British coal

Materials World magazine
,
1 Mar 2009

Michael Forrest visited the UK’s Daw Mill in north Warwickshire to find out about a planned expansion of the coal mine.

Daw Mill is the largest underground coal mine in the UK. It is fully automated, with a sophisticated computer monitored SCADA system that enables operators, and management at the surface, to monitor the performance of components, from individual rollers on conveyors to the cutting speed of the Eickhoff coal shearer.

Daw Mill is located in north Warwickshire, some 10km northwest of Coventry. A significant proportion of the county is underlain by the Warwickshire coal field whose strata dip to the southwest and continue under Oxfordshire at increasing depth. Underground coal mining in the area began in the early 1900s at the Kingsbury mine north of Daw Mill.

By 1927, it became necessary to sink a new shaft as the workings followed the seam south. This shaft became the Dexter mine, which, as it progressed, required an additional ventilation shaft sunk at Daw Mill, 4.8km further south. Site work began in 1956 and by 1958 it had been decided that Daw Mill would be a new mine and the following year a link at bottom shaft level was made to Dexter.

Daw Mill also became the centre for management and coal preparation. To maximise access, production and safety, a second 5.5m shaft was sunk to a depth of 556m, which was commissioned in 1971.

This southward expansion at increasing depths followed the seven high-quality coal seams that join together between Dexter and Daw Mill to make one 7.5m thick seam, the Warwickshire Thick Coal. This seam is worked out to the north and east, while to the west, a large down throw fault takes the seam to uneconomic workable depths. To the south, the full extent of the reserves are not known, but borehole information indicates that the seam starts to split and thin about 14km to the south and can be found at 1,200m below the surface at Kenilworth, some 13km away.

Rising production

During the 1970s, the mine gained from the introduction of modern technology and coal production rose consistently, only to be constrained by the capacity of the shaft. The solution was to drive a drift, an inclined roadway, to reach the workings, and install belt conveyors along its 2.5km length, allowing capacity to rise to 1.5Mtpa. This surface drift was part of a £35m refit and included coal preparation and loading facilities, workshops and stores. Personnel benefited from new baths, boilers and a canteen.

Wyn Griffith, Operations Design Engineer, says, ‘since the initial installation of the drift conveyor, technology has improved, and now the 1,050mm-wide steel cord belt has a capacity of 1,200t/hr and runs at 4.6m/s’.

Hit the road

The roadways in the ‘outbye’ districts of the mine are steel arched with corrugated steel sheeting or meshed lagging set between the arches, which are at one metre spacings.

The roadways on the coalface districts are of a rectangular shape, some four metres high and six metres wide, supported by roof and rib bolts. These are excavated by powerful ‘continuous miner’ development machines that are capable of rapid advance rates.

The gate roadway is made secure by numerous bolts that are drilled into the roof, some at a density of every 30cm. The steel roofbolts strengthen the strata and are set with resin. On the face side of the roadway the bolts are fibreglass, similarly encapsulated to the roofbolts.

Coal count

The coal is extracted by an Eickhoff SL500 coal shearer that traverses the coalface, cutting a five-metre high section with each pass. Overarching the shearer is a series of electro-hydraulically operated powered roof supports (PRS) over the roof and coalface that are advanced forward after each pass of the shearer. The void behind the PRS is then allowed to collapse.

The coalface is currently 295m long, but there are plans to extend the replacement coalface to 357m. Under the shearer is an armoured face conveyor (AFC) that receives and transports the coal along the coalface and deposits it onto the beam stage loader (BSL), which sits at 90° to the AFC. The coal is turned by a deflector plough on the AFC headframe and is transferred to the belt conveyor to begin its journey out of the mine. As the face retreats, this conveyor is shortened by a multibank storage loop.

Record breaking

Last year two significant records were broken at Daw Mill – the fastest million tonnes achieved and the European record for a single face of 3.2Mt. The 300s area of the mine has already been mapped out and development teams are constructing roadways to facilitate more mining. To maintain annual production in excess of three million tonnes will require at least 5,000m of roadways to access the coal and install equipment.

These investments are supported by proven reserves. Seismic surveys and borehole drilling has shown approximately 20Mt of extractable coal in the 300s area which extends over 15km2. These panels will be the next to be mined in a sequence that extends to 2014.

Beyond these reserves is a proven area designated as the 400s that extends to the outer suburbs of Coventry. Griffith notes that it would be logistically more effective to sink a new shaft to the south of the M6 to access and ventilate these reserves. But the cost of sinking and obtaining planning permission in time mean that faster ways of getting miners to the coalface will have to be found. Reserves in the 400s area are estimated at approximately 40Mt.

With declining North Sea gas (in the future most will come from Norway), as yet unbuilt nuclear stations, and limitations on the amount and availability of alternative energy sources, coal and its cleaner generation technologies and carbon sequestration, may yet maintain or even increase its 33% share of generating capacity. The UK does have the necessary coal reserves, remarks Daw Electrical Engineer Neil Battison, as well as the technology to mine it economically at the correct quality and, above all, safely.

Further information: UK Coal