Need of biofuel production
In the previous human history the fossil fuels are being used widely in the word by humans for all purposes such as cooking, heating, transportation, construction etc. but with the passage of time different problem arises due to more usage of fossil fuels. Burning of fossil fuels causes air pollution. When they burnt dangerous air pollutant like oxides of nitrogen, carbon go into the air and not only pollute it but also causes climatic changes, acid rain, green house affect , global warming, environmental fluctuations, health issues, increasing oil prices, fossil fuel depletion and many other. In the vehicles, burning of fossil fuel like petroleum, coal etc causes emission of carbon monoxide which is very dangerous for human health as well as for the health of animals and plants. The most important factor which make the scientists to think about another resource in place of these fossil fuels is that these fossil fuels are limited. They are not renewable resources. In fact they are non renewable resources. Therefore it is our urgent need to replace these non renewable resources i.e fossil fuels with that of some renewable resources and to lessen the dependency of humans on fossil fuels. It was estimated globally that almost 12.7 billions tons of oil reservoirs are being utilized which may include individually 32.4% oil, 27.3% coal and peat, and 21.4% natural gas, while biofuels and waste contributed with 10.0%. and in all these scenario transport sector consumed maximum oil reservoirs upto 61.5% of the total. This become an alarming situation for the whole that if these fossil fuels are not controlled for safe using then it may become depleted and all these situations resulted in a wide interest in renewable and environmentally friendly fuels.
First generation biofuels
First generation biofuels are those fuels which are generated from plant resources such as starch, glucose, different types of sugars, fats, vegetable oils etc. The conventional methods are utilized for the production of oil. Bioethanol is very important first generation biofuel which was being produced from starches and sugars. Sugar-based bioethanol are produced in Brazil while Starch-based bioethanol is produced in US China, Canada, France, Germany, and Sweden. In Brazil sugarcane is used as sugar-source while in US starch is consumed from corns and grains. Approximately 21 million ethanol is produced from sugars while almost 60 million ethanol is produced from starches.
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Ethanol production from Grains
Dry milling is very well known process of ethanol production which includes milling and liquefication of starch. The starch is then passed through hydrolysis stage in which starch is degraded into its components i.e glucose sugar. These glucoses monomers are then released into solution where they pass through fermentation in a fermenter by using yeast (Saccharomyces cerevisiae). The end product of this fermentation are ethanol and carbon dioxide. This mixture is then distilled to get pure ethanol. This purified ethanol is then dehydrated and stored to concentrations above 99.7% for fuel applications. In the bottom of the fermenter many solid particles are present which are considered as by product and these are then centrifuged to get some useful product which is reffered to as DDGS (Dried Distilled Grains Soluble) mostly used as protein feed for animals.
Ethanol production from Sugars
Sugar-based ethanol production is done by using sugarcane mostly in Brazil. Sugar in the form of sucrose contains glucose monomers. Glucose monomers are complicated enough that they can not be broken by yeast. So they have to pass through all the stages like water drying in which maximum water is dried from fresh stock up to 60% or more. Drying followed by hydrolysis which breaks down sucrose into simple sugar which is then fermented in the presence of yeast and the product which is obtained is ethanol and carbon dioxide and solid waste. This mixture is then distilled and purified ethanol is obtained. Carbon dioxide is being utilized by plants for the process of photosynthesis. The major difference of this process from that of grains that they include solid waste and waste water which is evaporated and solid waste is dried to get solid fuel which is used commercially.
First generation bioethanol produced from sugarcane replaces about 500,000 barrels of oil per day which is equivalent to approximately 3% of global gasoline usage and 0.7% of crude oil used globally. It is also believed that sugarcane ethanol could help replace 10% of global gasoline usage.
Limitation
Although first generation bioethanol production has many importance in replacing non renewable resources like fossil fuels. But this process is considered cost effective. All the process contains more amount than its commercially value of product or we can say that the profit margin is relatively considered as slow and all these fluctuations can make a pressure on economy process.
If the whole plant is being utilized in production of bioethanol then the feed source for humans becomes limited. Sugarcane is used as staple food crop or highly consuming food crop in many countries. This process also include large surface area for crop growth and sufficient amount for growing perspectives. All these conditions make first generation bioethanol production less familiar.
Second Generation Bioethanol Production
Second generation bioethanol produced from lignocellulosic biomass. Lignocellulosic contains cellulose, lignin and hemicellulose. Cellulose is the main component in plant biomass. Hemicellulose is simpler compound than cellulose and it occurs side by side with cellulose. Lignin is hard organic compound and is insoluble in water. It fills the empty spaces between cellulose and hemicellulose. Collectively cellulose, lignin and hemicellulose comprise plant biomass.
Second generation ethanol processes have technically no issues with feedstock supply. In second generation, inedible portion of food crops or waste is utilized as a feedstock to lessen the limitation of first generation bioethanol. In this perspective bagasse (which is leftover of sugarcane), straw, corn stover, wood waste and municipal and agricultural waste is used for the production of bioethanol. The goal of second-generation biofuel processes is to extend the amount of biofuel that can be produced sustainably by using biomass consisting of the residual non-food parts of current crops, such as stems, leaves and husks that are left behind once the food crop has been extracted, as well as other crops that are not used for food purposes (non-food crops), such as switchgrass, grass, jatropha, whole crop maize, miscanthus and cereals that bear little grain, and also industry waste such as woodchips, skins and pulp from fruit pressing, etc.
Interest in second-generation biofuels (from non-food feedstocks) has been driven by the need to find a broader range of feedstock and to allow production at a much greater scale to provide a greater proportion of future energy needs. The two main avenues for the production of second-generation biofuels are:
- biochemical: using microbes to convert cellulose and hemicellulose in the biomass to sugars for fermentation; and
- thermochemical: applying heat to gasify the biomass into a chemical feedstock that can be re-synthesised into fuels.
For second-generation ethanol, the biomass follows the biochemical pathway of pre-treatment, scarification (to liberate the sugars for fermentation to ethanol) and fermentation.
For the thermochemical pathway, the steps are drying (fresh biomass tends to be about 50% moisture) followed by gasification, conditioning the gas and various synthesis processes. Some of this work is being driven by a desire to reduce greenhouse gas emissions.
Because of the high compression ratio of diesel engines, diesel is a moderately better fuel than petrol with regard to greenhouse gases. The assumptions made in life cycle analyses are important. Some studies of ethanol from grains (including corn) have shown that it is mildly negative in terms of greenhouse gas performance, but most show it is fairly positive. Of particular interest is that “biomass to liquid” can achieve about 90% greenhouse gas reduction, similar to ethanol from wood.
Production of Second Generation Bioethanol requires following conversion technologies:
- Biochemical
- Thermochemical
- Gasification
- Pyrolysis
- Torrefaction
- Hydrothermal
1. Thermochemical
In this process the feed stock is heated at very high temperature in the presence of oxygen. The thermochemical processes resulted in different gases such as hydrogen, carbon monoxide, carbon dioxide, methane, other hydrocarbons, and water. There are also lower temperature processes in the region of 150–374 °C, that produce sugars by decomposing the biomass in water with or without additives.
2. Biochemical
In biochemical conversion technology, application of chemicals are being made necessary. Induction of chemicals to feedstock is basically way to hydrolyze the feedstock into its components i.e separation of cellulose, lignin and hemicellulose. After separation it becomes easier to produce alcohol from cellulose after fermentation process in the presence of yeast.
3. Gasification
Gasification is a process mainly based on heating at a very high temperature without combustion or in the absence of oxygen. Gasification is done basically for crude oil and coal or for the type of feedstock of agricultural waste, forestry, waste wood, energy crops and black liquor.
4. Pyrolysis
Pyrolysis is a well established technique for decomposition of organic material at elevated temperatures in the absence of oxygen. It is used to dissolve components of agricultural residues, forests and energy wood crops etc.
5. Torrefaction
Torrefaction is a form of pyrolysis at temperatures typically ranging between 200–320 °C. Feedstocks and output are the same as for pyrolysis.
6. Hydrothermal
Hydrothermal process includes feedstock in wet form. Temperature requires is less than pyrolysis but more than atmospheric temperature that is up to 400°C. The capability to handle a wide range of materials make hydrothermal liquefaction viable for producing fuel and chemical production feedstock.
Summary of Process
In Second generation, feedstock is first of all is dried up to 60% or more than it is grinded to fine powder in different forms of grinder machines that is vary from specie to specie. Then it is sieved to a very specific size. Then its is being transported to the factories, industries, where they are hydrolyzed and fermented in the presence of Yeast. After Fermentation the mixture is distilled and dried to get maximum concentration of bioethanol. Waste water is being evaporated. While other waste material is centrifuged and this time solid fuel may be obtained which is used commercially. It must be kept in mind that during transportation straws must not be transported because during transportation straws can go into environment and can cause air pollution. While storage it must be carefully stored in the form of straws. Large components are difficult to store as they can be damaged by some fungus.
Limitation
One of the major challenges of the lignocellulosic ethanol processes is obtaining sufficiently high sugar concentrations after the hydrolysis. This method is also considered very expensive and many cares must be taken into consideration. It is estimated that cost which is being applied in taking the feedstock, drying hydrolysis, transportation is much more than the product made. So this not considered as a reliable source for energy production tool worldwide.
References
- https://s3.amazonaws.com/academia.edu.documents/41536750/Thermophilic_ethanologenesis_future_prospects_for_second_generation_bioethanol_production.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1550861023&Signature=iDh6M7zUbAG7ampsm8a5ikErgI8%3D&response-content-disposition=inline%3B%20filename%3DThermophilic_ethanologenesis_future_pros.pdf
- http://www.redd-pac.org/papers/others/havlik_biofuels_energy_policy_2011.pdf
- https://en.wikipedia.org/wiki/Second-generation_biofuels
- https://ac.els-cdn.com/S0960852414001527/1-s2.0-S0960852414001527-main.pdf?_tid=5b2a1469-fc4b-4d23-8a43-83e850288c0c&acdnat=1550857474_60ead482f2a0f101df16b93c93532293
- http://biofuel.org.uk/first-generation-biofuels.html
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