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Dec-2008

Sulphur tail gas thermal oxidiser systems

SRUs (sulphur recovery units) in refineries and gas plants remove sulphur compounds from sulphur containing fuel gases and from natural gas and crude oil and produce elemental sulphur.

Ron Desai and Peter Pickard
UOP Callidus
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Article Summary
These systems also produce various gas waste streams. These waste streams must be destroyed to prevent environmental and health risks. Thermal oxidisers, or “incinerators” as they are also called, have been used for many years to oxidise remaining H2S, CO and other minor combustibles in these waste streams. This article discusses various particular design features of sulphur tail gas oxidisers.

Waste characterisation/emissions
The primary waste stream from a SRU is the sulphur tail gas stream. Other streams may be called, “pit vent” or “sweep air” etc. These other waste streams have very low mass flow compared to the tail gas stream and are not a major design consideration. These other waste streams can be injected into the oxidising chamber somewhere downstream of the burner.

H2S, sulphur vapour, SO2, COS and CS2 in the waste is oxidised (or burned) in the thermal oxidiser to form SO2 and a small amount of SO3 in the flue gas. SOx is an environmental design factor and a corrosion design concern. Unless the flue gas is treated with a downstream scrubber, which is very rare, all the SOx will exit the stack. Therefore it is important to note that a thermal oxidiser cannot control SOx emissions. The SOx emissions are completely determined by the sulphur content in the incoming waste.

The primary purpose of a sulphur tail gas oxidiser is to destroy H2S because it is a highly toxic gas, even in low concentrations. The specified H2S emission limit from an oxidiser typically varies from 5 to 10 ppmv.

In most cases, CO emissions limits are also specified. H2S is much easier to burn than CO. H2S autoignition temperature is 529ºF (276ºC) whereas for CO it is 1166ºF (630ºC). Emissions of CO are typically not guaranteed by an oxidiser manufacturer unless the operating temperature is above 1400ºF (760ºC). As with any combustion system, the higher the operating temperature, the lower the CO emissions. However, the higher the operating temperature, the higher the NOx emissions.

Low NOx fired heater burner technology is frequently confused with the capability of a thermal oxidiser burner. Can ultra-low NOx burner technology for fired heaters be applied to thermal oxidisers? The short answer to this question is, “no”. At UOP Callidus, thermal oxidiser burners are designed by the engineers in the Thermal Oxidiser Division, not the fired heater Burner Division. This is because the two types of burners are so different. Fired heater burners are sufficiently repetitive in design that they are assigned model numbers. However, this is not the case with the widely varying applications in the thermal oxidiser division. While a fired heater compared to a thermal oxidiser/waste heat boiler system may seem to be virtually the same, this is not the case. This is especially true in regard to NOx emissions and hydrocarbon destruction efficiency. This is easy to understand when one considers that the primary purpose of a fired heater is to increase the temperature of a process fluid, while the primary purpose of a thermal oxidiser is to provide high destruction efficiency of a combustible waste. The waste heat boiler in a thermal oxidiser system fulfils a secondary purpose. The boiler is an insignificant factor in the design of a thermal oxidiser burner and oxidizing chamber while the design of the burner is critical to fired heater design.

High destruction efficiency of waste hydrocarbons cannot be achieved unless the flue gas is maintained at a high temperature for 1 second in a refractory lined chamber before contact with the waste heat boiler. For this reason, the burner flame in a thermal oxidiser cannot be located close to the boiler tubes. However, in a fired heater, the heat transfer surfaces must be close to the flame in order to efficiently heat the process fluid.

Recent advances in ultra low NOx burner technology for fired heaters have focused on reducing the flame temperature via flue gas recirculation. The cool heat transfer surfaces near the flame in a fired heater burner make this method of thermal NOx reduction possible. However, this method is not possible in a thermal oxidiser where the entire combustion chamber is refractory lined and thus cool flue gas for recirculation into the burner is not available.

The only way to employ flue gas recirculation for NOx reduction in a thermal oxidiser is external to the burner and oxidiser chamber. This is commonly known as, “external flue gas recirculation”. A slip stream of the flue gas downstream of the boiler can be recirculated back to the burner via a recycle fan. However, the ducting, electrical power usage, larger plot space required and more complicated burner make this approach quite expensive and therefore rare.

The NOx emissions from a thermal oxidiser will depend on various factors such as the allowable CO emission limit, the CO content in the tail gas, the fuel gas composition, the waste gas composition and burner design. Typically, the NOx emissions will range from 0.06 to 0.1 pound of NOx per 1,000,000 Btu of heat released.

Although flue gas recirculation cannot be used to lower NOx emissions, UOP Callidus has developed a “tool kit” of thermal oxidiser burner design techniques over the years. All these methods involve a way to lower the flame temperature and may include such things as staged fuel gas combustion, water injection and steam injection. These techniques have been used for many years by several companies. UOP Callidus also has several other proprietary techniques to reduce NOx levels. These designs involve the various ways that tail gas can be used to cool the flame. Care must be taken in how tail gas is used to cool the flame. If the flame is cooled too much, CO emissions may increase. Experience, coupled with computational fluid dynamic modelling can result in designs that can lower NOx by 10 to 20%.

Corrosion
Due to the presence of SOx in the flue gas, care must be taken to prevent the steel shell of the oxidiser from getting below the dew point of the acid gas. Sulphuric acid can condense at very high temperature in flue gas. The acid gas dew point in the flue gas from a sulphur tail gas incinerator is typically between 300 and 350ºF (177ºC). Since thermal oxidisers are typically carbon steel with a castable (or brick) refractory lining, care must be taken to keep the steel temperature about 50ºF (28ºC) above the dew point in order to avoid corrosion from condensing acid gas. If it is not possible to keep the steel hot enough, a coal tar mastic lining or other high temperature acid resistant paint can be applied to the internal surface of the carbon steel shell before the refractory is installed. Although coatings are helpful, it only takes a small imperfection in the coating to allow acid to penetrate and cause corrosion. Therefore, maintaining the temperature of the steel above the dew point is the most reliable approach to corrosion prevention.
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