The viscosity of black liquor affects its handling properties and is a critical parameter for the control of a recovery furnace, in terms of the spray characteristics and the char bed size and shape. Many mills have increased the as-fired black liquor solids in order to improve their boiler’s thermal efficiency, increase liquor throughput and reduce environmental emissions. However, black liquor viscosity increases significantly at solids concentrations over 70%. If a mill is not equipped with a pressurized liquor handling system, in which high temperatures can be used to reduce viscosity, another viscosity reduction method such as alkali profiling may have to be used to facilitate high solids firing. There are other methods to reduce viscosity, such as liquor heat treatment or high temperature oxidation, but these involve capital cost. For more information on black liquor viscosity see my posts of May 26, June 9 and June 28, 2007.

One Canadian eastern mill has used temperature control to adjust viscosity [1]. The mill had problems with severe primary air port cracking and membrane bar corrosion. Investigation revealed that the severity of tube cracking and corrosion was related to the frequency and magnitude of the tube temperature excursions. It is known that there is a direct correlation between liquor viscosity and the current demand of the black liquor recirculation pump; i.e., the amperage can be used as an indirect measure of viscosity. The mill developed a system for controlling its black liquor viscosity based on this approach. The recovery boiler had been upgraded to have higher capacity and to burn liquor at as high as 76% solids. The intent was to fire at a solids content in the 74-76% range. To do this a 10 bar (150 psig) steam line was installed for the liquor heater, enabling the mill to increasing the firing temperature to 123 C. Because of the strong temperature dependence of viscosity, it was decided to install a control loop to maintain constant liquor sprays. The viscosity control was necessary, as changes in REA and wood species also affected the viscosity. The control loop was incorporated into the existing indirect liquor heater control system. A new in-line ammeter was installed on the liquor circulation pump, and a controller was used to monitor the amperage and control it at a set point. The output from the amperage controller was cascaded back to the existing cascade loop, which consisted of a temperature controller and a pressure controller. The steam pressure fed into the indirect heater was then adjusted, to bring the temperature of the liquor at the heater exit to the desired level; this maintained a constant amperage on the pump.

Subsequent tests showed that the control system worked well under high solids and high temperature conditions, and could handle process swings and respond well to changes in wood species. The mill reported that, since the installation of the control system, the liquor solids content has been consistently in the 73-75% range. The char bed has become more stable and the bed size remains in control. Operators still observe the bed size and shape to make sure the conditions are optimal. The mill also monitors thermal activity in the lower furnace, by recording the number of times the tube temperature spikes above 371 C and 482 C. Since the installation of the control system, the solids content of the liquor has increased to 73-76%, but the number of temperature spikes has significantly reduced compared to when the solids were 74-77%, and there was no temperature control. By lowering thermal activity, primary airport cracking has been diminished. The mill now has major shutdowns/inspection at an 18 months interval instead of 12 months.

1. McCabe, F.D., Mott, D., Savoy, D. and Tran, H., ”Controlling black liquor viscosity to improve recovery boiler performance”, pp. 427-432, Tappi 2007 Int. Chem. Rec. Conf., May 29-June 1, Quebec City, Canada.