HF hitting the roof?

Redland Roofing Systems, at its Bedworth Works near Coventry, produces clay roofing tiles using technology that has not changed dramatically since Roman times. Tiles are manufactured by firing clay, which has been prepared and conditioned for pressing into the finished profiles. Since the days of togas, emperors and perfectly straight highways, however, stringent regulations governing the release of toxic gas emissions to the atmosphere have been imposed.

The kiln typically runs at a temperature of 1,100°C. The flue gases that leave the kiln stack at temperatures between 100°C and 200°C contain hydrogen fluoride (HF) gas, which is evolved during the clay firing process in the kiln. Fluoride occurs naturally in clay, so the emission level during the firing process is dependent on clay type. EC regulations have assigned a maximum permissible concentration for HF releases to the atmosphere of 10mg/m3.

To ensure that the scrubber system is always operating at its optimal efficiency, the flue gases from the firing kiln need to be continuously and accurately monitored and, where necessary, excess HF must be removed before the flue gas can be discharged. This brings an added benefit to Redland in that it is always in a position to demonstrate compliance with the new limit values.

Laser monitor

HF and other acid gases in stack emissions can be effectively removed using absorption processes. The acid gas is typically removed during scrubbing by reaction with lime or calcium carbonate to form calcium fluoride.

Redland Roofing Systems needed an instrument which was robust and had a proven track record for effective monitoring in hostile environments such as exhaust stacks. ETi recommended its NEO (Norsk Elektro Optikk) cross duct laser monitor for measuring HF gas. The NEO cross duct laser is a single component instrument containing a tuneable diode laser which gives single line spectral absorption at very narrow bandpass (0.1nm). This system eliminates all interference, so the detection limit for a 1m path length of HF is as low as 0.01mg/m3. With no moving parts, the instrument is extremely reliable and can also be used for monitoring HCl, NH3, H2S or O2 by selecting a laser light wavelength corresponding to the particular single spectral absorption line.

During the tile firing cycle HF concentrations peak for about ten minutes in every hour, when a fresh batch of tiles enters the kiln. When HF levels exceed 8mg/m3, the fast response NEO monitor will allow enough time to trigger the dampers to divert a greater proportion of the polluted flue gas to the scrubbing stage of the process. Hence the 10mg/m3 limit value is never breached. As these concentrations decrease below the threshold value, less of the flue gases are passed through the scrubber thus maximising the life of the scrubber bed.

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