The flexible boiler operation in a wide range of load changes with considering the strength and environmental restrictions

Abstract

Thermal steam units cooperating in one power system with wind and photovoltaic farms must have high flexibility, i.e., operate over a wide range of load variations, typically between 40 and 100%. In addition, the start-up and shutdown time of the unit should be short. Thick-walled pressure components that limit the fast start-up of the units are turbines, steam boilers, and pipeline branches due to the occurrence of high thermal stresses in them. This paper determines the allowable heating rates of thick-walled components of a 97.22 kg/s natural circulation steam boiler. The boiler drum is the critical element, with a heating time of about 6000 s from the cold state. Computer simulations of the boiler combustion chamber were also performed at three different loads, i.e., 100, 60 and 40%. The calculations show that the maximum emissions of harmful substances do not exceed the permissible values given in European regulations: CO≤140mg/mn3 and NOx≤100mg/mn3. The maximum content of CO = 136.8 mg/mn3 occurs at 60% boiler load. The maximum content of NOx = 217.3mg/mn3 in the exhaust gas upstream of the SCR (Selective Catalytic Reduction) system occurs at 40% load, but downstream of the SCR system is reduced below the limit value 100mg/mn3.