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
isbn:
Ultimately, bioenergy production may increasingly occur in biorefineries where transport biofuels, power, heat, chemicals, and other marketable products could all be co-produced from a mix of biomass feedstocks. The link between producing energy and other materials deserves further attention technically and commercially.
Biomass-Derived Carbon
Biomass plays a unique role in the dynamics of carbon flow in the biosphere. Carbon is biologically cycled when plants such as soybean crops convert atmospheric carbon dioxide to carbon-based compounds through photosynthesis. This carbon is eventually returned to the atmosphere as organisms consume the biological carbon compounds and respire. Biomass-derived fuels reduce the net atmospheric carbon in two ways.
First, they participate in the relatively rapid biological cycling of carbon to the atmosphere (via engine tailpipe emissions) and from the atmosphere (via photosynthesis). Second, they displace fossil fuels. Fossil fuel combustion releases carbon that took millions of years to be removed from the atmosphere; combustion of biomass fuels participates in a process that allows carbon dioxide to be rapidly recycled to fuel. The net effect of shifting from fossil fuels to biomass-derived fuels is thus to reduce the amount of carbon dioxide in the atmosphere.
Because of the differences in the dynamics of fossil carbon flow and biomass carbon flow to and from the atmosphere, biomass carbon is often accounted for separately from carbon derived from fossil fuels.
See also: Biomass, Biomass – Pyrolysis.
Biomass – Direct Combustion
Direct combustion of biomass is a thermochemical technique in which the biomass is burned (combusted) in open air or in the presence of excess air. In this process, the photosynthetically stored chemical energy of the biomass will be converted into gases, unless char is also produced.
There are five thermal approaches that are commonly used to convert biomass into a renewable fuel: direct combustion, gasification, liquefaction, pyrolysis, and partial oxidation.
When biomass is heated under oxygen-deficient conditions, it generates synthesis gas that consists primarily of hydrogen and carbon monoxide. This syngas can be directly burned or further processed for other gaseous or liquid products. In this sense, thermal or chemical conversion of biomass is similar to that of coal.
Indoor direct combustion of biomass fuels in unvented cooking and heating spaces has caused considerable health problems to the direct users, primarily the women and children of developing countries. Biomass fuels, when used improperly in this manner, release considerable amounts of toxic or hazardous gases into the unvented area. These gases are typically: carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbon derivatives, organics, aldehydes, and trace amounts of aromatics and ketones. As the moisture content of the wood increases, and as other biomass fuels of lower energy content (such as animal and crop waste) are used, the emissions increase.
When the direct combustion of biomass is conducted in a well-ventilated area, biomass burning used for domestic stoves and boilers can be a sound substitute for combustion of conventional fossil fuel.
Most electrical power generation systems are relatively inefficient, due to the loss of a significant portion of energy, as much as half to two-thirds, in a form of waste heat. If this heat is used efficiently for industrial manufacture, space heating, district heating, or other purposes, the overall efficiency can be greatly enhanced. Therefore, smaller biopower systems are more suitable for cogeneration type of processes than much larger counterparts.
See also: Biomass, Biomass-Chemistry, Biomass – Combustion, Biomass – Gasification, Biomass – Liquefaction, Biomass – Pyrolysis.
Biomass – Drying
Biomass with a high moisture content such as sludge is often used as fuel in power plant without proper drying which reduces the efficiency of the boiler. However, by use of a drying process, moisture can be largely removed from biomass. The result is a reduction in the weight of the biomass. This leads to a reduction of the processing costs, as well as of the costs for storage and transport. The dried end product is frequently used as a plant nutrient or it can be used as a fuel. This, however, depends on the properties of the biomass that needs to be dried.
Using dry fuel in a direct combustion boiler results in (i) improved efficiency, (ii) increased steam production, (iii) reduced ancillary power requirements, (iv) reduced fuel use, (v) lower emissions, and (vi) improved boiler operation. One of the main reasons for these benefits is an increased flame temperature. With wet fuel, some heat of combustion is used to evaporate the water in the fuel. With dry fuel, all the heat of combustion goes into heating the air and products of combustion. As a result, dry fuels have a flame temperature on the order of 1,260 to 1,370°C (2,300 to 2,500°F), while green wood has a combustion temperature of approximately 980°C (1,800°F). In cold climates, the heat of fusion of any ice that may be mixed with the fuel will also have a significant effect on the flame temperature.
Dryers can be broadly divided into two categories based on how heat is provided for drying. In direct dryers, the material gets heat from direct contact with a fluid providing the heat – either hot air or hot steam. With indirect drying, the material being dried is separated from the heat source by a heat exchange surface.
Directly heated dryers can be further divided into two more categories: air and superheated steam dryers (SSDs). In air dryers, hot air is contacted with the material to be dried. The air loses its sensible heat and provides the latent heat of evaporation to dry the material. The air also removes the water vapor that is evaporated. The material can be agitated by some mechanical means or by the flowing air.
In superheated steam dryers, the heating fluid is steam rather than air, but the concept is the same. The superheated steam is contacted with the biomass material and loses some of its sensible heat to provide the latent heat of evaporation to dry the fuel. The steam, however, remains above its saturation temperature, so it doesn’t condense. The water vapor leaving the biomass is heated by the superheated steam, so the net result is a larger amount of steam at a lower temperature than when the steam entered the dryer. The excess steam is removed, and the remainder is reheated and recycled back to the dryer.
In the case of the indirectly heated dryer operated under a vacuum, or with a superheated steam dryer, the latent heat of evaporation of the water vapor is easy to recover because it is not diluted by air. With vacuum drying, this heat is available only at a low temperature, but with superheated steam drying, the dryer can be designed to produce steam at practically any pressure for use in other parts of the plant.
There are several steps to drying. First, the material must be heated from the temperature at which it entered the dryer, up to the wet bulb temperature, to produce a driving force for water to leave the wet material. Next, any surface moisture on the material is evaporated – this occurs fairly quickly. Once all the surface moisture is removed, the material must be heated to drive water from the inside of the biomass to the surface so it can evaporate. This occurs during the “falling rate period” when the rate of drying drops as the material becomes drier. During the falling rate period, the surface temperature of the material remains close to the wet bulb СКАЧАТЬ