Primer on supercritical steam

Sponsored by

Dr. Martin Alf and John Kern
Siemens Power Generation

Thermodynamics govern the ultimate performance of Rankine cycles whether they are used in a coal fired steam power plant or the bottoming cycle of a combined cycle plant. In order to increase the efficiency of those cycles, higher pressure and temperature steam conditions are desired. As a result, many coal fired steam plants being built and planned around the world today utilize "supercritical" steam conditions to achieve higher efficiency. To learn about this subject, EL&P spoke to Dr. Martin Alf and John Kern with Siemens Power Generation.

What is supercritical steam?

When the pressure in a boiler exceeds the "critical" pressure of 3208 psi, water and steam have the same density and no longer exist as separate phase states. Steam conditions above the critical pressure are referred to as "supercritical." Because the water and steam don't have to be separated in a boiler drum, once-through boilers of a Benson or Sulzer design are typically used to generate supercritical steam. These once-through boilers also make it easier to generate steam at higher temperatures, further improving plant efficiency.

What are the benefits when compared to subcritical units?

Typical steam parameters used in sub-critical reheat steam plants are 2400 psi/1000°F/1000°F while modern supercritical units are operating at steam conditions above 3900 psi/1075°F/1110°F. These more advanced steam conditions result in a net plant efficiency of up to 45 percent, which is a 3.5 percent improvement over a subcritical unit. Higher steam conditions yield higher efficiencies, which means that less coal is used to generate the same amount of electricity, compared to a subcritical unit.

Higher plant efficiency also reduces the production of all pollutants by up to 8 percent. This reduction translates into lower emissions of CO2 and mercury, which are difficult to control with current technologies.

How do supercritical steam conditions affect plant design and cost?

Once-through boilers used in supercritical plants have a number of differences from the drum-type boiler typically used in subcritical plants. In a once-through design, water is completely evaporated in a single pass through the boiler rather than re-circulating through the drum. Current designs for once-through boilers use a spiral wound furnace or incorporate vertical tubing with rifled inner surfaces to assure even heat distribution across the tubes. Since once-through systems don't have a drum to concentrate and remove impurities, they usually require a condensate polisher and use oxygenated water chemistry to maintain steam purity. Recent advances in high temperature materials (such as T-91), provide high temperature strength and improved corrosion resistance, permitting reliable operation in advanced steam cycles, thus improving thermal efficiency.

The design of the high-pressure (HP) element on the steam turbine takes into consideration the higher steam forces and higher pressure drop between HP inlet and exhaust. To accommodate this higher pressure, the number of stages in the turbine is increased and the cylinder wall is thickened. Rotor and cylinder components are upgraded to enhanced 10 - 12 percent chrome steels to withstand higher temperatures and pressures. Full-arc admission and simplified inlet features are used to take advantage of the flexibility and efficiency benefits provided by the sliding pressure capability of once-through boilers and to reduce solid particle erosion in the turbine.

While system processes and plant arrangements for balance of plant are basically the same, the higher supercritical pressures use smaller diameter pipes with thicker pipe walls. Additional systems such as the condensate polisher are required, but all of the coal handling and emissions control equipment can be sized smaller due to increased plant efficiency.

Because total plant costs vary considerably depending on project scope and specifications, it is difficult to make a direct comparison between sub- and supercritical capital costs. Most sources indicate that for a comparably sized plant, the EPC cost for a supercritical unit will be 2 - 5 percent higher.

How does the operation of a supercritical plant differ from a subcritical plant?

Operational differences stem mainly from the dissimilarity between a once-through boiler and a drum-type boiler. The drum boiler holds a greater stored water/steam volume, which allows larger load step-changes that can be helpful for operation on weak transmission grids. For ramping changes over large load ranges, as required for cycling operation, the once-through boiler is significantly faster due to its thinner walled components and smaller storage capacity. These design characteristics also allow supercritical plants to have a shorter start-up time. When combined with the sliding pressure capability of modern steam turbines, supercritical plants offer a higher degree of operational flexibility and reduced load efficiency than subcritical units equipped with a drum type boiler.

Operation and maintenance costs are about the same for the two designs. Supercritical units will have added costs for the condensate polisher and more sophisticated maintenance requirements, but will have lower costs for consumables such as limestone, ammonia and fuel due to higher efficiency levels.

How have recent supercritical plants performed?

Performance problems that plagued the second-generation supercritical plants that went into operation in the 1960s and 1970s have been largely resolved. These issues have not been seen on the third generation plants that have gone into service over the past 15 years. Evaluations of the availability and reliability of comparably sized sub- and supercritical power plants over the past 10 years using databases from NERC and the European agencies show no significant differences in performance between the two plant types.

What "factors" might impact the choice?

The choice between sub- and supercritical technology needs to be made early in the plant design phase and can be influenced by a number of site and operational considerations. The lower operating costs that directly result from higher efficiency supercritical steam cycles can be a strong influence if delivered fuel prices are above average or if the plant has to compete on "spark-spread" with other coal plants. In both cases, slightly higher supercritical capital costs will be justified by the more favorable operating costs.

In addition, if the plant has to compete in a market that might require cycling or reduced load operation, units with once-through boilers have a clear advantage.

The most important factor might be the potential for easier environmental permitting with a supercritical unit. Higher efficiency translates into reduced environmental impact. Less coal and water are used, smaller volumes of fly ash and scrubber waste is produced, and uncontrolled emissions of CO2 and mercury will be lowered. Exactly how environmental agencies will view this benefit is uncertain, but it is expected to become an increasingly important part of an effective permitting strategy.

Alf is the project manager for steam power plants in Erlangen, Germany. Kern is the director of steam turbine and steam plant marketing in Orlando, Florida. Siemens Power Generation's supercritical technology has achieved record efficiency levels of over 48 percent.

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