Compressed natural gas (CNG) transportation is used in very small systems in environmentally sensitive areas. Trucks, ships, or barges transport the gas from a remote well to a pipeline or from a pipeline to a customer location. Sometimes the gas is transported to remote filling stations for CNG-fueled vehicles. Large-scale transportation of CNG is not yet commercialized but is considered economically feasible and is being pursued actively by several companies.
In the 1960s, Columbia Natural Gas of Ohio tested a CNG carrier. The ship was to carry compressed natural gas in vertical pressure bottles; however, this design failed because of the high cost of the pressure vessels. Since then, there have been several attempts at developing a commercially viable CNG carrier. In the past five years, several competing CNG ocean transport designs have evolved. Each design proposes a unique approach to optimizing gas transport while using as much off-the-shelf technology as possible to keep costs competitive.
The CNG chain consists of the following components:
The production facility for CNG is simpler than other remote gas utilization options such as liquefied natural gas (LNG), gas to liquid (GTL), ammonia, or methanol. It typically consists of compression, cooling, dehydration, and possibly liquefied petroleum gas (LPG) separation. The extent of compression and cooling is different for the various CNG processes. The scope of the production facility depends on the quality of the gas and reservoir pressure, but is a small fraction of that of a comparably sized LNG or GTL facility.
A large portion of the CNG carrier’s capital cost is the gas containment system and associated safety and gas control systems. The means for transporting CNG differentiates the various CNG processes that have emerged over the last few years. These processes include:
- Coselle CNG carrier
- Volume-optimized transport and storage (VOTRANS)
- Coiled-pipeline (CPL) carrier
- Gas transport modules
- The pressurized natural gas concept
The central idea behind the Coselle CNG carrier, patented by Cran and Stenning Technology Inc., is to create a large but compact CNG storage with a pipe. A Coselle consists of several miles of small-diameter pipe coiled into a carousel, hence the name Coselle. Enersea Transport LLC is commercializing the VOTRANS technology. VOTRANS consists of long, large-diameter pipes encased in an insulated shell. The technology is different from other CNG concepts in terms of the lower compression requirements because of lower pressure and temperature of storage. C-Natural Gas’s CPL carrier uses a coiled-pipeline configuration, which is easily adaptable to existing maritime shipping with nominal modifications to the off-the-shelf ship design. The pressure and temperature at which CNG is stored vary depending on the CNG process. A typical range of storage pressures is 140 to 200 bar.
The CNG ship unloads gas into a pipeline at the receiving station. The CNG receiving terminal is relatively simple and includes a dock with high-pressure pipeline connections and possibly an expander to allow energy to be recovered from the high-pressure gas. A scavenging compressor may be needed to empty the ships below the pressure of the pipeline. This will make it possible to transport larger quantities of gas, which will reduce the number of ships required to transport a given quantity of gas.
Storage at the production and receiving terminal is required to maintain continuous operation. Assuming that the time between shipments is not great, a practical approach may be to have extra ships and keep them in the port for storage purposes.
The volume reduction for CNG depends on the conditions at which the compressed gas is stored but is typically in the range of 250 to 300, compared with gas at atmospheric conditions. CNG is considered a viable transportation option for markets that are 1000 km or less from the source of the gas. As the distance from the market increases, LNG or GTL becomes more favorable assuming sufficient volume of gas is available. The threshold volumes required for CNG are expected to be relatively smaller compared with LNG and GTL. CNG can handle gas volumes ranging from less than 100 MMscf/D to more than 1 Bscf/D. The CNG design is modular. By adding ships, the volumes handled can grow with growing demand. The CNG process is energy efficient with energy consumption approximately half of that of an LNG project and significantly lower compared with syngas-based generation routes. The fuel required for the compression of the gas at the production facility ranges from approximately 0.5 to 1.0% of the feed gas depending on the feed-gas pressure.
Additional fuel consumption during transportation is a function of the distance of the market from the source. The cost of transportation is dependent on specific-project conditions, shipping distance, and number of ships. A Coselle study indicates that the cost of transporting 300 MMscf/D over a distance of 1,100 miles is U.S. $1.4/million Btu excluding the cost of gas at the wellhead. Because there are no commercial CNG units in operation at this time and the technology is still under development, cost of these projects are not yet predictable and should be verified on a project-specific basis.
The key issue with CNG as a gas monetization option has been the ability to obtain financial backing for a real project. As with all new technologies that are not commercialized, CNG faces the first-adopter syndrome. It has been difficult to put a project together that is financially attractive and at the same time not too large to be considered too risky to be the “first mover.” One of the key variables that affects the cost of the CNG option is the cost of ships. The ship cost is a function of the amount of steel, which in turn depends on the safety factor used for the design of the containment system. Currently, there are no specific codes that govern CNG carriers. The regulations that determine the safety factor used to design the containment system could have an impact on the economics of CNG projects. Other issues with the CNG option include evaluation of controlled loading, unloading and emergency depressuring to confirm the use of carbon steel as the material of construction, and safety-related concerns.