Tejo Power Station operations

The basic principle of a steam power station is relatively simple: the combustion of fuel releases heat, transforms the water from the liquid state into vapor. This drives the turbine, which in turn, is coupled to the alternating current generator, producing electrical energy.

Due to the circumstances at the time of electricity in the " Central Tejo", however, was somewhat problematic. They needed, among other things, a complex internal circulation system for water and air supply, as well as for the preparation of the fossil fuel material, which in the case of the power station mainly consisted of coal.

Coal

The laden with coal freighter came mostly from the UK, went up the river and laid directly at the level of the steam power station; over narrow bridges between ship and quay, the coal was unloaded and stacked in the coal camp " Praça do Carvão " on different clusters. Here began the actual production process.

In tram the coal was pushed from the stockpiles to the screen and the coal mill of the burner system. A paddle wheel type it carried up into the mixing silos. There confounding coal types, so the fuel mixture could be matched for a perfect firing the boiler.

Mixing it up then it went by lifting blades further up to the conveyor system of the burner above the burner space. From here, the coal falls into the stoker and bulk pipes on the traveling grate inside the boiler, where it begins to burn slowly and reaches temperatures of about 1200 ° C.

The cycles of the boiler

The boiler is basically composed of three cyclic processes: water vapor, air - smoke and ash. The functions of each of these cycles are essential for the overall process, in which they complement each other. The task of the cycle water vapor is to transform the water from its liquid state into vapor; the ideal use of the cycle air - smoke affects the profitability of the boiler; Finally, the ash compartment, where not only the ash produced by combustion, but also unburned carbon residues are disposed of.

The necessary for steam generation water is treated and flows through a closed piping system through the preheater into the back of the feed water tank, the drum ( Portuguese: " Barrilete ") above the boiler. It is considered as a link, at the same time as it retains water and steam. From there, the water runs down the inside of the combustion chamber down in the so-called '' Bailey '' walls of the water-tube boiler. Made of cast iron, they are designed so that they keep the heat constant while circulating in many vertical tubes water evaporates. The mixture of steam and water increases again to the drum, where the steam flows to the superheater, a group of pipes, also in the interior of the combustion chamber. The high pressure (38 kg / cm ² and 450 ˚ C in a high pressure moment ) resets the wet steam into dry to. So he can be fed to the steam turbine in the turbine building.

Air supply is as important as water and steam for coal firing. The main part of the induced draft fan located at the rear of the boiler; to use the high temperatures of the exiting warm air, it will be blown by a Vorabsaugrohr the air preheater and passed from here through a fan for traveling grate to there to fan the flames. The smoke produced is ejected from flues through the chimneys, however, previously derived the residual heat to the furnace and filtered to reduce the pollutants.

The last cycle of materials, the ashes, takes place under the boiler. Here are three V-shaped container to catch the unburnt coal, burned only partially and the coal ash. The tray under the downpipes, that is, at the beginning of the combustion grate, coal collects those pieces that miss the ribbon at the feed; the average semi- charred, falling from traveling grate by vibration of the boiler; the thus recovered fuel comes back to the camp and is again integrated into the power system. The third and final ash hoppers under the boiler Stainless, was equipped with a shredder, the under -mixed water to cool the coal ash and soften. In these wagons was then carted out to the coal storage yard for ash depot.

Water treatment

As already mentioned, is fed to the boilers only clean water in a closed duct system; contrary to the obvious assumption in the central river water would be used by the Tagus for evaporation, tap water from the municipal supply system (as well as from an artesian well on the premises of the power plant ) consumed. From the huge depot on the high-pressure boiler room, the so-called " Castelo de Água " (= water castle ), where the water is stored, it comes in the water house " Sala de Água ". Here are three operations are performed: water treatment, preheating and pumping.

The treatment of the water is of crucial importance, since its impurities, harmful ingredients and gases dissolved therein to corrosion and leaks in pipes and turbines, as well as for the oxidation of lead pipes. In addition, deposits and scale formation on iron and steel can damage the system and reduce the efficiency of the power plant. Therefore, all specific for the power plant water must first be examined in the laboratory and subjected to a complex treatment and pre-treatment process, including Decanting, filtering, chemical Abgasung etc. before its feeding into the system in the form of pure H2O.

To increase the efficiency of combustion, the so- treated water is not preheated. For this, one uses the recovered turbine exhaust. In this heat exchange in the preheater to temperatures up to 130 ˚ C. result Then, the purified water has to be set only at a predetermined pressure before it can be fed into the boiler. Which take over the pump in the water chamber at a pressure of 52 kg / cm ², is sufficient to overcome the back pressure of the boiler drum.

Turbo-generators

The superheated steam produced flows under high pressure (38 kg / cm ²) to the turbo - AC unit, wherein it emits the absorbed energy in the form of kinetic energy of the steam turbine, and the power is again converted by the connected alternator into electrical energy. A turbo-generator set comprises a turbine and an AC generator, hence the name turbo-generator. The turbine has eight wheels with two blade rings, and another seven with only one ring. Through the inlet valve of the boiler steam enters the distribution depot in the turbine housing. By controlled opening of the injectors, it flows from there on through the Venturi tube, and reaches the first rim with sufficient energy to accelerate the turbine at 3,000 rpm. The following wheels of the pressure of the steam decreases gradually in order to equalize the capacitor, but at a constant flow rate.

Thus, the turbine wheels are set in motion, which in turn bring about an engine the alternator to rotate and thereby produce electrical energy for distribution to the consumer, but also for their own use of the center. The star-shaped coiled alternator provides 10.500V three-phase current with a frequency of 50 cycles per second ( Hz). The excitation current of the generator is produced by an exciter, a directly coupled to the master station DC generator, a voltage of 170 volts and a current of 340 amperes performs at full power.

The power generated by the respective generator is directed to the output bus blocks. Each collective bar or rail is for a substation. From there, the variety of clients were provided. Apart from the main bus bar, designed for a substation with 10 kV power, for the power grid of the city of Lisbon, there were two other gang- bar, with a 3.3 kV and the other at 30 kV. The first and older of these two output busses fed the associated auxiliary drives of the headquarters, the second with 30 kV went from two lines, one after Marvila and from there to Vila Franca de Xira, while the other led directly to Santarém to the industrial plants upstream along to supply the Tagus.

Capacitors

Once the hot steam has turbine blades set in motion, flowing in the capacitors, and then again evaporate again in the liquid state into the steam boilers. The waste steam injected into the condenser and condensed in contact with the cold water -filled tube bundles within the Abdampfraums. The cooling water came from the Tagus. Three feed pipes and a return pipe forming a siphon, which is pushed into the water flow channels. This prevented the mixing of river water with the pure water used in the boilers. Because, as already explained, the river water flows through the cooling tubes, while the exhaust is in free space of the condenser.

The resulting by the condensation of the exhaust steam water is sucked from the condensate pumps and conveyed again to the kettle drums of the boiler, but not without last but not least, to go through only the preheater, tanks, feed pumps and the superheater. The recovery of the condensed exhaust steam and re-use as feed water for the boiler includes the water-steam cycle of a thermodynamic power plant, and as such was " Central Tejo" is no exception.