Supercritical path to Asia

By William L. Pomponi,
Alstom Power,
USA

Supercritical technology is finding its way in to the Asian power market in a big way. With the sector recovering from the economic downturn, an application that reduces costs and emissions is always welcome.

Supercritical technology is finding its way in to the Asian power market in a big way. With the sector recovering from the economic downturn, an application that reduces costs and emissions is always welcome.

The economic benefits that drove supercritical power plant cycle developments in the 1950s are driving a renewed and growing interest in the technology today. The higher steam cycle parameters associated with supercritical cycles result in higher plant efficiencies that directly translate into lower plant operating costs. As a result, the global power industry is in the process of developing a new generation of supercritical boiler designs that offer full sliding pressure, true cycling capability and simplified start-up systems, without the complex throttling valves found in past designs.

Supercritical designs, which currently represent approximately 20 per cent of today’s coal-fired steam plant market, continue to capture a growing share of the Asian boiler market. With most Asian economies slowly recovering from the region’s recession, new project enquiries increasingly specify supercritical technology and it is anticipated that the share of supercritical cycles will continue to increase.

Alstom studies have generally revealed that supercritical designs can improve plant efficiency and reduce fuel costs up to 17 per cent over subcritical designs. The corollary to higher efficiency is less fuel is burned, thereby creating lower emissions for the same unit of output. As a direct function of efficiency, CO2 is reduced up to 17 per cent, as are NOx and SO2 emissions. Particulate emissions are similarly affected.

With the largest global market share of supercritical units, Alstom’s advanced tangentially-fired designs can be found at ten Asian installations, including the first supercritical boilers ever built in China. In total, these installations represent nearly 16 000 MW in supercritical boiler capacity.

These ten projects include:

  • Two 600 MW boilers at the Formosa Plastics group’s Mai-Liao industrial complex in Taiwan;
  • Twenty 500 MW units at five power plants owned by the Korea Electric Power Corporation (KEPCO) and two 800 MW boilers at the Korea South East Power Co. (Kosepco) power plant in Yonghung, South Korea;
  • In China, two 600 MW units at Shidongkou, China’s first-ever supercritical units, plus two 600 MW units at the Hua Yang Electric Power Company plant in the Fujian Province, and two 900 MW boilers – the largest tangentially-fired boilers ever built in China – for Shanghai Electric at Wai GaoQiao.

Taiwan

Taiwan relies on imported fuels to meet most of its energy requirements. Imported oil is the dominant choice, representing roughly 50 per cent of the fuel used to produce power, followed by coal, which represents roughly 30 per cent of the nation’s total energy consumption.

One of the country’s first major independent power producer’s is the coal-fired Mai Liao power plant, located within the large Mai Liao industrial complex owned by Formosa Plastics Corporation. The plant, located on the southwestern coast, is part of an existing industrial complex that includes numerous utility and cogeneration facilities, as well as the Mai-Liao oil refinery and many petrochemical factories.


Figure 1. China’s WaiGaoQiao power plant in Shanghai houses two 900 MW supercritical boilers
Click here to enlarge image

Alstom has supplied the plant with a total of four power boilers that address Taiwan’s fuel cost and environmental concerns: two 150 MW circulating fluid bed boilers that will cleanly burn petroleum coke produced by the petroleum refinery; and two 600 MW supercritical units that will burn imported coals.

The two 600 MW supercritical units, the first went online in 2000 and the second is scheduled for 2002, were designed and supplied by Alstom and built by Formosa Heavy Industries. The two base-load units are owned and operated by the Mai Liao Power Corporation.

Each unit is capable of burning a variety of international coals to generate a main steam flow of 1950 t/h, steam pressure of 271 bar (3841 psig) and a reheat steam temperature of 566à‚°C (1051à‚°F). The critical in-furnace, emissions control is achieved through the use of Alstom’s TFS 2000 advanced tangential firing system for in-furnace NOx control, as well as a selective catalytic reduction (SCR) system for further NOx reduction, and a flue gas desulphurization system for sulphur dioxide removal.


Figure 2. KEPCO has opted for 20 units of 500 MW to power its supercritical production
Click here to enlarge image

To maximize the unit’s in-furnace NOx control, the TFS 2000 system minimizes the amount of oxygen in the combustion process through improved coal pulverization and classification.

Korea

South Korea uses a combination of nuclear, hydropower and oil, natural gas, and coal-fired technologies to meet its energy demands. Coal represents roughly 20 per cent of the country’s energy requirements, and most is imported from Australia, China and the USA.

Due to the growth in demand for new capacity in the 1980s, KEPCO, South Korea’s national utility, initiated a programme to reduce the country’s reliance on imported oil, increase use of coal, and increase electric power production. Hence KEPCO chose supercritical technology to power twenty 500 MW units.

The boilers, which are now fully operational, were designed by Alstom and co-manufactured, built by Doosan Heavy Industries & Construction co. (Doosan), and located at five KEPCO sites, including Poryong, Taean, Samchonpo, Tangjin and Hadong. The standardized design has not only reduced the country’s reliance on oil imports, but also tightly controls NOx and SO2 emissions; facilitates complex cycling and daily start-stop (DSS) duty needed to meet the peaks and valleys of the country’s electrical load demands.

Due to the success of the original 500 MW coal fired plant programme, Kosepco and Alstom developed in the late 1990s a more advanced 800 MW supercritical design. The first two such units, scheduled to begin operations in 2004, are currently under construction on the island of Yonghung, located just off South Korea’s western coast.

Like the 500 MW units, Kosepco specified a design that included a steam generator re-circulation system for fast startups, turbine bypass systems and a hybrid sliding pressure mode of operation, and, to address its citizens’ environmental concerns, low NOx firing equipment.

As a result, the Yonghung boilers were designed with a sliding pressure capability, which maximizes efficiencies not only at high loads, but at all loads. Specifically, each unit is capable of a fast start 90 minutes for DSS duty. In addition, the units are capable of producing load changes of five per cent per minute, even in the sliding pressure mode.

The units will burn imported coals with a moisture content as high as ten per cent by weight, an ash content up to 15 per cent and sulphur content up to one per cent. Like the boilers at Taiwan’s Mai Liao and China’s HYEPC plants, the Yonghung units are equipped with Alstom’s TFS 2000 in-furnace low-NOx technology and expected to achieve emissions well-below Korea’s 150 ppm (six per cent O2) NOx limit.

Overall, the boiler operating parameters are as follows: main steam flow, 2415 t/h; main steam pressure, 250 bar (3626 psig); main steam temperature, 569à‚°C (1056à‚°F); and reheat steam temperature, 569à‚°C (1056à‚°F).

China

Approximately 80 per cent of China’s electric power is generated by burning coal. With such rich indigenous coal resources, coal is expected to maintain its dominant role in electrical power production well into the future.

In 1998, the Shanghai Municipal Electric Power Company implemented two 900 MW supercritical boilers, the country’s largest supercritical units at the WaiGaoQiao Power Plant located in the harbour region of Shanghai. The units are scheduled to begin commercial operation in 2004.

Since the WaiGaoQiao plant will burn Chinese coals with an iron oxide (Fe2O3) content of 11 per cent, and a calcium oxide (CaO) content of 25 per cent, there is a high potential for slagging. As a result, Alstom based the tangential unit design on its prior experience in Germany with similar “high slagging” brown coals that required a large furnace with large cross-sections.

Therefore, a single-furnace with a low plane area heat release rate was proposed for the WaiGaoQiao plant. Each unit’s firing system will be fed by six vertical mills, with one standby mill. The unit’s concentric tangential firing system will allow it to operate with an excess air level of only 20 per cent when using high slagging Chinese hard coals, and result in NOx emissions of less than 650 mg/m3 s.t.p.

The WaiGaoQiao boiler steam design parameters are 258 bar (3722 psig) and 542à‚°C (1008à‚°F) for the superheated steam and 57 bar (818 psig) and 568à‚°C (1054à‚°F) for the reheated steam. During start-up and emergency conditions the HP pressure is controlled by a HP bypass valve that is designed for 100 per cent BMCR and able to avoid a boiler trip in case of a turbine trip.

Efficiency

These supercritical designs will provide more efficient, higher temperature steam cycles, with the corresponding environmental benefits and fuel savings. Each application will balance plant economics, design experience, and prudent engineering.

The selection of a steam cycle, whether supercritical or subcritical, will continue to be extremely fuel and site specific. However, many of today’s power plant developers are finding that their economic analyses favour supercritical technology. Despite the slightly higher initial investment cost, the calculated cost of electricity is lower for fuels exceeding $15-20/t delivered.

Additional savings can be realized through lower ash disposal costs, and reduced emissions. Ultimately, of course, each plant owner must choose the most cost-competitive solution based on a site-specific combination of fuel costs, operating profile, power revenues, and financing structure.

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