The fuels in Sweden are solid biofuels as well as different combustible wastes, including municipal solid wastes (MSW). The share of fossil fuels, coal or oil, is very small. The ash content in the fuels differs considerably. Clean heartwood as in wood pellets has an ash content of 0.2-0.5%. Wood chips obtained from forest residues have an ash content of 2-4% as the proportion of bark increases. Waste wood, such as construction and demolition debris, usually has higher ash contents. For peat, a figure of 5% is generally used, although the content can be much higher. Waste materials have comparatively high ash contents: 25% in MSW is quite common and various sludges in the pulp and paper industry have ash contents ranging between 10 and 50%.