Andrew Mcintosh

Low-temperature oxidation of single and blended coals

The oxidation of different types of coal under self-heating conditions is considered using the crossing point method performed in a cubical wire-mesh basket. This study investigates the effect of particle size and the physical structure of coals (including pore size and surface area) on the self-heating character of high and low rank Indonesian coals and their blends. The results confirm that both particle size and surface area give considerably different effects on critical ambient temperature, the activation energy and the product of exothermicity and the pre-exponential factor of low and high rank coals. It was found that the self-heating characteristics of high rank coals were strongly dependent upon the particle size of the coals. For low rank coals, the dependence was weaker. A coal bed with mixed sizes is thus more vulnerable than one with segregated sizes. This means that one must carefully consider the particle size distribution to judge the spontaneous ignition behaviour of coal. It is suggested that the potential for spontaneous combustion of blended coal is greater following the increased percentage of more reactive coal.

A computer model to assess fire hazards in tunnels (FASIT)

A computer model that simulates fire growth movement in tunnels is described, and a brief overview of tunnel systems is presented. The methods for predicting mass flows, velocities, smoke concentrations, and heat transfer are presented, along with a list of hazard output parameters. The validation of the model against experiment, and possible directions for future work are also presented.

Using the crossing point method to assess the self-heating behavior of indonesian coals

Indonesian coal, although generally low in sulfur and ash content, is a lower rank coal with a relatively high moisture content (5–18%). Consequently, it has a greater propensity to self-heating behavior, and the aim of this paper is to assess the hazard for three typical coals that are used both domestically and for export. Experimental tests are reported using a new technique (the crossing point method) to determine the activation energy and reactivity of the low-temperature slow oxidation reaction. This uses the transient temperature profiles to deduce the activation energy and reactivity of the sample. The method relies on finding the center temperature at the point when a flat profile is observed (Called the crossing temperature) and has been used successfully to ascertain the kinetic parameters of three Indonesian coal samples. It is found that the method is valid as long as the coal experiment is not greatly supercritical when it is then difficult to determine an appropriate crossing temperature. The method has the distinct advantage over the traditional Frank-Kamanetskii approach, in that only one oven experiment is needed for any one point on the kinetic plot, and that the results are not heavily dependent on the fan setting of the oven, or on sample size. A comparable experiment using the Frank-Kamanetskii approach of finding the critical temperature for different sample sizes showed good agreement for estimating activation energy.

A theoretical explanation of the influence of char formation on the ignition of polymers

A mathematical model is described for the behaviour of polymer combustion with char formation in thermally-thick conditions. A non-competitive single-step reaction is used, converting the polymer to char and volatiles. By examining this model numerically, the ignition behaviour of this system can be expressed as a function of the char yield. This simplified analytical approach yields good agreement with the numerical solution.

On the prediction of thermal runaway of coal piles of differing dimension by using a correlation between heat release and activation energy

Greater use of international coal and low-sulfur/low-rank coals brings new challenges in monitoring the spontaneous combustion behavior of these coals during transport in stockpiles, and in coal milling systems Extensive experimental work has been carried out to measure the kinetic parameters of low-temperature oxidation of a number of coals for varying particle fractions. A substantial set of experimentally measured values of activation energy, E a , and the product of the exothermicity and the pre-exponential factor, QA , have indicated a strong relationship, which is shared by coals of different ranks and of different particle size. This correlation for E a and QA leads to a new approach for estimating the thermal runaway behavior of coal piles based on small-scale experiments. By use of Frank-Kamenetskii theory, the correlation curve fit of E a versus QA can be used to estimate hazardous conditions at varying scales by small-scale oven heating tests measuring the heat release rate at low temperatures. The results suggest that the increasing size of the pile alters the particle size effect. For large coal piles, large particle size can increase the potential for spontaneous combustion. This new approach provides a more cost-effective assessment of the susceptibility of various types of coal to self-ignition.

The effect of heat sink additives on the ignition and heat release properties of thermally thin thermoplastics

Abstract The retardancy effect of additives, undergoing an endothermic decomposition in the solid phase, upon flammability properties of thermally thin thermoplastics is investigated. The criterion for ignition is that of a critical mass flux of volatiles pyrolysed from the solid into the gas phase. Strong coupling between the solid and gas phases is thus neglected in this model. The kinetic parameter values are set up in such a way that easy comparison can be made between the cone calorimeter and thermogravimetric analysis (TGA). The requirements for an additive to have maximal effect on ignition time and maximal mass loss rate are found to contrast, so that a simple heat-sink additive cannot maximise both.

The effects of an inert central core on the thermal pyrolysis of an electrical cable

A mathematical model is formulated to simulate pyrolysis of the insulation layer of an electrical cable. The cable is vertically orientated and the model resembles a cable flammability test similar to IEC 60332-1 (Test on Electrical Cables Under Fire Conditions). Only non-flaming pyrolysis is investigated in this paper. The composition of the cable is idealised such that it consists of an inert central core and an insulation layer. The configuration of the external heat source is also simplified such that only the truncated end of the cable sample receives heat. Pyrolysis is modelled as ablation which assumes a single constant critical pyrolysis temperature. Investigations are concentrated on the effect of the thermal properties of the central core on the pre-heating time and the initial stage of pyrolysis.

CFD Predictions of Low NOx Radial Swirlers With Vane Passage Fuel Injection With Comparison With Internal Gas Analysis Flame Composition

Radial swirlers with vane passage natural gas injection, similar to those used in some industrial low NOx gas turbines, were investigated for their flame structure both experimentally and using CFD. The radial swirler NOx emissions at 600K and 1 atmosphere pressure were shown to be 3–4 ppm at 15% oxygen at 1800K and 1–2 ppm at 1700K. These levels were similar to the best published low NOx emissions using any flame stabilizer design. A flame at O = 0.5 and 600K air temperature was investigated for its structure using a 10mm OD water cooled gas sample probe with a 1mm gas sample inlet on the upstream side of the probe. This showed that the mixing in the vane passage and outlet duct was very good. The maximum unmixedness at the first traverse location, 10mm downstream of the dump expansion zone, was 20% of the mean and the unmixedness was less than 5% within 30mm from the dump expansion. The flame structure was shown to involve a thick turbulence reaction zone of about 100mm thickness to the 90% heat release point. The CFD predictions were made using the RSM and k-e turbulence models and the flamelet combustion model with a strain rate library. The isothermal aerodynamics predictions were in good agreement with others for similar geometries. There was an inner and outer recirculation zone with a swirling shear layer between. The peak turbulent kinetic energy was predicted to be on the inside of the shear layer. The experimental results showed that the flame developed in this region of high turbulence and low axial velocities. The flamelet model was less successful at predicting the flame development. The NOx results were predicted to be 2ppm less than the experimental results, due to the shorter predicted heat release region with associated lower prompt NOx.Copyright

Ignition Properties of Thermally Thin Plastics: The Effectiveness of Non-Competitive Char Formation in Reducing Flammability

The retardancy effect of char formation upon the flammability of thermally nthin products is investigated. The char is formed in a single-step non-competitive scheme nand is assumed to be thermally stable. The criterion for ignition is that of a critical nmass flux of volatiles from the solid into the gas phase. Both steady-state and transient nformulations of the model are considered. In the high activation energy limit the critical nheat flux efficiency in the steady-state model is proportional to c/(1−c), where c is the fraction of char formed. In the transient model the efficiency in reducing the maximum nheat release rate, average heat release rate, and total heat released is given by c and is nindependent of activation energy and heat flux. The specific application that we have nin mind for our model is piloted ignition in the cone calorimeter.