HOW WE MAKE ETHANOL FROM CELLULOSE
The process developed by SEKAB E-technology consists primarily of four steps: pretreatment, hydrolysis, fermentation and distillation.
When the raw material comes to the facility it is taken to a receptacle in which it is pretreated with acid and steam at 200 degrees Celsius. This releases sugars from the hemicellulose. The process converts the material into a porridge-like so-called slurry. In the next step the acid in the slurry is nutralised so that the environment is right for subsequent reactions.
The mixture is then transferred to another vessel and enzymes, yeast and water are added. The enzymes break down the cellulose into sugars. Yeast ferments the sugar from both the hemicellulose and the cellulose into ethanol.
To access the ethanol the entire slurry is distilled and the ethanol is procured, cleaned and dewatered. What remains in the slurry is mainly solid lignin which is filtered off and dried. The lignin can be used as biofuel or refined into other products. Remnants in the form of dissolved substances remain in the water. These are retted using bacteria into biogas. It can be used to provide the plant with energy, or refined and sold.
The E-tech process – how we make ethanol from cellulose
In the first stage the raw material is pretreated with diluted acid (sulfuric acid or sulfur dioxide) and heat, which converts the hemicellulose into sugar.
Hemicellulose is more easily accessible and reactive than the long cellulose molecules and is composed of different monosaccharides, mainly mannose, glucose, xylose and arabinose. The pre-treatment liberates sugars and makes the cellulose available for enzymatic hydrolysis.
In the second step enzymes are added that hydrolyse the cellulose. Since cellulose is composed of glucose molecules that are linked together what remains after the process is glucose, i.e. ordinary sugar. This can be fermented into ethanol.
Simultaneous hydrolysis and fermentation (SSF)
SSF is an acronym for Simultaneous Saccharification and Fermentation, i.e. simultaneous saccharification with enzymes and fermentation in the same vessel.
To avoid having to separate the sugar solution from the solids before fermentation, one can let the hydrolysis and fermentation take place simultaneously. It gives higher alcohol content, but at the same time imposes higher demands on the yeast used in the process since the environment is inhospitable.
SSF and SHF
SSF and SHF are variants of the cellulosic process.
SHF is an acronym for Separate Hydrolysis and Fermentation. The method requires that the saccharification with enzymes and the fermentation of the released sugars takes place in two separate steps.
SEKAB E-technology masters both variants and their variations. Both techniques have their advantages and disadvantages depending on which raw materials one uses and how the co-products (lignin and biogas) are used. The choice is therefore specific to each plant.
Fermentation is a biological process in which microorganisms incompletely oxidise organic compounds. In this case, different sugars are converted by yeasts into ethanol.
Common yeast (baker’s yeast) converts 6-carbon sugar, hexose, rather than other sugars. Hemicellulose from agriculture and deciduous trees also contains a range of 5-carbon sugars, pentoses. It is therefore desirable that the yeast used can also convert these into ethanol.
SEKAB E-technology has collaborated with Taurus Energy to verify that their special yeast strains also function on an industrial scale. These yeast strains are capable of fermenting both pentoses and hexoses.
How we make use of the products
The ethanol is distilled
After fermentation the mixture is distilled and the ethanol can be purified and dewatered, sold or further processed.
Cellulosic raw material comprises largely of lignin, a substance that does not respond to saccharification in the process. It is separated by filtration, either before or after fermentation and distillation. Thereafter the lignin is washed and dewatered to a moisture content of about 50 percent.
Lignin has high energy content and can be used as a solid biofuel to provide energy to the plant. It can also be converted into various products such as polymers, adhesives and other products; this is an area in which there is ongoing research worldwide.
After distillation and filtration some dissolved substances remain in the process water that can be retted and converted into biogas using various microorganisms. The biogas can be used either to provide energy to the process, or it can be further refined into vehicle gas. E-Tech collaborates with researchers and commercial suppliers in the field to optimise biogas production from the cellulosic process.
This is how we make cellulosic ethanol by enzymatic hydrolysis
The raw material comes into the plant and is screened. A powerful fan blows it up to the roof via a pipe system
Hot steam is added to preheat the raw material and expel the air
Hemicellulose, which comprises various sugars is dissolved in acid at high temperature, 170-200 ˚ C
- Enzymatic Hydrolysis
Enzymes are added that separate the sugars from the cellulose.
Yeast is added and the dissolved sugar is converted to ethanol. Steps 4 and 5 take place in the same vessel.