Burning black liquor is a significant form of energy production in today's energy field. Black liquor is a pulp and paper by-product that is considered a renewable and carbon neutral fuel. At this point in time, more than 10% of Finland's energy consumption is covered by burning black liquor. A Recovery boiler is a complex production unit that has a very important role in the pulp mill`s energy production and its closed chemical cycle. The black liquor used as fuel for the boiler contains about half of the wood`s dry matter and almost all cooking chemicals and thus contains both inorganic and organic substances.

In the combustion process of the recovery boiler organic matter of black liquor burns and produces heat that is converted into very hot high pressure steam in the boiler`s water circulation. At the same time, in the inorganic pulping chemicals react and separate from the black liquor in the furnace. Sulfur is reduced to sodium sulfide and sodium forms sodium carbonate with carbon dioxide. The result of this reaction in the furnace, the amount of sodium sulfate reduced to sodium sulfide, is measured as reduction ratio. The chemicals recovered during the process can be reused in the production, which reduces costs and environmental emissions.

The reduction rate is an important indicator for assessing the operation of a recovery boiler. Poor reduction rate in process often indicates problems in combustion, which leads to chemical, and economical losses. Unbalanced combustion that causes poor reduction also affects the higher fouling of the boiler and increasing environmental emissions. The ratio of reduction is therefore an important measure in assessing the success of the combustion, which also affects the boiler´s operating mode and its adjustments. The target of the thesis was to find out a rapid, easy-to-use and reliable method for measuring reduction with visual analysis. The standard reduction measurement is based on glass tube sample of chemical smelt, which is examined in a laboratory. During the thesis, the glass tube and the reference smelt sample measurement results were compared and it was attempted to determine features indicating the degree of reduction of the sample. The second objective of the thesis was to define a system that could reliably repeat the measurement.

As a result, the laboratory result of the reference sample and the glass tube sample showed equivalence and a number of possible features indicating the reduction rate of the sample. This equivalence makes it practicable, that it can be possible to determine a measurement method based on visual analysis of the smelt sample. The purpose of the analysis is not to replace the laboratory results but to support the combustion control as a quick measurement with some inaccuracy compared to the laboratory assay. The thesis was commissioned by Andritz Oy. The research and measurement data and system details are confidential information and therefore they are not contained in the thesis.