Overview of Particulate Removal Technologies

Particulate removal requirements vary depending on the use of the product gas. For example, particulate levels must be reduced to below 50 mg/Nm3 for gas engines and below about 15 mg/Nm3 ( 5 im) for turbines, and to perhaps 0.02 mg/Nm3 for synthesis gas systems. The primary types of systems include cyclonic filters, barrier filters, electrostatic filters, and wet scrubbers. 

Cyclonic filters are a primary means of removing bulk particulates from gas streams. They rely on centrifugal force to separate solids from the gas by directing the gas flow into a circular path. Because of inertia, the particulates are unable to follow the same path and are separated from the gas 

Cyclonic filters (and closely related designs such as U-tubes) are employed as an initial gas cleanup step in most gasifier systems because they are effective and relatively inexpensive to operate. In circulating fluidized-bed or entrained-bed gasifiers, cyclones are an integral part of the reactor design, providing for separation of the bed material and other particulates from the gas stream. 

Cyclone filters are effective at removing larger particles and can operate over a wide range of temperatures, limited primarily by the material of construction. Cyclone filters are often designed as multiple units in series (multi-clones). They can remove 90% of particulates above about 5 microns in diameter at minimal pressure drops of 0.01 atm. Partial removal of material in the 1.5 micron range is also possible, but cyclonic filters become ineffective with sub-micron particles. 

Since cyclone filters can operate at elevated temperatures, the sensible heat in the product gas can be retained. Cyclone filters also remove condensed tars and alkali material from the gas stream, although the vaporized forms of those constituents remain in the gas stream. In practice, the separation of significant amounts of tars from the gas stream may be done sequentially by first removing particulates at higher temperatures, where tars remain vaporized. The gas stream is then cooled and condensed tars are removed. The step-wise approach reduces the tendency of particulate material to stick to tar-coated surfaces and contribute to plugging. 

---