Assessing the Smoke-Stack Performance with Boiler Unit Flue Gases Cooled below the Dew Point

The use of condensing heat exchangers (CHEs) in gas-fired boiler units helps cool the flue gases below the dew point. One of the issues that has to be settled in the case of CHEs installed downstream of boilers is to ensure that the flue gas removal stacks will operate without steam condensation on their inner surfaces. To protect smoke stacks against hydrate corrosion, bypassing of part of combustion products not cooled in the CHE is mainly used in practice. The article presents the results obtained from computations of the heat-transfer processes in the combustion products cooled in a CHE as they move in a reinforced concrete smoke stack fitted with clamped lining, which is protected against hydrate corrosion by bypassing. The computations are carried out for three operation modes of the 180-m high smoke stack, through which flue gases are removed from three power-generating boilers of the BKZ-420-140 NGM type installed at the Samara combined heat and power plant (CHPP), a branch of Samara PAO T Plus. The peculiarity and complexity of the computations are connected with the fact that that the flue gas’s thermophysical parameters and motion velocity in the smoke stack vary during the flue gas cooling process. The parameters’ variation pattern depends essentially on the fraction of gases directed in bypass of the CHE. A mathematical model and computer program are developed for computing the heat-transfer processes in flue gases moving in the smoke stack with CHEs installed downstream of the boilers and with the smoke stack protected against hydrate corrosion by the bypassing method. It has been determined that, for a 180-m high three-layer reinforced concrete smoke stack operating at an outdoor air temperature of –30°С and boilers operating at the nominal load, the fraction of bypassed gases makes 30–35%. With the boilers operating at partial loads equal to 75 and 60% of the nominal value, the fraction of bypassed gases makes 35–40 and 40–45%, respectively. The use of condensing heat exchangers in boiler units results in that the levels of temperature difference, free temperature deformation, and thermal stresses in the smoke stack’s structural elements are reduced by a factor of 1.33–2.80 depending on the fraction of gases passed through the CHEs, thereby enhancing the flue gas removing smoke stack performance reliability.