טכנולוגיות ייצור והולכה
Development of a new Combined Cycle start-up procedure which improves reliability of HRSGs and reduces fuel consumption and NOx emission
One of the major problems that develops during the operation of combined cycles is vulnerability of the Heat Recovery Steam Generator (HRSG) to cyclic thermal loads, especially when combined cycles operate with frequent starts and shutdowns. The source of the problem is insufficient steam flow during the initial stage of the start-up procedure caused by high thermal inertia of HRSG.
The low steam flow cannot cool the super-heater and re-heater tubes, and hence they experience thermal shocks during the steep rise of the flue gas temperatures in the gas turbine exhaust duct (inlet to HRSG) After a certain number of start-ups and shut downs (thermal cycles) the super-heater and re-heater tubes develop cracks (particularly in locations of super-heater tube-to collector weld joints). The drum is also susceptible to thermal fatigue if the load variations are steep. In addition to these problems, the traditional drum-type HRSG requires prolonged time for cold and even warm start-ups. Start-up periods are characterized by excessive consumption of fuel and increased emission of pollutants to the atmosphere. In general, this type of HRSG has high O&M cost and from the reliability point of view is often considered a “weak link” in the combine cycle if it is operated in the cyclic regime.
In summary, traditional HRSGs have two startup problems: thermal shocks (reducing reliability and life-cycle duration) and a long heating procedure leading to increasing fuel consumption and emissions. The objective of this research is to develop a new combined cycle start-up procedure, which improves reliability of HRSG operation with frequent start-ups and reduces its O&M cost. Additional expected benefits of the proposed start-up procedure are reductions in fuel consumption and NOx/CO emissions.