Isooctane combustion

Figure 5.9 Comparison of isooctane conversion achieved in a microchannel steam reformerwith integrated catalytic burnerfor isooctane combustion (case A), and a microchannel steam reformersupplied withenergyfrom hotcombustion gasesfed into heating channels (case B) [383] S/C ratio was 3.0 in both cases the air excess was 20% in case A and 94% in case B.

Octane number is a measure of a fuel s abiUty to avoid knocking. The octane number of a gasoline is deterrnined in a special single-cylinder engine where various combustion conditions can be controlled. The test engine is adjusted to give trace knock from the fuel to be rated. Various mixtures of isooctane (2,2,4-trimethyl pentane) and normal heptane are then used to find the ratio of the two reference fuels that produce the same intensity of knock as that by the unknown fuel. 

Isooctane 2,2,4-trimethylpentane bums very smoothly in internal combustion engines and has an octane rating of 100. 

Mention should be made of studies of slow, controlled combustion of alkanes, where formation of oxetanes can be detected. For example, oxetane is observed during combustion of propane, while 2-f-butyl-3-methyloxetane and 2-isopropyl-3,3-dimethyloxetane are observed from combustion of isooctane. While the yields are extremely low, the presence of these compounds, along with the other products found, have provided evidence for the mechanism of combustion. The oxetanes are believed to result from rearrangement of peroxy radicals in the radical chain process (equation 114) (70MI51300,73MI51301). 

The halogen compounds used were methylene dichloride, chloroform, carbon tetrachloride, ethylene dichloride, ethyl bromide, ethylene dibromide, bromoform, methyl iodide, and ethyl iodide. The hydrocarbons selected for their interesting combustion properties were hexane, 2-methylpentane, 2,2-dimethylbutane, hex-l-ene, heptane, methylcyclo-hexane, isooctane, diisobutylene, benzene, toluene, m-xylene, and ethylbenzene. 

Detailed comparisons for near adiabatic combustion of isooctane are shown in Figures 3 and 4. Figure 3 shows temperature and Figure 4 shows CO, CO2, O2 and unburned hydrocarbons as functions of equivalence ratio. The quality of these comparisons is very good and is similar to that obtained for the toluene experiments. Soot was not observed to form in measurable quantities for iso-octane mixtures which could be stably burned in the Jet-Stirred Combustor. 

In this paper we report on factors which affect the conversion of fuel nitrogen to TFN in laboratory jet-stirred combustors which serve to simulate the primary zone in a gas turbine. The independent variables in the experiments were fuel type (aliphatic isooctane vs. aromatic toluene), equivalence ratio (fuel-to-oxygen ratio of combustor feed divided by stoichiometric fuel-to-oxygen ratio), average gas residence time in the combustor, and method of fuel injection into the combustor (prevaporized and premixed with air vs. direct liquid spray). Combustion temperature was kept constant at about 1900K in all experiments. Pyridine, C5,H5N, was added to the fuels to provide a fuel-nitrogen concentration of one percent by weight. 

The energy required to overcome the barrier to reaction is called the activation energy and is usually given the symbols a or AG. An energy level diagram for a reaction such as the combustion of isooctane is shown below. 

Energy changes in the metabolism of glucose and the combustion of isooctane, a high octane component of gasoline (Opener, Section 6.4)... 

TABLE 11.2 Stoichiometric Combustion Air Requirements for Pure Liquid Alcohols, MTBE, ETBE, TAME, Isooctane, and Gasoline"... 

Using tables of thermodynamic data, it is possible to work out the energy differences for many different reactions at different temperatures. For example, for the combustion of isooctane, AC (at 298 K) = -1000kJmorf... 

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