Hydrocarbon fuels pyrolysis

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

Muradov, N., C02-free production of hydrogen by catalytic pyrolysis of hydrocarbon fuel, Energy Fuel, 12, 41,1998. 

In earlier sections of this chapter, the role that particulates play in a given environmental scenario was identified. This section will be devoted exclusively to combustion-generated particulates whose main constituent is carbon. Those carbonaceous particulates that form from gas-phase processes are generally referred to as soot, and those that develop from pyrolysis of liquid hydrocarbon fuels are generally referred to as coke or cenospheres. 

Investigators have used the words carbon and soot to describe a wide variety of carbonaceous solid materials, many of which contain appreciable amounts of hydrogen as well as other elements and compounds that may have been present in the original hydrocarbon fuel. The properties of the solids change markedly with the conditions of formation and, indeed, several quite well-defined varieties of solid carbon may be distinguished. One of the most obvious and important differences depends on how the carbon is formed carbon may be formed by a homogeneous vapor-phase reaction it may be deposited on a solid surface that is present in or near the reaction zone or it may be generated by a liquid-phase pyrolysis. 

Following the conceptual idea introduced by Milliken [68], Takahashi and Glassman [53] have shown, with appropriate assumptions, that, at a fixed temperature, i/c could correlate with the number of C—C bonds in the fuel and that a plot of the log ipc versus number of C—C bonds should give a straight line. This parameter, number of C—C bonds, serves as a measure of both the size of the fuel molecule and the C/H ratio. In pyrolysis, since the activation energies of hydrocarbon fuels vary only slightly, molecular size increases the radical pool size. This increase can be regarded as an increase in the Arrhenius pre-exponential factor for the overall rate coefficient and hence in the pyrolysis and precursor formation rates so that the C/H ratio determines the OH concentration [12]. The 4>c versus C—C bond plot is shown in Fig. 8.14. When these... 

However, 2,3-benzofuran was detected in emissions from a Swedish floor finish used on domestic flooring (van Netten et al. 1988), and in emissions from the pyrolysis of silk (Junk and Ford 1980), and in combustor flue gas emissions from fluidized-bed coal combustion at a concentration of 900 ng/g (Hunt et al. 1982). Exhaust produced by an automobile burning simple hydrocarbon fuels contained 2,3-benzofuran at concentrations ranging from less than 0.1 to 2.8 ppm (Seizinger and Dimitriades 1972), but an analysis of air in a highway tunnel in use by both diesel- and gasoline-powered vehicles indicated no... 

Carbon formation is also different for diesel and gasoline. The long chain hydrocarbons present in diesel or kerosene fuel are more difficult to reform than the shorter chain hydrocarbons present in gasoline, while aromatics in gasoline hinder the overall reaction rate. An example is found in the results of Ming et who showed that SR of n-Ci required a higher steam/ carbon ratio to avoid coke formation than i-Cg. The cetane number of the feed had little effect on carbon formation. Carbon formation can often be attributed to fuel pyrolysis that takes place when the diesel fuel is vaporized. This is considerably decreased when the steam content in feed increases. 

In an early investigation on fuel cell applications, Callahan studied catalyzed pyrolysis of various hydrocarbon fuels (combat gasoline, Sunoco 190 gasoline, JP-4 jet fuel, and diesel fuel) for onboard H2 generation. Ni supported on... 

Williams, P. T. Taylor, D. T. 1993. Aromatiza-tion of tyre pyrolysis oil to yield polycyclic aromatic hydrocarbons. Fuel, 72, 1469-1474. 

Another process that is listed in Table 39 is the C02-free production of hydrogen via thermocatalytic decomposition of hydrocarbon fuels. The process involves a single-step decomposition (pyrolysis) of hydrocarbons over carbon catalysts in an air- and water-free environment. This approach eliminates the need for a water-gas shift reactor, CO2 removal and catalyst regeneration, which significantly simplifies the process60. 

Hot-gas filtration can reduce the ash content of the oil to less than 0.01% and the alkali content to less than 10 ppm - much lower than reported for biomass oils produced in systems using only cyclones. Diesel engine tests performed on crude and on hot-filtered oil showed a substantial increase in burning rate and a lower ignition delay for the latter, due to the lower average molecular weight for the filtered oil (36). Hot gas filtration has not yet been demonstrated over a long-term process operation. Pyrolysis oils are not miscible with hydrocarbon fuels but with the aid of surfactants they can be emulsified with diesel oil. 

One of the most critical issues in developing catalytic reformers, especially for the reforming of hydrocarbon fuels, is the risk of carbon deposition on the catalyst surface and consequent catalyst deactivation. Carbon formation can occur in several regions of the steam reformer where hot fuel gas is present. Natural gas for example will decompose when heated in the absence of air or steam at temperatures above 650 °C via pyrolysis reaction as shown in Equation 2.4. 

Fuel combustion can lead to pyrolysis of hydrocarbon fuels and soot formation, important in diesel engines and contributing to particulate emissions. A proportion of this soot finds its way into the lubricating oil and increasing use of EGR systems to reduce particulate and gaseous emissions has led to higher levels of soot loading in lubricants. 

High-temperature pyrolysis reactions of hydrocarbons are responsible for the production of PAH and solid carbon black particles, soot. This phenomenon is common in diffusion flames where, at high temperatures and without oxygen, hydrocarbon fuel aggregates follow pyrolysis and condensation paths with the formation of heavy aromatic structures. Many PAH s identified in aerosols have been found to be mutagenic and are certainly important soot precursors. This formation of carbonaceous particles has recently become one of the main topics in chemical reaction engineering, especially in the field of pyrolysis and combustion of hydrocarbon fuels. This interest rises from environmental concerns about PAH and soot particle emissions because of their dangerous impact on the human health (Oberdorster et al., 2004). 

Muradov, N. "Thermocatalytic Production of Hydrogen via Pyrolysis of Hydrocarbon Fuels from Methane to Residual Oil", Amer. Chem. 

Krukovsky, VK., Kolobova, E.A., Lubchanska, L.I., Nikishkov, B.V (1987), Complex Plasma-Chemical Conversion of Solid Hydrocarbon Fuel in Water Vapor, in Plasma Gasification and Pyrolysis of Low-Quality Coals, p. 81, Energy Insitute, ENIN, Moscow. 

The TR7 oil is superior to TR12 and both are much superior to pyrolysis oils as feedstocks for conventional catalytic hydrotreating to produce hydrocarbon fuels. 

Ligno-cellulosic biomass is a resource from which liquid hydrocarbon fuels potentially may be derived. Pyrolyzing the wood yields gas and liquid products, but a relatively large percentage of the original wood carbon can be lost to a low value char by-product. Furthermore, like the model oxygenates described above, the EHI of the pyrolysis liquid products is substantially less than 1. 

Hydrocarbon yields can be increased significantly when wood pyrolysis liquids are coprocessed with methanol over ZSM-5 catalyst vs separate processing of the two streams over the same catalyst. Thus, coprocessing pyrolysis liquids with methanol produced from char gasification is one means of producing hydrocarbon fuels from wood. Results obtained in this study provide a basis for comparison with other processing schemes such as wood gasification followed by either Fischer-Tropsch synthesis or methanol synthesis plus Mobil s MT6 process. 

The raw materials of the polymer industry are obtained by pyrolysis (or cracking ) of oil. In earlier times, they were similarly obtained by pyrolysis of coal, which leads to the formation of coke, tars and town gas . Waste hydrocarbon polymers are similar in chemical structure to mineral oil and on heating to high temperatures they crack to give a mixture of lower molecular weight hydrocarbons (see, for example. Table 4,4), some of which have utility both as chemical feedstocks e.g. the olefins) and the rest as fuels. Pyrolysis can be carried out successfully... 

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