Flame Arresting Mechanisms and Flameproof Device for VAM Mitigation

The overall goal of this project was to study the gas flammability limits, and flame propagation and extinction mechanism, and to develop a flameproof device for the ventilation air methane (VAM) mitigation units. The following research tasks were successfully completed:

· Modelling and experimental work on the ignition and flammability limits of various mixed gases, including the effects of N2 and CO2;

· Modelling and experimental work on the flame propagation and extinction mechanism of the mixed gases;

· Determination of flame propagation speed, and a series of flameproof devices (flame arrestors);

· Preliminary design of the flameproof devices for the mitigation units with an air flow range of 10-20m3/s.

Ignition and flammability limits

There are limited studies reporting the effects of N2/CO2 on flammability limits. In particular, no existing work can be found dealing with flame arresting for use in VAM mitigation.

Flame propagation and extinction

Efforts have been made to understand the flame propagation and quenching mechanisms. A series of flame quenching experiments were carried out with parallel channels to build the relationship of quenching length, flame speed and gap width. Effects of fuel and inert gas were also considered. An empirical equation, h=A·L^0.5 + B, was obtained through the analysis of experimental data. A and B are a function of the flame propagation speed, and h is the gap width of small channels in the parallel plate. From the equation, quenching length and gap size can be predicted under specific flame speed, which would be very useful for the selection or design of flame arrestors.

To better understand the flame arresting mechanism in narrow channels, numerical simulations were carried out by using a computational fluid dynamics (CFD) program FLUENT. The physical models of 3D single triangular channel and single parallel plate gap were built, and verified by the experimental results and data from the literature. Key factors including inlet velocity, channel height, channel geometries and wall temperature were studied to understand their effect on the flame quenching process in narrow channels.

Clearly, the experimental results showed that the addition of CO2 enhances the flame quenching significantly. This could be explained as CO2 leads to a substantial decrease in flame speed, due to its higher heat capacity than N2.

Flameproof tests

Usually, maximum experimental safe gap (MESG) of 0.9mm is used for most of the commercial flame arrestors for the methane lines. This channel size, however, is not preferable in terms of reducing pressure drop and managing dusts when such a flame arrestor is used for coal mine ventilation air with a high air flow rate.

Clearly, in order to design or select a flame arrestor for its application to successfully arrest the flame in a cost-effective manner, the flame propagation speed needs to be determined correctly with consideration of ventilation air characteristics, mainly higher flow rate resulting in pressure drop, and higher dust loading resulting in dust clogging. There will be significant benefits if the flame arrestor can be installed at a place where the flame propagation is under deflagration rather than detonation. It has been challenging to determine the flame propagation speed even though number of tests on two test rigs of DN50 and DN150 were conducted, and all possible flame propagation data for large pipes from available literature reviewed. The most reliable way is to conduct the flame propagation test in actual size pipe for the proposed ventilation air application, but it was too expensive for the project. Hence, three ranges of flame propagation velocities are selected: 200-300m/s, 400-600m/s and 700-900m/s. A series of flame arrestors (7 DN50, 3 DN150) were selected or fabricated for the experiments. For two new DN50 flame arrestors, when the channel size was enlarged for the dust clogging issue, it still arrested a flame with a propagation speed up to 1,064m/s as long as the channel length was increased to meet the quenching.

Preliminary design of a flameproof device

The preliminary design of a flameproof device was carried out for the VAM mitigation units in a flow rate of 17m3/s, and it was mainly used to indicate the structure and dimension of the flameproof device. Two configurations were discussed. The first was to use a set of DN500 flame arrestors (four or more) to form a branching configuration. The second was to use a single flame arrestor with approximate DN1,200 mm.

Based on the research outcomes of this project, the following R&D would be needed to develop a suitable commercial scale flame arrestor for the ventilation air flow.

· Studying a flame arrestor with injecting an inert gas, such as CO2 in upstream when needed to arrest the flame from the VAM units.

· Determining the flame propagation speed in actual size pipe for the proposed ventilation air application.

· Optimising the flame arrestor element structure and dimension associated with the flame speed, dust clogging, pressure drop and cost.

· Support from an experienced manufacturer would be necessary to develop a commercial scale flame arrestor suitable for the VAM mitigation application under available standards.