An effective pollution control device
Over a period of time, baghouse has overtaken ESP in pollution control. In many cement plants, ESPs are getting replaced with a baghouse. Let us see the advantages of baghouse.
A baghouse, which uses 'filter bags', is an air pollution control device that removes particulates out of air or gas released from chemical processes or combustion for electricity generation. Cement and power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers and other industrial companies often use baghouses to control emission of air pollutants. Baghouses came into widespread use in the late 1970s after the invention of high-temperature fabrics to be used as filter media capable of withstanding temperatures over 3000 C.
Functioning of baghouses typically has a particulate collection efficiency of 99 per cent or better, irrespective of particle size. However in case of electrostatic precipitators, the performance may vary significantly depending on process and electrical conditions.
Most baghouses use long, cylindrical bags or tubes made of woven or felted fabric as a filter medium placed in a very big chamber. (For applications where there is relatively low dust loading and gas temperatures are 120°C or less, pleated, nonwoven cartridges are sometimes used as filtering media instead of bags. Typically dust-laden gas or air enters the baghouse through hoppers large funnel-shaped containers used for storing and dispensing particulate and is directed into the baghouse compartment. The gas is drawn through the bags, either on the inside or the outside depending on cleaning method, and a layer of dust accumulates on the filter media surface until air can no longer move through it. When sufficient pressure drop (delta P) occurs, the cleaning process begins. Cleaning can take place while the baghouse is online (filtering) or is offline (in isolation). When the compartment is clean, normal filtering resumes.
Baghouses are very efficient particulate collectors because of the dust cake formed on the surface of the bags. A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter must be cleaned periodically.
Baghouse types - Cleaning methods:
Baghouses are classified by the cleaning method used. The three most common types of baghouses are mechanical shakers, reverse gas, and pulse jet.
Performance of a baghouse
Baghouse performance is contingent upon inlet and outlet gas temperature, pressure drop, opacity, and gas velocity. The chemical composition, moisture, acid dew point, and particle loading and size distribution of the gas stream are essential factors as well.
Fabrics are designed to operate within a certain range of temperature. Fluctuation outside of these limits even for a small period of time,can weaken, damage the bags.
Baghouses operate most effectively within a certain pressure drop range. This spectrum is based on a specific gas volumetric flow rate.
Opacity measures the quantity of light scattering that occurs as a result of the particles in a gas stream. Opacity is not an exact measurement of the concentration of particles; however, it is a good indicator of the amount of dust leaving the baghouse.
Gas volumetric flow rate:
Baghouses are created to accommodate a range of gas flows. An increase in gas flow rates causes an increase in operating pressure drop and air-to-cloth ratio. These increases require the baghouse to work more strenuously, resulting in more frequent cleanings and high particle velocity, two factors that shorten bag life.
Fabric and filter bag
Fabric filter bags are either oval or round tubes, typically 15-30 feet and 5 to 12 inches in diameter, made of woven or felted material. Depending on chemical and/or moisture content of the gas stream, its temperature, and other conditions, bags may be constructed out of cotton, nylon, polyester, fibreglass or other materials.
Nonwoven materials are either felted or membrane. Nonwoven materials are attached to a woven backing (scrim). Felted filters contain randomly placed fibres supported by a woven backing material (scrim). In a membrane filter, a thin, porous membrane is bound to the scrim. High energy cleaning techniques such as pulse jet require felted fabrics.
Woven filters have a definite repeated pattern. Low energy cleaning methods such as shaking or reverse air allow for woven filters. Various weaving patterns such as plain weave, twill weave, or sateen weave, increase or decrease the amount of space between individual fibres. The size of the space affects the strength and permeability of the fabric. A tighter weave corresponds with low permeability and, therefore, more efficient capture of fine particles.
Reverse air bags have anti-collapse rings sewn into them to prevent pancaking when cleaning energy is applied. Pulse jet filter bags are supported by a metal cage, which keeps the fabric taut. To extend the life of filter bags, a thin layer of PTFE (teflon) membrane may be adhered to the filtering side of the fabric, keeping dust particles from becoming embedded in the filter media fibres. Some baghouses use pleated cartridge filters, similar to what is found in home air filtration systems.
Reverse Air Bag House (RABH):
The typical RABH is modular in construction, with four or more independent modules - the modules being set in pairs, when the gas flows are on the higher side. The sealed air gap between the modules, adds to the insulation to increase operating economy. The extra space created by the hoppers provides a large passageway between rows down the middle of the system. This passageway is divided into three sections horizontally to make the inlet plenum, outlet plenum, and the reverse-air plenum.
Reverse Gas Flow:
Each module is periodically and automatically shut-down for a brief reverse-air cleaning as per the system logic. Clean hot gas is drawn from the outlet plenum by the reverse-air fan into the reverse-air plenum during cleaning. During the process, the outlet damper is closed and the reverse-air damper is opened, letting in the reverse air in the opposite direction from the normal dusty gas flows. This action slowly collapses the bags breaking up the dust cake on the inner bag surfaces allowing the dust to get discharged to the hopper.
Several Industrial units - cement, steel and power plants - had installed electrostatic precipitators (ESPs) for emission control. These ESPs were designed for earlier emission norms of about 150 mg/Nm3 now the revised norms for Pollution Control Boards are at stringent level of about 30 mg/Nm3. In addition industries also have to deal with changes in process inputs, inferior quality coal and increase in capacity which causes higher pollution. Installing a new ESP is costly and in most of the cases unfeasible due to limited space in the plant. Add to these the costs of long shutdowns and production loss. These old ESPs can be retrofitted to fabric filter by combining the functions of an ESP and a bag filter. Dust emission is reduced to about 10 mg/Nm3. For industries like cement, the product achieves futuristic environment norms and product recovery.
Emissions control is a hot topic in the cement industry. Cement plants are generally driven by production numbers, but if they fail to comply with the new NESHAP (National Emission Standards for Hazardous Air Pollutants) regulations, production could be stopped immediately, introducing a slew of cost factors that affect plants' bottom line.
What's more challenging for cement plants is how compliance is calculated, with the MACT (Maximum Achievable Control Technology) approach, which sets compliance limits in line with the top 12 per cent of plants. Plant compliance is calculated not at a given moment, but on a 30-day rolling average.
Prior to NESHAP, plants had to prove their compliance based on annual tests. The MACT approach to setting the new limit at the average of the best 12 per cent is a significant change. What makes the change even more challenging are plants having to now prove their compliance on a continuous 30-day rolling average.
As industry and baghouse needs have evolved, Gore has introduced new developments in filtration technology that deliver industry-leading performance and reliability. To meet the requirements of the industry are brought in by the manufacturers e.g. products include our GORE Low Emission Filter Bags, which are seam-taped to block emissions, ensuring cement plants meet NESHAP and other regulations.
DeNOx Catalytic Filter Bags are designed to destroy NOx and NH3 at levels similar to an SRC tower, but at a much lower investment cost. GORE Low Drag Filter Bags are proven to increase throughput by promoting greater airflow. The same manufacturer has Mercury Control System (GMCS) is a unique fixed sorbent system for capturing elemental and oxidised gas phase mercury from flue gas streams and reduces SO2 concentrations. One would normally expect not to have any process changes in the running plant.
Baghouse operators purchase filter bags primarily for particulate collection. This is a primary filtration factor. As the particulate is collected, the resistance to flow, differential pressure, increases, causing higher fan energy costs. This is a secondary filtration factor. When the filters have holes so they are no longer as efficient at particulate capture as they need to be or when the differential pressure of the filter bags is too high, the filter bag life is over. Although life is usually one of the major factors considered during a filter selection, it is defined by the primary and secondary factors. Gore provides the optimal balance of extremely high filtration efficiency very low resistance to flow and extremely long filter life. Typically, filter bags will provide five-year effective filter bag life in a pulse jet cement kiln baghouse.
Gore recently introduced GORE Low Drag Filter Bags - a step change in industrial dry filtration. An entirely new class of membranes developed by Gore acts as a true surface filter in fume and fine powder applications. The low drag technology, "Drag" is defined as the resistance of a filtration material to airflow. The new materials are inherently less resistant to airflow and are therefore more efficient with respect to the amount of energy required to drive air through them during filtration. The key is improved cleanability, without sacrificing durability or particle capture efficiency.
The dust collection on fabric filters through the following four mechanisms:
- Inertial collection: Dust particles strike the fibres placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
- Interception: Particles that do not cross the fluid streamlines come in contact with fibres because of the fibre size.
- Brownian movement: Submicrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces.
- Electrostatic forces: The presence of an electrostatic charge on the particles and the filter can increase dust capture.