Conversion of Tashkent power plants into power heat generation for district heating purposes in Tashkent

Vladimir Bulanin, PhD, Nikolai Barmin, MSc

Tashkent Power Plant - Uzenergonaladka

The USSR's Power Programme [1] provides a further combined electrical and thermal power production development as one of the main fuel saving procedures. The combined electrical and thermal power generation is carried out by putting into operation of new extraction turbines and by converting ageing condense turbines as well. The condensing turbine conversion is usually carried out according to the plans of Harkov's Branch of the Sojusenergoremont Central Engineering Office (HBSCEO). These plans are based on the technical specification of the corresponding power plant.

The Tashkent power plant, with 12 power generating units of the same type with TGM-94 boiler and K-160-130 turbine, total power of 2000 MW, is favourable to conversion into power-and heat load operation among 27 other power plants ( with 90 power units ) The scientific and technical council of the USSR's Ministry of Energy has recommend that power plant to the city of Tashkent as the base source of total thermal power of 2400 MW.

The Power plant is situated about three km from one of the largest cities in the USSR, having a population of more than two million, mainly living in blocks of flats. District heating with an opened hot water supply system carried out from heat generation plants (district heating plants ).

The Tashkent power plant was constructed without considering its further enlargement of floor area, conversion, or, for example, power and heat load operation. So, there is an increasing demand of generating a wide array of new technical solutions of the issue, which are caused among other factors by: 
    • limited possibilities for heating steam extraction from turbines;
    • uncovered and uncomfortable configuration of operating equipment;
    • inadequate unit capacity of manufactured heating equipment;
    • plant location not far from city potable (drinkable) water pipe lines ;
    • peak heating sources in heat consumption centres, etc 
The Tashkent power plant conversion is carried out to fit the developed design, considering the above-mentioned facts. As is shown in economical and technical calculations it makes sense to use of all turbine equipment for one-tube district heating from the power plant to the city's peak heating sources. The amount of feed water varies equalling the city hot water supply consumption in district heating which is 3.6-4.2 m3/s. Under these conditions water supply load over the course of the year may change in the range of 800-1200 MW, water temperature changing from 60-65 oC in summer to 120-130 oC in winter for district heating load in partial discharging. Besides, about 300 MW of thermal energy is transferred to heating of the neighbouring regions by two-tube district heating network. So that the total thermal power 2 400 MW may be reached, as mentioned above, that equals the combined thermal plant capacity with six T-250/300-240 turbines.    When developing technical specifications one of the main problems is the location of the water heater. Usually it is manufactured and delivered as a unit with an extraction turbine and is placed close by. However, there is no room in the K-160-130 turbine cell for installing a water heater, required by the safety regulations and operations. That is why after a thorough study of the situation, the decision concerning the installation of the heater in a separate building has been made, and in doing so the water heaters are connected to each turbine by individual steam lines. There was no place on the plant sits for a water treatment plant of total capacity of 4.2 m3/s (15 100 ton/h ), and so we had to place it outside the plant. The above mentioned and other facts imposed a great responsibility on the operating personal, who had to participate immediately in realisation of the operating plant's reconstruction and conversion. A comprehensive analysis of these problems will be omitted in the present article ( it may be a special subject for discussing ). We will dwell on the basic flow diagram of the Tashkent power plant conversion.

In the first place, the conversion of the basic flow diagram of the plant seems relatively simple but there are facts, which complicate the matter, as follows:
    • total combination of interconnected facts;
    • non-standard character of conversion;
    • lack of analogues in domestic practice of power engineering.
So, originally HBSCEO accepted feed water heating as follows:
    • condensers of two turbines ( the first stage of heating ),
    • water treatment (TW) - vacuum deaerators ( heating up 15 oC ) ( the second stage of heating);
    • main water heater, connected to regulated extraction of eight turbines (the third stage ) 
District heating feed water which is heated in the main water heater up to 120-130 oC is used as heating agent of a vacuum deaerator. But such an arrangement of the vacuum deaerators (DV) results in a vital water temperature rise ( 15 oC ) before the second stage of heating ( up to 68 oC ) and the condenser pressure rise from 14 to 32 kPa as well. So the powerful pressure rise in a condenser reduces the combined generation of electric power and simultaneously increases fuel consumption more than 60 000 tones per year.

Besides, the low water temperature before VD does not ensure its normal operation.

Collaborative investigation [2], which was carried out by the Tashkent Power Plant and Uzenergonaladka, made it possible to conclude the expediency examination of successive multistage feed water heating up to 50-53 oC in the condensers of different turbines, using the whole condensers surface. That the basic flow diagram of the turbine condensers, are used as main water heaters, contributes to more effective use of steam heat drop available in a turbine and more flexibility in operation as well. It is appropriate to carry out further heating by steam of higher potential from newly created extractions on the rest of the turbines.

The suggested base flow diagram of TW connection before the first stage of heating by the Tashkent Power Plant deserves important consideration. The advantage of the base flow diagram is chemically treated water supply to the first stage of heating, and considerable reduction in the number of transit pipelines of 1200 mm diameter on the plant territory as well. In this case effective decarbonizator operation is needed to ensure removal of dioxide from the feed water at a temperature lower than 18 oC.

An illustration (made by Tashkent power Plant) gives a more refined version of the base flow diagram of the successive multistage district feed water heating in the condensers of all plant turbines. The rated values of water flow rate and temperatures in different places of the diagram at nominal winter operation are presented.

  Base flow diagram of the district heating feed water heating after the Tashkent Power Plant conversion (alternative): 

1 - condenser converted for feed water heating;  2, - turbines with steam extraction for one-tube district heating; 3 - turbines with steam extraction for two-tube district heating; 4 - district heating department; 5 - district heating water pump; 6 - main feed water heater for one-tube district heating; 7 - feed water line for two-tube district heating; 8 - main feed water heater for two-tube district heating; 9 - feed water line for two-tube district heating; 10 - recycling line; 11 - temperature regulator; 12 - circulation water inlet; 13 - heat-carrying water supply in summer operation

 In winter potable quality water at the rate of 4,6 m3/s and temperature of 5 oC is supplied to the mixing unit before TW for the district heating. To ensure optimal TW operation the part of chemically treated water, which was heated in condensers up to 50-53 oC, comes back via pipeline 10 to TW. Regulator 11 maintains water temperature before TW at (16 + 2 oC) level by changing heating water rate from 0 in summer (when raw water temperature is 18 oC) up to 1.7 m3/s in winter. That technical solution allows to maintain a constant water temperature before the first and the following three condensers, when water is being heated successively inside them. Hence the district heating hot water load following the water rate (consumption) is made possible by synchronous (simultaneous) changing of waste steam rate to condenser 1, in such a manner that the condenser inlet and outlet temperatures maintain on the operating level.

Water heated up to 50-53 oC is supplied to vacuum deaerators where it is degassed and heated up to 65-68 oC. District heating feed water, which is heated in the main water heaters (MWH) up to 120-130 oC, is used as heating agent of VD.

After VD the feed water is supplied by the circulating pump 5 to main water heater 6, which are fed from extractions of turbines 5-10. Then the feed water is supplied to the header of the common plant district heating. A portion of feed water, about 0.3 m3/s is supplied via pipeline 7 to the return pipe of the two-tube network to compensate the waste of district heating water. The main flow of the feed water is supplied to the district heating plants which are the peak heating sources (PHS), and placed close to the customers areas. There the feed water is supplied to the outgoing or incoming pipeline of the two-tube district heating network and reheated in the water heating boiler up to the required temperature according to the load curve of 150-170 oC. 

As a result of the conversion of all turbines into power and heat generation units, the total thermal power plant output in winter is equal to 2400 MW during which about 800 MW is generated in four turbine condensers at the expense of the complete use of waste heat with a minimum electrical output reduction. When this takes place heat output is carried out as follows:
    • the independent two-tube district heating of the neighbouring region - 350 MW;
    • one-tube city district heating - 3050 MW, including hot water supply - 1000 MW and district heating - 1050 MW. Since the total fuel consumption in the plant has not changed after its conversion, the district heat will be generated at the expense of the waste heat reduction with cooling water in turbine condensers from 2800 up 900 MW. Thanks to the above mentioned facts the fuel saving will reach about 0.7 - 1.0 million tons a year. The fuel saving would be account according in [ 3 ] more closely.
In summer to ensure the heat power generation of 900 MW for the city hot water supply consumption only five turbines are needed, three of them are operated at successive water heating in the condensers and two turbines with extended range of the changing of stem pressure in the turbine extraction. Main water heater 6 is turned off the steam lines, and the last portion of the feed water after circulation pump 5 is supplied to water heater 8, where it is heated up to 110-117 oC and directed through line 13 to VD as a heating agent. The hot water two-tube district heating is carried out via pipe line 7 from the one-tube system.

Operation parameters of the turbines and of the base flow diagram of the feed water heating for nominal winter and minimum summer load are presented in the table. The minimum summer load deals with operating conditions when the temperature of raw water is considered to be 18 oC.

 

 *When circulating water is directed into the second part of the condenser, the total cooling water consumption would equal 5.9 M3/s

** With account of TW loses

At the first stage of heating the pressure in the condenser does not actually differ from the one in nominal operation (see the table). In summer operation ( when the feed water flow rate is reduced to 3.6 m3/s one half of the condenser is disconnected the feed water line and connected to the circulating water line, having a temperature of not more than 18 oC. It allows turbine 1 operation at nominal electrical output.

In the second and third stages of heating the steam pressure in the condensers increases up to 9.5 and 11.3 kPa accordingly. However, considering the large steam flow rate in the condenser (95 kg/s) and the relatively low temperature of the exhaust stage ( lower than 50 oC ), an operation like this will ensure the required long-term reliability.

The steam pressure in the condenser of the fourth stage of heating exceeds the alarm level (14.3 kPa). Besides, at that pressure the sixth stage low pressure cylinder (LPC) does not generate effective power. So to ensure the tolerance temperature of the exhaust stage the removing of the moving blades of the sixth stage of the LPC of turbine number 4 has been considered.

  The fifth stage of heating consists of district heating units (deaeratotor, circulating pump, main feed water heater), which are fed by steam from turbines reconstructed for the plans of HBSCEO on two modifications:
  • for turbines numbered 5-10 regulated steam extraction with pressure variation range of 0.34-0.54 Mpa is arranged;
  • for turbines numbered 11 and12 regulated steam extraction with pressure variation range of 0.24-0.53 Mpa is arranged. The distinction is due to utilisation of turbines nos11 and 12 for heating of feed water (up to 110-117 oC), used as a heat-carrying agent in deaerators in summer. This solution improves efficiency of total base flow diagram of the feed water heating.
When developing the base flow diagram of the city district heating particular attention has been given as follows:
    • reliable heat transmission from the power plant and heating plants to customers in an emergency;
    • the possibility of circulating water feeding to condensers of turbine no 1-4 makes possible:
    • the base flow diagram survivability and flexibility;
    • the possibility of increasing heat delivery by stages with minimum decreasing of power plant electrical output. So , - turbines operation when feed water is absent;
    • - mutual reservation of turbines nos. 1-4 in summer;
    • the minimum decreasing of the total Power plant electrical output in the increased load period and at the staged conversion;
    • change-over of any of turbine to electrical load curve operation.
One of the turbine condensers of the final stage of heating is considered to reserve the base flow diagram of feed water preheating (condensers of turbines of nos. 1-4).

A condenser of that turbine is converted by analogy with condenser of water preheating stage, but at nominal operation circulation water pumped through it.

The unit configuration of the district heating equipment is pioneered into domestic practice. The same type units (deaerator, district heating pump, main water heater) placed in a separate building outside, will ensure systematic uniform construction of the makeshift wall of that building, and put into operation the district heating equipment simultaneously with the increasing heat load.

All technical solutions have been adopted according to comprehensive investigations of alternative designs. When calculating it was assumed that the turbine parameters and technical data after conversion would correspond to the average effectiveness of the turbine which was made during overhaul repair. The coefficient of cleanliness of the condenser tubes is taken to be 0.8. The nominal rate of the steam flow through the turbine is taken to be 95 kg/s, which corresponds to nominal operation of turbine K-160-130 at nominal electrical load of 160 MW [4]. According to procedure [5] the feed water mean arithmetic temperature difference is taken to be 15 oC.

In parallel with the above mentioned procedures, when doing combined conversion of the plant, of the new technical solution, which are not being presented in this article, will be used.

The one-tube district heating system operation will be put into operation after installing of pipelines supplying district heating feed water to peak heating plants at the rate of 0.9 - 1.1 m3/s. That rate through one half of condenser K-9115 HTGZ will ensure water velocity in the condenser tubes of 0.6-0.7 m/s and an acceptable heating steam heat exchange as well.
 
 

Conclusions 

1. The connection of the Tashkent Power Plant to the city district heating will allow to integrate existing and designing sources of heat in a large district heating complex with the Power plant design heat output of 2400 MW. Doing so the Power Plant output will decrease about 500 MW. In summer the Plant turbines will cover actually all the load of the hot water supply system (about 900 MW). Simultaneously, the Power Plant would participate in the regulating operation of the Uzbekistan Power system in distribution of the electrical load from 700 to 1800 MW.

2. For the first time in the domestic power engineering the condensers of a large condenser turbines such as K-160-130 HTGZ will be used as the preheating stage of a feed water for the one tube heat transmission.

3. The arrangement of feed water treatment and its heating up to 120-130 oC with consumption of 4.2 m3/s when operating the Power Plant for the city district heating ( population being 2 million ) is a very complicated scientific-technical problem. When doing conversion, design and experimental operation it is necessary to carry out investigation to ensure reliable and effective plant operation in the district heating system and to accumulate experience for other plants. 

4. Thanks to combined generation of heat and electrical energy in the Tashkent Power Plant the fuel saving will equal
0.7 - 1.0 millions tons per year, or 25-30 % of the fuel consumption.

 Bibliography  1. USSR Power Programme for a Long Perspective. Moscow.: Politizdat., 1984. 
2. V. Bulanin., N. Barmin. The Design of the Optimal Base Flow Diagram of the District Feed Water Heating in Condensers of Turbine K-160-130. Moscow.: Electrical Stations no 8, 1985.
3. V. Bulanin., E. Rodimkin. Method of Steam Turbine Unit Power Balance Analysis. Moscow.: Electrical Stations no 11.
4. Normative Net Energy Curve of Turbine K-160-130 . Moscow.; ORGRES, 1975.
5. Vacuum Deaerators Adjustment and Operation Procedure. Moscow.; Sojuztechenergo, 1980.
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