Włodzimierz Kordylewski

Evaluation of kinetic parameters of coal ignition

Abstract A method is presented of evaluating the kinetic parameters for the ignition reaction of coal based on the Semenovs thermal ignition theory adopted to coal particle ignition. The evaluated apparent activation energy, E, for bituminous coal, anthracite and bituminous coal char are in the range 46–103 kJ mol−1. A verification was made by comparing the measured induction periods for the coal particles with those calculated numerically. The oxidation rate is controlled by diffusion into pores.

Critical parameters of thermal explosion

Abstract An analysis of the critical conditions of thermal explosion based on bifurcation theory for nonlinear elliptic equations is presented. It has been proved that the value of the Frank-Kamenetsky parameter δ equals the principal eigenvalue of the equation of heat conduction linearized about the point of bifurcation of the solution u(δ), which correspond to self-ignition and extinction points. From this two computational procedures for determining critical conditions of self-ignition or extinction have been worked out. Both procedures are illustrated with examples of numerical calculations.

Influence of ducting on explosion pressure: Small scale experiments

Abstract This paper presents the results of experimental studies on the venting of gaseous and dust explosions in a 22 1 spherical vessel connected to a duct. Earlier observations of the strong sensitivity of gaseous explosions to the duct length have been confirmed and related to the acoustic oscillations in the system. The origin of the acoustic oscillations is not clear and their occurrence can be prevented by initial turbulence. Helmholtz oscillations have not been detected in the supercritical venting of an explosion. At a very early stage of the explosion in the vessel a brief secondary explosion occurs but its influence on the explosion course is not clear. Because of the absence of acoustic oscillations in dust explosions their violence is much less affected by the length of the connected tube.

Influence of ducting on the explosion pressure

Abstract In spite of many investigations of pressure generation due to gaseous explosions in a vessel there is no reliable correlation between the experimental data and the theoretical models. It was shown that such factors as the presence of obstacles [1] and the acoustical feedback [2] can lead to a dramatic increase in pressure in the vessel in comparison with the theoretical predictions based on the one-dimensional laminar flame propagation speed [3]. In this short communication we report another example of the unusual increase in the explosion parameters due to ducting. It was found that for a relatively short (12 diameters) tube connected to the vent of the combustion vessel there is a strong maximum of the maximal pressure and the maximal pressure rise of explosion. This is a preliminary report and we cannot give an explanation of the observed phenomenon.

The influence of self-heating on the second and third explosion limits in the O2 + H2 reaction

Abstract The interaction of self-heating and branched-chain reaction in the vicinity of the second and third explosion limits of the H2 + O2 reaction is studied numerically. We assume the vessel walls are reflective and use Baldwins scheme appropriate to an aged boric acid coating. We confirm the dominance of chain branching at the second limit. An unexpected feature, however, is the bistability of the stationary-state radical concentration (i.e., there are two solutions for each vessel temperature below the limit). As Ta is increased, the two solutions move closer together and coalesce at the point of ignition. This allows the limit to be identified mathematically as a limit-type bifurcation. The computations reveal that at higher pressures, self-heating shifts the point of explosion to lower vessel temperatures. Self-heating influences the reaction rate mainly via the kinetics of hydrogen peroxide decomposition. A simplified scheme of the leading reactions responsible for reaction near the third limit can be represented in the form of a closed cycle with (i) initiation; (ii) branching and progation steps; (iii) termination accompanied by significant heat release; and (iv) formation of H2O2. There is feedback in this cycle as the heat evolved in (iii) increases the rate of thermal initiation (i) which occurs principally by H2O2 decomposition. These kinetics lead to critical conditions analogous to the Semenov criterion for thermal explosion and give a value of the apparent activation energy for the third limit of 242 KJ mol−1. The numerical calculations involving the complete Baldwin scheme of reactions lead to an apparent activation energy very close to that determined from the simplified scheme of reactions.

Effectiveness of Polish lignites as reburn fuels

The effectiveness of four Polish lignites as reburn fuels was studied in the laboratory-scale drop-tube furnace. The established rank of the lignites and chars used correlates with concentration of calcium in lignite. A comparison of the reburning effectiveness of partially devolatilized chars with that of the parent lignites showed that the parent lignites achieved a higher NO reduction than the chars.

Dynamic responses to perturbations in the non-isothermal CSTR: Times to ignition and times for temperature decay—I. Adiabatic operation

When a simple exothermic reaction is carried out in an open system, discontinuous jumps between stationary states (ignitions and extinctions) may occur in response to continuous variation of control parameters such as inflow-temperature or average residence-time. This paper examines the dynamics of such jumps for reaction in a CSTR operating adiabatically. Two problems are analysed: (a) the dependence of time-to-ignition on the degree of supercriticality and (b) the decay of small perturbations to the steady state for marginally sub-critical conditions (“critical slowing down”). It is shown that both the time-to-ignition (and time-to-extinction) and the decay to a stationary state obey universal formulae characteristic of the category of instability, increasing rapidly as criticality is approached: time-to-ignition ∝ (degree of supercriticality) -1/2 We illustrate our results chiefly by reference to a single, first-order, deceleratory reaction. We at first exploit the exponential approximation to the Arrhenius temperature law but the treatment is quite general and can cope with any temperature-dependence of reaction-rate.

An Analysis of Critical Ignition and Extinction Diameters of Solid Particles

This paper gives an analysis of the influence of solid matter consumption on the critical diameters for the ignition and extinction of a single particle of solid fuel. The Thomas theory of ignition with reactant consumption for the pre-mixed system was adopted for obtaining the formula for the critical diameters of ignition and extinction. The calculations made for the coal particle show that consumption deflects the critical diameter of extinction about two times stronger than that for ignition

The kinetics of combustion of coal volatiles

Abstract The thermal explosion theory of volatile matter evolved from a coal particle has been adopted to evaluate the activation energy of the combustion of volatiles. The apparent global activation energies calculated from the experimental ignition of three bituminous coal types were in the range 60–70 kJ mol−1. By analogy with rapid coal pyrolysis this suggests that a sequence scheme reaction describing the oxidation of volatiles can be approximated by a global single-step process having a lower activation energy than the combination of any single volatile matter flame component.

Dynamic responses to perturbations in the non-isothermal, continuous-flow stirred-tank reactor (CSTR)—II. Nonadiabatic operation: The general case

Abstract When an exothermic reaction is carried out in an open system, discontinuous jumps between stationary states may occur in response to continuous variation of control parameters such as average residence-time. Our previous paper examined the dynamics of such jumps for reaction in a CSTR operating adiabatically. Here the general case of marginally supercritical conditions during non-adiabatic operation in a CSTR is considered. It is again found that: (i) times to ignition and (ii) times for decay to a stationary state both obey universal formulae characteristic of the category of instability, lengthening rapidly as criticality is approached. For example: time-to-ignition ∝ (degree of supercriticality) − 1 2 . We illustrate our results chiefly by reference to a single, first-order, deceleratory reaction with the usual strong dependence on temperature, but the treatment is quite general and can cope with any concentration-dependence and any temperature-dependence of reaction rate.