Sulphur Burning - Sulphur Furnace
March 9, 2002

Introduction Cold Shell Design Hot Shell Design Sizing Criteria Baffle Walls Length to Diameter Ratio

Associated Links Sulphur Guns Sulphur Furnace Materials of Construction Sulphur Systems

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Introduction

A sulphur furnace is generally a large horizontal cylindrical vessel of carbon steel lined internally with refractory brick.  An air stream enters the furnace at one end and liquid sulphur is fed in at the same end through a sulphur gun.  Sulphur atomization is achieved usually by a simple spray nozzle or spinning cup.  The furnace is closed coupled to a boiler in which much of the heat of the sulphur combustion is removed.

A sulphur furnace provides for the complete combustion of molten sulphur with oxygen to sulphur dioxide according to the following reaction:

            S(s) + O2(g)  =  SO2(g)                         H_{f @ 25}°C = -70.94 kcal/mol

The reaction is highly exothermic resulting in a large temperature rise.  A side reaction of sulphur dioxide with oxygen forms sulphur trioxide.

            SO2(g) + ½ O2  =  SO3(g)                    H_{f @ 25}°C = -23.45 kcal/mol

The design of the sulphur furnace must achieve good gas mixing and full combustion of sulphur prior to leaving the furnace and entry to the boiler section.   Sulphur droplets impinging on baffle or checker walls will vapourize immediately and burn to sulphur dioxide.  The internals of a sulphur furnace are important to ensure complete combustion of sulphur to sulphur dioxide.

Unburned sulphur that does impinge on the carbon steel surfaces of downstream boilers, ducting and heat exchangers will corrode the steel.  Evidence of this type of corrosion will be evident during equipment inspections.

Cold Shell Design

This design maximizes the lining thickness with no insulation on the outside of the shell.  The cold shell minimizes shell expansion.  The disadvantage is the potential for corrosion at the brick/shell interface due to condensation of acid on the cold shell.

The shell temperature will be a function of the operating temperature of the furnace, lining thickness and ambient conditions and will be in the range of 100 to 150^oC (212 to 302^oF).   This temperature is higher than generally permitted from a safety point of view for surface temperature of equipment.  Some form of protection or barrier other than insulation should be provided to prevent burns from accidentally contact with the hot shell.

Basis	12% SO2, 25oC Ambient Temperature, No Wind
Furnace Temperature	1093oC (2050oF)
Fire Brick	4½”	9”
Insulating Brick	9”	9”
Shell Temperature	106oC (223oF)	103oC (217oF)

Hot Shell Design

This design has a thinner lining and a thin layer of insulation on the outside of the shell.  The temperature at the brick/shell interface is maintained above the dew point of the gas which minimizes the risk of corrosion.  The disadvantages are larger shell expansion and large differentials between summer and winter operation.

The shell temperature should be maintained at approximately 200 to 250^oC (392 to 482^oF).  A thin layer of insulation 13 mm (1/2") with cladding is applied to the outside of the furnace.

Basis	12% SO2, 25oC Ambient Temperature, No Wind
Furnace Temperature	1093oC (2050oF)
Fire Brick	4½”	9”
Insulating Brick	9”	9”
Insulation	½”	½”
Shell Temperature	252oC (485oF)	243oC (470oF)

Sizing Criteria

The sulphur furnace should be sized for the approximately 0.094 m³/tonne (3 ftª/ton) per day of acid production based on a maximum gas strength of 12% SO2 and a plant located at sea level.  For a 2000 tonne per day acid plant, the furnace volume required is 188 m3 (6639 ft3) minimum.  This criteria provides sufficient residence time for the complete combustion of sulphur to sulphur dioxide provided that the internals of the furnace are design properly.

For plants at higher elevations the furnace volume needs to increase due to the greater gas volume at the lower barometric pressure.  The reason is to maintain the same gas residence time in the furnace.

For furnaces producing gas strengths higher than 12% SO2, additional furnace volume or residence time is required to achieve complete combustion of sulphur to sulphur dioxide.

The type of sulphur gun and the degree of sulphur atomisation will also affect the size of the sulphur furnace.  A high efficiency sulphur gun capable of atomizing the sulphur to fine droplets will allow the sulphur to burn more quickly thus reducing sulphur furnace size.

Baffle Walls

There are two types of baffles typically used in a sulphur furnace; Checker Walls and Segmental Baffle Walls. 

Checker walls are constructed by leaving a space between each brick to form a large number of pigeon holes.  The gas flows through the openings in the wall and the brick surface provides incandescent surfaces.  The dead space or volume created by a checker wall is minimal since the gas flows through the wall across the entire cross-section of the furnace.

Segmental baffle walls are similar to baffles in a heat exchanger.  The gas is diverted through the baffle window which creates turbulence and mixing.  The first baffle is generally an underflow baffle with subsequent baffles forcing the gas over the baffle wall (overflow baffle).  Segmental baffles in a furnace create a dead zone similar to the dead zone created in shell and tube heat exchangers. 

A typical furnace will have two or three baffles.  Sufficient turbulence and mixing is provided to ensure complete combustion of the sulphur to sulphur dioxide.

Length to Diameter Ratio

The furnace length to diameter ratio should be in the range of 2:1 to 4:1.  The overall diameter of the furnace is limited by the maximum diameter that can be effectively bricked.  When a furnace is at its operating temperature the metal shell will expand away from the refractory lining creating a gap between the shell and refractory lining.  The refractory lining is left unsupported when this occurs and the problem is magnified the larger the furnace diameter.  In a properly designed and installed refractory lining this is not a major problem but when there are defects in the installation or design, the result may be collapse of the refractory lining.

The maximum furnace diameter is limited to about 15 ft (ID brick) due to mechanical limitations with the brickwork.