Black Liquor % Solids - What Do We Really Know?
Importance of Inorganic, Organic, and Lignin Contributions to Total Solids

Abstract
Black liquor % solids has been an important liquor parameter since the introduction of the first recovery boiler. The
principal reason for this is that black liquor, as a fuel, contains a significant amount of water. This water can create a
safety concern for recovery boiler operation because of a phenomenon known as a smelt /water explosion. Water also
significantly influences the boiler efficiency and can lead to corrosion and emission issues. At lower solids a mill will
typically measure the solids using density from Baumé sticks. At higher solids operators typically use a benchtop
moisture analyzer such as a Computrac to determine solids content which evaporates out the water from the sample
relatively quickly. Online refractometers are extensively used for as-fired black liquor solids determination. Some
mills also use boiling point rise calculations for solids determination. These readings are validated using lab tests
employing the Tappi 650 standard which dries the sample for a minimum of 6 hours at 105 degrees C. The variability
of the results from these different test procedures is quite often significant, particularly in multi-species mills.
This paper will compare the different solids test method results and how solids content impact black liquor (BL)
viscosity, a separate liquor parameter that trends well with % solids. It will discuss the accuracy of tests. It will raise
some questions as to whether tests that are designed to measure the water content can provide sufficient information
to allow the optimization of the evaporator and recovery boiler operations. Do we need a fuller breakdown of the
solids content into organic and inorganic? Do we need a lignin determination? The use of on-line FT-NIR
measurements has already provided inorganic and organic content as well as lignin determination since 2008. Using
mill data, this paper will suggest new observations concerning % solids and suggest potential ways to improve our
understandings of issues facing evaporator and recovery boiler operators. 

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