The power station was the first to be equipped with a new generation of Mitsubishi gas turbines. Mitsubishi Heavy Industries installed a combined cycle power plant (Unit 4) that has a turbine inlet that can handle temperatures of up to 1,450°C.
“The power station was the first to be equipped with a new generation of Mitsubishi gas turbines.”
Before the turbine began commercial operation in 1999, the plant had a capacity of 2,990MW. The existing installation comprised two 350MW steam turbines (Minato units one and two), two 600MW steam turbines (Units 1 and 2) and a 1,090MW CCGT (Unit 3). Unit 3 was completed in 1984; it has six MW701D gas turbines and was Japan’s first large CCGT plant.
HIGASHI NIIGATA UNIT 4 PLANT MAKE-UP
Higashi Niigata Unit 4 plant comprises two power trains, each with two 270MW Mitsubishi 701G1 turbines. The turbines are operated with a reduced inlet temperature of 1,400°C (design rating is 308MW at 1,500°C). Each gas turbine has a heat recovery steam generator with a reheat temperature of 566°C. The steam from each pair of HRSGs drives a 265MW two-cylinder steam turbine generator.
Overall plant efficiency is around 50%. This can be compared with the performance of the older Unit 3 plant, which operates with a combined-cycle efficiency of 44%. In single-cycle operation the 701G1 has a thermal efficiency of 38.7% compared with 37% for its predecessor. Mitsubishi claims that the 701G is capable of offering better than 58% efficiency in combined cycle plant. Tohoku expects that the new plant will deliver savings of around 180,000t of LNG per year compared to the old plant.
The Higashi Niigata plant uses natural sea water to cool steam produced by the generation process and turn it back into water. The return sea water vaporises the LNG fuel for the gas turbines. To eliminate a fouling problem in the water outflow, Tohoku Electric selected Intersleek, an innovative non-toxic foul release system from International Protective Coatings, to coat the walls and ceiling of the plant’s circulation water outflow concrete culvert.
Some anti-fouling systems contain damaging biocides and have poor performance levels, requiring frequent recoating. First pioneered in the early 1980s, Intersleek is biocide-free and has been used for 25 years in the marine and power industries. The Intersleek range provides effective, long-lasting fouling protection for steel and concrete structures without relying on biocides.
WORLD’S FIRST DLN COMBUSTOR
MHI introduced the world’s first operational DLN combustor in its 1450K Class gas turbine at Higashi Niigata Power Station in 1984. The DLN1 combustor used a hybrid burner with an air bypass arrangement for part loading. NOx levels at gas turbine exit were some 75ppm, reducing to less than 10ppm after the SCR system was installed in the HRSG.
The DLN1 combustor incorporated a double chamber for pilot and main burners in an annular arrangement. The combustor liner had a conventional film cooling system. However, for higher temperature engines, the air requirement for lean pre-mix burners and film cooling is excessive.
The next development, DLN2, in 1992, therefore used a single chamber to reduce cooled surface area. It also incorporated advanced types of liner cooling, known as PLATEFIN and MTFIN. DLN2 also has a multi-nozzle system allowing dual-fuel operation. NOx levels of 25ppm have been achieved on the MHI 1600K Class ‘F’ engine using DLN2.
701G1 HAS LARGER COMBUSTION CHAMBER
The MHI 701G1 combustion chamber and first- and second-stage blades and vanes are very similar to those used in the prototype 501G design, but the size has been increased to give a higher output. The 701G1 is a single-shaft machine with the generator at the compressor end of the turbine to allow the turbine exhaust to be directed into the heat recovery steam generator. Small diameter tubing is used in the HRSG superheaters and reheaters to improve heat transfer. The reheat temperature is 566°C.
Supercritical steam temperatures are now almost standard in Japan to ensure plant efficiency. The compressor operates with a 19:1 ratio, and has variable pitch inlet guide vanes to improve control and performance at part loads. Air is bled off at three stages in the compressor and is used to provide cooling for the first three stages of turbine blades.
“Tohoku expects that the new plant will deliver savings of around 180,000t of LNG per year compared to the old plant.”
The turbine combustion chamber uses closed circuit steam cooling to maintain temperature between 1,500°C and 1,600°C. This ensures that NOx emissions are controlled to 25ppm or better. The first- and second-stage blades have a thermal barrier coating as well as being air-cooled.
The third and fourth stage turbine blades are shrouded as they are on the earlier 701F design. The cooling air from the turbine blades is passed through a filter and external cooler before discharge.
The turbine blades and vanes are made from new nickel alloys developed by MHI in conjunction with Mitsubishi Steel Manufacturing Company. The MGA2400 alloy used for the vanes has improved welding characteristics. The MGA1400 alloy used for the blades has good creep strength. The first two stages of blades use directionally solidified alloy to further improve creep strength.
TOTAL GENERATING CAPACITY OF 12.5GW
Tohoku Electric Power Company is one of the ten Japanese vertically integrated electric utilities that enjoy regional monopolies. Tohoku Power has a capacity of 12,500MW, which is made up from a balanced portfolio of hydro, geothermal, nuclear and conventional thermal plant. It serves the north of Honshu, the main Japanese island. The area is relatively rural, but there are a number of large towns including Niigata.