Vertical tubes improve supercritical systems

Between 1995 and 1999, 19.4 GWe of supercritical coal-fired power plant capacity was commissioned in OECD countries compared to just 3.0 GWe of sub-critical capacity. The growing use of supercritical cycles is being driven by the need to minimise the environmental impact of power generation from coal.

Modern supercritical technology has recently been largely restricted to Japan and Western Europe, with some activity in the US, but now it is now being transferred to other regions. Between 1991 and 1998, the proportion of supercritical pulverised-fuel-fired boilers, as a fraction of the total installed utility boiler capacity, was 5 per cent in non-OECD and 56 per cent in OECD countries.

One current estimate of future supercritical plant sales predicts world-wide a steady rise from the present 5GWe per annum (10 per cent of coal fired plant) to 25-40 GWe per annum (50 per cent of coal-fired plant) within 20 years.

Mitsui Babcock has long held a licence from Siemens for the once-through Benson technology. The technology has been applied to supercritical boiler designs in plants such as the Meri Pori power station in Finland (Figure 1).

The Meri Pori plant features a Benson-type, supercritical once-through boiler, producing 440 kg/s of live steam at 240 bar and 540°C, with reheat steam conditions of 397 kg/s at 46 bar and 560°C. Mitsui Babcock boiler technology was selected by the plant owners, IVO and TPO, the Mitsui Babcock technology being supported by the Benson technology from Siemens KWU.

The boiler is a two-pass design, manufactured by Mitsui Babcock, Tampella (Mitsui Babcock’s then licensee) and Rafako in Poland. The furnace walls are a spirally-wound construction to just below the furnace arch, the furnace hopper and upper furnace being formed by vertical tubes. The boiler is furnished with 30 low NOx burners, and over-fire air for further NOx reduction. Boosted by the low temperature cooling water from the Baltic and the advanced steam conditions, the overall efficiency of the plant was an international leader for coal-fired plant with 43.5 per cent, based on LHV. Since entering operation its annual availability has been close to 100 per cent.

To date, all Mitsui Babcock plants, including Meri Pori, have been based on the spiral-wound furnace arrangement. However, Mitsui Babcock now designs and has bid supercritical plants based on a new vertical ribbed-tube furnace. The internal tube ribbing allows the use of lower mass flow rates and hence vertical furnace wall tubing.

Since the introduction of pulverised-coal-firing in the 1920s, there has been a move to larger and more efficient units. In recent years, power stations with sub-critical steam cycles have increased from about 37 per cent (1970) to over 40 per cent efficiency (net, LHV) (1990). This improvement has been due to optimisation of the cycle (eg, number of feedheat stages) and the boiler combustion performance, with very little change to final steam conditions (166 bar, 568°C/568°C). The improvement in efficiency over recent years is shown in Figure 2.

Since 1990, efficiencies have increased from 42 per cent to 47 per cent in Scandinavia and development projects in Europe plan extremely high steam conditions (383 bar, 702°C/720°C/720°C) offering as much as 55 per cent efficiency.

Power station efficiency improvement is dominated by the condenser pressure, but this is a function of the cooling water temperature and hence location dependent and difficult to alter significantly.

The number of stages of steam reheat also has a large impact on cycle efficiency, but the decision to use two stages is associated with additional capital costs. The increased investment may only be warranted where there are very low condenser pressures (eg, plants in Finland with cold sea water cooling) and relatively high fuel prices.

Excess air and final flue gas exit temperature are important to the boiler efficiency, but these are small contributors to the overall cycle efficiency. Increasing the final steam temperature and pressure can attain very significant increases in the power station efficiency.

Overall, 3 percentage points can be gained by moving from standard sub-critical conditions to 300 bar, 600°C/600°C, which are in the supercritical regime (see Figure 3).

Spiral wound furnace

Supercritical conditions are experienced above 221 bar. At these conditions, there is no distinct phase change on boiling and the latent heat of vaporisation is zero. Together with the higher pressures, these factors mean some changes to the sub-critical drum boiler design are required, but the systems remain basically very similar.

There is no longer the ability to separate steam from water and hence the steam drum becomes redundant. Supercritical boilers need to be once-through, ie water enters the economiser, passes to the furnace walls and on to the superheat surfaces; there is no recirculation.

Some changes are required to the furnace wall configuration. There is a lower fluid mass flow in the furnace wall compared to a natural circulation unit of the same evaporation (no recirculation of water), but the same furnace volume needs to be enclosed.

Mitsui Babcock’s supercritical boiler employs the standard two-pass configuration offering the benefits of:

A lower capital cost than tower boilers, particularly for larger units due to a superstructure height about two thirds of that required by tower boilers;

Optimum gas velocity in each tube bank as the gas cools;

Easier dust collection than for tower boilers, as more is carried out of the unit to the precipitators;

Vertical superheater platens provide more effective heat transfer and are more tolerant to slag build up, allowing higher furnace exit temperatures and the use of a wider range of coals including those of high slagging propensities.

Furnace walls

The furnace walls are of a membrane construction in which the lower furnace tubes are usually inclined, forming a spiral pattern around the furnace. Fluid flow enters the ash hopper tubes and exits the furnace wall at the top of the boiler. The spiral wound tubing means each tube experiences a similar heat input, as each tube passes through mid-wall, corners, burner areas etc.

Inclined spiral wound tubes are used in the zone of the highest heat flux in the furnace (Figure 4). These allow the low water flow of a once-through unit to be used at sufficient mass fluxes to keep the tubes cool, despite operating in the highest heat flux zone, and yet still enclose the furnace volume. The high fluid mass fluxes improve heat transfer. The inclined wall tubes are not self-supporting as vertical tubes are and so Mitsui Babcock uses a welded strap arrangement to adequately support the spiral zone. Typical boiler materials for furnace walls and other components are given in Table 1.

Superheater & reheater

The superheater and reheater for advanced steam conditions are similar to those in conventional sub-critical plant. The superheater tubes are designed to operate at some 35°C above the final main steam temperature. The higher steam temperatures impose additional stresses on the superheater material, and increase the rates of both fire and water side corrosion. To offer good service lifetimes, austenitic materials such as Esshette 1250 may be used.

Esshette 1250 was used by Mitsui Babcock in the superheaters and reheaters for all the coal fired 500 MW and 660MW class boilers in the UK. T91 and T92, 9 per cent chromium ferritic/martensitic steels, can be used for moderate steam conditions. The company has used T91 at Dandong, Dalian, Fuzhou, Heze and Liaocheng in China and in numerous retrofit applications in the UK.

The reheater pressure is about one quarter that of the superheater and this allows either less advanced materials or, more commonly, a higher steam temperature to be used. The reheat temperature is typically 20°C greater than the superheat temperature.

Vertical ribbed tube

Through its Benson licence with Siemens, Mitsui Babcock has developed a boiler design where there is no need to incline the furnace tubing. A special vertical tube is used with an internal spiral ribbing (Figure 5) – called “rifling” by Siemens (see following article). Early work on the tubing was carried out by Mitsui Babcock and the Central Electricity Generating Board (CEGB) in the UK and more recently by Siemens to optimise the rib geometry. To enhance heat transfer in the zones of highest heat flux, Mitsui Babcock has used ribbed tubing with a different rib profile in natural circulation boilers for many years.

The internal ribbing improves the heat transfer by throwing the water droplets against the wall of the tube through increased turbulence and the induced swirl. The improvement in heat transfer makes it possible to use these vertical tubes at the low fluid mass fluxes required for once-through operation, even in the regions of highest heat flux, without risk of overheating.

The tubing has been tested as individual tubes and panels in existing furnaces. Mitsui Babcock is now offering a vertical tube supercritical boiler with full commercial guarantees. The vertical ribbed tube furnace benefits from lower capital costs than the spiral wound furnace because: l The tubes are self-supporting so boiler support becomes much simpler.

The transition headers at spiral/vertical interface are no longer required.

Ash hopper tubing geometry is simplified.

The corners are easier to form.

The operational costs are similarly reduced for the vertical ribbed tube boiler because:

while the pressure drop of ribbed tube may be greater than smooth tube, the tube lengths are reduced and the number of parallel paths is increased, providing an overall reduction in the boiler pressure drop.

The auxiliary power load of the boiler is reduced, giving a higher plant output and higher efficiency.

The reduced pressure drop also gives each tube the very desirable “positive flow characteristic” with regard to heat fluxie, as the tube receives more heat, the fluid moves through it more rapidly, providing increased cooling for the metal. This effect minimises temperature differences between tubes, making mixing devices unnecessary and allowing lower design temperatures.

Tube repair is cheaper and easier.

Material limits on efficiency

Currently, materials limit steam conditions to the order of 300 bar, 600°C/620°C giving a net thermal efficiency of 45 per cent (LHV) based on an inland location. Some of the highest steam conditions currently demonstrated are for the Nordjyllandsværket plant (285 bar, 582°C/580°C/580°C), which gives an efficiency of 47 per cent (LHV) in its cold sea water setting.

Avedøre 2 in Denmark is one of the most advanced plants under construction in Europe (305 bar, 582°C/600°C), with an electrical generation efficiency of 49 per cent. The materials for furnace walls are probably currently the limiting factor. A collaborative European project involving Mitsui Babcock is developing boilers with very high steam conditions (383 bar, 702°C/720°C/720°C) offering a potential efficiency of 54 per cent (LHV).

Furnace walls need high-temperature creep-resistant ferritic steel capable of being welded without requiring post-weld heat treatment. T23 and T24 are probable candidates.

Superheater and reheater elements for over 600°C need ferritic steels with improved steam side corrosion. Above 650°C, austenitics are required with improved fireside corrosion resistance, and high nickel alloys are required for 750°C (metal temperature).

Main steam and reheat pipework and headers need high-strength ferritic steel that can operate at 650°C, and a high nickel material or novel pipework design capable of 700°C.

R&D and demonstration

The Mitsui Babcock deployment strategy for supercritical boilers is shown in Figure 6.

The UK Department of Trade & Industry is funding a project between the Thermal Power Research Institute, China Power Engineering Consulting Corporation Ltd in China and Mitsui Babcock. This is to study the cost and technical benefits of the vertical ribbed tube furnace design in China.

The project will use today’s supercritical steam conditions for developing countries (250 bar, 540°C/570°C). Technology transfer tasks cover an examination of the market for supercritical boilers, a workshop to discuss barriers to the introduction of clean coal technologies and reciprocal visits to allow Chinese engineers to become fully acquainted with the Mitsui Babcock technology.

The European Commission has recently selected a collaborative project for potential funding, Innovative Supercritical Boilers for Near Term Global Markets (ISB-2000). Mitsui Babcock will lead the project with Siemens (Germany), PowerGen (UK), ENEL (Italy) and IST (Portugal) as partners. The project will focus on the optimum steam cycle using the limits of available materials (300 bar, 600°C/620°C). The project will develop reference designs for 600 MW class boilers, comparing the standard spiral wound furnace with the vertical ribbed tube variant and a novel style of horizontal boiler.

In parallel with the R&D activities of Phase One, the consortium aims to identify potential demonstration sites in China. A further European supported project is planned (Phase Two) to aid rapid market penetration of developed technology. This will involve demonstration of the selected best supercritical boiler option from Phase one in an emergent country with strong power generation capacity growth.

Application of generic Benson technology in supercritical boilers would benefit from the use of vertical internally ribbed tubing in place of the traditional spirally wound tubing in terms of both plant cost (simpler supporting arrangement), running cost (reduced feed and start-up circulating pump power), and operability (stable flow characteristic). However, no demonstration of this configuration has yet been built.

Worldwide, there are a number of major collaborative research programmes aimed at raising net generating efficiency towards 50 per cent and beyond. These include Marcko (Germany), Cost 522 and the Thermie 700 advanced power plant programme in Europe; EPRI 1403-50 in USA; and programmes supported by CRIEPI in Japan.

The European Commissions’s Cost 522 programme is aimed at developing and testing the emerging boiler materials, particularly focussing on a 650°C target steam temperature.

The Thermie 700 advanced power plant is possibly the most ambitious programme. This is 40 per cent funded by the EC, and includes 40 companies from several European countries. The objective of the project is to raise the steam conditions of supercritical PF plant to 383 bar, 702°C/720°C/720°C, with associated benefits in terms of fuel consumption and emissions.

A number of techno-economic problems need to be solved to achieve these steam conditions.

The project aims to develop a boiler design for operation in about 15 years. For these conditions the design will be dependent on the use of nickel alloy materials. Work to identify and test candidate materials is underway. Demonstration of critical components is expected in the next two years
Tables

Candidate materials for supercritical boilers