Ash Products for Nutrient Recycling in Forestry : Leaching and Drying Characteristics

Abstract: As the demand for carbon dioxide neutral energy sources increases, forestry becomes more intensive. Entire trees are removed from the felling sites threatening the nutrient balance of the forest soils. By recycling wood ash produced from the combustion of clean wood biofuels back to the forest, it is possible to maintain the nutrient balance of the soil and also to counteract its acidification. To avoid negative effects, e.g. burn damage on mosses and lichens caused by the high pH of the ash, it is important that the ash is treated and hardened in order to lower its reactivity and to prolong its dissolution rate. The ash chemicals content are determind and the reactivity and dissolution properties are evaluated by laboratory leaching tests. Pulp and paper industries generate not only ash but green liquor sludge also as residual material. These nutrient-rich materials could be put together into granules and be recycled on forest soil. The applied leaching tests vary in their performance and duration. Therefore the focus of this work is put on both making granules of ash and green liquor sludge and developing a method for the testing of these.The aims of this thesis are: to investigate how leaching tests influences the dissolution of ash products; to find out how drying techniques affect the product's leaching behaviour and to find an appropriate technique for mixing and agglomerating wood ash and green liquor sludge in suitable proportions.The leaching tests compared were: up-flow percolation, batch leaching and leaching using a magnetic stirrer. This study includes analyses of pH and concentrations of Na, Ca, K, Mg, Mn, Al, Cu, Fe, P and Zn in the eluate from each leaching test. The results show that the batch leaching test at L/S 10 has the ability to dissolve 70-90 % of K and Na from the ash products. The upflow percolation test releases 55-100 % K and Na, but takes several weeks to perform. The leaching test with a magnetic stirrer results in low leached amounts of elements and underestimation of the dissolution potential of the material. The test seems to be best suited for its original purpose, i.e. control of the reactivity of wood ash.Four drying techniques were applied on wood ash pellets: drying at room temperature, drying by hot air (60 and 130 C) and drying by flue gas. The ash pellets dried in flue gas showed the significantly lowest values of pH and electric conductivity. This makes flue gas drying an advantageous method, for ash products intended for spreading in forest. Regarding the elements studied, the flue gas drying does not contaminate the ash material.Processing the ash and green liquor sludge using an intensive mixer offers effective blending and a quick granulation with a short process line. With the intensive mixer it is possible to granulate a mixture of maximum 65 % sludge (wet weight basis) and 35 % ash (dry weight basis).

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