Oxygen and combustibles measurement in recovery boilers

August 2002 Analytical Instrumentation & Environmental Monitoring

The recovery boiler in the Kraft process is one of the more difficult analysis applications in industry. The determination of the levels of both oxygen and combustibles is critical. The process involves co-firing natural gas with concentrated black liquor. This is the liquid left over from cooking wood pulp in a digester. Burning this mixture produces large quantities of particulate matter, such as sodium sulphide and sodium carbonate. The bulk of these are recovered in the molten form, smelt, from the bottom of the boiler and re-used in the digester. Significant amounts of these compounds are carried along with the flue gas. This causes both a sampling problem and an environmental/safety problem.

To prevent high levels of particulate from leaving the boiler, some device must be used to collect them. Typically, this will be an electrostatic precipitator or a wet scrubber. We will concern ourselves with applications involving precipitators. Electrostatic precipitators are devices that rely on the mutual attraction between particles of one electrical charge and a collecting electrode of opposite polarity. These can handle large gas volumes at high efficiencies and temperatures with a very low pressure drop. These typically involve plates and wires as positive and negative electrodes. These will operate at very high voltages, 20-100 kV below ground potential, with 40-50 kV being fairly typical.

With these very high voltages present in the precipitator, combustible gases should be kept from entering the system. Due to the aggressive nature of this process, it is difficult to find an analyser that offers the response time and ruggedness that this application requires. The analyser is the Ametek Thermox WDG-HPIIC. This analyser uses an armoured probe and the unique oxygen and combustibles combination. The oxygen signal is used to control the air to fuel ratio. The combustible detector is used to monitor what actually reaches the precipitator.

(Extreme variations in black liquor BTU content would cause rapid fluctuations in the CO signal; this would cause problems in the precipitator.)

For more information: Mike Andrews, OEN Enterprises, 011 792 2859, [email protected], www.oenenterprises.co.za

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