Advancements in fabric filtration technology
The article highlights the new emission standards and online reporting protocol for cement plants and captive power plants. It also covers the advancements in filtration technology to reduce dust, SOx, NOx, Mercury, Dioxins and Furans, heavy metals from the Kiln gases and also dust and SO2 from CPP.
Stringent dust emission regulations are introduced in early 1980s where the emission standards were 250mg/Nm3. During this period, most of the cement plants adopted electrostatic precipitators (ESP) for all applications. Subsequently, when the emission standards were reduced to 150mg/Nm3, Cement plants have upgraded the ESP with modern controllers, additional fields, change of electrodes. When dust emission standards have reduced to 50 mg/Nm3, cement kilns have shifted to reverse air baghouses with fibre glass bags and other applications like coal mill, cement mill, raw mill etc., shifted to low temperature bags like polyester/acrylic bags with or without antistatic treatment. However, the clinker cooler and captive plants continued with ESP technology.
In 2014, first time in India, gaseous emission standards (SO2, NOx, VOC, Mercury, NH3, heavy metals, dioxins and furans) for cement kilns and simultaneously dust emissions standards reduced to 30mg/Nm3 instead of 50mg/Nm3, which are on par with the global best practices. Apart from the emission standards, CPCB issued a direction on 5th February 2014 about the online reporting of emissions (both Stack and ambient air) and effluents from 17 categories of industries. Further, CPCB released guidelines for continuous emission monitoring system during July 2017.
The notification calls for online reporting of emissions from all process stacks and ambient air quality stations to SPCB and CPCB on 24 X 7 basis and stringent reporting and compliance standard. In 2015, CPPs emission reduction standard (Dust, Sox, NOx & Mercury) were introduced with varied emissions based on the vintage of the plant and also size of the plant.
Compliance timelines in both the cases i.e., Cement Plants, March 31, 2017 and for CPPs up to December 7, 2017. In both the cases, industry faced many technical as well as financial challenges to complete these projects. Based on the industry request, the Cement plants have been given time line extension up to August 31, 2018 and for the power plants, time line extension is not yet finalised.
The monitoring reporting protocol also is a major challenge in terms of technology option. As per the standard the emissions are measured on a 15 minute average and non-compliance alerts has been given to the companies. By design, emission from ESP varies with the process conditions and also emissions goes up while cleaning system of the electrodes takes place, especially in the outlet field. This becomes a big challenge when complying with the 15 minute duration constant emission from any ESP. Global compliance standards takes 1 day average or 3 day emissions average or 30 days rolling average to issue compliance alert and also to action of noncompliance. Indian standard on monitoring reporting protocol is the toughest standard at this moment.
The reporting issues posed biggest challenge for the technology selection for the control equipment. From early 1980s, there is a rapid advancement in terms of fabric filtration technology and currently newer fabrics and membranes have been developed to reduce the emissions to below 5 mg/Nm3 with a lower pressure drop and guaranteed longer life of up to 6 to 8 years. Apart from dust, the current advanced filter media is capable of reducing heavy metals, Dioxins, Furans, Mercury removal.
Latest environmental regulations
MoEF&CC has issued notification on revised emission norms to cement plants on August 25, 2014 against various parameters such as PM, SO2 & NOX emissions with varied compliance timelines for various parameters from January 1, 2016 to June 1, 2016. Final compliance timelines extension is further extended August 31, 2018.
Filtration technologies adopted by cement
Indian cement industry is very progressive and is continuously adapting to the latest technologies to make the Cement Industry more efficient and green with less environment footprint. In the same spirit, Cement Industry is first one to adopt filtration technologies like Pulse Jet Bag House (PJBH), reverse air bag house and hybrid filters for controlling of dust emissions from stack.
Advent of new fabrics which can withstand higher temperatures and tough working conditions, controls and advance electrical systems provided the opportunity to reduce the dust emissions to very low levels. Cement Industry embraced these technologies that helped industry today in achieving consistent and lower stack emissions of 30 mg/Nm3.
To meet the online reporting requirements, cement plants have installed Continuous Emission Monitoring Systems (CEMS) and continuous ambient air quality monitoring stations (CAAQMS). Among various Industrial sectors, Cement Industry is the first one to move ahead with on line reporting of their dust emissions performance. This has demonstrated the willingness of Cement Industry to be more transparent in disclosure of their dust emissions performance.
Advancement in filtration technologies in fabric filters
There are several conditions like change in process/raw material, wrong selection of fabric, and other design issues, abrupt changes in process operational parameters, poor diagnosis including lack of automation, improper maintenance, operational incompetency etc. resulting in poor performance of bag filters.
To address these issues, various advancements took placed in fabric filter technology and details of these developments are as given below:
- Advancements in filter fabrics w.r.t., temperature with stand ability, chemical resistance, etc., giving an opportunity to select application specific fabric
- Higher fabric area weight (density) fabrics
- Economic viability of Polytetrafluoroethylene (PTFE) lamination technology
- Low pressure filtration
- Technological innovation in Pulse valves
- Pulse valve failure detection system
- Automation in back leak detection through Invention of bag leak detection systems along with latest controls
- Remote diagnostics
- Computational Flow Dynamics (CFD) studies of the bag house to correct the flow distribution and prevent the bag failure
- Constant Pressure drop based bag cleaning system
- Tall bags of 10-15 metres
- Mandatory precoating of bags
- Operating bag house at least 20 deg C above Water / acid dew point temperature
- Residual analysis
- Mandatorily adopting all seasons weather enclosure
Advancements in filter fabrics:
Greater advancement took place in Filter fabrics w.r.t., temperature and tough working conditions with stand ability, filtration efficiency improvement etc., giving an opportunity for fabric manufacturers to develop various fabrics with different surface finishes and characteristics. Notable advancements in this direction are advent of Polyimide (P 84) fibres and micro denier fibres. Fabrics made out of polyimide fibres are having temperature with stand ability of approximately 260 degree and being a felt fabric can withstand rough handling and working conditions. This has helped Cement Industry to consider these fabrics in place of glass with/without PTFE membrane which are very fragile and delicate fabrics for Kiln application which has resulted improved operational efficiency of Kiln Bag Houses.
Similarly, PANOX and PYRON fibersare Oxidised Polyacrylonitrilefibers do not burn nor melt nor char. These fibres in blend with Nomex can bring in extra heat protection in Clinker cooler bag filter application.
Advent of micro denier fibres which are light in weight and high bulk, water repellent helped in developing fabrics which are more efficient with respect to filtration efficiency and more durable. The most common types of microfibres are made from polyesters, polyamides (e.g., nylon, Kevlar, Nomex), or a conjugation of polyester, polyamide, and polypropylene. This has resulted in achieving better permeability with better dust dislodgement characteristics, thereby enhancing the bag filters / bag houses performance.
Apart from dust filtrations, recent trends are towards use of Catalyst powder along with PTFE Powder during the manufacture of the felt. The resultant bag with membrane lamination can be effective in controlling the particulate emissions besides controlling Dioxins and Furans. This is also self-regenerated catalyst and is effective at temperature above 200 degree C.
Fabrics with high fabric area weight (density):
With the advent of finer fibres like micro denier fibres gave an opportunity to develop higher density fabrics with same / lower thickness than the traditional fabrics. Currently fabrics are available with 600 to 750 grams / sq m fabric density with the similar thickness, flexural characteristics as against traditional fabrics with density of 500 grams / sq m. These high density fabrics are more robust are able to give higher bag life even in touch working conditions.
Economic viability of PTFE membrane technology: Both the PTFE membrane manufacturing and lamination technologies have become more commercially viable.
This has resulted in industry adopting fabrics with PTFE lamination which helps in better permeability, dust dislodging, less pressure drop, lower energy consumption and improved productivity. Industry is looking at this technology wherein reduction in pressure drop and increased productivity up to 10-15 per cent can be tapped from the existing filter.
Low pressure filtration:
One of the latest technological advancements in bag filters/bag house are low pressure filtration which uses filter bags cleaning pressure of 0.8 bar as against 4 to 6 bar pressure used in traditional Pulse jet bag houses / bag filters. This low pressure filtration is achieved by using physical flow model study / Computational Flow Distribution studies to achieve optimum gas flow distribution, energy efficient roots blowers, specially designed cages and filter bags, advanced pulse valves etc., This helps bag houses in achieving low energy consumption, lower outlet emissions, extended bag life & overall reduced operational cost.
Technological advancement in pulse valves:
Greater Technological advancements took place in pulse valves, which resulted in enhancing pulse valves performance wrt, its ability to take higher flow / valve, Longer life, ability to have consistent performance due to pressure variations and contamination, very fast and repeatable response time for quick and accurate purges, reliable performance in harsh environment conditions, self-cleaning ability, less consumption of air, faster response time for more efficient duty cycles and higher impact force when blowing. This has resulted in improving operational efficiency and lower energy consumption both in existing and new filters.
Pulse valve failure detection system: These systems will identify the operational failures with Solenoid valves and convey the same to the plant personnel to enable them to replace/repair the solenoid valves immediately. This protect the filter bags from negative impacts like dust build up, blinding etc., due to non-pulsing which in turn increases the Pressure drop and higher power consumption.
Automation in back leak detection through Invention of bag leak detection systems along with latest controls: The integrated Bag cleaning mechanism monitors the dust emission on continuous basis. In case of spike in dust emission due to broken bags, the associated solenoid is automatically disabled to avoid flexing of damaged bag, thereby avoiding the enlargement of hole. The controls have features to continuously adjust pulse off time to maintain differential pressure at single set point within a narrow band ¦ 2.5mmW.C. The solenoid activation pulse output can sense the short or open solenoid with instant failure detection and row identification. The system can also identify leaking or ruptured, stuck open or closed diaphragm with instant failure detection and row identification
Latest automation provides an opportunity for us to have all the bag house operational data like Differential Pressure, Temperature, Pulsing cycle, Dampers position, dust build-up in hoppers, healthiness of dust handling systems, filter bags, solenoid valves, outlet emissions, etc., and fine tune various parameters from the plant control room / local control panel. This helps us in ensuring the operation of bag house at the optimum level to enhance its performance with respect to bag life, differential pressure, energy consumption, etc. and maintaining consistent outlet emissions.
Computational Flow Dynamics (CFD) studies of the baghouse to correct the flow distribution and prevent the bag failure: CFD is a branch of Fluid Dynamics that uses numerical analysis and data analysis and data structures to solve and analyse the problems that involve fluid flows. CFD helps in designing the Air Pollution control systems with better efficiency, minimise cost of product development and design the systems in much smarter method. Modern CFD programs permit the simulation and analysis of flows on the computer. The computer-supported analysis enables examination of the dynamics of flowing media and provides a computer model which represents the examined conditions of an installation. The special strength of CFD simulations lies in the fact that 'trial and error' experiments, which are practicable in reality only with great effort, can be limited by CFD to the most likely solutions of the problem and with a minimum of effort.
CFD speeds up project work in conceiving and realising industrial dust removal installation, but it also serves as a tool for basic advancements. With a suitable choice of the simulation model, optimisation possibilities close to the installation can be found. The key to efficiently solving tasks is the networking of the CFD program with the CAD system. Nevertheless a simulation program is only as good as the user who serves it. The model construction, the simulation realisation and evaluation need
a lot of experience.
Typical CFD outcomes are as given below:
Constant pressure drop based bag cleaning system: Current bag cleaning systems are automated to maintain constant pressure drop across the bag house. This helps in operating the bag house with consistent performance with respect to pressure drop, energy consumption.
Usage of taller bags of 10-15 m long:
Advanced pulse valves along with latest ventures are facilitating in effective cleaning of longer bags up to 10 - 15 meters long. This helps in having new bag houses with lesser foot print, converting ESPs to bag houses / hybrid filters.
Mandatory pre-coating of bags:
Dust with finer particles of 0.5 microns or smaller can leak right through pores of a new bag working their way deep into the media to the point of blinding, or clogging, the filter and slowing or stopping airflow through the bag house / bag filter, which in turn affect the performance of bag house and lead to higher power consumption. Pre-coating can reduce or prevent the permanent failure of new filter bags. Built up of pre-coating material as initial dust cake on the media, prevents dust particles from flowing into and blinding the media. Pre-coating ensures that air flows freely through the dust collector, improving filtering performance while extending the bag life. Pre-coating of new filters provides other benefits like improving the dust collector's initial filtration efficiency at start-up, ease of cleaning, better dust cake release etc.
Operating bag house at least 20 deg C above water / acid dew point temperature: It is mandatory to operate the bag house at least 20 degree C above water / acid dew point temperature to avoid condensation of water / acids on the filter fabric which in turn will damage the filter bags and leading to higher pressure drop, higher energy consumption and higher emissions (if the bags are damaged due to acid attach).
Residual bag life analysis:
To ensure consistent performance of the bag house, it is essential to periodically check the filter bags for residual life and to replace the bags before the failure happens. This will help in preventing the higher emissions from the bags and to ensure consistent performance of the bag house.
Mandatorily adopting all seasons weather enclosure:
Bag houses / Bag filters are prone to water seepage through the top doors, top roof during the rainy season which will impact the performance of bag house by blinding the bags due to water condensation. Hence it mandatory to provide weather enclosure on all the existing/new baghouses to prevent moisture ingress and to ensure consistent performance of the bag house.
Conversion of RABH to PJBH/additional module:
When the plant capacity is enhanced, few of the options available for accommodating the higher gas volume is either converting reverse air baghouse to pulse jet baghouse or addition of module which will facilitate in creating more filtration area and thus accommodating the additional gas volume.
Current constraints and design challenges for bag filter suppliers
In spite of technological advancements in fabrics and bag house technology, still the bag filter / bag house suppliers continue to face few challenges as given below:
- Online maintenance of the filter
- Emissions exceeding beyond the permissible limits
- Ability to install new filter with least shutdown time
- Minimising the ID fan energy consumption and compressed air consumption
- Consistent longer bag life
- Ability to install fabric filter in ESP Casing
- Ease of bag house maintenance / bags replacement
- Fabrics which can withstand consistent operating temperatures > 260-280 deg C
- Catalytic filter media
- Expenses Cages
Advancements in ESPs
In spite of the advancements in Fabric Filters, Electrostatic Precipitators are still preferred over Fabric Filters especially for high temperature applications like Clinker Cooler and treatment of flue gas in Power Plant. One of the main drawbacks of ESP is that it is highly influenced by the process parameters. Small changes in the operation conditions - flue gas temperature, dust/flue gas characteristics have enormous impact on the ESP efficiencies.
Majority of recent advancements in ESPs are as given below:
- Smart Controllers for conventional transformer rectifier (TR sets)
- Three Phase transformers
- High frequency transformers
Smart Controllers for conventional TR sets: Optimum power to the ESP is a key in achieving the maximum dust collection in the different fields of the ESP. Adapting smartly to changing process conditions, reducing the impact of sparking in the field as well as back corona occurrence thereby improving the energising level helps in reducing emissions to desired levels. Fast response to sparking condition thereby always maintaining peak power levels is an inherent feature of these controllers.
Manufacturers are developing better products and software for combating back corona especially in Indian coal scenario. Advanced Algorithms for automatic detection / control of back corona with very high pulse blocking ratios has been effective in mitigating impact of back corona. Reliability has been another issue which has been addressed with the selection of superior components. With ESPs designed with 3-4 electrical fields, failure of one controller can impact collection area of around 25 to 33 per cent. Sectionalisation of mechanical field by splitting into two fields (either across gas flow or in direction of gas flow) to increase number of electrical fields, can result in substantial improvement in the ESP performance.
Three Phase Transformer:
Recent trends for improvement in power levels have been the increasing up-gradation of existing transformers (single phase 415 V) with Three Phase transformers. Sparking in the field is predominantly due to the peak KV reaching spark over level across the collecting and discharge electrodes. With conventional TR sets, the average KV is 60 to 90 per cent of the peak KV. With conversion to Three Phase transformers, the average KV can be more than 90 per cent of the peak voltage thereby drastically increasing the energisation levels of the ESP, thereby reducing emission levels. Equipment suppliers are willing to offer 20 to 30 per cent reduction in emission for up-gradation with three phase transformers
High Frequency Transformers (HFTR):
In conventional TR sets, the power level to the TR set and thereby to the ESP is controlled by the firing angle of the Thyristor (SCR) - point in the AC power cycle where the voltage is applied to TR set. Once the Thyristor start conducting, it can be stopped only during cross over to the reverse cycle. This limits the point at which the conduction of the SCRs can be stopped, rather there is no control. With the advent of IGBT's the start / stop of the conduction of the device can be controlled. It is possible to provide more precise control of the ESP parameters such as the output voltages and currents. It is also possible to make a rapid increase or decrease in voltage and to provide a very fast response to load changes.
The HFTR supply uses an IGBT converter which supplies the primary of transformer with 5 kHz - 20 kHz AC. (Conventional TR set are controlled at mains frequency i.e 50 Hz). Due to these advantages it is possible to suppress the supply quickly in the case of sparking, reducing the spark energy and the quantity of ionised gasses produced by the electric arc. Similarly the recharging is also faster. Reduction in the spark energy is many times compared to conventional SCR solution.Thus HFTR can comfortably operate with 50 to 100 sparks per minute without significant loss of corona power and very close to flash over levels unlike traditional Sets.
The lower quantity of ionised gasses produced by the spark contribute to much shorter de-ionisation intervals, required to quench sparking and evacuate charged particles in order to reinstate the voltage and proceed with the operation.
Since the average and peak KV being very close, they can operate at significantly below flash over levels in case of combustible and explosive applications thereby reducing chances of fire and maintaining the desired efficiency.
As a result, the collection efficiency and energy efficiency of the electrostatic precipitator can be increased many fold by applying high frequency high voltage power supply.
Hybrid Dust Collectors: ESP-Bag filter
Another approach quickly gaining widespread usage especially for CPP Boiler is Hybrid Dust Collectors. For a typical four field ESP, the outlet two fields can easily be converted to bag filter by simple modification of the ESP internals. The Coarse particles are easily collected in the inlet fiels and the fine particles which are comparatively difficult to collect in ESP are collected by the filter bags. The cost for new bag house is reduced as in most cases the existing ESP casing / ducting & hopper / dust conveying system is used. Also the operating cost is reduced as the DP is on lower side as dust loading is quite low.
HFTRs and Three Phase work the best in inlet fields. So in Hybrid filters providing HFTR / 3 Phase in the field-1 is a great idea! Benefits a) In case of minor bag failures emissions will not rise alarmingly. b) The requirement of cleaning bag is less since it is exposed to less dust, meaning longer bag life and less compressed air usage.
Closed Loop Energy Management System
The importance of closed loop energy management systems with opacity monitors is being looked at seriously. Although the limited electrical fields do not give enough room for energy management, this will slowly become the norm rather than exception.
Another off shoot of new emission norms is that a lot more care is taken in dust and ash conveying, especially false air leakages through them. This not only reduces emissions, but saves power also. Care is also taken to avoid dust build-up in hopper so as to avoid tripping of the fields due to 'hopper
Environmental Protection and continuous adoption of environment abatement technologies continue to be the primary focused area of Cement Industry to comply with environment regulations and to beyond the regulatory regime. Various technical advancements in filtration technology indicated above are clearly demonstrating their significance for new emission regulations by overcoming constraints like layout constraints, longer shutdown timelines, reduced financial resources requirements, etc.
We thank Dilip Sakphara - Managing Director and Rushabh Sakhpara - Business Development -MaxTech Industries and Dr VS Rajan - Chief Technical Advisor - Supreme NonWoven Industries Pvt Ltd for providing technical inputs in drafting this article.
The article is authored by:
KN Rao, Director - Energy, Environment & Sustainability, ACC Limited