Sustainable whole-tree harvesting practice requires that nutrient removal from the forest is compensated. Woodashes contain all the nutrients, except for nitrogen, that are found in unburned fuel and can also increase soil pH, whichmakes ash recycling a natural way to stabilize the nutrient balance and counteract the acidification of forest soils thatoccurs due to intensive forest management. Several methods for processing ashes into spreadable products have beendeveloped. The aim of this paper is to compare these methods. The study mainly focused on an economic evaluation ofproduction, transportation and the spreading of self-hardened ash, ash pellets and ash granules. Self-hardened ash isgenerally considered to be the cheapest alternative to manufactured ash products, but these results imply that the most costeffective alternative is ash pellets. Around 27% of total costs could be earned from recycling the ash by producing pelletsand 8% if granules are produced instead of self-hardened ash. This partly depends on the higher density of the pellets andgranules and a significant reduction in the number of transportation operations. The reduction in transportation operationsand diesel consumption also has major environmental benefits. Furthermore, it is more efficient to produce granules andpellets than it is to produce self-hardened ash and it is also easier to produce a reliable product of an appropriate size,shape and texture for a market that has well defined requirements.

Companies that generate a large amount of wood ash will need an industrial process to agglomerate the ash and lower its reactivity, because untreated ash is a dust hazard for workers and is difficult to spread evenly on forest soil. In addition, untreated ash can cause burning damage to vegetation owing to its alkalinity and rapid release of salts. Production of large amounts of wood ash agglomerates demands an effective dehydration process. The reactivity and release of inorganic constituents from wood ash pellets dehydrated at room temperature using hot air and flue gas was investigated. Our results imply that flue gas-treated pellets have significantly lower reactivity in terms of pH and electrical conductivity, and release less Ca2+ and more Mg2+ compared to pellets dried at room temperature or in hot air. Ash pellets dehydrated in hot air are very reactive, and release more Ca2+ than pellets dried in other ways. The formation of syngenite during the flue-gas treatment decreases K+ release from the ash pellets.

Use of biofuels in the form of logging residues is increasing in the European countries. This intensive forestry, where entire trees are removed from the felling sites, may contribute to a negative nutrient balance in the forest soil. Recycling of ash from the combustion of clean wood fuel, sometimes in combination with limestone or additives/binders, back into the forest soil could maintain the soil nutrient reservoir intact. Before spreading ash, it is important to determine its contents and, particularly, its decomposition pattern using reliabl laboratory leaching tests. In this study, mineralogy and the leaching of Na, Ca, K, Mg, Mn, Al, Cu, Fe, P, and Zn from wood ash pellets and granules, produced both from green liquor sludge and fly ash, are examined by XRD and by subjecting these substances to three different laboratory leaching tests: upflow percolation (CEN/TS 14405), batch leaching (SS-EN12457), and a new Swedish leaching test using a magnetic stirrer. Mineral phases such as quartz, ettringite, calcite, gehlenite, and aphtitalite were identified in the ash granules and in the ash/green liquor sludge granules, by means of XRD. Six additional minerals were detected in the granules of ash only, and another six in the ash/green liquor sludge granules. At L/S 2, the batch leaching test resulted in the highest amounts of elements leached and the upflow percolation test the lowest. At L/S 10, both the batch leaching test and the upflow percolation test resulted in high amounts of elements leached. The batch leaching test at L/S 10 complies quite well with the percolation test and could be suitable for ash/green liquor sludge granule evaluation in daily practice. The magnetic stirrer test seems to underestimate the release potential of elements from granules. The batch test is simple to perform, and has the ability to dissolve 70–80% of the elements with the highest mobility from the materials under study.