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Old and New Cycles

1.Introduction

In past papers I have tried to focus on subjects that will be of interest to both electric utility and facility energy management readers. For a while I have wanted to write a paper on combined cycle power plants (a.k.a. combined cycle gas turbines or CCGTs). However these are not particularly applicable to facilities, except the largest ones. Recently it occurred to me that if the paper were broadened to include the two prime movers in CCGTs, which are combustion turbines and steam turbines, as separate electric generators that these are frequently used in facilities, and are still applicable going forward.

In a paper last fall, "Alternatives for Alternative Energy" (link below), we covered steam and gas turbines briefly, but only in the context of renewable energy. Here we will focus on the technologies without considering whether the fuel is renewable or not.

http://www.energycentral.com/c/pip/alternatives-alternative-energy?utm_medium=update_email&utm_campaign=weekly&utm_content=200777&utm_source=2018_01_03

Furthermore I believe it will be interesting to briefly explore the history of each of these turbines, and the evolution of combined cycle plants.

2.History

Frequently the evolution of various machines occur as developments that they are dependent on evolve. This is the case with the three electric generation technologies described below. The main supporting technologies in these cases were metallurgy and metal fabrication techniques.

2.1.Steam Turbines

Many think the oldest steam engine is the reciprocating engine invented by Thomas Newcomen and James Watt in the 18th century. That's almost true, if you add the phrase "…to perform useful work" The oldest engine was a steam turbine, the aeolipile, described by Heron of Alexandria in 1st-century Roman Egypt.[1]

After the aeolipile, the only other steam device before the development of the reciprocating engine was in the 16th century when a device similar to the aeolipile was described being used to rotate a spit in Ottoman Egypt.

Although several steam turbine designs were proposed (including one by the above James Watt), none were able to be commercialized until in 1831 William Avery of Syracuse, New York created a design similar to the aeolipile. About 50 of these were produced and primarily used to drive saw mills, cotton gins and woodworking shops.[2]

The first steam turbines that were relatively close to modern designs were developed during the 1880s by Carl de Laval of Sweden (diagram below). From 1889 to 1897 de Laval built many turbines with capacities from about 15 to several hundred horsepower. His 15-horsepower turbines were the first employed for marine propulsion in 1892.2

The main issue with the de Laval turbine was that it operated a very high rotational speed, and the metals of that period had difficulty coping with the centrifugal force. In 1884 Charles Parsons developed a multistage steam turbine that operated at a more reasonable speed, and also paired these turbines with electric generators. In the next few years Parsons made many incremental improvements to his designs, and by the end of the 19th century "C.A. Parsons and Company" was producing Megawatt-scale turbine-generators.

C. A. Parsons and Company became Reyrolle Parsons in 1968, merged with Clarke Chapman to form Northern Engineering Industries in 1977, became part of Rolls-Royce plc in 1989, and still survives today as a division of Siemens.[3]

2.2.Combustion Turbines

One would think that combustion turbines (a.k.a. gas turbines) would have followed soon after steam turbines. After all, both use a compressible gas (steam and combustion-heated air) to extract energy by passing this through turbine blades. However, there is one major challenge with combustion turbines. Even though pressurized steam turbines operate at temperatures over 800°F, in a combustion turbine the turbine inlet temperature is much hotter and can be above 2,000°F.[4] Early steam turbines challenged the capabilities of metals used for turbine blades, and these metals certainly were not ready for the temperatures of combustion turbines.

A gas turbine (as used for electric generation) has a very simple design. There are basically three components – the compressor, the combustor and the turbine. Both the compressor and the turbine are rotating machines, and they typically are on same shaft. Where the turbine is used for a generator, the shaft typically extends to the generator, unless the generator is offered for both 50 Hz and 60 Hz operation. In the latter case gearing is used between the turbine and the generator.

The idea of a combustion turbine has been around almost as long as the steam turbine, via various descriptions and patent-filings. The compressor was developed first along with a lower-temperature turbine in a turbocharger. Sanford Moss, who worked for GE, developed the turbocharger: "Moss built a high-RPM supercharger, driven by engine exhaust flow, and tested it in 1918 at Wright Field in Dayton, Ohio. As a result of this test, the government awarded its first [turbocharger] contract to GE."[5] A turbocharger's turbine is driven by the exhaust of a piston engine (which is cooler than a combustion turbine). The turbine is directly coupled to a compressor that compresses air before it enters the piston engine's combustion chambers. This either increases the power of the engine and/or (especially for aircraft) increases the altitude at which it can operate.

Because the final development of the combustion turbine occurred in the late 1930s, and coincided with the approach of WWII, a major boost to combustion turbine technology occurred with the development of the aircraft turbo-jet (in Germany and England initially).

The first combustion turbine used for electric generation was a 4 MW utility power generation gas turbine from Brown, Boveri & Company for an emergency power station in Neuchâtel, Switzerland in 1939 (figure below)[6]. The only major difference of this design vs. more modern combustion turbine is that the three components are separate (even though the compressor and turbine are on the same shaft as the generator) rather than being tightly integrated.

Three major variants of the combustion turbine as used to produce power have evolved. The first to evolve was the industrial turbine as developed by BBC and described above.

At the same time the precursor of the second variant was evolving. Aircraft turbojets initially entered full production in 1944 with the Junkers Jumo 004, which powered German military jets. Aircraft turbojets eventually evolved into the aero-derivative combustion turbine in 1963 when Pratt and Whitney introduced the GG4/FT4. Aero-derivative turbines were smaller, lighter and more expensive than industrial turbines, since they used more expensive flight-rated metals. They offered several advantages, including higher efficiency, easier maintenance and fast-start / fast-ramp capabilities. The last capability made them ideal for peaking and fast frequency regulation service.

The third variant is the microturbine. The precursor to microturbines were the auxiliary power units used (mainly) by aircraft to generate power and hydraulic pressure when the primary turbojet engines were not operating. These were used in various other similar operations, but were quite expensive compared to other alternatives like piston-engines. In 1981 Allison developed a microturbine that generated electricity for U.S. military deployments. Approximately 200 generators were delivered to the U.S. Army, and since then, more than 2,000 units have been provided.[7]

In 1978 the deregulation of the electricity market led to the hope that microturbines could be used for distributed generation. The Gas Research Institute started a program in 1980 to develop a cost-effective microturbine for distributed generation. The program was abandoned in 1990 (due to the apparent high-price of the design), and then restarted with additional partners a few years later. The program bootstrapped some early commercial manufacturers of microturbines, including Capstone Turbine Corporation (then NoMac Energy Systems).

2.3.Combined Cycle Power Plants

It wasn't long after the development of the early commercial combustion turbines before the initial versions of the combined cycle plants appeared. These were basically retrofits of conventional steam power plants where the combustion turbine provided additional electric power plus additional heat to the conventional boiler to improve the plant's efficiency. Several sources identified the first combined cycle plant as Oklahoma Gas and Electric Company Belle Isle Station. There, a 3.5 MW combustion turbine used the energy from its exhaust gas to heat feedwater for a 35-MW conventional steam unit in 1949.[8] Note that much of the information in this section comes from the prior reference.

In the 1950s and 1960s most combined cycle installations were similar to that described above in that the plants retained conventionally fired boilers, and the exhaust of the gas turbine was used to provide additional electric power plus supplemental heat for the boilers.

Boiler manufacturers introduced spiral-finned tubes in 1958. This increased the efficiency of heat transfer between the exhaust from a combustion turbine and the steam in the boilers. In the 1960s dedicated heat recovery boilers were used, initially in combined heat and power plants, and towards the end of the decade in power (only) plants. Ultimately this led to Heat Recovery Steam Generators (HRSG) used in modern combined cycle power plants.

3.Application Details for Three Cycles

As stated in the introduction we will look at applications for steam turbine electric generation, gas turbine electric generation and combined cycle electric generation. Since we covered the last of these technologies at the end of the prior section, and since it used primarily by electric utilities (and power producers for utilities), we will cover this in the first subsection below. Thereafter we will cover steam turbine and combustion turbine generation, and primarily focus on facility applications. In the final subsection we will cover some interesting variations of these technologies.

3.1.Electric Utility Combined Cycle Generators

Modern combined cycle plants have increased their efficiency to over 60%, and have been redesigned to assume some new roles. These mainly include rapid-start and rapid-response cycling in order to supplement intermittent renewables like wind farms and solar power plants. Modifications made to do this include:8

  • Air-cooled combustion turbines operate without the complexity of being linked to the steam cycle. Prior to this modification, some combustion turbines used steam-cooled hot gas path components.
  • Combustion turbines with optimized active clearance control to prevent blade rubs during rapid startup
  • Exhaust stack dampers that close to retain the heat in the combustion turbine and HRSG, to permit a rapid start with less thermal shock
  • Optimized combustion turbine control that integrates the start-up ramp rate of the combustion turbine with steam turbine thermal stress control
  • Standby heating systems for the HRSG and condensate processors (polishers) to reach steam purity specifications more quickly
  • The addition of an auxiliary boiler to maintain condenser vacuum during shuts downs
  • The integration of the combustion turbine dry low NOx combustion system and the selective catalytic reduction system located in the HRSG, resulting in improved emissions during start up and cyclic operation
  • HRSG drums with lower thickness to accommodate rapid heating and cooling
  • Double casing high and intermediate pressure steam turbines for improved clearance control and higher ramp rates during fast starts and cyclic operation

Natural gas fueled combined cycle power plants produce more electricity than any other generation technology in many areas. In the U.S. in 2016 combined cycle represented 53% of all gas-fired generation.[9] As older coal-fired power plants are retired, the percentage of gas-fired generation will increase (currently 42% of the U.S. fleet).

Natural gas combustion is much cleaner than coal combustion, both from the standpoint of greenhouse gases and other pollutants. See the table below.[10]

Coal (anthracite)

228.6

Coal (bituminous)

205.7

Coal (lignite)

215.4

Coal (subbituminous)

214.3

Diesel fuel and heating oil

161.3

Gasoline (without ethanol)

157.2

Propane

139.0

Natural gas

117.0

Pounds of CO2 emitted per million British thermal units (Btu) of energy for various fuels:

As far as "other pollutants", modern combustion turbines produce less than 10 ppm of carbon monoxide and less than 20 ppm of NOX in their exhaust. The pollution from coal-fired power plants varies depending on the age of the plant and retrofits, but it should suffice to say that there are potentially many pollutants at levels that pose a serious health risk to downwind residents. See the EPA document (AP42) section linked below for details.

https://www3.epa.gov/ttn/chief/ap42/ch01/final/c01s01.pdf

I live in California. The figure below shows the different fuel used for electric energy in my home state.[11] In 2016 natural gas generated almost 50% of the total energy provided. The natural gas fueled generation consisted of about 54% combined cycle, 26% independent gas turbines (a large majority appear to be peakers) and 20% independent steam turbines (mainly legacy base-load plants with a few cogeneration plants).The peakers and legacy plants are being slowly retired.

3.1.1.Facility Steam Turbine Electric Generation

Industrial steam turbine electric generators (hereafter steam gen-sets) have a lower electrical output than gas turbine electric generators (gas gen-sets), with designs ranging from 50 kW to 50 MW. Steam gen-sets are very useful in facilities that already use steam for industrial processes, environmental heating and/or cooling processes. In such cases steam gen-sets are frequently used at the front-end (between the boiler and the process) or back-end (between the process and the condenser) of such processes. In the former application they can be used in lieu of pressure reducing valves (see third bullet below) In many of these cases the application is combined heat (for the process) and power (CHP) or combined cooling, heat and power (CCHP).

Efficiency depends on the specifics of an application, but is generally in the range of 10% to 30% (note that this includes the thermal-efficiency of the steam generation and condensation). CHP or CCHP applications can provide up to 90% efficiency.

Both gas turbines and steam turbines can be used to directly power rotating machinery like pumps and compressors. Many major industries use steam. These include auto assembly, breweries, oil & gas extraction, wood products, paper and pulp, glass production, metal production, food/beverage manufacturing and hospitals.

Other advantages of steam gen-sets include:

  • If a facility already has a requirement for steam, and the boiler meets all regulatory standards, adding steam gen-sets should require little or no additional regulatory approval.
  • Most boilers achieve maximum efficiency around 50%-load, and the efficiency decreases with reduced load, ending up below zero if the boiler is banked (no steam output, but kept hot). Operating a steam turbine to generate power on the low end of the efficiency curve increases overall steam-output efficiency and results in the most efficient power generation.
  • Backpressure turbines can be used where the pressure of steam coming from a boiler is too high for the process where the steam is being used. The normal practice is to use pressure reducing valves, but this wastes the energy removed during the pressure reduction. Per DOE backpressure turbines can be considered where a pressure reducing valve has constant steam-flow of at least 3,000 pounds per hour and when the steam pressure drop is at least 100 psi.[12]

3.1.2.Facility Combustion Turbine Electric Generation

The main new factor causing facilities reexamine natural gas based generation is the dynamics in natural gas pricing over the last decade brought on by "unconventional" natural gas sources. The figure below is from the U.S. Energy Information Administration’s 2017 Annual Energy Outlook Report.

 Annual average Henry Hub natural gas spot market prices,
1990–2040 (2015 dollars) per million Btu)

Electrical output of industrial gas gen sets range from 5 MW to around 60 MW. The installed price varies quite a bit depending on regulatory approval requirements, but is about $1.25 per watt for a 5 MW unit down to about $0.85 per watt for a 50 MW unit. Efficiency ranges from about 30% for a 5 MW unit to about 40% for a 50 MW unit. All numbers are for electrical output only. Efficiency (and cost) increases substantially when gas turbines are used for combined heat and power (CHP) or combined cooling heat and power (CCHP). Efficiency can be up to 85% depending on specifics.

3.2.Technology Variations

Two variations of turbines are described below. Both of these are useful today, given the right application.

3.2.1.Steam-Injected Gas Turbines (STIG)

STIG technology can absorb excess steam (e.g., due to seasonally reduced heating needs) to boost power production by injecting steam into a gas turbine. The size of typical STIGs starts around 5 MW. STIGs are found in various industries and applications; especially in Japan, Europe and the United States. For example, International Power Technology installed STIGs at Sunkist Growers in Ontario, California in 1985. This STIG uses the exhaust heat from a combustion turbine to turn water into high-pressure steam, which is then fed back into the combustion chamber to mix with the combustion gas. The advantages of this system are:

  • Added mass flow of steam through the turbine can increase power by about 33%.
  • The machinery involved is simplified by eliminating the additional turbine and equipment used in combined-cycle plant.
  • Steam is cool compared to combustion gases helping to cool the turbine interior.
  • The system reaches full output more quickly than combined-cycle unit (30 minutes versus 120 minutes).

3.3.Recovered-Heat (Organic Rankine-Cycle) Generator

This technology is virtually identical to a steam turbine electric generator at the block-diagram level. The main difference between the two technologies is that the organic Rankine cycle uses an organic fluid (refrigerant) instead of water/steam. This enables it to operate from very low heat sources (50°F to 100°F over ambient), albeit at very low efficiencies. Nevertheless it does produce useful power.

This technology is only applicable to industrial processes with a low-quality (temperature) waste-heat output. Industrial processes that have waste-heat streams useful for generation include gas compressor stations, incinerators, and processes at paper mills, oil refineries, cement factories, chemical plants, glass plants and metal refineries.

 

[1] Wikipedia Article: "History of the steam engine", https://en.wikipedia.org/wiki/History_of_the_steam_engine

[2] Encyclopedia Britannica Article on "History of steam turbine technology, https://www.britannica.com/technology/turbine/History-of-steam-turbine-technology

[3] Wikipedia article on: " C. A. Parsons and Company", https://en.wikipedia.org/wiki/C._A._Parsons_and_Company

[4] DOE Office of fossil Energy, "How Gas Turbine Power Plants Work", https://energy.gov/fe/how-gas-turbine-power-plants-work

[5] Wikipedia article on Sanford Alexander Moss, https://en.wikipedia.org/wiki/Sanford_Alexander_Moss

[6] MIT Gas Turbine Laboratory, Early Gas Turbine History, http://web.mit.edu/aeroastro/labs/gtl/early_GT_history.html

[8] IMIA 2015 Annual Conference Working Group Paper 91, "Combined Cycle Power Plants", https://www.imia.com/wp-content/uploads/2015/10/IMIA-WGP-09115-CCPP-Combined-Cycle-Power-PlantsFinal-1.pdf

[9] U.S. Energy Information Administration, "Today in Energy, Natural gas generators make up the largest share of overall U.S. generation capacity", Dec 17, 2017, https://www.eia.gov/todayinenergy/detail.php?id=34172

[10] U.S. Energy Information Administration, "Frequently Asked Questions, How much carbon dioxide is produced when different fuels are burned?", https://www.eia.gov/tools/faqs/faq.php?id=73&t=11

[11] California Energy Commission, Electric Generation Capacity & Energy, http://energy.ca.gov/almanac/electricity_data/electric_generation_capacity.html

[12] United States Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Energy Tips, Steam, Replace Pressure-Reducing Valves with Backpressure Turbogenerators, 2010, https://www.nrel.gov/docs/fy11osti/49435.pdf

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