HomeWorld RegionsEuropeSupercritical CFB boiler technology enters utility-scale territory

Supercritical CFB boiler technology enters utility-scale territory

With commercial operation of the worldࢀ™s largest and first supercritical CFB boiler with once-through technology at the Lagisza power plant in southern Poland only months away, PEi takes a look at this key project à‚— a significant milestone in the greater implementation of CFB technology in large power plants.

Heather Johnstone, Senior Editor

Although Poland is one of the few countries in Europe that is currently a net exporter of electricity, there is growing concern, especially amongst energy analysts, that the country could face blackouts in the not too distant future unless its aging power plant fleet is either upgraded or replaced. It is estimated that 60 per cent of Polandࢀ™s generation assets have been in operation for over 30 years and will soon become obsolete.

Having said that, Poland does have several large-scale projects underway that will add significant amounts of new installed capacity over the next three years. One of these is the Lagisza power plant, where a 460 MW unit will replace two of the plantࢀ™s seven power blocks that were built back in the 1960s, and increase its installed capacity from 840 MW to over 1000 MW.

What is particularly interesting about the Lagisza project, however, is that the 460 MW unit features not only the worldࢀ™s largest supercritical circulating fluidized bed (CFB) boiler, but also the first incorporating once-through technology.

Project background & update

In 2001, Poludniowy Koncern Energetyczny SA, one of Polandࢀ™s largest power generators, began a tendering process for the delivery of a supercritical once-through boiler for a 460 MW unit at its Lagisza plant, located near the city of Katowice in southern Poland.

In the December of the following year, Foster Wheeler Global Power Group was selected as the boiler supplier. PKE subsequently decided on CFB combustion technology rather than pulverized coal (PC) combustion because the former offered a lower total plant investment, better net plant efficiency and fuel firing flexibility.

The Lagisza power plant is a world-first, i.e. the largest supercritical CFB boiler incorporating once-through technology
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The scope of Foster Wheelerࢀ™s work, which has been jointly carried out by Foster Wheeler Energia Polska sp. z o.o. of Poland and Foster Wheeler Energia Oy of Finland, comprised the turnkey delivery of a boiler island, including engineering and design, civil works and foundations for the boiler, boiler house enclosure with steel structures, boiler pressure parts, auxiliary equipment, main steam piping to turbine and reheated steam piping, coal bunkers and fuel feeding equipment, electrostatic precipitator and cold end flue gas heat recovery system, boiler controls, instrumentation, erection, construction, start-up, and commissioning. The ࢀ˜Full Notice to Proceedࢀ™ was received in December 2005 and construction at the site began the following February.

To-date the construction phase of the project is 99 per cent complete, and commissioning has begun. Between February and June of this year hydrostatic testing of the pressure parts, including the boiler and main steam and reheat piping, was conducted in three phases. In the second half of August, all the components on the water/steam side of the boiler will be acid cleaned and in September, the main steam and reheat piping to the turbine will be cleaned by steam blowing. The first-fire of the new unit will take place in October, and will be on auxiliary fuel, with the first solid fuel fire occurring in November. PKE is currently constructing the electrical connection to the national grid, and the project is on schedule for synchronization to the grid in December. The first commercial operation of the Lagisza power plant is still on schedule to take place mid first quarter next year.

High Plant Efficiency

The supercritical steam conditions (i.e. 27.5 MPa, 560 à‚°C) mean the new unit at Lagisza is extremely efficient. The boiler efficiency is further enhanced by a special back-end cooling system, which cools the flue gases down and recycles the low temperature heat back into the steam cycle, thereby recovering valuable heat that would otherwise be lost.

The calculated net plant efficiency for Lagisza is 43.3 per cent on an LHV basis, which is a significant improvement on the average efficiency of the European fleet, which is estimated to be around 36 per cent.

Fuel flexibility

The primary fuel for the new unit at Lagisza is bituminous coal, which will be supplied from local mines, but the boiler design is optimized for combustion of additional fuels, such as wet coal slurry that is also available in large amounts from the local coal mines. Because of the CFB technology characteristics, this wet coal slurry can be combusted with the primary fuel at up to a 30 per cent share by fuel heat input. Coal washing rejects can also be burned in the form of dry coal slurry granulates. Finally, the boiler is also designed to utilize biomass fuels up to ten per cent by weight of fuel input, if so desired by the customer.

Meeting (and exceeding) Emission limits

The emission requirements of the European Unionࢀ™s Large Combustion Plant Directive, with regard to sulphur dioxide (SO2), nitrogen oxides (NOx) and fine dust particles has had a profound effect on coal fired power stations. The Lagisza boiler, however, not only meets these requirements, but will exceed them. According to Foster Wheeler, emissions of SO2 and NOx will be half of the required limit, i.e. half of 200 mg/m3n.

It is possible to achieve these low emission levels because of two key features of the CFB process, namely in-furnace capture of SO2 by feeding limestone into the furnace and low NOx production because of an inherently low combustion temperature and staged combustion. Particulate emissions from the unit are controlled by an electrostatic precipitator.

Key boiler design feature

The boiler design for the Lagisza CFB is based on well-proven Foster Wheeler CFB technology, and utilizes the experience gained from over 300 reference units.

One of the key design features of the Lagisza boiler is on the water and steam side, and is based on the low mass flux BENSON once-through technology licensed by Siemens AG, Germany. This technology is ideal for CFB boilers because it utilizes vertical furnace tubes à‚— a typical arrangement in natural circulation boilers à‚— rather than the traditional, more complex spiral wound tubing found in the majority of other once-through designs.

The furnace heat flux (heat absorbed per surface area) in CFB boilers is very low and uniform compared to PC boilers. In addition, BENSON low mass flux design provides a unique ࢀ˜self protectingࢀ™ feature, which reduces the risk of overheating any one tube.

Much like a natural circulation design, if any one tube receives a higher heat flux, the water/steam flow will naturally increase, cooling the tube and minimizing the increase in the tube metal temperature. This effect is a direct result of the increased buoyancy characteristic of the steam/water mixture in the low mass regime. In a nutshell, the vertical tube arrangement produces a high efficiency, low-pressure drop, self-compensating boiler design.

Of particular interest is that the new unit at Lagisza is the first supercritical CFB boiler being built in the world that has incorporated the vertical tube rather than the spiral design.

Flue Gas Heat Recovery System

Another important design feature of the new unit at Lagisza is the flue gas heat recovery system (HRS), which improves the boiler and power plant efficiency by decreasing the flue gas temperature to 85 à‚°C. The system recovers heat from the flue gases and results in an improvement of 0.8 percentage points in total plant efficiency.

The HRS is located in the clean gas area after the electrostatic precipitator and induced draft fan. The cooling of the flue gas takes place in a heat exchanger made of PF-plastic tubing to avoid corrosion problems. After the HRS, the flue gas flows to the cooling tower via glass fibre duct.

A primary water circuit transfers the recovered heat from the HRS to the combustion air system and heat is transferred to both primary and secondary air. As the combustion air temperature before the rotary air preheater is increased, the air is not able to absorb all of the heat available from the flue gases. Therefore part of the flue gas is directed to a separate low-pressure bypass economizer where the heat from the flue gas is used for the heating of the main condensate.

Dynamic Simulations

It is important for all utility power plants to meet current demands for the load following capabilities, load cycling, fast start-ups and high availability. For the Lagisza power plant the electrical grid operator has set up a specified test programme for the verification of unit capability for the primary and secondary controls (electrical grid frequency control) and for the tertiary control (scheduled load changes).

For the Lagisza design, simulation models have been applied for the analysis of boiler transients and control system responses. The above mentioned load change test programme, which will be performed during the commissioning phase, has already been demonstrated in a process simulator environment.

These dynamic simulation models have shown that the unique control features of a CFB boiler combined with standard control can be used to make the CFB once-through comparable to conventional PC boilers for step and ramp load changes. However, unique to the CFB once-through is that it can accommodate disturbances from the process (fuel rate and quality) because of the stabilizing effect of the inventory and flywheel of circulating solids.

The simulation models have demonstrated that with a tightly controlled balance between firing rate, heat transfer, water/steam flow and properly coordinated turbine and boiler controls, the Lagisza CFB boiler is able to fulfill the response requirements of the electrical grid authority.

scaling-UP Supercritical CFB boilers

The Lagisza 460 MW project represents a major evolutionary step towards achieving the greater implementation of CFB technology in utility-scale power plants, and no doubt many utilities and plant owners will.

The next logical step is to design even larger boiler sizes and higher plant efficiencies and even lower emissions. This is exactly what two teams at Foster Wheeler à‚— one in Finland and the other in the USA à‚— are doing.

By end of 2009, the company plans to be able to offer the market both 600 MW and 800 MW single units. The 600 MW CFB unit is likely to be based on the Lagisza design, while the 800 MW looks set to feature a number of unique design features.

In parallel, the scaling-up the steam conditions to advanced or ultra supercritical, i.e. 600 à‚°C is also being pursued by Foster Wheeler.