Page 160 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

270 Fuel Chemistry

TABLE VI Typical Yield from a Barrel of Gasoline 1. Specific gravity—ratio between the weight of any
volume of oil at 60 F to the weight of equal volume
◦
Product Yield (gallons)
of water at 60 F
◦
Gasoline 19.5 2. Viscosity—a measure of resistance to motion of a
Distillate fuel oil 9.2 fluid
Kerosene 4.1 3. Flash point—The temperature at which the vapors
Residual fuel oil 2.3 generated “flash” when ignited by external ignition
Lubricating oil, asphalt, wax 2 source
Chemicals for use in manufacturing 2 4. Pour point—The temperature at which the oil ceases
(petrochemicals)
to flow when cooled under prescribed conditions.

A. Combustion of Liquid Fuels
boiling points, these are separated into different fractions
Fuel oil-fired furnaces, diesel engines, and distillate fuel-
inarefinery. The most common refining operations are
fired gas turbines utilize fine liquid sprays to increase the
distillation, cracking, reforming alkylation, and coking.
rate of evaporation and combustion rate of the fuel. In gen-
The demand for various products changes with the season
eral the combustion of a liquid fuel takes place in a series
and the lifestyle of the society. Typical yield from a barrel
of stages: atomization, vaporization, mixing of the vapor
of crude oil is shown in Table VI.
with air, ignition, and maintenance of combustion (flame
stabilization). Recent advances have shown the atomiza-
tion step to be one of the most important stages of liquid
XI. PROPERTIES FOR UTILIZATION fuel combustion. The main purpose of atomization is to
increase the surface area to volume ratio of the mixture.
Themajorityofproducts(firstfour)areburntinvariousde- For example, breaking up of a 3-mm droplet into 30-µm
6
vices.Gasolineisamixtureoflightdistillatehydrocarbons drops results in 10 droplets. This increases the burning
with a boiling range of 25–225 C consisting of paraffins, rate by 10,000 times. The finer the atomization spray the
◦
olefins, naphthenes, aromatics, oxygenates, lead, sulfur, greater the subsequent benefits are in terms of mixing,
and water. The exact composion varies with the sea- evaporation, and ignition. The function of an atomizer is
son and geographic location. During summer months low twofold: atomizing the oil and matching the momentum
volatile components are added to reduce the vapor pres- of the issuing jet with the aerodynamic flow in the furnace.
sure, whereas in winter months low boiling components The atomizers for larger boiler burners are usually of
are added to make it more volatile. Under the 1990 Clean the swirl pressure jet or internally mixed two fluid types,
Air Act Amendments (CAAA), the U.S. Environmental producing hollow conical sprays. Less common are the
Protection Agency (EPA) developed reformulated gaso- externally mixed two fluid types. The principal consider-
line (RFG) to significantly reduce vehicle emissions of ations in selecting an atomizer for a given application are
ozone-forming and toxic air pollutants. RFG is required turn-down performance and auxiliary costs.
to be used in the nine major cities with the worst ozone There are differences in the structures of the sprays be-
air pollution problems. Similar to normal gasoline, RFG tween atomizer types which may affect the rate of mixing
will contain oxygnates. Oxygenates increase the combus- of fuel droplets with the combustion air and hence the
tion efficieny and reduce emission of carbon monoxide. initial development of a flame.
Table VII provides a comparison of properties for various For distillate fuels of moderate viscosity, (30 mm 2
−1
gasolines. sec ) at ordinary temperatures, a simple pressure atom-
Diesel fuel is also a mixture of light distillates but with ization with some type of spray nozzle is most commonly
higherboilingpointcomponentswithaboilingpointrange used. Operating typically with a fuel pressure of 700–
of 185–345 C consisting of lower volatile and more vis- 1000 kPa (7–10 atm) such a nozzle produces a distri-
◦
cous compounds. The average molecular weight is ap- bution of droplet diameters from 10 to 150 µm. They
−1
3
proximately 200. range in design capacity of 0.5–10 or more, cm sec .
Kerosene fuels are used in jet engines. Kerosene fu- A typical domestic oil burner nozzle uses about 0.8 cm 3
els have a wide range of boiling points. The aromatics in sec −1 of No. 2 fuel oil at the design pressure. Although
kerosene are limited due to their tendency to form soot. pressure-atomizing nozzles are usually equipped with fil-
Residual fuel oils are classified into five categoroies. ters, the very small internal passages and orifices of the
Some of the important properties are listed here for good smallest tend to be easily plugged, even with clean fuels.
atomization. With decreasing fuel pressure the atomization becomes