A correctly selected air pollution control system will not only control air emissions successfully, but also contribute to the cement plant’s overall efficiency.
It is a fact that emission limits in the cement industry have become and will continue to get more stringent. Although there are some differences, depending on which part of the world one looks at, the limits are undoubtedly heading downwards. A direct implication of this is that the dust control activities of cement manufacturers will have to be improved globally.
In cement plants across the world, cyclones, electrostatic precipitators (ESPs), and baghouses, either alone or in combination, are widely used dust control technologies, each with their own benefits. The well-known and established ESP technology is capable of handling dust emission limits of 5- 10 mg/Nm3.
ESPs in the cement industry can be operated up to a service temperature of around 450°C, so there is no cooling of the gas needed for kiln exit gases and those of the clinker cooler. In the case of bypass filters, depending on the kiln exit temperature, cooling of the gases with air or water may be required. One of the most compelling arguments for an ESP installation is that ESPs are very easy to operate.
Moreover, maintenance is relatively simple and the cost reasonable, due to fewer components involved as compared to a baghouse, for example. On the other hand, the space requirements for an ESP filter are huge and, if one wants to lower the dust emissions even further, the filter becomes very large, and the consumption of electricity very extensive. The extension of an ESP to meet new emission limits could imply higher fixed costs and increased operating costs.
In most cases, the option of expanding the ESP filter is not available, as space is often the limiting factor in existing facilities. However, depending on the desired lower emission targets, ESP technology may not even be able to reach those new levels.
Time and again, changes in regulations and lower emission limits have forced the cement industry to look for other solutions. Lower emissions, in the range of about 3-5 mg/Nm3, can be achieved with baghouses, which also have a much smaller footprint than ESPs. No wonder that one of the industry’s responses to the challenge was the conversion of an existing ESP installation, either in full or in part, into a baghouse with a fabric filter. This type of filter conversion has become common practice in the cement industry, and with new limits in place, many more retrofits are expected worldwide in the coming years. However, the right implementation is crucial to reaping the benefits of such a retrofit. Some of the problems encountered can be traced back to fundamental differences between the two technologies. Two of these basic differences are the direction of flow of the flue gases and the operating temperature. An ESP filter requires a horizontal flow of the flue gases going through the collecting plates. In a baghouse, the flue gases go through the vertically hanging bags.
Therefore, in a baghouse filter, the gas flow should be vertical. An ESP installation can be operated at approximately 450°C, whereas in the case of a baghouse, the temperature is limited by the kind of filter media used. Maximum continuous operating temperatures for fabric filters are 250-260°C; cooling of the flue gases is therefore required. The actual filter media may be a fabric cloth made of either a needle felt or a woven fibre glass. Both fabrics could be equipped with an expanded polytetrafluorethylene (ePTFE) membrane material. Due to the very small pore size of the membrane (1 – 2 µm), lower emission rates of about 3-5 mg/Nm3 can be achieved.
The crystallite melting point of PTFE material is 327°C and a potential active continuous service temperature of 288°C seems possible. However, practical continuous filtration operating temperatures are between a maximum of 250-260°C. In order to protect the fabric filtration media, valuable heat energy has to be wasted due to the cooling of the flue gas. In many cases, where cooling is done by air, about 30-50 per cent of the air going through a fabric filter baghouse is the air required for cooling the flue gas to a desired temperature.
Cooling of flue gas can be avoided, if the filter medium can withstand higher temperatures, presenting a number of opportunities:
The volume of air can be reduced, which saves electricity costs on the fan motor. Increased production capacity may become possible without having to scale up the ID fan capacity. The clean gases are higher in temperature and therefore do not need to be heated for potential SCR NOx reduction treatment. This will save on fuel consumption and therefore on cost.
The thermal energy from the clean hot gases can be reused as thermal energy for drying raw material or coal. Those clean hot gases could also be used to generate electricity.
The use of latest state-of-the-art high efficiency pollution control equipment like ESPs and bag filters has made it possible for the cement industry to be well within the particulate emission norms. The recently prescribed norms for PM, SO2 and NOx are at par with the stringent EU norms. However, Indian cement plants need adequate time to implement measures in order to comply with the norms (particularly with SO2 and NOX) in a gradual phased manner, given the fact that there is a need for availability of good quality coal, ammonia, equipment design modifications and concerns on health hazards associated with use of ammonia in NOx reduction technologies.
The cement industry has installed Continuous Emission Monitoring Systems (CEMS) in most of the kiln stacks and opacity monitors in most of process stacks. CEMS facilitates tracking of SO2 and NOX emissions in real time on a continuous basis and enhance the accuracy of reporting. It also helps to identify the base line emissions and deviations from expected regulatory norms with a view to take corrective measures wherever necessary.
Various types of Air Pollution Control Equipments (APCEs) are used in cement plants to control the particulate emission to the atmosphere such as cyclone and multi-cyclones, wets crubbers, ESPs, fabric filters/bag filters or gravel bed filters.
The use of more advanced technologies, such as pulse jet bag houses, has been extended in manufacturing units of cement industry. The methods employed for fugitive dust control in cement industry includes water spray system, green cover, tree plantation, exhaust ventilation system and proper house-keeping. Filter bags are a critical component in assuring that the fabric filter will be able to meet environmental regulations and plant process demand. Though filter bags are usually the first place plant personnel look when problems occur, there are other areas to consider. The bag house equipment design, such as how the bags are cleaned and how the dust is removed from the bags are equally important.
The maintenance practices such as filter bag installation, start-up and shutdown procedures are important to attain optimal performance. One of the biggest issues that cement plants face with fabric filters is the ability to distribute gas equally to all of the compartments without causing high velocity problems, which can quickly cause filter bags to break.
To ensure the fabric filter meets operational goals and complies with environmental regulations, it needs the support of associated services, including qualified technical support, filter training, lab testing, spare parts, make and hold agreements, and ongoing customer service. This requires an experienced solution provider who has both operational know-how and technological manufacturing competencies. Filter bag engineers should be able to evaluate all process conditions, including gas temperature, air flow, volume, and more.
Here, FLSmidth Airtech has engineered the best design to meet these challenges head on by utilising an exclusive design of gas distribution screens and side inlet that reduces velocity and allows dust to pass directly into the hopper. This contributes directly towards longer filter bag life.
The feature of equalised and low velocity gas streams provides extremely important benefits for the cement plant such as cost reductions in the compressed air energy used for filter bag cleaning.
Fiberglass is the most common media for filter bags in kiln filters for good reason. Its temperature resistance of up to 260°C (500°F) can withstand the hot gases of the kiln, providing flexibility to plant operations as higher temperature ratings allow for improved throughput and production.
However, fiberglass bags are relatively fragile and must be handled very carefully, especially during installation. Consequently, it is recommended that installation of fiberglass bags is left to experienced, qualified personnel.
A potential drawback of fiberglass bags is their relative inability to withstand ‘overpulsing’ or excessive cleaning. Here, efficient and careful cleaning of the bags is an important factor in preventing failure. An excessive amount of cleaning pulses will often lead to premature failure, so it is necessary to find the optimal frequency to ensure the longest possible service. Qualified fabric filter technicians can provide best-practice recommendations.
The typical media for clinker cooler filters is aramid. This sets a maximum temperature limit of 204°C (400°F) and a constant of 190°C (375°F) since above this, aramid bags will fail. Bag failures can lead to unnecessary stoppages of the clinker cooler, because the defective bags need to be located and exchanged. A particularly useful device is a broken bag detector that can alert the plant operators in the event of a broken bag as well as pinpoint the location.
This minimises production loss and provides better conditions for finding and replacing the bags efficiently and safely. In addition, if fabric filter performance is not cost effective, several filter media upgrades are available.
In finish mill filters, polyester is one of the most widely used media because of its high availability and low cost. However, hydrolysis is a common problem for this process. Polyester becomes brittle when exposed to moisture and temperatures around dew point, approximately 100°C (215°F). A better option is an acrylic filter bag because it operates well in high moisture applications. Other benefits of acrylic filter media are good resistance to moist mineral and most acids as well as an excellent resistance to organic solvents.
Typical filter media for coal mill dust collectors include polyester, acrylic and aramid. With a stainless steel scrim, these new and improved media replace the traditional, blended fabric with carbon fibres, also known as epitropic filters. The conductive scrims dissipate static consistently throughout the filter bag at a lower cost. Static electricity in the coal mill filter could ignite coal dust and cause a fire or explosion. Safety measures preventing static electricity discharge therefore need to be in place to reduce the potential for explosions and fires. These measures include using stainless steel or copper grounding wires sewn to the filter bag or semiconductor filter bags.
It is not uncommon to see undersised dust collectors in applications such as silos, pack houses, belt transport and conveying systems. High differential pressure issues in these units, designed for a specific grain loading, are usually caused by system overloading as production expands. The issue can be solved by retrofitting the design, increasing the size of the collector or using a pleated filter bag design instead. This last option can be the most cost effective because the same housing and other components are used.
Although particulate monitoring systems are generally purchased to monitor environmental emissions, many users also utilise these instruments as preventative maintenance tools. The ability to predict when a filter is likely to fail and to be able to identify which row or chamber is at fault has provided users with a proven method to not only reduce the environmental impact and clean-up costs associated with large-scale emission events but also to make significant savings in spares, maintenance times and lost production.
To achieve this, the selected monitoring technique must be able to accurately track the very dynamic dust emissions created during a bag filter cleaning cycle. To these ends UK-based PCME recommends Electrodynamic units in preference to Optical or Triboelectric systems.
As a filter is reverse jet cleaned, any defects in the filter membranes are exposed resulting in relatively high dust peaks. By monitoring these peaks in real time using the Predict software package, it is possible to identify potential problems within the filter before they result in breaches of environmental limits.
The cleaning signature of the bag house is made easily identifiable by the input to the monitor of the filters cleaning pulses via Auxiliary Input Modules. Additionally further outputs maybe taken from pressure sensors within the bag house to assess the caking of the filter elements, thereby allowing the operator to reduce bag wear and compressed air usage and allowing the optimisation of the filter system.
Predict provides the possibility to observe filter problems remotely and check maintenance work to ensure correct performance of the filter. The use of Predict has proven the ability of a monitor not only to be used for environmental compliance but also to be used as a significant aid to plant maintenance and to also enable users to greatly reduce the instances of catastrophic filter failure.
These two complimentary monitoring techniques are used as they offer the best monitoring solutions in the widely different conditions found in these two locations. Electrodynamic sensors have a proven capability to monitor the extremely high dust loads found Pre-filter, providing a reliable, rugged monitoring solution whereas Optical sensors are chosen for chosen for use.
Post filter as a result of their capability to measure extremely low dust levels (0.1 mg/m3 utilising pro-scatter techniques) and their low maintenance requirements. The ability to observe in real time the performance of the filter allows the operator to adjust operating parameters to optimise not only filter efficiency, but also reduce operating costs, extend the filters operating life and decrease the environmental impact of the process.