Methane combustion constants

Time-dependent Catalytic Activity - The strong variation of activity as a function of time on stream is a typical feature of the methane combustion reaction on most Pd catalysts. Several different transient phenomena have been reported. In some cases, the activity is initially low, or even zero, but then it increases with time on stream. In other cases, the activity starts high but then it drops to a lower steady state value. The approach to the steady state has also been found to vary greatly. In some cases, it reaches a relatively constant value in a few minutes. In other cases, the activity still changes after several hours on stream. 

Figure 24.1 Numerical calculations of combined methane combustion and steam reforming, (a) Outlet conversion versus channel half-height (b) wall temperature as a function of dimensionless reactor length calculation results determined at constant inlet velocity [33],...

Staying with our methane example. Figure 9.9 illustrates how rich and lean mbctures alter the heat evolved. The heat released at stoichiometric equivalence was previously calculated (Figure 9.3) note that the heats of formation for water (in gas or liquid form) and methane are constants in this combustion and the contribution of elemental O2 s zero. As a result, the expression for calculating in order to compare the three reactions depicted in (he figure can be simplified as ... 

Heat Release Rate From Fuel Gas. The fuel gas used in these tests was a mixture of natural gas supplied by the local gas company. This gas mixture contains approximately 90 percent methane and small fractions of ethane, propane, butane, C02, and nitrogen, as analyzed by Brenden and Chamberlain (6). Although composition of the gas changes with time, the changes were small in our case. A statistical sample of gross heat of combustion of fuel gas over several months showed a coefficient of variation of 0.7 percent. Also, the gross heat of combustion of natural gas reported by the gas company on the day of the test did not vary significantly from test to test. Thus, we assumed that the net heat of combustion was constant. 

Methane leaks from a tank in a 50 m3 sealed room. Its concentration is found to be 30 % by volume, as recorded by a combustible gas detector. The watchman runs to open the door of the room. The lighter mixture of the room gases flows out to the door at a steady rate of 50 g/s. The flammable limits are 5 and 15 % by volume for the methane in air. Assume a constant temperature at 25 °C and well-mixed conditions in the room. The mixture of the room gases can be approximated at a constant molecular weight and density of 25 g/mol and 1.05 kg/m3 respectively. After the door is opened, when will the mixture in the room become flammable ... 

The aim of the present work was to design and operate an apparatus in which stationary combustion and flames can be produced and sustained to pressures of 2000 bar and with environmental temperatures up to 500°C. Visual observation of the interior of the reaction vessel should be possible. Arrangements had to be made by which a gas flow of only a few microlitres per second could be fed steadily into the reaction vessel at pressures to two kilobar. A similar provision was necessary to extract small samples for product analysis at constant conditions. The principle of design and operation will be described. First results will be given for experiments with oxygen introduced into supercritical water-methane mixtures. 

Sventoslavsky (Ref 2a) developed in 1908 a method of calculation of heats of combustion which later proved to be in agreement with the method developed by Kharash (Ref 4). Kassatkin Planovsky gave a good description of Sventoslavsky s method (Ref 7, p 31). Later, Thornton (Ref 2b) has shown that the molar heat of combustion at constant volume of any saturated hydrocarbon at room temperature is approximately 52.7 kcal for each atomic weight of oxygen required to burn it. For example, methane, which burns according to the equation ... 

CASH CBM CBO CBPC CC CCB CCM CCP CDB CEC CFBC CFC CFR CMM COP CSH CT Calcium aluminosilicate hydrate Coal bed methane Carbon burn-out Chemically-bonded phosphate ceramics Carbonate carbon Coal combustion byproducts Constant capacitance model Coal combustion product Citrate-dithionate-bicarbonate Cation exchange capacity Circulating fluidized bed combustion Chlorofluorocarbon Cumulative fraction Coal mine methane Coefficient of performance Calcium silicate hydrate Collision theory... 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

For the combustion of 1 mole of methane at 25°, we find by experiment (corrected from constant volume to constant pressure, if necessary) that the reaction is exothermic by 212.8 kcal. This statement can be expressed as follows ... 

The proportionality constant K includes the rate constant of the combustion, thermal capacity of the pellistor, and the factor related to the diffusion of the gas. Ideally, all terms on the right-hand side of (3.18) except Co are constant. For safety applications, the response of the pellistor is expressed on the scale of %LEL, which for methane is % 100 T, F.T. = 5% v/v in air. On the LEL scale, the dynamic range is between 10% and 100%. The response time is around 1 s. 

A simple model of the chemical processes governing the rate of heat release during methane oxidation will be presented below. There are simple models for the induction period of methane oxidation (1,2.>.3) and the partial equilibrium hypothesis (4) is applicable as the reaction approaches thermodynamic equilibrium. However, there are apparently no previous successful models for the portion of the reaction where fuel is consumed rapidly and heat is released. There are empirical rate constants which, due to experimental limitations, are generally determined in a range of pressures or concentrations which are far removed from those of practical combustion devices. To calculate a practical device these must be recalibrated to experiments at the appropriate conditions, so they have little predictive value and give little insight into the controlling physical and chemical processes. 

Measurements have also been made of the speed of the uniform movement in mixtures of air with each one of the hydrocarbons of the paraffin series up to and including pentane. The determinations were carried out with horizontal glass tubes, 2-5 cm. in diameter,1 and the results are shown diagrammatically in fig. 26. With the exception of methane, the maximum speeds are approximately the same, namely, about 82 cm. per second. The value for methane is rather lower than this, being 67 cm. per second. Owing to the few data available for the thermal constants of the paraffin hydrocarbons, it is not easy to explain this difference. In each instance, the mixture having the maximum speed of flame contains more combustible gas than is required for complete combustion. 

Tsang has prepared a series of publications on data required for modelling the combustion of methane [45], methanol [53], propane [54], isobutane [55] and propene [56]. The treatment is less detailed than in the case of the CEC publications there is less evaluation of the primary data, often previous evaluations are accepted, and there is no graphical presentation of results. However, data on more than 500 reactions are considered and, where there are no experimental data, estimates of the rate constants are given. Unimolecular reactions are also treated slightly differently from the CEC evaluations as described in Section 3.3. 

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