Surface belt conveyors are mining machines that consume up to 10% of electricity in lignite mines. In Poland, coal conveyors with a constant belt speed are typically used. By applying new techniques for the construction of conveyor drive systems that allow for belt speed regulation, electricity consumption can be reduced by up to 50%. Coal transport is highly energy-intensive, and this process must currently be strictly monitored in terms of minimizing electricity consumption. Poland is one of the largest producers of lignite in the world, and with an annual output of approximately 62 million tons, it is also an important climate factor contributing indirectly to the reduction of CO2 emissions.
Coal conveyor line with adjustable belt speed conveyors
Many mines still use medium-voltage 6kV slip-ring motors with starting resistors, and sometimes squirrel-cage motors with a hydrodynamic clutch, which ensure a smooth start of the coal conveyor but without the possibility of adjusting the belt speed.
Conveyors without belt speed regulation should mainly be used for overburden transport and operate at nominal load. Operating a conveyor with a nominal belt load ensures the lowest energy intensity of transport. In many coal transport lines in Polish mines, conveyors with a constant belt speed are used.
The power of motors in the drive stations of overburden and coal conveyors ranges from 250 kW to 1.2 MW, in quantities of 1 to 4, depending on the length of the conveyor and the width of the belt. The mining power supply network is an IT network. The motors of belt conveyor drive stations are usually powered by 6 kV, and for lower power ratings, by a low-voltage network (for example, 3 x 0.5 kV).
Due to the varied geological structure of coal seams, it is not possible to ensure a constant output of the coal excavator to simultaneously provide a nominal load for a belt conveyor with a constant nominal speed (5.24 m/s). In such applications, to ensure nominal belt filling, it becomes necessary to adjust the conveyor belt speed to the instantaneous output of the excavator (photo 1).
The article presents a study of an actual set of coal conveyors with adjustable belt speed, commissioned at the “Konin” Lignite Mine at the beginning of 2006 at the new “Drzewce” open-pit mine. This is currently the only application of this type in Poland. Globally, they are being used more and more frequently.
The coal line contains a set of 7 belt conveyors of various lengths, from about 200 m to about 2,000 m, horizontal or inclined. In the analyzed coal line, the conveyor drive stations are equipped with energy-efficient squirrel-cage motors and electronic frequency converters, allowing for the regulation of the motor rotational speed, and thus the conveyor belt speed.
The “Konin” mine, one of three lignite mines in Poland, extracts annually, and therefore transports approximately:
[65-70] million m3 of overburden (x1.5 tons),
[90-95] million m3 of water (x1.0 tons),
[10-11] million tons of coal.
Coal extraction at the “Konin” mine accounts for nearly 20% of national production. The “Drzewce” open-pit mine, where conveyors with adjustable belt speed were used, has a coal line approximately 5.9 km long (figure 1).
The “Konin” mine also operates 3 other open-pit mines with conveyors with non-adjustable belt speed (v = 5.24 m/s) and 320kW/6kV slip-ring motors, with a total length of coal conveyors on the order of 20 km and overburden conveyors with a total length on the order of 35 km. The energy intensity of the coal line at the “Drzewce” open-pit, operating with the belt speed of all conveyors reduced to 2.5 – 3 m/s, is almost 50% lower than the energy intensity of conveyors operating at a constant speed of 5.24 m/s in the other open-pit mines.
The issues of coal transport costs in an open-pit mine using conveyors with non-adjustable and adjustable belt speed will be the subject of analysis in the further part of the article. The primary goal of this analysis is to demonstrate the necessity of rapid modernization of conveyor drive stations that do not provide the possibility of belt speed regulation. According to the author, conveyors with non-adjustable, constant (nominal) belt speed should be urgently eliminated from coal transport lines.
The article comes from the journal “Logistyka” 4/2009.