OPTIMIZING PROCESS CONDITIONS
Dry coal feed entrained-flow slagging gasifiers operate at temperatures of typically 1500°C. At this temperature the oxygen consumption is high, but nevertheless there is still some moderator required. The challenge is to operate with close to the minimum amount of gasifying agent required, as this reduces the amount of expensive oxygen per unit product gas. But there is more: the less oxygen that is used, the more steam we need. This is not so bad, as oxygen is more costly than steam. However, there is a more important advantage that becomes clear when one looks at the two most relevant gasification reactions for this type of gasifier:
C + Vi02=C0 -111 MJ/kmol (2-1)
and
C + H2O^CO + H2 +131 MJ/kmol (2-5)
The heat balance dictates that most of the gasification will be accomplished via reaction 2-1, but every carbon that can be gasified should be gasified with steam via reaction 2-5, as this reaction yields two molecules of synthesis gas per atom of carbon with cheap steam, but reaction 2-1 yields only one with expensive oxygen. More about optimization will be discussed in Section 6.8.
In order to optimize the process conditions in a dry coal feed entrained-flow slagging gasifier, first the equations are solved for the heterogeneous case, as this gives the minimum amount of oxygen per mole synthesis gas. Then in order to obtain a realistic operating window, the calculations are repeated for a somewhat higher C02 content in the gas for the homogeneous case, and that is then made the set point for the further operation.
Although most entrained-flow gasifiers operate with a surplus of gasifying agent (blast), there are and have been exceptions. One example is the first slagging stage of a two-stage gasifier where, also because of a lack of gasifying agent, carbon slips to a second nonslagging stage where it is further converted with steam. Such a gasifier is discussed in Chapter 5.
