Sizing up particulate separation

Basic theory

How the particles are separated from the flue gases depends to a significant degree on the relative size and density of the particle, and the temperature/viscosity of the flue gas. The main separation mechanisms are:

Sieving

Inertia

Electrostatics

Diffusion

Generally, large, high density particles can be separated by exploiting their inertia or through sieving techniques. Smaller, lower density particles are not as susceptible to inertial separation or sieving from a gas stream but tend to be separated by other means. Their separation can be either through diffusion to collecting surfaces due to Brownian motion, or through electrostatic separation due to particle/collector charge.

One or more of these mechanisms form the basis on which particulate collection devices are designed. There are many different devices in operation ranging from simple particle fall-out vessels to highly sophisticated filtration systems. The main ones are described below.

Cyclones and multi-cell cyclones

Cyclones of all types rely on separating particulate from a gas stream by inertia; the momentum of the moving particulate being high enough for it to break away from the gas stream lines. This is achieved by applying a force to the particulate material, usually in the form of a centrifugal force generated by making the gas stream change direction. Gas entering the cyclone is induced to rotate within the cyclone forming a vortex which applies a centrifugal force to the gas stream, forcing the larger/denser particles outward on to the inside wall of the cyclone.

A number of cyclones may be fabricated in parallel with common inlets and outlets and dust collection hoppers. These are known as multi-cell cyclones and offer the same function as single cell cyclones in a more compact unit; lack of space often being a problem in process buildings.

Cyclones are relatively small, and cheap to install and operate. Construction is simple, with no moving parts, and they can be fabricated in a variety of materials including ceramics and metals although mild/steel and stainless steel is typical for flue gas applications. The main disadvantage is their performance which, in practice, is often no better than 100% collection of particles down to 50 microns, becoming progressively less effective as the particle size becomes smaller. When viewed in the light of current emissions legislation, this severely limits their application.

Electrostatic precipitators (ESPs)

Electrostatic separation is a technique involving two processes, particle charging and particle collection. The flue gas is passed into an electric field produced between an array of wire cathodes and plate anodes. Corona discharge around the wire cathodes causes ionisation of gas molecules and emission of electrons which are attracted to the plate anodes. Some of these electrons collide with the fly ash due to Brownian motion and field induced motion, negatively charging the particles. These then (migrate) toward the plate anodes, where they form a dust cake. At regular intervals the plate electrodes are mechanically rapped to dislodge the dust cake, allowing it to fall into the collection hopper(s) beneath the electrode array.

In practice, ESPs are large vessels containing a number of wire cathodes and plate anodes hung vertically. Direct current is applied across the wires and plates from large transformer/-rectifier sets which provide the electrostatic field and the corona discharge current. Particle migration velocities, under the influence of the electric field, tend to be low. This results in ESPs being very large, particularly in cross-section, to allow very low gas velocities through the device. It is common practice to place more than one wire cathode and plate anode set in series to increase efficiency.

ESPs can collect a wide range of particulate material effectively to very low particle sizes. Against this is their size and high capital cost, and their performance is also sensitive to process conditions and in particular fuel/waste type, flue gas temperature and flue gas humidity.

Fabric, metallic, ceramic filters

Filtration relies on a number of separation mechanisms to remove particulates from flue gas streams. These include sieving of large particles on the filter surface, inertial separation of smaller particles on individual fibres/yarns in the filter medium and diffusional separation of very fine particulate by Brownian collision with individual fibres also within the filter medium. However, once a fly ash filter cake has formed on the surface, it is this cake that performs the filtration function.

Filtration media vary in construction and material type. Typically, they are woven and felted fabrics in various fibre/yarn materials including glass fibre and a range of high performance polymeric fibres which enable the filter to operate at temperatures up to 260oC. There are, however, applications in waste incineration where temperatures are in excess of this and, to meet this need, filters are now being made from metallic and ceramic materials in flexible and rigid formats.

The main features of fabric filtration are the filter casing, gas inlet and outlet ducts, a solids collection hopper, the filter medium (normally comprising vertically hung cylindrical filter elements), support cages and a cleaning system to remove the filter cake. There are various types but the ones used in Europe are typically of the outside-in, pulse-jet type. In this design, dirty gases flow from the outside of the filter bag to the inside and the filter cake is periodically removed by injecting a pulse of high pressure air into the clean side of the filter element.

Filters are very efficient collectors of all particulate down to fine particle sizes but are large physically and involve high capital outlay. Operating costs can be significant if care is not exercised at the design stage to correctly specify the filter plant, but with careful operation and maintenance good performance and long life is achievable.

Wet gas de-dusters (scrubbers)

Wet gas de-dusting involves mixing and contacting the particulate in the flue gas stream with a washing liquor within a scrubber vessel. The particulate are retained in the washing liquor, and the particulate free flue gas is allowed to exit the de-duster to atmosphere. A variety of scrubber designs exist, many of which arose in the chemical process industry, including the venturi scrubber, the plate column scrubber, packed bed scrubber and the foam scrubber.

All scrubbing systems require ancillary equipment to circulate and treat the washing liquor. This treatment plant will normally include a sedimentation/filtration step, to remove the collected particulate, as well as chemical treatment to collect any soluble materials the de-duster may have picked up from the flue gases.

The performance of wet de-dusters is variable changing with a number of parameters including types of washing liquor, flue gas flowrate and washing liquor flowrate, but is limited in many designs to the capture of larger particulate capable of inertial separation. The additional problems of liquid effluent treatment frequently mitigate against their use.

Design and selection

Particulate collection equipment should always be considered an integral part of the whole process and is designed and evolved with the rest of the waste combustion plant, not simply added as an afterthought. There are many examples where this advice has not been heeded and the resulting under performance has been costly. Poorly designed equipment is generally expensive to maintain and operate, and process downtime can become significant.

There are many issues to consider during the selection process including the waste stream, the combustor type, the heat exchange design and the process. The choice of ‘dry’ filters/ESPs or ‘wet’ de-dusters will often depend on existing site facilities, able to treat either the dry solids from the filter/ESP or the wet effluent from the scrubber.

There is a continuing need to improve and develop particulate collectors as legislation becomes increasingly stringent. Already, for some processes, current emission limits can be as low as 10mg/Nm³ and there are moves to introduce tighter limits on the capture of very fine particulates below 5 microns. Cyclones and conventional wet gas de-dusters will struggle to meet these new standards, leaving filters/ESPs as the BATNEEC option.