Clean, optimised CCGT solutions – three UK plants compared1 November 2008
The contracts for three major gas fired projects in the UK have been awarded to Alstom. They are all based on the KA26 package (GT26 in combined cycle mode) but with subtly different requirements leading to quite different train configurations and other plant components. The company’s ‘total plant’ approach plays an important part in meeting the brief for clean and efficient solutions.
With annual GDP growth stable at
2-3%, the UK’s electricity demand is not expected to increase beyond a modest 1-1.5% per annum over the next 10 years, but there is nonetheless a large new build requirement owing to the necessity of replacing the slew of old plants that will be decommissioned over the next 5 to 10 years. This includes several coal-fired power plants, as a result of the European Directive on large coal plants, which is related to emissions, as well as almost all nuclear reactors scheduled to progressively shut down before 2023. Moreover the country’s gas-fired power plants built during the 1990s ‘dash for gas’ will be 15 - 20 years old during the next 5 -10 years, and could be targets for replacement.
This situation was recognised some years ago and several new combined cycle power stations are being built. The contracts for three such projects were awarded to Alstom. They are all based on GT26 gas turbines, in a CC combination designated KA26 by Alstom, but with different requirements leading to subtly different solutions. They are:
• Staythorpe, a high efficiency and low emissions plant (Figure 1)
• Grain, the UK’s largest CHP plant (Figure 2)
• Langage, a fast-track power plant meeting stringent environmental criteria.
These projects, totalling nearly 4000 MW, are all KA26 single-shaft or multi-shaft power plants, and will be supplied under a turnkey engineering, procurement and construction (EPC) contract.
The choice of the KA26 technology, combined with the company’s trademarked (and unique) Plant Integrator system, has enabled Alstom to meet a diverse set of customers specifications. It has also, based on the wide operating flexibility of its sequential combustion GT26 gas turbine, contrived to minimise the impact on the environment by achieving significant CO2 emission reductions.
Alstom has to date built and commissioned 16 combined cycle power plants in UK, which along with the three CCPP's being built by Alstom mentioned above, equate to a total generation capacity in excess of 12 GW, more than 40% of the overall CC total.
The UK power market
The UK has for the first half of this decade seen a lull in new fossil-fired power build, with the emphasis shifting towards renewables, mainly wind farms. It is expected that these and other low carbon technologies will play an increasing role in the longer term, but it remains the case that around 90% of the UK's energy needs are currently met from oil, gas and coal and that the UK's dependence on these fuels is likely to persist for decades.
In 2008, the UK's electricity generation mix was 32% gas-fired, 39% coal fired, 15% nuclear, 4% renewables and 10% others (oil, imports etc). The UK currently has approximately 75 GW of electricity generation capacity to satisfy demand, which amounts to about 63 GW for the winter peak and a 20% margin. However, much of this capacity is middle-aged and will face closure in the next 10-15 years.
This is driven in many cases by EU environmental legislation aimed at reducing SOx and NOx emissions. The UK will need to make substantial investment in new generation plant over the next two decades to replace old plant and meet the expected increases in electricity demand. The average electricity consumption growth over the last decade has been approximately 1.1% per annum or about 0.77 GW in additional production capacity per year.
It should also be remembered that the UK is one of the most de-regulated and volatile power markets in the world. Low wholesale pricing levels in the UK saw the financial collapse of a large number of Independent Power Producers (IPP's) at the beginning of this millennium, and resulted in the market’s consolidating into a smaller number of larger integrated energy companies, which have a broader, more balanced risk profile.
UK government policy is aimed at reducing the UK's overall energy use by way of greater energy efficiency, encouraging low carbon forms of generation and providing an investment framework that encourages investment in the appropriate power generation technologies. The regulatory body that has the responsibility of managing the UK's energy markets is Ofgem (the Office of Gas and Electricity Markets), which is also responsible for assuring the security of the energy supply, in addition to ensuring a competitive energy environment and enforcing regulations that meet the UK government's energy goals.
In 2005, the UK introduced the EU's Emission Trading Scheme (ETS), which for the first time imposed a cost on power generators for emitting CO2. The ETS sets out to ensure that power generation investment decisions reflect the EU's carbon objectives by requiring power companies to take in to account the cost of carbon emissions in their decision making.
The EU has also launched its Large Combustion Plant Directive (LCPD) which sets out to make power generators either (i) agree to take on the task and associated investment of cleaning up the emissions from their coal- and oil-fired plant (thereby ‘opting-in’) or (ii) choose to ‘opt-out’, resulting in the power generators being limited in the number of hours they can operate between 2008 and 2105 and being obliged to close by 2015. Currently, about 11 GW of opted-out coal- and oil-fired stations are scheduled to be closed by 2015, and about 20 GW of opted-in coal-fired stations will have restricted operation post-2016. In addition, around 7 GW of nuclear power generation capacity is also scheduled to close between now and 2020 based on their current lifetimes.
This being so, the UK foresees the need to construct around 20-25 GW of new power stations between now and 2020, with an additional 10 GW needed to top this up to 2030. In some quarters it is thought that this estimate is inadequate, that the full 35 GW is needed by 2020.
CC as the bridging technology
Gas fired combined cycle power plants are still seen to offer the best all-round solution to satisfying the UK's short- to mid-term power needs, especially when taking into consideration the additional financial burdens imposed on the power generators by the ETS and LCPD. The key drivers in favour of the gas-fired combined cycle power plants in today's market environment compared to other major power generation technologies are:
• They are best understood in terms of risk
• ETS / LCPD requirements have a much lower investment impact compared to coal-based generation
• Short construction times, compared to other major power technologies (coal and nuclear)
• Highly competitive cost of electricity in combination with wide operational flexibility.
Four large CC plants
The second half of this decade has seen a revival in the construction of combined cycle power plants in the UK, brought about by the consolidation of its power market. There are currently four large combined cycle power plants under construction in the country, all utilising advanced-class high efficiency gas turbines. Three of them have been awarded to Alstom under EPC contracts. They are Langage, an 847 MW CCPP (KA26-2 multishaft, 2 x GT26 gas turbines, 1xSTF30c steam turbine) near Plymouth, for Centrica; Grain, a 1275 MW CCPP (3xKA261 single shaft power trains using 3xGT26 GTs and 3xSTF15c STs) in south east England for E.On; and Staythorpe 1650 MW CCPP (4xKA26-1 single shaft power trains using 4xGT26 GTs and 4xSTF15c STs) for RWE npower.
KA26 combined cycle plants
The above mentioned projects Langage, Grain and Staythorpe have in common the need to satisfy the stringent UK Grid Code requirement, the environmental constraints, and all applicable codes and standards including the latest health and safety requirements. In addition, different local and project specific requirements and customer preferences had to be considered. These relate to plant arrangement, architectural treatment, redundancy philosophy, far-field noise limitation, and special waste water and cooling water discharge.
Plant concepts – Grain and Staythorpe
A KA26-1 single shaft arrangement is the solution decided on for these two plants. Grain is composed of 3 identical single shaft power trains while Staythorpe is four. Each single shaft power line consists of one GT26 gas turbine, one HRSG, one floor mounted two-casing triple pressure reheat STF15c steam turbine and one hydrogen cooled TOPGAS turbogenerator coupled on one side to the cold end of the GT and on the other to the steam turbine through a synchronous self-shifting clutch. Each power train is designed to stand alone, that is, there is no steam cross over system – the steam produced by each turbine exhaust feeds only one ST.
Plant concepts - Langage
Langage is based on one KA26-2 ICS multi shaft configuration. ICS (Integrated Cycle Solution, a technology owned by Alstom) is an optimised water-steam cycle designed to enhance thermal efficiency. The KA26-2 consists of two GT26 gas turbines, two HRSGs, and one floor mounted three casing triple pressure STF30c steam turbine. Each GT and the ST drives an identical TOPAIR turbogenerator.
Plant concepts - general
Both basic arrangements are very flexible. They are built around a number of modules, which readily permits customisation and adaptation to project-specific applications, conditions and requirements. This flexibility is best illustrated with the Langage project, which had to be arranged in a limited area (Figure 3). While the water-steam cycle configuration is triple pressure reheat for all three projects each cycle is individually designed to optimise the individual parity factors – evaluation of the best techno-economic balance between investment and operational cost to achieve efficient energy production at a low cost.
For Langage the design also incorporates the installation of supplementary firing, which permits delivery of the required 850 MW in all conditions, but can also allow the generation of more than 30 MW additional power with practically the same very high efficiency.
The power plant cooling is strongly site dependent:
• Grain has direct cooling, with cold water extracted from the cooling water of the existing Grain power station
• Langage has air-cooled condensers, because the water available at the site is not sufficient for cooling purpose
• Staythorpe has hybrid, low plume cooling towers.
The Grain power plant will also provide process heat to the LNG vaporisers of the nearby LNG terminal With a maximum thermal output demand of 341 MWt Grain will be the largest combined heat and power plant installed in the UK and will achieve a fuel utilisation of over 70%. To ensure that the LNG vaporisers will receive the required heat in all circumstances, the station has the capacity to supply 227 MWt of heat to Grain LNG facility at a delivery temperature of 42 °C. For this application an innovative process is employed in which heat from the steam condensing process is utilised.
The condenser of each block is direct cooled by seawater. In HP mode, one side of the condenser can also be circulated with demineralised water while the other side remains cooled by the seawater. The warm demineralised water is pumped in a closed circuit to the LNG vaporisers.
‘Plant Integrator’ approach
Power companies today take a ‘big picture’ approach to their investment in that they look at the whole plant and its functional requirements over its entire life cycle. Alstom has redefined the way it approaches power plant design, construction and maintenance in order to align with these functional requirements. Through its ‘Plant Integrator’ approach it aims to deliver optimised solutions that are specifically designed to create added value, and the three examples described here illustrate how that can be done.
Different approaches to plant design
Every power plant in the world has been designed by engineers who integrated the systems and the components. Often they are the operators themselves (for example, major utility companies) sometimes architect engineers, OEMs, or construction companies.
There are two main business models for building a power plant. In the first, suppliers bid for their components within the frame of a precise specification. In the second model suppliers bid for the entire plant or part of a plant against an overall functional specification and intend to execute the project at minimum cost, buying components, systems and services at the lowest possible price.
The Plant Integrator approach is a method by which the advantages of both traditional models can be combined: the excellent quality of the components obtained by the first approach, and the commercial effectiveness of the second approach. The central idea involves seeing and meeting the customer’s functional requirements, that is, helping to optimise electricity production in a way that gets the maximum value from the investment. Alstom has developed the capability to accompany and assist the customer during this process of assessing the complete plant. By integrating the plant components, the complete plant can be optimised to create the best fit for the customer's needs and the market conditions.
When power companies look at the whole plant over its entire life cycle, they can see a number of lifetime benefits that can actually be measured as 'added value'. If power companies are prepared to go through this process, there are several benefits that can be achieved. Looking at the complete picture will allow, for example, faster completion of the project, improved performance, reduced risks and lower costs.
Benefits of the total plant approach
To evaluate and calculate this value, NPV calculation methods and tools are used. There are six key areas to increase the net present value: investment, lead-time, performance, availability and operational flexibility, retrofit and life extension. There are other areas that are sometimes more qualitative, for example the compliance with or anticipation of environmental regulations or the ability to accept fuel flexibility. Nevertheless these conditions can always be translated into one or more of investment cost, time, performance, availability, retrofit and life extension.
Other things being equal, taking a Plant Integrator approach can create value through lower investment. Having a contractor with in-house architect engineer capability that is also designer, manufacturer, and supplier of all the major plant components can reduce costs in the project development phase. It is then possible to co-ordinate between the business units and align project schedules in order to purchase materials and equipment at the same time.
Because each plant is designed to the customer’s specific needs, Alstom will know in advance what equipment will be installed, allowing optimisation before the project is launched. Because the bulk of the design can be done before the contract is signed, lead-time is reduced. Architect engineers have to rely on external suppliers and therefore have to wait until the supplier is selected before they can engineer at the required level of detail.
Pre-fabrication also saves time during erection. To give an example, the 1200 MW Cartagena combined cycle plant in Spain was built in just 23.5 months, some 3 months faster than a typical comparable project. In spite of the very tight site area, site logistics and schedules could be agreed early enough between the different Alstom businesses working on site, so that construction work could be accelerated and compensate for the longer construction time imposed by the local constraints. For example, modularisation of the HRSGs, so that they arrived at site as complete packages, contributed to the construction time and costs savings at site.
The Plant Integrator approach always looks at the plant boundaries, even when an equipment-only contract is being considered. Using data from the customer such as gas or feedstock costs and the revenues from the plant, computerised NPV calculations for up to 100 000 different configurations show what makes the most sense for the customer. Case studies show that sometimes it is worth spending a little more at the outset in order to reap the rewards over the lifetime of the plant.
As an example, at the 840 MW combined cycle power plant KW Emsland in Germany efficiency was maximised to take advantage of a tax exemption on the gas fuel. The efficiency increase was achieved through the specific once-through design of the HRSG, which had already been successfully implemented by Alstom in steam plants and in the 60 Hz combined cycle market. Although it made the HRSG slightly more expensive, at current gas prices it will provide the operator, RWE Power, with significant savings over the 20-year lifetime of the plant.
Within the current business models most customers want the lowest bid for a given performance but the answer is not necessarily to provide the specified equipment (eg the turbine) at the cheapest price. It is about achieving the best plant performance, which sometimes calls for arbitrage between components. Alstom defines this overall system approach as ‘The optimum of the sum is more than the sum of the optima of the parts’. It is not necessarily a move from a ‘component market’ to a ‘plant market’. It means better performance and better output can be achieved if the components are matched to the overall plant specifics – as defined by the customer's needs.
Availability and flexibility
By using a system across its plants that collects data related to how plants are operating and on events related to reliability, Alstom has accumulated a good track record on availability for its components, resulting in increased plant availability. For example at the Castejon combined cycle plant in Spain, the company achieved 97% availability during more than 4.5 years of commercial operation. Guaranteeing availability and reducing outage time translates into savings that are quantifiable over the plant life. And supplying equipment to a project and engineering the overall plant design gives the plant contractor a unique insight that enables it to optimise when, where, and what maintenance will be needed across the whole plant.
New low load concept
In deregulated markets there is an increasing need for flexible operation. In some deregulated markets flexible operation may call for production to be decreased, or generation to be interrupted, at night. The Plant Integrator capability coupled with the GT26’s particular features and its sequential combustion system mean the water-steam cycle can be varied to follow the operation of the GT, and mirroring this in the control system allows Alstom to offer a new low load concept under which the plant can be run at 15% load during the night, thus avoiding complete shutdown while complying with emissions regulations. NPV calculations show that by this method significant savings can be made over the life of the plant while income is derived from providing frequency support.
Using the plant integrator approach also helps life extension decision-making by offering functional guarantees for the upgraded plant that it will operate at a specified performance for a predetermined lifetime.