Submerged combustion vaporisers for LNG distribution facilities
Submerged combustion vaporisers, used to vaporise LNG stored under low temperature, are ideal for peak shaving and base load facilities
Olavo Cunha Leite
Selas Fluid Processing Corporation (Now Linde Engineering North America)
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Cryogenic and low temperature fluids are stored economically in the liquid state and pumped to vaporisers prior to use. Natural gas is liquefied for safer and more efficient handling and is used to supplement fuel gas in peak shaving facilities and for base load facilities in areas deficient in fuel gas. Liquefied natural gas (LNG) must be re-gasified for send-out into the distribution header. Vaporisers are classified into three categories:
- Heated type, with integral or remote heat source
- Ambient type, with heat sources such as air and water
- Process type, with heat from thermodynamic or chemical process.
Submerged combustion vaporisers are heated type units in which the heat source is integral with the exchanger and are used to vaporise and to heat LNG stored in containers under low temperatures, prior to distribution in base load or peak shaving plants (Figure 1). These compact design vaporisers are indirect fired heaters with the burner and heat exchanger parts of the tank assembly, and are used for heating and/or vaporising, in addition to LNG, several other cryogenic fluids, including propane, oxygen, nitrogen, ethylene and ammonia.
The design of vaporiser is dependent on several operating parameters, such as process fluid, flow rates, temperatures, pressures and viscosity. In the USA, LNG facilities are subject to insurance, federal, state and local regulations, ASME pressure codes and National Fire Protection Association (NFPA) standards.
The submerged combustion vaporiser system includes the burner(s), a high temperature flue gas distributor or a downcomer, the bath/exchanger section, concrete pit or metal tank, and the exit stack. The hot combustion product, after leaving the high heat release burner(s), is exhausted through either a downcomer or a distributor into the spargers, designed and arranged to prevent direct impingement of combustion products on the tube bundle, providing direct contact heating of water bath.
After leaving the bath tank, the flue gas enters the disengagement section to remove, by gravity, the larger water droplets before it exits the stack. Water entrainment of exhaust should be reduced, limiting the stack velocity and a mist eliminator pad may be added near the stack base.
Submerged exhaust burner designs
The true submerged burner design is based on an immersed burner, with combustion taking place under the liquid surface, and the products of combustion are directly discharged against the liquid bath. The exhaust burner(s) design (Figure 2) is based on one or several small downfired high velocity (HV) burners located on the tank deck, with the combustion taking place above the liquid level, firing into a common downcomer to maintain uniform distribution.
The HV burners are compact and operate soot-free with high back-pressure, offering full heating capacity within a few seconds. The products of combustion are exhausted directly through a water cooled downcomer with spargers into the liquid bath below the liquid level. The multi-burner units provide the flexibility to operate the remaining burners when a burner failure or loss of flame occurs during production or during start-up, allowing time to review and fix the problem, minimising vaporiser down-time. The semi-submerged single burner design (Figure 3) is based on a larger capacity single burner, partially immersed, with combustion taking place under the liquid level. The burner is constructed in three major sections, with the combustion air upper volute and the exhaust immersed lower volute connected by a partially immersed conical combustion section cooled by a water jacket.
The gas nozzle injects the fuel gas together with a portion of the combustion air into the conical combustion chamber. The water is purged before the combustion and the products of combustion are directly exhausted through the bottom volute tangential outlet and the distributor duct fitted with spargers into the liquid bath to improve its temperature uniformity.
Low combustion chamber metal temperatures minimise corrosion and eliminate any refractory need, allowing larger all-metal burner and combustion chamber due to the low skin temperature. The formation of NOx is reduced due to the progressive mixing of the combustion air and fuel gas in the burner combustion chamber. To reduce further the NOx levels, a water injection spray can be installed in the fuel gas nozzle, although it increases the risk of corrosion. The larger single burner can substitute the multiple burner design, simplifying the control scheme and reducing the number of components.
Both of these exhaust burner designs are successfully used on submerged combustion vaporisers. Post-mixed burners, both high velocity and modified vortex types, are selected due to their ability to operate efficiently with high back pressures and they should be sized to provide an extra 15 per cent heat output at peak duty.
Flue gas distribution and exhaust
Either the distribution duct header or the downcomer, fitted with spargers, collects the hot flue gas from the burner and exhausts it into the water bath, within the weir area and below the process tube bundle, providing uniform distribution of the froth around the process exchanger tubes.
A weir is placed to confine the froth lift, providing high turbulence and recirculation of water through the open bottom of the weir and the base of the tube bundle assembly. The flue gas, after passing the tube bundle and the disengagement section, exhausts through the stack.
The primary purpose of the stack is to provide dispersion, not creation of draught, due to the higher pressure drop across the burner, distributor and submergence. To achieve good dispersion of the flue gas pollutants, the design exit velocity of the stack should be high, being generated by the mechanical draught system.
Vaporisers use short stacks and the water entrainment of the exhaust gas must be reduced, consequently the stack velocity should be limited to 30ft/sec. A mist eliminator pad may be added near the stack base to further reduce the droplets in the exit flue gas.
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