Surprises in Calculations
When calculations are carried out for a gasifier assuming heterogeneous reactions, it may come as a surprise that sometimes the quantity of moderator required is negative. The reason may be that the fuel and/or the blast contain so much ballast in the form of ash, water, steam, or nitrogen that the desired temperature can only be obtained by consuming more oxygen than is required to gasify all carbon in the fuel. What happens in the calculations is that the algorithm tries to reduce the amount of moderator, but only when the moderator becomes negative is a mathematically valid solution obtained. In other words, one has reached the area below the abscissa in Figure 2-5. The logical way out is to set the quantity of moderator at zero, but then there is one equation too many, as the ratio of moderator to product gas is set to zero. Again, the solution is found in using the two equilibrium constants of the CO shift and the steam-methane reforming reactions instead of the three for the heterogeneous gasification. Examples where this situation may occur are:
• The use of low-rank coal in a dry coal feed entrained-flow slagging gasifier.
• The use of a coal/water slurry in an entrained-flow slagging gasifier.
• The use of air as blast in entrained-flow gasifiers.
In all cases where no moderator is required, the process control becomes much simpler, just as in a furnace where only two streams have to be controlled instead of three.
