In any combustion process, an amount of oxygen reacts with an amount of fuel. If the amount of oxygen exactly matches the amount of fuel, combustion is complete. This is called stoichiometric combustion.

When there is a shortage of oxygen, combustion is incomplete and a mixture remains that still contains fuel. That is combustible. With this incomplete combustion of hydrocarbons, not only carbon dioxide but also toxic carbon monoxide remains. A shortage of oxygen is therefore inefficient and dangerous.

An excess of oxygen results in soot formation, which causes problems with emission requirements. A second disadvantage of too much oxygen is that the air is heated. This is at the cost of process energy and therefore less efficient. Controlling the combustion process to keep the chemical reaction stoichiometric is therefore very important.

Photo: This burner is controlled very accurately based on stoichiometric calculations.

Output-based proces management

An excess of oxygen is only inefficient; a deficiency is dangerous as well as inefficient. That is why every controlled combustion process is regulated with an excess of oxygen of a few percent. For this control an oxygen meter is placed in the flue gas channel. When the oxygen percentage is too low, air will be added. When the oxygen percentage becomes too high, the amount of air will be reduced.

Proces control based on input

For quiet combustion processes, end result control is a fine solution. There is one important variable that is not taken into account: all stoves are leaky. As the chimney draws, there is a danger that false air is sucked in from outside and measured as residual oxygen in the flue gas. It is therefore worth serious consideration to regulate the fuel gas and combustion air volume based on stoichiometric calculations in addition to the oxygen measurement in the flue gas duct.

Thermal oxidizers

Controlling combustion to be stoichiometric does not only occur in classical burner stoves such as for steam generation or process heat. In almost every industrial plant, the measurement and control of stoichiometric processes in thermal oxidizers comes into play. These installations burn excess process gases to reduce harmful emissions. The thermal oxidizer burns process gases with a feed gas. This may be natural gas or LPG, but also residual gas from the process or biogas. The aim is to emit only CO2 and water.

For stoves where residual gases from processes are burned and where the composition can vary, a pro-active control loop will be built in in addition to the oxygen control. Thereby it is desirable to determine the composition of the fuel in addition to the quantity measurement. On this basis the amount of oxygen for stoichiometric combustion can be determined.

A regenerative thermal oxidizer designed to convert chemical and organic waste with a process of controlled oxidation based on energy recovery.

Photo: A regenerative thermal oxidizer designed to convert chemical and organic waste with a process of controlled oxidation based on energy recovery.

Which flowmeter for stoichiometry?

There are several options for measuring these types of gases. Thermal insert flow meters are used because they directly measure gas mass and therefore monitor the composition. For the fluctuation in the composition of the fuel gas, placing an inline gas chromatograph in the fuel gas line is also a good solution. Another interesting alternative is the use of an ultrasonic flow meter in the fuel gas line. In addition to the flow rate, this instrument also determines the VOS (Velocity of Sound). This can be combined with pressure and temperature, so that the average molecular weight and the composition of the gas can be calculated via models. In any case, the ultrasonic flow meter will be able to provide additional information about the fuel gas and make the control much more accurate. Measuring fuel gas also gives the overall system a much better response. In this way, efficiencies varying between 97% and 99.9% can be achieved.

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