1.1 Supercritical Technology

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   

 

The vertical water wall design uses rifled tubing for improvedcooling effect  with uniform temperature across the walls and is also operating satisfactorily.  Its advantage lies in ease of construction and maintenance. Keeping in view  the fact that various manufacturers have standard waterwall configurations  which are proven,both the optionsviz. spiral and vertical tube designs have  been included. 

1.2.3    Boiler start up circulation systems  

Supercritical  boiler  starts  operating  in  the  once  through  mode  beyond  a  particular minimum load of say 30 to 40 %. Below this load, it operates in the  circulation mode and needs a separator and circulation system for water steam  separation; the separated water is circulated back to the boiler. Generally, two  types of circulation systems are in use. In one of the systems, separated water  from the separator is led to the deaerator/ condenser and is circulated to the  feed  water  system  through  boiler  feed  pump.  This  system  is  simple  and  relativelyinexpensive but involvesloss of heat from boiler during cold start-  up. In other system a circulation pump is provided to circulate the water from  separator directly to the economizer.  This preventsheat loss from boiler  during cold start- up but adds to cost. Both systems have also been provided in  some of the supercritical units to improvereliability. Other provenstandard  systems for boiler startup drain circulation system are also acceptable.   

An alternate drain connection to main condenser has also been envisaged to  enable start up of steamgenerator even when the Start up drain recirculation  pump is not in service and for initial flushing of boiler to achieve water/ steam  quality.  

1.2.4    HP turbine extraction 

In the sub-critical units upto 500 MW, the highest pressureextraction in the  regenerative  feed  heating  cycle  is  from  the  HP  Turbine  exhaust.  This  conventional design with highest feed water extraction from CRH line is able  to  achieve  a  final  feed  water  temperature  of  about  255⁰C.    Designs  with  extraction from HP turbine are available leading to increased final feed water  temperature of about 290⁰C or higher. The higher feed water temperature due  to HP extraction leads to a marginally betterturbine cycle heat rate. It also  involves  additional  heaters.  Keeping  in  view  the  advantages  of  higher  efficiency, design with HP turbine extraction has been adopted. 

1.2.5    Boiler feed pump configuration 

A number of configurations viz. 2x50% TDBFP+2x30% MDBFP, 2x50%  TDBFP+1x50%MDBFP, 2x50% TDBFP+1x30% MDBFP, 3x50% MDBFP  are in use for boiler feed pumps in large size units. The normal practice being  followed in the country for 500 MW units is to provide 2x50 % turbine driven  Boiler feed pumps (TD-BFP) and 1x50 % motor driven BFP (MD-BFP). The  above configuration has the advantage for having same pump for both TD- BFP  and  MD-BFP  leading  to  interchangeability  of  spares  etc.  and  better 

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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.