Coal combustion problems: Sulfur, sodium and vanadium are present in residual fuels in concentrations that depend on the source of the coal feedstock, contamination in handling etc. Concentrations of sulfur in residuals varies from 0.3% to 0.5% in some coal and it may be up to 2% to 3.5% for coal. Vanadium and sulfur vary from 20 ppm to 30 ppm in some coal to as much as 900 ppm