Black liquor is a waste liquid from kraft wood pulping. It contains used inorganic chemicals and organic wood residue. Black liquor is burned in recovery boilers to recover the pulping chemicals for reuse and to generate steam for power generation and mill consumption. Because of this dual function the operation of a recovery boiler is particularly demanding. Successful combustion of black liquor depends extensively on the characteristics of the droplets formed at the boiler firing nozzle. Droplet formation and size, in turn, depend on the viscosity of the liquor. Existing sensors and instrumentation do not allow on-line measurement of liquor viscosity. This is mostly because, at high concentrations, this liquid is very viscous, potentially non-newtonian, and highly scaleforming. A considerable amount of research on the properties and behavior of kraft black liquor has been sponsored by DOE. These studies have revealed the dependence of the drying and burning characteristics on the size of the droplet, and hence on the viscosity of the liquor. The research has also produced important new knowledge on the basic physical properties of black liquor. The development planned herein will be based on this work. A reliable means of measuring black liquor viscosity at high concentration will provide a major missing element for achieving direct, on-line control of the combustion process in recovery boilers. It will facilitate the use of higher-than-present liquor dry solids contents in the firing of recovery boilers. Because of the higher efficiency, a saving of other, primarily fossil, fuels will be realized. An additional benefit will be improvements in the operating safety of the boilers.Anticipated Results/Potential Commercial Applications as described by the awardee: There are several commercial and operational benefits to be derived from a black liquor viscosity meter. First, there are the energy savings due to optimal firing control of recovery boilers, in particular at black liquor dry solids contents higher than present practice. Today, most kraft recovery boilers are fired at 63-68% liquor dry solids. Recent developments in evaporation technology have shown the possibility of achieving 75-80% dry solids content. The increased boiler thermal efficiency, if the liquor with high dry solids could be fully utilized, converts into savings of fossil fuels. Second is the increased safety and reliability of kraft recovery boilers through better control of the firing process. Potentially, this would contribute to a reduction of the risk for unexpected, violent recovery boiler explosions. Finally, there will be savings in energy costs in other industries (oil, sugar, etc.) where accurate viscosity measurements provide an opportunity for improved process control. Topic 2: Advanced Industrial Sensors and Control Systems
