Encyclopedia of Renewable Energy. James G. Speight
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Название: Encyclopedia of Renewable Energy
Автор: James G. Speight
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119364092
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Ethanol can also be produced by synthesis from the chemical compound ethylene, which is derived from crude oil or natural gas, or by the fermentation of carbohydrates.
Methanol (methyl alcohol, wood alcohol, CH3OH) is mainly manufactured from natural gas, but biomass can also be gasified to methanol. Methanol can be made with any renewable resource containing carbon such as seaweed, waste wood, and garbage. Methanol is stored and handled like gasoline because it is produced as a liquid. Methanol is currently made from natural gas, but it can also be made from a wide range of renewable biomass sources, such as wood or waste paper.
Methanol can be used as one possible replacement for conventional motor fuels and, like ethyl alcohol, has a high octane rating; hence an Otto engine is preferable. If an ignition booster is used, methanol can be used in a diesel engine.
Methanol also offers important emissions benefits compared with gasoline. It can reduce hydrocarbon emissions by 30 to 40% with M85 and up to 80% with M100 fuels. Methanol costs less than gasoline, but has lower energy content. Taking this into account, costs for methanol in a conventional vehicle are slightly higher than those for gasoline.
However, methanol is not miscible with hydrocarbon derivatives and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. Anhydrous ethanol, on the other hand, is completely miscible in all proportions with gasoline, although separation may be effected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol. Also, benzene or Acetone can be used.
See also: P-Series Fuels.
See also: Alcohols.
Algaculture
Algaculture is a form of aquaculture involving the farming of species of algae. The majority of algae that are intentionally cultivated fall into the category of microalgae (also referred to as phytoplankton, microphytes, or planktonic algae).
Macroalgae (seaweed) also have many commercial and industrial uses, but due to their size and the specific requirements of the environment in which they need to grow, they do not lend themselves as readily to cultivation. However, as the algae grow and multiply, the culture becomes so dense that it blocks light from reaching deeper into the water. Direct sunlight is too strong for most algae, which need only approximately 10% of the amount of light they receive from direct sunlight.
Algae can be cultured in open-ponds which are vulnerable to contamination by other microorganisms, such as other algal species or bacteria. Thus cultivators usually choose closed systems for monocultures. Open systems also do not offer control over temperature and lighting. The growing season is largely dependent on location and, aside from tropical areas, is limited to the warmer months.
Open pond systems are cheaper to construct, at the minimum requiring only a trench or pond. Large ponds have the largest production capacities relative to other systems of comparable cost. Open pond cultivation can exploit unusual conditions that suit only specific algae and can also work if there is a system of culling the desired algae and inoculating new ponds with a high starting concentration of the desired algae.
Enclosing a pond with a transparent or translucent barrier effectively turns it into a greenhouse. This allows more species to be grown; it allows the species that are being grown to stay dominant; and it extends the growing season – and if heated, the pond can produce year round.
Algae can also be cultured in a photobioreactor (PBR) which is a bioreactor that incorporates a light source. However, because photobioreactor systems are closed, the cultivator must provide all nutrients, including carbon dioxide. A photobioreactor can operate in batch mode, which involves restocking the reactor after each harvest, but it is also possible to grow and harvest continuously. Continuous operation requires precise control of all elements to prevent immediate collapse. The grower provides sterilized water, nutrients, air, and carbon dioxide at the correct rates. This allows the reactor to operate for long periods.
See also: Algae, Algae Fuels.
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds, which are photosynthetic but lack the many distinct organs found in land plants. Microalgae are organisms that are less than 0.4 mm in diameter and include the diatoms and cyanobacteria and are capable of photosynthesis. Macroalgae are organisms such as seaweed.
Algae are not highly differentiated in the way that plants are, and they lack true roots, stems and leaves, and a vascular system to circulate water and nutrients throughout their bodies. They can exist as single, microscopic cells; they can be macroscopic and multicellular; live in colonies; or take on a leafy appearance as in the case of seaweeds.
The general term algae includes prokaryotic organisms cyanobacteria, also known as blue-green algae, as well as eukaryotic organisms (all other algal species). Algae lack the various structures that characterize land plants, such as leaves, roots, and other organs that are found in vascular plants. Many algae are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus.
The majority of algae live in aquatic habitats and these organisms can thrive in freshwater lakes or in saltwater oceans. They can also endure a range of temperatures, oxygen or carbon dioxide concentrations, acidity, and turbidity. An important contribution of algae to the environment and well-being is the generation of oxygen through photosynthesis.
Algal biofuels are a promising replacement for fossil fuels. All algae have the ability to produce energy-rich oils, and several microalgal species naturally accumulate high levels of oil in their dry mass. Moreover, algae are found in diverse habitats and can reproduce quickly and also efficiently use carbon dioxide. Algae help to keep atmospheric carbon dioxide levels stable by storing carbon dioxide in organic materials that include crude oil and inorganic carbonate rocks. Green algae, diatoms, and cyanobacteria are just some of the microalgal species that are considered good candidates for the production of biofuel.
See also: Algae Fuel, Aquatic Plants, Biomass.
Algae Fuels
Algae fuel is a biofuel that is derived from algae, which are photosynthetic, eukaryotic, plant-like organisms that use chlorophyll in capturing light energy, but lack characteristic plant structures such as leaves, roots, flowers, vascular tissue, and seeds. The production of algae to harvest oil for biofuels has not yet been undertaken on a commercial scale. But algae potentially can be grown commercially in environments such as algae ponds at wastewater treatment plants and the oil extracted from the algae and processed into biofuels. During photosynthesis, algae and other photosynthetic organisms capture carbon dioxide.
The benefits of algal biofuel are that it can be produced industrially, thereby obviating the use of arable land and food crops (such as soy, palm, and canola), and that it has a high yield of oil when compared to other sources of biofuel. Thus, algaculture, unlike food crop-based biofuels, does not entail a decrease in food production, since it requires neither farmland nor fresh water.
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