon dioxide steps from their knowledge that when absorbed, the gas will increase the acidity of the liquid while reacting with water. The increasing acidity of the ocean which has resulted due to increase in absorption of CO₂ by ocean water from the atmosphere is based on the same principle.

15 different types of ionic liquids with different concentrations were tested by the researchers. They found cholinium lysinate, which is a mixture of amino acids lysine and choline, to be the most suited for the fermentation and enzyme mixtures available commercially. They tested this with different pressures and concentrations of carbon dioxide.

The ideal range of pH, for microbes and enzymes, was achieved when 145 pounds per square inch of CO₂ was used. By doing this, the scientists were able to get over 83 percent of the theoretical ethanol yield, from the initial level of glucose present in the biomass.

The technique can be implemented in the production of ethanol in the immediate future, confirmed the scientists.

There are many challenges left, but we’re very proud of how quickly this line of research has progressed.

Seema Singh, Director of Biomass Pretreatment, JBEI

The scientists also added that the next action is to use this technique in the production of “drop-in” improved biofuels that can be used as a substitute for diesel, aviation and automotive fuels.

The research was headed by JBEI’s postdoctoral researcher, Jian Sun. N.V.S.N. Murthy Konda, Jian Shi, Ramakrishnan Parthasarathi, Tanmoy Dutta, Feng Xu, and Corinne Scown are the co-authors of the study.

DOE’s Office of Science has supported JBEI.