Gas Analysis
The coal footprint was already discussed in Section 2.3.3. This footprint is only relevant for fluid-bed and entrained-flow gasifiers, as it assumes chemical equilibrium between all major gas components under conditions where methane is the only hydrocarbon that is present. It was concluded that, in general, the methane content in the gas is the best indicator for monitoring the temperature of a nonwater-slurry feed gasifier. In fact, the only reason to calculate the methane content in entrained-flow gasifiers is for process control reasons, as the impact of the methane content (only a few hundred ppmv) on the total mass and energy balance is negligible.
Studying the coal footprint taught that the C02 content of a gas is a dangerous indicator, as the iso-C02 lines run almost perpendicular to the isotherms (see Figure 2-6). Those experienced in furnace control engaged in the start-up of a gasifier may have a tendency to look only at the C02 content in the gas. And there are entrained-flow gasifiers where this works very well, namely with coal-water slurry fed gasifiers. Control is relatively simple in this case, as the only variable that has to be watched is the ratio between the coal-water slurry and the oxygen. And if the reaction takes place in a brick-lined reactor where the heat loss is low, one neither has to worry about the heat loss in the reactor nor about the methane content in the gas. The coal footprint is then simple as it has only two dimensions (no heat loss as an additional variable), and the methane content in the gas is always very low because of the high water concentration, so there is no need to use it for control purposes.
With dry-coal feed entrained-flow gasifiers, life becomes more difficult, in comparison with coal-water slurry-fed systems, now three flows of reactants have to be monitored. Furthermore, dry-coal feed entrained-flow gasifiers so far always feature a membrane wall and hence also the heat loss plays a role. The methane content in the gas analysis must be checked with calculated values based on the mass and heat balance and chemical equilibria, as the methane value depends on variables as the feedstock, reactor geometry, and freezing-in temperatures of the various reactions.