Temperature Control
To use a temperature measurement for process control is especially difficult in entrained-flow gasifiers where the only option is an indirect temperature measurement based on the composition of the product gas. Moreover, the control is complicated by the very short residence time of 1-3 seconds. Operating for a short period at a temperature below the melting point of the ash immediately leads to the buildup of solid slag on the walls and on the bottom of the reactor. Membrane-wall reactors are worse in this respect than brick-lined reactors, as the membrane walls are relatively cool (250-300°C, depending on steam pressure) and keep their heat only for a very short period. Because most modern gasification plants have entrained-flow gasifiers, the discussion on process control will concentrate on this type of gasifier.
Fluid-beds are already less difficult to control because of the much lower temperatures of 900-1150°C and the longer residence time.
The easiest gasifiers in terms of control are moving-bed gasifiers because of their very long coal/char residence time of about one hour. On the other hand, the gases have a short residence time and leave the reactor at relatively low temperatures of below 600°C. Measuring the reactor outlet temperatures gives a fast indication about the gasification temperatures in the bottom of the reactor.