Application note

Flow control in combustion analysis

Accurate & reproducible flow control

An OEM manufacturer of elemental analysis equipment requested Bronkhorst to provide a solution for the controlled supply of gases to the combustion oven and the detection part in combustion analysis. Accurate and reproducible gas flows of the carrier gas are crucial here for the combustion.  

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What is combustion analysis?
Within the field of elemental analysis, combustion analysis is a technique to measure the total content of elements such as carbon, hydrogen, nitrogen, sulphur and oxygen in a sample. It can be used to clarify the behaviour of plastics, to establish the quality of (bio)fuels, to confirm the nitrogen content in soil or to verify the quality of food. In combustion analysis, the sample is burned with oxygen and the combustion gases are swept along to a detector for determining the total amounts of these gases and elements.

Flow control in Combustion analysis for soil sample

Application requirements for combustion analysis

To allow a controlled combustion, oxygen as a reaction compound needs to be dosed accurately. Accurate and reproducible helium flows are necessary as carrier gas for the combustion products to the detector. Furthermore, flow controllers are required that can cope with small sample quantities, and that are small themselves to reduce the equipment footprint in the analytic lab.

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Important topics

  • Accuracy and reproducibility of elemental analysis
  • Small footprint of flow devices
  • Low costs

Process solution for flow control

The Bronkhorst solution consists of three mass flow controllers of the EL-FLOW Base series, that are controlled by a PLC inside the analysis equipment. A sample is accurately weighted and fed into the combustion oven of the analyser. One of the mass flow controllers is used to supply oxygen as a reactant in the combustion process. Oxygen needs to be dosed accurately to save catalyst, and the flow through the combustion oven needs to be constant

In the combustion oven, the elements carbon, hydrogen, nitrogen and sulphur are combusted to their respective oxidised forms CO2, H2O, NO2 and SO2

Flow scheme Hydrogen storage in metal hydride
Flow scheme

The second mass flow controller introduces a helium flow into the combustion oven, which serves a as carrier gas for these combustion gases. Water (H2O) is removed in a water trap downstream of the combustion oven, and the helium flow transports the combustion gases through a gas chromatographical (GC) column which separates these constituents from each other. Then the gases are conveyed to a thermal conductivity detector (TCD) to quantitatively determine the amount of each of these gases, and in turn of the corresponding elements. The third mass flow controller supplies a helium gas flow to the thermal conductivity detector.

For the analysis of oxygen in a sample, the analyser needs to be operated in a reducing mode without supplying gaseous oxygen. Helium is still used here as sweep gas. The oxygen is converted into a compound that is separated in the chromatographical (GC) column to be detected by the thermal conductivity detector.

To be able to accurately determine the composition of very small samples, the Bronkhorst devices, EL-FLOW Base and MANI-FLOW solutions, can accurately and reproducibly control small gas flows in the range of 100 ml/min. Moreover, the compactness of the gas flow devices themselves makes them a very useful part of the analysis equipment to reduce the footprint in the lab, savings costs.  

Prior to using flow controllers, the analysis equipment applied a pressure difference as a driving force for the combustion gases to be transported through the process. Indeed, the combustion analysis process is more accurate by using flow devices.

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