Bioethanol
Bioethanol (ethanol derived from agricultural feedstock) has been a commercial fuel since as far back as 1931. While the US and European gasoline markets incorporate ethanol as a blending component, South Africa has traditionally used it as an industrial solvent and an additive in alcoholic beverages. This ethanol is sourced from food crops, including sorghum, maize, wheat, barley, and sugar cane. Countries like Brazil, the United States, and Australia are leaders in bioethanol fuel usage and dominate bioethanol production, using sugar cane and maize as their production feedstocks.
Ethanol plays a dual role in the fuel industry. First, it serves as an “oxygenated additive” which helps fuel burn more cleanly and efficiently. Secondly, and perhaps more crucially in today’s environmentally-conscious world, ethanol serves as a substitute for a portion of the fossil fuel component in vehicle fuels. By doing so, it diminishes our reliance on non-renewable resources, paving the way for a more sustainable energy future.
Not all additives are as beneficial as ethanol. Some refiners use methyl tertiary-butyl ether (MTBE) to achieve similar effects. However, MTBE has shown to contaminate groundwater, leading to serious environmental and health concerns, especially in the US. As a result, its use is on the decline, with many turning to the more sustainable and eco-friendly ethanol.
Adding to ethanol’s appeal is its high octane rating: a higher octane rating means the fuel can withstand more compression before igniting, allowing engines to run more efficiently and extract more energy from each drop of fuel.
Conceptual outline of the sorghum-to-bioethanol process:
Sorghum-to-bioethanol process explained:
Feedstock undergoes a size-reduction step, ensuring that it is more manageable, which optimises the efficiency of ethanol production. Specifically, agricultural residues (leftover plant materials) are finely ground to achieve consistency in particle size.
After undergoing a quality control check, this biomass moves forward to the starch conversion phase. Any airborne particles or ‘dust’ generated during these processes are captured and contained using a filtration system.
By introducing dilute sulfuric acid to the biomass, a chemical reaction called hydrolysis takes place. Hemicellulose (one of the components of the biomass, comprised of complex sugar chains) gets broken down, releasing simpler sugar forms. These include five-carbon sugars (like xylose and arabinose) and six-carbon sugars (such as mannose and galactose). A small fraction of another component, cellulose, gets converted into a simple sugar called glucose during this phase.
The process further requires special enzymes, called cellulase enzymes, to break down the cellulose in the biomass. These enzymes can be grown specifically for this purpose or might be sourced from commercial enzyme companies.
During the cellulose hydrolysis step (also termed cellulose saccharification), the remaining cellulose in the biomass is further broken down, this time into glucose. This is achieved using the cellulase enzymes mentioned earlier. As these enzymes act on the cellulose, they release glucose. This method is both energy-efficient and produces a high glucose yield.
Glucose, derived from the biomass conversion process, undergoes fermentation to be transformed into ethanol. Fermentation involves chemical reactions, primarily facilitated by yeast or bacteria, that convert sugars into ethanol and carbon dioxide. Simply put, these microorganisms “eat” the sugars and produce ethanol as a result.
The hemicellulose component of biomass is notably rich in five-carbon sugars, commonly referred to as pentoses, with xylose being the most prevalent. In the specific process of pentose fermentation, xylose is acted upon by certain bacteria, such as Zymomonas mobilis, or other specially engineered bacteria to produce ethanol.
The end product of both glucose and pentose fermentation is termed “ethanol broth.” To extract pure ethanol, it is separated from other components in the broth. Following this, a dehydration process ensures the removal of any residual water, yielding pure ethanol.
The plant employs a sophisticated multi-pressure distillation system, optimised for energy efficiency. This design ensures economical utilisation of heat energy, subsequently lowering steam consumption. A unique feature of this system is its multiple distillation columns, each operating at varied pressure levels. This configuration allows one column to be heated using the vapours from another, streamlining the process and enhancing efficiency.
Following distillation, hot silage is temporarily housed in an intermediate storage tank. From this holding, it is directed to decanters where the process of solid separation occurs, effectively removing and isolating these solids. These extracted materials are then blended with concentrated silage in preparation for the drying phase, which takes places in a rotary drum dryer fueled by natural gas.
After drying, the product is carried by conveyor to the storage area or pelletizer. Here, this dried yield, known in industry terms as DDGS (Distiller’s Dried Grains with Solubles), undergoes conditioning through the introduction of steam and water in specific humidifiers. Following intensive mixing, the humid DDGS is pressed into pellets. These pellets are cooled and transported, finally reaching a dedicated location either for direct client collection or for onward distribution.
