Advances in Chemical Recovery

As the industry moves toward closed-mill systems, research in chemical recovery is focused on reducing emissions, minimizing recovery costs, and limiting the buildup of nonprocess elements, which can increase particle deposition on recovery equipment and interfere with its performance.

Current Research

Control of evaporator fouling

Increased deposition fouls evaporator surfaces and reduces capacity. The two main types of fouling in black liquor evaporators are soluble scale deposition in the higher solids effects and concentrators and calcium carbonate deposition in the medium- to high-temperature effects. Most of our work is focused on reducing soluble scale deposition in falling-film units. As we studied the solubility of sodium carbonate and sodium sulfate in black liquor, we found that fouling tends to occur in the evaporator effects where the transition from burkite precipitation to sodium carbonate precipitation occurs. Collaborative work with Georgia Tech in this area is focused on understanding the crystallization behavior of sodium sulfate and sodium carbonate from black liquor.

Recovery boiler capacity

Because the chemical recovery boiler is the single most capital-intensive component in the kraft pulp mill, we are conducting research aimed at increasing boiler capacity and reducing the need for new boiler equipment. Work continues on controlling the formation of fine particles along with studies on the composition of these particles, their deposition on heat transfer surfaces, and the hardening of deposits. The research has defined a new category of particles in the 10-30 micron size range. These particles are formed during char burning, apparently by fragmentation of the char particles. The mass of particles formed increases rapidly with char-bed temperature. At typical char-bed temperatures, the mass of these particles formed is about the same as the amount of "fume" (submicron condensation aerosol particles) generated.

The composition of the fine "fume" particles in recovery boilers depends upon both black liquor composition and the concentration of sulfur dioxide in the combustion gas along the gas path in the boiler. Black liquors higher in potassium and chloride generate "fume" particles with higher concentrations of these elements. Higher bed burning temperatures increase the total "fume" generated but decrease the concentrations of potassium and chlorine volatized. High sulfur dioxide concentrations in the upper furnace and superheater region react with carbonate and chloride in the "fume" particles, converting them to sulfate.

Work on sintering and hardening of deposits in recovery boilers shows that dust composition plays an important role in how fast these deposits harden. Deposits high in chloride and potassium harden rapidly, while those with no carbonate or chloride are the slowest to harden. Sintering and hardening of all dust compositions increase rapidly with temperature. Work continues to determine the role of sulfur dioxide concentration on fine particle composition, sintering, and hardening.

New projects focus on improving black liquor combustion and controlling metals emissions from recovery boiler stacks.

Control of nonprocess elements

As fresh water use is reduced in pulp and paper mills, one consequence is increased scale and chemical use due to the buildup of both organic and inorganic matter in process streams. In this project, we are developing tools to predict the accumulation levels of inorganic nonprocess elements in kraft pulp mills and bleach plants. These inorganic elements are often bound to wood pulp fibers or complexed with dissolved organic matter. Therefore, our approach is to characterize quantitatively both the solubility of the inorganic matter as inorganic species and organometal complexes and the adsorption of metal ions on wood pulp fibers.

The tool that has been developed for this work is a chemical equilibrium predictor for pulp- mill and bleach-plant streams. It includes databases with free energy data and activity coefficient parameters for the inorganic species of interest (the carbonates, sulfates, sulfides, hydroxides, and silicates of Al, Ca, Fe, K, Mg, Mn, and other trace metals), for soluble wood-derived organic species and their organometal complexes, and similar data that describe adsorption of metals on wood pulp fibers. The tool has been evaluated as a predictor for the solubility of both high-concentration sodium and potassium salts as well as trace species in pulping and bleach-plant liquors with good results. The tool has been coupled with a commercially available process simulator, IDEAS, for evaluating strategies to manage the buildup of nonprocess elements in pulp-mill and bleach-plant process streams.

Odor reduction using green liquor dregs for TRS (Total Reduced Sulfur) removal

Our early results indicate that green liquor dregs can reduce hydrogen sulfide emissions in a waste gas stream by a factor of 1000. The goal of this project is to move toward implementation by conducting a pilot study at a kraft mill where actual TRS emission sources will be treated. The mill-scale trial will look at several different TRS point sources, including the digester blow tank, evaporator hotwell, recovery boiler stack, dissolving tank vent, slaker vent, and lime kiln exhaust.

Electrolytic causticizing of kraft smelt

A process for direct electrolytic conversion of the sodium carbonate in kraft smelt to caustic has been evaluated in a one-year study and was shown to be feasible. This single-step electrolytic process, invented previously by researchers at IPST and Georgia Tech, would replace conventional equipment between the smelt spout and the digester with a unit similar to an aluminum smelter. Advantages include a 40% reduction in energy demand for causticizing, elimination of the equilibrium limit to causticizing, and elimination of sulfur gas and dust emissions with elimination of the lime kiln.