461080 Pretreatment of Black Liquor to Improve the Recovery Boiler Operation
Black liquor is an intermediate stream that is formed during the production of kraft paper pulp where wood is used as raw material. It contains most of the lignin from the wood but also some parts of the hemicelluloses and celluloses. In addition, it also contains the ingredients of the cooking liquor used to deliberate the different wood components. This makes the black liquor highly rich in alkali and sulfur. It is not uncommon with a portion of 20 wt% for sodium and 7 wt% for sulfur on dry material basis. In order for the pulp mill to close its material balances and to produce necessary steam and electricity, the black liquor is combusted in a recovery boiler. Before the combustion, the black liquor is concentrated to about 80% before it is fed into the boiler through the liquor guns.
During combustion, there are three distinct phases for each individual droplet. These are: drying, pyrolysis and combustion. Due to the high concentration of alkali metals (sodium and potassium), the amount of these that are entrained in the gas flow or just evaporated due to the high temperature is rather large. This creates problems in the boiler due to deposition of material on the heat exchangers, which might cause plugging and thus decrease the heat transfer but also by blocking the passage and create an unwanted pressure drop. Another problem is the corrosive combination between alkali and chlorine (chlorine compounds are present in the original wood material and ends up in the black liquor) which are corrosive and thus detrimental for especially the super heater tubes inside the boiler. Most of the sodium reacts rapidly and forms sodium carbonate or sodium sulfur oxides such as sodium sulfates or sodium thiosulfates. Thereafter, the pyrolysed droplet starts to combust but as a design feature of the recovery boiler, the droplet should contain some carbon and not be completely burned out when it reaches lower part of the boiler in order to reduce the sulfur compounds to sulfides.
The thermal pretreatment has been performed in our laboratory on black liquor from two Swedish kraft pulp mills at different temperatures and heating rates. In combination to laboratory test for the amount of sodium leaving the black liquor during pyrolysis, we have also performed thermodynamic modelling of the lower part of the recovery boiler to investigate if all sodium would stay in the melt or if it is evaporated in this last stage. It has been previously showed that the sulfur oxide compounds in the black liquor will mainly form hydrogen sulfide in oxygen lean environment at temperatures below 600°C which is the condition for mild pyrolysis but the equilibrium also shows that it will completely form sodium sulfide at high temperatures (above 800°C) which is the condition in the smelt bed.
Within this project we have showed that the thermal pretreatment is a possible way to minimize the sodium loss due to entrainment and evaporation during the free fall period and that the equilibrium calculations indicates that this higher amount will stay in the molten phase and thus increase the direct recovery of sodium from the boiler. This could give higher efficiencies and a lower cost for boiler operation due to lower need for soot blowing steam, possible higher temperatures in the steam cycle and less maintenance.
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