Bio-based Isobutanol — a versatile, viable next generation biofuel

From fuel blendstock to operations to infrastructure compatibility, Isobutanol is a cost effective, drop-in renewable alternative with broad optionality for use in fuel and chemicals

Richard Kolodziej, Wood Group Mustang
Jeff Scheib, Gevo Inc

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Article Summary

The success of any new industry is its ability to innovate and grow; relative to renewable fuels, future growth may require the evolution away from first-generation products, such as ethanol and/or biodiesel, to next-generation products, such as isobutanol.

Isobutanol, a form of biobutanol, has many outstanding characteristics that allow it to:
• Be used “as Is”, as either a solvent or as a gasoline blendstock
• Be readily converted, through known processes, to a variety of hydrocarbons for use in the petrochemical and/or refining industries
• Be efficiently and effectively used in existing production, distribution, marketing and end-user assets

This article will highlight the technology, the feedstocks and the market-growth opportunities for isobutanol, by reviewing Gevo, Inc. a company bringing isobutanol and/or its derivatives to market beginning in 2012.

Technology Pathway – Bio-isobutanol via an integrated fermentation/separation process
The pathway to make bio-isobutanol is via fermentation, paired with using an integrated separation technology to optimise production. This approach, developed over the past seven years, has been successfully proven at bench scale, at a pilot plant and a 1.0 MMGPY demonstration plant. In May 2012, Gevo announced that it started up the world’s first commercial bio-based isobutanol production plant in Luverne, Minn, a planned 18 MMGPY facility.

Bio-isobutanol fermentation is quite similar to the existing ethanol process; ethanol plants can be repurposed to make isobutanol relatively easily and cost effectively with two key modifications:
1.  Modified Biocatalyst – Isobutanol is a naturally occurring product of the fermentation process, found in many items such as bread and scotch whiskey; however, it’s commercial use to date has been limited. However, through innovations in microbiology/biochemistry, traditional yeasts have been modified, which make possible a much higher selectivity in producing isobutanol (i.e., turn up the yeast’s ability to make isobutanol while also limiting the ethanol production pathway).

2. Unique Proprietary Separation – As the isobutanol is produced, a stream is taken from the fermentation broth where the isobutanol is removed and the remaining broth returned for further conversion. This has the effect of keeping the isobutanol concentration below the biocatalyst toxicity level, but allows improved conversion.

With mainly just these two additions to existing facilities, one can see how the project completion time and CAPEX to make bio-isobutanol can be significantly lower than having to build a greenfield plant. A plant conversion can nominally be 20-40% of the CAPEX of a greenfield bio-isobutanol plant. As fermentation ethanol plants have been shutdown or underutilised due to recent, poor economics (i.e. the ethanol subsidy is now gone, and the regulation “blend wall” has effectively been reached), the ability to re-purpose these plants to isobutanol becomes an attractive opportunity.

Upon fermentation plant conversion, the plant capacity will be ~80% on a volumetric product yield basis (compared to ethanol), but comparable on an energy equivalent basis (isobutanol contains more energy than ethanol). So, the utility requirements and OPEX are comparable to ethanol production (which again, limits CAPEX requirements.)

There is over 20 BGPY of existing fermentation ethanol capacity in the world, mostly in North and South America. Gevo is a leading company in bio-isobutanol, and its plans for the foreseeable future are to convert ethanol plants to make isobutanol. Gevo’s business model is based on the flexibility to buy the ethanol plant assets, JV with the current plant owner for the conversion or license the isobutanol production technology to ethanol plant owners.

Figure 1 illustrates the isobutanol plant conversion. The “before” picture is a photo of Gevo’s Luverne, Minn. facility while it was a 22 MMGPY ethanol plant. The “after” picture is photo of the plant as it has been repurposed to make up to 18 MMGPY of isobutanol.  Gevo’s proprietary separation package addition (the second required addition noted above) is trademarked as GIFT®, which stands for Gevo Integrated Fermentation Technology®, a modular unit which ties into the fermentation and distillation processes.

Gevo’s fermentation process is designed to convert feedstocks of all types: grains, sugar cane, non-food-based materials and/or cellulosics. Basically, anything that can be converted to a fermentable sugar can potentially be used, whether it be a traditional C6 sugar, such as glusoce, or a C5 sugar, such as pentose. The issue of feedstock selection is one of economics, but Gevo’s isobutanol pathway technology can be put into yeasts that can digest C6 or C5 sugars. In fact, at bench scale, they have produced cellulosic isobutanol, using a mixed stream of C5 and C6 sugars.

Bio-isobutanol has Versatility
One of the main reasons that converted plants have such good projected economics is that bio-Isobutanol is so versatile as a platform molecule. In the chemicals arena, it can be: sold as solvent product itself (e.g., paints) and/or, through dehydration to isobutylene, converted into materials such as butyl rubber and paraxylene and other derivatives for use in market applications such as tires, plastic bottles, carpets and clothing. For fuels applications, isobutanol can be blended in as a low vapour pressure gasoline component, and/or used as feedstock 
to make other transportation fuels (e.g., iso-
paraffinic kerosene for use as bio-jet) or 
other renewable products (e.g., renewable heating oil).

Bio-isobutanol — the Gasoline Blendstock
Bio-Isobutanol’s properties as a gasoline blendstock can best be understood by comparing some of the blending properties to ethanol and 
alkylate. Figure 2 summarises some key aspects in the comparison.

Compared to ethanol, isobutanol has a much lower RVP and about 30% more energy content.  The Blend Octane of isobutanol is high as well (although slightly lower than ethanol). Isobutanol also has less oxygen content than ethanol, so more isobutanol can be blended into gasoline for a given oxygen content. And, more blend volume plus more energy content means more renewable identification number (RIN) generation. See Figure 3 for a RIN comparison summary.



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