Wet Solids Removal
In most existing plants the solids are washed out with water in Venturi scrubbers or wash towers. This scrubbing takes place below the dewpoint of the gas so that the finest solid particles can act as nuclei for condensation, thus ensuring that all solids are removed efficiently. The wet removal of solids causes them eventually to appear in the filter cake in the water treatment. The disadvantage of wet solids removal is that it is more difficult to separate the ash from compounds containing lead, zinc, cadmium, arsenic, and others, and thus increases the amount of chemical waste.
In this chapter the control of the gasifier itself will be discussed. This excludes the process control of, for example, an IGCC where the gasifier is often closely integrated with the ASU and the CC. However interesting the complex control of such systems is, this will not be discussed as it falls beyond the scope of this book.
6.8.1 Gasification Temperature Measurement
When operating a gasifier, one wants primarily to control the temperature. As discussed in Chapter 2, the temperature is the principle variable determining the gas composition.
The temperature is decisive in relation to the ash-rejection regime, whether slagging or nonslagging. Ultimately, too high a temperature will destroy the integrity of the reactor containment, be it refractory or membrane wall.
Unfortunately, any accurate measurement of a gasifier temperature is extremely difficult, both for a number of practical reasons and, surprisingly, also for one theoretical reason. Let us look at the latter first. The problem is that where there are still solid particles present that contain carbon, the measurements are influenced by the phenomena of the “chemical wet bulb temperature.” This effect was first explained by van Loon (1952, pp. 17-34), who showed that at the surface of the solid carbon only endothermic reactions with H20 and C02 occur, whereas in a sort of halo around the particles part of the CO and H2 formed by these reactions, react exothermically with oxygen to form H20 and C02 (see Figure 6-10). This mechanism renders pyrometers of little use for exact measurement, since one cannot establish whether the temperature measured is dominated by the relatively cool but more strongly radiating solid particles or by the hot gases in the halo.
The practical problems of temperature measurement are particularly relevant to entrained-flow slagging gasifiers. Partly this is caused by the harsh conditions of high temperatures per se, but also by the fact that slag can attack ceramic protective sheathing around the thermocouple, causing erosion damage to thermocouples by the ash and slag and allowing hydrogen to penetrate into the metals of the thermocouples and causing faulty readings. Where nitrogen or other purge gas is used to protect the thermocouple assembly, local cooling occurs, which gives rise to understated temperatures. A further disadvantage of thermocouples is that their exact location has a significant influence on the accuracy of measurement. In refractory-lined
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Figure 6-10. Van Loon’s Gasification Model |
gasifiers, the tip of the assembly is typically located slightly withdrawn into the wall, so as to protect it from slag or other erosion damage. The actual temperature measured is closer to that of the refractory rather than that of the reactor core and thus highly dependent on the extent of the depth of withdrawal from the reactor space.
Despite these disadvantages, platinum-rhodium thermocouples are still the most common device currently used for gasifier temperature measurement. It is accepted that real accuracy of temperature measurement is less important than consistency. For the gasifier operator, who has set his feed inputs on the basis of other parameters, a continuous and steady temperature reading is more important than the absolute value shown.
Nonetheless, investigations continue to develop alternative methods, not least because in oil gasification thermocouple life can be run-length determining. Systems under consideration or development include the following:
• Pyrometers. Texaco has used a pyrometer in its pilot unit for several years, and this will shortly be tested in a commercial environment (Leininger 2002). The principle advantage of such a system is that the sensor is located outside the reactor and not subject to the harsh environment. The necessity to ensure pressure integrity, including a high pressure nitrogen purge, does make it expensive, however. Interestingly, the actual temperature measured is dependant on the gasifier fuel. With gas firing, the visible path reaches to the opposite wall of the reactor, so that the temperature measured is that of the refractory. Depending on the degree of solids in the reactor, the visible path may reach only to the center, that is, the hottest location in the reactor, or even less where the temperature is cooler again. Furthermore, the nitrogen purge can cool the slag around the line of sight of the pyrometer leading to a loss of reading. Interruption of the nitrogen purge can solve this problem online, a distinct advantage over thermocouples, which generally require a reactor shutdown for replacement. Interpretation of such a loss of reading does require additional temperature measurements, so that any commercialization of pyrometry is likely to be in addition to rather than as a replacement of thermocouples.
• Steam make in the membrane wall. This measurement is of course limited to reactors having a membrane wall or water jacket. As was already mentioned in Section 6.4.2, the steam make in the membrane wall is a valuable indicator for both the heat loss through the wall and for the reactor temperature. It has the advantage of being an integral measurement of the temperature anywhere near the wall of the reactor. It is fast, with a response time of less than a minute, and very reliable. It does not, however, provide local temperature values.
• Heat flux measurement. This measurement comprises installing a small piece of membrane wall in the wall of a reactor and measuring the increase in water temperature of a known flow of water through the membrane wall. It can give a fast—10-30 second response time—indication of the local temperature.
• Microwave-based measurements have also been considered.
• Other devices. A new system based on temperature-dependant changes in the optical properties of single-crystal sapphire is under development. Despite some promising results for other applications, this sensor will still have to survive the difficult reactor environment, and the fundamental uncertainties of temperature measurement in a gasifier will remain (Pickrell, Zhang, and Wang 2002).
• Using the gas analysis. Using the methane or the C02 content in the gas can give a valuable indication about the reactor temperature. The advantage of this method is that it gives an integral measurement of the temperature at the reactor outlet. But it does not give an indication about local hot spots. Moreover, the measurement has a certain time lag that with modern gas analyzers can be reduced to less than a minute. For the interpretation of the gas analysis, see Section 6.8.3.