PVC5: Alcohol fuels from sugars
Fermentation of cellulosic sugars to ethanol
Cellulosic ethanol is chemically identical to first generation ethanol (i.e. CH3CH2OH). However, it is produced from different raw materials via a more complex process (cellulose hydrolysis).
In contrast to first generation bioethanol, which is derived from sugar or starch produced by food crops (e.g. wheat, corn, sugar beet, sugar cane, etc.), cellulosic ethanol may be produced from agricultural residues (e.g. straw, corn stover), other lignocellulosic raw materials (e.g. wood chips) or energy crops (miscanthus, switchgrass, etc.).
The first step in the processing of lignocellulosic feedstocks to ethanol is a pre-treatment consisting of a physico-chemical step and an enzymatic liquefaction step, and fractionates the feedstock into its three main components (cellulose, hemicellulose and lignin). The most common method is the steam explosion with or without an acid catalyst but also acid and base treatment and organosolv processes have been or are in use. The nature of the pre-treatment has large impact on the accessibility of the still (partially) crystalline, de-lignified cellulose for saccharification while hemicellulose is mostly hydrolysed to sugars and oligomers and dissolves at this stage. Depending on the pre-treatment used, different amounts of inhibitors are formed that can be detrimental both for enzymatic hydrolysis performance and for yeast fermentation. A detoxification step might be necessary, although it is nowadays generally avoided under demo and pre-commercial scale.
After hydrothermal treatment, additional water is added to the mixture of solids and liquids resulting from the pre-treatment after which hydrolysis and saccharification of the cellulose and hemicelluloses oligomers take place. This step uses specifically developed enzyme cocktails, but also acid hydrolysis has been used. The enzyme treatment results in a pumpable slurry and unhydrolysed solids. After separation of solids and liquids, the slurry is either fermented in the same vessel (Simultaneous Saccharification and Fermentation (SSF)) or in a downstream fermenter (Separate Hydrolysis and Fermentation (SHF)). Lignin is separated before or after fermentation and usually dried to be used as a fuel for the process and/or for power generation. The cellulose- and hemicellulose-derived C6 sugars are fermented by yeast strains derived from traditional yeasts used for the production of wine, beer or bread, while for the fermentation of C5 sugars genetically modified yeasts have been developed in the recent years. After the fermentation has been finalized, the ethanol is recovered by distillation and dehydration as described for sugar and starch ethanol in EVC 3.
There is a complex trade-off between the water addition, the viscosity, the enzyme consumption, the ethanol concentration achievable and the possible inhibition of the ethanol and the energy required for the downstream processing. At present, the technology can give up to 300 liters of ethanol per tonne of agricultural waste of which a significant part is derived from the C5 sugars, i.e. an efficiency to biofuels in the order of 30 %.
Lignocellulosic ethanol is an advanced biofuel in the EU.
There are, and also have been, a number of developments worldwide for lignocellulosic (i.e. second generation) ethanol, many of which have reached demonstrations at TRL 6. However, only a few of these developments have reached industrial scale at TRL 8. In the last decade, six plants at industrial scale (25 000 - 90 000 tonnes/year product capacity each or 13 000 - 45 000 toe/year (toe, tonnes of oil equivalents) have been built, one in the EU (Beta Renewables), two in Brazil (Raizen, Granbio) and three in USA (Abengoa Hugoton, Dupont and POET Liberty). However, the Abengoa and Dupont plants are closed, the POET plant is back to R&D work, and the Beta Renewables plant was closed ut has been taken over by Versalis who has announced that operations will be resumed. The Beta Renewables and the Granbio plants are based on the same technology but different feedstocks, agricultural residues and sugar cane bagasse, respectively.
A number of smaller facilities is operational in Europe, with capacities ranging from 100 – 19 000 tonnes/year, including Borregaard Industries (Sarpsborg, Norway, running since the 1930s), Domsjoe Fabriker (Örnsköldsvik, Sweden), St1 (Kajaani, Finland), Chempolis (Oulu, Finland), Clariant (Straubing, Germany), IFP (Bucy-Le-Long, France), SEKAB (Örnsköldsvik, Sweden) and AustroCel Hallein (Hallein, Austria).
Furthermore, there is one plant in construction in the EU by Clariant, and in India the government has instructed oil companies to invest in twelve plants, of which a few plants are already under construction and others are in different stages of planning. These projects are based on both Indian (Praj, IBC) and foreign technologies. Announcements have been made for further plants based on the technologies of Clariant, St1, SEKAB, and others. More details are provided in the report “Current Status of Advanced Biofuels Demonstrations in Europe”, published by ETIP Bioenergy in March 2020.
Fact Sheet: Ethanol
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.
Some bacteria naturally produce butanol and yeast can be engineered to produce butanol instead of ethanol. This pathway can be used for producing both n-butanol and iso-butanol, the latter also having a high value as a chemical building block.
Another development is to use an engineered microorganism to produce iso-butene that can be the basis for chemicals but also oligomerized and hydrogenated to e.g. gasoline. Since iso-butene is a gas that separates from the broth, this facilitates the product separation and upgrading, as well as limiting any product inhibition issues.
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”
 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/