CHAPTER 1
INTRODUCTION
1.1 Supercritical Technology
The current thrust of thermal power development in the country is on supercritical units so as to improve the conversion efficiency and reduce carbon footprint. A number of power generation utilities are going for supercritical technology and a large numberof supercritical unitsof 660/800 MWsize are already under construction. Apart fromBHEL and L&T, several other manufacturers are setting up facilities for manufacturing supercritical boilers and turbine generators in the country. Considering these developments, this document on “Standard technical features for BTG system of supercritical 660/800 MW thermal units” has been prepared with a view to evolve common understanding amongst utilities, manufacturers and consultants on design and sizing philosophy for supercritical units. The objective is to incorporate broad functional aspects deemed necessary for specifying major quality and performance parameters unambiguously; and at the same time provide flexibilityto the manufacturers. Steam generator and auxiliaries, being the major focus area for supercritical units, have been dealt with in more detail. This document is not intendedto be detailed specification for use as bid document.
The generation efficiency of coal fired stations depends on the steam parameters adopted - higher the steam parameters, higher is the efficiency. It is with this objective that the steam parameters have been constantly raised from 60 kg/cm 2 for 50 MW units to 170 kg/cm2 for 500 MW units. Supercritical technology implies use of steam pressure beyond the critical point of water/steamwhich is about 225 kg/cm2. Thus, supercritical units use higher steam parameters of over 240 kg/cm2 with various combinations of temperature and pressure. This has been made possible largely through developmentsin materials technology to withstand the higher temperatures and pressures in the boiler.
World over the supercritical technology has been driven by the need to achieve higher efficiency in order to reduce specificfuel consumption and green house gas emissions. Supercritical technology is an established and proven technology with over 500 supercritical units operating worldwide and reliability and availability of supercritical units being at par with that of sub- critical units.Ultra supercritical parameters with pressure of 250-300 kg/cm2 and main steam/ reheat steam temperatures of 600/6100C are also being adopted. Research is underway to further increasethe steam temperatures to 7000C.
Whilst the earlier supercritical units installed in the countryadopted steam parameters of 247 kg/cm2, 535/5650C, higher steam parameters of 247 kg/cm2, 565/5930C are being adopted for later units and have been adoptedin this
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Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal Units
document. The Central Electricity Authority (Technical Standards for Construction of Electrical Plants and Electric Lines) Regulations, 2010, stipulate the maximumturbine cycle heat rate for supercritical units as 1850 kcal/kWh with turbine driven BFP and 1810 kcal/kWh with motor driven BFP and this would require adoption of minimum steam parameters of 247 kg/cm2, 565/ 5930C at turbine inlet. Efficiency improvement of about 2.38 % over the present 500 MW sub-critical units is expectedwith these minimumsteam parameters. Parameters higher than above may also be adoptedto achieve better heat rate/ efficiency as per standard practice of OEM.
Supercritical technology being a recent introduction in the country,a brief introduction of this technology along with implications on design/construction has also been covered hereunder.
1.2 Implications on Design/Construction
Adoption of supercritical technology involves several design/construction changes intrinsically associated with this technology. Some other issuesalso emanate due to largerunit size of supercritical units.These are discussed as under:
1.2.1 Evaporator design
Unlike 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 point for phase change like the drum in sub-critical boiler acting as evaporation end point. Therefore the standard circulation system (natural/assisted), which relies on the density difference between steam and water and steam separation in drum is no longer suitable for supercritical units. Instead, supercritical units necessarily use a once-through type of boiler.These boilers also operate in subcritical recirculation mode, subcritical once- through mode and supercritical mode under different pressure regimes.
Many types of supercritical once through boilerdesign exist. While some allow complete variablepressure operation, where the pressureacross whole boiler is varied (reduced at low loads),others operate at fixed evaporator pressure and thus involve loss of energy for part load operation. Due to requirement of cyclic operation, variable pressure type evaporator system has been adopted in this document.
1.2.2 Water walls design
Supercritical units deploy spiral wall furnaceusing smooth tubes or vertical wall furnace with rifled tubes. Spiral wall furnace increases the mass flow per tube by reducing the number of tubes needed to envelopthe furnace without increasing the spacing between the tubes. It also leads to uniform heat absorptionin each tube rendering the spiral wall system less sensitive to changes in the heat absorption profile in the furnace. However,it involves a complex support structure and is relatively difficult to construct and maintain.
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Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal Units
inventory management. For large size supercritical units also, the same configuration i.e. 2x50 % TD-BFP and 1x50 % MD-BFP has been adopted. Alternate provision of 3x50% MDBFPshas also been suggested. However, this shall be resulting in increased auxiliarypower consumption and reduced net unit output.
1.2.6 Design pressure of HP heaters and feed water piping
In case of sub-critical units, feed regulating station is generally located at down stream of HP heaters, and HP heaters and feed water piping from BFP discharge to boiler inlet are normally designed for the shut off head condition of BFPs. However,in case of supercritical units,such a design criteria may lead to extremely high design pressure rating for HP heaters and lead to extremely high thicknesses for pipes and heater tube sheet etc. Thus, in supercritical units, feed regulating station is located at upstream of HP heaters and no isolation valve is provided at economiser inlet. The feed water piping and HP heaters are designed as per design pressure of the boiler with provision of pressure relief valves across HP heaters or media operated three way valves are provided at inlet/ outlet of HP heater(s) so as to prevent BFP shut off pressure from being communicated to downstream piping system and HP heaters.
1.2.7 Water chemistry
Unlike the sub-critical units that offer flexibility for water chemistry
correction in the boiler (drum), the supercritical units require necessary quality
correction of condensate to ensure final steam quality. High chemical
concentration in the boiler water and feed water cause furnace tube deposition
and allow solids carryover into the superheater and turbine. Further, dissolved
oxygen attacks steel and rate of attack increases sharply with rise in
temperature. Accordingly, water chemistry of boiler feed water is maintained
using combined water treatment (oxygen dosing and ammonia dosing in
condensate and feed water system).Oxygenated treatment (OT) using high
purity DM water minimizes corrosion and flow accelerated corrosion (FAC) in
the feed watertrain. Provision for dosingof ammonia and hydrazine (all
volatile treatment) is also made during start up and chemical excursions.
Further, the units are also provided with 100 % condensate polishing units to
achieve requisite condensate quality to the regenerative feed heating systems.
1.2.8 ID fan selection
Normal practice in the country has been to provide radial type Induced Draught (ID) fans for upto 500 MW unit size as radial fans are considered more reliable specially under conditions of high dust loadings (and consequent high wear of fan). However, radial fans of high capacity(for 660/800 MW unit size) may not be available and hence axial type variablepitch ID fans have been adopted.These are more efficient and lead to considerable power savings. Also with considerable improvements in ESP performance, problems of fan wear etc. are not expected to be significant.
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Standard Technical Features of BTG System for Supercritical 660/800 MW Thermal Units
1.2.9 Materials
High steam pressure and temperature parameters adopted in supercritical boilers require use of improvedmaterials to withstand the severe operating conditions. Gas side corrosion & erosion and steam side scaling and exfoliationare some of the major issues in material selectionfor coal-fired boilers. Higher temperature leads to creep,high temperature oxidation and accelerated attack of materialsdue to the presence of aggressive corrosive species, such as sulphur and chlorine, in the coal.
Ferritic, austenitic, or nickel-based alloy with mechanical strength at high temperatures are used in supercritical boilers. Materials being used are T11, T12, T22, T23, T/P91, T/P92, TP-304H, TP-347H and super-304H or equivalent. The relative use of these materials for various surfaces depends on the steam parameters adopted and also on design philosophy of the manufacturer. The high temperature superheater sections normally require advanced materials; howeveruse of advanced materials in other sectionscan provide design flexibility (e.g., thinner piping/headers for cycling service), though they may not be essential in those areas. Thus sufficient flexibility has been providedfor choice of materials for various equipments/ sections and piping to enable design freedom to the manufacturers.
