DIESEL PARTICULATE FILTERS     

Diesel particulate filters (DPF) also called as ‘particulate traps’ have been developed to filter out PM from the diesel exhaust gases to meet very stringent emission limits. Alumina coated wire mesh, ceramic fiber, porous ceramic monoliths etc., have been studied as filtration media. Presently, ceramic monolith of honeycomb   type structure is used to trap the particulate matter as the gas flows through its porous walls.  These filters are also termed as ‘ceramic wall flow filters’.
A ceramic honeycomb type particulate filter is shown in Fig. 6.14.  In this cellular structure, alternate cells are plugged at one end and open at the opposite end. The exhaust gas enters the cells that are open at the upstream end and flows through the porous walls to the adjacent cells. The adjacent cells are open at the downstream end from where the filtered gas exits .to the atmosphere. Flow path of gas through walls of the filter is also shown on Fig 6.14.

Some advantages of wall flow filters over other filtration media are;

• The wall flow  filters  have a large filtration surface area per unit volume
• The pore size of walls can be controlled to provide gas flow without excessive pressure drop.
• Very high filtration efficiencies close to 98% are possible with ascceptable pressure drop.