ENOTEC – Solutions for drying processes

AQUATEC measures the oxygen content in industrial drying processes in air or non-flammable gases (flue gases). The water vapor concentration is proportional to the displaced amount of oxygen. The exact water vapor concentration can be calculated using the oxygen reference value. If drying does not take place in ambient air, the oxygen content in the process is measured and analyzed before the actual drying takes place.

In addition, ENOTEC products offer a wide range of applications in various incineration plants, as they can be used in flue gas temperatures of up to 1,400 °C, with high dust loads and in corrosive or reducing exhaust gases. The spectrum ranges from coal and gas-fired power stations, waste incineration plants, shipbuilding, refineries and crematoria to cement and steel works and the chemical industry. Wherever combustion needs to be controlled efficiently, quickly and precisely.

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Drying with hot flue gas:

In a hot gas generator, hot flue gas is generated by a burner. As this flue gas contains more or less oxygen, depending on the excess air from the burner, the O₂ value must be measured at the inlet to the drying system and after the drying system. In the example above, the inlet value measured with AQUATEC 1000 is 15 vol. % O₂ and the value downstream of the drying system is 7.5 vol. % O₂.

The reduction in O₂ can only have been caused by an increase in water vapour in the flue gas mixture. As the O₂ value has halved in this example, the water vapour content in the flue gas/water vapour mixture after the dryer is 50 % by volume.

If the material to be dried is material from which CO and / or hydrocarbons escape during drying, the COe content must also be measured with a COMTEC 6000 probe to ensure that the reduction in the O₂ value is not due to a smouldering fire (e.g. CH₄ or CO).

This application note describes the need to measure O₂, humidity and COe at different installation locations within a drying process (drying of organic material such as coal, waste, sewage sludge, wood, etc.).

The free oxygen in vol. % is a measure of the inert mode of operation, i.e. the O₂ content measured ‘in situ’ must be below a certain limit value (e.g. lignite or peat less than 8 vol. %). If the product to be dried (e.g. sludge) contains a lot of water, this water is vaporised and displaces the air, which significantly reduces the O₂ content in the water vapour/exhaust air mixture: depending on the tightness of the system and the water content in the sludge, down to 0 %. Coal dryers, for example, have a water content of 50 to 70 % in the dryer exhaust air.

In order to accelerate the drying effect, a warm flue gas mixture (hot gas generator or hot flue gas from the incineration plant) is mixed with a partial flow of the dryer exhaust air, as shown in the picture above, or the dryer is heated indirectly, e.g. with heat transfer oil.

Measurement task: In principle, the hot ZrO₂ sensor can only be used to measure in inert (non-flammable) gases. If only the water molecule is evaporated from the product to be dried, the ambient air in the dryer (with 21 vol. % O₂) is diluted with water vapor molecules (H₂O). If, for example, 50 % of the ambient air has been displaced by H₂O molecules, the mixture of 50 % air and 50 % H₂O contains only half the atmospheric oxygen (10.5 % by volume O₂). If the temperature in the dryer volume is above 100 °C, 100 % water vapor may also be present and the O₂ measured value drops to 0 %. Cell Surface Protection (CSP):

Drying with hot air:

If hot dry air is used for drying, which is heated indirectly with a heat exchanger, an AQUATEC 1000 O₂ measurement is required downstream of the drying system.

In this case, the O₂ inlet value can be assumed to be 20.95 vol. % O₂. Here too, the value of the reduced O₂ is a direct measure of the water vapor content in the outlet air. If dry air is not used as the drying gas, an AQUATEC measurement must also be installed upstream of the dryer.

During the drying of sewage sludge, a large number of hydrocarbons evaporate, so that organic molecular structures are also present in the H₂O exhaust air mixture in the range of several 100 ppm. This, together with a low O₂ content, can destroy the sensor due to the reducing conditions.

ENOTEC has developed ‘CSP’ as a protective circuit. If the mV sensor signal rises above 120 mV (0.1 vol. % O₂), the ‘classic’ voltage sensor (O₂ change generates a voltage (mV) that increases with decreasing O₂) is operated as a current sensor. In current mode, oxygen ions are ‘pumped’ from the reference air side to the sample gas side by an applied pump voltage until the mV voltage signal of 120 mV remains constant.