Combining Gas-to-Wire Technology With Carbon Capture and Storage
Natural gas, coupled with carbon capture and storage (CCS), could be a cheap and reliable low-carbon energy source in the long term and a critical technology for climate change mitigation while delivering sustainable power. Gas-to-wire (GTW) is the process of generating electricity from natural gas at or near the field, different from producing electricity at a centralized power plant. This paper presents the results of a prefeasibility study of applying an integrated GTW/CCS scheme to a gas-production field.
In the GTW process, a gas turbine is placed close to the field and all or part of the field production is directly converted into electricity for use onsite or for sale to the local market. This production scheme is completely different from the traditional one because the natural-gas turbine is close to the fossil-fuel fields, regardless of whether they are far from or right next to the consumer market.
For a low-emissions process, the carbon dioxide (CO2) produced by the power plant can be captured, injected, and stored underground.
The GTW scheme can be particularly useful for marginal fields and associated natural gas development when volumes available are too small (between 10 Bcf and 1 Tcf) to be exploited by use of traditional transport technologies. GTW can have economic and environmental merits, too, especially if the CCS process is associated with it.
The application of GTW could help solve the natural-gas-flaring problem, especially if the natural-gas field or the associated gas is located far from the consumer market.
Fig. 1 (above) shows a simplified scheme of GTW, including the process of capture, injection, and storage of the CO2 emitted by the power plant.
The integrated GTW/CCS process can be summarized as follows:
- The produced gas is separated, processed, and sent to the power plant.
- The electricity produced by the gas combustion in the turbine can be used to feed the field facilities or delivered to the market. The economic convenience of the market-sale option depends on the relative length of the electric line compared with a pipeline for gas transport.
- For a low-emission integrated process, the flue gas produced by the power plant can be sent to a capture plant, where CO2 is captured, compressed, and injected underground for permanent storage, while the exhausted gas (composed primarily of nitrogen) is released into the atmosphere. In this way, the transportation and storage costs are reduced and the “not in my backyard” (NIMBY) effect is limited because everything remains confined to the field and the only output is electricity.
For this exercise, combined-cycle-gas-turbine (CCGT) technology is used for the gas-to-power conversion because of its high efficiency, which allows better exploitation of the natural gas resources, although at higher costs. The CCGT system, in fact, includes one or more gas-turbine modules coupled with a heat-recovery steam generator, where additional power is produced from the hot exhaust gas from the gas turbine that otherwise would be discharged into the atmosphere. Typically, two-thirds of the power is generated by the gas turbines and one-third is generated by the steam turbine.
In addition, CCGTs have reduced startup times, are able to respond quickly to changes in electricity demand, and may operate at reduced capacity with limited reductions in efficiency. Additional benefits are associated with low CO2 emissions—300–400 g CO2/kW-h vs. 700–900 g CO2/kW-h for coal-fired plants—and with reduced emissions of all other air pollutants that are emitted at fractional levels compared with other combustion-based power-generation systems.
Compared with coal-fired plants, natural-gas plants, in general, are more flexible to technical and environmental factors. Nevertheless, the plant selection is a function of different parameters, the main ones being the natural gas volume available, its composition (gas turbines are more sensitive to acidic gases compared with reciprocating engines and microturbines), its pressure (gas turbines need a fuel-compression system), and the geographical location (efficiency derating, depending on environmental conditions, could penalize gas turbines more than reciprocating engines and microturbines).
Three technologies, at different maturity levels, are currently applied for capturing CO2 from a point-source emitter.
- Post-combustion is the most mature technology, commercially available in large scale (500 MW) but at high cost, where CO2 is separated from flue gases, typically through chemical absorption with amines. It is applied to combined-cycle plants fueled by natural gas and coal-fired plants working with vapor cycles in supercritical conditions. Because of the energy needed for solvent regeneration, it is an energy-intensive process.
- Precombustion capture is a mature commercial process widely practiced around the world for natural-gas processing, where CO2 is separated from the fuel upstream of the combustion process, usually through physical absorption by solvent mixtures. It is less energy-intensive than post-combustion with amines but less competitive because of its lower efficiency and higher investment costs.
- Oxycombustion is conducted with oxygen as combustor (instead of air). Consequently, the flue gas consists of virtually pure CO2 that can be compressed and injected into an oil or a gas field with minimal processing. The technology has seen some industrial applications (e.g., in the steel industry) but not at large-scale CCS plants.
Three options exist for the storage of CO2. Geological storage is the most common, while oceanic and mineral storage are less viable for economic and environment reasons.
Depleted oil and gas fields (DOGFs), saline aquifers (SAs), and unmineable coal seams are the alternatives available for geological storage. DOGFs are safe sites for confining CO2 because they held oil, natural gas, and CO2 for ages. SAs are promising, with important storage capacity worldwide, but less is known about them. Unmineable coal seams have poor storage potential.
CO2 captured and used for enhanced oil recovery can remain stored permanently in the reservoirs, with approximately 50–60% of that injected being retained. This option combines environmental benefits with economic advantages.
Wire Transmission Lines
Two methods exist for electrical transmission: direct current (DC, produced by batteries, solar, and fuel cells) and alternating current (AC, produced by most power plants). While AC transmission is the most widespread system, because it allows for easy distribution of power, DC transmission offers an alternative that mitigates many limitations of AC transmission. DC transmission is especially useful for long transmission distances where AC lines are unworkable and costly.
Because the electricity produced by power plants is usually AC, it must be converted for transmission by DC. AC electric power enters the system, where it is converted to high-voltage AC by use of standard AC transformers before finally being converted to DC by use of a circuit referred to as a rectifier. Electricity is then transferred through the DC power cables and converted back into AC by an inverter.
AC transmission is favored for relatively short distances (up to 500 km, as a function of power delivered and location), while DC transmission generally is more convenient for transmission over long distances (> 1000 km), for offshore applications, in regions where right-of-way is a constraint, for connections between unsynchronized power grids, and for bulk transmission. Under these conditions, despite the complexity associated with voltage steps, DC transmission is more convenient than AC transmission.
GTW is a mature and consolidated technology, potentially competitive in the case of stranded natural-gas assets, for gas-flaring management, and for fields next to existing electricity-transmission lines. In developing countries, it can be a means to provide energy access to local populations.
Natural gas is a low-carbon fuel, and its sustainable use could be improved by coupling a CCS process to a power plant to capture the CO2 produced and store it underground. CO2 transport and storage costs consequently would be reduced, and, because everything remains confined inside the field and the only output is electricity, the NIMBY effect would be limited.
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