Published in American Longwall Magazine
The admirable increases in productivity, out put and safety experienced by longwall mines in the United States are a result of cooperation between mine operators, equipment manufacturers and the mine workforce. Also, mine operators now have a better understanding of mine geology and have integrated the use of more reliable, higher-powered longwall and panel development equipment into their mine plans.
Historical performance
The average output from individual US longwall mines in 2004 is projected to reach 4.2 millions tons, compared to 1.9Mt in 1989. However, the number of longwall mines has decreased from a high of 75 mines in 1989 to 45 mines in 2004 (see figure 1). While the current number of longwall mines is the lowest in 15 years, it is not expected to decrease further in the foreseeable future.
In fact, there are plans for new longwall mine development in Illinois, Kentucky and the Pennsylvania/West Virginia coal fields. Also, several longwall mines that were idled as a result of geological problems or fires are scheduled to reopen in 2004.
Longwall mines that were idled but have recently reopened include Gary No. 50 (WV), Mine 84 (PA), and Loveridge No. 22 (WV).
Production from US longwall mines has outpaced production from non-longwall US underground mines for the fifth straight year. Production from longwall mines currently accounts for 51% of total US underground mine production and 17% of total US coal production.
Production from longwall mines over the past four years is evenly divided between union and non-union operations, compared to 1989 when only 25% of longwall production was from non-union mines (see figure 2).
Geographically, the majority of longwall mine production is from eastern states. Production from eastern mines, however, has decreased from a high of 86% in 1991 to the current level of 70%. Western longwall mine production increased from 16% to 30% during the same period.
Current productivity (saleable tons per man-hour, including surface workers and preparation plant personnel) at longwall mines is 60% higher than from non-longwall mines. After a period of steady increases in productivity, longwall mines reached a peak of 5.52t per man-hour in 2000 and have maintained close to that level of productivity through 2004. Non-longwall mine productivity has remained at about 3.4tpmh since 1998 (see figure 3).
Productivity from western longwall mines has increased from 4.1tpmh in 1989 to the current level of 9.8tpmh and is continuing to show growth, while productivity from eastern longwall mines has increased from 2.4tpmh in 1989 to the current level of 4.5tpmh, but has not shown real growth since 2001 (see figure 4).
The number of employees at both longwall and non-longwall mines has decreased from 79,000 in 1989 to 38,000 in 2004. The majority of losses have been at non-longwall operations.
Longwall panels in the US are becoming wider and longer (see figure 5). The average longwall panel dimension in the US in 1989 was about 700ft wide by 5800ft long. Both width and length steadily increased to the 2003 average dimension of 940ft by nearly 10,000ft. New longwall panels, 1500ft wide by 15,000ft long, are currently being planned.
Underground longwall and non-longwall coal mines are considerably safer today than in the past. While the US Mine Safety and Health Administration (MSHA) does not maintain safety records comparing longwall and non-longwall mines, MSHA records indicate that the total US underground Non-Fatal Incidence Rate (NFIR) in 2003 was 6.79 compared to an NFIR of 13.12 in 1989. NFIR is calculated as the number of injuries x 200,000 divided by the number of man-hours worked.
Present day statistics
Details relating to US longwall mines operating in 2003 include:
Seam thickness ranges from 48-216in.
Cutting height ranges from 60-156in.
Panel widths range from 710-1100ft.
The longest panel is 15,000ft.
About 75% of longwall panels are developed with three entry gateroads, 17% with four entry gateroads and only 8% with two entry gateroads. (The two entry gateroad systems are used in Utah only).
Depth of shearer cut ranges from 30-48in.
All longwall faces are cut with shearers except for one WV mine which uses a plow.
The majority of the shearers are Joy 7LS series.
Roof support yield ratings range from 620-1280t.
The majority of face conveyor chains are 42mm, twin strand, in-board. Three mines use 48mm, triple strand chains.
Face conveyor speeds range from 225ft per minute to 450fpm.
About 53% of longwall mines operate at 4160v while the remaining 47% operate at 2300v.
For a comprehensive list of US longwall mines operating as of June 30, 2004 please see page 64.
Future challenges
Technological factors which have enabled US longwall mine production and productivity to increase in recent years include:
Increased shearer horsepower, allowing faster tram and cutting speeds.
Improvements in strength and reliability of equipment, allowing wider and longer panels.
* Wider (two meter) shields, which minimize the number of shields, the number of components to maintain and the number of units to move.
Automation and fiber-optic control.
Walk-through roof bolters with full-mesh capabilities and feedback drilling control, providing faster and safer gateroad development.
According to equipment manufacturers, future challenges for longwall mining include providing thin-seam (50in) cutting height face systems and complete face automation.
The primary physical problem with current longwall systems operating in thinner coal seams is the minimum operating height of existing systems is in the mid-60in range. In order to mine thin seams, existing systems are forced to cut considerable rock to provide sufficient machine clearance. In addition to reducing production, cutting rock can also create refuse disposal problems.
One potential way to mine thinner coal seams could be the use of advanced design low-profile plow systems, but with increased horsepower and fully computerized remote control, including the ability to sense coal-to-rock interfaces and automatically adjust cutting height. Full automation, potentially eliminating the need for personnel at the face, will be necessary as a result of the high speed of the plows, which...click here to read on.
