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Reconstruction of the K4 steam boiler in Cinergetika, a.s. Ústí nad Labem

Rated boiler parameters:

 

Boiler steam output: 75 t/h 
Superheated steam temperature: 460°C
Superheated steam pressure: 6,35 MPa 
Feedwater temperature: 105°C
Palivo: hnědé uhlí o výhřevnosti 16,9 MJ/kg

 

The client’s specification requested a boiler reconstruction with the aim to enhance the rated output from 75 t/h to 90 t/h. At the same time, the repair of the pressure systems of all superheaters and the water heater was in progress.

The reconstruction involved boiler revitalization and the installation of new jet powder burners improved conditions for low-emission combustion and for effective boiler operation.
 

The actual boiler reconstruction was related to the replacement of powder burners, superheaters and the water heater, and in particular, the parts of boiler whose lifetime had already expired and which often caused unavailability of the boiler during its operation. In practice, this involved the replacement of all heat transfer surfaces of the input, roof and output superheater and water heater. Since the combustion process has changed as a consequence of the reconstruction (removal of flue gas recirculation and introduction of after-combustion air), the concept of both superheaters was changed to achieve sufficient temperature of the superheated steam even at partial outputs of the boiler.
 

A part of the boiler revitalization was also the renovation of the evapourator gasket between the combustion chamber and the second draught primarily to eliminate hot fuel gas penetration from the combustion chamber to the second draught.
 

The supply of a sufficient quantity of combustion air to the boiler is the limiting factor for the enhancement of the rated output of the K4 boiler from 75 t/h to 90 t/h. This problem was solved in Cinergetika by removing the supply of recirculated flue gases to new burners and modifying the supply of combustion hot air. Furthermore, the jet blowers for the supply of combustion hot air were built in thus ensuring good fuel burnout and low generation of CO. These modifications increased the total discharge area of the combustion chamber apertures and decreased resistance of air routes and thus improved the efficiency of the boiler operation.
 

 

The installation of one zone of after-combustion air in the boiler side-walls provided substantial improvement of theconditions for low-emission combustion as well as for the observance of emission limits for NOx nitrogen oxides and CO. This primary measure for low-emission combustion is based on the creation of slightly reducing the atmosphere in the area of the burners in the combustion chamber, where denoxidation takes place so that NOx nitrogen oxide emissions in flue gases behind the boiler are decreased substantially. The reduction of the CO content in fuel gases is achieved by after-combustion air supply, which also ensures good fuel burnout in the combustion chamber.

After-combustion air also creates good conditions for low residual ash in flue dust, and at the same time, eliminates fouling of the combustion chamber walls to a certain extent.

Therefore, the boiler operation complies with the emission standards even at increased output.

The boiler reconstruction also involved the delivery of an advanced control system enabling the reliable functioning of all important boiler control circuits. This is particularly related to combustion control, which is very important both for reliable and stable operation, and for the achievement of under-limit values of NOx nitrogen oxide and CO carbon monoxide emissions in the control range of the boiler at 50 to 100%.
 

Solver: Dr. Ing. Pavel Nekvapil
Realization: Ing. Davel Depiak

References