AD700 innovations pave the way for 53 per cent efficiency

The culmination of the European power industry’s AD 700 development programme is anticipated to be construction of a full scale pulverised coal power plant, with 700°C steam and efficiency (before any provision for carbon capture and storage) in excess of 50%. The hope is to have the plant in operation by 2014, with E.On’s planned “50 plus” plant at Wilhelmshaven a contender – although a go/no go decision has yet to be made on that project. However, AD700 - still pretty much on schedule after 10 years, an impressive achievement in itself - has already yielded a number of innovations and spin-offs that promise significant improvements in ultrasupercritical pulverised coal generation technology in general, including more conventional units currently at the detailed design stage with steam temperatures somewhat below 700°C.

The four phases of AD700 are outlined in the panel, pp 16-17.

Tables 1 and 2 summarise the outcome of the AD700 Phase 1 investigations, in terms of planned main steam parameters and net efficiency targets, respectively.

Also coming out of Phase 1 were proposals for optimising the layout of “conventional” ultrasupercritical power plants. One result was the “compact design”, Figure 1, and the in-line arrangement of boiler and steam turbine, Figure 2. The steam line design for Dong Energy’s planned 2 x 800 MW Greifswald power plant (Figure 3) will be based on these design principles (and this plant design also incorporates a low mass flux vertical tube Benson boiler).

Another consideration (particularly with the prospect of more renewables on the grid) has been increased flexibility, with the following goals: minimum load, presently 20%, target 10%; lowest Benson operation load, presently 30%, target 20%; and load following capability, presently 4%/min, target 5%/min.

Three development tracks

But perhaps the most important spin-offs from AD700 to date have been some significant new ideas for improving the water/steam cycle in pulverised coal plants.

The potential for efficiency increases is large. The improvements can be grouped into three main tracks, the “temperature track”, the “Carnot track” and the “component track.”

Figure 4 shows net efficiency versus maximum steam temperature for a number of supercritical power plants (bottom curve) and the ideal Carnot process (top curve). The curves clearly demonstrate a development track where higher process temperature drives cycle efficiency upwards, called the temperature track. For the past 20 years, it has been a very successful development track where main and reheat steam temperatures rose by ~60K to around the 600°C mark, resulting in a heat rate improvement of more than 3%.

In Figure 4 the efficiency gap between the Carnot cycle and actual supercritical water/steam cycles can be seen as two gaps of almost equal size, the upper gap illustrating the lack of thermodynamic completeness and the lower gap corresponding to internal and parasitic losses in the various components of the plant. The curve in between has been established by setting all equipment (turbine, boiler, pumps, fans, etc) efficiencies at 100% and parasitic losses at zero, and then calculating the net efficiency, which corresponds to a plant with ideal components.

The upper gap, the “Carnotisation gap”, at steam temperatures of around 600°C, is around10 percentage points. Furthermore, Figure 4 also shows that closing the Carnotisation gap would correspond to a development along the temperature track of some 200K! Therefore, the potential efficiency improvements along the Carnot track, are very large, even if the Carnotisation gap cannot be closed completely.

The lower gap which is also in the range of 10 percentage points at 600°C - corresponding to internal and parasitic losses associated with main power plant components, we call the “component gap”, and reductions in these losses constitute a third development track, the “component track”.

At present, it seems that efficiency improvements in individual power plant components comprising the water/steam system and reductions in parasitic losses have reached a stage where only minor improvements can be expected due to high additional investment costs and diminishing returns.

It may be concluded from this simple analysis that besides the temperature track, the Carnot track is the only remaining development track that can still offer solid improvements in plant efficiency. Furthermore, developments along the Carnot track could start now and achieve returns in the short term as they entail a relatively modest engineering effort, notably by the turbine manufacturers.

The Master Cycle

In Europe, the double reheat plant never became very popular. However, Elsam commissioned two double reheat plants in the in the late 1990s, coal-fired Nordjyllandsværket 3 and natural-gas-fired Skærbækværket 3. At 47%, Nordjyllandsværket 3 (Figure 5) still holds the world record net efficiency for a coal-fired power plant (see Table 3) and has an excellent operating record.

But as steam parameters get more advanced and steam temperatures grow, problems emerge with the superheat of some of the very hot turbine bleeds. This “superheat problem” is illustrated in Figure 6, where the expansion line of a conventional ultrasupercritical double reheat cycle operating at 600°C steam temperature is shown (red). Also the tapping points for steam bleeds for the regenerative condensate and feedwater heating train are indicated, and the large super heat of some of the bleeds is clearly shown (the super heat of the steam bleeds is equal to the temperature difference between the bleed temperature and the saturation temperature at the same pressure). Figure 7 shows the super heat of the five hottest bleeds of the conventional double reheat cycle for 600°C cycles, where super heat of the hottest bleed is more than 300K. The expansion line of the 700°C double reheat cycle is not shown, but Figure 8 shows super heat of the top bleeds, and it demonstrates that the super heat of hottest bleed around 400K!

Danish experience suggests that double reheat cycles offer a potential heat rate improvement of about 3% and with present and future coal prices of more than 100 U$/t, they will be competitive. But the superheat problem prevents further development of double reheat cycles. However, there may be a way round this. Investigations carried out as part of AD700 have suggested a new step along the Carnot track based on a simple modification of the conventional double reheat water/steam cycle. Called the Master Cycle, the basic idea is to shift the IP turbine bleeds from the IP turbines to a separate turbine, called the tuning turbine (“T turbine”), which is fed by steam from the first cold reheat steam line and is used to drive the feed pump turbine (see Figure 9).

The name reflects the improved possibilities for tuning and optimising the steam cycle. The expansion and the bleeds of the T turbine are shown in green in Figure 6. It can be seen how effectively the T turbine reduces super heat. The reductions are also shown in Figures 7 and 8.

Figure 10 shows the Master Cycle in a little more detail, with the T turbine working as a 100% feed pump turbine.

Although the modifications entailed by the Master Cycle are relatively simple, they seem to have a deep impact on the water/steam cycle.

The Master Cycle solves the superheat problem by introducing a relatively small additional steam turbine fed by steam from the first cold reheat steam line and serving as a source for the bleed steam for those heaters which normally bleed off the IP turbines (IP bleeds). The T turbine has many similarities with a feed pump turbine, but steam expansion through the T turbine ends in an IP heater, thus avoiding the complicated condenser and cooling water systems of a conventional feed pump turbine.

The steam path of the T turbine is unconventional as the first stages experience a relatively high volume flow compared with a conventional feed pump turbine. However, as expansion progresses, more and more steam is extracted for the regenerative heaters and the final stages only experience the steam flow for one heater. This also means that the main problem in relation to the T turbine design is not limiting the length of the last stage rotating blades as for a conventional feed pump turbine; the problem is rather one of getting sufficiently long and effective blades.

The T turbine could work alone delivering all of its power to a generator, but it is ideal to have the T turbine replace the conventional turbine drive for a 100% feed pump.

Finally, it is important to note that shifting the IP turbine bleeds to the T turbine, which is fed from the cold reheat steam line, means that heat uptake is shifted from the two reheaters to the more effective furnace walls and superheaters as the steam flow through the reheaters is reduced by the amount of bleed steam shifted to the T turbine. This means significant reductions in boiler and steam piping costs.

The main thermodynamic parameters for a Master Cycle operating at 600°C are shown in Table 4.

Calculations done by DONG Energy indicate that the Master Cycle can improve the heat rate by some 3% and efficiency by about 11/2 percentage points, relative to a conventional single reheat cycle, at affordable costs.

Unfortunately, the concept does not have the same potential for single reheat cycles as the super heat problem only appears once in the case of the single reheat cycle.

Towards the AD700 demo plant

With the AD700 technology, efficiencies of 50% can be reached on inland locations and up to 53% on North European coastal locations with double reheat and the Master Cycle.

Support from the European Commission for AD700 has always been considered important as a sort of guarantee of political acceptance. But it did not prove possible to support research on fossil fuel technology under FP6. On the other hand, the Technology Platform set up by the Commission for the Zero Emission Fossil Fuel Power Plant (ZEP) has had extensive support and commitment from the power industry and the Commission has followed i ts recommendations. So fossil fuel efficiency improvements can be supported under FP7, although there is a strong focus on CO2 capture and storage.

In addition a power plant pre-engineering study has been initiated, called NRWPP700, with the aim of providing a solid basis for economic and technical decision making about 700°C power plants. The study, which began in October 2006 and is planned to run until the end of this year, will establish final specifications and includes detailed design for a full-scale demonstration plant. It must also come up with solutions to some of the materials problems identified thus far in AD 700. The project is supported with regional funds from Brussels via the government of North Rhine-Westfalia (hence the name of the study). More than 70% of the money comes from the Emax group of utilities, with co-ordination by VGB.

The pre-engineering study, which includes consideration of the Master Cycle, is the last step before the full-scale demonstration plant. Key issues are optimisation of the boiler design, welding procedures and non-destructive testing techniques, issues that are also being studied in the German national programmes MARCKO 700 and COORETEC, where material qualification and fabrication of components similarly have a central role including qualification of Nimonic 263 and Alloy 740.

The AD700 project has brought the European power industry to a position where a decision on constructing the very first 700°C coal-fired power plant, with an efficiency of more than 50%, can be made on a sound basis. In a press release of October 2006, E.On has declared its willingness to build a full-scale (550 MWe) demonstration plant based on AD700, on the coast at Wilhelmshaven, Germany.

Applying AD700 findings to conventional units

But in addition findings from the AD700 programme to date can also be applied to more conventional ultrasupercritical plants. In particular the invention of the Master Cycle, currently the subject of a feasibility study by Dong Energy, indicates good potential for further improvements. Optimisation of the regenerative feedwater preheating system improves the entire cycle; it makes it possible to realise double reheat systems at reduced cost, and it can provide a more appropriate steam supply to a post combustion carbon capture installation.

The concept of a compact design is also generally applicable to a wide range of steam plants, promising considerable savings.