Gas turbines performance: Ready for the next boom

Industrial gas turbine performance by OEMs and the bold moves taken by GE and Westinghouse to produce larger units were discussed in the third part of this series. More innovative models and modifications were introduced by Brown Boveri and GE in the coming years, for greater efficiency.

Brown Boveri, a pioneer of the industrial gas turbine, introduced many innovations and made the first gas turbine locomotive for the Swiss Federal Railway in 1941. The engine had an efficiency of 18 %. A full report of this locomotive was published in R. Tom Sawyer's The Modern Gas Turbine in 1945. Brown Boveri delivered the first reheat gas turbine 35 years ago to the Huntorf, Germany air storage facilities, CAES Power Plant now owned by E-on. It was rated at 290 MW and later uprated to 321 MW. The design was based on the GT-13 gas turbine with a single welded rotor, but without an air compressor. The inlet pressure to the first combustor was 609 psia from the storage cavern. This pressure is within the reach of today's axial compressor technology.

Development of GT24 and 26 models

This early background led to the development of the GT24 and 26 models introduced in 1994. GE moved forward with a 150 MW Frame 7 - F design in 1997. It had all the latest developments such as single crystal first stage blading, TBCs and a higher firing temperature. Sales were brisk at first, but the first units placed in operation had a serious design flaw. The turbine discs got overheated.

GE had a costly field modification to make on all operating units, and production was held up. After the problem was corrected, sales slowed down on all gas turbines due to the unstable natural gas prices that fluctuated from 4 to 14 dollars per million BTU. Gas turbines for coal gasification did not take off, and only a few were placed into operation with help from DOE.

Industrial aero gas turbines

Pratt and Whitney has sold a number of JT 8D twin packs rated at 25MW each at 38 % efficiency as back up to support wind farms and solar energy. GE’s LM 6000 40 MW units are also being used for the same purpose.

There is another natural gas burning gas turbine power producer – the new GE LMS 100 – a 100 MW intercooled gas turbine introduced in 2003 by GE Energy’s packaging operation in Houston, Texas. This unit has a GE Frame 6 FA front end compressor running at 5000 RPM connected to an aero LM 6000 gas generator that can be quickly taken out and another spare put back in for quick maintenance. An intercooler cools the partially compressed air, at about 3 ½ atmospheres and 335 ^o F, to around 100 ^o F before entering the gas generator.

The cooled air is much denser, and the flow increased to about 450 pounds per second. The cooled air supercharges the gas generator with considerably more mass flow. The overall cycle pressure ratio is about 44. A 3600 RPM heavy duty power turbine expands the hot gas from the gas generator to produce 100 MW of power, an increase of 60 MW from that of the GE LM 6000 version. The efficiency is 45 %, and the unit’s part load efficiency is also good. There are about 40 units running now.

LMS 100 picks up despite slow start

The unit was slow to catch on in the beginning due to the US glutted gas turbine market following the 1999 to 2002 boom and bust when US natural gas became scarce and high priced wherein the IPPs started going belly up. The year 2003 was not a good year to introduce a new concept, which was half-areo and half-HD with inter cooling.  It was a crockogator or a swoose with hard to believe output and efficiency. Even so, GE packaging sold an average of five units per year for eight years. 

Sales have now taken off, thanks to fracking and the new West Texas shale gas production. This gas turbine fits quite well to back up wind farms and solar energy and to provide intermittent power to existing grids. In the past, before the large amounts of renewable, utility planners added low cost frame gas turbines to meet seasonal peaking demands. Now grids also need highly flexible operating technology to serve the very volatile, real time load demands of the grid.

The LMS 100 with its quick start, and fast ramp capability is well suited to provide these new grid requirements for dynamic intra-hour flexibility. GE packaging (GE Power and Water) recently announced the sale of 8 units for the CPV Sentinel project for installation near Palm Springs, California. A unit has also been sold to El Paso Power, and soon, 6 more units were sold to The Los Angeles Department of Water and Power (LADWP). The Houston factory is now loaded with orders.

Six dry units for Los Angeles

This last batch of six LMS 100 dry units for the LADWP will replace two 50 year old gas fired steam units at the Haynes plant in Long Beach, California. They are scheduled for start up by the end of 2013, a short period of time for power plants.  Ronald Nichols, general manager of LADWP cites four good reasons for selecting the LMS 100s: (1) rapid 10 minute start up time for renewable energy backup, (2) the dedicated goal of eliminating the ocean water cooling (once through cooling),  (3) the protection of fish and other aquatic life (federal and state environmental regulations),  and (4) the aging gas fired steam plants.

LADWP already has two combined cycle power units ordered in 2005 for the 1,524 MW plant to reduce ocean water usage. The LMS 100 IC units will also have a better heat rate of about 15 % than the replaced units.

This unit can be readily made into a reheat version to produce perhaps 130 MW at about the same efficiency. The existing LMS 100 has a low exhaust temperature of about 775 ^o F and is not well suited for a combined cycle. The reheat version would have an exhaust temperature of around 1200 ^o F,  ideal for a combined cycle with an efficiency level of around 60 % if some of the heat rejection from the inter cooler can be used. The heat rejected by the inter-cooler accounts for lower combined cycle efficiency than for the regular gas turbine. The part load efficiency would be very good. The 0 to 100 % load in perhaps only 20 minutes would be attractive. The present LMS 100 can achieve full load in only 10 minutes.

The power turbine would require cooling, perhaps with steam, which would have to be developed, as well as the reheat combustor. GE has the know-how through the “H” gas turbine program for steam cooling.

Advent of GE90 for aircrafts

General Electric jet engine group in 1995 came up with a larger fan engine designated as the GE90.  It was introduced for the Boeing 777 aircraft and had a rating of 78,000 pounds of thrust. It had a pressure ratio of 42. Improvements in rating and efficiency took place for the next nine years. In 2005, GE produced a new GE90 engine called the GE90 - 115B with a pressure ratio of 45. This engine produced 127,900 pounds of thrust on the test stand to become the largest fan engine in the world. GE has produced over 1000 of the 90s so far.

Modifications for greater efficiency

This large fan engine has never been offered to date to generate electrical power.  It could be modified like the LM 6000 to produce much greater output by taking the fan off and adding front end compressor blading and modifying the low pressure turbine section. The output would be about 80 MW at an efficiency level of around 48 %. The 115B could also be made into an inter-cooled unit incorporating a GE Frame 7FA or FB front compressor and applying inter cooling. Two HD power turbines could be designed, one for 3600 RPM and one for 3000 RPM. The cycle pressure ratio would be about 55; the output would be about 200 MW at a cycle efficiency close to 50 %. It would be similar to the LMS 100 but twice as big.

An inter cooled unit could then be made into a reheat gas turbine integrated cycle to produce perhaps 400 MW where steam is used to cool the reheat combustor and gas turbine such as done for the GE “H” gas turbine. The exhaust temperature would be about 1350 ^o F. The HRSG and steam turbine would use the new GE "FlexEfficiency" design for 1100 ^o F or even higher for greater output and efficiency of about 60 %. It would have good part load heat rate, quick start up, and a fast overhaul time. It is doubtful, however, that this arrangement will ever be built because it would be placed in direct competition with the new “FlexEfficiency” units.

Rolls-Royce’s new version

Rolls-Royce is furnishing a version of its large Trent engine for the US Navy's latest war ships, the outcome of the DDX program. Alstom and R-R could team up through their ongoing technology program to make such a machine. R-R now knows about reheat combustor design and large HD power turbines.

Rolls-Royce is abreast to the shale gas surge and has several power units to offer.  R-R has uprated the RB211 and calls it the H63 model. This engine is rated at 44 MW with an efficiency of 41.5 % to compete with the GE LM 6000. There is also the Trent 60 with a present rating of 64 MW at an efficiency of 42 %. This unit can be uprated to 80 MW to match the GE 90, if offered by GE. R-R also plans to offer the marine inter cooled MT-30 rated at 38 MW in the near future.  

Another gas turbine boom is on the way, hopefully to stay this time. All manufacturers and designers think so, and are gearing up their factories for the coming boom. Environmentalists, however, have mixed feelings about this situation. They like burning low CO₂ emission gas fuel versus coal, but they question the wisdom of fracking. They are pushing and pulling at the same time.

A final word about jet engines

Over the years, fuel efficiency gains have steadily taken place and continue to do so. Late in the 60s, the high bypass fan jet engines were introduced for aircraft.  Efficiency jumped a whopping 25 %. The C-5A large Air Force transport used the GE FT-6 (military version was the T-39 engine) developed through a program sponsored by the government and P & W jumped ahead of GE to power the first Boeing 747 jumbo jet with its version. Rolls- Royce has its big Trent that is powering our large planes and will drive our new naval war ships.

Over the past 50 years through the NASA E (energy efficient engine) program, significant advancements were made, and they continue today. The GE 90 and large P & W engines came out of this program. This past year has seen the introduction of the P & W PW-1100 geared front fan engine for the airbus 320 and possibly for the Boeing 737. Another 15 % improvement in heat rate is here. Also Snecma has its LEAP X engine ready to go matching Pratt and Whitney’s efficiency. These advancements are remarkable. A significant saving in jet fuel and crude oil will be realized. These advancements are being passed on to the industrial versions of the gas turbine. Higher firing temperatures are now possible to give higher efficiencies.

In the next part of the series, the author explains how the combined cycles can be optimized for maximum output, and sets a new benchmark of combined cycle efficiency to be achieved in the next decade.

Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.