(418b) Dynamic Modeling and Control of Heat Recovery Steam Generator and Steam Turbine Units as Part of IGCC Power Plants

Integrated gasification and combined cycle (IGCC) power plants, with integrated air separations units, have the potential to sequester carbon dioxide at a lower annualized operating cost than similarly equipped pulverized coal-fired power plants.  The synthesis from the coal gasifier is combusted in a turbine to yield one level (cycle) of electric power. The heat from gas turbine exhaust gases, raw-syngas (from gasifier) and various other sources are used to generate steam for a multistage steam turbine to generate another level of power (thus, the notion of combined cycle). Heat Recovery Steam Generators (HRSG) are composed of drum boilers at three different pressure-levels, a number of heat exchangers, condenser, the spray valves and actuators. Conventionally, PID-based controllers with regulator tuning based on standard parameters and simple procedures have been used to cope with fluctuations in energy demand and grid regulation. Hence, at partial loads the combined cycle power plants suffer significant loss of efficiency, and therefore prefer full-load operation. In addition, an excess-air-ratio lower bound set due to limiting NOx production, inherently has a direct effect on the load rates of the HRSG, so as to extract the maximum flue gas energy.

In this study, we implement a model predictive control (MPC) strategy to control specific temperature nodes on the flue gas and steam lines inside the HRSG. We initially base this study on a single-pressure boiler operation, to understand the operability limitations and identify the controllable temperature and pressure nodes to the process, and later extend to a more "plant integrated" three-pressure level operation. We exploit the rigorousness of Aspen Plus/Dynamics pressure-driven simulations for thermodynamic calculations (including fluid compressibility and specific heat at elevated pressures) and realistic boiler and heat-exchanger designs. We employ lower level PID-based control loops at various places for fast and stable regulatory layered operation, including the boiler drum levels.  For instance, the "fast" regulation of the steam temperature at the turbine inlet (HP and IP stages) is performed by desuperheating the corresponding steam fluxes using a similar approach. As a further study, we also compare the responses of a fixed-pressure mode operation attained by means of steam turbine throttle valves to that of a conventional sliding pressure mode operation, both implementing the MPC method.