A process engineering approach to the ALSTOM gasifier problem

The approach made here to designing a coal gasifier control system has its roots in process engineering. The favoured solution is largely based around conventional single-input single-output loops which control gas calorific value with air flow, gas pressure with steam flow, gas temperature with char flow, and bed mass with coal flow. Evolution towards this solution is discussed in relation to three intermediate schemes (in some cases involving non-linear control elements) that were found viable but less effective in terms of the defined performance criteria. The favoured solution fully meets the required performance in both the 100 and the 50 per cent load cases but at 0 per cent load, while responses are all stable, there are constraint violations which are transient following the pressure step disturbance but sustained for the sinusoidal disturbance. An important operational issue to be addressed in relation to the proposed scheme is the management of automatic-manual transfers for some loops to avoid possible system instability. During the study a number of issues, including equipment design and measurement precision, were identified as important. Their influence on controllability of the gasifier, and on the robustness of the proposed control scheme, is discussed.