Dynamic responses of key performance parameters to O2/coal ratio fluctuations in a commercial Shell coal gasifier: Insights from a pseudo-2D reduced-order model.

• Very slow dynamics of slag layer govern the responses of slag thickness and heat duty. • Characteristic time of heat duty was 4 min, half or less than that for slag thickness. • Lower operating load and slag fluidity extend the characteristic times. • Short O/C fluctuations minimally impact slag thickness, less than Q wall. • Long, large O/C dips form thick slag layers, complicating bottom slag discharge. In commercial coal gasifiers, the oxygen/coal (O/C) ratio is the key operation variable representing the reaction stoichiometry and is carefully controlled considering coal properties and operation load in commercial coal gasifiers. However, the O/C ratio fluctuates frequently from the target value, which causes variations of the gasification performance. In this study, the dynamic response of a commercial Shell coal gasifier to changes in the O/C ratio were investigated using a pseudo-2D reduced-order model (ROM) with focus on three key performance parameters: exit gas temperature (T g,exit), heat duty (Q wall), and slag thickness at the slag tap (δ tap). The ROM was validated for two actual operation cases of the plant and used to analyze the response of the parameters to various simulated cases with step changes, peaks, and dips in the O/C ratio. T g,exit responded almost immediately to the change in the O/C ratio; however, the slow change in the slag layer on the wall with a residence time >20 min considerably influenced the dynamic responses of Q wall and δ tap. The characteristic times of Q wall and δ tap at an operation load of 85 % were approximately 4 and 9 min, respectively. Therefore, usual fluctuations in the O/C ratio with periods much shorter than the characteristic times have a minimal impact on δ tap , which is considerably smaller than that of Q wall monitored by the operators. However, larger fluctuations with periods comparable to the characteristic times can cause changes in Q wall and δ tap significantly larger than the variations in the O/C ratio. In particular, a long, large dip in the O/C ratio should be avoided for coal with low slag fluidity at low loads to prevent the formation of an excessively thick slag layer, which can cause issues in the bottom slag discharge. The dynamic behaviors of the key parameters provide physical insights for improved operational control of the gasifier in which Q wall is the only parameter measured directly. [ABSTRACT FROM AUTHOR]