Modelling and control of industrial gas headers to improve flow stability and reduce pollution

Abstract

The thesis is developed in three sections to realise the objectives of using the inherent storage capacity in industrial gas headers to mitigate pollution due to gas emissions and improve the flow stability of consumers.

The first section develops a first principles nonlinear model to capture the transient flow and pressure behaviour of gas in industrial pipelines for the use in simulation and control applications. Hyperbolic partial differential equations describe the pipe pressure and flow profiles, and composition analyses are used to develop the physical properties of the gas appearing in the pipe segment equations. The spectral element method (SEM) is used to discretise the spatial profiles within the pipeline, and a model verification is done on the SEM tuning parameter choices. The model is developed into a compact state-space description for improved usability. Furthermore, the developed model achieves good accuracy when validated on real process data of an industrial methane rich gas (MRG) network and can be used for simulation and model-based control applications.

The second section uses the first principles model of the pressure and flow dynamics inside a gas header to obtain the optimal pressure measurement location for buffering control. The pressure states at the collocation points are estimated using an extended Kalman filter (EKF) and subsequently used as potential measuring locations. A staggered regulatory pressure control scheme is used to evaluate the closed-loop performance of the objective function in a self-optimising control (SOC) framework. It is found that there are opposing goals in the optimisation scheme and that the final pressure location selection is dependent on the economic importance of these goals. The economic goals which cause opposing control objectives include minimising gas emissions, providing consumer stability, and limiting expensive supplier use.

The third section investigates various regulatory and advanced control schemes which can be applied to industrial gas headers. The intention is to exploit the buffering capacity for pollution control as well as improve flow stability for consumers. The control schemes are compared using a Monte Carlo simulation on a simulated case study and a sensitivity analysis is done to evaluate the impact of variations in the gas properties on the cost functions. A compensated linear model predictive controller (CLMPC) is implemented on a real industrial header and compared with standard proportional–integral (PI) control. It is found that the CLMPC reduced emissions and improved consumer stability by intelligently utilising the available pressure buffering capacity in the industrial gas headers.