Important advances are being made by Kvaerner in the battle to reduce carbon dioxide (CO2) emissions offshore. The company has developed a method for removing CO2 from the exhaust of gas turbines which is suitable for use offshore. It is also progressing in its efforts to create energy from gas turbine exhaust gases through air bottoming cycle technology.
The big incentive for reducing CO2 emissions in Norway is the CO2 tax, which in 1996 cost the offshore sector as a whole almost NKr 2.8 billlion - equivalent, Kvaerner calculates, to $53/ton of emissions.
Gas turbines on offshore installations are also one of the leading sources of CO2 emissions in Norway, accounting for about one fifth of the country's total.
A joint industry project will now be established to test the system, according to project manager Olav Falk-Pedersen. A demonstration unit will be operated at Statoil's Kaarstoe laboratory until 2000, and the first full-scale unit could be in operation by 2001, he says.
An initial feasibility study based on the use of an LM2500 PE gas turbine indicated that the best method would be an amine absorption process combined with 40% recycling of the exhaust gases. The study showed that by putting the flue gas from the gas turbine through a waste heat recovery unit, the turbine efficiency could be increased from 35% to 42%, thus providing the energy for the removal process while also increasing the net power output available for platform functions.
The study also indicated that an 86% reduction in CO2 emissions could viably be achieved using the amine process. In the case of the LM2500 PE, this is equivalent to removing 97,200 tons a year of CO2 that would otherwise be released into the atmosphere.
The big question, according to Falk-Pedersen, was how the amine process, which in onshore plants uses heavy and bulky equipment, including large absorption and desorption columns, could be practically applied on an offshore installation.
The solution was to replace the conventional packed columns with a membrane. Kvaerner made the breakthrough in collaboration with Gore & Associates, a membrane specialist which developed a PTFE membrane which fulfilled the necessary requirements.
In the membrane module, CO2 from the gas stream is attracted through the membrane micropores into the absorption liquid, though no liquid penetrates through into the gas stream.
Compared with a conventional absorption column with a 4.5-meter diameter, height of 21.8 meters and weight of 70 tons, the cube-like membrane absorber is modestly proportioned, measuring 5 x 4.5 x 4 meters and weighing 24 tons.
Savings of similar proportions are available for the desorber, in which the CO2 is removed from the absorption liquid. While the equivalent desorption column is 22 meters high, 2.2 meters in diameter and weighs 20.5 tons, the membrane version measures only 1.6 x 1.6 x 3 meters and weighs seven tons.
The end-product of the process is clean CO2 gas, ready to be compressed and disposed of. But the by-product is a relatively large volume of amine waste, and efforts are now being made to reduce this waste, Falk-Pedersen says.
The technology is also being developed
for the treatment of natural gas, and a demonstration project is about to start at a North Sea gas terminal.
Again, the technology offers considerable weight and space saving in an offshore context. In the case of Statoil's Sleipner West treatment platform, which includes conventional CO2 absorption technology, Falk-Pedersen reckons the weight of the absorber could be reduced from just over 500 tons to 150 tons.
ABC technology
Another Kvaerner initiative is to use air bottoming cycle (ABC) technology to achieve greater fuel efficiency - and hence a reduction in CO2 emissions per MW produced.A demonstration ABC turbine using an LM2500 gas turbine as the energy source is to be installed and operated at a plant in Norway. The NKr 200 million project will run for three years.
According to project manager Donald Laing, ABC technology holds out the promise of reducing CO2 emissions by up to 25%. It has been shown to increase the fuel efficiency of an LM2500 from 35% to 45%. It is both smaller and lighter than a combined cycle system, Laing says.
The process involves using the hot exhaust gases from the parent LM2500 turbine to heat air compressed by the ABC turbine. The hot pressurised air is expanded through the ABC turbine to produce electrical power through a generator.
The ABC turbine is similar to a gas turbine but has no fuel system. Its design is somewhere between that of an industrial and an aero-derivative version.
Being independent of the parent turbine, the system is suitable for retrofit, Laing says.
For an newbuild application, the two turbines could be placed back-to-back to drive a single generator. Compared with the gas turbine on its own, this combination would be about 50% longer and a little wider.
ABC technology will pay for itself through the CO2 tax savings it achieves and reduced fuel consumption, Laing says. The pay-back period will vary depending on the application, but is likely to be between three and six years.
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