Tailgas Thermal Oxidation
Regardless of the type of sulfur recovery process used (Modified Claus Process, Cold Bed Adsorption Process, or Modified Claus Process with Tailgas Cleanup), thermal oxidation of the tailgas is almost always the final processing step. Although catalytic oxidation of hydrogen sulfide (H2S) and other sulfur compounds is possible, thermal oxidation is by far the most widely used process due to its lower capital cost. In addition, the lower utility consumption of the catalytic process can often be offset by adding waste heat recovery to the thermal process.
Although many sulfur plant designers are experienced at designing thermal oxidizers to destroy H2S, recently there has been much attention focused in the U.S. on other contaminants found in sulfur plant and TGCU tailgas, such as carbon monoxide, nitrogen oxides, and hydrocarbons (particularly aromatic hydrocarbons). Field testing has shown that thermal oxidizers designed for H2S destruction are often woefully inadequate for destroying these other contaminants.
Ortloff has helped several plant operators to determine the source of these heretofore overlooked contaminants, and to redesign their thermal oxidizers to destroy the contaminants. As a result, Ortloff is uniquely qualified to design thermal oxidizers for compliance with today’s more stringent environmental regulations. Our thermal oxidizers are designed with the proper attention to feed introduction, burner design, operating temperature, and residence time to ensure almost complete destruction (95% or higher) of all these contaminants.
Thermal Oxidation without Waste Heat Recovery
For small thermal oxidizers (usually 20 LT/D or smaller), the capital expense of waste heat recovery is usually not justified by the value of the steam that could be produced. For these plants, a conventional incinerator is normally used.
1 – Heat is usually provided to the thermal oxidizer by combustion of fuel gas in a natural-draft burner, although forced-draft burners are sometimes used. The fuel gas flow rate is adjusted to maintain the furnace temperature at 1000°F or higher to ensure complete incineration of the sulfur compounds. Higher temperatures are usually required if carbon monoxide and and/or hydrocarbons in the feed to the thermal oxidizer must be incinerated. The burner operates with excess air to provide the free oxygen needed to incinerate the tailgas.
2 – The tailgas mixes directly with the burner effluent to heat the gas and mix it with the oxygen leaving the burner. The incinerator stack often includes a larger diameter bottom section to provide residence time for the high temperature oxidizing atmosphere to incinerate all of the sulfur compounds to sulfur dioxide (SO2).
3 – The hot effluent is then dispersed to the atmosphere from the top of the stack. The stack is normally equipped with ladders, platforms, and sampling ports to allow periodic stack testing. Environmental regulations may also require continuous monitoring of SO2 emissions.
Thermal Oxidation with Waste Heat Recovery
Cooling the thermal oxidizer effluent before dispersing the gas to the atmosphere is an efficient method of producing steam. If there is a use for the steam generated, the cost of the waste heat boiler can normally be justified by the savings in boiler fuel due to the steam produced from this “waste” heat, and by the cost savings from eliminating the refractory lined stack. (An externally insulated stack is used to disperse the effluent instead.)
Ortloff pioneered the use of waste heat boilers on the effluent from tailgas thermal oxidizers in the early 1970’s. Water-tube boilers are used to efficiently recover the maximum amount of heat, while careful design and attention to detail avoid potential sulfuric acid attack in the boiler or the downstream vent stack. Since sulfur plant tailgas and TGCU unit effluent ordinarily contain a considerable amount of combustibles, it is often possible to generate more steam in a thermal oxidizer waste heat boiler than is possible burning the same amount of fuel gas in an ordinary boiler. The incremental income from the steam produced will typically pay for the extra capital investment in less than a year.
1 – Heat is provided by combustion of fuel gas in a forced-draft burner. The burner operates with excess air to provide the free oxygen needed to incinerate the tailgas.
2 – The tailgas mixes directly with the burner effluent to heat the gas and mix it with the oxygen leaving the burner. The mixture enters a refractory-lined furnace to provide residence time for the high temperature oxidizing atmosphere to incinerate all of the sulfur compounds to SO2.
3 – The hot furnace effluent enters a water-tube boiler to generate high pressure (250 PSIG or higher) steam as the gas is cooled.
4 – The cooled boiler effluent is then dispersed to the atmosphere from the top of an insulated stack. Either the stack or its inlet line is normally equipped with sampling ports to allow periodic stack testing. Environmental regulations may also require continuous monitoring of SO2 emissions.
The decision of whether or not to apply waste heat recovery to thermal oxidizers depends on plant size and utility costs. Ortloff has performed many economic evaluations comparing thermal oxidizers with and without waste heat recovery, allowing us to quickly advise our clients on the optimum choice.