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Tail Gas Incineration

The Direct Fired Heater in VMGSim offers a wide variety of options to quickly model combustion systems, including gas incinerators. For example, it allows you to directly specify the outlet temperature and oxygen concentration and it automatically back calculates the fuel and air flows. This unit operation was designed for Claus plant applications but it can be used with any property package. This article describes the capabilities in VMGSim to model a thermal incinerator from a sulfur plant.

This article describes the capabilities in VMGSim to model the material and energy balance of an incinerator for the waste gas from a sulfur plant. The final part of this article describes the steps required to implement a more rigorous kinetic model that becomes a requirement when trying to optimize the operation. The article focuses on the Claus property package but the same methodology can be used for other gas feeds.

Background

The main purpose of a sulfur plant is to convert H2S to elemental sulfur. The tail gas from this process contains mainly H2O, CO2 and N2 but it will also contain residual amounts of sulfur components such as H2S, COS, CS2 and SO2 along with CO and H2. The content of these components depend on the acid gas being treated and the performance of the different units treating the gas.

Environmental regulations require the destruction of the sulfur components before releasing to the atmosphere. This final destruction of the contaminants is commonly done in an incinerator that discharges into a tall stack that releases the products to the atmosphere.

The typical waste gas going to an incinerator does not contain enough combustibles to achieve the temperatures required to sustain a flame and achieve effective destruction of the contaminants. A separate fuel stream is necessary for the proper operation of the incinerator. This fuel stream typically is burned with air at the inlet of the unit and is then mixed with the waste gas for incineration.

The two key operational parameters of an incinerator are temperature and the oxygen concentration in the outlet. The main goal is reach ppm levels of H2S, COS and CS2 depending on the jurisdiction while using the least amount of fuel. A thermal incinerator operates at around 1000-1200 F (550-650 C) (1) but under certain jurisdictions with tighter control for H2S and/or CO the operation may require much higher temperatures up to 1500 F (800 C).

Simulation

For this example we will use the Claus property package and a fuel, air and tail gas streams as follows:

Acid_Gas_1.png

For the incinerator we will use a Claus Direct Fired Reheater from the Claus stencil (this unit operation can be used with any property package). Go to the Settings tab and make sure the “Process Side is Reactive”. Connect the streams to the corresponding ports and the unit operation will solve right away by using Gibbs minimization.

Acid_Gas_2.png

Acid_Gas_3.png

The outlet O2 composition is 2.69 % and the outlet temperature is 1080 F. Assume we want to optimize the operation and we want an outlet of 2 % O2 and at 1040 F (this value assumes that the temperature will drop 40 F in the stack before it goes to the atmosphere at 1000 F). This can be simply done by removing the flow specifications for air and fuel and specify these values directly in the form:

Acid_Gas_4.png 

VMGSim will automatically do all the internal iterations to calculate the flows of air and fuel. These changes in specifications result in a lower consumption of fuel and air.

So far, we have seen how in very few steps we can get a complete material and energy balances around an incinerator. These calculations are all based in equilibrium and should not be used for detailed prediction of emissions. Equilibrium values will assume that H2S, H2, CO, COS and CS2 will be consumed completely. In reality, H2S, COS and CS2 will be in trace amounts and should be closely monitored. CO will not be destroyed and for these simple calculations it may be more conservative to set it as non-reactive. H2 is a meaningful source for combustion energy and assuming complete consumption may lead to incorrect estimation of fuel flow.

Final Thoughts on More Rigorous Kinetic Modeling

A more rigorous approach requires the kinetics of the key components. Once the kinetics are gathered a suitable approach would be to stop the equilibrium reactions for the key components in the direct fired heaer (COS, CS2, H2S, CO):

 Acid_Gas_5.png

 The next step is to connect the outlet to a plug flow reactor (PFR) to model the incinerator kinetics and then connect to another PFR to model the stack. It is better to model the stack as a separate unit to allow for different geometry and for heat transfer calculations. A sample flowsheet would look as follows:    

 Acid_Gas_6.png

Acid_Gas_7.png

When using kinetic parameters it is always important to validate the data and if necessary tune the parameters for specific operation. Also keep in mind that even the best kinetic model will not take into account operation anomalies such as faulty burners or “channeling”.

Please contact your local VMG office for more information. 

Raul Cota, Ph.D., P.Eng., VP Research & Development

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