In developing large-capacity gas turbines as the main machines in combined cycle power plants, MHI has been making every effort to increase thermal efficiency.

Click here to enlarge image

Since the 1980s when the commercial operation of power generation plants with a combustion temperature of 1100ºC began, combustion temperatures have been increasing at a rate of approximately 20ºC per year and now reaches 1500ºC. However, further increases come up against a number of technical issues including reducing NOx emissions and the need for higher strength materials. At the same time, the period of high economic growth in which a specified increase in demand for power could be expected has come to an end. New products must be created that are capable of coping with current social and economic conditions.

In addition, global environmental concerns are becoming greater, and to reduce CO2emissions it is indispensable to develop more highly efficient power generation plants.

Increasing cycle temperature

According to the second law of thermodynamics, when the maximum temperature of a thermal cycle is raised, the efficiency of energy conversion is increased. It is an ideal combination in which the gas turbine uses high temperature heat energy at 1100ºC, while the steam turbine recovers power from low temperature heat energy at 600ºC or less. By using heat cascades in the combination of two temperature ranges, thermal efficiency can be increased more in combined cycles than in simple cycles.

Mitsubishi Heavy Industries (MHI) has developed a variety of advanced technologies aimed at increasing peak temperatures. In one, the high temperature parts of a gas turbine are cooled by using the steam cycle. The gas temperature at the outlet of a combustor (first vane inlet) and the temperature on the downstream side of the outlet can be increased by adopting this system for cooling the combustor. In addition, by cooling steam at the first vane, the inlet gas temperature of the first blade can be further increased. By using the cooling effect of steam’s large specific heat, the mixing of cooling air into the main gas flow can be reduced and the maximum cycle temperature can be increased using conventional materials.

Figure 1. Operating experience of G-series gas turbinesClick here to enlarge image

Since 1997, when G-series gas turbines with this configuration started operation in the verification power plant of MHI Takasago Machinery Works, 18 G-series gas turbines have gone into commercial operation. Their cumulative operating time currently exceeds 240,000 hours. Their high reliability through an improved design relies on MHI’s extensive experience in the verification power generation facility.

MHI is working to develop systems achieving combustion temperatures of 1700ºC and a combined cycle thermal efficiency of 62 to 65 per cent (LHV) as the next standard values. To achieve this, technological breakthroughs based on new concepts that differ from simply extending conventional technologies are required in the form of new materials, cooling technologies, aerodynamic technologies, and low NOx combustion technologies. MHI is leading a development programme sponsored by the Japanese government.

Fuel flexibility

As well as natural gas, gas turbines have been fuelled using excess refinery gas and gas from ironworks blast furnaces. In the future, the use of excess gas is expected to expand. Around 40 per cent of Japanese coal imports, including that used for manufacturing coke, is consumed in ironworks. The amount of CO2 generated per heating value of coal is about 1.5 times that of natural gas, and an increase in energy efficiency in ironworks would contribute significantly to reducing CO2 exhaust.

MHI has realized blast furnace gas-fired combined plants with combustion temperatures ranging from 1100 to 1250ºC and has also put into commercial operation the world’s largest 1300ºC class blast furnace gas-fired combined plant, which began operation in July 2004. The CO2 exhaust from the plant has been reduced by approximately 25 per cent, compared with conventional boiler firing plant. Overseas, the consumption of energy per unit iron manufactured in the ironworks of China is said to be approximately 150 per cent that consumed in ironworks in Japan, and MHI is actively making efforts to save energy in the ironworks of China.

Figure 2. Variations in the demand curve must be metClick here to enlarge image

Regarding IGCC, MHI commercialized its 300MW M701F VR IGCC in 2003. This was a combination with oxygen blown vacuum residue gasifier. Since June 2003, the plant has been successfully accumulating operating hours. NOx levels at base load condition were lower than 25 ppm(at 16 per cent O2) with stable combustion. Total operating hours as of April 2005 are 16 500 hrs.

In addition, work on a coal gasification combined cycle plant which is currently under development as a national project started construction last year and operation is scheduled to begin in 2007.

In the future, the demand for highly efficient power generation using various alternative fuels is expected to increase. In addition to conventional fuels, there are fuel gases that are generated from natural waste such as biogas, and low pollution liquid fuels such as DME (dimethyl ether) as well as GTL (gas-to-liquid) artificially produced from natural fossil fuels. These fuels are expected to be used in the future. MHI has also been active in performing combustion tests of DME in a national project to verify the practicality of such fuels.

Peak power generation

The unit capacity of large gas turbines has grown, due to increases in combustion temperature. At present, MHI’s maximum unit capacity is 330 MW for the M701G2 turbine, while for single-shaft combined power generation it is 500 MW. But there must be sufficient demand for 500 MW turbine power generation plants for such plants to be commercially viable in the long term.

Though a variety of large new power sources was planned during the era of sustained economic growth, in the present rapidly varying situation, in which economic growth has slowed and various social circumstances affect the peak demand for power, a middle range capacity thermal power station capable of being constructed in a shorter period with smaller investment than a large capacity machine is needed.

Figure 3. SOFC and gas turbine combined systemClick here to enlarge image

In addition differences in demand between day and night tend to increase with the spread of air conditioners. As a result, the expectations of gas turbine combined plants, which play an important role as controllable thermal power plants, are becoming greater.

In the future, intermediate capacity, high efficiency plants with a low utilization factor of approximately 40 per cent will be required. So combined cycle gas turbines will diverge into base load operating systems that emphasise efficiency, and middle range machines with high efficiency, minimized initial costs, and high operating maneuverability.

New cycles

Although an increase in combustion temperature leads to a rise in overall thermal efficiency, the completely effective use of heat energy with a maximum possible combustion temperature of 2000 to 2500ºC which can be achieved by fossil fuels involves many technological issues that must be solved.

MHI has been examining the prospect of combining existing gas turbines with other cycles, to realize further increases in thermal efficiency. The combination of gas turbines with fuel cells uses the fuel remaining after most is chemically converted into electricity in a gas turbine combined cycle to increase overall thermal efficiency.

Figure 4. Cycle combinations will be needed to extract more powerClick here to enlarge image

A national level was also carried out to investigate a CO2 closed cycle in which air is not used as a dilution medium for combustion, flue gas is recirculated, and NOx is not emitted as exhaust.

Conclusion

In this analysis, the two points concerning the fuel flexibility and intermediate capacity peak power generation were cited as near future trends, while the two points concerning further increases in the combustion temperature of gas turbines and challenges to new cycles were cited as mid- and long-term trends.

In recovering energy obtained from fossil fuels, it is important to efficiently convert the thermal energy of the fossil fuels with a maximum combustion temperature of 2000 to 2500ºC to electric power through cascades achieved by combining cycles. The means for accomplishing this goal include: increasing the maximum temperature of the present 1500ºC gas turbines; achieving a new cycle to recover thermal energy at the 2000ºC level by other methods; and developing a combined cycle to recover thermal energy without combustion (such as a fuel cell system).

In any event, gas turbines are very effective machines for efficiently converting temperatures of 1100 to 1500ºC or higher on a large scale, and it can safely be said that, in the examination of various cycles, gas turbines will play an important role as an effective means of converting combustion energy in the future.

The cost of power generation plants is expected to increase as cycles multiply in number and increase in complexity. On the other hand, issues concerning the global environment are becoming ever more serious, and an increase in thermal efficiency will become an important challenge as fossil fuel resources are used up. If a scheme for compensating additional costs can be established through new business opportunities such as CO2 ECO Right, problems associated with increased costs due to the increased complexity of cycles can be solved and the advancement of power generation plants can be accelerated.