Published in American Longwall Magazine
In just its fourth year of operation, the BHP Billiton-run mine, 16 miles west of Farmington, will this year produce more than seven million tons of steam coal from a single longwall face and two continuous miner units.
Automated shield initiation/pan push, including gate-end cut-outs, auto height control, memory cut and complete data transmission to the surface are said to have demonstrated high levels of reliability. The mine’s high-tech monitoring facilities – designed to help manage weak roof conditions, spontaneous combustion, and hydrogen sulfide and methane gas - are seen by industry watchers as a template for the future.
San Juan received its mining approvals in October 2000, continuous miner (CM) development began in February 2001 and the longwall started in October 2002 – two years to the day after approval.
BHPB recorded its best month of production in August this year when it produced 785,316 tons. Mining of the current panel is due to finish in February/March 2005 with a 30-day changeover to panel no 4.
The mine was established to replace production from two surface mines and is the only coal source for the adjacent San Juan Generating Station. Underground access is via a highwall in one of the opencuts. The mine layout makes efficient use of CM development with east-west mains and 23 longwall panels; eleven to the south and 12 to the north of the mains.
Panels are typically 1000ft wide and range between 5000-12,000ft long, containing 2.5-6 million tons of coal each. The seam is12.5-16ft and the cutting height varies between 10-13ft. Roof and floor coal are left on the longwall and CMs are used to maintain required quality and roof control.
Longwall equipment includes a Joy Mining Machinery 7LS5 shearer, Joy 1150-ton-yield capacity roof supports and Joy armored face conveyor, stageloader and crusher. The roof supports are remotely controlled with Joy’s RS20 electro-hydraulic controls. The Joy AFC utilizes dynamic chain tensioning – crucial in high power/large chain link faces. Load sharing and soft start functions provided by Joy’s TTT couplers are said to have maximized chain longevity with chain elongation of less than 1% recorded after two panels on the 3300hp installation.
Sixty-inch-wide panel belts deliver coal to a 72-inch mainline system. The conveyor systems were supplied by Continental Conveyor. Panel belts utilize two 500hp Rockwell VFD drives, while the three 750hp main belts utilize Voith TPKL fluid couplings. Both systems incorporate Falk gearboxes. The total longwall system capacity is 4000t per hour.
Headgate computers have been built into a monorail-suspended “pod” type system, designed principally by the mine’s longwall automation specialist, Mike Harvey. Multiple screens allow longwall personnel to access real-time monitoring of shield, shearer and AFC health, pump condition and power centers as well as outbye services such as belts, water and compressed air. They also provide access to the mine’s general computer network for use of parts manuals, work order systems, written plans and standards, and the input of operating statistics.
One of the most impressive features of the mine is the surface monitoring capability in the control room. It comprises several servers that collect and interpret real-time data from a vast array of sensors, cameras and tube bundles located underground, including the longwall server. The production server monitors power centers and allows pit-top personnel to monitor belt status and start or stop the belt by remote control. This data is a key management tool and enables remote fault identification which in turn saves time deploying staff.
A mechanic can determine the nature of a problem before physical inspection takes place, and technical support can be provided from a distance, eliminating travel from the time it takes to troubleshoot a problem. Further, the information available is used to monitor critical trends such as shield set pressures and roof loading patterns. The computer system is programmed to flag when data trends move beyond pre-set bracketed limits, allowing early intervention on a wide set of parameters. It also plays a critical role in finding the cause of equipment failures.
Hundreds of temperatures, pressures, amps, cylinder positions and other data points are monitored and recorded on an often enough basis to allow reconstruction of events leading to an unexpected event. An emulsion system is similarly monitored, allowing staff to view total volumes and tank capacities. The system has the functionality for emulsion flow to be reversed if the operating height of supports is being lowered, creating excess fluid. Abnormal flows and fluid use trigger alarms leading to an investigation into the cause.
Because San Juan's coal is prone to spontaneous combustion, gas monitoring is extensive and includes monitoring of the current and two previous gobs. The latter are kept sealed and pressurized. The mine operates a “bleederless” ventilation system and an exhausting mine fan. Other than two longwall mines in the western coal fields of the US, all longwalls in the country operate a bleeder ventilation system.
At San Juan, gob pressure is monitored constantly as are current and forecast barometric pressures. The system can predict upcoming changes in gob conditions, allowing early intervention. If barometric readings are falling, excessive seal pressure is relieved through the operation of gob vent borehole pumps (GVBs) connected to each of the sealed and active gobs. When the readings are rising, nitrogen injection is used to maintain pressure and an inert atmosphere behind the seals.
Nitrogen is also injected into seals immediately behind the active longwall face. This injection facilitates a change from atmospheric oxygen levels on the longwall face to a low oxygen environment in the gob. Oxygen is displaced, rather than being consumed by low-level oxidation. As a result, carbon monoxide levels also remain low.
Nitrogen is provided from a nearby Praxair nitrogen generation facility. The plant was situated adjacent to the minesite to provide nitrogen for mine activities and service the regional natural gas industry. As a “base load” client, the mine has available enough stored nitrogen to flood an entire panel and access flow rates up to 4000 cubic feet per minute if required in an emergency.
All components of the system, including nitrogen flow, GVB flow and quality and gob pressure and quality are monitored and controlled from the control room. Overall, the system has proven...click here to read on.
