Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 72

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.

Algae 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.