Selection of proper filter media
The fabric selection is based on process parameters with normal and worst duty conditions.
To improve stack emissions day by day, stringent air pollution control norms imposed by CPCB have become a priority for cement plants operations. In the last two decades, pollution control norms and trends in India are reducing in every sectors. This awareness is a prime requirement of every human being. The term, particulate matter (PM) describes suspended solids in air, regardless of its origin, covering vide diameter range right from 0.01 microns (virus, tobacco smoke) to 10-100 microns in the form of dust, sooth and ash. The basic classification for PM2.5 and PM10 are more concerned as far as health and airborne diseases caused to human beings. These sub micron particles when inhaled during normal breathing settles down in respiratory tracks, lungs, and leads to cardiopulmonary diseases or even lung cancer.
Now the challenges are to control, monitor suspended sub micron particles in ambient air. At the same time, in all types of industries, a notification is given by CPCB to control, monitor sub micron particles of PM10 and PM2.5. If any violation of these standards found for particular industry, then the central government has been given full rights to the CPCB authority to shutdown/lock that industry, without giving any reason.
To implement such plans, a timeframe was given to industries to refurbish their process equipment to meet new standards. The conventional technology of APC (air pollution control) equipment like ESPs, cyclones and multi-clones are now absolute to meet new emission standards. The solution is to convert all these conventional equipment to new highly 99.99 per cent efficient bag filters, bag houses and hybrid-bag filters technology. The designing of such high efficiency bag filters and bag houses is a challenge to all OEMs.
The internal construction design engineering, gas distribution, low power consumption, selection of proper filter bag media and its surface treatments are the major key areas where system designer has to be worked out in-depth.
Multi cyclones, cyclones and ESP's can't bring down emission <20 mg/nm3. So what is the solution? Only bag filters and bag houses are capable of giving constant emission less than 10-20 mg/nm3.
In non-woven fabric filtration techniques, the conventional fabric cannot meet new emission standards. So, ePTFE membrane lamination, which is nothing but a micro porous thin film, which is thermo-bondable process, was developed. In the past one decade, most of the conventional media filter bags were replaced by membrane laminated bags. In cement plants, the major use of filter bags are from polyester, either it is in nuisance areas or to control fugitive emission. The main process bag filters or bag houses where product recovery is a prime requirement, upgraded nonwoven fabric with ePTFE lamination is widely selected.
The fabric selection is based on process parameters with normal and worst duty conditions. Chemical or surface finishes are required to enhance bag life as well as to get nearly ZERO emission, It is a task in front of technical felt manufacturers. The fibre selection - right from its shape, denier, length, and during manufacturing to maintain uniform permeability, maximum tensile/bursting strength, consistency in GSM and proper blending during manufacturing, is a key factor to get best emission and bag life results. Moreover, surface finish and chemical treatment also plays an important role. ePTFE membrane lamination on nonwoven or woven fabric is quite a bit an advance technology to improve quick release of dust from surface of filter bags to achieve low filtration drag.
Not only fibre and fabric selection is a criteria for selection of fabric suitable for particular application, but additional drill down process parameters like gas composition, particle size distribution, dew point, CFD study, VDI testing becomes an advance technical validation checkpoints before selecting the right fabric. Even MIT flex testing, which decides or project what will be the life of filter bag.
From years together, in cement plants from crusher to packing plant, bag filter sizing and fabric selection is fixed. To meet new challenges of emission norms, an environmental/performance auditing should be done. The audit report results will give a direction for selection of best and suitable fabric considering power savings in ID fans and compressor air consumption. From auditing report where is the scope of optimisation in bag filter operation will bring daily cash savings in power. The reduction in stack emission by replacing conventional filter bags by membrane laminated bags will give extra collection of fine dust, which can be recycled.
For cement industries, how to reduce power consumption per tonnes of clinker production and reduction of pulverised coal as a fuel is a key factor to sustain in the competitive market. So, using alternative fuel resource (AFR) has become mandatory to run kiln with combination of AFR and coal. In this process, how to handle doxins and furans as well as sulphur contains in limestone in dust laden flue gases in bag filters and bag houses have become an additional challenges to filter bags manufacturers apart from new emission norms.
To handle such types of gaseous conditions, a vast R&D is going on catalytic fabric filters where two layer filter bags concept is under study. Outside layer is taking care of particulate emission and inside layer of bag acts as a catalytic filter. The catalytic filter is nothing but a converter of doxin, furan gases to HCL, CO2 and some less percentage of H2O.
The other method to neutralise such toxic gases is by adsorption method, where activated charcoal/lime is injected in gases entering into bag filter. In this process, highly contaminated doxin waste gets collected in bag filter hoppers. To handle such future gas compositions from kilns, filter bags manufacturers will have to geared up for development of combo type fabric to meet new challenges of emission as well as to handle toxic gases in stacks.
Industrial Filters & Fabrics (P) Ltd is the only company in India, having full range of nonwoven fabric manufacturing facility, in line with largest bags as well as cage making capacity and back up with technical support services. At IFF, a continuous R&D in fabric with new combination of woven as well as nonwoven fabric will be the most important factor to remain and grow in air pollution control business.
About the author
The article is authored by Shrikant Satpute, who is Vice President - Technical & Field Services at Industrial Filters & Fabrics (IFF). Satpute is in the field of industrial air pollution control systems for the past 34 years. He was closely associated with major OEMs in APC equipment and system manufacturers in India. During his period, most of the projects, which were first time of its application were successfully executed with his dedicated project team.