Syngas Cooling in Oil Service
There are a number of syngas cooler designs available for oil gasification service that have given excellent service over many years and that, in contrast to many designs for coal service, are relatively inexpensive. They all have a number of common features, not least that all are fire-tube designs. As discussed in Section 5.4, oil gasifiers are operated so as to leave a certain amount of residual carbon in the gas, and this ensures that the ash passes through the syngas cooler as a dry particulate. The heat transfer in the cooler takes place through a number of coils, which are designed with a gas velocity of 25-35 m/s and arranged vertically in a water chamber. The selection of gas velocity, tube size, and helical arrangement are chosen to ensure that ash and soot particles are transported through the waste heat boiler with neither fouling nor abrasion of the tubes. The tubes have two or three reductions of diameter over their length, so as to ensure that the velocity is maintained in the design range over the length of the tube. The very high gas temperatures and heat fluxes at the inlet demand extreme attention to detail in the design, and the solutions of this issue represent an important differentiation between the various designs.
Borsig Design
The syngas cooler design, which over the years has established itself as the standard for Texaco plants, was originally developed by Steinmuller (Figure 6-8) and is now manufactured and marketed by Borsig. Borsig’s reference list includes 24 units with steam pressures as high as 110 bar.
The salient features of this fire-tube boiler are the coils and the hot gas inlet zone. The coils are made as individual “candles” mounted vertically and in parallel in the water bath. Each coil has its own “tube sheet” in the form of a double-walled tube. Boiler water flows through the annular space between the two tubes by forced circulation to provide intensive cooling. Recent developments include the use of stiffening ribs on the water side and a ceramic coating on the gas side of the inlet zone.
Shell Design
Shell has its own proprietary design for its SGP oil gasification process, which is used in about 135 installations worldwide (see Figure 5-27). Generally, these coolers are designed for the production of saturated steam (up to 120 bar), but designs can include a superheater as was installed for the natural gas fired unit in Bintulu. A design for a residue-based application with superheater has been announced (de Graaf and Magri 2002).
The Shell syngas cooler addresses the same issues as those described above, namely safe passage of the solids through the boiler coils and intensive cooling of the gas inlet zone. The Shell coil design integrates the coils into a single helix rather than having each coil separate. This keeps the radii of curvature larger than in the Borsig design, and thus the potential ovality of the tubes after bending is less. Since the mechanical design of the coils is defined by the maximum external pressure from steam side to gas side, this makes design for very high pressures somewhat easier.
Shell employs a patented double-tube sheet, one to provide the mechanical rigidity required, and in front of it a thin heat shield supported by the tubes. The incoming fresh boiler feed water cools the intermediate space.
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Figure 6-8. Borsig Syngas Cooler (Source: Becker 1969) |
A recent entry into the market for oil gasification syngas coolers is Alstom. For the tubes Alstom uses a multiple-candle concept similar to that of the original Steinmuller design. Alstom has its own solution to intensive cooling of the inlet zone (Alstom undated).