Ametek Process Instruments - THERMOX

The Other Side of Zirconium Oxide case study


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Regular Fuel Efficiency Applications
Separate Oxygen and Combustibles Sensors In standard flue gas applications for Thermox analyzers, the user generally tries to extract the maximum energy from the fuel and transfers as much of that energy as possible to the process. These applications require the process to be operated as efficiently as possible by keeping both oxygen (excess air) and combustibles (incompletely burned fuel) to a minimum. The best way to do this is to measure both oxygen and ppm combustibles simultaneously, using two separate detectors in one analyzer. Thermox WDG-IVC and WDG-HPIIC analyzers have both oxygen and combustibles detectors. Here the term combustibles describes the background amounts of CO and H2 which occur during normal combustion, typically 50-200 ppm.

What applications maintain excess air conditions?

Most standard heating processes including boilers operate with excess air (excess oxygen)—for example:

  • Steam or power boiler
  • Process heater
  • Sludge incinerator
  • Reformer furnace
  • Recovery boiler, etc.

Excess Fuel or High Millivolt Applications Oxygen or Combustibles from ZrO2
Some applications use the flue gas not only to generate heat, but also to alter the properties of the product exposed to it. Here fuel
efficiency is not as important as the oxidizing or reducing properties of the flue gas. This is called heat treatment and is used on many metals, ceramics, glasses and other materials. By adjusting the air/fuel ratio to the burner, the flue gas is controlled to be as oxidizing or reducing as needed. The exact air-to-fuel ratio which gives theoretically perfect combustion is called stoichiometric (approximately 9.5 parts air to 1 part fuel for methane). Using more air than the stoichiometric amount creates an oxidizing effect; less air than stoichiometric creates a reducing effect.

This latter condition is known as sub-stoichiometric (other terms include excess fuel, reducing or Lambdaglass. In some applications, such as bright annealing, a highly reducing atmosphere is required, but the product would be damaged by carboncontaining gases so a flue gas from fossil fuels cannot be used. In such cases a mixture of nitrogen and hydrogen (or cracked ammonia) is commonly used. In many applications, it is easier and less expensive to use a flue gas as the furnace atmosphere. The zirconium oxide cell can also be used to follow sub-stoichiometric as well as oxidizing atmospheres.

What applications run under excess fuel conditions?
In the following applications, it may sometimes be desirable to run sub-stoichiometrically and to have some way of knowing at all
times how oxidizing or reducing the process is:

  • Heat treatment of metals
  • Wire manufacturing
  • Heat treatment of ceramics and porcelains
  • Primary stage of a thermal oxidizer (incinerator)
  • Manufacture of specialty glass, e.g. colored glass
  • Primary stage of a low NOx burner

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