Bag Filters
Bag Filters (BFs) have been used in the cement industry well before Electrostatic Precipitators (EPs) were developed. BFs use a filtering medium, the bags, to separate the dust particles from the exhaust gas. While the exhaust gas can pass the bag tissue the dust particles are captured on the bag surface.To further reduce pressure drop and dust emission of a BF, bags with special surface treatment or cleaning support (e.g. acoustic horns) can be installed. The main disadvantage of bag filters is the high pressure drop over the whole filter (8 – 20 mbar) causing comparatively high power consumption at the filter fan. Although BFs have a very high efficiency, their performance deteriorates formechanical and process reasons. Bags have a limited life and will give high emissions through the development of pinholes, the occasional tearing of a bag or the failure or gradual deterioration of the seal at the fixing point of bags. It is important to prevent hot particles being carried in the gas stream into the filter, and to avoid severe temperature surges, which can cause disastrous failure.Large modern BFs can be divided into separate compartments which can be separately isolated. This increases the size and cost of the filter but it allows on-line maintenance. For the operator the problem is that, if the emission monitor indicates an increase in emission, and this increase has occurred progressively through a multiplicity of small holes/leaks, it can be time consuming and expensive to find and rectify the faults.The main advantage of bagfilters is that the dedusting efficiency is always very high even if the process parameters are changed (e.g. during start up or shut down of the kiln, clinker cooler or cement mill, during switching from compound to direct operation (raw mill on -> raw mill off), during occurrence of CO peaks or any other process disturbance).The BFs can be subdivided into reverse gas BFs and pulse jet BFs. The dedusting efficiency of the two systems is very similar.
Reverse Gas BFs
The main difference between a reverse gas BF and a pulse jet BF is the cleaning method for the bags.The reverse gas cleaned bag filters usually contain woven filter bags (usually glass fibre bags with special surface coating). The raw gas enters the bags from the bottom. It flows from the bag centre to the outside of the bag. The dust is deposited on the inner surface of the bag. Removal efficiency is improved and maintained by these particulate deposits (residual dust cake). With time more and more particulates are deposited on the bags and increase the system resistance to the gas flow (pressure loss).To allow the filter fan to operate within the design parameters and to reduce the fan power consumption, this dust cake must be partly removed. Bag cleaning methods must be designed properly – not over-cleaning or under-cleaning. Otherwise increased dust emission or high pressure loss result.The bag cleaning process is triggered either by a timer or, better, when the pressure drop over the bag filter reaches some predetermined level. A reverse air bag filter consists of several compartments, usually more than ten. When the bag cleaning process is started, the outlet valves of one of the compartments are closed (off-line cleaning). Then, an auxiliary fan forces a relatively gentle flow of filtered gas backwards through the compartment and bags to be cleaned. This causes the bags to partially collapse inward, dislodging the dust cake. This falls through the bags, the thimble and the tubesheet into the hopper. Metal anticollapse rings sewn into the bags along their length prevent complete bag collapse. One square meter of a bag can filter up to 45 m3/h of exhaust gas.Reverse gas bag filters are very well established in North and South America for kiln exhaust gas dedusting. The operating and investment costs of reverse gas BFs are usually higher compared to pulse jet BFs and EPs because of the filter size and the more expensive bags.
Pulse Jet Bag Filters
The application of pulse jet BFs for dedusting of air from mills, coolers and transport systems is already standard. However, the dedusting of kiln exhaust gas with pulse jet BFs is very new and only started a few years ago. They have some advantages because of the reduced space requirement and reduced investment and operating cost compared to reverse air BFs. Pulse jet BFs for kiln exhaust gas dedusting are mainly used in Europe.Pulse jet cleaned BFs normally employ felted fabrics of various types. The raw gas enters the bags from the outside. The cleaned gas flows through the centre of the bag to the clean gas plenum and from there to the stack. The dust is deposited on the outer surface of the bag. To prevent bag collapse during filtering, metal cages are inserted inside each bag. Just like the reverse-gas cleaned bag filter, periodic bag cleaning is required to remove excess residual dust cake. This is accomplished by pulsing compressed air down into each filter bag. Bag cleaning can be carried out either with the compartment isolated or not isolated (on-line or off-line cleaning). One square meter of a bag can filter up to 90 m3/h of exhaust gas .
Electrostatic Precipitators
EPs were developed for use in cement production, initially on kilns, in the 50s and 60s. EPs use electrostatic forces to separate the dust from the gas. Discharge electrodes under high negative voltages (50 – 100 kV) emit electrons which settle on the dust particles. The now negatively charged particles are directed towards and separated on the collecting electrodes because of the electrical field between the discharge and the collecting electrodes. Then the dust particles accumulated on the collecting electrodes are discharged to the dust hoppers by electrode rapping.The efficiency of EPs can be reduced during changes of the process parameters (e.g. during start up or shut down of the kiln, during switching from compound to direct operation and during occurrence of CO peaks). EPs are easy to maintain although on-line maintenance is not possible.The main advantage of the EPs is the low pressure loss over the filter and therefore a reduced power consumption of the filter fan. The other advantages are the separation of coarse and fine particles in the filter for efficient evacuation of circulating elements and a constant pressure loss over the filter which allows constant high clinker production.There are also means available to upgrade and optimise already existing EPs. Especially improved exhaust gas conditioning (e.g. conditioning tower) and modern control systems for EP energization have been shown to be very effective.
Other Abatement Techniques
Cyclones and gravel bed filters are not installed any more for the final dedusting stage because of their reduced efficiency and high operating cost.CyclonesCyclones have been used in the distant past to reduce material loss from kilns and more recently high efficiency designs have been installed on clinker coolers. They are not capable of achieving the emission levels now needed for final exhaust and they are not now installed.Gravel Bed FiltersGravel bed filters have given good service on clinker coolers but their maintenance costs are high and their efficiency is not comparable with that obtainable from EPs and BFs. They are no longer installed.
