Submerged combustion, as its name implies, is the combustion of gas or fuel oil in such a manner that the hot combustion product gases are released under the surface of a liquid. In this way, the energy released by the combustion process is transferred by direct contact with the liquid (see Direct Contact Heat Transfer). Although it is possible for the burner itself to be submerged, most systems are arranged with the burner above the liquid level and a submerged exhaust system. In the Figure 1, the exhaust gas is released into the annulus between the downcomer and draught tube. This promotes good mixing between the hot gas and the liquid and produces intense circulation of the liquid in the tank.
The first patents in the field were granted at the end of the nineteenth century but it was the development work of Swindin (1949) in the 1930s and 40s which led to the first commercial applications of the process.
Submerged combustion is used in two classes of Evaporators, direct and indirect. In the first, it is used to concentrate corrosive or toxic materials. In the second, water is heated which, in turn, is then circulated over a tube bank containing the liquid to be evaporated.
While the advantages of submerged combustion are the absence of fouling or corrosion and the ability to handle highly viscous liquids or liquids containing up to 40% solids, the disadvantages are the contamination of the liquid by combustion products.
Proprietary burners have been developed for submerged combustion applications using a variety of liquid or gaseous fuels. The requirements are complete soot-free combustion within a small chamber volume with low residence times at above atmospheric pressure. High velocity vortex-type burners are employed utilizing, for liquid fuels, recirculation of the hot gases to aid fuel vaporization. Multiple burner arrangements are possible with either separate or common downcomers.
Because of intensive gas/liquid contact, the dew point of the released combustion gases approximates to the vapor pressure of the liquid (water) at the operating temperature. The water vapor in the combustion gases will condense below the dew point (~60°C for natural gas and 50°C for gas oil) and the heat transferred to the liquid (water) will exceed the lower calorific value of the fuel. At higher water temperatures the efficiency falls off until a maximum temperature of 90°C. At this temperature all the sensible heat in the combustion gases is used to evaporate water to saturate the gases.
A partial list of applications of submerged combustion:
Fish stick liquor
Carbonation of liquids
Treatment of sewage sludges and cellulose effluents
Stripping of phenols/H2S
Inert gas generation
A major application of submerged combustion is the vaporization of liquid cryogens such as nitrogen, oxygen, natural gas (LNG), petroleum gas (LPG), ethylene, ammonia etc. Such fluids are often transported as liquid but are required for use as a gas. Because of safety requirements indirect heat transfer rather than direct firing from the combustion source is often essential. In the submerged combustion vaporizer, water acts as the heat transfer agent.
Referring to the earlier diagram, a tube bank through which the fluid to be vaporized flows, is inserted into the riser section between the downcomer and the draught tube. The issuing combustion products produce a rapid circulation of water over the coils and considerable turbulence at the heat transfer surface. This enhances the heat transfer coefficient on the outside of the tubes and prevents ice formation. Edwards (1967) described the so-called Sub-X° unit developed by the Thermal Research and Engineering Corporation in the U.S. This design is now marketed in the U.S. and elsewhere by T-Thermal. In the United Kingdom the technology was licensed and developed by Thurley International Ltd. [Thurley (1970)] and since 1984 by Kaldair Ltd. with its range of "TX" vaporizers.
In the Sub-X° design the combustion chambers are located above the liquid surface and exhaust into a common rectangular downcomer unit. The combustion products are sparged out through a series of short stub pipes into the zone between the downcomer and weir and below the tube bundle. The weir and the flues are free-standing in a rectangular tank. These units are available and operating with thermal capacities up to 75 M Btu/hr (22 MW) and vaporize up to 100 tons/hr of LNG. The water temperature for such units is between 15-52°C with thermal efficiencies of 90-99%. Condensation of the water vapor from the combustion product gases occurs and make-up is not required. The tank water becomes saturated with carbon dioxide and care with the choice of materials and with water treatment is required. Over 250 units of varying designs and ratings are in use world wide.
Swindin, N. (1949) Recent developments in submerged combustion, Trans. Inst. Chem. Eng., 27, 209-221.
Edwards, R. M. (1967) Efficient new heat exchanger suited to LNG vaporisation, The Oil and Gas Journal, 96-99, Oct. 2nd.
Thurley, J. (1970) Liquid natural gas vaporisation, Natural Gas—LNG and LPG, Oct.
T-Thermal (Europe), GOC House, Blackwater Way, Aldershot, Hants, GU12 4DR.
Kaldair Ltd., Kaldair House, Langley Quay, Waterside Drive, Langley, Berks SL3 6EY.