Activation and reactivity of Estonian oil shale cyclone ash towards SO2 binding

Abstract

Electricity production in Estonia is based mainly on the pulverized firing of low-grade local fuel—Estonian oil shale—and it is concentrated mainly in two big power plants, the Balti and the Eesti power plants. Estonian oil shale is characterized by a low calorific value (8–10 MJ kg−1) and a high content of mineral matter (65–70 %, carbonates and sandy-clay minerals in about equal amounts). The sulphur content is around 1.5 %. At the Eesti Power Plant, the Alstom semi-dry DeSOX system was recently installed on four power units by 180 MWel each to guarantee deeper binding of SOx from flue gases. Commercial lime in addition to oil shale ash is used there as a binding agent. Considering that cyclone ash there contains about 20–25 % free CaO and its ability to bind acidic gases has been proved earlier, the idea to replace expensive commercial sorbent with ash was raised. Hence, activation of ash is needed. The aim of the present work was the comparative investigation of the efficiency of activated ashes and the commercial lime used in the DeSOX system. Initial ash, dry ground and semi-dry ground ashes with two different amounts of water—5 and 7 % by mass—and commercial lime as sorbents for SO2 binding were studied. The experiments for testing the reactivity of sorbents towards SO2 binding were carried out with the Setaram Labsys 2000 thermoanalyzer under isothermal conditions. The model gaseous mixture used contained 1 mol% sulphur dioxide in nitrogen. The temperature was varied between 80 and 700 °C. A Pt-multiplate crucible was used with 80 ± 0.5 mg samples. The results obtained indicated that ash activation by semi-dry grinding increases noticeably the reactivity of it towards SO2 binding, and as a result additional commercial lime will not be needed.