SO₃ Injection Systems

Molten Sulfur SO₃ System | DSI™ Technology

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A precipitator will not operate effectively if sufficient SO₃ is not present in the flue gas. SO₃ reduces the sparkover voltage of the flyash layer by lowering the resistivity. Some SO₃ is produced naturally by the boiler in very small quantities from sulfur in the fuel. When low sulfur coal is burned, SO₂ and SO₃ production is minimal. Therefore, SO₃ must be added to restore precipitator collection efficiency. All of the SO₃ attaches to the flyash and does not increase SO_x emissions. SO₂ is introduced to a catalyst and is oxidized forming SO₃ which is injected into the flue gas at levels normally less than 20 ppm.

The SO₂ is generated via combustion of elemental sulfur in either dry or molten sulfur feedstock options. Feedstock options are evaluated to meet site specific needs.

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Molten Sulfur Based SO₃ System

The system consists of a sulfur storage and handling facility, an SO3 production unit, SO3 piping and distribution probes.

Tanker trucks deliver molten sulfur which is unloaded directly into a storage tank by a transfer pump. The insulated tank is steam heated which keeps the sulfur liquid. It has control valves for maintaining the required heat input and instrumentation for monitoring level and temperature.

Sulfur from the storage tank outlet flows through steam jacketed piping to the steam heated sulfur metering pumps. A heated, lighted and ventilated enclosure houses the pumps, power switching and process controls for delivering the required amount of sulfur upon demand.

All of the equipment to produce the hot gaseous SO3 is contained in separate heated, lighted and ventilated enclosures. As a unit, they contain the central control/power distribution panel, air inlet filter, process air blower, electric air heaters, sulfur burner, and catalytic converter.

The blower delivers filtered air at a constant volume to the air heaters. The heaters initially preheat all equipment to the temperature required for combustion of sulfur to SO2 and catalytic conversion of SO2 to SO3. As the flow of sulfur increases, power to the heaters decreases. After the heaters are completely turned off, the gas temperature is controlled by regulating the flow of process air entering the catalytic converter. The converter is a multi pass unit designed to maximize SO2 to SO3 conversion efficiency by introducing air from the blower ahead of each pass.

The outlet of the converter connects to the insulated hot gas piping and injection probes. The probes are installed directly into the flue gas stream and provide the final SO3 gas distribution. The probe design prevents corrosion, includes erosion protection and each is equipped with a thermocouple to indicate gas temperature.

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DSI™ Technology

Combines the advantages of dry sulfur storage and molten sulfur metering. Outside the United States, sulfur is delivered in dry form and then melted for various processes. Using this tried and true proven method, dry sulfur is converted to molten sulfur for conventional process metering. This takes advantage of dry sulfur storage and handling while maintaining the good burn characteristics of molten sulfur and accurate reliable metering. With the proper provisions, the system could accept both dry and molten sulfur feed stock so that the process is not dependent on one source or method of sulfur delivery.

The Sulfur Melter is a stand alone unit which physically separates the dry sulfur feedstock and the molten sulfur pumps. A Sulfur Melter reduces the amount of steam jacketed piping, and eliminates unloading of hot molten sulfur as required by traditional molten sulfur feedstock. Dry sulfur can be stored and handled on an as-needed basis. Liquid sulfur is metered and burned while buffering the dry sulfur storage from the molten sulfur process.

The sulfur melter is designed specifically for molten sulfur handling. Standard features include an insulated melter tank, High/Low level alarms, temperature sensors, vents, removable sulfur screen, and removable steam coils. Sulfur Melter capacities are sized to suit and typically range from 5 to 10 tons. Controls for dry feedstock and molten sulfur handling are provided to easily integrate into existing molten sulfur or dry feed SO₃ Production Systems. Controls allow for both OEM local control or Distributed Control Systems. In a retrofit situation, dry sulfur storage silos may be reused or designed for additional on site supply of sulfur feedstock. Existing Sulfur metering pumps may be reused or submersible or displacement pumps added.

Benefits of Dry Sulfur Storage:

• Eliminate unloading of hot molten sulfur from a tanker truck to a molten sulfur storage tank.
• Reduce costly steam jacketed interconnecting process piping.
• Feedstock can be purchased in 50 lbs. man sacks or 1 ton super sacks as-needed vs. 20 ton tanker tuck.
• Dry Sulfur can be stored indefinitely without need for steam heating.
• More flexibility in sulfur storage location and equipment layout. Does not require road or rail access and can be located near injection point.
• Ideally suited for small to medium sized SO₃ Flue Gas Conditioning System and can be scaled for larger systems during peak utility loads or scrubber outages.
• Quick startup from cold start in under 6 hours.

Benefits of Molten Sulfur Metering:

• Reliable operation, accurate sulfur metering, and repeatable results.
• Utilizes proven Molten Sulfur Metering process equipment.
• SO₂/SO₃ production equipment is buffered from dry feedstock handling.
• Burning molten sulfur results in a controllable steady SO₂/SO₃ production process.
• High metering turn downs are easily achieved.

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