A novel control system at DOFASCO's 2 natural gas mixing station

This paper describes a novel automatic control scheme that has been successfully implemented to control the 2 natural gas mixing station at Dofasco (a fully integrated steel producer located in Hamilton, Ontario, Canada). The purpose of the mixing station is to supplement natural gas into the coke oven gas grid to ensure a reliable supply of gas fuel to the end users. Due to the many constraints associated with controlling the station, it was initially thought to be too complex to be successfully controlled automatically. Under a manual operation, it was very difficult to consistently provide optimal control, with the result that more than desired amounts of gas flaring occurred. The final control scheme that was implemented was composed of two standard Smith Predictors. The logic of switching between the two controllers is rule based and is implemented by a multiplexer. The control scheme also incorporates feedforward logic to compensate for large, quick swings in gas fuel demand. The full control scheme was implemented in August 1997 on a DCS using its standard software. Many significant direct and indirect benefits were realized from this application. This paper highlights the unique challenges of controlling the process, details on the final design of the control scheme and the benefits achieved. The main contribution of this application is to illustrate that the use of many simple controllers can be made more powerful by letting process conditions dictate the need for selecting the active control mode.