Studying the possibility of separate and joint combustion of Estonian shales and oil shale retort gas at thermal power plants

Abstract

Results from investigations of joint and separate combustion of shale with a low heating value and oil shale retort gas (OSRG) are presented. The question about the possibility of further using shale as basic fuel is presently placed on the agenda. This matter is connected with the fact that the environmental regulations are imposing increasingly more stringent limits on emissions of harmful substances and that a decrease in the shale heating value is predicted. An adequate mathematical model of one of the TP-101 boilers installed at the Estonian power plant was developed and verified for carrying out investigations. Criteria for determining the reliability, efficiency, and environmental safety of equipment operation were formulated based on the operating chart, regulatory documents, and environmental requirements. Assessment of the possibility of boiler operation and the boiler unit as a whole in firing shale with a low calorific value has shown that despite fulfilling the required superheated steam parameters, quite a number of limitations relating to reliable operation of the boiler are not complied with. In addition, normal operation of forced-draft equipment and mills is possible only at low loads. For operation with joint combustion of shale and OSRG, the fractions of degraded-quality shale and OSRG (by heat) at which reliable and efficient operation of the boiler and boiler unit is ensured in the entire working range of loads with fulfilling the environmental standards are determined. Proposals on modifying the equipment for joint combustion of shale and OSRG are formulated. Boiler operation with firing OSRG as main fuel was modeled for three versions of furnace waterwall thermal efficiency with a view to estimate possible changes of boiler operation in carrying out waterwall cleaning operations. Calculation results have shown that operation of the boiler and boiler unit meeting the elaborated criteria is possible in the entire working range of loads with essentially increased the air excess factor at the furnace outlet in low load modes.