Heat recovery steam generators design options and benefits

The heat recovery steam generator (HRSG) is a heat exchanger designed to recover the exhaust ‘waste’ heat from power generation plant prime movers, such as gas turbines or large reciprocating engines, thus improving overall energy efficiencies. James Hunt looks at the recent evolution of technologies involved.

HRSGs can be used to generate steam for district heating or factory processes, or to drive a steam turbine to generate more electricity. Used in cogeneration, HRSGs help bring overall plant efficiency to 85%”90%, and the economic and environmental benefits are well recognized. In a recent Chinese triple-pressure HRSG application, an exhaust gas flow of 702 kg/s was cooled from an inlet temperature of 596à‚ºC to 119à‚ºC at the HRSG outlet before exhausting to stack. Total heat removed was 371 MWth.

There are several types of HRSG, but the basic construction techniques are largely similar, comprising banks of tubes mounted in the exhaust path. Exhaust gases at temperatures of 430à‚º”650à‚ºC heat these tubes, through which water is circulated. HRSGs mainly absorb heat from the hot exhaust in the flue gases by convection heat transfer, but, in certain sections, heat is transferred by both radiation and convection. The water is typically held at high pressure to temperatures of around 200à‚ºC, boiling to produce the steam.


The HP steam drum from the University of Florida Cogeneration Facility HRSG
Click here to enlarge image

HRSGs are large and complex, and designing, manufacturing, delivering and erecting one is a major undertaking. Big gas turbine HRSGs often exceed maximum size limits for overland shipping of shop-manufactured units. Therefore, they can be supplied modular, or packaged, to minimize shipping problems and site work. Packaged HRSGs are supplied as fully assembled units, but are usually smaller (for 20 MW applications or less).

HRSG design and construction

HRSGs typically comprise three sections: low pressure (LP); reheat/intermediate pressure (IP); and high pressure (HP) ” so this is a triple pressure system, which maximizes plant thermal efficiency. Each section has a steam drum and an evaporator section where water is converted to steam. The steam then passes through superheaters to raise the temperature and pressure past the saturation point. Diverter valves regulate inlet flows, allowing the gas turbine to continue to operate when there is no steam demand, or if the HRSG needs to be taken offline.


Above left: Schematic of the Rya CHP plant in Gothenburg, showing the arrangement of three gas turbines with their HRSGsà‚ etc source: austrian energy & environment cz s.r.o
Above right: The three HRSGs at the combined cycle plant in Ribatejo, Portugal. The plant is equipped with Siemens SGT5-4000F gas turbines.
Click here to enlarge image

These sections, at the various stages, relate to four major components ” evaporator, superheater, economizer and steam drum.

Evaporator

This is where the heat from the gas turbine exhaust turns the water in the tubes into steam. The evaporator is so important that it defines the overall HRSG configuration. Since the inlet and outlet temperatures are both close to the saturation temperature for the system pressure, the amount of heat that may be removed from the flue gas is limited.

Superheater

This dries the saturated steam from the steam drum ” perhaps only heated a little above saturation point, but sometimes to a much higher temperature for extra energy storage. The superheater is usually positioned in the hot gas stream before the evaporator. The type used depends on the evaporator type.

Economizer

This preheats the feedwater, and it replaces steam removed via superheater or steam outlet, and also because of water loss through blowdown. The economizer is conventionally fitted in the path of the colder gas downstream of the evaporator. The type of economizer also depends on the evaporator type, with configurations being typically similar to those of superheaters.

Steam drum

There are usually two ” HP and LP. The internals include primary separators (baffles or centrifugal) and secondary separators, as well as the channels or plenums needed to collect the steam and water mixture from the risers.

Other main components include: inlet duct; distribution grid; burner; stack and pre-heater. The remaining parts are the structure upon which everything is mounted: the casing, suitable flexible and expansion joints, all ductwork, the stack with damper, acoustic shroud and silencer.

Plant emission control equipment may be fitted to the HRSG, but can significantly alter the layout. Such equipment may include selective catalytic reduction (SCR) to reduce NOx emissions. SCR works optimally at temperatures between 340à‚ºC and 400à‚ºC, so is commonly situated in a split evaporator section. However, a relatively recent design advance has been NOx catalysts operating at a relatively low temperature (180à‚º”260à‚ºC). These can be placed between evaporator and economizer. Carbon monoxide (CO) oxidation units may also be fitted.

To improve the thermal efficiency, the flue gas needs to be cooled to a low outlet temperature to recover as much heat as possible. Too low a temperature, however, may result in fuel sulphur condensing as highly corrosive sulphuric acid.

Typically, HRSG manufacturers can supply units designed for use with gas turbines ranging from 5″250 MW with steam parameters to 500 tonnes/hr and pressures to 135 kg/cm2g at around 540à‚ºC temperature.

Evaporator types

D-frame
This comparatively compact type is ideal for HRSGs used with smaller gas turbines and reciprocating engines. Being compact, it can usually be shipped fully assembled. The type’s main disadvantage is that the tubes are v-shaped, and this may result in the HRSG module exceeding shipping limitations where a large gas flow is required.

I-frame

I-frame evaporators can be built as multiple axial modules, or as multiple lateral modules, and can accept any gas flow, and are economical to build and ship. As a result, they have become very popular. Tube bundles can be arranged to contain one, two, or three rows of tubes/header.

O-frame

This type has probably been used for longer than any other. Because the upper header is, effectively, also the steam separation drum, and because of the riser arrangement, more than one evaporator can be connected to the same steam drum. Some of the tube rows can be used as downcomers rather than risers (or the downcomers can be outside the gas pass). The design is, therefore, suitable for large gas flow HRSG modules that are still relatively easily handled.

Horizontal tube

This natural circulation horizontal tube design is suitable for heat recovery from gas turbines, as well as recovery from process plant flue gases. The tubes are large in diameter, and often long too, so this type may have shipping restrictions due to its size. This type of evaporator is generally cheaper to manufacture than the others.

Natural or assisted circulation?

European and Far Eastern HRSGs have tended towards assisted circulation, while those in North America generally use natural (thermosiphon) circulation. Both are proven, with similar thermal performances, but the trend today is toward natural circulation as the most cost-effective and reliable design.


The HP feedwater electric pumps at The University of Florida Cogeneration Facility HRSG
Click here to enlarge image

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Assisted (or forced) circulation HRSGs

These usually feature a vertical exhaust gas path, with the exhaust stack above horizontal heat exchanger tube banks. The water is distributed inside an inlet header to the various parallel circuits, and the steam”water mix is collected in an outlet header, then returned to a drum for steam”water separation. Theà‚ horizontal tubes are supported by drilled tubesheets, hung from a steel structure. This arrangement is often tall (some tubes have been 30 metres long or more), and may notà‚ be suitable where there are height restrictions. Ground levelà‚ pumps circulate the water”steam mixture in the evaporators. Circulation ratios for safetyà‚ are generally less than 3:1″5:1, but lower ratios à‚ may suffice for certain fired heater cases.


The HRSG under construction at the Sappi Paper factory in Gratkorn, Austria. The HRSG is particularly flexible, capable of gas turbine, supplementary firing and fresh air operation.photo: aalborg engineering, denmark
Click here to enlarge image

Assisted circulation HRSG advantages:

  • faster start-up from cold
  • more flexible in response to steam condition changes
  • smaller footprint
  • more easily fitted with extra tube banks without increasing footprint
  • generally lower water content
  • can be mounted directly above gas turbine (if space is restricted).

Disadvantages:

  • circulation pumps consume electricity and may compromise reliability
  • very tall
  • horizontal evaporator tubes can become ‘dry’, leading to local tube overheating, or corrosion
  • not so easily drained.

Natural circulation HRSGs

These generally have a horizontal exhaust gas path, so that the heat exchanger tube banks are arranged vertically (or inclined), with the vertical tubes usually top supported. This allows for unrestricted downward expansion. Water”steam mixture circulation is achieved using natural rising characteristics (thermal lift through differential density). It is replaced by water from the drum by gravity flow. Circulations may vary from tube row to tube row, and circulation ratios range from 8:1 to 15:1.

Natural circulation HRSG advantages:

  • easy to provide water-cooled combustion zone or furnace
  • lower ” may be more suitable for earthquake zones
  • if exhaust stacks from several HRSGs are collected to a common area, ductwork can be at ground level
  • no circulation pumps ” less energy consumed and more reliable.

Disadvantages:

  • superheaters can be more difficult to drain
  • tube withdrawal may not be easy
  • start-up times from cold may be longer
  • more cleaning usually required
  • takes up greater footprint.

Whichever type of HRSG is chosen, it must operate in any required mode during the plant’s typical 25 year life ” even if there are many starts, which can significantly add to stress build-up, metal fatigue, leaks in tubes and so on.

Tube materials

Tube materials vary according to the operating temperature(s). Most HRSGs recover heat from a comparatively low temperature gas (540à‚ºC or lower), although, where supplementary firing is used, gas temperatures will be significantly higher. Carbon steel tubes usually suffice for evaporator and economizer. However, higher alloy content tubes (e.g. 18 Cr 10 Ni Ti) with thinner tube walls can sometimes be more economical than lower-cost tubes with thicker walls. HRSG tubes are often finned ” sometimes helically ” to add heating surface. Superheaters, which are in the hottest part of the exhaust gas flow, generally use T11 tubes.

As oxides and carbon deposits from the exhaust gases slowly build up on tubes and other parts, heat transfer efficiency can fall considerably, which is why HRSGs must be regularly cleaned.

Ducting and casing

HRSGs have a lot of ducting. This transfers flue gas, or bypasses it, connects plenums to stacks, and distributes combustion air to burners. Dampers and louvres control flows. Pressure lossesà‚ can be considerable and may reduce plant efficiency. Ducting/pipework penetration seals must not leak, yet are a regular source of problems. Casings may be of ‘hot’ or ‘cold’à‚ types. Cold casings need more careful insulation techniques. Ceramic fibre case insulation provides a higher efficiency and makes casings safer for operation and maintenance personnel.

The ‘pinch point’ and other effects

To achieve optimum HRSG heat balance, many variables have to be taken into account. As the evaporator inlet and outlet temperatures are close to the saturation temperature for the HRSG system pressure, the amount of heat that can be recovered from the flue gas is limited because of the ‘pinch’ (or ‘approach temperature’). This effect is important because the ‘pinch’ varies considerably ” HRSGs in process plant applications have a significantly higher ‘pinch’ than HRSGs used in cogeneration. This phenomenon affects coil arrangements and gas/water flows. There are other process approach temperatures, but they have relatively little effect on HRSG design.

HRSG heat balance also depends upon prime mover exhaust mass flows/temperatures, max/min steam flow and temperature requirements, flue gas (exhaust) thermal properties, back-pressure, noise and emissions limits, and also the approach temperatures. Any cycling must be taken into account, as well as any simple-cycle operation. Another consideration is whether supplementary firing might be required (see below).

Supplementary firing

Supplementary (or ‘duct’) firing uses hot gas turbine exhaust gases as the oxygen source, to provide additional energy to generate more steam if and when required. It is an economically attractive way of increasing system output and flexibility. For example, 100 MM BTU burners can produce about 13 MW of extra power.

Supplementary firing can provide extra electrical output at lower capital cost and is suitable for peaking. A burner is usually, but not always, located in the exhaust gas stream leading to the HRSG. Extra oxygen (or air) can be added if necessary. At high ambient temperatures, a small duct burner can supplement gas turbine exhaust energy to maintain the designed throttle flow to the steam turbine. While HRSGs do not form unwanted emissions, supplementary firing itself can produce emissions, such as NOx. SCR equipment will reduce this.

The benefits of supplementary firing in a large combined cycle or cogeneration plant depend on capital, fuel and electricity production costs, overall plant design and hours run. However, although supplementary firing produces more steam during periods of high demand, it is less thermally efficient than unfired plant because the extra fuel required by the burner(s) adds power in a Rankine cycle ” less efficient than combined-cycle. However, there is mitigation to some extent because of HRSG heat recovery.

Another reason for the lower thermal efficiency compared with the base plant, is that the steam turbine is sized for maximum supplementary firing, so is less efficient at the ‘part load’ that the base plant otherwise operates at. Even so, the price of power can make supplementary firing very profitable.

Another economic and environmental advantage of supplementary firing is that it is possible to use a wider range of fuels, such as hydrogen, biogas, digester, blast furnace, landfill and coal gasification gases. As an example, a large gas turbine-powered US petrochemical plant was fitted with HRSG supplementary firing to burn volatile organic-compound-laden off-gases, which otherwise had to be destroyed for environmental reasons. The gases were fed to a supplementary burner and injected into the natural gas flames, oxidizing the combustible components of the waste gas stream.

In brief, therefore, supplementary firing directly into the gas turbine exhaust has the following benefits:

  • greater plant output
  • better control of plant thermal output
  • more efficient process steam production
  • steam production at reduced gas turbine load (or even shutdown)
  • can compensate for changing ambient conditions
  • can burn fuels unsuitable for gas turbines.

However, capital, fuel and operating costs, as well as maintenance, all increase. Typically, duct burners add 10%”15% to HRSG cost. The economic benefits, therefore, depend on the trade-off between periods of high and low electricity prices.

Maintenance

Excellent preventative measures and good regular maintenance are crucial for high efficiency and a long, reliable HRSG life. Electronic monitoring systems, such as ABB’s Optimax, provide automatic monitoring and failure prediction of critical components. Eventually, however, repair or replacement may be necessary. Manufacturers usually provide a complete turnkey service including all maintenance functions. Note that low gas turbine loads can result in unexpected HRSG problems occurring, such as economizer fatigue.

Oxygen scavengers remove excess oxygen from water circuits, avoiding accelerated corrosion, and water pre-treatment reduces scale formation. Such measures will improve up-time and reduce maintenance costs. Protection can be achieved by maintaining water OH alkalinity above 250 mg/litre, so that it is not corrosive to wetted steel internals.

Recent examples

In the 400 MW CCPP plant in Castejon, Spain, CMI HRSG supplied a vertical gas path natural circulation HRSG with three pressure levels, plus reheat for a 265 MW gas turbine. This latest HRSG is suitable for use with the most powerful gas turbines, and for heavy cycling.

The Skanska Brasil LTDA /Construcoes e Comercio Camargo Correa consortium has awarded Vogt Power International a $9 million HRSG contract for the Cubatàƒ£o combined cycle thermal power plant project in Sàƒ£o Paulo, Brazil. Vogt Power is providing a three-pressure non-reheat, natural circulation HRSG with an independent condensate pre-heater, installed behind a GE-7FA gas turbine.

A Far East dairy uses four gas turbines. Up to 400 GWh/year of electricity is produced, with HRSGs providing superheated high-pressure steam. Extra electricity is produced by a back-pressure steam turbine ” total plant output 70 MW. Excess HP steam is converted to saturated LP for other uses. At peak periods, supplementary firing is used. The two HRSGs each produce 17 tonnes of 40 bar superheated steam/hour without supplementary firing (37 tonnes each with supplementary firing). This is a Todd Energy/Fonterra Co-generation owned plant.

New developments

Conventional HRSGs have thick-walled components. Load changes have to be relatively slow and infrequent to avoid stress damage. Such constraints can seriously limit the gas turbine’s quick-starting capability. However, the latest gas turbines have made a relatively recent development feasible. This is the ‘once through steam generator’ (OTSG) which has neither conventional steam drums, nor interconnected pipework. OTSGs can operate at supercritical pressures and are, therefore, very important for modern high-efficiency, lower-emission power plants. Thermal lag is greatly reduced and faster cold starts are possible.

Such an OTSG is used in the 51 MW Whitby Cogeneration baseload plant in Ontario, Canada ” powered by Rolls-Royce Trent gas turbine. This plant has to be flexible because the steam needs vary significantly. The gas turbine’s exhaust heat generates up to 60,000 kg/hr (132,000 lb/hr) process steam at a net plant heat rate of 5340 Btu/kWh without duct firing (64% efficiency). Supplementary firing is used when more process steam is required. With duct firing, 82,550 kg/hr (182,000 lb/hr) of process steam is produced by the OTSG at 204à‚ºC steam conditions.

In another development, Siemens plans to overcome the reduced thermal efficiency of supplementary firing with its Complementary Fired Combined Cycle (CFCC) design. This uses fractionally sized gas turbines exhausting into HRSGs associated with their base gas turbine(s). Such a configuration offers, claims Siemens, a very high peak loading efficiency, plus the possibility of increasing the level of power augmentation because of its impact on the HRSG. This system can be applied to new plant, or be retrofitted into plants with (or without) existing supplementary firing.


James Hunt is a UK-based writer on energy and electrotechnical issues.
e-mail: cospp@pennwell.com

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