Abstract—Due to the low heavy metal concentrations, the bottom ash from a 32 MW municipal district heating plant was determined to be a potential forest fertilizer as such. However, additional Ca would be needed, because its Ca concentration of 1.9- % (d.w.) was lower than the statutory Finnish minimum limit value of 6.0-% (d.w.) for Ca in forest fertilizer. Due to the elevated As concentration (53.0 mg/kg; d.w.) in the fly ash from the 32 MW municipal district heating plant, and Cr concentration (620 mg/kg; d.w.) in the ash fraction (i.e. mixture of the bottom ash and fly ash) from the 6 MW municipal district heating plant, which exceed the limit values of 30 mg/kg (d.w.) and 300 mg/kg (d.w.) for As and Cr, respectively, these residues are not suitable as forest fertilizers. Although these ash fractions cannot be used as a forest fertilizer as such, they can be used for the landscaping of landfills or in industrial and other areas that are closed to the public. However, an environmental permit is then needed. Keywords—Ash, fertilizer, peat, forest residue, waste I. INTRODUCTION LL extraction of biomass from the forest removes nutrients. During the combustion of forest residues and peat, the nutrients and acid-buffering substances are con- centrated in the ash. This makes the ash potentially suitable as a compensatory fertilizer to replace the lost nutrients and acid- buffering capacity in forest soil. For the sustainable utilisation of biomass in dedicated combustion plants, the most beneficial approach involves the return of the ash discards to the field or the forest as a relatively low-grade fertiliser, based principally on the lime, potassium and phosphorus contents of the ash [1]. The dumping of ash in landfills conflicts with public efforts to reduce waste disposal to landfills through increased recycling and reuse. The EU Landfill Directive (1999/31/EC) has resulted in the stricter regulation of landfills [2]. Many of H. Nurmesniemi (Ph.D.) is with the Stora Enso Oyj Veitsiluoto Mill, FI- 94800 Kemi, Finland (e-mail: hannu.nurmesniemi@storaenso.com). M. Mäkelä (M.Sc. Tech) is with the Aalto University, School of Chemical Technology, Department of Forest Products Technology, P.O.Box 16300, FI- 00076 Aalto, Finland (e-mail: mikko.makela@aalto.fi). R. Pöykiö (Ph.D.) is with the city of Kemi, FI 94100 Kemi, Finland (corresponding author to provide phone: +358-16-259 673; fax: +358-16- 259 481; e-mail: risto.poykio@kemi.fi). O. Dahl (Professor) is with the Aalto University, School of Chemical Technology, Department of Forest Products Technology, P.O.Box 16300, FI- 00076 Aalto, Finland (e-mail: olli.dahl@aalto.fi). these cannot meet the current requirements and are consequently being shut down. The cost of running landfills has also increased because of the new regulations. This has made it economically feasible to return forest fuel ash to the forest and in so doing complete the cycle of nutrients and restore the acid-buffering capacity. The large and increasing volume of ash residues arising from Finnish pulp and paper mills is motivating a search for alternative disposal options to landfill. The utilization of solid wastes and residues, such as ash, allows industry to implement the 3R principle of the reduction, reuse and recycling of materials as beneficial products [3]. Therefore, there is a growing trend towards the utilization of ash residues in Finnish pulp and paper mills. Careful attention to ash chemistry can maximize the potential of ash residues for beneficial application instead of the relying on landfills for their disposal. II. BACKGROUND A. An overview of Finnish national legislation on ash utilization in forestry In Finland, national legislation regulates the utilization of ash in forestry. The Fertilizer Product Act (539/2006), the Degree on fertilizer products (12/07 and 9/09) and the Degree on the operations concerning fertilizer products and their supervision (13/07) set guidelines for ash recycling and fertilizing in forestry [4-7]. According to these regulations, only wood, peat or agrobiomass ash is permitted to be used as a raw material for agroash and forest ash. Air pollution control (APC) residues are not allowed to be used as fertilizer products. According to Finnish legislation, “agroash” refers to ash 1) to be used as such as a fertilizer product, 2) to be used in agriculture, horticulture, landscaping and forestry, and 3) having a minimum P+K content of 2-% and at least 8-% of Ca. “Forest ash” refers to ash 1) to be used as such as a fertilizer product, 2) to be used only in forestry, 3) having a minimum P+K content of 1-% and at least 6-% of Ca, and 4) having a maximum Cl content of 2-% [8]. According to the procedures of the Finnish Ministry of Agriculture and Forestry [4-6], which sets statutory rules for fertilizer products in Finland, the average maximum load of cadmium due to the use of fertilizer products must not exceed 1.5 g of cadmium per hectare per year. The maximum load of cadmium due to the application of fertilizer products as used batches and as cycles of use may be: 1) no more than 6 g per hectare at an Hannu Nurmesniemi, Mikko Mäkelä, Risto Pöykiö, and Olli Dahl Comparison of The Fertilizer Properties of Ash Fractions from Medium-Sized (32 MW) and Small-Sized (6 MW) Municipal District Heating Plants A World Academy of Science, Engineering and Technology International Journal of Chemical and Molecular Engineering Vol:5, No:3, 2011 252 International Scholarly and Scientific Research & Innovation 5(3) 2011 ISNI:0000000091950263 Open Science Index, Chemical and Molecular Engineering Vol:5, No:3, 2011 publications.waset.org/11180/pdf