Analytical Measurements Protect Recovery Furnace And Boiler In A Pulp and Paper Mill
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Recovery Boiler operations can be improved considerably by using continuous analytical measurements. The information they collect can be used to optimize black liquor conversion and energy extraction in the furnace without compromising safety and reliability of boiler tubes and other components. The Recovery Furnace oxidizes concentrated black liquor, thereby generating feedstock for green liquor and the rest of the Kraft process, and simultaneously producing steam for millwide use. The furnace is optimized by controlling excess combustion air levels to maximize smelt recovery, prevent corrosion, and maximize steam production. The boiler is optimized for longevity by monitoring the quality of water used to produce steam. This also protects the boiler tubes against corrosion and pitting due to harmful mineral deposits. Effective analytical measurements can assist in optimizing these operations. Additionally, accurate analysis of both combustion flue gases and boiler water can be used to prevent explosive conditions.
Furnace Operations
While traditional natural gas or oil burner arrangements may be used for boiler start-up, the black liquor reaction is exothermic, and once started, it is self-sustaining. The black liquor is viscous, and great care is exercised to maintain at least a 50-60% solids content. The liquor is injected from above through oscillating nozzles, and forms a molten bed in the bottom of the furnace. Fuel control is comparatively poor, and fuel/air ratio control is difficult. Partially pyrolyzed liquor can form localized pockets of explosive gases, so reliable gas analysis is important to not only maintain efficiency and optimize conversion, but also to prevent explosions.
In-situ zirconium oxide oxygen sensing technology works well in this application. Typically, the heated sensor is placed onto the end of a probe that is 3-9 feet long, minimizing problems involved with filter plugging, since there is no sample transport required. It should be noted that since the sensor is heated to approximately 1500° F, the sensing cell will burn any combustible components in the flue gases, and read the remaining O2. Excursions of high CO or other combustible components will result in a depleted O2 reading, so a low O2 alarm should be utilized in the DCS control system for optimum control.
A passive hastelloy filter (diffuser) protects the sensing cell from particulate matter, and can endure the high temperatures and corrosive attack inside the recovery boiler. The buildup of salt cake on this in-situ filter (diffusion element) can pose a problem over time, reducing the speed of measurement response. The best way of diagnosing a plugged diffuser is a flow test with calibration gas. This can be done on-line, but the boiler operator should be notified so that any control loop can be placed into manual for safety considerations. Use bottled calibration gas with a different O2 value than the normal operating range (for instance, 8% O2). Once the O2 readings have stabilized at the bottled gas value, remove the cal gas, and note the time it takes to return to the normal process value.
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