Comparative investigation of drum-type and once-through heat recovery steam generator during start-up

• First start-up comparative study of once-through and natural-circulation heat recovery steam generator.
• HP heat exchanger surface is 8% lower for the natural-circulation heat recovery steam generator.
• Faster pressure build-up in the once-through evaporator.
• High temperature gradients in the HP drum wall.
• Uniform wall temperature re-establishes more than 90 min faster in the HP separator.

This study investigates the impact of the design of a heat recovery steam generator (HRSG) on its dynamic behaviour under the boundary condition of a gas turbine start-up. For that purpose, a validated HRSG model with three pressure stages and reheater section is modified by replacing the once-through evaporator in the high pressure circuit with a natural circulation evaporator, including the associated control circuits. Both models are designed to supply equal steam mass flows with equal steam parameters (temperature, pressure) at full load, which enables a balanced assessment of the two technologies. After an extensive description of the modelling approach and its practical realisation, detailed simulation results for start-up procedures from warm and hot initial conditions are presented. Differences in the transient behaviour of the HRSGs are highlighted and discussed. In industrial practice, frequent start-ups cause increased material fatigue, which in turn has an adverse effect on the operating lifetime of a power plant. Hence, the present work is complemented by an analysis of the temperature gradients in the most critical components with respect to thermal stress. Results generally show similar responses of the high pressure systems to gas turbine start-up with the exception of accelerated pressure build-up in the once-through evaporator. Greater temperature deviations are observed in the natural-circulation HRSG across the wall of the high-pressure drum.