Saga targets gas turbine exhausts in emissions crackdown

Combined cycle technology recovers waste heat to create electricity while lowering carbon dioxide emissions. [45,726 bytes] Saga Petroleum is aiming to set new environmental standards offshore on its Snorre B platform, with carbon dioxide emissions a primary target. This move is not a response to regulatory pressure, nor in expectation of reaping economic benefits.

Saga Petroleum is aiming to set new environmental standards offshore on its Snorre B platform, with carbon dioxide emissions a primary target. This move is not a response to regulatory pressure, nor in expectation of reaping economic benefits.

Instead it is a "voluntary" approach, which reflects the growing priority given to environmental considerations by companies operating on the Norwegian shelf. Future Saga projects such as the Halten Bank South development will embody the same policy.

The company is planning a two-fold attack on carbon dioxide emissions, according to Svein Lange, systems section manager in the engineering department.

One will be the use of combined cycle technology to recover the heat contained in the exhaust gases of the gas turbines and put it through a steam turbine to generate additional electricity without creating additional carbon dioxide emissions. No supplier has yet been designated.

The second is the membrane contactor technology for removing carbon dioxide from the exhaust gases, which Kværner is developing under a joint industry project backed by, among others, Saga.

This technology, which uses an amine absorption process in conjunction with the contactor, is capable of capturing up to 85% of the carbon dioxde content of the gases, Lange says.

The proposal is to put a pilot membrane contactor plant on one of Snorre B's two gas turbines, but before making a firm decision, the company will wait to see the early results from a demonstration unit which Kværner is about to bring into operation at Statoil's Kaarstoe gas terminal. It will also further evaluate the economics and financing of such a unit on the platform, Lange says.

Both turbines will also have low nitrogen oxide burners which will reduce NOx emissions by some 80%. Installing all this equipment will not benefit the platform economics, according to Lange - overall it is expected to have a negative net value, even though it will reduce the licensees' carbon dioxide tax liability.

In evaluating new technology of this kind, it is important to look at the total system and to integrate and optimize energy demands and consumption, Lange says. An example of this is a proposal to use the B platform plant to generate power for the A platform as well. Although the whole of A's power needs could not be met in this way, it would enable one of the three turbines on the A platform to be closed down, thus achieving a net reduction in carbon dioxide emissions.

Another emerging technology which interests Saga is the use of the membrane contactor to remove carbon dioxide from natural gas. Here again, Kværner has just started a demonstration unit, this time at Mobil's Sage natural gas plant in Scotland. This would be of particular interest for the Halten Bank South development, where the carbon dioxide content of the gas is as high as 5%.

A pilot project is also taking place on Snorre A for the injection of produced water into the reservoir. The water is cleaned to a certain standard before being injected. If the results are successful, the process will be adopted permanently on both platforms.

Gas flaring on Snorre B will be minimised by the use of a closed flare system, Lange says. This means there will be no constantly burning pilot flame, and low-pressure gas will be recycled.

Statoil launches R&D program to limit carbon dioxide output

Statoil has set itself the stiff target of cutting carbon dioxide emissions from its operated fields on the Norwegian shelf by 30% over the next 10 years.

To enable this target to be reached, the company is implementing an aggressive three-year program of technology development costing NKr 600 million. Half of the cost will be contributed by Statoil, and the remainder, it proposes, from industry and the authorities.

There are five main areas to the technology development program, according to Nils Foldnes, Manager for Conceptual Development for the carbon dioxide program:

  • Carbon dioxide reduction at source
  • Development of a land-based gas turbine concept which facilitates carbon dioxide removal and disposal
  • Development of air bottoming cycle (ABC) technology
  • Longer-term research and development projects
  • Industrial utilization of carbon dioxide.
Within the technology development group, a number of teams are now at work with a brief to look at the conceptual development of relevant new technologies, including development and implementation, qualification and systems engineering.

Statoil's initiative is not driven by current regulatory requirements, though in the light of the Kyoto agreement and a general trend towards the use of legislation to impel industry to a better level of environmental performance, this situation could change before long.

To put Statoil's self-imposed target in perspective, it should be remembered that a tax has been levied on carbon dioxide emissions on the Norwegian shelf since 1991. "Since we pay a carbon dioxide tax, we have been doing energy optimization for some time," Foldnes says.

"As a result, we already have quite efficient production from an emissions point of view."

Meanwhile, the campaign is not restricted to the technology development group. Instead, Foldnes explains, all units, whether offshore platforms or onshore plants, must themselves take the responsibility for finding ways to achieve the target in their own particular operations. A degree of urgency is involved - by September costed plans must have been produced, and detailed design of the first offshore modifications is scheduled to start in October.

While the toughest problems will relate to modifying existing operations, the opportunity is being grasped to take best available environmental practice on board in current development projects. In this case, the technology development group reviews proposed development plans, commenting on carbon dioxide issues and looking for ways to reduce fuel demand and to capture and dispose of carbon dioxide.

In the case of the Åsgard B platform, the construction of which begins this year, it has been possible to identify a 30% reduction in carbon dioxide emissions compared with the use of conventional technology. Implementing these measures will not require significant levels of extra investment, Foldnes says.

In terms of existing operations, one of the main areas of focus is the Kaarstoe gas terminal, where a conceptual design study for a carbon dioxide capture and disposal plant is being performed. The study covers two gas turbine drives for gas export compressors for the Åsgard field, and a new 350 MW combined cycle power plant. The carbon dioxide reduction potential is about 1.3 million tons/yr.

A qualification test of membrane contactors developed by Kværner for carbon dioxide removal is also due to start in about mid-year. The test plant will have a 500 KW gas motor generating 3,150 kg/hour of flue gas with a 6% carbon dioxide content.

Another attractive option, when, for example, the power output of an installation is increased to cater for the needs of a satellite tie-back, is to build in a steam cycle, Foldnes says. This uses the waste heat of the gas turbines to create additional power, thus increasing the efficiency of power generation.

Another method of increasing fuel efficiency is the air bottoming cycle (ABC) technology. Here again Kværner is due to start a demonstration project this year.

Subsea processing and injection is another method which provides a net saving on topside power requirements. The world's first unit, developed by ABB, will be installed next year on Norsk Hydro's Troll Oil II development. Statoil, one of the Troll partners, is now doing conceptual design for a similar pilot unit for the Statfjord North field.

Another novel idea is to save the power directed into gas injection operations through direct gas injection from a high-pressure well to a low -pressure well, Foldnes says. It may also be possible, using expansion processes to capture the energy contained in the wellstream as it arrives on the platform. At present, this energy is dissipated in the treatment process.

The carbon dioxide captured by removal processes represents a problem in that it has to be disposed of - injection under the seabed is one current solution. But it may be possible to take advantage of the situation, for example by injecting it into the reservoir to enhance oil recovery.

Flaring will continue to be minimized. It has already been eliminated as a continuous activity on the three Gullfaks platforms, and a similar move is being planned on other fields.

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