500 MWe Sub-critical to Advanced Supercritical Boiler Retrofit to

Reduce CO2

Keith W. Morris, P.Eng

Doosan Babcock Energy America LLC

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Abstract Retrofitting carbon abatement to existing sub-critical coal-fired power plants minimizes capital expenditure and maximizes the use of existing infrastructure which leads to environmental benefits being realized faster and more widely. Two approaches – short term and longer term are possible. In the short term, efficiency improvements through converting from sub-critical to super critical steam cycle and biomass co-firing can deliver substantial reductions in CO2 emissions. Longer term, as regulations and infrastructure permit, carbon dioxide capture and permanent underground storage (CCS) is possible using technology such as oxy-fuel firing. The paper describes the results of a detailed Front End Engineering Design (FEED) study to evaluate and optimize how this sub to super critical retrofit is applied to a real 500 MWe sub-critical plant. The paper explores the phased approach to CO2 reduction and the design strategy of “future proofing” the plant to accommodate CO2 capture at a later date, as regulations and economics dictate. Advanced steam conditions of 4,200 psig and 1117°F/1135°F would place this plant at the forefront of supercritical technology. Naturally it will feature DeNOx and DeSOx equipment from the out-set, and will be CO2 capture-ready. CO2 emissions will be reduced further by co-firing with carbon-neutral biomass fuels. Technology considerations are discussed along with initial economics to retrofit the boiler and turbine to meet current and plan for future emissions regulations. This paper discusses the design challenges that have been encountered for the upgrade, together with the solutions employed.

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Introduction Many coal-fired power plants in Europe are overdue for replacement due to worn-out parts or failure to meet new emissions standards. However, numerous major components may be re-used and the plant re-born to match today’s higher emission standards and improved thermal efficiencies. Retrofit to ‘state-of-the-art’ supercritical steam conditions and emissions abatement technology is a cost-effective way to extend plant life by 20 years or more without compromise. Scottish & Southern Energy (SSE) in association with Doosan Babcock Energy believe that cleaner, more efficient coal-fired power stations will complement the expansion of renewable energy technologies by making a significant contribution to reduced emissions at lower cost than comparable new build. Scottish & Southern Energy initiated a Front End Engineering Design (FEED) contract with Doosan Babcock Energy for a carbon capture ready 500 MWe clean coal plant at its Ferrybridge Power Station in West Yorkshire, UK. The existing station, comprising 4 x 500 MWe Doosan Babcock Energy boilers, was originally built in the 1960s. The one- year FEED study will deliver project costs and timescales required to remove the sub-critical components and associated equipment and replace them with ‘state-of-the-art’ advanced supercritical components. Final go-ahead is expected late in 2007 with the aim of being fully operational by 2011/2012, saving approximately 600,000 tons of carbon dioxide a year. Initially, the focus of attention is Unit 1, although the expectation is that the solution would also be adopted on the other 3 units at site. This represents a particularly challenging project as the intention would be for the station to remain in commercial operation while the retrofit of a particular unit is in progress. Basis of Retrofit Ferrybridge power station consists of 4 x 500 MWe units originally supplied in the mid 1960’s. The station is equipped with two stacks. Units 3 and 4 at Ferrybridge share one stack and are ‘opted-in’ to the new European Large Combustion Plant Directive (LCPD), they are currently being fitted with Flue Gas Desulfurization (FGD) equipment in order to comply. Units 1 and 2 share the other stack and are currently ‘opted-out’, restricting further operational life to only 20,000 hrs, commencing January 2008. Retrofitting advanced supercritical boiler/turbine technology has advantages in minimizing costs, reducing planning consents and re-using existing infrastructure, compared with new build ‘green field’ plants. The key benefit of retrofitting an existing sub-critical plant with advanced supercritical boiler/turbine technology is the improvement in cycle efficiency that can be achieved. In the Ferrybridge project, Doosan Babcock Energy will be contributing their POSIFLOW TM advanced supercritical boiler technology.

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State-of-the-art steam conditions of nominally 4,200 psig and 1117°F main steam and 1135°F reheat steam, at the turbine inlet are being utilized. A life extension of 20 years has been used as the basis of the design. The above measures will improve the overall cycle efficiency from 36% (net) for the existing sub critical units to over 42.3% (net, Higher Heating Value basis) for the supercritical retrofit. This latter figure incorporates the impact of SCR DeNOx and FGD equipment which are not fitted to the existing sub-critical unit. Ferrybridge Advanced Supercritical Boiler Retrofit The existing coal fired boilers were supplied by Doosan Babcock Energy and are of the sub-critical, natural circulation, single reheat, balanced draft type. These boilers have proven to be rugged and reliable over the many intervening years. Originally based loaded, they have adapted well to the demands and changes in operating regime that have been necessary in order to perform within the confines of an often changing electricity supply market. The boilers have been well maintained over the years and the basic configuration of heating surfaces and equipment has remained unaltered from that adopted for the original design. However, higher statutory emissions standards are now being demanded. For older existing plant the provision of additional emissions clean-up equipment represents a significant capital cost and also impacts plant economics in terms of increased ‘in-house’ electricity usage as well as running costs for consumables. A cost-effective way of extending plant life is retrofitting to adopt supercritical steam conditions. The resulting higher cycle efficiencies inherent with supercritical steam conditions mean that less fuel needs to be fired in order to generate a given MWe output, as compared with existing sub-critical plant. The lower fuel input translates immediately into reduced emissions in terms of tons per hour of flue gas produced, and additionally results in smaller capacity emissions clean-up equipment. One of the key imperatives of the advanced supercritical retrofit at Ferrybridge is to utilize as much as possible of the existing plant still deemed usable. For the boiler, the following key components were identified: existing steel structure and boiler house, air-heaters, electrostatic precipitators and draft plant, as well as coal mills, pf pipework and burners. Furnace Design The existing Ferrybridge boilers have one particularly unusual feature in that the lower furnace is wider than the upper furnace. The 48 coal burners are all located in the furnace front wall and are arranged in 4 rows high, each row comprising 12 burners. The requirement to accommodate 12 burners in one horizontal row set the width of the lower furnace. Optimum heating surface arrangement and available sootblower travel dictated the width of the upper furnace. See Front Elevation in Figure 1 below.

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Figure 1 Existing Boiler Elevations

Side Elevation

Front Elevation

Sub-critical, natural circulation boiler Steam Conditions: Main Steam 2400 psig/1054 °F Reheat Steam 625 psig/1054 °F Feed Water: 489 °F

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The existing sub-critical boiler features a natural circulation system which gives rise to a fluid flow rate at full load through the furnace tubes of about 4 times that of the superheater outlet steam flow. The advanced supercritical retrofit boiler, being a once through unit, has approximately the same fluid flow through the furnace tubes as leaves the superheater outlet. The challenge for all once through boiler designs is, therefore, to provide adequate heat removal to the fluid within the furnace tubes to prevent excessive tube wall metal temperatures. The majority of once-through boilers in operation around the world use high furnace tube mass flux rates of 410 lb/ft2-s, or more, to maintain satisfactory tube temperatures. The achievement of such mass velocities necessitates either a small furnace perimeter, or more usually, the adoption of a spiral tube arrangement. For the advanced supercritical retrofit at Ferrybridge, Doosan Babcock Energy will be employing their world leading low mass flux furnace tube technology (POSIFLOWTM). In this design mass fluxes within the furnace tubes are about 205 lb/ft2-s at full load, enabling a reasonable furnace perimeter to be adopted, while maintaining a vertical tube configuration. The low fluid mass fluxes are realized by using carefully optimized furnace tubing featuring internal ribbing. Doosan Babcock Energy currently have 3 New Build orders for this type of boiler, 2 of these are 800 MWe units operating at advanced supercritical conditions. Despite the deployment of this technology, it was recognized that the lower furnace width of the existing boiler resulted in an unacceptably large furnace perimeter. Fortunately, the adoption of supercritical steam conditions benefits the overall cycle efficiency by approximately 15% compared with the existing sub-critical unit. Therefore, resulting fuel flows are correspondingly reduced. As a consequence, the existing complement of 48 burners can be reduced. A comparison of the existing and proposed supercritical retrofit data is shown in the tabulated data (Table 1) below; 100% Load Comparison Table 1

Existing Sub-critical Boiler Advanced Supercritical Retrofit

Gross Generation MWe 505 520 Net Generation MWe 480 480 No. Mills Installed 8 6 No. Burners Installed 48 36 Coal Flow to Boiler t/hr 215 183 No. Mills in Service 6 5 Coal flow/Mill t/hr 35.8 36.5 Burner Thermal Input MWt 36.1 36.6

The adoption of a 36 burner configuration for the supercritical retrofit furnace enables a lower furnace perimeter that is completely acceptable in terms of tube mass flux conditions. As a result the idiosyncrasy of the existing boilers whereby the lower furnace is wider than the upper furnace has been eliminated, resulting in a more practical arrangement for a supercritical unit, as illustrated in Figure 2 overleaf..

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Figure 2 Retrofit Boiler Elevations The major benefits of using the POSIFLOW TM vertical tube low mass flux furnace design are:

• Significantly lower boiler pressure losses (130 psi lower than for the high mass flux spiral furnace tube design) resulting from the low water mass flux, which leads to reduced boiler feed pump power consumption and improved station net heat rate.

• Simple boiler furnace wall framing design and construction as the furnace wall tubes are naturally self-supporting, requiring a less complicated furnace framing structure.

• Simple boiler furnace tube construction. These benefits translate into reduced operating costs as well as easier construction. This latter point is particularly important in a retrofit situation where space and time constraints are at a premium. Fuels and Combustion The supercritical boiler will fire a variety of fuels including: UK domestic coal, blends of imported coals (e.g. South African/Indonesian) and additionally, up to 25% biomass (by heat input). The nominated design coal is from the UK Kellingley mine. Details of this coal are given below in Table 2.

C BOILERL

C S

IDE

WAL

LL C

SID

E W

ALL

L

VIEW ON BOILER FRONT

EL.+120'-0"

EL.+120'-0"

EL.+120'-0"

EL.+120'-0"

EL.+120'-0"

EL.+41'-6"

EL.+0'-0"

Side Elevation Front Elevation

Supercritical POSIFLOWTM boiler

Steam Conditions:Main Steam 287.5 bar/603 °CReheat Steam 59 bar/613 °CFeed Water: 305 °C

C BOILERL

C S

IDE

WAL

LL C

SID

E W

ALL

L

VIEW ON BOILER FRONT

EL.+120'-0"

EL.+120'-0"

EL.+120'-0"

EL.+120'-0"

EL.+120'-0"

EL.+41'-6"

EL.+0'-0"

Side Elevation Front Elevation

Supercritical POSIFLOWTM boiler

Steam Conditions:Main Steam 287.5 bar/603 °CReheat Steam 59 bar/613 °CFeed Water: 305 °C

Steam Conditions Main Steam 4200psi/1117 ºF Reheat Steam 860psi/1135 ºF Feedwater 581 ºF

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Table 2

PROXIMATE ANALYSIS (as received basis)

Kellingley (UK) Coal

Wt. % Moisture 12.60 Wt. % Volatile Matter 27.48 Wt. % Ash 14.80 Wt. % Fixed Carbon 45.12 Higher Value HHV (Btu/lb) 11,075 ULTIMATE ANALYSIS (as received basis)

Wt. % Carbon (C) 59.74 Wt. % Hydrogen (H) 3.90 Wt. % Nitrogen (N2) 1.32 Wt. % Oxygen (O2) 5.24 Wt. % Sulphur (S) 2.00 Wt. % Chlorine (Cl) 0.40 Wt. % Fluorine (F) Moisture 12.60 Ash 14.80

This coal is typical of those mined locally, and has been regularly fired on the existing sub-critical boiler plant. While representing a mid-range bituminous coal, the sulfur content, and more especially the chlorine content, are important factors from the point of view of heating surface material selection and the avoidance of corrosion. The combustion system (coal mills, pf pipework and burners) of the existing sub-critical boiler was identified as being suitable for re-use in the supercritical retrofit. The existing boiler uses 8 ring and ball type mills (designated ‘A’ through to ‘H’), each equipped with an integral static classifier. For full boiler load only 6 mills are required. This arrangement therefore represents an (n+2) mill configuration whereby 2 mills are either on maintenance or standby. The mill turret is connected to two large diameter pf pipes which, closer to the boiler front, trifurcate to feed the individual burners. In this manner, each coal mill is associated with 6 burners. This same philosophy of associating one mill with 6 burners has been carried through into the supercritical retrofit boiler where only a maximum of 6 mills is required. Existing maintenance practices at site have been very good and the mill availability is high at around 95 %. The supercritical retrofit scheme aligns with the concept of (n + 1) mill configuration, which has been widely applied throughout the power industry. Although, on more recent ‘new build’ projects the trend is towards no spare standby/maintenance mill capability. However, as 8 mills per boiler are already in situ at Ferrybridge, the two surplus mills, (in this instance mills C and F), will be linked to the pf pipework of mills (B+G) and D respectively. The surplus mills C and F would provide back-up in the event that mills B, G, or D become unserviceable, thereby providing even greater availability and more flexibility in terms of mill maintenance schedule. The mills serving the existing boiler are already being used for biomass co-firing up to around 5% (by heat input). Experience has shown that firing rates of 5% biomass are sustainable long term while at rates of 10% or more problems can occur.

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Biomass increases the internal recirculation rate of material within the mill and at high flow rates, the cellulosic component inherent within most biomass materials can accumulate on the mill grinding components causing problems. For the supercritical retrofit at Ferrybridge, the biomass co-firing capability through the milling plant will be retained at around 5% by heat input. The additional 20% biomass firing requirement of the supercritical retrofit will be achieved by direct injection of biomass into the pf pipework system. This avoids the need for additional dedicated biomass burners. Additionally, burner control and burner management issues that would arise from the use of dedicated biomass burners are also avoided. The Ferrybridge boilers were retrofitted in the 1990s with Doosan Babcock Energy Low NOx burners. The burner windbox arrangement for the supercritical retrofit boiler will differ from that of the existing boiler. Therefore, while the existing burners will be retained and re-used for the supercritical retrofit an improvement in the form of an actuated secondary air shut-off sleeve will be added to each burner. A boosted over-fire air system has recently been installed by Doosan Babcock Energy on the existing boilers at Ferrybridge to further reduce NOx levels. Once again, this system will be largely retained for use on the supercritical retrofit boiler. Boiler Envelope and Heating Surfaces The overall envelope dimensions of the supercritical retrofit boiler match very closely with those of the existing boiler. The only exception being that the unusually wide portion of the lower furnace of the existing boiler (discussed previously) has been eliminated. The boiler support hanger locations and associated attachment points on the existing steel structure, therefore, require the minimum amount of modification. The boiler pressure parts are being designed to BS EN 12952. The plant is expected to be mostly base loaded for some years following the retrofit, but is being designed for a more variable load regime should this subsequently be the case, especially in mid to later life. Main steam and reheat steam temperatures will be maintained down to 50% load, with flue gas biasing being employed for reheat steam temperature control. Broadly, the heating surface arrangement is very similar to that of the existing boilers. The heating surface in the rear pass has been configured to be at the highest possible elevation so that ductwork to and from the SCR plant (not present on the existing boilers) can be accommodated. The total weight of the pressure parts associated with the supercritical retrofit boiler is lower than that of the existing boilers. The re-use of the existing boiler steel structure, including the support deck, main columns, etc, has been made possible. The disposition of localized loads is not identical, and therefore, a small amount of reinforcing in a few areas will be necessary. Importantly, it will not be necessary to alter the boiler structure foundations or civil works in any way. The pressure part materials are relatively standard. Candidate materials for the upper parts of the furnace walls include T23 or equivalent material. Membrane panels manufactured from this material have been manufactured and tested by Doosan Babcock Energy several years ago with good results. For those components subject to high steam temperatures, notably the platen and final superheaters and

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the final reheater, austenitic stainless steels including Esshete 1250 (which has over 40 years service on the existing sub-critical boilers) is being considered. Emissions Control Equipment As part of the retrofit, additional flue gas clean-up plant in the form of an SCR and FGD will be added. The FGD plant will be a comparatively normal addition and does not need discussion within this paper. The arrangement of the existing Ferrybridge plant is relatively compact. The adoption of a ‘cold-end’ SCR plant was discounted on the basis of its impact on overall cycle efficiency. However, the inclusion of the more conventional ‘hot end’ SCR design posed particular problems. The original plant layout utilized 3 electrostatic precipitators situated between the rear of the boiler house and the stack. To comply with more stringent dust emission requirements, 2 additional electrostatic precipitators were subsequently installed on steelwork above the 3 original precipitators. In more recent times a sixth electrostatic precipitator was added, again mounted on steelwork at a higher elevation, above all five of the previous precipitators. This latter sixth precipitator is situated adjacent to the rear wall of the boiler house and occupies the position that would normally be taken by an SCR. For Unit 1 the possibility of locating the SCR reactor to the side of the boiler house was investigated and a possible scheme detailed. However, on the basis that subsequent units at Ferrybridge would also undergo a supercritical retrofit, replication of this scheme would be impossible. Therefore, due to the lower flue gas (and ash) flow rates applicable to the supercritical design, the potential for removing the sixth precipitator was considered. Discussions with the precipitator plant original equipment manufacturer confirmed that it would indeed be possible to dispense with this precipitator, and by making some changes to the internals of the remaining five precipitators, the required dust emission limit of 0.11 gr/Sft3 could be achieved This option for locating the SCR plant has been selected, as not only does it allow for replication on subsequent boiler units, but it also results in a more symmetrical arrangement of the flue gas ductwork. Additionally, the structural steelwork on which the sixth precipitator was mounted is readily re-usable with only minor modifications for the SCR reactor. This arrangement is illustrated below in Figure 3. It will be noted that the routing of the ductwork from the boiler exit to the SCR reactor and then returning to the boiler house to mate with the existing 3 regenerative air-heaters was a difficult challenge, but one that has been successfully overcome.

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Figure 3 Ductwork to/from SCR reactor Carbon Dioxide Emissions The adoption of advanced supercritical steam conditions and biomass co-firing will deliver substantial reductions in CO2 emissions. These are already integral features of the Ferrybridge supercritical retrofit. Looking forward, as regulations and infrastructure permit, carbon dioxide capture and permanent underground storage is possible. In order to meet this potential requirement, another key ingredient of the work Doosan Babcock Energy is undertaking is the facility to deliver the Ferrybridge retrofit plant CO2 capture ready. An oxy-fuel firing capability is being considered in this respect. What is Oxy-fuel Combustion? Oxy-fuel derives its name from the two main constituents of the combustion process, oxygen and fuel. Combustion processes are normally the result of firing a fuel using air as a source of oxygen. (Figure 4)

AIR

FUELFLUE GAS

TO ATMOSPHERE

Figure 4 Conventional Combustion

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In oxy-fuel combustion (Figure 5), the air that would normally be delivered to the furnace is separated into oxygen and nitrogen in an Air Separation Unit (ASU). Following which only the oxygen is supplied to the combustion process. This oxygen stream must be diluted by recycling flue gas back to the furnace to regulate the combustion temperatures achieved in the furnace. Hence, by utilizing oxy-fuel combustion, the generated CO2 from the combustion process can be captured directly through compression without the need for any absorption processes. Other emission control requirements such as FGD and SCR can be bypassed as NOx and SOx can be captured directly in the compression system. The field of CO2 capture and storage is still in its infancy and while many commentators make reference to designs being ‘capture ready’, few describe what this actually means in practice. In the case of the Ferrybridge supercritical retrofit plant Doosan Babcock Energy are addressing five key issues in this respect.

1) Anticipated new output and generating efficiency. Carbon dioxide capture from boiler flue gases requires significant amounts of energy (both electrical and process steam) to be expended. Detailed calculations have been undertaken and the overall impact on the power plant cycle investigated. Headline results are tabulated below:

AIR

FUELFLUE GAS

ASU INERTS

OXYGEN

CO2 CAPTURE

Figure 5 Oxy-fuel

Combustion

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Table 3

Air Firing Oxy-fuel

Gross Plant Output (MWe) 520 508

ASU Power (MWe) - 63

CO2 Compression Power (MWe) - 46

Boiler/Steam Cycle Auxiliary Power (MWe)

40 36

Overall Net Plant Output (MWe) 480 363

Specific CO2 Emission (lb/kWh) 1.73 0.20

A striking feature from the above tabulation is the significant reduction in

electricity that is available for export (Overall Net Plant Output). The loss of revenue from this product stream would have to be compensated by the value of the CO2 reduction achieved in order to make the capture process economically viable.

2) Evidence that sufficient land space is available

The plot plan of the equipment associated with carbon dioxide capture should not be under-estimated regardless of the technology route chosen. In the case of oxy-fuel firing, space will be required for the air separation units, liquid oxygen storage facility, as well as the CO2 clean-up plant. Based on currently available technology, the required overall area necessary to accommodate these items is around 200,000 ft2 per boiler.

3) Estimated down time between shutdown and completion of re-commissioning with capture

For the oxy-fuel plant, the majority of the equipment can be installed while normal operation continues uninterrupted. The connections for flue gas recycle ducts, gas ducting to the CO2 capture plant and steam/water cycle connections are all the kind of activities undertaken during normal overhaul outages.

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4) Completion of an environmental impact appraisal for the change to

capture operation This item is being considered as part of the Ferrybridge FEED activities.

5) Evidence that the technologies intended to be employed are commercially proven

As noted above, all CO2 capture technologies are still under development. Pilot plant testing, for example, the Vattenfall 30 MWt oxy-fuel demonstration unit will commence operation in early 2008. Additionally, Doosan Babcock Energy will shortly be conducting oxy-fuel trials on the 90 MWt Multi-fuel Burner test facility (MBTF) at their Glasgow (UK) site. By the time Ferrybridge is considering implementing CO2 capture it is envisaged that the technology will be commercially proven.

CO2 reduction is gaining increasing exposure on the European Government agenda. Large stationary fossil-fired power plant installations are an obvious area where CO2 capture equipment could be effectively deployed. The work Doosan Babcock Energy has undertaken as part of the Ferrybridge FEED project will provide a good foundation and site specific information on which the plant owners (Scottish & Sothern Energy) will be able to make informed decisions at a later date, once conditions have matured, to enable the technology to be employed.

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SUMMARY The work that Doosan Babcock Energy has undertaken during the execution of the Ferrybridge FEED project has shown that the retrofit to state-of –the-art supercritical steam conditions is readily achievable. The retrofit will deliver significant reductions in emissions compared with the existing sub-critical plant. Some aspects of the design work have proven to be challenging, even more so than on a comparable ‘new-build’ project. The overall solution developed demonstrates a high level of innovation in numerous areas. While plant additions, plant modifications and retrofits can all too often appear particularly unsightly, the solution for the Ferrybridge supercritical retrofit is elegant in that it falls within the envelope of the existing equipment. The only exceptions being the addition of new FGD plant (required for compliance with statutory emission regulations) and perhaps at a later date the CO2 capture plant. The precepts of (1) maximizing use of the existing plant, and (2) minimizing planning consent issues, have been met. The project schedule anticipates full release towards the end of 2007, with detailed design and procurement of long lead time components commencing in 2008. Manufacture would then follow. It is anticipated that site work would commence during the latter part of 2009. The removal of the existing sub-critical components and their replacement with supercritical components would occur over an 18 month period. Following several months of commissioning, it is anticipated that Unit 1 would be ready for commercial operation at the beginning of 2012. It is important to note that during the implementation phase of the retrofit, right up until deconstruction commences, the plant remains in full commercial operation. (The 20,000 hour operational limit imposed by the Large Combustion Plant Directive for ‘opted-out’ plant will not have been reached within the timescales proposed) Loss of generation revenue only occurs during the approximately 18 month period during which the sub-critical components are removed and the new supercritical components are installed and commissioned. This represents a very significant cost saving compared with a ‘new build’ plant Indications are that the overall capital cost of the project is very much in line with original expectations, not including the significant escalation in material prices. Retrofitting offers a faster lower cost alternative to new build as a way of achieving advanced supercritical performance. It is less intensive in terms of equipment supply, which, in the current busy market place is an important consideration. The Ferrybridge project has already sparked significant interest in similar supercritical retrofit FEED projects by other utilities within the UK, and should be of interest to power plant owners operating in other countries as well.