Submerged combustion is a highly efficient and safe method to heat and evaporate a wide range of aqueous solutions and slurries, even those with a high tendency to scale, corrode, or erode.
It involves the direct contact between combustion gases burned in a submerged combustion chamber and the liquid. Thermal efficiencies in excess of 99 per cent on HHV basis are feasible, therefore less fuel is required and less greenhouse gas emissions are produced than conventional heating methods.
The Submerged Combustion Components
- Tank: Used to contain the liquid and support the combustion chamber. Material selection is based on the liquid properties.
- Fuel Source: Often natural gas. Other fuels like propane, butane, or No. 2 fuel oil can be used. Innovative fuel sources can be applied as they emerge.
- Combustion Air: Usually from a centrifugal or positive displacement blower, the air source must overcome resistance to deliver air at the appropriate pressure.
- Combustion Chamber: Usually cylindrically-shaped and especially designed to ensure complete combustion of the fuel gas is achieved before the gases leave through openings specifically sized for each application.
- Exhaust Outlet: Exhaust gas is vented to a safe location using a stack.
- Burner Control and Safety: CSA, NFPA, FM, and CEN safety requirements set operational standards for submerged combustion systems, but specific tools can further enhance safety. Flame detectors and scanners provide ongoing monitoring, while mass-flow measuring devices track fuel and airflows.
- Process Control: Effective control allows oversight of the burner firing rate, the container’s liquid level, and any other process variable of interest.
The Process
The process begins with starting the combustion air blower, which must overcome the hydrostatic liquid head in the combustion chamber. Afterwards the burner will ignite and a flame is established.
Combustion gases exit the combustion chamber and bubble through the liquid, transferring their sensible heat energy to the liquid. Some combustion water is condensed, transferring latent heat as well. The gases leave fully saturated and at the same temperature as the liquid, carrying sensible heat, and latent heat from the water vapour.
The saturated gas contains latent heat relative to the humidity ratio (Table 1), with enthalpy increasing rapidly with temperature. At higher liquid temperature limits, submerged combustion’s efficiency decreases. In order to maintain high thermal efficiency, a heat recovery unit is added to the process flowsheet. This unit operation can be internal or external to the submerged combustion tank.
This phenomenon of latent heat carried by the exhaust gas makes it impossible to boil water by submerged combustion at a certain temperature below the boiling point. The amount of energy contained in the water vapour in the saturated gas is equal to the amount of energy that is brought into the liquid by the burner. At sea level, pure water can only be heated to about 85°C. This makes submerged combustion an effective evaporator.
Applications
Submerged combustion can be used to heat and evaporate any aqueous solution or slurry; however, it is often found to be the best available technology when dealing with difficult streams that pose a significant challenge to conventional equipment.
Heating
- Potash brine
- Sulphuric acid heap leach solution
- Log chest water containing suspended wood fibre and dissolved lignin
- Process water containing high levels of chloride
- Sea water for LNG vapourization
- Wastewater aerobic activity
- Municipal sludge – Class A biosolids
- Fresh water for process use
- Iron ore concentrate slurry
Evaporation
- Iodine brine
- Produced water from oil and gas wells
- Brine from cowhide curing
- Reverse osmosis rejects
- Salt whey
- Calcium chloride solution
- Metal acid leach solution
- Landfill leachate
Looking Ahead
Submerged combustion offers an effective method of heating difficult solutions with high thermal efficiency. Applying it can yield plant operators cost savings, higher efficiency, and lower emissions.
For more information about submerged combustion, contact Osama Shenouda, CEO, Inproheat Industries Ltd., at oshenouda@inproheat.com. Or visit them online at Inproheat.com.
