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Useful Heat Production

There are two district heating stations at the plant employed for the heating of the towns of Velenje and Šoštanj, and hot water supply to all the factories in the Šaleška valley. We transfer the necessary thermal energy to the users and systematically deposit the harmful by-products of burning, we filter the flue gasses for SO2 and the rest is exhausted into the higher strata of the atmosphere.

The main and only distributor of heat energy provided by the Šoštanj thermal power plant is the Komunalno podjetje Velenje (Public utility company Velenje) – Energetika (Energy) business unit. This unit which has a 265MW capacity line, with a guaranteed supply of 219MW - calculated for accounting purposes*, operates on four different pressure and temperature regimes. With a distribution network of 141.5km, it supplies 90 percent of the population of the Šaleška valley, including remote settlements. The principal network that extends east and west of the Šoštanj plant has a maximum flow of 2,115m3 of water per hour with a temperature of 140 °C that is reduced to 80 °C on the return line.

The district heating system was built from 1959 on when the three-pipe heat system, then supplied from the Velenje thermal power plant, came into operation. Every three or four years the system was extended and modernized and in 1995 it was completed with the connection of the Topolšica resort locality.

The district heating stations can produce parallel output into the principal line which is necessary in the winter, or one of them can be shut down as we do in summer. The stations produce heat according to the parameters set by the distributor. The maximum possible quantity of heat produced is 500KWh. On average, we produce from 360 to 400 million KWh. A power plant with a cogeneration facility is more profitable as the energy efficiency is increased this way.

District heating station one, which is the older one, has a capacity of 90MW. To produce heat it needs fresh steam directly from boilers 1-4. This steam must be reduced in pressure and cooled to a suitable temperature, so it can be used for water heating in the heat exchangers. This way we waste a part of the superheat energy, which could be used to drive the steam turbines and produce electric energy. The station can also be supplied by the steam exhaust conduits of generating unit four. This steam has already given away its superheat energy on the turbine and could produce some more mechanical energy on lower pressure turbines, but it is much more profitable to exploit the heat of this steam for district heating. This way we avoid losses that would occur if the steam continued its way into the condenser. If at all possible we use the exhaust steam from the turbines of gen. unit four for the operation of district heating station one, but because of the operating regime of the power plant this is not always possible. The schematic shows the cooling water from the principal line, pumped by the circulation pumps, flowing through the condense cooler and then flowing through the low-pressure heat exchangers. These heat exchangers give the water the bulk of the heat and then the water goes on to the high-pressure heat exchangers where it is further heated to a temperature set by the distributor. The district heating steam cools down to water during its flow in the heat exchangers and is pumped back to the condenser of the turbine from which it was extracted after use. The district heating water temperature is 140 °C in the winter and 120 °C in the summer and the pressure varies between 16 and 20 bars according to the temperature. The return water has a temperature of 80 °C and a pressure of 8 bars.

District heating station two was built more recently and is more efficient; therefore we operate mainly this heating station. It has 110MW of power installed in the heat exchangers. It is fed with the steam from the high-pressure turbine that flows back to the boiler for reheating. This steam has already released its superheat energy and is cooled on the turbine from 540 to 340 °C. The secondary supply of steam comes from the steam turbine exhaust conduit named A4; this steam has also already released most of its energy on the turbines and has cooled down to 240 °C, a temperature suited to heating hot water of the principal district heating conduit. There is also a pipe from the generating unit four steam line, which has a purpose built conduit for district heating station two. This steam is from the A8 outlet and goes to the reheater.

When building the desulphurisation facility we needed to find a way to cool down the flue gasses so they could be processed in the scrubber. This was achieved by installing a bank of heat exchanging tubes, where the water from the primary water heating system flows. Here the water receives its first cycle of heating and then continues on to the heat exchangers in the district heating station. Thus, we recover a large quantity of thermal energy that would otherwise be lost through the chimneystack.