Primary method if used in pulverized oil shale-firing boilers in operation enables to achieve the target value of SO2 specific emission 400 mg/nm(3). It will also be possible to meet the SO2 specific emission limit value (200 mg/nm(3)) set by the European Union for the new-installed solid-fuel boilers by further optimization of technological parameters of pulverized oil shale firing and construction of oil shale boilers on the basis of primary methods. Optimization would make it possible to design a pulverized oil shale-fired boiler for super-critical and ultracritical steam parameters and to enhance the efficient and environmental-friendly use of oil shale to a considerable extent.