Flameless combustion of wood: charring and heat release characteristics

Introduction. The article presents the results of a research on the two stages of thermal decomposition of timber. The first stage of thermal decomposition is flame combustion, which is followed by a transition to flameless combustion due to the formation of a char layer on the surface of wood. The flameless process is accompanied not only by heterogeneous combustion, but, at least, three reactions: pyrolysis, thermal oxidative destruction of wood and oxidation of resulting coke.Goals and objectives. The goal is to identify the criteria of charring and heat release under the influence of an external radiative heat flux on samples of coniferous and deciduous species of wood using a standard flowthrough calorimeter with a focus on flameless combustion. Methods. A standard OSU flow-through calorimeter, produced by Atlas (USA), was used to identify heat release characteristics under the influence of external radiative heat fluxes that had the density of 20, 35 and 52 kW/m2. The lower limit of heat, released in the complete combustion of samples, that had a char layer, was identified using bomb calorimeter C-5000.Results. The co-authors analyzed the charring process and characteristics of heat release using samples of wood species that were 10 and 25 cm thick. Wood samples were exposed to combustion under the influence of an external radiative heat flux that had the density of 20, 35 and 52 kW/m2 subsequent to the results of tests, conducted using the OSU calorimeter. The co-authors evaluated the charring velocity and the coke layer thickness for the cases of flame and flameless combustion; efficient combustion heat release and the combustion completeness coefficient, as well as the sample shrinkage. The co-authors demonstrated that a transition from flame combustion to heterogeneous combustion occurs upon completion of the quasi-neutral burning of wood samples, which corresponds to the final point of heat release velocity curves and marks a transition from the behaviour of a thermally thick material to that of a thermally thin material.Conclusion. The obtained experimental data allow to forecast a change in the physical and heat engineering properties, characteristics of heat release in the processes of flame and flameless combustion of different wood species with account taken of the char layer formed on its surface under the influence of various heat fluxes.

Delay time of gases's catalytic heterogeneous ignition on various sizes's catalyst particles

In this work, catalytic ignition delay time of combustible gas's small impurities in air on a spherical metal particle of various diameters is analytically determined by the example of gas-air mixtures's flameless combustion with hydrogen impurities on a platinum particle. It is shown that stable flameless combustion is observed after an induction period for particles of a certain range. It has been established that catalytic ignition time of gases is divided into three stages: 1. inert heating, the duration of which still depends on the combustible gas concentration; 2. the stage of self-acceleration and catalyst temperature rise during the course of the catalytic reaction in the transition region; 3. stage of diffusion inhibition and reaching stable catalytic combustion. The characteristic relaxation time was used in a dimensionless form. To determine the duration of the second stage, a modified Frank-Kamenetsky approach is applied. The duration of diffusion inhibition stage in the dimensionless form is practically independent of catalyst particle's diameter, although the catalytic combustion temperature decreases with an increase in the catalyst diameter. Heat transfer by radiation, the role of which increases with the growth of the catalyst size, is included in the effective heat transfer coefficient, which allows maintaining the classical ideology to solving the problem of the induction period.

Modelling adiabatic flame temperature for methane with an overview for advanced combustion process: flameless combustion

In this study, we have carried out to modeling by mathematical equations the environment of the combustion chamber under some conditions using the software MATLAB in order to make an adequate algorithm of resolution and get solution of the different equations that taken part in the phenomenon of combustion, so the first was to identifies the kind of the fuel to use in that model (taking Methane as fuel combustion) and minimizing the reduction of novice gas burned is in priority, for this, we want to establish the optimal values to take to preserve the environment, especially from CO2 and CO emissions, secondly, its nature according to the equivalent ratio (lean, stoichiometric or rich mixture), all variations of equivalent ratio, the third idea is to retrace the ways of a different product of the reaction and see their variations compared to the equivalent ratio, once traced, we can improvise which exact place in the reaction, a product will be finished either in the form of a gas or to decompose in order to bind to another and form another component. We also discussed the percentage of O2 and H2O emissions for an interesting viewpoint of the environmental aspect of hydrocarbon’s chemical reaction. Another additional part will be dedicated to the process of flameless combustion to write its mathematical equation, compare it with the so-called traditional one, and see the variations in the temperature according to the equivalent ratio.