Working out which cutting system is best for a longwall is a difficult job as hundreds of complex parameters need to be reckoned with. Researchers at the Excavation and Mining Equipment Group (BGMR) at RWTH-Aachen University in Germany have developed ProSL – a calculation program to simulate the production capacities of various shearer loader cutting sequences.
To reduce production costs and improve productivity in a longwall mine, the two most significant key parameters are installed power to the face and longwall face length. This has resulted in up-to-date systems producing unsurpassed production rates and face lengths in excess of 400m.
To use longwall technology at its maximum efficiency, however, the most appropriate cutting sequence must be selected and this is what ProSL was designed to do.
Today, the most popular cutting sequences are uni-directional (uni-di) and bi-directional (bi-di). More sophisticated cycles have also been developed over the past decade. These new methods, adjusted to individual geological and technical conditions, have led to significant increases in production.
Operations that have successfully used these new methods include Peabody Energy’s Twentymile mine in Colorado, with its partial web sequence, and the partial opening (Opti Cycle) sequence at Eyesizwe's Matla mine in South Africa.
There are five commonly used cutting sequences.
The bi-di cutting sequence uses two sumping operations at the face ends in a complete cycle that is accomplished during both the forward and return trip. The whole longwall face advances during the completion of each pass. The advance distance is two web distances per cycle. The leading drum of the shearer cuts the upper part of the seam while the rear drum cuts the bottom and cleans the floor coal.
General industry perception says the main disadvantages of this method are the unproductive periods resulting from the face end activities, and its complex operations. The trend in recent years, therefore, has been to increase face length to reduce the relative impact of sumping operations in favour of a longer production time.
In contrast to the bi-di method, the shearer loader cuts the coal in one single direction when in uni-di mode. On the return trip, the floor coal is loaded and the floor itself cleaned. The haulage speeds on the return trips are restricted only by the operators’ ability to move along the longwall face, or that of the haulage motors in a fully automated operation. The sumping procedure starts near the head gate. The low utilisation of the machine because of cutting just one web per cycle is the main disadvantage of the sequence, and the coal flow can be quite irregular depending on the position of the shearer in the cycle.
3. Partial web
The main benefit of half or partial web cutting sequences is the reduction of unproductive times in the mining cycle, resulting in high utilisation of the machine. This is achieved by cutting only half a web in mid-face, with bi-directional gate sequences. The full web is mined at the face ends, with lower speeds allowing faster shearer operations in both directions in mid-seam. Besides realising higher haulage speeds, the coal flow on the armoured face conveyer (AFC) is better balanced for shearer loader trips in both directions.
4. Partial opening
The advantage of the partial-opening cutting sequence is the face is extracted in two passes. The upper and middle part of the seam is cut during the pass towards the tailgate. In the last part of this trip, the equivalent of a machine length, the leading drum is raised to cut the top coal to allow the roof support to be advanced. On the return pass, the bottom coal is mined with the advantage of a free face and a smaller proportion of the leading drum cutting coal. As with the trip to the tailgate, the leading drum has to be raised a machine length ahead of the head gate.
5. Partial web-partial opening
On the first pass the shearer cuts the middle part of the coal only with a partial width of the drums. The bottom coal is also mostly cut during the first pass. Prior to mining the top coal that has been left in place, the machine has to raise the leading drum a machine length before reaching the tailgate. During the return pass, not only the top coal is mined as the whole web is sliced out to reach the full drum depth.
Using a series of parameters researchers were able to conduct theoretical comparisons of productivity between different shearer loader cutting cycles. Results were based on three main parameter sets: face length varied from 150-400m; the seam height was set from 2-6m; and coal hardness was simulated with specific energy values of 0.2, 0.3 and 0.4kWh/m³
In the model, researchers used the latest generation Eickhoff SL300 and SL500 series shearer; maximum web to be cut was one meter; and AFC data was taken from DBT’s PF4 series.
The model calculated the individual time based on the haulage speed for each sub-operation, as in sumping processes, production runs and unproductive time during the swivelling of the drums while changing directions.
The fastest haulage speeds possible and unproductive times determined the cycle time that was used to calculate the tonnage of the cutting sequence in tons per hour.
The capacity calculations revealed uni-di cutting sequences to be the least productive. The bi-di cutting sequence increased its productivity with longer face lengths but did not catch up with the partial web and partial web-partial opening cutting sequences with variable cutting depth. The latter sequences appeared to be best practice benchmarks. The partial opening cutting sequences showed good performances with short faces and in soft coal. With increasing geological and geotechnical boundaries, its capacities dropped down to the lower level of uni-di systems.
Starting point influence
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