Comparison between single-shaft and mutli-shaft gas fired 800 MWel combined cycle power plant

ABSTRACT

Multi-shaft and single-shaft configurations allow customization to optimize plant performance, capital investment, construction and maintenance access, operating convenience, and minimum space requirements. Technical comparison between both configurations at partial loads has not been published before. This paper will primarily address a comparison between the 2 configurations based on thermodynamic simulation results for a gross power capacity of approximately 800 MWel at ISO conditions.
This capacity has been chosen based on power market requirements. The analysis approach for each configuration is divided into 3 components: (1) Performance, (2) Plant configuration, and (3) Environmental impact. The first component dealt with plant gross power output, plant gross efficiency, plant auxiliary power demand, plant generator losses and plant shaft power. The second component dealt with space limitations and extension capability. The third component dealt with specific emissions of NOx and specific emissions of CO2. The thermodynamic simulations have been carried out using Thermoflow at base load and part load respectively. The results show that the single-shaft configuration is more suitable with regards to performance, NOx specific emissions, CO2 specific emissions, start-up and extension possibilities. The multi-shaft configuration is more suitable with regards to space limitations, steam turbine shaft power, availability, and reliability.

1. Introduction

It is known from literature that many authors have discussed the principles of the Combined Cycle Power Plants (CCPP).
However, many worldwide gas turbine manufacturers and combined cycle developers have mentioned only general nomenclature regarding their products (e.g. the recent examples from. Until now, no publication has addressed a technical comparison between the 2 configurations of the CCPP from a non- manufacturers point of view. Depending on the combination of the turbines and generators, systems can be classified into 2 categories (single-shaft and multi-shaft configurations). With a single-shaft configuration, the gas turbine is directly coupled to a steam turbine with a single common generator, and with a multi-shaft configuration the steam turbine and a gas turbine are coupled to their own generators respectively. The signed Kyoto protocol which called for the reduction of CO2 emissions would favor the execution of efficiency increasing measures for fossil fueled power generation. The latest trend in CCPP configurations, high efficiencies of up to 60%, low air emission
rates, and modest space limitations are the driving forces for the anlaysis this paper is to document. This evaluation will compare both combined cycle configurations for a gross power generation capacity of approximately 800 MWel (single-shaft and multi-shaft). Factors such as consideration of performance, environmental impact, availability, reliability, extension possibility, and space limitations will be considered for both configurations. An investigation of combined cycle configurations with F-series gas
turbines (SGT5-4000F gas turbine, formerly known as V94.3A) was conducted. Commercial thermodynamic simulation software was used to simulate the power plant performance under ISO conditions. The initial analysis approach of both configurations is briefly described below, including the design philosophy of each
configuration option. Next, evaluation of the simulation results will be presented, and then overall findings and conclusions will be made.

2. Analysis approach

The analysis approach for both CCPP configurations is addressed in Table 1 as

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The  steam  turbine  choice  depends  on  the  condenser  back pressure,   mainly influenced   by   the   cooling   condition.

A  combined  HP/IP,  double  flow  LP,  induction  and  condensing reheat type turbine was selected for better performance with the desired low back pressure. In the induction part, additional power by means of low pressure steam was generated. The condensing
part  exhausted  steam  in  a  partially  condensed  state  at  the condenser back pressure. In the reheat part, steam flowed from the high pressure section of the turbine and was returned to the HRSG,  where  additional superheat  was  added. The steam then went back into the intermediate pressure section of the turbine and continued its expansion. The optimum bottoming cycle was selected to attain the maximum thermal efficiency by recovering the heat effectively from the turbine exhaust gases. Optimization of the plant cycle resulted in selection of a superheat/reheat and low pressure cycle or Triple Pressure System with double feeds to the  steam  turbine.  The  steam  pressure  and  temperature  were determined  based  on  the  limitation  of  the  steam  pressure requirement for natural circulation of the Heat Recovery Steam Generator  (HRSG),  and  the  erosion  of  the Low  Pressure  Steam Turbine (LPST) last stage. The high, intermediate, and low pressure steam conditions were: 125 bar/565           C; 30 bar/565 C; and 4.4  bar/235 C. At part load the deaerator pressure will be kept above the atmospheric value to improve the overall efficiency and also to remove the dissolved gases from the feed water. Although supplementary  firing  is  an  effective  means  of  increasing  plant capacity, it significantly reduces the plant efficiency and therefore increases the emissions. No supplementary firing was considered to reach the best efficiency at the required output power. Based on the  flow of exhaust  gases, HRSG  is  categorized  into  horizontal type. The HRSG pinch points were optimized and selected based on the modern design of the CCPP. The HP, IP and LP Pinch points were optimized through multiple runs of the simulation software to get the highest process efficiency at a moderate HRSG surface.
The cooling cycle was designed as a closed cycle system by means of a condenser with wet cooling towers. Wetness fraction of the steam was kept to less than 12% at 0.045 bar back pressure. This pressure could be achieved at ISO ambient conditions through an optimum design of the cooling system such as the water approach to wet bulb temperature was designed at 7 K with cooling water temperature rise in the condenser at 10 K at 3 K degradation. Common steam cycle components such as feed water pumps and deaerators for the 2 HRSGs were considered for the multi-shaft configuration. The exhaust system of the HRSGs included a by-pass stack by which steam from the HRSGs is fed into the multi-shaft configuration utilizing a common header to the steam turbine. In the single-shaft configuration, steam from each HRSG was fed to a separate steam turbine with no interconnection between the 2 steam cycles. The exhaust system for the single-shaft configuration did not include a by-pass stack, as only one of
the single-shaft trains will be in operation and the second will be in an outage condition. A single reheat combined cycle system was considered to improve the thermal efficiency.  In this system, steam is typically supplied from the steam turbine back to the
HRSG, where it is then piped to the Intermediate Pressure steam turbine. Both are heated and returned to the steam turbine. This configuration does not exist with a single-shaft combined cycle, where one steam turbine and one HRSG receive heat from the gas
turbine which is supplied directly from the one steam turbine to a single HRSG. However, application of a reheat steam cycle to a multi-shaft  combined  cycle  has  typically  been  restricted  to a configuration in which cold reheat steam is distributed to each of only 2 HRSGs for reheating and subsequent return to the steam  turbine.  This  restriction  is  a  result  of  the  complexity, expense,  and  operability  problems  that arise with  multi-shaft configuration. In other words, the distribution and collection of reheat steam requires more complicated design in terms of piping, valves, and other auxiliary equipment.  For  the  single-shaft configuration   hydrogen-cooled generators   were   considered, whereas for  the multi-shaft  configuration  air-cooled  generators were  considered  for  the  gas  turbines  and  hydrogen-cooled generators  for  the  steam  turbines.  The consideration of the generators types was made based on the output range limitations of both types.  For increased flexibility in plant operation, the single-shaft configuration can be equipped with flexible couplings and/or synchronous-self-shifting (SSS) clutches. In this system the generator is located in the middle of the single-shaft train (between gas turbine and steam turbine), with one end connected to the cold end of the gas turbine and the other end to the steam turbine through the SSS clutch. It engages and disengages automatically during start-up and shut down of the steam turbine.

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This  further  allows  independent  operation  of  the  gas  turbine during start-up or steam turbine by-pass operation. The need for an auxiliary boiler is thereby eliminated, which would be required if no clutch were used.  This results in increased operating flexibility and reliability. The plant configuration design options are illustrated in Table 3.

2.2.  Benson once-through steam generator (OTSG)

Due to the restriction of the daily start and stop (DSS) and the limitation  of  the  partial  load  operation,  the  application  of  the Benson once-through steam generator (OTSG) is not discussed in this paper.

3.  Plant configurations

Combined cycle power plants can be configured in numerous ways  depending  on  the  number  of  gas  turbines.  The first consideration is whether to arrange the plant as multi-or single-shaft. Multi-shaft configuration in this context means the application  of  an  independent  gas  turbine  (GT)  and  steam  turbine generator  (ST) set will be located in separate buildings  (multi-shaft, Fig. 1), or on a single-shaft to a common generator. In the later case, there will be a single generator and a single generator transformer for both GT and ST, Fig. 2. These configurations may, of course, be optimized by taking into consideration the manufacturers’  standard  design  and  final  arrangement  of  external facilities.

3.1.  Single-shaft configuration

With  the  single-shaft  configurations,  each  power  train  is designed  to  house all  the  main  components  in  the  same building  with  a  compact  arrangement.  Fig. 3 shows simple schematic for single-shaft configuration (2GTs 2STs).  This

Optimizes engineering in terms of accessibility, maintainability, and health and safety criteria.  Sufficient space for routing major steam piping and cabling is required within the GT area

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to prevent interference with the inlet duct of the GT inlet air filter,  especially  in  a  single-shaft  configuration.  The  required area for the single-shaft configuration  (2 single-shaft trains) is  approximately 20%  higher  than  that  of  the  multi-shaft configuration (e.g. 2GTs ş 1ST). This difference is due mainly to the  additional  steam  turbine  and  auxiliary  systems.  If  an auxiliary  boiler  is  considered,  this  will  increase  the  space requirement.   The   expansion   possibility   of   a   single-shaft configuration to add a second unit is beneficial. The design of the additional single-shaft train will be identical to the first by applying  standard  design.  Further  benefit  is  realized  for  the technical  and  economical  assessment,  since  both  are  already known.  Finally,  each  single-shaft  train  can  be  dedicated  to supply  power  to  the  net  or  be  disconnected  from  the  net independently.

3.2.  Multi-shaft configuration

In  a  multi-shaft  configuration, the  plant includes 2 separate power islands; a gas turbine area and a steam turbine area.

In  a  multi-shaft  configuration,  the  electrical  generators  are smaller,  making  the  arrangement  well-suited  to  unique  steam turbine  designs,  such  as  cogeneration  or  district  heating.  In addition, when 2 gas turbines are utilized, 2 air-cooled generators are required for the gas turbines and one hydrogen-cooled generator is required for the steam turbine as illustrated in Fig. 4.  The air-cooled generators are less efficient compared with the hydrogen-cooled generators. The output range of the air-cooled generators is between 175 and 375 MVA whereas the output range of the hydrogen-cooled generators is between 550 and 550 MVA. For these reasons, the air-cooled generators could not be utilized for the single-shaft configuration or the ST in the multi-shaft configuration.  The  expansion  possibilities  of a multi-shaft power plant means many components  (e.g. steam system, steam turbine, generator and step-up transformer) and associated  systems  need  to  be  oversized  to  accommodate  the
future expansion. This results in a decline in load efficiency due to the constant part load operation of the steam turbine/generator set. This will result in higher investment cost for the power plant.

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4.Performance results

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Both configurations have been evaluated in terms of the above mentioned approach aspects in Table 1. The calculations are made at both full load and part load operations under the same design conditions. In addition, the calculations included the cases where both GTs are in operation simultaneously or when one GT is off-line for both configurations respectively. The results are based on Lahmeyer’s database and may differ  from the latest design of  the  manufacturers.

4.1. Efficiency

Fig. 5 shows the results of the 2 configurations’ overall efficiency versus the overall output. The overall plant efficiency (e.g. gross efficiency) and the net efficiency are calculated by the following formulas:

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At base load (e.g. approximately 840 MW) the single-shaft had slightly  higher  efficiency  and  overall  output capacity  than the multi-shaft  by  approximately  0.05%  and  0.08%  respectively.  At partial loads, with 2 gas turbines in operation, both configurations have virtually the same overall results. This was due to the same steam turbine efficiency behavior of both configurations at this operating level. In the scenario of one GT in outage, the operational flexibility and the gross capacity of the single-shaft plant (one train) is higher than the multi-shaft. It is capable of running at 50% capacity with only one gas turbine in operation; whereas, the multi-shaft configuration can only reach 48.5% of the total capacity. This is due to the lower steam turbine efficiency of the multi-shaft configuration if one gas turbine is out of operation. In addition, one train of the single-shaft has approximately a 3% higher efficiency than the multi-shaft configuration at partial load when one gas turbine is in outage.

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4.2. Generator losses

The generator losses and decreased efficiency depend mainly on the generator cooling type. Hydrogen’s low density, high specific heat and  thermal  conductivity  make  it  a  superior  coolant  for rotating electrical machines and it lends itself to a compact, highly efficient, reliable design. The application of only one highly efficient hydrogen-cooled generator for the single-shaft had a 2-fold positive impact. First, there was a gain in overall efficiency and power output due to the fact that the hydrogen-cooled generator is more efficient than the combination of a hydrogen-cooled generator for the gas turbine and an air-cooled generator for the steam turbine. Second, there was the effect that the omitted set of associated isolated phase bus and generator step-up transformer meant less loss. This meant higher net power output and net efficiency. This leads to an overall incremental net efficiency gain, due to less  auxiliary  power  consumption  and  higher  generator  power train  efficiency of  about  0.1%  in  comparison  to the  multi-shaft configuration. Fig. 6 compares the shaft power of both configurations versus the generator losses.  From this figure, the most important results of the thermodynamic calculations at base load and part load are:

  • The generator losses for the multi-shaft configuration when 2 gas turbine in operation was approximately 20% higher than the single-shaft generator losses;
  • The generator losses for the multi-shaft configuration when one gas turbine is in outage was approximately 35% higher than the single-shaft generator losses;
  • The shaft power of the multi-shaft arrangement was approximately 0.25% higher than the single-shaft arrangement with 2 gas turbines in operation;
  • The shaft power of the single-shaft configuration is approximately 2.5% higher than the multi-shaft configuration when one gas turbine is in outage;
  • The efficiency of the hydrogen-cooled generators of the single-shaft arrangement was approximately 0.2% higher than the generator efficiency of the air-cooled generators of the multi- shaft arrangement.

For the multi-shaft configuration the bigger size of the steam turbine results in higher ST work output with lower mechanical losses and, therefore, higher shaft power compared to the 2 steam turbines of the single-shaft configuration. Nevertheless, 2 single-shaft  trains  will  have  approximately  0.08%  higher  gross output power than the multi-shaft configuration at base load due to the  lower  generator  losses  and  higher  generator  efficiencies compared with the multi-shaft configuration.

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4.3. Auxiliary power

Fig. 7 shows that, as long as 2 gas turbines are in operation, the auxiliary power of both configurations is nearly the same, but the   single-shaft   configuration   has   lower   auxiliary   power consumption from 0.05% at base load to 3% at 50% partial load, depending on the load conditions.  The major reasons for the reduced auxiliary power consumption are as follows:

A common lube oil system with redundant lube oil pumps requires only one lube oil pump operational as opposed to 2; One less generator with 2 less bearings translates into less friction loss and therefore into lube oil pump size, which is smaller than the sum of the 2 independent systems; A similar logic applies to the auxiliary cooling water system and its pumps, which do not have to carry the cooling load of an additional generator; The  one  less  generator  also  contributes  to  approximately 0.5-1% reduced cooling duty of the balance of plant; The avoided losses of the saved electrical equipment other than the isolated phase bus and the generator step-up transformer (e.g. the second excitation system, protection devices, etc.) also add  to  the  auxiliary  power  consumption  of  a  multi-shaft arrangement.

Furthermore, the single-shaft configuration has a clear advantage, especially if one of the 2 gas turbines is out of operation. In this case the full load efficiency of the smaller ST of the single-shaft train has approximately 20-30% lower auxiliary demands than the partial load auxiliaries of the big steam turbine of the multi-shaft configuration, which has to be operated in partial load at lower efficiency range. This again leads to an overall incremental net efficiency gain due to less auxiliary power consumption and higher generator power train efficiency in comparison to the multi-shaft configuration.

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5.  Availability and reliability

Availability reflects the percentage of the hours that a plant will be available to produce electricity. It depends on the percentage of planned and forced outages of ST and GT. Planned outages are planned maintenance activities, whereas forced outages are unexpected problems leading to a blackout of the GT or ST. It is calculated by the following formula:

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Reliability is a measure of the percentage of the time between planned overhauls. No values for reliability can be stated that will be valid for all cases, since factors such as preventative maintenance and operating mode have an impact. However, statistics indicate that similar type plants have the similar availabilities and reliabilities when operated under the same conditions. It is calculated by the following formula:

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In this paper the availability and the reliability for the main components of each CCPP configuration such as the gas turbine vs. steam turbine was considered. FOH and MOH together represent the unplanned outage hours, and MOH and POH together represent
the scheduled outage hours. MOH was not considered in the calculations, because they are very difficult to predict. The planned maintenance and forced outages all occur during the base load period hours. In order to simplify the calculations for availability
and reliability, the unplanned maintenance outage hours MOH was not considered. Furthermore, the outage factor for the balance of plant equipment was not considered in the calculations. According to the statistical data of the North American Electric Reliability Council (NERC), 2003e2007, the planned outage factor for the GT POF is estimated at 4.23% and the FOF is estimated at 2.86%. The POF for the steam turbine was estimated at 1.3% and the FOF was estimated at 0.5%. The data used for the calculations was an interpolation of the different data sources. The availability is calculated for both configurations at various phases of the year and led to the number of hours of power generation with the related number of GT and ST for both configurations as shown in the Table 3. For the single-shaft configuration, the forced and scheduled outages of GT and the corresponding ST occurred at the same time. In the case of a gas turbine trip or a HRSG trip the complete power train will drop
off-line. Consequently, the duration of the planned outages of the ST will be the same as for the GT and the percentage of forced outages is assumed to be the sum of the percentages of GT and ST.
For the multi-shaft configuration, the ST is in operation at the same time that one GT is in planned outage. During this period the ST will operate in partial load. The planned outages of the GTs are planned and will not occur simultaneously.  During these periods, the remaining HRSG cannot generate sufficient steam for the full load
operation of the ST. Furthermore, the planned outage of the ST is projected during the planned outage of the GT to minimize the partial load duration of the ST as shown in the Table 4.

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From Table 4, it is clear that a multi-shaft configuration has the lowest total blackout probability of 48 h/a compared with 60 h/a for the single-shaft. Furthermore, an additional advantage of the multi-shaft configuration is that the overall period of time in which the base load capacity is available is higher than single-shaft. From Fig. 8, the availability and reliability are slightly higher 0.1% and 0.3% respectively for the multi-shaft configuration. This was due to the different operation and outage scenarios for both configurations. The multi-shaft configuration has higher power output in one year due to the independent planned outage of the ST and GT which leads to higher availability and reliability. In practice, the forced outage  factor  of  the  balance  of  plant  system  leads  to  a  slight increase of the availability of the single-shaft configuration due to the independent balance of plant systems for each single-shaft train.

6.  Environmental considerations

Pollutant  emission  concentrations  and  emission  flow  rates during  operation  using  fuel  gas  is  a  Typical  GT  Manufacturer Guarantee Value. The sulphur dioxide (SO2) emissions are negligible because CH4 does not contain any sulphur. Nitrogen oxides (NOx) as the main pollutant does not exceed the emission limits of
50  mg/Nm3  at  15% O2  in dry exhaust gas. The NOx  emission is typically guaranteed by the GT manufacturers to emit 50 mg/Nm3 or  less,  which  complies  with  the  applicable  EU  emission  limit (Table 5). However, due to the use of Low NOx burners, the typical NOx emission of each GT at base load is constant at 25 ppmv at 15% O2 in dry exhaust gas. In the operating range of 50e100%, the Inlet
Guide Vanes (IGV) remain in premix mode to enable constant

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exhaust gas temperature. This mode of operation is achieved at or near the combustion  reference  temperature  design  point. Optimum emissions are generated in premix mode. The constant exhaust temperature will avoid increasing the NOx emissions. By designing the plant with fast-start capability, start-up emissions will be dramatically reduced. Table 5 illustrates the relevant NOx emission limits based on the gas turbines manufacturer (OEM) for base load operation and burning gas fuel. The Dry LowNOx operating modes from ignition to 50% load with the related NOx emissions are not available from the (OEM) for written publication. The NOx production dose not differs in both configurations but, the specific emissions of NOx kg/kWh change and is related to the power plant efficiency. Based on Table 5 and on the behavior of the Low NOx burners, it can be concluded that in the operating range 50e100% with 2 GTs in operation both configurations have the same specific NOx emissions. But, when one gas turbine is in outage the multi-shaft configuration at partial load has approximately 3% higher specific NOx emissions than one train of the single-shaft (see Section 4.1). This value of the specific NOx emissions shall be considered as an approximation.

The CO2 production dose not differs in both configurations but, the specific emissions of CO2 t/MWh change and is related to the power plant efficiency.  From the thermodynamic calculation results, for both configurations are illustrated in Fig. 9. At base load both with 2 GTs in operation both configurations have the same specific emissions of CO2. If one gas turbine is out of operation, the specific emissions of carbon dioxide of the multi-shaft configuration at partial load are approximately 6% higher than the single-shaft. This was due to the higher efficiency of  the single-shaft configuration  at  partial  load  when  one  gas  turbine  is  out  of operation.

7.  Findings and conclusions

It was concluded that each combined cycle configuration had specific advantages. The best approach needs to be evaluated for each  project  individually.  At  base  load  and  partial  load  the single-shaft configuration has better efficiency and output. This leads  to  fuel  cost  savings  for  the  single-shaft  compared  with

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multi-shaft.   Nevertheless,   the   multi-shaft   configuration   has better availability and reliability at base load. No one knows how fuel  and  electricity  prices  will  fluctuate  in  the  future,  and  as a result, which load regime a combined cycle power plant could be  operated  over  the  plant  lifetime.  On  the  other  hand, a  potential  plant  expansion  in  the  future  and  implementation time  play  important  roles  when  it  comes  to  decision  making. Single-shaft  configuration  has  better  expansion  possibilities  if required in the future. Based on the technical results, the single-shaft  represents  the  following  advantages  compared  with  the multi-shaft configuration:

  • Higher efficiency at base and partial load operations;
  • Higher output at base and partial load operations;
  • Faster start-up capability;
  • Lower generator losses;
  • Better maintenance access;
  • Better extension possibility without effect on the performance of the first train;
  • Lower specific CO2  emissions at partial load if one GT is in outage;
  • Lower specific NOx  emissions at partial load if one GT is in outage.

Note: Questions concerning the continued gas turbine operation in the scenario of a steam turbine in outage need to be answered by the  manufacturer  and  evaluated  carefully  with  regard  to  any interruption exposure.

The multi-shaft configuration represents the following advantages compared with the single-shaft configuration:

  • Higher steam turbine shaft power;
  • Higher availability and reliability at base load;
  • Lower space requirement;
  • Flexible open cycle operation when the steam turbine is in outage.

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