Investigation into enhancing the performance of underground filtration equipment using advanced dust capture techniques

This project is concerned with investigating ways of improving the performance of the MRDE irrigated filter (often called the Bretby Filter). This device is currently in use in all British Coal mines. It is a free standing unit, which is available as a number of variants and is capable of handling 4m3s-1 of air containing up to 2000 mgm-3 of dust. The main operational feature of the device is a mechanical mesh filter irrigated by a number of coarse water sprays. The action of these .sprays serves to keep the filter clean and to improve the performance of the filter.Normal assessment of its performance is in terms of its ability to handle respirable (as defined by the BMRC curve) dust, measured by comparing concentrations of respirable dust collected by elutriator samplers upstream and downstream of the filter. Measurements of total dust filtration efficiency and measurements of filtration efficiency as a function of particle aerodynamic diameter have also been carried out. It can achieve an overall filtration efficiency of the order of 95% for the respirable fraction of airborne dust.Whilst filtration efficiencies are generally adequate to maintain dust levels below control limits in most cases, there is nevertheless a need to improve on this performance for use where dust levels are high.The aim of the project was to utilise a range of particle capture techniques to provide an improvement in the filtration efficiency of the unit with a minimum increase in the power requirement.The potential of four different methods has been investigated. These are: 1) the introduction of fine droplet sprays to scavenge the dust, primarily by inertial forces; 2) the introduction of charged droplets and/or dust charging to scavenge by electrostatic forces; 3) the use of an acoustic field to form agglomerates from small particles; and 4) the introduction of new filter panels.Most of the experimental work was carried out using a 1A scale model of the irrigated filter. The air flow rate through this device could be varied with a maximum of 1 mV. A validation exercise was carried out to compare the filtration efficiency (total, respirable and size dependent) for the model against published results for the full scale unit. Measurements were carried out for a range of test rig air flow rates and for different challenge aerosols (coal dust, DIN70 dust and monodisperse aerosol). The model respirable filtration efficiency (RFE) was lower than measured in the full scale with a maximum of 93%. However, the form of the relationship was similar to that of the full scale in that the RFE decreased with total flow rate through the system and was higher for coal dust than for DIN70 dust. The differences between the model and full scale were attributed to the size distribution of the test dusts. In experiments to look at the size dependent filtration efficiency, good agreement was found between the results measured on the model and earlier estimates, based on coulter counter measurements. The filtration efficiency was found to be >95% for 3 �m particles but decreasing rapidly for decreasing particle size. We concluded that the performance of the model was similar to that of the full scale system and as such the model would provide a satisfactory test bed for evaluating the various enhancement methodologies.Of the various methodologies tested the use of droplets as inertial collectors proved to be the most successful. In the best combination, (at the highest total system air flow of 1.15 mV) penetrating dust concentrations were reduced, by40%, the RFE increasing from 94% to 96.5%. The nozzle types, nozzle position, water flow rate,-droplet size and initial droplet velocities have all been optimised. A theoretical model has been developed, based on initial impaction, which may be used to rank various spray arrangements. The measured reduction in concentration is generally lower than that predicted by the model.In the investigation of the use of electrostatic methodology, successful charging of droplets was demonstrated for a range of spray nozzle types. In the best electrostatic arrangement, a droplet charge to mass ratio of 7.41 x 10-6CKg was obtained although the maximum nozzle supply at which these levels could be sustained was 5 /hr-1. At this flow rate a reduction in downstream concentration (for a total system flow rate of 1.15 m3s-1) of 18% was achieved. This was for a positively charge droplet cloud. Similar reductions were measured for a negatively charged cloud. This was much larger than was found for this type of spray alone with no charge where a reduction of 8% was measured.Measurement of the coal dust test aerosol charge distribution was carried out. The net charge of the droplet cloud was found to be zero, but there was a distribution of charges around this which corresponded to a Boltzmann distribution.A charging grid was constructed which allowed a charge (either positive or negative) to be applied to the dust. Measurements of RFE carried out for droplets and dust charged to opposite polarities showed no improvement of the uncharged dust case. The reason for this is not currently understood.For the investigation into the use of acoustic fields for particle agglomeration, sufficient information was obtained to suggest that this may be a profitable area for future study. From the work carried out it is clear that acoustic agglomeration operates selectively on particles <5 �m in diameter and that only relatively low energy consumption (~ 400 W) is required. Both of these are excellent criteria for recommending further work in this area.The final area of investigation was in relation to filter panel design. In the conventional system, two panel types were available. One manufactured by Bondina was constructed from a matrix of synthetic fibres, impregnated with graphite. The second manufactured by Knitmesh, was constructed from a series of stainless steel mesh layers. Of these two, the first was in more common use. Measured filtration efficiencies were greater for the Bondina filter panel but the pressure drop across the panel was also larger (1.5 kPa at a flow rate of 1.15 mV1) compared to 0.9.Two new panels were obtained. One, manufactured by Knitmesh, was constructed from a mix of stainless steel and Teflon filaments. The second was constructed using Recticel foam. This foam is available in a set of grades, each have different filament sizes. A grade of foam was selected such that the pressure drop across was equivalent to the original Knitmesh panel. The pressure drop characteristic of the new Knitmesh (Knitmesh 2) panel lay between those of the Knitmesh 1 and Bondina. The filtration efficiency of the two new panels were measured. The Knitmesh 2 panel gave a higher filtration efficiency than the original Knitmesh panel but was still not as high as the Bondina. The foam panel, although not as efficent as either of the Knitmesh 2 or Bondina panels, was significantly better than the Knitmesh 1.Overall, the project has resulted in substantial improvements in the filtration efficiency of the MRDE filter. The largest improvements were achieved using swirl-air nozzles with which a reduction in downstream dust concentration of almost 50% was measured. Based on these results, recommendations have been made to implement the use of additional spray nozzles in the full scale unit. The recommendations cover nozzle type and location, water flow rates, droplet sizes and droplet velocities.Further recommendations have been made in relation to additional work in acoustic agglomeration and filter design and testing. ""