Cryogenic Vaporizers

 

Cryogenic Vaporizers - Design Principles

Single burner vaporizer

The submerged combustion vaporizer is an indirect fired heat exchanger with the burner and process tube coil contained within a single vessel. The design is based on the submerged exhaust principle whereby the burner combustion products are discharged into a water bath, which is used as the heat transfer media for vaporizing the LNG in the tube coil.

In the single burner submerged combustion vaporizer (SCV), combustion air is introduced into the burner at two locations. Most of the air enters the upper volute section (secondary), with the remainder (primary) being supplied to the region around the fuel gas injector. With this arrangement, the burner fires upward into the central section between the two volutes, where the combustion gases are co-reacted with the secondary combustion air. The secondary combustion air enters the upper volute via a tangential inlet, imparting a swirling motion to the air. This results in intimate mixing with the combustion gases rising from the burner and subsequent gas recirculation back along the axis of the burner before discharging into a set of sparger tubes located under the process tube bundle.

Injection of the products of combustion into the water bath forms a frothing two-phase mixture containing many small gas bubbles. The reduced density of the two-phase mixture creates a lifting action, causing the mixture to rise. The froth flows up through the tube bundle (vaporizing the LNG) and over the weir, where disengagement of the combustion product gases occurs. The water falls back over the weir and once again re-circulates up through the heat exchanger tube bundle. The frothing action is necessary, even at low heat duties, to maintain uniformity and to prevent icing in the tank. The water bath operating temperature is typically between 55-65 °F. Water vapor in the products of combustion is condensed in the bath, aiding SCV performance to an average thermal efficiency of over 99% on a gross heating value basis.

 

Cryogenic Vaporizers - Advantages

The patented Sub-X Vaporizer combines our direct fired heater experience with proprietary burner systems. Additionally, Selas Fluid's vaporizer employs the safety of steam heating, the high response characteristics of a direct-fired heater, and the following benefits unique to the Sub-X Vaporizer System: Reliability and Experience

Selas Fluid has more installation experience with submerged combustion cryogenic vaporizers than all major companies combined, with over 80% share of the world market Since our first installation in 1965, 95% of our vaporizers are still in service High Thermal Efficiency High heat flux with low approach temperature Average thermal efficiency over 99% Clean combustion with low emissions Temperature Uniformity Patented weir provides high turbulence/recirculation, resulting in temperature uniformity No ice buildup on tubes Fast Response Rapid startup and shut down without process upset Water bath mitigates process fluctuations during transition periods Safety No flame impingement on LNG coils No need for insulation for personnel protection Heat exchange medium is water, eliminating the hazards and handling of ethylene glycol or other fluids Automatic or semi-automatic operation, complete with safety interlocks Economy Fuel savings due to high thermal efficiency Single burner units offer lower lifetime maintenance Pre-piped and pre-wired stainless steel tank configurations available Compact footprint and layout Easily integrated with supplemental heat sources

 

Cryogenic Vaporizers - Research and Development

High excess air with enhanced controls. One recent success has been the employment of a High Excess Air (HEA) design with enhanced controls for primary air supply. HEA rates above 40% combined with the proper combination of primary and secondary air flow rates have significantly lowered CO by approximately 60%. Proper mixing and careful control of oxygen concentrations during the initial combustion reaction allows the burner to operate with lower CO levels without increasing NOx production.

Single point water injection dramatically reduces NOx emissions. By mounting a water spray at the tip of the burner nozzle, the flame temperature can be lowered, retarding thermal NOx. On average, NOx levels have been lowered by over 50% using this technology.

Further improvements to water injection systems are underway. CFD models of the burner have been created to simulate the burner temperature profile and identify new proprietary injection points designed to pinpoint high temperature regions surrounding the burner nozzle.

Innovations to achieve lower emissions. Two new technologies utilizing alternative burner designs are currently in development. First, an all metallic premix burner design will be located directly on top of the existing vaporizer tank. The burner will employ anti-flash back mixers and radial fuel injectors used for years in utility boiler applications to help prevent the Prompt NOx* mechanism.

Another relatively new technology employing our Thermatrix flameless thermal oxidation (FTO) process is being pursued as the most effective abatement technology yet. This technology has been used for over 15 years to oxidize organic waste streams. Patented in 1989, the process entails a lean pre-mixed air and fuel mixture introduced via dip tube under critical velocity profile into a bed of preheated inert media material; this stabilizes a reaction wave and provides the driver for the combustion process which will heat the water bath. A pilot unit with 50MM Btu/hr heat release is currently under construction at Selas Fluid's technology center. Expectations are that the FTO burner technology will lower both NOx and CO emissions into the single digit range without post-combustion treatment.

Ability to incorporate alternative energy sources.   As operating costs continue to rise, more resourceful ways to incorporate alternative energy sources are coming to light. Selas Fluid is designing SCV's to operate with supplemental hot water/air streams from a variety of sources:

condensing coils in the vaporizer tank for low pressure steam cycles

direct injection of heated flue gas to augment the burner firing rate

reverse cooling towers for supplemental heating of the bath water

gas turbine quench tower hot water flows

The use of these supplemental heating sources reduces the already low vaporizer fuel requirements and associated emissions.

* Prompt NOx is formed from molecular nitrogen in the air combining with fuel in fuel-rich conditions which exist, to some extent, in all combustion. This nitrogen then oxidizes along with the fuel and becomes NOx during combustion.

 

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