Design and implementation of multi-variable H_∞ robust control for the underground coal gasification project Thar

The energy per unit time is an important performance indicator in determining the performance of an underground coal gasification (UCG) site to produce electricity. In literature, model-based strategies are employed by considering UCG as a single input single output (SISO) system, in which only the heating value of syngas is maintained at the desired level by varying inlet gas flow rate. However, the energy per unit time is also dependent on the flow rate of the produced gas mixture. Therefore, in this work, a model-based multi-variable robust control design, based on H_∞ technique is proposed for the UCG process. The actual nonlinear model of UCG is very complex due to its 3D axisymmetric geometry, which makes the model-based control design a formidable task. Thus, a simple linear model with two inputs (flow rate and composition of inlet gas) and two outputs (flow rate and heating value of syngas) is identified by using subspace-based (N4SID) system identification technique. The linear model is then employed to design the H_∞ (S/KS mixed sensitivity) multi-variable robust controller. The simulation results show that the designed controller has achieved both robust stability and performance in the presence of modeling inaccuracies and external disturbance. Furthermore, the designed controller is also implemented on the actual nonlinear cavity simulator (CAVSIM) for the UCG process. The controller exhibits an adequate performance by tracking the desired set points for the heating value and flow rate of the syngas.