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Gas to power
One of the options for gas monetization is gas to power (GTP), sometimes called gas to wire (GTW). Electric power can be an intermediate product, such as in the case of mineral refining in which electricity is used to refine bauxite into aluminum; or it can be an end product that is distributed into a large utility power grid. This page focuses on electricity as the end product. The primary issues related to GTP are the relative positions of the resource and the end market and transmission methods. The scale or volume of gas and/or power to be transported influences each of these issues.
The most common method to generate power from natural gas uses gas turbine generators (GTGs), either in simple-cycle or combined-cycle configurations. Gas-turbine-based power generation has proven to be the lowest-life-cycle-cost alternative to date for large-scale electric power generation from natural gas.
Simple-cycle plants use GTGs without heat recovery. Combined-cycle plants use GTGs and recover the waste heat from their exhaust-gas streams with heat-recovery steam generators to make steam to run steam turbine generators, thus producing additional power. Simple-cycle installations are lower in capital costs but are less efficient (higher heat rate); whereas, combined-cycle installations have higher capital costs but higher efficiency (lower heat rate).
There are a number of categories of GTGs:
- Advanced units such as the F-class
- The so-called G-class and H-class turbines with steam-cooling features
Most GTGs fall into the standard category on the basis of their metallurgy and firing temperatures. The F-class and higher units are generally considered advanced technology units because of their higher firing temperatures and special blade-cooling technologies.
Aeroderivative units are typically more expensive on a unit cost ($/kW) basis and are more efficient than comparable GTGs. Aeroderivative units have the highest power density and typically are limited to approximately 50 MW in individual-unit capacity. They are used for power generation and mechanical drive applications, but are typically most prevalent in offshore platform and marine transportation applications in which power density is a significant issue. For GTP, the standard and advanced GTGs are the most likely candidates because of their individual size or scale and the large quantities of power generation involved. Most large power generation facilities are constructed on land. Offshore power generation, either on stationary platforms or floating vessels, is considerably more expensive in terms of unit cost, primarily because of the increased cost of the support structure and other infrastructure costs.
The standard way to transmit large quantities of electricity onshore uses high-voltage alternating current (AC) transmission lines. The power is stepped up in voltage with transformers at the generation sites, transmitted over the transmission lines, and then stepped down in voltage with additional transformers for distribution. AC power transmission is done in three phases at various standard high-voltage levels from 69 kV up to 500 kV.
The capacity and length of large AC transmission systems is limited by technical and economic factors. There are electrical losses in the AC transmission of power because of the inefficiencies in the transformers and simple line reactance and resistance (impedance). The primary alternative to high-voltage AC power transmission is high-voltage direct current (DC) transmission. High-voltage DC systems have been in commercial operation for approximately 30 years and are seeing increased application. The DC transmission system consists of:
- Transformers and converters to change the AC power into high-voltage DC power
- The transmission lines
- Additional converters and transformers to convert the DC power back into AC for local distribution
Offshore electric power transmission is usually by marine or subsea cabling. The transmission of limited power capacity over limited distances may use an AC cable system; however, AC cabling has limitations. Moderate- to long-distance marine transmission systems use high-voltage DC systems to manage the technical and cost issues. Subsea DC cables are simpler with fewer conductors.
The amount of power available from a fixed quantity of feed gas depends on several factors including the type of turbine, mode of operation, and transmission system. With regard to long-distance power transmission, there are general rules in relation to the “break-even” distance at which the DC alternative has an advantage over the AC alternative. For power transmission by subsea cable, either shore-to-shore or shore-to-platform, DC transmission is typically favored at distances longer than approximately 50 km (30 miles). For onshore transmission of large quantities of power, DC systems are typically favored at distances longer than 600 to 800 km (300 to 500 miles), depending on system capacity. These are general rules of thumb, and each specific application should be evaluated for its particular characteristics.
With regard to the economic merits of AC vs. DC transmission systems, initial-cost and operating-cost factors should be evaluated. The transmission lines for DC are less costly than AC; however, there are the added costs for the AC/DC conversion systems. Although there are some losses in the conversion of AC to DC and vice versa, the conductor losses for DC are lower. Therefore, the overall system losses for DC can be less than those of AC systems, particularly for long-distance transmission. The various factors have to be weighed to determine the best solution for any given application.
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