Fuel Deposition

The richest fossil fuel deposits are thought to occur at times when the earth was much warmer and contained much more C02 than today. 

The crust is the largest carbon reservoir in the crustal-ocean-atmosphere factory (8 x 10 Pg C including the sediments). Most of this carbon is in the form of inorganic minerals, predominantly limestone, with the rest being organic matter, predominantly contained in shale and secondarily in fossil fuel deposits (coal, oil, and natural gas). The oceanic reservoir (4 X lO" Pg C) and the terrestrial reservoir (2 to 3 x 10 Pg C) are both far smaller than the crustal reservoir. The smallest reservoir is found in the atmospheric, primarily as CO2 (preindustrial 6 x 10 Pg C, now 8 x 10 Pg C and rising). The flux estimates in Figure 25.1 have been constrained by an assumption that the preindustrial atmospheric and oceanic reservoirs were in steady state over intermediate time scales (millennia). 

This value helps predict the deposit-forming tendency of fuel. Deposits in oil burner systems can form hot spots on surfaces which can lead to stress, distortion, and even cracking of system components. 

Substituted and condensed thiophenes and thiophenols are usually the most abundant sulfur-containing compounds in refined fuels. These compounds are known to react with oxygen to form peroxides and eventually result in color bodies and gumlike fuel deposits. The reaction of thiophenol with a free-radical compound and oxygen is shown below ... 

The cloud point test is one of the most commonly used methods to evaluate the low-temperature characteristics of distillate fuel. The cloud point temperature identifies the point when wax begins to form into crystals large enough to become visible in the fuel. At this temperature, wax can settle from fuel, deposit onto fuel filters, and interfere with the flow of fuel through small tubes and pipes. During cold weather months, distillate fuels with lower cloud point values are refined and blended to minimize the low-temperature problems associated with wax. 

Fossil fuel deposits are not distributed evenly throughout the world. For instance, 65 percent of the world s recoverable petroleum deposits are in the Middle East, along with 34 percent of recoverable natural gas deposits. North America is relatively poor in petroleum and natural gas but has a bit more than one-fourth of the world s supply of coal. 

The origin of atmospheric methane can be estimated by determining its, 4C isotope abundance. This abundance should be the same as the l4C content of living plants, if methane is of biological origin or is provided by recently dead organisms. CH4 from fossil fuels or volcanic activity is practically free of radiocarbon, since fuel deposits are very old and their 14C content has already decayed. Measurement of... 

Heavy fuel deposits were expected in boiling systems, and therefore the initial studies of deposition and activity transport for power reactors concentrated on the CANDU-BLW concept until the fields at Douglas Point became a concern. The deposit thickness was proportional to iron concentration in the coolant and to the square of the heat flux (69) deposition was reversible and quickly reached a steady value set by the local conditions. The corrosion products initially deposit by hydrodynamic and electrostatic effects then boiling accelerates deposition by drawing water and its contained iron into the deposit to replace the steam that leaves. Local alkalinity gradients within the deposit determine whether iron crystallizes to cement the deposit or dissolves to weaken it, and erosion processes then define the equilibrium thickness (70), This model works well in explaining deposition under boiling conditions. 

Fig. 3-47 and Fig. 3-48 represent the behavior of n-hexacontane during the tests at 1 bar and 10 bar air pressure. The endothermic fusion peak at approximately 100 °C is not influenced by the increase of pressure. On the other hand, the exothermic oxidation peaks were shifted to lower temperatures. The first peak (low-temperature oxidation LTO) moves from 241 °C at 1 bar to 221 °C at 10 bar. In the range of fuel deposition, the peak at 334 "C (1 bai) disappears almost completely and may be recognized only in the shoulder of the LTO peak at 300 °C (10 bar). Also, the sharp peak present at 407 °C (1 bar) disappears. 

In tests on the higher boiling members of the homologous series of n-alkanes from n-triacontane up to n-hexacontane, two strong peaks are found which are easy to evaluate. These peaks are the LTO peak and the peak in the fuel combustion range. In the range of fuel deposition more than one peak appears. Evaluation of the numerous small peaks is problematic and rarely valuable. Evaluation is normally limited to one or two well-defined peaks. The peak temperatures of eight n-alkanes... 

Consequently the susceptibility to oxidation of elemental carbon depends strongly upon the history of its origin and of the nature of its surface. If the oxidation test of pure hydrocarbons is interrupted after the second reaction step (Fuel Deposition) then a glossy, black, lacquer-like residue will be found in the sample pan. The oxidation of the latter results in heats of combustion in the region of pure carbon. The susceptibdity to oxidation of the coke formed during Fuel Deposition will also be influenced by any residues of the original substances or their conversion products which are still present in the soot. 

The formation of the deposited carbon in the reaction range Fuel Deposition is a complex process. Conjugated diolefins react at temperatures of 550 °C up to 750 °C forming unsaturated cycloalkenes and aromatics requiring a weak activation energy of only 104kJ/Mol [3-32]. The primary reaction step of the pyrolysis of benzene at a temperature of 700 - 800 °C is the formation of biphenyl. The reaction continues to form pyrene (E 230 kJ/Mol). 

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