Regasification, also called vaporization, is the final step in the LNG process when LNG is returned to its original gaseous form. LNG is converted back to natural gas by carefully warming the liquid until it vaporizes. As the gas vaporizes, it expands in volume and is injected into a gas pipeline or distribution system for delivery to customers.
Regasification typically occurs at one of three points in the gas delivery system:
For LNG transported by ship tanker, it is usually regasified in an onshore terminal, although it may also be regasified in a ship offshore with the gas transported onshore by an undersea pipeline.
For LNG peaking storage facilities it is regasified at the facility.
For use as a backup source of gas during pipeline or distribution maintenance it is regasified by equipment typically hauled in the field on a large trailer.
For LNG shipped by tanker, the basic steps in the regasification process are:
Berthing and unloading of the LNG tanker
Storage of LNG
Delivery into the pipeline grid
The process is slightly different for offshore regasification. Each step is discussed in detail below.
Berthing and unloading
The tanker is parked and moored at a berth alongside the LNG terminal. The water in the berth must be deep enough to accommodate LNG tankers, which usually require a depth of 40 feet or more. Discharge or unloading lines are then attached mid-ship to the LNG tanker’s discharge valves. The discharge lines rest on a trestle built from the berth to the onshore storage tanks that protect and stabilize them. The discharge system includes high-volume pumps to accomplish the movement of the LNG from the ship to the shore. The marine facilities for each onshore regasification facility are site-specific and require custom design to fit each terminal.
Each regasification facility contains one or more specialized, full containment storage tanks capable of holding a minimum of one shipload of LNG. Onshore storage tanks are not only a holding place for LNG, but can also be used as blending facilities to adjust the heating content by mixing supplies from multiple shiploads of LNG. Typical tank sizes range from 55,000 m3 to 180,000 m3, but economies of scale have resulted in larger tanks being built in recent years, with 200,000 m3 tanks now commonly specified where space permits. Many terminals install multiple tanks to obtain the desired storage capacity. A facility’s storage capacity depends on a number of factors. Tanks are designed to allow for the largest LNG tanker possible to offload its full capacity plus additional capacity to allow LNG from an earlier cargo to be held in the event that multiple cargos are available before the vaporization facility is able to regasify the earlier cargo. If the terminal is in a region where market demand fluctuates, additional storage capacity may be built so that plant output can be matched to market needs, with the LNG stored in the tank until consumers require the gas.
The third step in the regasification process is vaporization. LNG receiving terminals typically use either an open rack vaporizer (ORV) or a submerged combustion vaporizer (SCV) to convert the liquefied gas back to a gaseous state. Other types of vaporizers include ambient air-heater vaporizers, combined heat and power units with gas-fired vaporizers, and shell and tube vaporizers. These are all much less common and are not discussed here.
ORVs are the most commonly found vaporizers and use seawater at ambient temperature as their source of heat. The LNG is vaporized using a heat exchanger that warms the LNG by passing it through radiator-like rows of tubes that are flushed with seawater. After the water is used to warm the LNG, it is collected and returned to the sea. Besides the ORV unit itself, required equipment includes large diameter intake and discharge pipes, pumping equipment, and water treating facilities. The ORV is made of aluminum to handle the extreme temperatures, and the rows of tubes are coated in zinc to resist corrosion from seawater. The water is chlorinated to protect the surface of the tube panel and any inside piping from algae growth. Water quality and quantity are critical to successful operation of the ORV system. The water must contain no heavy metals and be low in solids. Because of the vast quantity of water required – from 18,000 m3 to 65,000 m3 per hour – the amount of sea life destroyed by mechanical intake or the chemically treated water can have an ecological impact. In addition, since the millions of gallons of seawater are returned at 5 to 12 degrees cooler than the ocean’s ambient temperature, there is the potential for environmental impact from changing the natural temperature of the seawater around the facility.
The SCV vaporizes LNG contained in stainless steel tubes in a submerged warm water bath. A combustion burner fueled by natural gas provides the necessary heat to warm the water bath. The fuel source is usually low-pressure gas from facility boil-off. The water bath of an SCV uses significantly less water than the ORV since the water is contained and reused. Nevertheless, due to chemical reactions during the vaporization process, the water does require treatment before disposal. But since water is not continually being returned to the sea, SCVs are generally considered to be more environmentally friendly than ORVs, and most new North American terminals are using them to limit impact. Because of fewer equipment requirements, less space is needed to build an SCV. However, the increased cost of fuel (approximately 1.5 to 2.5% of the LNG received) offsets most of the capital savings.
Delivery into the pipeline grid
Before the gas can be delivered into the pipeline grid it must match pipeline grid specifications for temperature, pressure, and composition. Temperature is determined by the regasification process and pressure can be adjusted through pressure regulators. If the gas quality is not within pipeline tariff standards, additional measures must be taken to bring the gas up to marketability standards. The most common problem is that the Btu content of the regasified LNG may not match the Btu content requirement of the pipeline. In some cases this can be handled by blending the regasified LNG with natural gas from other sources. If this is not possible, it is then necessary to condition the gas.
There are three common ways to adjust Btu content prior to send-out: fractionation, injecting an inert gas such as nitrogen, and injecting compressed air. Fractionation uses traditional cryogenic gas processing techniques to remove ethane, propane, butanes, and heavier hydrocarbons in liquid form from the gas stream. To do this, cooling and pressure are used to mechanically separate components in a similar manner to the way in which they are removed in the liquefaction process. Facilities to perform fractionation are typically available only if the LNG is delivered to a region with gas production such as the Gulf of Mexico in the U.S. If such facilities are not available, another method must be used.
Nitrogen or air injection can also be used to dilute the Btu content in a given volume of gas. Since nitrogen is an inert gas, it simply takes up space in the gas. Compressed air, while more economical, adds compounds and elements to the gas stream that could be problematic. However, since LNG is so clean from contaminants, air injection is often an acceptable method for reducing Btu content while remaining within specifications for other contaminants.
Regasification in peaking storage facilities or in field equipment is a simpler process. LNG from a tank is piped through a vaporizer consisting of heat exchangers that use atmospheric air, hotwater/steam, and/or electric or natural gas heaters to warm the LNG. A control system is required to ensure pressure and temperature changes are monitored and carefully controlled. The gas can then be delivered into the pipeline system.