A SO year old power system has merit offshore. Featuring a semi-combined 44% thermal efficiency, the power density exceeds gas turbines by a factor of two. A 625 MW version has been proposed installed at Halten, Norway. The 4600 ton plant, using associated gas as fuel, transfers the power ashore as 400 kV DC. Combustion at 14.3 barg, 207 psig, reduces size and heat transfer surfaces. The concept competes with pipelining and power stations ashore. Electrical component developments and the elimination of the bulk of the oil process compressors give offshore an edge.
A slightly smaller version of this power plant is the basis for a 600 million SCFD nitrogen injection plant, proposed for the North Sea. The plant simply compresses its own flue gas. Residual O2 is removed to 1 ppm through catalytic combustion with fuel gas at 500°C. Water is removed to 25°C dew point by condensation. CO2 is removed by absorption to 2%, all as the flue gas is being compressed to 500 barg. Additional drying may be warranted.
A number of fast start-up power plants were installed in Europe between the two world wars. The principle has been readopted. Almost stochiometric pressurized combustion, a gas turbine and a water cooled combustion chamber yield high power densities and heat transfer rates. Downstream of the power turbine, a normal finned tube heat recovery surface is arranged as an economizer for feedwater preheating.
Oil field gas injection to enhance oil recovery has been limited by lack of suitable gases. Natural gas (NG), N2, CO2 and mixtures are used. NG price normally prohibits its use. Reservoir chemistry restricts CO2 use in limestone-rich reservoirs. Nitrogen dissolves poorly in water, and being practically inert, it is ideal for the reservoir rock. However, Na is difficult to separate from natural gas. In power plant use, large amounts of N. may be tolerated. In N. injection plants, it is even useful.
The leading process to date for N2 making is the liquid air distillation process. Its equipment is costly and complicated, involving compressors and -170°C cryogenic temperatures. With the nitrogen at hand, a sizable power plant is still required for N. compression, and for the (Linde) plant power consumption.
Another competing process is the membrane separation process. For membrane economy, the air pressure must be around 70 bar, 1000 psi. To achieve nitrogen enrichment to 98%, about four - 4 - times as much air must be compressed as the N. demand. Power requirements, therefore, becomes high. The required conventional power plants are bulky.
The large nitrogen demand invites new thinking. The process to be employed in the North Sea is scheduled for selection before May 2nd. Enrichment to 86% Na, 13% O2 and our process may have merit, as this eliminates the problem of the surplus steam. But the N2/fuel gas ratio will only be moderately improved, to 13 : 1.
