Excerpt from U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine by LT Hannah Kawamoto, U.S. Coast Guard Office of Operating and Environmental Standards, Deepwater Ports Standards Division.
Natural gas is odorless, colorless, non-toxic, non-corrosive, and, when supercooled to minus 260 degrees Fahrenheit, it turns into liquefied natural gas (LNG). Liquefying natural gas reduces its volume by more than 600 times, which makes it more efficient and practical to transport.
When liquefied natural gas reaches its destination, it is revaporized back into a gas, which is then linked to pipelines that transport the gas.
Best Available Commercial Technologies
The three sources of thermal energy typically used to warm LNG from a liquid to a gaseous state are ambient air, heat from combustion, and seawater.
Each system uses a vaporization process that passes the liquefied natural gas through pipes that are surrounded by a heating medium to transfer heat into the LNG. “Direct” heat is when the heating medium directly warms the liquefied natural gas. “Indirect” heat is when the heating medium is used to warm an intermediate (or secondary) medium that transfers the heat to the LNG.
Intermediate Fluid Vaporizers
An intermediate fluid vaporizer uses an intermediate heat transfer fluid (such as propane, refrigerant, or a water/glycol mixture) to revaporize LNG. Although refrigerant and propane have low flash points ideal for heat transfer, the operational risks are much higher when handling these types of fluids, and they are very costly. The water/glycol mixture has a high flash point, requiring a larger heat transfer area, which results in a larger system than the propane or refrigerant systems. However, the water/glycol fluid system is more cost effective and the associated operational risks are relatively low.
Ambient Air Vaporizers
Ambient air vaporization (AAV) technology uses ambient air as the thermal energy source to vaporize liquefied natural gas. The LNG is distributed through a series of surface heat exchangers, where the air travels down and out the bottom of the vaporizer.
This can be set up as either a direct heat or indirect heat system. AAV technology is best suited for areas with warmer ambient temperatures. Frost on the vaporizer is an issue because the LNG is vaporized directly against the air and the water vapor in the air condenses and freezes. Frost build-up reduces performance and heat transfer. Additionally, the ambient air vaporization system requires a significant amount of space.
Open Rack Vaporizers
Open rack vaporizers use seawater as the thermal energy source in a direct heat system to vaporize the LNG. To control algae growth within the system, chlorine is injected on the intake side of the system. The treated seawater is then pumped to the top of the water box and travels down along the outer surface of the tube heat exchanger panels, while LNG flows upward through these tubes and is vaporized. Because this technology relies on seawater as the primary heat source, it is only effective where seawater temperatures exceed approximately 63 degrees Fahrenheit.
Shell and Tube Vaporizers
Shell and tube vaporizers (STV) also use seawater as the thermal energy source. The liquefied natural gas passes through multiple tubes while seawater enters a shell surrounding the tubes.
A closed-loop system uses an intermediate fluid to transfer heat. The intermediate fluid flows through tubes in separate heating equipment to absorb heat, then the fluid passes through the STV unit to re-gasify the LNG. Since there are two heat exchangers, this requires a large amount of space.
Submerged Combustion Vaporizers
Submerged combustion vaporizers do not use seawater for LNG vaporization. Instead, the liquefied natural gas is warmed by flowing through tube bundles that are submerged in a water bath, which is heated by natural gas combustion. The submerged combustion burner emits hot exhaust gas that directly heats the water bath by bubbling through the water to an exhaust stack.
Waste Heat Recovery and Engine Cooling Technology
Deepwater ports can use regasification vessels equipped with revaporization systems to vaporize the LNG. Waste heat recovery and engine cooling technologies have been incorporated as part of the revaporization system to improve the efficiency and reduce the emissions of these regasification vessels.
Additionally, using engine cooling technology reduces the amount of seawater intake because instead of cooling the engines solely with seawater, cooled water from the LNG vaporization process is used to cool the engines. Additionally, any cooling systems can be tied into the intermediate fluid, such as the heating, ventilating, and air conditioning systems.
For more information:
Full article and “Environmental Protection” edition of USCG Proceedings is available at http://www.uscg.mil/proceedings/Winter2008-09/.
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Direct requests for print copies of this edition to: HQS-DG-NMCProceedings@uscg.mil.
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Tuesday, April 13, 2010
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