PVC6: Hydrocarbon fuels from sugars
Isolated sugars, today from crop or starch sources but in the future possibly also from lignocellulosic sources, are the starting point for a number of pathways to biofuels. There are two types of processes, one involving engineered microorganisms and one via aqueous chemical reactions.
Fermentation with engineered microorganisms
The fermentation route involves different developments. Engineered yeasts can be used to ferment sugar into a class of compounds called isoprenoids that have use for pharmaceuticals, nutraceuticals, flavours and fragrances and chemical intermediates, as well as fuels.
One of these isoprenoids is a 15-carbon hydrocarbon, beta-farnesene. It has been hydrogenated to farnesane, a compound accepted for 10 % blending in jet fuel as Synthesized Iso-Paraffinic fuel, (SIP) in the ASTM D7566 standard.
Aqueous phase reforming
Aqueous phase reforming (APR) utilizes heterogeneous catalysts including zeolites, metals and noble metals at temperature and pressure (200 - 250 °C, 3 - 5M Pa) to reduce the oxygen content of the carbohydrate feedstock. This involves hydrodeoxygenation reactions that consume hydrogen simultaneously produced in-situ from the carbohydrate feedstock. The product from the APR step is a mixture of chemical intermediates including alcohols, ketones, acids, furans and other oxygenated hydrocarbons as well as paraffins, which can undergo further catalytic processing using zeolites to generate a mixture of non-oxygenated hydrocarbons.
Platform chemicals
Yet another possibility is to convert the sugars into specific platform chemicals, such as 5-hydroxymethylfurfural (5-HMF), furfural or levulinic acid that can be further upgraded catalytically to fuel blending components or hydrocarbons. However, this pathway is preferably used at present to produce bio-based speciality chemicals rather than fuels.
Technology Readiness Level
Most developments at pilot scale have focused on sugarcane or crop starch, as sugar-containing products produced via enzymatic hydrolysis of lignocellulose contains more inhibitors and C5 sugars. A main supplier of lignocellulosic sugars from soft wood and hard wood is SEKAB in Örnsköldsvik in Sweden, who supplies these sugars to many projects in Europe for their research.
The biological technology that has reached the farthest is the Amyris technology to produce farnesene from sugars[1] a plant in Brazil with a capacity of 40 tonnes/day (some 12 000 tonnes/year or approximately 12 000 toe/year) has been operated and part of the products converted to bio-jet. However, the plant was sold to DSM and the Amyris and DSM activities are now focusing on high-value chemical specialities rather than on fuel at present.
Other developers including Gevo and Butamax have used fermentation technologies to produce iso-butanol at 5 000 tonnes/year (4 000 toe/year), and at 20 tonnes/year (18 toe/year) capacity, respectively. Both companies have plans for scale up at ethanol plants in the USA. Global Bioenergies, a French company, has developed a fermentation technology for producing iso-butene from sugars, and operates a 100 tonnes/year (108 toe/year) demonstration facility in Germany. The company has announced plans for a first industrial prototype in France. REG Life Sciences was pursuing a technology using bacteria to produce fatty alcohols at pilot scale. The company was acquired from REG by Genomatica in 2019, which may shift the product focus away from fuels. Overall this pathway can be said to be in the TRL 5-8 range.
Chemical conversion of sugars to hydrocarbons is pursued by e.g. Virent at up to TRL 5 pilot scale.
Other technology developers include Solazyme, Cozan, Amyris, Codexis, Dyadic, and Novozymes.
Acknowledgement: Large parts of the texts were taken from Lars Waldheim´s contribution to the report “The Contribution of Advanced Renewable Transport Fuels to Transport Decarbonisation in 2030 and beyond”
[1] SGAB Technology status and reliability of the value chains: 2018 Update. 28 December 2018. Ed. I Landälv, L Waldheim, K Maniatis. artfuelsforum.eu/news-articles/updated-sgab-report-technology-status-and-reliability-of-the-value-chains/