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Making the right selection

Hydrocarbon Engineering,

It is becoming increasingly important to save energy while reducing harmful emissions when using a pollution control device such as a flare stack. In refineries and chemical plants, proper flare stack design is essential to handle multiple waste streams for maximum destruction performance with minimum emissions. Since destruction efficiency and emissions are important criteria to consider when designing a flare, when the design is done correctly, dramatic operating savings will also be realised.

Waste streams are collected from the different processes around the refineries and chemical plants, and are sent to the flare stack for destruction. US EPA code 40 CFR 60.18 states that for optimum destruction efficiency in the flare, the waste stream must run at a minimum lower heating value (LHV) of between 200 - 300 BTU/ft3. Continuous monitoring of the waste stream is necessary to identify the minimum heating value and ensure proper combustion efficiency. In addition, by identifying the minimum heating value it can be determined whether the waste stream can be used as a standalone fuel source.

Types of measuring technologies

Several different technologies are available for measuring the heating value in a flare stack including:

  • Gas chromatography.
  • Thermopile.
  • Residual oxygen.
  • Microcombustion calorimeter.

Of these possibilities, the microcombustion calorimeter is the best choice.

Gas chromatographs are the most expensive to purchase and operate and not a suitable choice for this application. The sample is injected through a column and each gas passes through with a carrier gas at a different rate. The compounds separate out, depending on their retention time, which determines the gas composition. This method does not measure the heating value but rather speciates the sample. The GC is a batch type (not continuous) instrument with slow response time: the sample must travel through a column and this can take as long as 20 min or more for each sample. Another limitation: GC’s cannot measure heavy hydrocarbons.

Thermopile calorimeters mix and burn sample gas and air. Air flow is then regulated to maintain a constant exhaust temperature. Airflow variations provide an output signal that is proportional to the Wobbe Index. The Wobbe Index is the heating value divided by the square root of the specific gravity. A calculated (indirect) heating value measurement is provided only when an optional specific gravity meter is added. The biggest limitation to Wobbe Index is the measurement of fuel mixtures that are not merely natural gas. If heavy hydrocarbons are part of the gaseous stream additional challenges arise. This technology is susceptible to flameouts and is large and bulky with many moving parts increasing the maintenance burden.

Residual oxygen combustion calorimeters measure the excess oxygen content after combustion of the sample. Sample gas and air are heated and premixed and then enter a combustion chamber. Any residual oxygen is measured by a zirconium oxide or echem cell. The residual oxygen content provides an accurate measurement of the Combustion Air Requirement Index (CARI ) and correlates to the Wobbe Index. In order to obtain the heating value an optional specific gravity cell has to be added, hence not a direct measure of the heating value. In addition the system is large and bulky and subject to pressure variations.

Microcombustion calorimeters provide a direct measurement of heating value. Fuel is premixed with the process sample and completely incinerated by a carefully metered flame. A thermocouple measures changes in the flame temperature. An increase in the temperature is directly proportional to the heating value. This analyser provides a continuous, direct measurement of the heating value with a very fast response time. It has a universal response and is not susceptible to flameouts or pressurised systems.

Control Instruments’ CalorVal BTU Calorific analyser is a microcombustion calorimeter. Because of its unique construction and operating technology, it is the optimum analyser for directly measuring the heating value of varying waste gas streams for flare stack applications. Rugged and reliable, the CalorVal is built on a time tested field proven design, capable of withstanding the rigors of the flare stack environment.

Mounts at the base of the flare

The CalorVal is a compact design that can mount at the flare header, eliminating long and expensive heated sample lines and the need for a pump or other sample conditioning components. Mounting at the sample point decreases the sample transport delivery time resulting in the fastest response time possible. This allows the CalorVal to quickly respond to the flare stack heating value and adjust its fuel source as needed.

Fully heated assembly

A fully heated assembly prevents condensation of water vapour and other heavier less volatile hydrocarbons. Keeping all sample wetted parts of the sampling system and analyser at a high temperature will ensure that all combustible vapors are properly measured, eliminating inaccurate readings. The sample stays intact during measurement.

The analyser further avoids condensation and maintenance problems through its simple flow system. The CalorVal collects the sample using an aspirator driven system. There is no pump or other moving parts. This simple and extremely effective design requires very little maintenance, and its performance is unaffected by water, corrosives or other compounds in the sample stream.

Direct measurement/universal response

Unlike other sensor technologies, the CalorVal was developed to directly measure the total heating value. The proprietary technology provides the ability for the analyser to accurately measure a variety of combustible gases even though the analyser was calibrated on one specific gas, giving excellent cross calibration accuracy and minimizing reading errors.

The CalorVal gives a uniform response to a wide range of combustible gases and vapors, including heavy hydrocarbons, carbon monoxide, hydrogen and numerous other compounds found in waste gas streams. In addition, the presence of non-combustible compounds such as carbon dioxide, nitrogen and water vapour do not interfere with the BTU readings of the remaining combustible compounds.

Written by Control Instruments Corporation.

Edited by Claira Lloyd

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