Control strategy for a gasifier in IGCC power plant

Abstract

its dynamic response influences the dynamics of the downstream equipment and, in turn, affects the performance of the entire plant. Hence, this thesis intends to study the problem of gasifier control. Firstly, a $3 \times 3$ MIMO control problem is defined and three control strategies for the dry feeding type entrained-flow gasifier are studied. The conventional gasifier control practice is to fix the ratios of oxygen-to-coal and steam-to-coal during operation. It is argued that this strategy is insufficient to control the gasifier satisfactorily in the face of expected disturbances and load changes, and additional control loops need to be installed. Two strategies are tested. The first strategy cascades the existing ratio loops with additional PI control loops. The second strategy eliminates the ratio loops and then manipulates the individual flow rates directly. The cascade control strategy turns out to be the best option for the gasifier control. The common practice of fixing the input ratios can be improved by adding cascade loops to adjust the ratios based on feedbacks of the important CVs. Additionally, the ratio control strategy has advantages in controlling the multiple input streams of the gasifier, which brings feedforward effect in set-point changes. Secondly, for the first time, inferential control of slag thickness is studied. For a stable operation of the entrained-flow gasifier in IGCC power plant, special attention has to be given to slag thickness control. In spite of the importance of slag thickness control, there currently exist no studies on slag thickness control. The conventional control strategy for slag thickness is to maintain the operating temperature and introduce ‘flux’ of a fixed ratio to the fuel flow rate. In disturbance rejection tests, the conventional strategy failed to control the slag thickness in 8 scenarios of coal spec change. Two input disturbance model design strategies are proposed for the inferential control problem which intrinsically has more disturbances than measurements. The first strategy is to select the most likely scenarios, as many as measurements, and use a linear projection along the direction of the most likely scenarios. The second approach is to use the SVD for the linear projection to minimize the error over all possible scenarios. For the most likely scenarios, the first approach showed superior performances but the second approach gave a comparable level of performance. For the other cases that were not modeled, the second approach outperformed. In conclusion, the current control practice for gasifier is insufficient controlling not only the MIMO gasifier system but also the unmeasured slag thickness. The common practice of fixing the input ratios can be improved by adding cascade loops to adjust the ratios based on the feedbacks of the CVs. In order to control slag thickness, a model-based state estimation and inferential control are required. Also, an input disturbance model is required for LMPC to represent the disturbances of coal spec change. Two proposed design strategies for input disturbance model showed superior performances.

An energy revolution is underway but the search for clean, renewable energy sources is not progressing fast enough to meet the growing energy demands. Fossil sources still account for 80% of the world’s energy demand and it will continue to play a major role in the coming decades. Therefore, more attention is given to efficient and clean use of fossil fuel in the energy industry. Coal is an especially important energy resource in given its low price and stability of supply, yet coal accounts for 44% of the total carbon dioxide emission. Considering this, IGCC is an attractive option for clean and efficient use of coal. Despite its many advantages, the biggest challenge IGCC needs to overcome is its low availability. The low availability of the IGCC power plant usually is because of the difficulties in integrating two main technologies: gasification and combined cycle technology. The trouble-free operation of a gasifier is more important than other units in the IGCC system because it supplies the feed to the gas turbine