Shell Canada's Burnt Timber gas plant in Canada's Alberta foothills uses plant tail gas incinerators to oxidize sulfur compounds—including hydrogen sulphide (H₂S), carbonyl sulphide (COS) and carbon disulphide (CS₂)—that cannot be released directly into the atmosphere. These incinerators oxidize all sulfur species to sulfur dioxide (SO₂) prior to release in the flue gas.

A problem arose for the engineers and operators at the Burnt Timber Gas Plant when they realized the facility's existing oxygen flue gas analyzer was providing inaccurate readings. As a result, operators in the plant were unable to maintain proper air-to-fuel ratio control. Operators rely upon continuous measurement of oxygen to control the combustion process. Inaccurate oxygen flue gas readings cause operators to set the incinerator to burn excess fuel to ensure complete combustion of the sulfur species. Such inaccuracies also lead to excessive and costly maintenance time on the analyzer. This problem with the analyzer also appeared to be resulting in excess SO₂ emissions.

Because the oxygen and combustibles levels in the incinerator were fluctuating, it was difficult to maintain an air/fuel ratio sufficient for oxidizing the sulfur compounds to SO₂. Emerson Process Management's Oxymitter 4000 in situ oxygen transmitter was selected to replace the existing oxygen flue gas analyzer. The updated technology provides operators with accurate oxygen flue gas readings that can form the basis for a control strategy and ensure that the incinerator's air-to-fuel ratio is optimized.

Sulfur plant incinerator control is typically based on closed-loop control of the fuel gas flow providing the incinerator temperature required. Natural draft dampers that are manually opened or closed traditionally supply combustion air. Modern incinerators are designed to incorporate forced draft air for combustion air. This allows closed-loop temperature control where combustion air is provided, based on the required ratio to fuel gas. Closed-loop control based on excess oxygen can be used to optimize fuel gas control.

Ideal incinerator operation optimizes excess oxygen in the flue gas stream. This optimization ensures complete oxidation of all sulfur compounds to SO₂. Sulfur plant incinerators are typically operated with excess oxygen levels of 6% to 10%. The recommended operating range for excess oxygen is 2% to 5%. Operating above 5% will result in excessive use of incinerator fuel gas.

Since installation of the Oxymitter 4000, Burnt Timber has operated the incinerator's excess oxygen at 1% instead of 2%, directly resulting in reduced air blower requirements.

The most significant advantages of operating within the recommended range include:

• Proper oxidation of sulfur compounds;
• Reduction in fuel gas use; and
• Reduction in CO₂ emissions associated with decreased fuel gas consumption.

The new analyzer also showed operators at the Burnt Timber facility that the existing incinerator burner was not operating properly and causing violations of the plant operating permit. Replacing the burner stabilized the SO₂ emissions and eliminated plant operating permit violations.

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