Belgium has a diversified power generation mix that consists of nuclear, fossil, hydropower and natural gas fired power plants. The Belgian utility, SPE, and the privately-owned utility, Electrabel, formed the Coordination of Production and Transport of Electricity (CPTE) to achieve the most efficient use of power generation resources in the country. In 1997, 96 per cent of electricity supplied to the Belgian grid was produced by CPTE. The remaining four per cent came from IPPs.

Since 1992, this partnership has commissioned 360 MW of cogeneration power. The Langerlo and Ruien repowering projects will add 170 MW to the grid in 1998, and more than 200 MW of additional cogeneration power will be commissioned by the end of 1999. Table 2 provides some examples of recent cogeneration and repowering projects undertaken by Tractebel Energy Engineering (TEE) for CPTE.

It is projected that by 2005, there will be 15.8 GW of installed power in Belgium and 88 500 GWh of electricity produced. During this time period, Electrabel plans to build at least 100 MW of cogeneration annually that can be operated 8000 hours per year.

Parallel repowering

The 300 MW units at Langerlo and Ruien were designed in the early 1970s for oil firing, but were converted to coal or natural gas after the oil crisis. Because the furnaces of the existing boilers were not drastically changed, the power output had to be de-rated when firing with coal. The type of configurations at Langerlo and Ruien are ideal candidates for the basic repowering design TEE has developed.

TEE opted to undertake parallel repowering schemes at Langerlo and Ruien. In a parallel repowering, a combined cycle system is added alongside the existing unit, using the unit’s steam turbine. There are two forms of parallel repowering: eco-type and boiler-type. Langerlo and Ruien are undergoing eco-and boiler-type repowerings, respectively.

Eco-type repowering is considered to be the simplest and cheapest solution. In this process, heat from the gas turbine exhaust gases is recovered by two economizers which form the heat recovery boiler. Optimal use of the high temperature recovery is achieved by heating the high pressure feed water which bypasses the high pressure heaters of the existing cycle. The high pressure steam extractions on the steam turbine are considerably reduced, and this high quality steam is used to produce additional power in the steam cycle.

The remaining heat in the exhaust gases is used to heat up condensate water which bypasses the low pressure heaters. A disadvantage of the eco-type is that the load of the repowered unit has to remain high (170 MW at Langerlo) to ensure sufficient flow through the feed water heaters.

In the boiler-type repowering method, the heat from the gas turbine exhaust gases is recovered in an intermediate pressure steam generator. The generated steam is then injected in the cold reheat steam line or between two reheater sections, depending on the boiler configuration.

The remaining heat in the exhaust gases is recovered in a low pressure economizer which heats up condensate water bypassing the low pressure heaters.

Aeroderivative choice

When developing the repowering configurations for the Langerlo and Ruien sites, TEE reviewed the benefits of various gas turbines available on the market. Gas turbines with power outputs in the 80 MW range were not suitable because neither the eco-or boiler-type recovery cycle could reduce the flue gas temperatures to the 82°C range necessary to achieve a high cycle efficiency.

TEE ascertained that in the smaller-range gas turbines, aeroderivatives had advantages over industrial units because they have a better electrical efficiency, as well as lower exhaust gas temperatures.

In particular, the LM6000 PD proved to be a good choice for Langerlo and Ruien because the genset offers 45 MW and a net electrical efficiency higher than 41 per cent at 10°C average site temperature.

TEE also found the LM6000 to be an appropriate choice for its cogeneration projects because it can be operated at base load for a heat recovery steam production lower than 45 360 kg/h. Since steam needs govern the size of the gas turbine, such a low steam demand would require the selection of a smaller engine both with higher investment per kilowatt and lower efficiency. In contrast, increasing the electricity/steam ratio by using a high efficiency aeroderivative gas turbine results in lower steam cost for the heat host.

The LM6000 is one of the world’s most efficient simple cycle gas turbines. It features a hot or cold end drive option and operates with a directly coupled power turbine. There are currently 115 LM6000s in commercial power generation service throughout the world.

The original model LM6000s were designated LM6000 PA (equipped with a standard combustor) and LM6000 PB (DLE combustor). GE has developed the uprated version of these, the PC (standard combustor) and the PD (DLE combustor), bringing output and efficiency improvements.

Langerlo and Ruien will be equipped with the LM6000 PD. Tests carried out on this upated model in mid-1997 confirmed that the PD can provide more power than the PB. Depending on ambient temperature, the power increase achieved by the PD over the PB can range from five per cent to 20 per cent. A heat rate improvement of between four and six per cent has also been achieved.

The efficiency and power gains have been made through improvements to the last three stages of the low pressure turbine, without an increase in gas turbine firing temperature. The tests, carried out at GE Marine & Industrial Engines’ Evendale, Ohio, test cell, also demonstrated NOx levels of 25 ppm on natural gas and 42 ppm on distillate.

Best available technology

Located at Genk in Limburg Province, the Langerlo site will use two LM6000 gas turbines to repower two coal fired units.

The gas turbines were packaged by GE’s original equipment manufacturer, European Gas Turbines (EGT) of Lincoln, UK. The DLE system was selected since the Belgian government required that the best available technology be employed to reduce emissions.

The existing boilers have a steam cycle of 130 bar at 540°C. The units originally generated 300 MW; converted to coal they were limited to 230 MW. The eco-type parallel repowering chosen required the simple construction of a heat recovery economizer in the flue gas flow, and no physical interface was needed with the existing boiler.

Due to the capability of the existing steam turbine, no new steam turbine was required to achieve the combined cycle configuration that increases unit power output by 57 MW. This red- uced the investment cost per kilowatt. As a result, competitive kilowatt prices are expected to be achieved when compared to a new combined cycle plant with larger gas turbines.

The Ruien power station is located at Kluis-bergen in the East Flanders province. One 300 MW unit was converted to coal in 1986, giving a de-rating to 210 MW. The existing boiler has a similar steam cycle to the Langerlo units: 180 bar at 540°C. The unit is now being repowered using one LM6000 gas turbine, also packaged by EGT.

TEE opted for the boiler-type parallel repowering at Ruien since the plant requires higher operational flexibility than Langerlo. With this configuration in place at Ruien, the gas turbine can be operated at base load as long as the coal unit has not dropped below 75 MW. The repowering adds a total of 58 MW to this 210 MW facility.

Benefits brought

The repowering of Langerlo and Ruien has brought significant benefits:

  • No new operating personnel are required for additional power.

  • The relatively low exhaust temperature of the LM6000 made it possible to design the heat recovery generator for dry running. Therefore the gas turbine can be operated as a stand-alone engine when the existing coal fired unit is not in operation.

  • The compact gas turbine package and heat recovery generator can be located close to the existing equipment so there are short piping connections and very little floor space is used.

  • The construction of the repowering portion does not interrupt operation of existing equipment. Only a short outage is incurred when the new system is tied into the existing plant.

    The repowered Langerlo station is scheduled to begin commercial service this month, with the repowered Ruien plant to start up shortly afterwards in June. By using LM6000 gas turbine technology, both power stations should be considerably more efficient and more environmentally friendly.

    Table 1. Uprated LM6000 PC performance characteristics
    Table 2. Tractebel Energy Engineering recent repowering and cogeneration projects in Belgium