Parallel repowering steam turbines with gas turbines yield multiple benefits

Higher efficiencies and increased capability at existing sites are available with relatively low capital and time investments

By G?nter Bauer and John S. Joyce

Siemens

The best known and widely practiced form of repowering existing steam turbines is full repowering. In full repowering the fired boiler is replaced with one or more gas turbines exhausting through heat recovery steam generators (HRSG). The HRSGs are designed to raise sufficient steam at the proper conditions to obtain the highest feasible output from the steam turbine. A straight steam cycle is converted to an unfired combined cycle. This results in a boost in both station capability and efficiency.

Topping, sometimes called hot-windbox repowering, is a much less frequently implemented form of repowering existing steam boiler/turbine units. It involves the use of the exhaust gas from the new gas turbine as hot combustion air to burn the fuel in the existing boiler furnace. This is a conversion of a straight steam cycle to a fully fired combined cycle which also requires modification of the boiler island. It also results in both station capability and efficiency being raised but to a lesser extent than in the case of full repowering.

A similar increase in capacity and performance can be achieved more simply by parallel repowering an existing steam turbine because in most cases it does not necessitate modifying its boiler. Parallel repowering secures a second independent source of steam from an HRSG associated with the new gas turbine, which is designed to ensure full output from the steam turbine despite the thermal power output of the boiler being reduced. The boiler is unloaded because admission of supplementary steam into the steam turbine results in the superheater outlet pressure being lowered, thus substantially prolonging the operating life expectancy of the existing boiler.

Parallel repowering, a recently introduced concept, is highly versatile. It allows both coal and natural gas or fuel oil to be burned in the same combined-cycle block. The proportion of solid fuel to gaseous or liquid fuel can be freely determined by appropriate selection of the capability of the new gas turbine relative to that of the existing steam turbine.

Full repowering

The life expectancy of steam turbines is generally longer than that of boilers, especially coal-fired boilers. Hence the replacement of just the boiler with one or more gas turbines with HRSG to match the original or revised steam supply requirement of the existing steam turbine can be an attractive proposition in the case of many old conventional steam power plants. The retention of the steam turbine, complete with its steam-condensing and all other associated equipment, ensures continued use of most of the original investment in the station. The new investment in a gas turbine and HRSG is very low considering the greatly increased station generating capability and efficiency.

The ratio of gas to steam turbine capability in an optimized modern unfired combined cycle is roughly 65:35, i.e. the gas turbine generates almost two-thirds of the total block output (Figure 1). In addition to improving the station efficiency, typically from less than 38 percent to more than 50 percent, repowering can more than double, or even triple, the station output. The space occupied by the original boiler plant is usually adequate to accommodate the relatively compact gas turbine or turbines and associated HRSG. The new equipment (Figure 2) can be flexibly arranged within the confines of the station site relative to the existing steam turbine building because only steam and water pipework connects the HRSG with the steam turbine island.

Repowering is most popular in those countries where there is great difficulty in siting new power plants. It permits more intensive use of old station sites by increasing the installed capacity. Even trebling the station output does not necessarily increase the heat rejection to the neighboring body of water via the existing steam turbine condensation plant. There is generally no need to use a greater quantity of circulating water, or make-up water for cooling towers, or to return the original water quantity at a higher temperature.

In addition, the existing station transformers and links with the external electrical system can be fully retained; only the new gas turbine transformer has to be connected. Since gas turbines and their HRSGs require fewer tall structures than boilers, especially coal-fired furnaces and their stacks, repowering lowers the station silhouette.

Topping

Topping refers to the conversion of a straight steam cycle to a fully fired combined cycle. The exhaust gas from the gas turbine in a fully fired combined-cycle block is used as preheated vitiated air to burn the main fuel in the furnace of the fired boiler. In other words, the gas turbine assumes the role of the forced-draft (FD) fan in a conventional steam boiler/turbine unit. The process is fully fired because nearly all the oxygen in the gas turbine exhaust gas and any supplementary atmospheric air is consumed to support combustion of the main fuel, resulting in only 3 percent to 5 percent excess oxygen in the flue gas.

A conventional steam boiler/turbine unit can be topped by replacing its FD fans with a gas turbine and removing the air preheaters. With topping, roughly 40 percent more gas turbine exhaust than air is required to burn a given amount of fuel in the boiler furnace. This is because the exhaust gas is vitiated to contain only about 15 percent oxygen as against 23 percent by weight in air. This does not pose a particular problem when the fuel is natural gas or distillate. However, in the case of coal or other ash-bearing fuels, the gas velocity levels must be limited in order to prevent erosion of the tubes in the boiler furnace. This does not require the boiler thermal power output to be reduced by as much as 40 percent, but only by about 20 percent due to the hot combustion gas lowering the coal consumption.

Parallel repowering

Parallel-powered combined-cycle concept uses a steam turbine which is supplied from two separate and independent sources. The main source is usually a conventional coal or lignite-fired steam boiler, and the secondary source is the HRSG of a gas turbine (Figure 3). It links an unfired combined cycle with a straight steam cycle through a common steam turbine.

Since the gas turbine exhaust is not used to support combustion of the fuel in the furnace, it is not mixed into the flue gas flow but is expelled separately through the HRSG stack.

The greatest advantage of the parallel-powered combined-cycle principle is its versatility in design, fueling and operation. The ratio of gas turbine to steam turbine capability can be freely selected up to a maximum of about 1:1.5. Generally, however, gas turbines with much less capability than two-thirds that of the steam turbine are applied in order to secure a high proportion of coal relative to more expensive natural gas or distillate for optimum operating economy. The gas and steam turbines can be run independently of each other. Furthermore, only the flue gas flow from the coal-fired boiler may need to be cleaned in order to reduce pollutant emissions to below permissible limits.

Parallel repowering involves providing a new source of steam from a gas turbine HRSG to supplement the steam supply from the existing fired boiler to its steam turbine to achieve full use of the steam turbine and resultant high parallel-powered, combined-cycle efficiency. At the same time, it prolongs the boiler life expectancy. In contrast to full repowering, parallel repowering continues to use the old boiler, possibly in modified form, but at reduced thermal power.

As already stated, steam boilers generally have a shorter operating lifetime than the steam turbines they supply. In many cases their useful life can be extended by being operated at lower superheater output conditions than the licensed operating values, thereby alleviating the duty on the high-pressure parts which can greatly extend their remaining service life expectancy.

An example of parallel repowering is provided by the Mussalo No. 2 station in Finland. Unit 9 originally comprised a gas and oil-fired Benson reheat steam boiler and a Siemens three-casing steam turbine rated 160 MW when it was commissioned in 1973. In 1994 a 60 MW-class Model V64.3 gas turbine with a single-pressure HRSG was integrated into the steam turbine cycle (Figure 4). The HRSG superheats 24 kg/s of steam which is admitted to the intermediate pressure steam turbine, augmenting the hot reheat flow by 18 percent. The remaining sensible heat in the gas turbine exhaust preheats part of the turbine condensate flow. The steam turbine output is thereby increased by 25 MW to 185 MW. Despite the resulting increase in steam-condensing pressure from 0.026 to 0.05 bar, the net efficiency of the 249 MW combined-cycle block is 44 percent without any cogeneration of heating steam. This is four percentage points better than the original 160 MW steam turbine unit. In winter the preheating of turbine condensate in the HRSG can be reduced to heat water for district-heating purposes.

The overall efficiency of a parallel-repowered unit can be maximized by ensuring that the sensible heat in the exhaust gas of the selected gas turbine is fully used not only to raise superheated steam for the turbine, but also to preheat as much of the condensate and feedwater flows as possible in a custom-designed HRSG.

For the purpose of this article, a 70 MW-class Model V64.3A is selected to parallel repower a steam turbine with a rating of 350 MW, resulting in a ratio of nominal gas turbine power to total block power of 1:6. This ratio is used to illustrate the impact of the different parallel-repowering schemes. Their relative merits can, however, shift if a sizably larger gas turbine is selected for the same steam turbine.

The 350 MW reheat steam turbine supplied by a boiler with commonly adopted outlet steam conditions of 175 bar/542 C/540 C is assumed to be incorporated into a cycle with seven stages of regenerative preheating. The steam conditions at the turbine admission valves are 167 bar/538 C/538 C. The net efficiency of this typical coal-fired subcritical-pressure steam generating unit amounts to 40.28 percent based on the provision of highly effective flue gas cleaning-up systems. This thermal performance is strongly linked with the selected condensing pressure of 0.040 bar which corresponds to a circulating water inlet temperature to the condenser in the range of 16 to 17 C. It reflects a technological level which may be superior to that of many old units which have become candidates for parallel repowering.

Unlike topping, which requires the provision of large high-temperature gas ducting between the gas turbine and the boiler furnace, as well as a completely new combustion system to handle hot vitiated instead of preheated atmospheric air, parallel repowering only necessitates that the gas turbine HRSG be connected to the existing steam turbine unit by means of steam and water pipework. There are usually fewer physical obstacles to parallel repowering than topping old stations because the new gas turbine with its HRSG can be most freely located with respect to the existing steam power plant equipment. The simple steam and water pipe connections from the gas turbine HRSG to the existing steam turbine and its condensate/feedwater regenerative heating trains permit parallel repowering to be implemented with minimum operational disruption. The existing steam generating unit must only be shut down for a few weeks to connect the HRSG to its steam/water cycle.

Parallel repowering does not necessitate that any existing flue gas cleaning equipment be modified. However, if the pollutant emissions from a parallel-repowered unit have to meet more severe limitations, the new or enlarged cleaning systems need only be designed to handle just the flue gas from the coal-fired boiler. The reason for this is the gas turbine exhaust does not normally require any cleaning before being expelled separately through the HRSG stack to the atmosphere.

Conclusions

Full repowering is attractive because the capital required for the new gas turbine(s) and HRSG equipment is small compared with the resultant considerable increase in both station efficiency and capability. However, it possesses the disadvantage that it rules out the possibility of using solid fuels because only gaseous or liquid fuels can ensure satisfactory operation of the gas turbine(s).

Since optimum repowering more than doubles the station capability, the primary energy supply must almost be doubled. In many cases it may be difficult to secure an adequate supply of fuel gas or oil to replace entirely the previous lower coal requirement.

Like with topping, parallel repowering allows the continued use of coal or lignite as the main fuel which has to be supplemented by a relatively small supply of natural gas or distillate to operate the retrofitted gas turbine. Unlike topping, however, parallel repowering does not require a prolonged outage of the generating unit to implement the conversion to more highly efficient combined-cycle operation. In both cases a substantial increase of efficiency over that of the existing coal-fired generating unit can be gained. In addition, the new partial-load performance characteristic is much more favorable, yielding virtually constant efficiency over a wide operating range extending from about half to full load of the repowered unit.

All three basic forms of repowering enhance the efficiency of existing steam generating stations. They thus lessen the impact on the environment by reducing the pollutant emissions in addition to conserving the finite coal, oil and gas resources. The full or partial substitution of coal with natural gas or distillate further diminishes the CO2 release. The higher efficiency and the increased generation capability at existing sites, which are obtainable for relatively low capital investment, ensure in most cases greater operating economy.

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Authors

G?nter Bauer studied mechanical engineering with the emphasis on energy technology at the Engineering College in Regensburg, Germany. Since 1981, when he joined Siemens in Erlangen, Germany, he has worked on the design and application of large steam boilers, mostly for coal-fired generating stations. He is responsible for detailed boiler performance calculations and for the optimum integration of boilers into various thermal power plants.

John S. Joyce, B.E. read mechanical and electrical engineering at the University of Ireland in Dublin. Since 1954, he has been involved at Siemens in the design and application of nearly all aspects of fossil-fueled power plants. From 1970 to 1981, he was responsible for the marketing of large steam turbines in the US. Since 1981, he has been in charge of international marketing of turnkey gas turbine and steam turbine power plants, particularly GUD installations. He recently retired from Siemens.