Fuel mixing

The WP L cyclone boiler will bum I DE continuously with coal, as about 5% of its fuel mix, with htfle or no modification. By contrast, pulverized-coal boilers, which account for about 80% of the coal-fired capacity in the United States, probably caimot bum tire chips without significant modifications. In these boilers, which bum very fine coal particles in suspension, the heavy chips will fall from the area where best combustion occurs. 

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

The above discussion holds for dispersion by atmospheric turbulence. In addition, a momentum release of fuel sometimes generates its own turbulence, e.g., when a fuel is released at high pressure in the form of a high-intensity turbulent jet. Fuel mixes rapidly with air within the jet. Large-scale eddy structures near the edges of the jet entrain surrounding air. Compositional homogeneity, in such cases, can be expected only downstream toward the jet s centerline. 

If environmental and atmospheric conditions are such that vapor cloud dispersion can be expected to be very slow, the possibility of unconfined vapor cloud detonation should be considered if, in addition, a long ignition delay is likely. In that case, the full quantity of fuel mixed within detonable limits should be assumed for a fuel-air charge whose initial strength is maximum 10. 

Among the various selection considerations are specific combustion characteristics of different fuels. One of the combustion characteristics of gaseous fuels is their flammability limit. The flammability limit refers to the mixture proportions of fuel and air that will sustain a premixed flame when there is either limited or excess air available. If there is a large amount of fuel mixed with a small amount of air, then there is a limiting ratio of fuel to air at which the mixture will no longer sustain a flame. This limit is called the rich flammability limit. If there is a small amount of fuel mixed with excess air, then there is a limiting ratio of the two at which the flame will not propagate.This limit is called the lean flammability limit. Different fuels have different flammability limits and these must be identified for each fuel. 

It is the largest unit worldwide based on solid fuels. The plant has been operating since 1996 and can process up to 2600 ton/day of coal/petcoke fuel mixed with limestone (2 wt%) and produces 180,000 m3/day of raw gas. The annual production of slag (85% of the ash in weight) and fly ash (15% of the ash in weight) are respectively 120,000 ton and 12,000 ton per year. The demonstration project has now attained commercial development with a gross efficiency of 47.2%... 

Figure 6.10 Planar images of the air and fuel mixing pattern and reaction zone imaged by OH fluorescence of the ethylene flame at single-fuel injection low (a) and high (6) soot conditions...

Test Station, China Lake, Caiif (Refs 3 vapor clouds were once considered a way-out technique. A cloud of volatile fuel, mixed with air, is discharged and then detonated on a target, with the same violent expin that characterized grain-silo dust explns or blast, when a propane tank truck blows up. This kind of expln can level a city block. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Most important are the mechanisms of release, the release position relative to the surroundings, and atm conditions. These determine how the fuel mixes with air and its movement in the atm once a fuel/air cloud is formed. The conditions under which the fuel had been contained and its pressure-temp characteristics frequently determine if the cloud is predominantly a mist and whether it is dense or light compared to the atm... 

The function of a burner is to mix oxygen, in the form of air, with the fuel so that the fuel will burn more efficiently. Burners are available in a variety of different designs, all engineered with the intent to maximize air/fuel mixing efficiency, and in more recent times we have the added concern of also minimizing the formation of atmospheric pollutants. Please be aware that fuel will burn at the end of a pipe with no burner at all but the combustion will be far from efficient. 

In Diesel fuel combustion, the physical processes include metering and transportation of fuel and air into the combustion chamber, vaporization of fuel, mixing fuel vapor with air, and provision of an environment favorable for rapid chemical reaction. The chemical processes include self-ignition or autoignition of the fuel-air mixture, and extensive chemical reaction to liberate the potential energy in the fuel. The accomplishment of these processes is the basic consideration in the design of the Diesel engine. 

Recently, work on nitrogen oxidation in the combustion of a fuel mixed with air has become extensive in connection with pollution of the environment by toxic combustion products (including NO). An good review of this research was done by A. N. Heiherst and I. M. Vincent,7 and by A. Macek for coal combustion.8 Detailed experimental and theoretical work on the formation of nitrogen oxides in turbulent gas flames was performed by P. Moreau and R. Borghi.9... 

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