Combustion front

The in situ combustion method of enhanced oil recovery through air injection (28,273,274) is a chemically complex process. There are three types of in situ combustion dry, reverse, and wet. In the first, air injection results in ignition of cmde oil and continued air injection moves the combustion front toward production wells. Temperatures can reach 300—650°C. Ahead of the combustion front is a 90—180°C steam 2one, the temperature of which depends on pressure in the oil reservoir. Zones of hot water, hydrocarbon gases, and finally oil propagate ahead of the steam 2one to the production well. 

Maintenance and propagation of the combustion front are problems. This has led to a near-weUbore technology in which the same well is used for air injection and oil production. The combustion front needs to be propagated for a relatively short distance (275). 

A significant increase in light oil production can be achieved with air injection. A total consumption of 5% to 10% of the remaining oil in place can be expected to maintain a propagation of the in situ oxidation process. The flue gas and steam generated at the combustion front strip, swell, and heat the contacted oil. The light oil is displaced at near-miscible conditions with complete utilization of injected oxygen [1700]. 

The evolution in the composition of the residual organic matter is studied by OSA in the coke zone and in all the zone ahead of the combustion front. Whatever the oil, in each sand sample, the amount of gaseous hydrocarbons (SO) is low because all the light hydrocarbons have been stripped by the gas flow. 

The material balance is consistent with the results obtained by OSA (S2+S4 in g/100 g). For oil A, the coke zone is very narrow and the coke content is very low (Table III). On the contrary, for all the other oils, the coke content reaches higher values such as 4.3 g/ 100 g (oil B), 2.3 g/ioo g (oil C), 2.5 g/ioo g (oil D), 2.4/100 g (oil E). These organic residues have been studied by infrared spectroscopy and elemental analysis to compare their compositions. The areas of the bands characteristic of C-H bands (3000-2720 cm-1), C=C bands (1820-1500 cm j have been measured. Examples of results are given in Fig. 4 and 5 for oils A and B. An increase of the temperature in the porous medium induces a decrease in the atomic H/C ratio, which is always lower than 1.1, whatever the oil (Table III). Similar values have been obtained in pyrolysis studies (4) Simultaneously to the H/C ratio decrease, the bands characteristics of CH and CH- groups progressively disappear. The absorbance of the aromatic C-n bands also decreases. This reflects the transformation by pyrolysis of the heavy residue into an aromatic product which becomes more and more condensed. Depending on the oxygen consumption at the combustion front, the atomic 0/C ratio may be comprised between 0.1 and 0.3 ... 

Moreover, in case of a field application, a study of the oil produced and the organic matter from cores taken behind the combustion front, related to the analysis of the initial oil could provide information on the propagation of the combustion front. 

The conditions in the reaction zone determine the release rate of the N-precursors HCN and NH3 [11], Among these conditions are the properties of the fuel (e.g., N-content and particle size), parameters related to the combustion front (temperature and propagation velocity) and the gas composition in and directly above the combustion front. As the prediction of the mass fractions of the N-precursors is important for the final goal of this research, i.e., the prediction of NO formation of the complete furnace, a model is needed in which all these conditions are represented. 

Stationary, traveling wave solutions are expected to exist in a reference frame attached to the combustion front. In such a frame, the time derivatives in the set of equations disappear. Instead, convective terms appear for transport of the solid fuel, containing the unknown front velocity, us. The solutions of the transformed set of equations exist as spatial profiles for the temperature, porosity and mass fraction of oxygen for a given gas velocity. In addition, the front velocity (which can be regarded as an eigenvalue of the set of equations) is a result from the calculation. The front velocity and the gas velocity can be used to calculate the solid mass flux and gas mass flux into the reaction zone, i.e., msu = ps(l — e)us and... 

Currently, the ID model describes the essential features of the propagation of a combustion front in the reverse combustion mode. With an adapted version of the model, the combustion of biomass could be modeled accurately. To obtain... 

The conversion process occurs both on macro- and micro-scale, that is, on single particle level and on bed level. In other words, the conversion process has both a macroscopic and microscopic propagation front. One example of the macroscopic process structure is shown in Figure 10. The conversion front is defined by the process front closest to the preheat zone, whereas the ignition front is synonymous with the char combustion front. 

As the combustion proceeded, the bed surface regressed downwards and reached the first thermocouple (Tl), and after some time the second one (T2), and then the third one (T3). Since the time elapsed for the combustion front to regress between the thermocouples was measured (derived from temperature curves), and the distances between the thermocouples were fixed, the regression rate could be calculated. From the ratio between the air flow and regression rate the bed stoichiometric ratio could be obtained. 

At bed conditions SRbcarbon monoxide was formed due to lack of oxygen. At excess air conditions, SRb >1.00, measurements on the off-gas just above the fuel bed showed small amounts of CO. The distance from the bed surface to the gas probe, after the passage of the combustion front was to short for the reaction to go to completion. Measurements on the flue gases, carried out 0.96 m above the grate, showed no CO. 

Step one is, oxygen diffusion in the porous system of the particle inwards to the char combustion front and the reaction site, (2) adsorption of oxygen to the active sites on the intraparticle char phase, (3) oxidation reaction with carbon, and (4) desorption of... 

In these equations e denotes the unit of internal energy, p = pressure, v-specific volume and Q-chemical energy released per unit mass of substance. Subscripts 1 and 2 of eq (1) denote conditions ahead and behind shock front, whereas subscripts 2 3 of eq (2) denote conditions ahead and behind the combustion front. The internal energy, e, being a state function, can be expressed in terms of pressure, p, and speci-... 

Detonation, Predetonation Phase. This is an intermediate phase in the DDT (Deflagration to Detonation Transition) between deflagration (or combustion) and detonation Oppenheim (Ref 3, p 475) describes how during this phase a combustion front is accelerated by a shock process until the shock front is overtaken and a (CJ) Chapman-Jouguet detonation sets in. 

The detonation wave is a combination of a shock and combustion front, and has a constant width on the time-distance plot. Passage thru the intermediate state would require the attainment of extremely high peak pressure, and of wave-front velocities above the CJ value. Oppenheim quotes (Ref 3, p 476) some exptl evidence of these phenomena... 

Detonotion, Reaction Front in. It is generally agreed that a detonation is a combination of a shock front and a combustion front (Ref 1, p 126 Ref 2). Where combustion is the detonation reaction, the combustion front can also be called the reaction front. The two fronts do not always have the same velocity. At an interesting stage of the DDT (Deflagration to Detonation Transition), the shock front is still faster than the reaction front behind it (See under Detona-... 

In the overdriven state, the observed propagation velocity of the combustion front was found to be much higher than the steady-state velocity of spherical detonation. The overdriven state exists for only a short time, and the combustion front velocity then drops rapidly to the steady-state value. The rate at which this exceedingly high velocity decreases was found by experiment to be inversely proportional to the induction distance... 

The high flame front velocities prior to attainment of the steady state probably result from the transient conditions between the combustion front and shock front. Sufficient data were lacking to show whether the shock-heated gas ignited spontaneously, immediately behind the shock front, or whether the flame front overtook the shock front. In any event, the combustion wave finally moves along with the shock wave, thus forming a detonation wave... 

For this study mixts of CeH6 O and H O were detonated in a tube either by a shock wave or by a spark. The arrival of the pressure step was detd by a thin-film, heat-transfer probe with a rise time of 0.5 microsecs. The spectrograph viewed the passing deton wave thru a window slit and lens arrangement. Recording was accomplished by photomultiplier tubes. The deton waves observed consisted of a shock front followed by a combustion front and were classed as "strong , which is equiv to "unsteady or "decelerating detonation. Detailed structure of the detonations could not be resolved... 

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