The Path to Supercritical Power

As power generation technologies grow more sophisticated, control strategies and systems must keep pace for plant owners to take full advantage of the increased capabilities.

Because of its abundance and affordability, coal continues to be a major energy source for power producers worldwide. However, as carbon consciousness becomes more prominent, technologies for gaining efficiency and reducing emissions from coal-fired plants become more important. That is one reason why supercritical and ultra-supercritical boiler technologies are reemerging as new materials and designs help drive higher efficiency levels and ease of operation.

The term “supercritical” is used for power plants with design operating pressures above where normal boiling occurs. Water simply transitions from liquid to vapor without traditional nucleate boiling. For water, the supercritical point occurs at pressures in excess of 3,207 psi. Today’s supercritical units can achieve thermal efficiency of more than 45 percent, compared with a typical subcritical plant’s 30-38 percent.

Supercritical power generation units feature “once-through” boilers designed to operate with pressures from 3,500 to 4,000 psi, versus 1,800 to 2,500 psi for subcritical boilers. Higher firing temperatures and pressures translate into better efficiency, defined as more electricity generated per BTU of coal consumed. This is attractive to power producers, as these increased efficiencies translate into reduced fuel costs and emissions.

Supercritical technology actually dates back to the 1950s, but those first generation units were unreliable and difficult to control, suffering maintenance problems and material failures from operation at higher temperatures and pressures.

Now, thanks to improved boiler design, most notably the Benson once-through boiler, as well as advancements in metallurgy for stronger boiler tubing, a new generation of supercritical units have been gaining popularity over the last decade or so. The Benson design offers a number of advantages, including simplified start up and the ability to operate in sliding pressure mode.

The majority of new supercritical units are being built in Asia, especially China, which is building power plants at a rapid pace. These new plants will provide infrastructure to support booming economies and bring electricity to areas previously unconnected to the grid. In support of China’s growing power requirements, 90 GW of new capacity is planned over the next two years. Two noteworthy fast-track projects in China are Huaneng Power’s 4,000 MW Huaneng Yuhuan plant, and China Huadian’s Zouxian units 7 and 8, which are among the first ultra-supercritical units in China to have a 1,000 MW output rating

Question of control

For supercritical plants, the accuracy and resolution of the DCS (distributed control system) is more important than in subcritical units. A well-designed control system that provides tight regulation and the ability to hit and maintain setpoints can help utilities capitalize on the economic and environmental potential these units offer.

Better control allows power generators to capitalize on the heat capture capabilities of supercritical unit designs. Unlike a drum-type boiler, the once-through, supercritical boiler does not have a large steam drum to store energy. Because there is no energy reserve, the control system must match, exactly and continuously, feedwater flow and boiler firing rate (both fuel and air) to the turbine’s steam energy needs, to deliver the desired generator power.

The ability of the control system to control operations tightly leads to stable, steady-state operation, without oscillation. This is critical, as steady-state, base load operation is key to achieving supercritical unit efficiency.

And once there is confidence that the control system can keep plant operations tightly controlled without the need for frequent operator intervention, power generators can augment plant efficiency by applying other complementary advanced automation and control technologies.

Inchon application

The Yonghung Thermal Power Plant is located in Inchon, South Korea, and has a generating capacity of 1,600 MW. Yonghung units 1 & 2 came online in 2004; units 3 & 4, each capable of 870 megawatts, are currently under construction and slated to be operational by June 2008 and March 2009, respectively. Each supercritical unit has a 5,325,000 lb/hr once-through, singlereheat boiler and a tandem-compound, four-flow, single-reheat, regenerating and condensing turbine.

At the heart of the Yonghung Thermal Power Plant is an integrated control and monitoring system (ICMS) based on Emerson’s PlantWeb digital architecture with the Ovation control system. This manages all plant systems, including coordinated boiler and turbine control, burner management, data acquisition, motor control, and balance-ofplant processes. The Ovation system integrates the units’ processes and interfaces to the FGD (flue gas desulphurization), ESP (electrostatic precipitator), and auxiliary systems. The ICMS incorporates high-fidelity simulators that use Ovation hardware and software in conjunction with modeling software to simulate startups, verify operating procedures and test new applications software. The simulator design offers a realistic opportunity to train and prepare plant staff to handle any situation, which paved the way for easier commissioning and tuning, as well as earlier completion of the plant by approximately three months.

The ICMS uses AMS Suite: Intelligent Device Manager, which monitors the plant’s intelligent pressure, temperature, and level transmitters, and control valves. With it, plant personnel can predict and preemptively correct potential problems with Yonghung’s devices, reducing maintenance costs and contributing to plant reliability.

Another important component of the ICMS is SmartProcess optimization software. Since each power plant is different, effective operation is affected by a different set of variables, internal and external. Optimization software tailored to plant-specific needs can improve opportunities for achieving desired results.

SmartProcess software, for instance, optimizes process control and monitoring using advanced control techniques, fuzzy logic, advanced multi-regional model networks and model-based predictive controls to ensure attainment of increased plant efficiencies and decreased operating costs within operational and regulatory limits.

Optimization at work

The SmartProcess software was implemented approximately three months after Yonghung units 1 & 2 became operational. This enabled operators to measure plant performance first at “design conditions”, theoretically when a plant should be most efficient, then compare this baseline to plant performance after the optimization software was up and running. Comparing the data painted a clear picture of efficiencies directly related to the optimization software.

The software optimizes two processes that impact plant performance: steam temperature and combustion. Minimizing steam temperature variations increases efficiency by reducing boiler tube leaks and turbine blade fatigue, resulting in significantly reduced maintenance costs and outage requirements while improving ramp rates, which contribute to increased revenue.

Optimizing combustion control improves heat rate (boiler efficiency), which translates into reduced costs, as well as reduced emissions and controlled opacity levels. The focus was on minimizing CO (carbon monoxide) and LOI (loss on ignition) while controlling NOx (nitrogen oxides) below limits to meet the Yonghung plant’s objective of improving plant efficiency and maintaining good control response.

Receiving data updates from the ICMS every second, SmartProcess models the dynamic plant characteristics, optimizing on a continual basis, regardless of current unit activities like load ramping, mill swing and sootblowing, and generates optimal process setpoints and biases, integrated in real time with ICMS process controllers.

Although the optimization software manipulates numerous variables, operators are kept “in the loop” through viewing a diagnostic of all the control loops affected, as well as the permissive conditions for optimization activation, on one screen. Optimizing steam temperature considers two factors. The first is the dynamic characteristics of heat exchange rates between the flue gas and steam during changes in the combustion process such as changes in the fuel delivery system or burner configurations, or changes in unit heat transfer characteristics. The second is the time delay that is experienced between when these disturbances are initially introduced and when the final effect upon steam temperature is realized.

Two types of optimization techniques were used to model, predict and optimize these process dynamics effectively to minimize steam temperature variations: a model predictive controller models the process and generates supervisory level setpoints and biases for attemperation water sprays; and, fuzzy feed-forward models of the energy exchange are used for improved steam temperature control.

Combustion optimization decreases boiler NOx, CO emissions and opacity, and increases megawatt revenue through improved ability to respond to load changes, mill changes, and unforeseen events. The modeling and optimization software addresses non-linear or linear process dynamics to maximize quick and effective responses to disturbances and static process characteristics, and accurately predict steady-state responses.

Performance gains

The utilization of SmartProcess optimization software as part of the ICMS enabled the Yonghung plant in South Korea to meet its objectives for improving heat rate and reducing CO formation and LOI. The project illustrates how performance improvements can still be achieved even on a well-tuned, modern boiler within a highly efficient new plant.

The optimization software improved boiler efficiency by improving heat rate under some conditions by 0.44 percent. While that might not sound like a huge difference, an improvement of half a percentage point for an 870 MW boiler has major cost implications. Additionally, CO was slashed from 350-500 ppm to 50-60 ppm, a 90 percent improvement, while unburned carbon and LOI were reduced by 10 to 30 percent. And flue gas temperature decreased 1 to 3 °C, a telltale sign of improved boiler efficiency.

These improvements also benefit plant personnel because the software automatically optimizes boiler control, reducing an operator’s need to move the O2 bias to avoid CO formation. Therefore, not only is the automated system more accurate than manual adjustments, it also translates into a better and more efficient personnel deployment.

Because of the efficiency and environmental benefits they bring, supercritical technologies should continue to figure prominently in new base load plants being built in Asia and around the world. Power generators that choose this option must consider the control strategy carefully for these units. As recent projects demonstrate, leveraging such an advanced automation and control strategy can put power generators on the super-critical path to smooth, efficient unit start up, and the highest levels of unit reliability and availability.

Yonghung Thermal Power Plant

Subcritical drum boiler flow

Supercritical once-through boiler flow

Charlie Menten is a power generation consultant for Emerson Process Management.