The cold inlet water is first routed through the heat recovery unit, where the stack gas from the submerged combustion system mixes directly with the water. Multiple submerged combustion units can send their stack gases to the same heat recovery unit.
Advantages
In addition to the efficiency benefit discussed above, submerged combustion has
several additional advantages:
It is ideal for liquids that tend to foul or scale. Sludge can be heated without concern for fouling of tubes. Saltwater solutions can
be concentrated — as the water is evaporated, small amounts of insoluble salts, which would normally foul shell-and-tube heat exchangers in a matter of hours, may precipitate. Submerged combustion eliminates concerns for fouling of indirect heating surfaces. Although the cylindrical burner in the solution needs to be cleaned from time to time, the cleaning is far easier than hydroblasting tubes.
It operates near atmospheric pressure. Safety is improved because there are no high-pressure vessels. It requires minimal supervision. Unlike traditional boilers, which often require fulltime supervision, submerged combustion needs little attention. Automatic temperature controllers can adjust the fuel and air to maintain a temperature setpoint. Alternatively, burners can be configured to run at maximum
capacity full-time, especially when high volumes of water are being heated and the temperature increase is minimal. At a mine in Canada, 10 burners rated at 13 MMBtu/h each are used for 15,000 gal/min of brine with a temperature rise of 18°F. These burners run at maximum capacity. If brine flow decreases, burners are shut down to maintain a desired temperature.
Greenhouse gases are reduced. Higher efficiencies result in lower fuel consumption and lower CO2 emissions. Submerged combustion eliminates concerns for fouling of indirect heating surfaces. Although the cylindrical burner in the solution needs to be cleaned from time to time, the cleaning is far easier than hydroblasting tubes.
It operates near atmospheric pressure. Safety is improved because there are no high-pressure vessels. It requires minimal supervision. Unlike traditional boilers, which often require fulltime supervision, submerged combustion needs little attention. Automatic temperature controllers can adjust the fuel and air to maintain a temperature setpoint. Alternatively, burners can be configured to run at maximum capacity full-time, especially when high volumes of water are being heated and the temperature increase is minimal. At a mine in Canada, 10 burners rated at 13 MMBtu/h each are used for 15,000 gal/min of brine with a temperature rise of 18°F.
These burners run at maximum capacity. If hot water applications. In reality, 170–180°F is realistically achievable for water. Fluids with lower vapor pressures can be heated to higher temperatures. For example, an aqueous solution containing
30 wt.% magnesium chloride, which has a much lower vapor pressure than water, can be heated to above 187°F.
The heat of combustion causes the magnesium chloride concentration to increase because of the loss of water in the saturated stack gas. When the design of a submerged combustion system requires outlet liquid temperatures higher than 124°F, a heat
recovery unit will lower the stack temperature and improve.
Disadvantages and limitations
While the higher efficiencies make submerged combustion initially appealing, it is not the right technology for all applications. Consider the following disadvantages:
Cooling water may be needed. For large burners (larger than 8 MMBtu/h), cooling water may be required. The heat transferred to the cooling water can be recovered, if the water can be sent into the tank being heated. For concentrating
aqueous solutions, the heat will be lost. Cooling water may require additional pumping, additional water consumption, and more problems with keeping burners operating (since cooling water temperature must be maintained below a certain level to keep burners running). Chemical or mechanical cleaning of the heating jacket may be required to remove mineral deposits. Separate heat-recovery equipment may be necessary. Separate heat recovery is required to maintain efficiencies greater than 90% above 124°F. Additional equipment means more capital investment. The maximum water temperature is 180°F. For most aqueous applications, submerged combustion is limited to 180°F due to the vapor pressure of water. For solutions with lower vapor pressures, bench or pilot testing is required to evaluate the benefits of submerged combustion.
It is not a steam replacement. Submerged combustion will not necessarily replace steam, unless steam is used to heat or evaporate water. Submerged combustion produces a low pH. In aqueous solutions, a low solution pH is created by bubbling CO2 from the exhaust gas through water. The slight positive pressure of the tanks keeps the CO2 in solution, where carbonic acid is formed. However, measurements show that the pH quickly returns to that of the original solution when the pressure is released.
Vibration control is required. For a 13.5 MMBtu/h burner, the cylindrical combustion chamber extends more than 6 ft below the point of attachment to the lid. The blower is
designed to evacuate this cylinder to allow combustion. The combination of combustion and positive pressure creates waves in the chamber, and the overall vibration can be violent if the unit is unsupported. Additional supports may be required for the bottom of the cylinder and the top. One installation without supports at the bottom of the cylinder showed cracking of the cylindrical walls and required substantial repair.
It cannot be used with flammable solutions.
Implementation
If submerged combustion appears to be the right technology based on a consideration of the above criteria, the first step is to determine the heating load. One must calculate the heating load required in the process and apply an appropriate efficiency to find the total heat required. Fuel savings can also be calculated using an appropriate efficiency for other technologies. With the recent escalation of natural gas costs, justification for submerged combustion is far easier than it would have been several
years ago.
The material of construction must also be considered. Carbon and stainless steels have been used in many applications, but more corrosion-resistant alloys could be used. Vibration control must be paramount in the design of a system, especially above 8 MMBtu/h. For burner capacity, the critical design issue is flame length. If the flame length is too long (meaning the cylindrical burner chamber is too short), the flame will be cooled prematurely causing high CO problems. Consider the need for environmental permitting of a submerged combustion system, and allow adequate time for state
and/or federal approval.
One design for submerged combustion used a mechanically linked air and gas valve. The system was tuned like any typical natural gas burner, where either the gas or air
flowrates are adjusted to minimize CO and to minimize excess oxygen in the stack gas. However, the problem with this traditional method is that when the ambient air temperature changes either seasonally or from day to night, the air-to-fuel mixture would be adversely affected. One new submerged combustion design provides for an automatictuning system to adjust the air flow and the gas flow independently using pressure differential.
For heating water and most aqueous solutions, vendors should have adequate experience. However, for evaporative applications or exotic solutions, pilot testing is highly recommended to minimize risks. Vendors of submerged combustion equipment can assist with the details (e.g., burner selection, process control, safety considerations,
etc).
Concluding thoughts
As energy consumers search for more efficient methods to produce steam and heat, submerged combustion could be considered as an alternative to steam. The higher efficiencies often result in a payback period of less than two years, even for replacements of existing lowerefficiency boilers.
Literature Cited
1. Panz, E. L., and J. L. Jachniak, “The Green Solution for Heating
Clean, Contaminated or Corrosive Solutions and Slurries,” presented
at Combustion Canada 99 — Combustion and Global Climate
Change, Calgary, Alberta, Canada (May 28–29, 1999).
2. Perry, R. H., and D. Green, eds., “Perry’s Chemical Engineers’
Handbook,” 6th ed., McGraw-Hill, New York, NY p. 11-35 (1984).
JASON L. BAGLEY, P.E., is a product development engineer at Great Salt Lake
Minerals (765 North 10500 West, Ogden, UT 84404; Phone: (801) 732-
3341; Fax: (801) 731-4881; E-mail: bagleyj@compassminerals.com), where
he is responsible for field troubleshooting, quality assurance, process
engineering and new product development. He received a BS in chemical
engineering from the Univ. of Utah.
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