It has been estimated that fuel accounts for about two thirds of the lifetime costs of a typical gas turbine, so it is no wonder that gas turbine makers have spent so much time, money and effort trying to raise efficiencies. But sadly a number of recent projects have demonstrated just how quickly the expected advantages of high efficiency can be wiped out if you run into a reliability problem.
An excellent paper by Bruno Schroeder and Klaus Schippers of RWE presented (by Schroeder) at the recent Power-Gen conference in Milan gives a very frank account of RWE's recent experience with their machines, and should be required reading for gas turbine vendors everywhere - particularly their marketing departments.
RWE is well qualified to comment on gas turbines as its fleet includes Siemens, Alstom and GE designs. The authors agree that the technology is attractive but it is necessary that "the advantages can actually be realised in practice, ie in operation." Unfortunately their experience demonstrates that there has been a gap "between expectations and reality."
The authors admit that part of the problem is what they call the "race for technological market leadership", which has been initiated to some extent by the operators themselves. This race is bound to have an impact on reliability, say Schroeder and Schippers, because the "manufacturers do not have the time required to conduct detailed prototype tests and the high gas turbine capacities no longer permit comprehensive test bed runs."
For the typical combined cycle plant considered by Schroeder and Schippers, it only needs about nine days of outage to eat up the advantage of a one percentage point increase in plant efficiency. RWE's experience suggests that this is all too easily done.
At Ludswighafen, which is equipped with two 13E2 gas turbines, a number of major problems were encountered during commissioning, including fuel supply system failures, combustion pulsations, aka "humming" (in the annular combustion chamber used for the first time in the 13E2), difficulties in meeting specified NOx emissions levels, and subsidence of the hot end turbine bearing.
Despite these difficulties, key project target dates were met and contractually defined availability levels reached. However, five years on, "stable combustion under all operating conditions and load gradients has still not been achieved." Furthermore, "no short-term solutions can be expected" given the complexity of combustion pulsation problems, which are "essentially the result of ever higher combustion chamber loading [MWth/m3] accompanied by ever lower emission limits."
In April this year a major turbine blade failure occurred at the plant, which is still under investigation.
Humming, again associated with the introduction of an annular combustion chamber, was also among the problems encountered at the Dormagen combined cycle plant, which has two V94.2A gas turbines. The combustion pulsations encountered were severe enough to cause "lifetime-shortening shutdowns of the gas turbines."
Modified (HR3 type) burners were installed to address the problem and major portions of the commissioning programme had to be carried out again. In summer 2001 all the burners had to be modified once more to "remedy an expansion obstruction." These and other problems meant that acceptance was delayed by 11 months. However, the plant now seems to be operating well, even though the natural gas preheating system has not yet operated as planned and combustion chamber pulsations still occur when outdoor temperatures are low and under certain load conditions.
Schroeder and Schippers conclude that reality has proved to be rather different from what operators had in mind, with commissioning of gas turbines sometimes looking more like "continued prototype testing."
As well these problems encountered at the beginning of many a combined cycle plant's operating life, there also seems to another set of worries looming about end-of-life. There are growing concerns about premature ageing and reduced longevity, mainly as a result of cycling operation. This was highlighted by several other speakers at the Power-Gen event. Having gone through the traumas of commissioning new combined cycle plants with high-efficiency gas turbines, plant lifetimes may turn out to be as little as a dozen years or due to the need to replace hot gas path components. This looks like another area that the gas turbine operator must grapple with where great expectations and reality are showing signs of diverging uncomfortably.
Lofty ambitions
There are high expectations of another sort surrounding a large Australian solar chimney project currently in the early planning stages. This 200 MW plant - essentially a very large greenhouse attached to a giant chimney with Kaplan-type turbines running in the updraught - will entail building nothing less than the world's tallest structure, a pipe1 km high and 130 m in diameter.
A prototype solar chimney operated in Spain in the 1980s but the Australian project represents a power scale up of 4000 times, which is ambitious, to say the least.
The designers say generation costs for power from their solar chimney plant might be "merely 20 per cent more expensive than that from coal", which presumably qualifies as competitive in the world of renewables.
But the promoters of the project also see other benefits: "The monolithic size of our first 200 MW power stations will capture worldwide attention and attract significant added value through tourism and associated benefits."
And the developers cannot be accused of understating the attractions of their technology. "With solar chimneys," they say, "the greatest problems of our times, environmental exploitation and the poverty and population explosion in the Third World, could be solved immediately and peacefully." If only it was as easy as that.
Linkedin
