(184w) Development of Advanced Model-Based Controllers for Optimal Load-Following Operation of the Supercritical Pulverized Coal Power Plants

High penetration of variable renewable generators into the electricity grid has resulted in changing operating requirements of traditional power generation plants, particularly in super- and sub-critical pulverized coal plants. A number of these coal-fired plants were originally designed to operate under base-load condition, but are now being forced to load follow. This leads to low efficiency, high operation and maintenance (O&M) costs, and potential increases in environmental emissions. Optimal operation during load following is necessary for the commercial viability of these plants.

High-fidelity, plant-wide, dynamic modeling of supercritical pulverized coal (SCPC) plants is critical for studying optimal load-following operation, but is lacking in the existing literature. It is desired that the model is accurate during load-following operation when significant changes in the operating conditions do occur. Under sliding-pressure operation during load-following, the boiler can transition to subcritical condition. Due to high nonlinearity in the physical properties of steam and due to the heat transfer characteristics under part-load operation, the temperature profile in the boiler can change considerably. Thus a model that can capture these characteristics during load-following operation is desired. Furthermore, due to the high heat and mass integration in the SCPC plant, it is critical to consider the dynamics of the entire plant.

Dynamic models can be highly useful for developing efficient control strategies for SCPC plants [1-2]. However, due to the considerable differences in the time scales of the key phenomena and due to the transport delay, development of efficient controllers under load-following operation becomes challenging. Uncertainties in the heat transfer model in the boiler due to the time-varying fly ash deposit on the tubes and due to the complex multi-dimensional convective, radiative and conductive hear transfer can lead to poor performance of the model-based controllers.

In this work, a plant-wide dynamic model of a 640 MW SCPC power plant is developed following the cost and performance baseline studies conducted by the National Energy Technology Laboratory [3]. Using the Aspen Custom Modeler software, high-fidelity process models are developed for the once-through pulverized coal boiler, the steam turbine system with consideration for fixed and sliding pressure operation, the surface condenser, the selective catalytic reduction unit, and the heat exchangers in the feed-water heating section. These custom models are compiled into the complete plant-wide SCPC steady-state Aspen Plus and Aspen Plus Dynamics models considering steam extractions and other auxiliary operations.

The steady-state SCPC model is validated against the NETL Baseline Report. A regulatory control layer is designed for disturbance rejection and plant stabilization. A supervisory control layer is first developed following the industry-standard coordinated control technique. Finally, both linear and nonlinear model predictive controllers under state and rate-of-change constraints are developed. The state and rate-of-change constraints help to ensure that the creep and fatigue damages due to load-following are reduced. Several load-following scenarios are evaluated using the dynamic SCPC model. The presentation will include comparisons of control performance by studying the tradeoff between the efficiency, ramp rate, state, rate of change, and environmental emission.

Bibliography

[1]	J.-Z. Liu, S. Yan, D.-L. Zeng, Y. Hu and Y. Lv, "A dynamic model used for controller design of a coal fired once-through boiler-turbine unit," Energy, no. 93, pp. 2069-2078, 2015.
[2]	W. Shinohara and D. E. Koditschek, "A Simplified Model Base Supercritical Power Plant Controller," in Proceedings of the 35th Conference on Decision and Control, Kobe, Japan, 1996.
[3]	National Energy Technology Laboratory (NETL), "Cost and Performance Baseline for Fossil Energy Plants Volume 1a: Bituminous COal (PC) and Natural Gas to Electricity," U.S. DOE/NETL, 2015.