Gas Cleaning Filters
Filters are one of the most common methods for removing particulate matter and are effective for a very wide range of particle sizes. Barrier filtration occurs when a gas stream passes around fibers or granules or through a porous monolithic solid [1].
Particulate matter is removed during filtration by a combination of four different mechanisms:
1. Diffusion
2. Inertial impaction
3. Direct interception
4. Gravitational settling
There are different types of filtration devices that can be used in biomass syngas cleaning. These include: fabric filters, rigid filters, and both fixed — and moving-bed granular filters.
Fabric filters are effective in removing particulate matter even smaller than 1 mm to concentrations less than 1 mg/m-3. However, the operation temperature of the fabric filters are generally limited to around 250°C, which classifies them as a warm gas cleanup method [25]. Filters made from ceramics and metals can stand much higher temperatures, and these are also known as rigid filters. They have advanced in recent years to the extent that they can remove 99.99% of particulate matter smaller than 100 mm while operating at temperatures beyond 400°C [26]. Candle filters are another type of high temperature gas cleaning devices. These filters are prepared in the form of hollow tubes, and are primarily composed of porous ceramic materials. In this technique, dirty gas passes through the outside of a long, closed-end tube (or cone), depositing the particles on the surface before exiting through the top of the tube. The resulting accumulation of particulate matter, known as filter cake, is periodically removed with a reversed pulse of gas, typically nitrogen. Several candle elements are placed in parallel to form an array so that several filters are always operating while others are being cleaned. Candle filters are commonly constructed of clay-bonded silicon carbide (SiC) as well as materials such as monolithic and composite ceramics [27,28].