Going supercritical: once-through is the key

The pilot supercritical waste heat recovery steam generator test rig at CMI's Seraing works in Belgium, provides ample evidence of the advantages of once-through supercritical HRSG (heat recovery steam generator) design. The rig is based on a typical full-size single pressure level assisted-flow CMI vertical-gas-flow HRSG, but with the notable absence of the large circulation pumps, and the replacement of the separation drum by a dry separator.

There is nothing new in once-through boiler technology in power generation, which has been used in large coal fired utility boilers for decades, particularly in the USA. Neither is supercritical steam technology new on this side of the business, but both are relatively new developments in gas turbine combined cycle system development.

Combined cycle heat recovery technology has primarily seen improvement through the transition from single pressure level HRSGs to multiple pressure level HRSGs by increasing the steam pressures and temperatures and by incorporating reheat into the steam cycle. Higher steam temperatures are a natural result of increases in gas turbine exhaust temperature. The introduction of reheat has allowed higher steam turbine outputs and fume recovery and higher steam pressures to be used without encountering the problems of high moisture content in the steam turbine exhaust.

Once-through circulation can be exploited at high supercritical steam pressures to gain higher overall plant efficiency. The concept may well be beneficial for the high subcritical pressures that are typically encountered in repowering applications.

Simplicity

The OTSG is the simplest design possible to recover waste heat from a gas turbine. It is configured as a serpentine tube bundle with parallel circuits. Water enters at one end and exits as superheated steam. The circulation ratio is one and there is no need for circulation pumps. Since the phase change takes place in the heat exchanger in a once-through boiler, there is no need for either steam drum or blow-down tank. Drum, level controls, blow-down, drum chemical injection, a great many valves and other components are eliminated in the OTSG design.

The advantages are most marked with larger unit sizes. Since it takes a steam turbine of over 250 MWe to take full advantage of supercritical operation, a combined cycle system combining three large gas turbines, or at least two, with a single steam turbine is the best arrangement

New large industrial gas turbines coming on the market such as the upgraded GT26 and GT24, the V94.3A and V84.3A, and the GE and Westinghouse Mitsubishi "G" and "H" machines with unit outputs of more than 250 MWe, are all moving to exhaust temperatures well above 600°C and can benefit from higher pressure supercritical HRSG operation.

For the steam cooled "H" series machines, compact once through steam generators are particularly suitable for generating the cooling steam flow required for simple cycle operation. They are equally beneficial for the kind of superheated steam injection gas turbine cycles that are now penetrating the US cogeneration market.

The pilot plant

The CMI pilot plant rig cost some BEF 120 million. Instead of a full gas turbine, a fan is used, which has duct burners in front of it to raise the gas to temperatures representative of gas turbine exhaust. In effect, it is equivalent to 17 MWt gas turbine discharge. The equipment is able to supply some 65 000 Nm³/h of hot gas at 650°C, about the same as the exhaust temperature of the ABB GT 24 and GT 26 and other advanced industrial gas turbines.

The rig is designed to represent an entire combined cycle unit. The steam turbine is replaced by a steam conditioning valve and a pressure reducing valve so that total independence is preserved in respect of any external disturbance The tests to be carried out will tend to cater for future industrial applications in which gas turbine and steam turbine perturbations are to be eliminated as far as is possible.

A simple, single pressure level system is used with two heat exchangers designed for supercritical pressures up to 240 bar. The first has been designated as the econo-vapo because of its preheating as well as its vaporising functions. The main characteristics are summarised in Tables 1 and 2.

The pilot project started operational testing in November 1997, and stabilised run tests at subcritical pressures of 60 and 150 bar were carried out in April 1998. Since then, supercritical test runs at up to 240 bar to confirm flow stability characteristics and control system function have been carried out, and start-up procedure with the separator optimised.

An ejector pump design which may be introduced to replace mechanical circulating pumps on assisted circulation HRSGs, or to supplement start-up pumps on natural circulation HRSGs in certain cases, has also been tested.

Tests of a patented start-up sequence, which excludes the separator, for which the pilot plant is fitted with special circuitry, are continuing, and additional heat exchange coefficient tests are planned using special facilities built into the transition duct.

High pressure development

It has been pointed out (in papers by Pierre Dechamps and Jean-Francois Galopin of CMI) that the introduction of multiple pressure once-through technology allows lower stack temperatures, therefore recovering more

energy from the exhaust gases. It also allows better matching of the gas curves with

the water/steam curves in the heat exchange diagram.

The CMI team has carried out an objective analysis to compare the relative merits of supercritical schemes with subcritical schemes on a fair basis. They used the developed ABB GT26(A) for the initial study. This machine is well known for its unique application of internal gas reheat. The generic combined cycle characteristics are listed in Table 3.

Seven different combined cycle schemes based on this turbine were studied:

Table 4 lists the combined cycle plant characteristics optimised for maximum efficiency.

Operation

The outlet steam temperature can be reached using two types of regulation depending on the boiler time constant: the first one is feed-forward and has the heat input as outlet variable whereas the second one has the feedwater flowrate for regulating variable.

The response is immediate in the first case but requires a source of heat which can have a fluctuating power output, which is probably not desirable in combined cycles in which a gas turbine is the heat generator. Particular attention will then have to be taken to the feedwater flow regulation through a control valve if it is done at the economiser or preheater inlet. There might be some advantage in inserting an injection point at the inlet of the last superheater for quick response.

These modes of regulation cannot ensure the right temperature at all heat exchanger outlets for every single gas turbine load. It may be that injection will be needed at some point of the process if part loads are to be matched. The optimisation of its location will depend on the ability for the heat exchanger materials to withstand the calculated temperatures. The capability of the steam turbine to cope with these heat and flow modifications will have to be studied, especially in cogeneration applications.

No interaction with normal drum level control regulation exists since the separator will be dry during nominal conditions. The separator is just a kind of water buffer for transient operation – a level controller acts only on the downstream water control valve and has no feedback on the feedwater flow except for emergency purposes.

Two options exist for the design of the vaporiser – cocurrent or countercurrent with respect to the direction of flow of the fumes. This choice will define the amount of water which will be fed to the separator during start-up and/or transients. The cocurrent option gives the biggest water volume between the first evaporation bubble and the separator outlet since this bubble will take place at higher fume temperatures, ie at the exchanger bottom. But for the OTSG the countercurrent option has been selected in order to minimise the separator volume, and as a consequence diameter and thickness, while maintaining the stability of the system. Also, the fumes to water temperature curves of the countercurrent option offer more efficient heat recovery.

The use of once through circulation in a single high pressure level cannot be extended directly to multiple pressure level boilers with a low pressure circuit (LP). A study of the LP circuit stability has shown that the required steam velocities were much too high in the vaporiser and would have brought erosion problems.

Water quality is very important in supercritical conditions. Unlike assisted or natural circulation boilers, in which the separation of water from steam is carried out in the drum, no continuous blowdown exists in forced circulation boilers except during start-up. Hence salts can indeed deposit in the heated section if they are not removed initially. The main problems are due to internal corrosion and tube overheating if heat fluxes are high.

Oxygen values will then have to be kept below 10 ppb as early as the start-up procedure. Polishing or chemical control then becomes of primary importance.

Operating experience

Performance so far with the test rig, as for example reported at the 12th CEPSI conference in Pattaya, Thailand, 2 to 6 November 1998, has been encouraging. The operating characteristics of such a system have many attractive features in addition to the higher thermal efficiency and compact dimensions, especially at part load.

The CMI study assumes that the HRSG should work at subcritical steam conditions below about 70 per cent of full load. This implies that the HRSG is able to work in a sliding pressure mode, which gives the best part load performance for the overall cycle when the load is reduced.

A once-through HRSG can handle much quicker transients than natural or assisted circulation systems, because the limiting characteristic is the maximum temperature gradient induced stress in the HP steam drum. In the once-through HRSG the separator takes the place of the HP drum. The separator can be made out of 800-1000 mm diameter tubing, whilst the diameter of a typical HP steam drum is some 2600 mm. The thickness of the walls and the corresponding temperature induced stress is therefore greatly reduced, allowing faster transients.

Dynamic stability has already been demonstrated. Figure 5 shows graphical printouts of typical variables during experiments with low speed start-up procedure at pressures up to 210 bar. Table 5 shows the main performance parameters of the boiler at various loads. The authors noted that the design inlet gas temperature of 650°C which has been selected does not imply that supercritical steam generators require such temperatures. Indeed they would work very well at 600°C, which is more typical of today's large industrial gas turbines.

Substantial field operating experience has been built up with once through boilers in larger and smaller applications. Slightly more expensive materials may be required, but these are more than offest by system simplification, reduced space requirements, lower material inventories, and higher efficiencies.

When it comes to commercial realization, it is likely that the once-through supercritical HRSG configuration will be implemented as an integrated module. CMI has recently developed this concept, in which everything can be shop manufactured and tested before delivery, so that the whole assembly can be reduced to optimum compactness, which reduces erection time and greatly simplifies commissioning.

An assisted circulation dual pressure boiler arranged within the integrated module concept. This is not an OTSG but it illustrates the approach. Two to three heat exchangers are grouped together in tube bundles which are spaced for easy inspection. Finally.
Tables

Table 1 Basic characteristics of the CMI supercritical HRSG pilot plant
Table 2 Main heat exchange surface area design data
Table 3 Generic combined cycle characteristics
Table 4 Generic combined cycle characteristics
Table 5 Performance parameters in the boiler at various loads