Encyclopedia of Renewable Energy - James G. Speight

Скачать книгу

endothermic steam-carbon reaction is primarily responsible for cooling effects in furnaces and the presence of moisture is believed to cause heat generation at the surface of the bed and in the combustion by virtue of the (endothermic) formation of carbon monoxide and hydrogen in the bed which then burn at the surface. On the other hand, the presence of water vapor appears to assist in the formation of carbon dioxide:

images

      Moisture appears to play a more integral role in the combustion of hydrogen-deficient carbonaceous fuels (such as coal) than has been generally recognized. The carbon-steam reaction to produce carbon monoxide and hydrogen (which are then oxidized to the final products) is an important stage in the combustion sequence as is the carbon monoxide shift reaction to yield carbon dioxide and hydrogen:

images

      Since the whole system involves reaction (and equilibria) between the fuel (i.e., carbon), water, carbon monoxide, hydrogen, and carbon dioxide, the rapid rates of the reactions render it difficult (if not impossible) to determine precisely which of the reactions are the major rate-controlling reactions. In addition, the heterogeneous nature of the system adds a further complication.

      The presence of inert gases would usually be expected to dilute the reactants and therefore diminish the reaction rates, such inert materials may actually, on occasion, accelerate the reaction(s). Indeed, the addition of nitrogen to the reaction mixture can be as effective as the addition of oxygen. The nitric oxide formed in the mixture is believed to act as a catalyst:

images

      See also: Combustion.

      The term fuel additives applies to chemicals that are intended to improve the capabilities of a gasoline-fuel or diesel-fueled engine while it is running or in use. Fuel stabilizers are designed to keep the fuel in a functional condition when being stored for a long period of time without use. Chemicals may also be added to liquid fuels such as heavy oil and crude shale oil to improve the transportation properties. Pour point depressants have been successful in some instances but are disadvantageous because they only change the physical characteristics of the oil and not its chemical properties.

      In modern fuels, a combination of several chemical additives is used in order for the fuel to meet the desired performance level. For example, the use of antiknock additives permits greater efficiency and higher power output because of the higher compression ratios they produce. In certain diesel engines, the higher cetane fuels have shorter ignition delay periods than lower cetane fuels. Several different additives have been tried to increase the cetane number of diesel fuel and lubricity improvers as well as friction modifiers both work through the action of film formation on the metal surfaces. However, biodiesel is, to a point, capable of self-lubricating, thereby reducing fraction in diesel engines.

      See also: Fuels.

      Add-On Environmental Control Methods

      There are four general processes used for emission control: (i) adsorption, (ii) absorption, (iii) catalytic oxidation, and (iv) thermal oxidation.

      Adsorption is a physico-chemical phenomenon in which the gas is concentrated on the surface of a solid or liquid. Subsequently, the captured gas can be desorbed with hot air or steam either for recovery or for thermal destruction. Usually, activated carbon is the adsorbing medium, which can be regenerated upon desorption. Adsorbers are widely used to concentrate a low gas concentration prior to incineration unless the gas concentration is high in the inlet airstream. Adsorption also is employed to reduce odors from gases which have potential odor problems. The only major limitation for an adsorption system is the requirement for minimization of particulate matter and/or condensation of liquids (e.g., water vapor) that could mask the adsorption surface and drastically reduce its efficiency.

      Absorption differs from adsorption in that it is not a physico-chemical surface phenomenon, but an approach in which the absorbed gas is ultimately distributed throughout the absorbent (liquid). The process depends only on physical solubility and may include chemical reactions in the liquid phase (chemisorption). Common absorbing media used are water, caustic, sodium carbonate, and nonvolatile hydrocarbon oils, depending on the type of gas to be absorbed. Usually, gas-liquid contactor designs which are employed are plate columns or packed beds.

      Catalytic oxidation is used predominantly for destruction of volatile organic compounds (VOCs) and carbon monoxide, these systems operate in temperature regime of 205 to 595°C (400 to 1100°F) in the presence of a catalyst. Without the catalyst the system would require much higher temperatures to operate. Typically, the catalysts used are a combination of noble metals deposited on a ceramic base in a variety of configurations (e.g., honey-comb-shaped) to enhance good surface contact. Catalytic systems are usually classified based on bed types such as fixed-bed (or packed-bed) and fluid-bed. These systems generally have high destruction efficiencies for most volatile organic compounds, resulting in the formation of carbon dioxide, water, and varying amounts of hydrogen chloride (from halogenated hydrocarbons).

      Thermal oxidation systems without the use of catalysts, operate at temperatures in excess of 815°C (1,500°F). These operating temperatures are 220-610°C (395-1100°F) higher than catalytic systems.

      Particulate matter control (often referred to as dust control) has been one of the primary concerns of industries, since the emission of particulate matter is readily observed through the deposition of fly ash and soot as well as in impairment of visibility. Differing ranges of control can be achieved by use of various types of equipment. Upon proper characterization of the particulate matter emitted by a specific process, the appropriate piece of equipment can be selected, sized, installed, and performance tested. The general classes of particulate matter control devices are: (i) cyclones: wet, dry, axial flow, multi-cyclones, (ii) fabric filters, (iii) wet scrubbers, and (iv) electrostatic precipitators.

      Fabric filters are typically designed with non-disposable filter bags. As the dusty emissions flow through the filter media (typically cotton, polypropylene, Teflon, or fiberglass), particulate matter is collected on the bag surface as a dust cake. Fabric filters are generally classified based on the filter bag cleaning mechanism employed and operate with collection efficiencies above 99%.

      Wet scrubbers are devices in which a counter-current spray liquid is used to remove particles from an airstream. Device configurations include plate scrubbers, packed beds, orifice scrubbers, venturi scrubbers, and spray towers, individually or in various combinations. Wet scrubbers can achieve high collection efficiencies at the expense of prohibitive pressure drops. These units for particulates operate by contacting the particles in the gas stream with a liquid. In principle, the particles are incorporated in a liquid bath or in liquid particles which are much larger and therefore more easily collected. Other techniques include high-energy input venturi scrubbers, electrostatic scrubbers where particles or water droplets are charged, and flux force/condensation scrubbers where a hot humid gas is

Скачать книгу