Beneficial uses of pulp and paper power boiler ash residues

Ash residuals generated from recovery and power boilers combusting wood residues, sludges, or auxiliary fuels constitute a major fraction of the solid residues produced by pulp and paper mills. Generation rates in Canada, and likely elsewhere, for ashes of different types have increased substantially since the mid-1990s. Landfilling is the primary disposal method, but there are many potential beneficial applications for these ashes. Large-scale opportunities include land application and construction. Smaller-scale applications exist within both wastewater treatment systems and the papermaking process. Ashes from wood-fired power boilers are generally more suitable for land application than those from coal combustion, as they contain fewer metals at lower concentrations (except for cadmium). The major benefit of land application arises from the neutralizing properties of ashes, as they provide alkalinity to the soil. Compared to fly ashes, bottom ashes have higher bulk density, lower carbon content and few, if any, dioxins and furans. Land application of ashes produced from salt-laden hog fuels at coastal pulp and paper mills is regulated for dioxins and furans. However, steps can be taken to minimize the generation of such chlorinated organics, making these ashes suitable for land application.