Wood Ash Database

– Wood for Energy -

Ash types

General informationAndersson, S., Karltun,E. et al. Wood ash - properties and ecological consequences of recycling to forest.

Wood ash contains all macro and micro nutrients except N and S in proportions relatively close the proportions found in the biomass fuel. The concentration of the elements varies considerably between ashes, mostly because of varying amount of charcoal due to incomplete combustion, but also because fuel contaminants like soil, or sand material from the incineration bed in fluidized bed combustion reactors can be found in the ash.

The amount of ash generated by incineration depends significantly on the fuel source, i.e. which part of the tree that is used. Data describing the three species Norway spruce, Scots pine and Downy Birch shows that stemwood contains the lowest DW (dry weight) percentage of ash, while bark generates a considerably higher amount per weight unit. The largest relative percentage of ash, however, is derived from needles and leaves.Werkelin,J. 2002. Distribution of ash-forming elements in four trees of different species.

As well as depending on the fuel source, the final chemical properties of wood ash also depend on incineration conditions, additives, contaminants and storage conditions Hjalmarsson, A. et al. 1999. Handbok för restprodukter från förbränning. . One of the most influent factors on the concentration of different elements in ash is the amount of charcoal which varies due to incomplete combustion. In fluidized bed combustion, fuel contaminants like soil or sand materials from the incineration bed can be found in the generated wood ash.

Also, ash can be categorized into different types. The main categories are bottom, fly and mixed ash. Further information about each type of ash is presented on its respective page.

Along with oxygen, the major components of wood ash are calcium (Ca), potassium (K), magnesium (Mg), silicon (Si) and phosphorus (P) Nilsson, J., Timm, B. 1983. Environmental effects of wood and peat combustion. Summary and conclusions. Kofman, P. 1987. Wood-ashes from chip fuelled heating plants: Chemical composition, possibilities of application. Eriksson, J., Börjesson, P. 1991. Wood ash in forests. Holmroos, S. 1993. Karaktärisering av vedaska. Steenari, BM. et al. 1999. Evaluation of the leaching characteristics of wood ash and the influence of ash agglomeration. Ash is generally very low in nitrogen (N) because it is vaporised during combustion and considerable part of the sulphur (S) may also be lost. Trace elements found in ash include As, Ba, B, Cd, Cu, Cr, Ag, Mo, Hg, Ni, V and Zn, of which B, Cu, Mo and Zn can also be regarded as micronutrients Booth, C. et al. 1990. Changes to forest management and silvicultural techniques necessitated by forest energy production. International Energy Agency/Bioenergy Agreement, Activity A1. Conventional Forestry Systems..