Mining technology

The lignite of the Mátra and Bükkalja was formed some 5 to 8 million years ago in the so-called Pliocene Ages, at the end of the tertiary period of recent epoch in historic geology. The occurrence of Mátra-Bükkalja was emerged from the destruction of the subtropical marsh flora accompanying the creeping bays getting sweeter in the Pannonian inland sea when marsh cypress (Taxodium), leafy trees (e.g. alder, willow) and huge volume of sedge and reeds were accumulated. The lignite represents the initial phase of turning into coal and the plant components in them can be recognised well. Due to the characteristics of seam formation, the quality of lignite is characterised by a comparatively low calorific value and a high ash and moisture content. There are sandy, muddy and clayey sediments between and above the lignite seams.

The lignite production in Visonta and Bükkábrány takes place with surface/open-pit mining technology. According to surface mining technology, first, the overburden layers (clay, mud, sand, etc.) above the lignite seams have to be removed, then, after lignite is excavated, dead materials have to be backfilled into the open pit of the mine. As soon as one of the trenches is depleted, the mine goes further on and any exploited overburden material is to be put into the hollow of the previous area.

Three years before coal production in the area to be excavated, the preliminary drainage of the area started. During which the water from the cover and intermediate water-storage layers was drained to the deeper water-storage formations, from where the subterranean waters were raised to the surface by submersible pump wells installed at the edge of the open-pit mining area.

The production of overburden layers is primarily performed with bucket-wheel excavators. Bucket-chain excavators are also used periodically in overburden production. It is also necessary to continuously involve one-bucket excavators during specific technological processes.

Coal excavation is performed with bucket-chain excavators and one-bucket excavators. The bucket-chain excavators and bucket-wheel excavators dump the excavated material onto a conveyor line. The frontal, moorband and dump-side conveyor lines make up the conveyor systems consisting of elements with belts of 1200, 1400, 1600 and 1800 mm width. In the system of conveyor lines transporting coal, there are slope bridges of Hungarian development and production for bridging over great depths of the system of slopes in the mines and for crossing conveyor lines transporting overburden, which are capable of bridging over a level difference of 22 to 25 meters.

Placing overburden material is performed with spreader equipments. The excavators, conveyor lines and overburden spreaders are connected together into chains of machines, conveyor systems. The so-called direct-dumping system operating without any conveyor lines and consisting of only an excavator and a spreader deserves to be mentioned especially.

Lignite is transported to the crusher row, then to the power plant on a conveyor line from Visonta mines. The crushing equipment operating in Bükkábrány is capable of separating big size fraction, so it is possible to use lignite for residential purposes. Leaving the crushing equipment, the crushed lignite is placed on the coal storage providing delivery for railway, from where the coal is delivered to the power plant by wagons of 55 to 60 tons. The reception of lignite takes place in rotating unloading devices for wagons. The delivery of the coal to the coal storage area of the power plant or directly to the power plant units is accomplished on conveyor lines from the unloading devices.

Surface mining activity of Visonta started on 23 July 1964 with opening the mining pit K-I, which was followed by exploiting the Western mine, then the mine K-II. In the Southern mine overburden production was completed at the end of 2020, coal excavation was completed in the first quarter of 2020. As a continuation of the mining field K-II (Eastern), excavation of the mine Eastern-III (K-III) began on 8 September 2015. The coal from the mine K-III is transported to the power plant on a conveyor line of 11.8 km length.

The other mine of the power plant, Bükkábrány mine is located in Borsod Abaúj Zemplén county, between the municipalities of Bükkábrány, Mezőnyárád and Vatta, predominantly in the vicinity of farmlands, areas under agricultural cultivation. The mine was opened in the spring of 1985, where overburden removal and coal production started in a north-easterly direction, in the so-called mine É (Northern). In 1991 the Southern mining field was also involved in coal production, which is still being excavated.

Part of the mining activity is making the area suitable for recycling, during which technical landscaping and later biological landscaping is performed.


Technology of the power plant

Mátra Power Plant had two energy generating units of 100 MW installed capacity each and three energy generating units of 215 MW installed capacity each until 2006. The units consist of steam generating boilers, turbo-generator equipment groups, cooling systems and flue gas desulphurization equipments.

After the investment of the gas turbines, the installed output of units IV and V reached 232MW + 33MW each, and by the retrofit overhaul of the turbine of unit III, its installed output was increased to 220MW. Thus the power plant’s total installed capacity reached 950 MW.

The daily lignite usage of the power plant is 20 to 25 thousand tons. The coal excavated in the mines of Visonta and crushed to 0-60 mm in the crusher as well as the crushed coal arriving in wagons from Bükkábrány are placed on the coal storage area of the power plant, which is capable of storing 200 thousand tons of fuel. The various quality of the coal quantity deposited on the coal storage area can be levelled out.

Fuel is delivered to the boilers by means of an internal system for coal transportation. All the five boilers are of coal-dust firing, with two draughts in a semi-outdoor design. The heat quantity breaking loose when coal dust is burnt will generate steam in the vaporising systems of the boilers. The length of the pipe system in the steam generating boiler of the 215 MW unit reaches 45 kilometres.

The energy of the high-pressure and high-temperature steam provided by the steam generating boilers will be transformed into electricity by means of turbo-generator equipment groups. The steam turbines are of three and four casings. First, any steam coming from the boiler goes to the high-pressure stage of the turbine, then, after being reheated in the boiler, to the medium-pressure and low-pressure stages of the turbine. The steam coming from the final stage of the turbine is driven to the condensers. The cooling water of the condensers is recycled to the ‘Heller – Forgó’ – type closed towers with air cooling, while the cooling water for condenser no. 3 is cooled in open cooling towers with artificial draft and water-film cooling. In order to provide the increased capacity due to the gas turbine investment, a so-called hybrid-cooling was executed by expanding the air condensational cooling of units IV and V by wet cooling.

Electricity is produced by the generator on common shaft with the turbine. The generator consists of stator and rotor. The direct current driven to the coil of the rotor moving together with the shaft of the turbine generates a magnetic field which generates three-phased alternating current in the coils shifted at 120° from one another in the stator. Electricity is connected to the national grid through high-voltage transmission lines and a transformer station installed 4 km from the power plant in Detk.

Water management of the power plant is characterised by water saving, since there is no watercourse of higher yield nearby. Therefore supplying the technological water demand of the power plant is primarily solved by recirculation (e.g. steam generation, steam condensation of the units, etc.), which provides approximately 99 % of the total water consumption of the power plant. Thus, the use of fresh water is not more than 1% in respect of the total water consumption. The provision of raw water consumption of the power plant is ensured by the reservoir of 8.5 million m3 capacity in the outskirts of Markaz. The drainage area of the reservoir is 50 km2. In addition to surface water streams, filling the reservoir and replenishing water are accomplished from the water pumped by dewatering wells of Visonta mine. The water of the quality required by the industrial water systems of the power plant is provided by its own water supply plant.