Recovery boilers benefit from multivariable control

“It’s possible to configure multivariable control so that both steam-flow and firing-rate limits are respected.” International Paper’s Andrew Jones discussed model-predictive rate control at this week’s Foxboro User Group conference in San Antonio.

“The heart of a paper mill is the recovery boiler. It must run well for the mill to run well,” said Andrew Jones, senior engineering fellow, International Paper. The boiler provides power, steam and recovery of chemicals, and it reduces the toxicity of black liquor by burning it, with efficient recovery to steam. “But if it’s not done correctly, the boiler fouls, plugs up and may shut down the mill for several days.”

Jones spoke to attendees of his session, “Comparison of recovery boiler rate control strategies,” at the 2018 Foxboro User Group conference this week in San Antonio.

“Boiler control becomes a balance of efficiency and reliability,” said Jones, who’s worked with various incarnations of Foxboro, Invensys, AVEVA and Schneider Electric systems for more than 12 years, using multivariable predictive control (MPC) and various control schemes.

Jones described four ways to control the flow of black liquor to the burners: volumetric flow rate control, dry solids firing rate control, heat input (steam flow) control and multivariable predictive control.

Volumetric flow rate control

“This is the traditional method, where control is based on upstream and downstream inventories of black and green liquor, with no consideration of the impact on the boiler due to variations in the composition of the black liquor,” Jones said. Firing conditions may vary considerably with liquor strength (65% to 80% solids, with the balance mainly water). The heating value is not considered, which varies with the species and operating conditions, as well as percent solids, typically 6,000 ±300 Btu/lb. Auxiliary fuel, usually natural gas, may be added as needed to improve stability and boost steam output.

“Volumetric flow rate control carries the risk of overfiring, which can foul and plug the boiler,” Jones said.

Dry solids firing rate control

“Many mills have changed over to dry solids firing rate control, and all of our mills run at least this level of control,” Jones said. The dry solids firing rate is the product of volumetric flow rate, liquor density and weight fraction of solids in the black liquor solution. Liquor density can be measured online but is typically estimated using an empirical equation where density is a function of liquor temperature and solids fraction. “Dry solids control does well as a green solids inventory control, but you may need to vary the flow to account for black liquor variation,” he added.

Liquor strength (percent solids) are taken into account, but the heating value, the organic vs. inorganic solids and the auxiliary fuel are not, so steam output at a given flow rate may vary considerably, and “there is still a risk of overfiring, but it’s an improvement over volumetric flow rate control,” Jones said.

Heat input control

This strategy is based on steam flow rate, “so it’s harder to implement,” Jones said. The concept is to control black liquor flow rate on steam rate. It allows the boiler to run closer to its maximum continuous rating (MCR), but it’s difficult to implement with simple PID control because of steam header pressure fluctuations and downstream disturbances. “In cases where the black liquor is weak, it can overfeed, which can cause poor flame quality or even a blackout,” he said. “So, you may also need a blackout control.”

On the plus side, black liquor strength, organic vs. inorganic content and auxiliary fuel are all taken into account, and there is no risk of overfiring, providing limits are set correctly.

Multivariable control

Here, heat input (steam flow) control is implemented with additional variables taken into account, typically firing rate (dry solids). “It’s possible to configure multivariable control so that both steam flow and firing rate limits are respected,” Jones said. “In a typical configuration, the active limit would be the firing rate, but if the MCR is reached, the controller would keep steam flow at MCR and lower the firing rate.”

This control strategy also can allow for the effects of other variables. “For example, on an older boiler with manual port rodding, we were able to control for changes in behavior that happened when the ports were rodded out,” Jones said.

To explore the effects of different control schemes on real equipment, International Paper compared efficiency and fouling potential on four boilers before and after implementing multivariable control.

Steam flow control implemented as part of a multivariable predictive controller increases thermal efficiency of a recovery boiler by stabilizing combustion. An increase of 1% to 4% in black liquor-generated steam production was observed on four recovery boilers. “An increase of 4% in steam flow efficiency easily justifies the cost of the system, even without the increased capacity,” Jones said.

Multivariable steam flow control reduces standard deviation of steam production by 10% to 30%, and eliminates MCR excursions except for short transients, which reduces the potential for fouling and plugging, increasing reliability.

On the other hand, “volumetric control does have the advantage of making a consistent droplet size and trajectory,” Jones said. “If other, higher-level schemes are having to vary volumetric rates significantly, this could detract from boiler stability. So, it is still good practice to have a percent-dry-solids control loop to reduce the need for large changes in volumetric flow.”