The process developed by Sekab E-Technology consists mainly of four steps: pre-treatment, enzymatic hydrolysis, fermentation and reprocessing.
When the raw material has come into the plant, it is transferred to a vessel where it is pre-treated with acid and steam at a temperature of 200 degrees. This releases sugars from the hemicellulose. The process converts the material into a porridge-like slurry. In the next step, the acid in the slurry is neutralised so that the environment is right for later reactions.
The mixture is then transferred to another vessel and enzymes. The enzymes break down the cellulose into sugar. The sugar can be separated and used in other process solutions. If ethanol or any other chemical using yeast is to be produced, yeast and perhaps water are added. The yeasts ferment the sugar from both the hemicellulose and the cellulose into ethanol or other chemicals.
To access the ethanol, the entire slurry is distilled and the ethanol is taken care of, purified and dewatered. What remains in the slurry is mostly solid lignin which is filtered off and dried. The lignin can be used as biofuel or processed into other products.
In the remaining water there are residues in the form of solutes. These are broken down into biogas with the aid of bacteria. It can be used to supply the plant with energy or refined and sold on.
In the first step, the raw material is pre-treated with dilute acid (sulphuric acid or sulphur dioxide) and steam up to about 170 – 200 degrees depending on the raw material, which converts hemicellulose into sugar.
Hemicellulose is more readily available and reactive than the long cellulose molecules and is composed of various monosaccharides, mainly mannose, glucose, xylose and arabinose. The pre-treatment releases sugar from the hemicellulose and makes the cellulose available for enzymatic hydrolysis.
In the second step, enzymes are added which hydrolyse the cellulose to sugars. Since cellulose mainly consists of linked glucose molecules, glucose, i.e. ordinary sugar, remains after the process. This can be fermented into ethanol.
SSF and SHF are variants of the enzymatic hydrolysis process.
Simultaneous hydrolysis and fermentation (SSF)
SSF is an abbreviation for Simultaneous Saccharification and Fermentation, i.e. simultaneous saccharification with enzymes and fermentation in the same vessel.
In order to avoid having to separate the sugar solution from the solid material before fermentation, one can allow hydrolysis and fermentation to proceed simultaneously. This produces higher alcohol content, but at the same time puts higher demands on the yeast used in the process, since the environment is inhospitable.
Separate Hydrolysis and Fermentation (SHF)
SHF is an abbreviation for Separate Hydrolysis and Fermentation. The method means that the saccharification with enzymes and the fermentation of released sugar takes place in two separate steps.
For those customers who want to make other chemicals from the sugar, SHF can be advantageous as one acquires a sugar stream, which can then be further processed, e.g. concentrated, purified or some other process step.
SEKAB E-Technology masters both versions and their different variations. Both techniques have their advantages and disadvantages depending on what the sugar is to be used for, which raw materials are to be used and how the co-products (lignin and biogas) are to be 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 to ethanol or other chemicals by yeast fungi.
Ordinary yeast (baker’s yeast) converts six-carbon sugars, hexoses, rather than other sugars. Hemicellulose from agriculture and deciduous trees also contains several different five-carbon sugars, pentoses. Consequently, it is desirable that the yeast can also convert them into ethanol.
SEKAB E-technology has collaborated with Taurus Energy and other yeast manufacturers to verify that their special yeast strains also function on an industrial scale. These yeast strains are capable of fermenting both hexoses (six-carbon sugar) and pentoses (five-carbon sugar).
The ethanol is distilled
After fermentation, the mixture is distilled and the ethanol can be purified and dewatered, sold or refined.
Filtration/Lignin separation
Cellulose-containing raw materials consist largely of lignin, a substance that cannot be broken down into sugar in the process. It is separated by filtration either before or after fermentation and distillation. Then the lignin is washed and dewatered to a water content of about 50 per cent.
Lignin has a high-energy content and can be used directly as solid biofuel to provide energy at the plant. It can also be refined into various products, such as polymers, adhesives and more, an area in which research is being carried out all over the world.
Biogas
After distillation and filtration, some solutes remain in the process water that can be broken down to biogas with the help of various microorganisms. The biogas can either be used to provide energy in the process, or further refined to compressed natural gas. E-tech collaborates with researchers and commercial suppliers in the field to optimise the production of biogas from the cellulose process.