THE SINTERING OF ESTONIAN OIL-SHALE ASHES

Utilization of Estonian oil shale in thermal power plants causes many operational problems. These problems include slagging, fouling and cor rosive-erosive wear of steam boiler heat transfer surfaces. Part of su lphur dioxide produced is absorbed in calcium oxide in the boiler flue gas ducts. The conventional technique of pulverized firing (PF), howe ver, does not provide absorption of extremely high amounts of formed s ulphur dioxide, regardless of large amounts of calcium oxide in the fu el ash. In fluidized bed combustors (FBC), on the other hand, most of the sulphur may be absorbed by the ash of high calcium oxide content, without application of any separate ale gas treatment system or additi onal use of sulphur oxide absorbents. This research work deals with th e sintering behaviour of Estonian oil shale ashes. The effect of diffe rent gas atmospheres and temperatures on the ash sample sintering was paid special attention to in the course of this study. The different t ypes of ashes tested included: (i) ash collected from the cyclone of a PF oil shale boiler, (ii) ash collected from the electrostatic precip itator of the same boiler, and(iii) oil shale ash prepared by standard laboratory procedures. The ashes were tested for their compression st rength after a sintering test under laboratory conditions. First, cyli ndrical pellets were made from the ash samples. Then the pellets were exposed to various atmospheres, and various temperatures in the range of 300-1050 degrees C, in a tube furnace for four hours. After the exp osure, the pellets were tested for compression strength, and the pelle t-crushing stress was taken as an indicator for the degree of sinterin g.The compression tests were complemented with chemical analyses of he at-treated ash pellets. Changes of mass find dimensions of the heat-tr eated pellets were recorded as well. Some experiments were also made t o simulate the impact energy of different ash particles, and to invest igate the buildup of ash deposits on the heat transfer surfaces of boi lers. This was done by varying the pellet-making pressure. The tests s howed that significant sintering occurred under most of the conditions tested. The sintering of oil shale ash was found to depend on both he at-treatment temperature and gas atmosphere. Higher pressure at pellet -making resulted in some cases in increased sintering.