Published in the December 2005 American Longwall Magazine
Emerald is owned by Emerald Coal Resources and Cumberland is operated by Cumberland Coal Resources, both affiliated with Foundation Coal Corporation.
The industry trend of widening the longwall face has some obvious benefits but certainly doesn’t come without its challenges.
With this in mind, Emerald decided to approach the project with eyes wide open by completing a detailed study into the engineering issues for both mine and equipment design.
The United States has been fairly conservative in face length, with mines averaging between 900ft and 1000ft.
In comparison, under much more difficult geological conditions, Germany operates faces more than 1500ft wide.
Australia has also taken the plunge, with Xstrata widening New South Wales mine Ulan’s face to more than 1300ft this year.
Pennsylvania mines Emerald and Cumberland were successful operations before the face extensions. However longwall productivity has been enhanced since the widening of both faces in late 2003. In 2004 Emerald produced 5.7 million tons and Cumberland 5.2Mt.
Widening the face is one way of cutting gateroad development – one of the greatest impediments to further longwall productivity improvement.
A case study showed increasing face width by 19% would save 13% in development footage, and increasing face width by 38% would save 26% in development footage.
Wider faces reduce the costs of continuous miner development. The productivity of the longwall is improved because a higher percentage of time is spent with the shearer in the middle of the face and less time is spent cutting wedges at each gate end.
Construction work associated with development – ventilation, belt, power, track installation and retreat, tailgate support and longwall moves – are all reduced. Resource recovery can be lifted by 1-2% because of the different ratio of longwall to continuous mining.
On the financial side, although the timing of capital is more front-end loaded, total capital spending in the long term is similar because the wider panels retreat at a slower pace, resulting in fewer shield cycles per year and extending the life of the shield.
At face value the benefits of widening the longwall face are encouraging. However, with the benefits come the cons – specifically roof control and ventilation.
Generally, for operations in a depth of cover ranging from 600ft to 1200ft, any face width above 768ft was already supercritical and there was no additional loading expected on either the roof supports or on the gate-road pillars.
Looking at ventilation, the liberation of methane and the resistance to the ventilating circuit is directly proportional to the width of the face. In the Emerald case, methane liberation was expected to increase by about 2.8%, and the pressure drop about 0.04in for every 100ft of panel width. This added up to a requirement for an additional 1500 cubic feet of air per minute for each 100ft of face width.
The 50,000 to 60,000 cubic feet per minute of airflow across the face was estimated to be more than adequate to handle the additional methane and overcome the minimal pressure drop. Existing gob vent borehole locations were calculated as adequate to remove the expected additional methane in the gob.
Once Emerald had addressed the design issues it then turned to equipment design concerns. For its case study, the shearer was projected to cut 7.5ft high by 42in wide. The armored face conveyor (AFC) was assumed to be 45in wide. The coal was estimated to weigh 85 pounds per cubic foot in place and 58 pounds per cubic foot on the conveyors.
The belt haulage system was capable of handling a maximum of 3500t/hour without using any horsepower reserve. The chain friction factors were calculated with 0.38 for the top strand with coal on the pan, 0.3 without coal, and 0.4 for the bottom strand. The hydraulic system, one of the most critical parts in the design issues, was assumed to be able to cycle two shields at the same time.
Emerald completed hydraulic flow simulations with DBT and DMT in Germany. The mine staff decided it need to guarantee a pressure drop of not more than 30 bars at the maximum shearer speed of 60ft/min and two shields moving at the same time.
With this in mind, Emerald increased the pump capacity and introduced for the first time 100 gallons per minute pumps to better cover the inefficiency factor of the existing 80 gallons per minute pumps. But there was also the need to add hoses in the monorail system and in the AFC cable tray in the face to compensate for the expected pressure losses.
Additional ladders in the face between the main lines and the shields were also recommended. The monorail and cable trays had to be modified to accommodate the additional lines. Also, switching to a pipeline system instead of hoses could have a positive impact on the expected pressure drop of at least 10 bars. Nevertheless, Foundation decided not to consider the option at the time.
The calculations of the power requirement of the AFC indicated that several changes had to be made to the motor dimensions and chain diameter. To calculate overall haulage capacity, it was assumed a shearer speed averaging 35ft/min. The 1250ft faces required three 1250hp motors, and 45-series Controlled Start Transmission (CST) gearboxes. The 1450ft face required three 1650 hp motors, larger 65-series CST gearboxes and 48mm face conveyor chain.
In addition to the proven and widely used CST technology, the possible use of water filled couplings or Variable Frequency Drive (VFD) technology was assessed. CST technology proved the only viable option.
Once the mine staff completed the design and equipment changes, the Emerald and Cumberland wider faces were put to the test in late 2003.
“The experience with having mined four longwall panels 1250ft wide can be described as very successful,” an Emerald representative said. The mines did not experience any ventilation or roof control problems and the equipment design proved correct.
The longwall float times have improved dramatically, along with longwall productivity.
