A comparison of the reactivity of solid wastes formed during electricity production by firing pulverized Estonian oil shale was prepared and some specific characteristics of oil shale ash were pointed out. Some problems related to the selection of the sorbent injection point are discussed. Industrial-scale experiments for desulfurization of flue gases by recurrent use of cyclone ash and mud from an ash hydrotransport water sedimentation pond were carried out, with positive results, at the Baltic Power Plant in co-operation with Eesti Energia SC, St. Petersburg State Technical University and Tallinn Technical University.