Hi formulation of controller synthesis for emission reduction in the control of utility boilers

The recently developed Hi control methodology addresses the problem of synthesis of a controller which minimizes the magnitude of the transfer function between the disturbance and the output error. This makes the methodology directly suitable for the environmentally sensitive controller synthesis. Indeed, if as a part of the normal operating requirements of a boiler one specifies the emission levels for several pollutants, such as CO, NOX, particulate, and metals, it is then natural to design the controller which minimizes the deviations from these levels induced by the disturbances, such as change in the load demand. A very important feature of this methodology is in its capability to provide a means of attaining the best minimization in the presence of significant modeling errors. This feature becomes critical in the emission-sensitive boiler control, since the emission dynamics are usually poorly known and approximately modelled. Another important feature of the Hi design framework is its ability to address emission specifications even for the on-line unmeasurable able quantities, such as metals, as long as there is a model relating emissions to the real-time measurable variables, such as steam temperature, pressure, flow rate, and excess oxygen. This paper presents the mathematical formulation of the environmentally sensitive controller design framework based on the Hi methodology and discusses problems needed to be solved in order to make the proposed methodology possible.