Standard Technical Features of BTG System for Supercritical 660 ...

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Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal Unitsdocument. The Central Electricity Authority (Technical Standards forConstruction of Electrical Plants and Electric Lines) Regulations, 2010,stipulate the maximum turbine cycle heat rate for supercritical units as 1850kcal/kWh with turbine driven BFP and 1810 kcal/kWh with motor driven BFPand this would require adoption of minimum steam parameters of 247 kg/cm 2 ,565/ 593 0 C at turbine inlet. Efficiency improvement of about 2.38 % over thepresent 500 MW sub-critical units is expected with these minimum steamparameters. Parameters higher than above may also be adopted to achievebetter heat rate/ efficiency as per standard practice of OEM.Supercritical technology being a recent introduction in the country, a briefintroduction of this technology along with implications on design/constructionhas also been covered hereunder.1.2 Implications on Design/ConstructionAdoption of supercritical technology involves several design/constructionchanges intrinsically associated with this technology. Some other issues alsoemanate due to larger unit size of supercritical units. These are discussed asunder:1.2.1 Evaporator designUnlike at sub-critical pressures, there is no co-existence of the two phases,water and steam at supercritical pressures and there is no fixed transition pointfor phase change like the drum in sub-critical boiler acting as evaporation endpoint. Therefore the standard circulation system (natural/assisted), whichrelies on the density difference between steam and water and steam separationin drum is no longer suitable for supercritical units. Instead, supercritical unitsnecessarily use a once-through type of boiler. These boilers also operate insubcritical recirculation mode, subcritical once- through mode andsupercritical mode under different pressure regimes.Many types of supercritical once through boiler design exist. While someallow complete variable pressure operation, where the pressure across wholeboiler is varied (reduced at low loads), others operate at fixed evaporatorpressure and thus involve loss of energy for part load operation. Due torequirement of cyclic operation, variable pressure type evaporator system hasbeen adopted in this document.1.2.2 Water walls designSupercritical units deploy spiral wall furnace using smooth tubes or verticalwall furnace with rifled tubes. Spiral wall furnace increases the mass flow pertube by reducing the number of tubes needed to envelop the furnace withoutincreasing the spacing between the tubes. It also leads to uniform heatabsorption in each tube rendering the spiral wall system less sensitive tochanges in the heat absorption profile in the furnace. However, it involves acomplex support structure and is relatively difficult to construct and maintain.2
Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal UnitsThe vertical water wall design uses rifled tubing for improved cooling effectwith uniform temperature across the walls and is also operating satisfactorily.Its advantage lies in ease of construction and maintenance. Keeping in viewthe fact that various manufacturers have standard water wall configurationswhich are proven, both the options viz. spiral and vertical tube designs havebeen included.1.2.3 Boiler start up circulation systemsSupercritical boiler starts operating in the once through mode beyond aparticular minimum load of say 30 to 40 %. Below this load, it operates in thecirculation mode and needs a separator and circulation system for water steamseparation; the separated water is circulated back to the boiler. Generally, twotypes of circulation systems are in use. In one of the systems, separated waterfrom the separator is led to the deaerator/ condenser and is circulated to thefeed water system through boiler feed pump. This system is simple andrelatively inexpensive but involves loss of heat from boiler during cold startup.In other system a circulation pump is provided to circulate the water fromseparator directly to the economizer. This prevents heat loss from boilerduring cold startup but adds to cost. Both systems have also been provided insome of the supercritical units to improve reliability. Other proven standardsystems for boiler startup drain circulation system are also acceptable.An alternate drain connection to main condenser has also been envisaged toenable start up of steam generator even when the Start up drain recirculationpump is not in service and for initial flushing of boiler to achieve water/ steamquality.1.2.4 HP turbine extractionIn the sub-critical units upto 500 MW, the highest pressure extraction in theregenerative feed heating cycle is from the HP Turbine exhaust. Thisconventional design with highest feed water extraction from CRH line is ableto achieve a final feed water temperature of about 255 0 C. Designs withextraction from HP turbine are available leading to increased final feed watertemperature of about 290 0 C or higher. The higher feed water temperature dueto HP extraction leads to a marginally better turbine cycle heat rate. It alsoinvolves additional heaters. Keeping in view the advantages of higherefficiency, design with HP turbine extraction has been adopted.1.2.5 Boiler feed pump configurationA number of configurations viz. 2x50% TDBFP+2x30% MDBFP, 2x50%TDBFP+1x50% MDBFP, 2x50% TDBFP+1x30% MDBFP, 3x50% MDBFPare in use for boiler feed pumps in large size units. The normal practice beingfollowed in the country for 500 MW units is to provide 2x50 % turbine drivenBoiler feed pumps (TD-BFP) and 1x50 % motor driven BFP (MD-BFP). Theabove configuration has the advantage for having same pump for both TD-BFP and MD-BFP leading to interchangeability of spares etc. and better3
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Standard Technical Features of BTG System for Supercritical 660 ...

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