Benefits abound with novel CHP scheme

E.ON UK is building a gas fired combined heat and power (CHP) station at the Isle of Grain in the UK that will supply condenser waste heat to a nearby LNG plant, heat which ordinarily cannot be usefully recovered due to its low temperature. Graham Geraghthy, Dave Draper and Roger Parkinson explain the workings of the new plant, which E.ON says will be one of Europe’s largest and most efficient CHP plants.

By: G. Geraghthy, D. Draper & R. Parkinson, E.ON Grain project engineering team, UK

Utility giant E.ON is constructing plants to help provide new generating capacity in the UK. They include a 1275 MW gas fired CHP station at the Isle of Grain in Kent, which is part of the company’s plan to build 18 plants with an aggregate generating capacity of more than 13.5 GW over the next five years. This is part of a €60 billion ($85 billion) strategic investment programme until the end of 2010. Part of E.ON’s growth strategy is driven by the continued rise in demand for power in the UK and the pending closure of many of the country’s older generating stations, such as the MAGNOX nuclear plants and large coal or oil thermal units that have opted out of the provisions of the European Union Large Combustion Plant Directive (LCPD). It is expected that around 19 GW of generating plant will close by 2020.

E.ON received planning approval for the Grain CHP station in 2006 and signed a full turnkey EPC contract with Alstom Power in February 2007 for the construction of three 425 MWe gas fired units. Some 341 MW of waste heat energy will be recovered from these units for use at the nearby National Grid Grain LNG storage and regasification facility.

CAD visualisation of the new Grain CHP plant
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The new station is being constructed on the site of E.ON’s existing oil fired power station on the River Medway, about 16 km north east of Rochester. The original power station was built in the 1970s, and today two of the units are retained by E.ON as part of their operational portfolio. However, their use is limited to a maximum of 600 GWh per year. These units are currently scheduled for closure by the end of 2015 as required by the LCPD.

The output of the existing Grain oil fired power station owned by E.ON is limited to 600 GWh
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The new CHP station (see artist’s impression below) will be built next to the original power station and will share certain features of the original infrastructure, such as the existing cooling water intake and outfall structures and part of the existing 400 kV substation.

A major BP oil refinery on the island closed in 1982, but Grain is again becoming a focal point for energy and infrastructure projects. For example, the nearby Thamesport has become the UK’s third largest container port.

A major LNG importation and storage facility has also been constructed by National Grid Grain LNG Limited. When the final phase of that facility is completed in 2010/11, the total annual LNG throughput will be 14.8 million tonnes per year. With gas demand in the UK expected to increase by at least 15 per cent over the next ten years, and indigenous supplies from the UK Continental Shelf in inevitable decline, it is widely expected that LNG will play an increasingly important role in securing the nation’s long-term gas requirement.

Additionally, Grain has also been selected by Anglo-Dutch joint venture Britned for construction of a 1 GW AC/DC converter station, which will eventually link the Netherlands and the UK via a 260-km subsea HV DC cable.

Plant configuration

The basic Grain CHP station configuration consists of three Alstom KA26-1 single-shaft units based on the advanced-class GT26 gas turbine. Each single-shaft power train consists of one Alstom GT26 gas turbine equipped with a sequential lean-premix, dry low-NOx combustion system, a floor-mounted, three-casing, triple-pressure reheat steam turbine and one common hydrogen-cooled generator. Figure 1 shows the plant’s configuration.

Figure 1: Configuration of the Grain CHP plant
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The GT26 uses Alstom’s sequential combustion system. This consists of an environmental combustor and a sequential environmental combustor. This applies the thermodynamic reheat principle to the gas cycle, which leads to better gas turbine performance compared with the conventional approach when using the same firing temperature.

The water-steam cycle utilizes a state-of-the-art triple-pressure reheat cycle with a drum-type heat recovery steam generator (HRSG) which allows optimum utilization of the energy in the gas turbine exhaust gas. The HRSG is of a horizontal gas flow design that uses natural water circulation. To accommodate the requirement for good cycling operation, the HRSGs have key design features such as the inclusion of single-row harps with good weld accessibility, stepped component thickness, enhanced drain systems and a top-supported pressure part arrangement.

The existing 400 kV overhead line connections will export electrical power from the generators. One bay of the existing 400 kV National Grid Grain substation, which is air insulated, will be modified to receive power from the first generator unit. The two subsequent generators will connect to a new gas-insulated 400 kV substation that National Grid will construct at the site of the existing oil-fired power station.

Natural gas will be the primary fuel of the turbines. There is no requirement for distillate operation. As part of the enabling works, E.ON UK has placed separate contracts for the construction of a 3.5-km gas pipeline to connect the CHP station to the high-pressure National Grid transmission system at the Isle of Grain.

When operating in full CHP mode, at least half of the waste heat energy from the power station condensers will be supplied to Grain LNG Ltd as low-grade heat for regasification of LNG stored at the LNG terminal. The station also has a conventional once-through condenser cooling water system that uses sea water as the cooling medium. A new cooling water pumping station will be constructed next to the existing pump house, and the two forebays will be interconnected by means of a syphonic crossover pipe. The original Grain pumping station used a submerged intake in the River Medway for cooling water abstraction. The interconnection of the two forebays in this way avoids the requirement for construction of additional marine works.

Ground conditions at the site are generally poor because alluvial deposits overlay London clay. All heavily loaded structures will be piled, and it is estimated that 3000 piles will be required.

The CHP station will be fully automated to provide safe and reliable centralized control of the CHP assets. An ABB 800xA system will provide a single centralized, unified operator interface for all areas of plant. The system, in combination with intelligent instrumentation and fieldbus technology, will also provide advanced diagnostic and maintenance facilities.

Turnkey approach

Alstom won the turnkey EPC contract for the CHP station. Notice to Proceed was signed on 25 May 2007, and commercial operation of the first unit is expected in October 2009, with the third and final unit being delivered in January 2010.

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Alstom will design, supply, install and commission the entire power plant, including the gas turbines, steam turbines, condensers, heat recovery steam generators, step-up transformers, plant control systems, civil works and balance of plant.

The final design of the CHP station was based largely on Alstom’s standard KA26-1 reference plant design and included agreed modifications to meet E.ON’s particular technical requirements. The advantage of this approach is that it allows for delivery of a plant with optimal performance, shortened installation times and reduced overall costs.

CHP scheme

When operating in combined-cycle mode, the CCGT units will have an excellent net full load combined-cycle efficiency of 58.6 per cent. In CHP mode, the station will allow recovery of up to 341 MW of thermal energy from the condenser cooling water system for supply to Grain LNG. This will increase the overall net station efficiency from 58.6 per cent to 72.6 per cent, and will result in one of Europe’s largest and most efficient CHP stations.

The Grain CHP scheme differs from most conventional CHP schemes that normally rely on the use of export steam extracted from the steam cycle for the delivery of heat. The extraction of steam in this manner reduces the amount of useful work that could otherwise have been produced by full expansion through the steam turbine. The Grain CHP scheme is novel in the way that it manages to retain all of the useful steam turbine work and then further extracts the latent heat of condensation of the low-pressure steam condensing in the power station condenser for recovery and delivery to Grain LNG.

Large quantities of waste heat are normally discharged by power station condenser cooling systems, but the temperature of this waste heat is normally too close to that of the surrounding environment to be of any practical benefit. However, the proximity of the adjacent Grain LNG site that stores large quantities of LNG at a temperature of -162 à‚°C provides a natural opportunity for recovery of condenser waste heat in the form of a CHP scheme.

Grain LNG Ltd uses natural gas as a fuel in the vaporization process to heat the stored LNG before it flows into the national transmission system in periods when the system price is favourable. The LNG is stored in cryogenic tanks and is heated inside submerged combustion vaporizers (SCVs). These rely on the products of gaseous combustion to heat a water bath that surrounds a stainless steel tube bundle that contains LNG on the tube side. With a typical SCV system, about 1.6 per cent of the send-out gas is consumed in the vaporization process. This is a potential loss of revenue and also results in the generation of carbon dioxide (CO2) as a combustion product.

During CHP operation, the once-through sea water condenser cooling water system will be isolated. The condensers will be purged of sea water and the SCV water baths at the Grain LNG facility will be connected to the power station condenser water boxes by two 2.5 km supply and return pipes with a diameter 1.4 m. This will form a closed circuit of demineralized water that will be continuously circulated by dedicated circulation pumps. This closed-circuit cooling water scheme will condense the LP turbine exhaust steam and convey the recovered waste heat energy to Grain LNG. This heat would otherwise be passed into the River Medway if the once-through condenser cooling water system were to be used in the conventional way.

The CHP plant will develop in two stages. Stage 1 will deliver 227 MW of heat to satisfy the immediate requirements of the Grain LNG phase 1 and 2 developments. Stage 2 will coincide with completion of Grain LNG’s phase 3 expansion and will provide a total of 341 MW of recovered heat energy.

Operation of the CHP scheme requires degradation of the condenser vacuum to achieve a condenser outlet water temperature of 42 à‚°C. The large temperature difference the stored LNG and the demineralized water leaving the condenser provides the driving force for the waste heat recovery and transfer process.

The amount of heat required by Grain LNG will be nominated in advance, and this will be regulated by controlling the number of recirculating pumps in operation and modulation of flow control valves at the inlet to each SCV.

Development of the CHP system design has presented E.ON with interesting environmental and technical challenges, some of which include:

  • Construction of two 2.5-km corrosion-resistant supply and return pipes across the site of a former oil refinery. The pipes must also pass under an existing LNG cryogenic unloading line and public highway.
  • Environmental mitigation of the heat pipe route, which is home to populations of protected species such as great crested newts and water voles.
  • Material selection and water chemistry specification required to limit the chloride content of the demineralized water circuit and eliminate any possibility of stress corrosion cracking the stainless steel SCV tube bundle.
  • Modification of the existing Grain LNG plant and SCV control system, which will be live and operational during the modification process.

Mutual benefits

The CHP scheme was developed jointly with Grain LNG on the principle that the project would provide mutual benefits to both parties. This was captured in a heat supply agreement that will set the tariff and govern the sale of waste heat from E.ON to Grain LNG.

Grain LNG will benefit from a reduction in the amount of gas that it uses when operating the SCVs in combustion mode, which will reduce their operating costs. When phase 3 is complete, the company’s annual CO2 emissions will also drop by up to 0.5 MTe per year. An additional environmental benefit is the reduction in the amount of heat energy discharged to the river Medway from the Grain CHP station’s conventional once-through cooling water system when operating in CHP mode.

From E.ON’s perspective, the recovery of 341 MW of low-grade waste heat increases the basic CCGT net efficiency from 58.6 per cent to 72.6 per cent when in full CHP mode. However, the recovery of this energy requires a significant capital investment, and the economics of the scheme were supported by a number of government policy instruments. These include the facility to register Grain as a partially qualified CHP plant, which provides for additional carbon allowances to be issued under phase II of the UK National Allocation Plan (August 2007) and entitlement to Levy Exempt Certificates for each MWh of good quality CHP produced.

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