Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ can produce up to 60% of their biomass in the form of oil. Because the cells grow in aqueous suspension where they have more efficient access to water, carbon dioxide, and dissolved nutrients, microalgae are capable of producing large amounts of biomass and usable oil in either high rate algal ponds or photobioreactors. This oil can then be turned into biodiesel. In fact, seaweeds, which are macroscopic, multicellular marine algae, may offer a particular useful source of biofuels, since they lack lignin and likewise do not require land, fresh water, or fertilizer. One complication is that approximately one-third of the sugars in seaweed take the form of alginate and microbes have not been able to convert it into ethanol.

      In the process of oil production, the algae is harvested from the growing process as algae paste after which water is removed by heat drying or de-watering presses. Centrifuges are also another way in which the algae past can be de-watered. The finished product is algae oil in a form that is then suitable for use in the transesterification process to make biodiesel fuel.

      The production of biofuel from algae does not reduce atmospheric carbon dioxide (CO₂), because any carbon dioxide taken out of the atmosphere by the algae is returned when the biofuels are burned. They do however eliminate the introduction of new carbon dioxide by displacing fossil hydrocarbon fuels. Also, algal fuels do not affect fresh water resources and they can be produced using ocean and wastewater. Algal fuels are also biodegradable and relatively harmless to the environment if spilled.

      Open-pond systems for the most part have been given up for the cultivation of algae with high-oil content. Open systems using a monoculture are also vulnerable to viral infection. The energy that a high-oil strain invests into the production of oil is energy that is not invested into the production of proteins or carbohydrates, usually resulting in the species being less hardy, or having a slower growth rate. Algal species with lower oil content, not having to divert their energies away from growth, have an easier time in the harsher conditions of an open system.

The preference toward microalgae is due largely to its less complex structure, fast growth rate, and high oil content (for some species). Some commercial interests into large scale algal-cultivation systems are looking to tie in to existing infrastructures, such as coal power plants or sewage treatment facilities. This approach not only provides the raw materials for the system, such as carbon dioxide and nutrients; it also changes those wastes into resources. However, there is interest in using seaweed for biofuels, probably due to the high availability of this resource.

      Algae fuel is also very appealing in terms of its emissions as well. The combustion of algae fuel produces less carbon monoxide, unburned hydrocarbons, and harmful pollutants compared to crude oil-derived diesel fuel as well as emits no sulfur oxides. Replacing fossil fuels with algae could substantially reduce carbon dioxide emissions.

      In addition, microalgae has a strong impact on wastewater, and systems for producing microalgae have the ability of being able to use saline waste, as well as carbon dioxide streams, as an energy source. This is because the algae from microalgae bioreactors is capable of capturing organic compounds and heavy metal contaminants in wastewater. As a result, the production of algae has the side effect of being able to recycle formerly unusable water. In fact, not only does this process clean waste water, but it also recovers phosphorus from the waste water. Phosphorus is a highly limited resource, so much so that the last reserves of phosphorus are estimated to have already been depleted.

      Biogasoline is gasoline produced from biomass such as algae. Like traditionally produced gasoline, the constituents contain between 6 carbon atoms (hexane, C₆H₁₄) and 12 (dodecane, C₁₂H₂₆) carbon atoms per molecule and can be used in internal-combustion engines. Biogasoline is chemically different from biobutanol and bioethanol, as these are alcohols, not hydrocarbons.

      See also: Algae, Aquatic Plants, Biomass.

      Algae Fuels – Extraction

      Algae fuels can be recovered by three processes: (i) physical extraction or (ii) chemical extraction, and (iii) enzymatic extraction.

      In the first step of physical extraction, the oil must be separated from the rest of the algae. The simplest method is mechanical crushing. When algae are dried it retains its oil content, which can then be recovered using an oil press. Many commercial manufacturers of vegetable oil use a combination of mechanical pressing and chemical solvents in extracting oil. Since different strains of algae vary widely in their physical attributes, various press configurations (such as the screw, expeller, and piston configurations) work better for specific algae types. Often, mechanical crushing is used in conjunction with chemical solvents.

      Ultrasonic extraction is a type of physical extraction that can greatly accelerate extraction processes. Using an ultrasonic reactor, ultrasonic waves are used to create cavitation bubbles in a solvent material. When these bubbles collapse near the cell walls, the resulting shock waves and liquid jets cause those cells walls to break and release their contents into a solvent. Ultrasonication can enhance basic enzymatic extraction. The combination sono-enzymatic treatment accelerates extraction and increases yields.

      In chemical extraction, chemical solvents are often used in the extraction of the oils. A common choice of chemical solvent is hexane, although other hydrocarbon solvents can also be used. Another method of chemical solvent extraction is Soxhlet extraction in which oils from the algae are extracted through repeated washing, or percolation, with an organic solvent, under reflux in specialized equipment. The value of this technique is that the solvent is reused for each cycle.

      Supercritical carbon dioxide can also be used as a solvent. In this method, carbon dioxide is liquefied under pressure and heated to the point that it becomes supercritical (having properties of both a liquid and a gas), allowing it to act as a solvent.

      Enzymatic extraction uses enzymes to degrade the cell walls with water acting as the solvent. This makes fractionation of the oil much easier. The enzymatic extraction can be supported by ultrasonication.

      See Algae, Algae Fuels, Aquatic Plants, Biomass.

      Algal Blooms

Algal blooms refer to the rampant growth of certain microalgae, which in turn leads to the production of toxins, disruption of the natural aquatic ecosystems, and increases the costs of water treatments. The blooms take on the colors of the algae contained within them. Graham states that the main toxin producers in oceans are certain dinoflagellates and diatoms. In freshwaters, cyanobacteria are the main toxin producers, though some eukaryotic algae also cause problems. Under natural conditions, Graham notes that algae use the toxins to protect themselves from being eaten by small animals and only need a small amount to protect themselves.

      The main cause of algal blooms is nutrient pollution in which there is an excess of nitrogen and phosphorus, which can push algae toward unrestrained growth. The phenomenon is caused by a variety of human activities in which the fertilizers used in agriculture and animal manures are rich in nitrogen, while improperly treated wastewater is high in both nitrogen and phosphorus.

      Alkali Washing Process

      Alkali washing (often referred to as caustic scrubbing) for hydrogen sulfide removal with caustic scrubbing is only economical in small amounts if hydrogen sulfide is present and suitable means of disposing the spent solution are available. The chemistry is simples and to some extent, depending upon the concentration of hydrogen sulfide in the gas stream, efficient. Thus:

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