What is Bioethanol?
What are the benefits of Bioethanol?
Instead of using acid to hydrolyse the biomass into sucrose, we can use enzymes to break down the biomass in a similar way. However this process is very expensive and is still in its early stages of development.
Wet Milling Processes
The fructose and glucose sugars then react with another enzyme called zymase, which is also contained in the yeast to produce ethanol and carbon dioxide.
The chemical reaction is shown below:
The fermentation process takes around three days to complete and is carried out at a temperature of between 250C and 300C.
Fractional Distillation Process
The principle fuel used as a petrol substitute for road transport vehicles is bioethanol. Bioethanol fuel is mainly produced by the sugar fermentation process, although it can also be manufactured by the chemical process of reacting ethylene with steam.
The main sources of sugar required to produce ethanol come from fuel or energy crops. These crops are grown specifically for energy use and include corn, maize and wheat crops, waste straw, willow and popular trees, sawdust, reed canary grass, cord grasses, jerusalem artichoke, myscanthus and sorghum plants. There is also ongoing research and development into the use of municipal solid wastes to produce ethanol fuel.
Ethanol or ethyl alcohol (C2H5OH) is a clear colourless liquid, it is biodegradable, low in toxicity and causes little environmental pollution if spilt. Ethanol burns to produce carbon dioxide and water. Ethanol is a high octane fuel and has replaced lead as an octane enhancer in petrol. By blending ethanol with gasoline we can also oxygenate the fuel mixture so it burns more completely and reduces polluting emissions. Ethanol fuel blends are widely sold in the United States. The most common blend is 10% ethanol and 90% petrol (E10). Vehicle engines require no modifications to run on E10 and vehicle warranties are unaffected also. Only flexible fuel vehicles can run on up to 85% ethanol and 15% petrol blends (E85).
What are the benefits of Bioethanol?
Bioethanol has a number of advantages over conventional fuels. It comes from a renewable resource i.e. crops and not from a finite resource and the crops it derives from can grow well in the UK (like cereals, sugar beet and maize). Another benefit over fossil fuels is the greenhouse gas emissions. The road transport network accounts for 22% (www.foodfen.org.uk) of all greenhouse gas emissions and through the use of bioethanol, some of these emissions will be reduced as the fuel crops absorb the CO2 they emit through growing. Also, blending bioethanol with petrol will help extend the life of the UK’s diminishing oil supplies and ensure greater fuel security, avoiding heavy reliance on oil producing nations. By encouraging bioethanol’s use, the rural economy would also receive a boost from growing the necessary crops. Bioethanol is also biodegradable and far less toxic that fossil fuels. In addition, by using bioethanol in older engines can help reduce the amount of carbon monoxide produced by the vehicle thus improving air quality. Another advantage of bioethanol is the ease with which it can be easily integrated into the existing road transport fuel system. In quantities up to 5%, bioethanol can be blended with conventional fuel without the need of engine modifications. Bioethanol is produced using familiar methods, such as fermentation, and it can be distributed using the same petrol forecourts and transportation systems as before.
Bioethanol Production Ethanol can be produced from biomass by the hydrolysis and sugar fermentation processes. Biomass wastes contain a complex mixture of carbohydrate polymers from the plant cell walls known as cellulose, hemi cellulose and lignin. In order to produce sugars from the biomass, the biomass is pre-treated with acids or enzymes in order to reduce the size of the feedstock and to open up the plant structure. The cellulose and the hemi cellulose portions are broken down (hydrolysed) by enzymes or dilute acids into sucrose sugar that is then fermented into ethanol. The lignin which is also present in the biomass is normally used as a fuel for the ethanol production plants boilers. There are three principle methods of extracting sugars from biomass. These are concentrated acid hydrolysis, dilute acid hydrolysis and enzymatic hydrolysis.
Concentrated Acid Hydrolysis Process The Arkanol process works by adding 70-77% sulphuric acid to the biomass that has been dried to a 10% moisture content. The acid is added in the ratio of 1.25 acid to 1 biomass and the temperature is controlled to 50C. Water is then added to dilute the acid to 20-30% and the mixture is again heated to 100C for 1 hour. The gel produced from this mixture is then pressed to release an acid sugar mixture and a chromatographic column is used to separate the acid and sugar mixture.
Dilute Acid Hydrolysis The dilute acid hydrolysis process is one of the oldest, simplest and most efficient methods of producing ethanol from biomass. Dilute acid is used to hydrolyse the biomass to sucrose. The first stage uses 0.7% sulphuric acid at 190C to hydrolyse the hemi cellulose present in the biomass. The second stage is optimised to yield the more resistant cellulose fraction. This is achieved by using 0.4% sulphuric acid at 215C.The liquid hydrolates are then neutralised and recovered from the process.
Enzymatic Hydrolysis Instead of using acid to hydrolyse the biomass into sucrose, we can use enzymes to break down the biomass in a similar way. However this process is very expensive and is still in its early stages of development.
Wet Milling Processes
Corn can be processed into ethanol by either the dry milling or the wet milling process. In the wet milling process, the corn kernel is steeped in warm water, this helps to break down the proteins and release the starch present in the corn and helps to soften the kernel for the milling process. The corn is then milled to produce germ, fibre and starch products. The germ is extracted to produce corn oil and the starch fraction undergoes centrifugation and saccharifcation to produce gluten wet cake. The ethanol is then extracted by the distillation process. The wet milling process is normally used in factories producing several hundred million gallons of ethanol every Year.
Dry Milling Process The dry milling process involves cleaning and breaking down the corn kernel into fine particles using a hammer mill process. This creates a powder with a course flour type consistency. The powder contains the corn germ, starch and fibre. In order to produce a sugar solution the mixture is then hydrolysed or broken down into sucrose sugars using enzymes or a dilute acid. The mixture is then cooled and yeast is added in order to ferment the mixture into ethanol. The dry milling process is normally used in factories producing less than 50 million gallons of ethanol every Year.
Sugar Fermentation Process The hydrolysis process breaks down the cellulostic part of the biomass or corn into sugar solutions that can then be fermented into ethanol. Yeast is added to the solution, which is then heated. The yeast contains an enzyme called invertase, which acts as a catalyst and helps to convert the sucrose sugars into glucose and fructose (both C6H12O6).
The chemical reaction is shown below: The fructose and glucose sugars then react with another enzyme called zymase, which is also contained in the yeast to produce ethanol and carbon dioxide.
The chemical reaction is shown below:
The fermentation process takes around three days to complete and is carried out at a temperature of between 250C and 300C.
Fractional Distillation Process
The ethanol, which is produced from the fermentation process, still contains a significant quantity of water, which must be removed. This is achieved by using the fractional distillation process. The distillation process works by boiling the water and ethanol mixture. Since ethanol has a lower boiling point (78.3C) compared to that of water (100C), the ethanol turns into the vapour state before the water and can be condensed and separated.
Bioethanol (from sugar and starch crops) On a world level, bioethanol is the most used biofuel. It is produced from sugar-containing agricultural products such as sugar cane (Brazil), corn (United States), wheat, sugar beet, waste from sugar refineries, or sweet sorghum. The predominant technology for converting biomass to ethanol is fermentation, which is a mature bio-chemical technology. In this process, the biomass is decomposed using micro organisms (bacteria or enzymes). Then, yeast converts the sugars present in the biomass to alcohol. Finally, the ethanol is distilled and dehydrated to obtain a higher concentration of alcohol to achieve the required purity to make the bioethanol suitable for the use as automotive fuel. Ethanol is best used in a spark ignition or Otto engine because of its high octane rating, implying very good anti-knock characteristics. However, when applying bioethanol as a petrol substitute, the lower vapour pressure and volumetric energy content (ca. two-third of that of petrol) should be taken into account. Moreover, ethanol is corrosive towards certain kinds of plastics and metals, but this does not cause problems in low-proportion blends with petrol. At present, bioethanol is applied in several EU Member States, however mostly in the form of its derivative ETBE (Ethyl Tertiary Butyl Ether), for example, in Spain and France. The reason for this is to avoid corrosiveness problems as a result of the presence of water in ethanol/petrol blends. ETBE is perfectly mixable with petrol (ETBE is allowed in blends up to 15% in European petrol) and improves the combustion properties of petrol. Bioethanol can be used in blends up to 20% with fossil petrol without necessary engine modifications, but currently only blends up to 5% in European petrol are allowed. In Sweden, a high-proportion bioethanol blend E85 (85% ethanol and 15% petrol) is being used in Flexible Fuel Vehicles (FFVs) with modified engines that are able to run on either E85 or petrol, or any mixture of the two.
Bioethanol (from lignocellulosic biomass) Lignocellulosic or woody biomass is considered a future alternative for the agricultural products that are currently used as feedstock for bioethanol production, because it is more abundant and less expensive than food crops, especially when waste streams are used. Furthermore, the use of lignocellulosic biomass is more attractive in terms of energy balances and emissions. Lignocellulosic biomass consists of three main components, i.e. carbohydrate polymers called cellulose and hemicellulose that can be converted to sugars, and a non-fermentable fraction called lignin that can be utilised for the production of electricity and/or heat. Although the decomposition of the material into fermentable sugars is more complicated, the fermentation, distillation and dehydration process steps are basically identical for bioethanol from either agricultural crops or lignocellulosic biomass. Various process configurations are possible for the production of cellulosic ethanol, however, the most common method combines cellulose hydrolysis and fermentation of five- and six-ringed sugars in the same reactor (Simultaneous saccharification and co-fermentation, SSCF). In a more advanced process called Consolidated Bio-Processing (CBP), which will take long development, enzyme production, hydrolysis and fermentation all take place in the same vessel.
One of the main challenges for the production of ethanol from woody biomass is the development of an efficient pre-treatment process in order to break up the fibre structure of the biomass. There are several methods being developed: mechanical, thermal, chemical and biological processes and combinations thereof. However, none of them has proven to be suitable so far, due to their high costs, low yields, produced waste or undesired by-products. Another important research topic concerns the current high costs and low productivity of the enzyme cellulase, which is needed to convert cellulose to glucose in enzymatic hydrolysis. An alternative, acid hydrolysis, is available but for the hydrolysis of cellulosic materials it is capital intensive and has a negative effect on sugar yield. Although the performance of enzyme cellulase has improved rapidly over the past years, further improvement is still needed. Finally, a suitable organism for the fermentation of five-ringed sugars, which are present in hemicellulose, needs to be found or developed. Recently a breakthrough has been achieved by genetic modification of industrial yeast, but its robustness needs to be improved for industrial process conditions. At present, most research on cellulosic ethanol takes place in the US. There is one pilot production facility, which is located in Sweden.
Another technology for producing bioethanol from woody materials is the gasification of biomass, followed by fermentation to ethanol using anaerobic bacteria. This eliminates the need for hydrolysis to break up the cellulose and hemicellulose fractions of the biomass. Furthermore, in this process also the lignin fraction can be converted into ethanol. The development of this process is still at lab scale. Another process still under development is the gasification of biomass combined with catalytical processes to produce bioethanol, which especially gains more attention in the United States. For gasification to produce bioethanol the gasification process and the development of catalysts especially need more research.
