Personal Sampling of Dust in Coal Mines
Last year's review reported on the difficulties experienced with the CIP10 personal dust sampler when used in field trials at some UK coalmines and the subsequent laboratory trials to establish the cause. Laboratory tests were then carried out at HSL with a small personal cyclone sampler to see if it could be relied on in the arduous conditions found in coal mines before restarting the field programme.
The wind tunnel tests on the performance of the personal cyclone were completed in 2002. It was found that the size selective performance of the cyclone was affected by the high wind speeds that may be found on some coal faces close to the shearer and therefore experienced by the shearer drivers. This manifested itself as a bias of at least 30% for the cyclone when sampling typical mine dusts in winds from 1 to 8 m/s. This was considered to be unacceptable by the DMCIAC/MIC working group advising on the development of new inhalable dust regulations.
Following experience gained in the outdoors environment, the use of a cowl to reduce the wind speed around the cyclone entry was investigated. The final version was made from stainless steel, and was designed such that the standard plastic cyclone could be easily popped in and be fastened with simple elasticated straps. The presence of this cowl reduced the bias to a maximum of 13 % for wind speeds from 1 to 8 m/s. Before restarting the main underground trials phase, a limited set of trials with the cyclone personal sampler was carried out in one colliery. In the first set of tests, the cyclones were mounted in the return roadway side by side with MRE 113A samplers. Sensible respirable dust levels were recorded. The second set involved sampling shearer drivers on 15 consecutive shifts on one coal face. No evidence was found of large particle transfer to the filter cassette and some useful experience was gained with the harness required to mount the cyclone and associated pump.
The main underground trials with the cowled cyclone sampler started early in 2003 and four large mines were sampled by mid-April. Although reasonable and understandable dust exposures were obtained, a sealing problem with the aluminium cassette used in the cyclone to hold the filter was discovered. This was solved with the insertion of a simple PTFE O-ring, but unfortunately the data from those four mines was considered to be unreliable and was not accepted for use to propose exposure limits. Reliable data were subsequently obtained from seven large mines and three small mines. The datasets both for respirable dust and respirable quartz look to be very consistent and understandable and, based on these results, the DMCIAC has proposed limits for both respirable dust and respirable quartz. Progress has also been made with sourcing a lightweight intrinsically safe personal sampling pump and considerable improvements have been made to the design of both the cyclone cowl arrangement and the harness for carrying the sampler in the light of comments and suggestions from the miners.
Diesel Engine Exhaust Emission Monitoring
The owners of a non-coal mine had employed consultants to carry out an assessment of exposure to diesel engine exhaust emissions (DEEEs) in the mine. They used the HSE booklet ('Control of Diesel Engine Exhaust Emissions in the Workplace') as their guide on how to assess workplace exposure to DEEEs and, within the limits of this guide, produced a good report. However, this demonstrated the limitations of the HSE guidance because no actual measurements of DEEEs were made.
HSE then funded a project with HSL to carry out further monitoring in the mine including collection of diesel particulate samples for organic carbon and elemental carbon analysis. Elemental carbon (EC) provides an excellent surrogate measure of exposure to DEEEs and allows comparisons to be made with other mines and industries. As a guide, any results above 200 µg/m³ EC would be classed as high exposure. The aim is to get all exposures below 150 µg/m³ EC with an ultimate target of below 100 µg/m³ EC.
The mine was worked by bord and pillar methods with the mineral being liberated by blasting. Up to six diesel powered vehicles operate in the mine, including two new dumper trucks which continually transport the broken mineral from the production area to a stockpile at the surface, and a large and relatively new Cat 938G front end loader.
The measurements in the production area were all above 300 µg/m³ EC and in some cases well above. The main reason for the high results was considered to be inadequate ventilation and the mine has since addressed this and the situation is much improved. It was noteworthy that the filtered air cabs on the dumper trucks and the front end loader were very effective at reducing the driver's exposure to DEEEs so that in both cases, measurements inside the cab were 80% lower than outside.
Whilst at the mine the opportunity was taken to demonstrate how exposure to DEEEs can be estimated on-site by simply measuring the 'blackness' of a diesel particulate filter sample using a Bosch Smokemeter Evaluator. Non-coal mines are encouraged by the Mines Inspectorate to use this simple on-site method to monitor exposure. With regular monitoring, any high exposures can be quickly spotted and the causes attended to.
Diesel Fumes/Particulates in Mines - Phase 4
Part 1: Diesel Particulate Filter
Tests have been carried out by HSL on the diesel engine test bed facility at Buxton to investigate the possible use of a catalytic diesel particulate filter (CDPF) and its potential to reduce exposure to diesel engine exhaust emissions (DEEEs) in mines, especially coal mines. This type of filter not only traps the diesel exhaust particulates but, when exhaust temperatures are favourable, regenerates itself by oxidising the trapped particles to carbon dioxide. The main objectives were to determine how effective the filter was at reducing particulate emissions, to confirm that the oxides of nitrogen (i.e. nitric oxide (NO) + nitrogen dioxide (NO2); often referred to as NOx) were not increased, and to verify that the back pressure on the exhaust system was not compromised by inclusion of the CDPF into the exhaust line.
The results show that over all engine modes the particulate concentration was reduced by over 90% and that carbon monoxide and hydrocarbons were also reduced to very low levels. The results also showed that the total NOx concentration was not increased by inclusion of the filter but that the ratio of NO to NO2 did alter. Normally raw exhaust contains only NO so the total NOx is essentially NO. However, with the filter in line and when exhaust temperatures are in the range 200 - 400 ºC the catalyst oxidises some of the NO to NO2 and the NOx can be as much as 50% NO2.
The overall conclusion was that the filter has much to offer in terms of reduced particulate emissions in mines but that some further trials should be carried out to confirm that the filter regenerates regularly and that there is no risk of increased back pressure. Increased back pressure is not only damaging to the engine but increases the exhaust emissions. British Gypsum has agreed to install the filter on one of their vehicles for underground field trials.
Part 2: Guidance Booklet
A guidance booklet titled 'Controlling and Monitoring Exposure to Diesel Engine Exhaust Emissions in Non-Coal Mines' has been drafted by HSL. The leaflet provides non-coal mines with information on how and where to measure DEEEs and the equipment required, as well as practical advice on ways and means of reducing exposure. The guide also includes information on legislation on diesel particulate exposure and the health risks associated with exposure. Comments from management at a salt mine and a gypsum mine have incorporated into this latest draft and which should be published on HSE's website in 2004.