Gasifier Outlet Temperature as a Function of the Gas Analysis
Calculating the gasifier outlet temperature from the gas analysis is not so simple. The reason is that it is not possible to measure the composition of the gas when it leaves the reactor. The best one can do is a “postmortem” when the gas has been cooled down. There have been attempts to devise a means to draw a gas sample directly after the gas has left the reactor through a cooled, high-alloy, thin tube and then perform the analysis. The idea is that by freezing-in the equilibria between the various possible reactions, a fair analysis will be obtained. However, the problem is that high-alloy steels mostly contain nickel or other metals, which may catalyze reactions between the various gas components. Moreover, by drawing the sample gas through a thin capillary tube, the exposed metal surface is relatively large and makes the situation only worse. Finally, it is difficult to avoid fouling of the entrance of the capillary tube and keep it open.
Because of these difficulties it is more practical to analyze the gas after it has been cooled down by quenching or by indirect cooling. In the case of single-stage entrained-flow gasifiers, the analysis has to be corrected for the fact that, for example, the CO shift reaction freezes in at a temperature below that prevailing at the outlet of the gasifier itself. For entrained-flow gasifiers the reactor outlet temperature is about 1500-1550°C. In most cases it may be assumed that the freezing — in of the CO shift occurs at a temperature of 1300°C. This causes a small part of the CO that is present in the gas leaving the reactor is converted into H2. This reaction is exothermic, and hence this heat effect increases slightly the duty of the subsequent gas-cooling train.
Cooling the gas by quenching with water will also result in some CO shift. In general, there is always some CO shift taking place until the point where the temperature has dropped to about 1300°C. Only below 1300°C the cooling will be purely physical.
The temperature at which the CO shift reaction freezes can be calculated from the gas analysis of the cooled gas. The first step is to correct the gas analysis for the fact that water has condensed out of the gas, as the gas analysis always takes place after the gas has been cooled below its dewpoint. From this corrected gas composition the kp value of the CO-shift reaction can be calculated.
Certainly where the gas leaving the gasifier has been cooled by quenching with water, it will be found that the corresponding temperature Teq associated with this kp value is much higher than the outlet temperature of the gasifier. This is logical, as much of the water was added to the gas below the temperature at which the CO shift reaction freezes and has only increased the water content of the gas without having any chemical effect. In a trial-and-error calculation, it is now possible to subtract quench water from the gas in such a way that the mass and energy balance tally. The temperature of the water and of the gas after the gasifier reactor must of course be known before this exercise can be carried out. Subtracting the water results in a higher temperature of the gas in the now “shortened” quench and in a lower—but still too high—Teq corresponding to the newly calculated kp value. Hence the gas temperature and the equilibrium temperature come closer together. By repeating this procedure there comes a moment where both temperatures become equal, and this is the temperature at which the CO shift reaction was frozen in (see Figure 6-11).
With the gas analysis it is thus possible to calculate the freezing-in temperature of the CO shift reaction accurately, but it says nothing about the outlet temperature of the gasifier. It is possible, though, to make an element balance over the reactor, and then the outlet temperature can be calculated to within about 30°C. A computer program for calculating the freezing-in temperature of the CO shift equilibrium from the gas analysis is provided in the companion website. It is given for both water quenching and indirect cooling. The indirect cooling can be either a radiant boiler or a gas quench.
The freezing-in temperature of a suitable methane containing reaction can also be calculated.
The best reaction to use (see also Section 2.2.2) is:
CH4+C02 5 2CO + 2H2 (2-10)
The advantage of this reaction is that there is no water present.
The data given for the coal footprint in Figures 2-5 and 2-6 have been obtained taking into account the effect of the adjustment of the CO shift reaction. These data show that, although it has been assumed that the freezing temperature of this reaction is the same in all calculations, the composition of the gas after cooling still clearly reflects the differences in the reactor outlet temperature.
Thus far one may wonder why the methane content in the product gas has been taken into account, as in entrained-flow slagging gasifiers it is only a few hundred
ppmv and hardly plays a role in the overall mass and heat balance. Moreover, forgetting about methane greatly simplifies the calculations. The very reason is process control. In the preceding discussion on the coal footprint it was already mentioned that the iso-methane lines run more or less parallel to the isotherms and hence were a good indication of the outlet temperature of the gasifier. In fact, this is only partly true. They are a valuable indicator of this temperature, but when one calculates what the outlet temperature is on the basis of the gas analysis, one finds values that relate to different temperatures than can be reasonably expected. However, introducing correction factors with the calculated temperatures based on the other gas components, the methane content becomes an extremely valuable indicator of the reactor outlet temperature.