Pulping methods and conditions have a pronounced effect on the viscosity of black liquor. For example, black liquors from (PS+AQ) pulping are lower in carbohydrate content, and are expected to have lower viscosities (if the liquor REA stays relatively constant). They also have lower organic/inorganic ratios and lower heating values. If liquor firing parameters are held constant, the likely effect in the recovery boiler is the formation of smaller droplets. Smaller droplets have a higher propensity for being entrained in the flue gas, and could contribute to carryover problems and boiler plugging. Reducing the liquor firing temperature by a degree or two can increase viscosity and remedy these problems. For boilers which have problems maintaining a hot bed, this may not be an option, as decreasing the liquor firing temperature could also lower the bed temperature. The alternative is to increase the viscosity by alkali profiling, or to adjust both the liquor gun firing angles and liquor firing temperature or pressure, to compensate for the changes in droplet size distribution. If the mill has excess evaporation and steam capacity, the black liquor solids can be increased to increase viscosity. This has the added advantage of increased thermal efficiency and liquor throughput, and reduced TRS emission.  

If the REA content is not controlled during the cook, higher viscosity black liquor may result. Alkali profiling, and firing at higher temperatures, are two available options for reducing liquor viscosity and any operating problems that might be encountered. When the firing temperature is increased, there is always the possibility of liquor flashing. Flashing decreases the flow of black liquor though a nozzle at a given pressure, and decreases the median droplet size of the spray, which can increase carryover and boiler plugging problems. 

Some mills fire soap with the black liquor; this will increase the viscosity of the liquor and lower its swelling index. High viscosity liquor would generate larger droplets upon firing. These slow-burning (lower surface area) larger droplets may reach the bed before drying, and cause blackouts or smelt-water contact. Increasing the firing temperature by a few degrees (depending on the level of soap) can reduce the liquor viscosity and improve the situation. High viscosity liquor will also reduce the capacity of the evaporators. 

The viscosity of black liquor determines its handling properties, and is a critical parameter for the control of a recovery furnace. Viscosity is also an important factor in black liquor evaporation, as it affects the heat transfer rate. An increase in viscosity can lower the evaporation capacity. Softwood black liquors (e.g., pine) generally exhibit near-Newtonian behaviour (i.e., viscosity is independent of shear rate) but some hardwood black liquors (e.g., birch) can be thixotropic (i.e., viscosity is both time and shear dependent).

The viscosity of black liquor is affected by such factors as the wood species, the solids content of the liquor, the cook’s kappa number, the residual alkali and sulphidity of the liquor, as well as the temperature. As high solids firing can improve a boiler’s thermal efficiency, increase liquor throughput and reduce environmental emissions, many mills have increased their as-fired liquor solids concentration. However, black liquor viscosity increases significantly at solids concentrations over 70%, where NOX emissions also increase. If a mill is not equipped with a pressurized liquor handling system, in which high temperatures can be used to reduce viscosity, a viscosity reduction method may have to be employed in order to facilitate high solids firing. A viscosity of 200 cp and over is considered high, since 200 cp is a common practical limit for centrifugal pumps.

The simplest and most cost effective way of keeping viscosity in an optimal range (if the solids are not too high) is by alkali profiling. All you need is a suitable benchtop viscometer for black liquor, and a reliable way of measuring residual effective alkali (REA) and black liquor solids. To control black liquor viscosity in real time, you would need an on-line viscometer (or an on-line probe which provides an indirect measure of viscosity) and an on-line REA probe in a control loop.