Sliding Mode Control of Underground Coal Gasification Energy Conversion Process

The control of highly complex and nonlinear underground coal gasification (UCG) energy conversion process is a challenging job. As the process occurs under the surface of the earth, so it is either impossible or very expensive to measure all the important parameters, which further complicates the control design. The input of the UCG process is the flow rate of the injected air and the heating value of the product gas is the output. In this paper, a sliding mode control (SMC) algorithm is designed for a simplified model of an actual UCG process in order to maintain a desired constant heating value. The relative degree of the sliding variable is zero, because the input is readily available in it. As the heating value is the only measurement available, the trivial control design is not possible. Therefore, the time derivative of the control is selected as the system input, and then the relative degree becomes one and the conventional SMC may be implemented. This approach let us maintain the output at the desired level and provides insensitivity with respect to different types of uncertainties. The stability of the zero dynamics is proved, which ensures that the overall system is stable. The simulation results demonstrate the robustness of the SMC design against the input disturbance and the modeling inaccuracies.