Hybrid power for Mexico City

Increasing competition in power generation is bringing new life to older steam power plants and utilities are becoming increasingly involved in the re-engineering of their steam turbine plants. Since steam turbines can remain operational for many decades, it is a very attractive proposition for older plants to be repowered, essentially providing the means to convert a high cost producer into a low cost one, with the potential for rendering an existing power plant competitive in the deregulated and merchant markets of the future.

Taking into account repowering, an existing plant may have an actual value much higher than its book value. Repowering enables this hidden value to be realised.

Repowering steam plants with topping gas turbines and heat recovery steam generators increases power plant output and efficiency and reduces environmental effects. A further advantage of this option is that it usually only entails a modest amount of extra cooling water, which can sometimes be a bottleneck for new capacity additions.

In the context of repowering, however, every cycle is different and there is no standard solution for every plant. But experience shows that certain configurations are more competitive than others. For example, often the best way to minimise capital costs is to do as little alteration to the existing cycle as possible.

Repowering projects can be essentially split into two main types: full repowering; and hybrid.

Full repowering

Full repowering consists of decommissioning the existing boiler, and replacing it with a highly efficient gas turbine and a heat recovery steam generator.

With full repowering, an optimal match between the energy available from the exhaust gas of the gas turbine, and the steam generated by the existing boiler is achieved. In addition to that, the existing configuration of the water-steam cycle should be carefully considered, as the loading of the steam turbines will not be the same in the new cycle as it was in the old.

To maximise efficiency of the new HRSG, condensate and feedwater preheating is moved from the existing condensate and feedwater heaters into the HRSG. Therefore all existing heaters are taken out of operation. The feedwater tank, feedwater pumps and condensate pumps may be re-used depending on the economics. As there is no extraction steam the steam flow increases on its way through the turbine because of the addition of IP steam and LP steam.

The new flow distribution means that the HP turbine is operating at part load. Therefore if the HP is designed without a control stage (full arc admission), it is more efficient to operate the repowered plant at sliding pressure with the turbine inlet valves fully open, rather than operate at the original live steam pressure with permanently throttling inlet valves. If the HP turbine is designed with a control stage (partial arc admission), it has to be determined on a case by case basis whether it is more efficient to operate at full live steam pressure or, eg, with three of four valves fully open and sliding pressure. Differences in efficiency and/or output are relatively small.

When operating at sliding pressure, the HRSG can on one hand be designed for a lower pressure, but on the other hand lower pressure also requires larger volumetric flows and larger pipe diameters. For the HP turbine control stage, sliding pressure with all valves fully open means less stress, if that is a concern.

Because extraction steam for the preheating of condensate and feedwater is no longer needed, all extraction lines are cut off and flanged off close to the turbine and drained. The operation of a steam turbine without its extractions of course changes the axial thrust. Each case has to be analysed carefully to find out whether safe operation is possible or whether corrections are necessary. In most cases where the partial turbines are designed with individual balancing pistons there is no problem to be expected. Adjustment is possible by, for example, removing one stage of the blading.

The results show that even with a relatively old steam turbine, a good net plant efficiency can be achieved. Further gains can of course be made by performing a turbine retrofit, eg an LP turbine retrofit with a larger exhaust area together with a condenser retrofit.

The advantage of full repowering is the high efficiency that the plant achieves. When considering that for most conventional or combined cycle plants fuel costs are around 60 per cent of the cost of electricity, this efficiency jump considerably reduces generation costs.

Hybrid repowering

With hybrid repowering, the existing boiler is used, but with the addition of a gas turbine and an HRSG. The hybrid approach provides the most flexible option, in terms of specific fuel cost and absolute power output. There are three main modes, and many other possible modes. The three main modes are:

• Original mode - conventional steam cycle. The plant can still be run in its original mode, utilising the boiler 100 per cent, but with the gas turbine out of operation.

• Hybrid mode - all systems (new gas turbine and HRSG plus old boiler and steam turbine) are used to provide maximum power output. The existing boiler is run at part load, and the reduced heat input is compensated for by the exhaust gas of the gas turbine through the HRSG. Therefore the steam turbine is running near full load. Additionally the gas turbine is running at full load to provide extra capacity.

• Combined cycle mode - the existing boiler is not in operation, but the gas turbine is run at full load, providing steam to the steam turbine through the HRSG. The steam turbine is not used at its maximum load, but the efficiency of the plant is at its highest.

Switching between these modes provides flexibility. The time needed depends on each mode, but typically mode changes that involve the starting of the gas turbine take around 30 minutes.

With a repowered plant using the hybrid option various start up modes are available:

• Start up of the steam turbine and the boiler while the HRSG is not in operation. This is the conventional start up with bypass operation until the steam requirements are met and the steam turbine can be loaded.

• Start up of the gas turbine, HRSG and steam turbine, with the boiler not in operation. This is the combined cycle start up mode with bypass operation until steam requirements are met and the steam turbine can be loaded.

• Start up of boiler, while gas turbine, HRSG and steam turbine are in operation (ie if fuel and load regime require use of the boiler it can be started separately). The steam is bypassed to the condenser until the required pressure and temperatures are reached. Simultaneous start up of boiler and HRSG is not required since the gas turbine always has the faster start up sequence.

• Start up of gas turbine and HRSG, while the steam turbine and boiler are in operation.

Choosing the right option

Deciding whether to repower at all, and, if so, choosing which option, depends heavily on a plant-specific economic evaluation, and analysis of the market conditions the plant faces, both now and in the future.

Typically, full repowerings are most competitive when some or all of the following market conditions are present:

• A secure supply of natural gas at a known cost.

• The existing plant is running on natural gas. The heat rate improvement of around 20 per cent is directly translated into fuel cost.

• The boiler is near the end of its lifetime, or needs major repairs.

• The existing plant does not meet emissions regulations, and needs an investment in NOx and particulate control abatement equipment.

The hybrid option, on the other hand, is appropriate when slightly different market conditions prevail, namely:

• Where both fuels for the hybrid cycle are available, but where there are often seasonal fluctuations between the prices. This can lead to fuel arbitrage.

• Where there are large fluctuations in power demand. This is often coupled with large fluctuations in electricity price. A hybrid plant can provide a wide variation in generating capacity, which can be optimally utilised in a merchant environment.

• In areas where the need to maintain local employment (especially in coal mines) conflicts with the need to meet tightening emissions regulations and generate competitively. The hybrid plant retains some degree of reliance on the original fuel, but integrates it with modern technology to provide clean and competitive power.

The Valle de Mexico case

The Valle de Mexico project is a good example of the hybrid repowering concept in practice.

In autumn 2000, Alstom signed a contract worth around r200 million with Comision Federal de Electricidad (CFE), the Mexican state utility, for the repowering of unit 4 at Valle de Mexico. The plant is about 38 km north of Mexico, at an altitude of 2283 m.

Under the contract, Alstom is supplying three GT11N2 gas turbine generator units and three heat recovery steam generators, the electrical and control system, auxiliary equipment, along with engineering, site preparation, civil works, erection and commissioning.

The repowered unit 4, which will provide electricity to Mexico City, is to be fully operational in the middle of 2002. It will be the first combined cycle plant in Mexico where the design allows operation in pure combined cycle or hybrid mode. In hybrid mode, the three new installed gas turbine generator units and three HRSGs will operate at baseload while the existing boiler will run at 50 per cent load so as to ensure that the existing steam turbine unit is running at full load.

The repowering of Valle de Mexico unit 4 will increase generating capacity from the original 300 MW, with 36.8 per cent efficiency, to about 550 MW, with 45 per cent efficiency, at the design point and during hybrid operation.

In combined cycle mode, ie with the three new heat recovery steam generators but without the original boiler, efficiency rises to 50.6 per cent, with a capacity of 373 MW.

The water/steam cycle is a dual pressure reheat cycle, which gives optimum utilisation of the exhaust gas energy.

The gas turbines are designed to use natural gas as fuel. No water or steam injection is required to match the required emission levels.

The generators are arranged in parallel, feeding two step up transformers (one of three windings and one of two windings).

The electrical equipment for the gas turbines and heat recovery steam generators is located within container modules, which are placed close to the gas turbines. The common electrical and electronic equipment is placed in separate modules.

The new plant will be operated from the existing control room, where new operator stations have been installed.

Project schedule

The notice-to-proceed was issued in September 2000.

The project is being carried out in three stages:

• First phase: open cycle operation of gas turbine No. 1, by mid-March 2002.

• Second phase: open cycle operation of gas turbines 2 and 3, by mid-April 2002.

• Third phase: combined cycle and hybrid cycle operation - by August 2002.