Effect of Various Parameters in Recovery Boiler Operation
This part of the series deals with the various parameters of the chemical recovery boiler that are different from conventional coal and oil fired boilers. As the combustion in black liquor fired chemical recovery boiler takes place under a reducing atmosphere, the effect of various parameters are critical.
The basic input parameters to the recovery boiler are:
- Black liquor quality which includes concentration (% dry solids), temperature, Higher Heating Value, composition, % organic to inorganic ratio, and other physical properties
- The feed water temperature and feed water chemical quality
- The combustion air temperature which plays a major role in bed stabilization
The important properties include liquor composition, total dissolved solids, ultimate chemical composition, specific gravity and density, viscosity, specific heat, thermal conductivity, boiling point, surface tension, calorific value, latent heat of vaporization and solubility characteristics. Other properties that are mostly qualitative in nature, and not genera,l and very often used for comparison purposes are precipitation point, swelling volume ratio, foam index, etc.
Black liquor is distinctly alkaline (pH varies from 10.5 to 13.5), but not caustic owing to the fact that a large part of the alkali is present in form of neutral compounds.
The lignin has intense black colour shading to reddish brown on dilution and retains a dark straw to yellow colour even when diluted to 0.04% with water.
It is foamy at low concentrations. Black liquor from the sulfate process is generally foamier than that from the soda process. The foaming increases with an increase in resin content of wood used for pulping.
The amount of total solids in black liquor depends on the quantity of alkali charged to the digester and the yield of pulp. Under average conditions, black liquor going to evaporators will contain 14-18% solids for wood and bamboo.
In general, the inorganic compounds in black liquor tend to decrease specific heat and thermal conductivity, increase density, specific gravity, viscosity, boiling point elevation, and have practically no effect on surface tension.
The organic constituents of black liquor tend to decrease specific heat, thermal conductivity, and surface tension, and increase density and viscosity values.
The data also shows that there are considerable differences among values for the different liquors attributed to the diversity of organic constituents in black liquors caused by the variation in pulping species, pulping conditions, and pulp yields.
The black liquor obtained from agricultural residues, wheat straw, rice straw, and bagasse, etc. are characteristically different.
Black liquor generally contains 50%-70% organics and 30%-50% inorganics. It also contains minor amounts of impurities such as lime, iron-oxides, sodium chloride, and alumina.
High silica content is a major obstacle in any recovery process. The concentration of silica is particularly high in rice and wheat straw black liquors. The presence of silica leads to problems related to scaling, clarification, and precipitation.
The non-wood fiber black liquors have a high percentage of silica: 4 - 6% (even more) in case of straw and 1.% in case of bagassee. Silica enters both as intrinsic and external silica with raw material and cooking liquors. The magnitude of silica for different liquors is: rice straw 3 - 16%, wheat straw 3 - 6%, bamboo 2 - 5%, bagasse 1 - 3%, and eucalyptus 0.1 - 0.8%.