Synthesis Gas Coolers
When the gas from a slagging gasifier is quenched to about 900°C, or where the gas is produced at temperatures in the 900°C to 1000°C range, it has to be cooled further before the gas can be treated for use. Two aspects of gas cleaning have to be considered carefully and intimately with the design of this section of the cooling system. These are particulate removal and condensation, whether it be condensation of tars from biomass gasification, for example, ammonium chloride from coal gasification, or simply water.
The first cleaning stage after the syngas cooler comprises the removal of any solids present in the gas. Effective solids removal is only possible at temperatures below 500°C, whereas for the removal of acid gases and ammonia the gas has to be further cooled to essentially ambient temperatures.
The lowest-cost method of cooling the gas is to continue quenching to the temperatures required for the gas cleaning. This practice is only possible with a water quench. It results in the gas being loaded with even more steam which then has to be condensed out when acid gases and ammonia are removed from the gas.
In a typical syngas cooler the gas is cooled from 900 to 300°C. At 900°C there are no sticky ash or slag particles left in the gas, and at 300°C there is as yet no chance of NH4C1 deposits. This is the ideal temperature range for raising good quality steam and for preheating the clean gas that is obtained after the near ambient temperature gas treating. Such preheating is beneficial, for example, when the gas is later used as clean fuel gas in a combined cycle power plant. Whether such preheating is economic is another matter, as it requires an expensive gas-gas heat exchanger. For corrosion and other material reasons, the metal temperatures should not exceed 500-600°C. When steam is to be made, the highest temperature gas is therefore used evaporation, followed then by a superheating section, and finally for further evaporation and water preheat. The temperature range of 300 to about 40°C is used for water preheat.
Where a gas quench is used, all the sensible heat in the gas leaving the gasifier is used for raising additional steam, which results in high efficiencies of the IGCC power station. The drawback is that this also results in the highest cost syngas cooler.
The syngas cooler is often one of the most expensive items in a gasification complex. Expensive high alloy steels have to be used in many places, as all the contaminants are still present in the gas. There is fly slag, which leads to erosion. There are also sulfur compounds, chlorine compounds, and so on. Frequent rapping of the boiler internals may be required, for example, to remove deposits from the boiler tubes. In order to accomplish this, expensive penetrations have to be made through the pressure wall of the syngas cooler.
As in so many occasions in gasification, there is the classical trade-off between efficiency and capital cost. Water quenching is cheap, but then efficiency is reduced; whereas with a syngas cooler, especially in combination with a gas quench, the capital costs are high but so is the efficiency.
There are two principle designs for syngas coolers: water-tube boilers and fire-tube boilers. Both have been operated successfully in various plants. Fire-tube boilers are lower in cost but have certain limitations, particularly with high-pressure steam. In practically all applications, the steam pressure is greater than the gas pressure, so that the tubes are subjected to external pressure. Depending on the details of the individual design, maximum steam pressures for fire-tube boilers he between 100 and 150 bar. An advantage of fire-tube boilers is the well-defined flow of the gas in the tubes, but the inlets need to be designed carefully in order to ensure that the dustladen gas does not cause erosion. Another detail to which attention must be paid is the adequacy of the cooling at the inlet where the heat fluxes are very high. In the field of oil gasification, fire-tube boilers are used almost exclusively, and some examples are discussed in Section 6.6.3.
With water-tube boilers the local flow pattern around the tubes is less even than a fire-tube boiler, and there can be areas of almost stagnant gas with the attendant risk of dust accumulation. A number of designs include rappers to shake off any dust (Keintzel and Gawlowski 1993). On the other hand, at the HTW plant at Berrenrath, rappers originally included as part of the design were dispensed with after tests showed them to be unnecessary. In fact, at this plant both fire-tube and water-tube boilers were tested in parallel, and both were deemed to be satisfactory. The conclusion of these parallel tests was that economics would be the deciding factor in syngas cooler design selection (Gorges, Renzenbrink, and Wischnewski 1998).