Process Indicators
When calculating the coal footprint it is very useful to draw also the iso-C02 and iso-CH4 lines (Figure 2-6). The fact that the iso-C02 lines run more or less perpendicular to the isotherms and the iso-CH4 lines more or less parallel, shows that the CH4 content of the product gas is a better indicator for the temperature at which the gas leaves the gasifier than the C02 content. The latter can only be used in cases where the oxygen/steam ratio is fixed, or where no steam is required as moderator for the gasification.
Another good indicator of the gasification temperature is the heat flux through the reactor wall. Where a tube-wall or a jacket is used to protect the reactor wall, the steam make in the protection is a very valuable indicator for the reactor temperature (provided that this steam production is not integrated with other steam systems in such a way that it can not be measured properly).
For the gasifier performance, the measurement of the C02 and CH4 content in the product gas, together with the heat flux through the reactor wall, are the best indicators. In practice, data will be used that are gathered from experience. The calculations will
give good leads during start-up and the initial nonoptimal operation of the unit, but for the fine-tuning the real data are more valuable, as will be explained in Section 6.8.
