DEDUSTING EQUIPMENT OPTIONS FOR CEMENT KILN

Transcription

1 DEDUSTING EQUIPMENT OPTIONS FOR CEMENT KILN Dilip Sakhpara General Manager W.L. Gore & Associates - India Presented at the Ninth NCB International Seminar on Cement And Building Materials, New Delhi, November 2005 Abstract: The Indian cement industry is at present going through rapid growth in production and simultaneously improving the de-dusting systems to meet lower emission levels. In this article, author has reviewed various options available for cement plants in various situations. 1) New Green field cement project 2) Upgradation of existing kiln for higher production rate. 3) Replacement or modifying the existing equipment for reduced emissions without any production enhancement. Typically Indian cement industry had predominantly used Electrostatic Precipitators (ESP) till about Now, most of the new plants are opting for baghouse for cement kiln dedusting over ESP. Over the last few years, imported membrane type filter bags have become cheaper due to lower import duty and favourable exchange rate. At the same time, the steel prices have more than doubled. In today s situation, membrane filter bags offer lower Capital as well as lower operating cost compared to conventional filter bags. Detailed cost analysis and references for both pulse Jet and Reverse air baghouse and types of filter bags is described in detail in this article. Any one involved in the selection of dedusting equipment for cement plant would find the article very useful. 1

2 I. Introduction : The PTFE membrane filter bags were first introduced in India in 1993 by Author. Since then, many cement plants have used membrane filter bags in all primary bag filter applications in cement plant. However, the general mindset has been that the PTFE membrane filter bags are more expensive than conventional bags and therefore should be used in only special situations. Over the last 12 years, import duty has dropped from 85% to 15% and steel prices have jumped from Rs.20,000/ton to more than Rs.40,000/ton. This has changed the capital and operating cost structure for the baghouse. Indian cement Industry is known to adopt new technology readily. The cement Industry is also one of the most proactive industry and has been voluntarily trying to reduce the pollution levels to below 50 mg/nm 3. This has also necessitated changes in the selection of the equipments. Let us look at all the different scenarios that cement plant engineers are faced with. II. New Green field project New cement plants are now designed for 1 to 3 Million MT/year capacity. All new cement plants are also using the latest dry technology, where the hot gases from Kiln are used for its heating value in raw mill and/or coal mill. However, the raw mill and the coal mill are typically designed for less than 24 hours of operation. This can lead to high variation in the inlet parameters for the dedusting unit. This makes it difficult for ESP to provide consistently low emission. The complications are further increased as the new larger capacity plants are having two raw mills, kiln and coal mill all connected to the same de-dusting unit. Thus, the de-dusting equipment has to be designed to provide high collection efficiency in all different situations. Assuming 50 gm/nm 3 dust load at the inlet of the dedusting equipment, collection or filtration efficiency of more than 99.9% is required to get outlet emission of less than 50 mg/nm 3. The cost of the ESP for such high efficiency is much higher than a baghouse. Additionally, the ESP goes through CO trips, which lead to higher emissions. Thus, Baghouse has become the preferred technology over ESP in most countries around the world, including India. III. Cement plant with a Baghouse : a) Capacity Enhancement : Let us look at a case where the plant is having an existing Reverse Air Baghouse fitted with conventional bags. In such a case, the plant has options to a) Change the type of filter bags partially or fully to PTFE membrane type b) The plant can carry out mechanical modification of the baghouse to add more compartments and bags to increase the airflow capacity. Based on various analysis carried out by the author, it is always found that the option of changing the filter bags to membrane bags is the best solution. A case study of a similar situation for a plant in India is described later. b) Improving the baghouse performance : The conventional bags filter on the principle of depth filtration. This means that the emission rate and pressure drop are interrelated. The Baghouse performance can be improved in terms of emission rate, pressure drop, maintenance and down time by replacement with membrane filter bags. IV. Cement plant with an Electrostatic Precipitator (ESP) : If the existing de-dusting system is an ESP, then its performance will deteriorate due to lower residence time for the dust in the electrostatic field, with increased airflow and production rate. This will lead to increased emissions. The ESP performance can be improved by : 2

3 a) Partially or fully converting the ESP in to a baghouse : This is generally preferred because, it is lower cost, utilizes the existing location, civil structure and dust transportation system. This modification requires longer shutdown period (20 to 30 days), as the components of the existing equipment are to be used. The longer shutdown may not be a problem if there are major modifications to be carried out in other equipments also for the production increase. b) Replace the existing ESP with a brand new baghouse or ESP : This option is feasible only if there is sufficient space available. This modification can be completed in a relatively shorter shutdown period, as the new equipment can be brought in line within a short period of 7-14 days. V. Fabric - Fiber Options : Let us now look at the fibers available for conversion in to fabric for use as filter media. The maximum temperature of the gases coming out of the preheater Fan is generally 280 to 350 deg C. Since there is no fabric available for these temperatures, the gases are cooled by atmospheric dilution air or water cooling to below 250 deg C. Fiberglass fabric can withstand maximum continuous temperature of 260 deg C and maximum temperature surge of 288 deg C. Therefore, the baghouse set point is designed to maximum 240 deg C and the gases are cooled to 240 deg C. If the temp at the preheater Fan outlet is about 280 deg C, the preferred method of cooling is dilution air. This way a gas conditioning tower (GCT) or water spray system is avoided. However, if the temp is well above 300 deg C and water is freely available at low cost, then water cooling (using a GCT or by water spray in the down comer duct) is an option. From the Table -1, one can see that among the high temperature fabrics, fiberglass is the lowest cost fabric available today. However, fiberglass is very brittle fabric with low flex durability 1. Hence if used in aggressively flexing system such as pulse-jet filter the bag life is less than one year. This is why Reverse air technology has been a preferred technology till now. Type of fibers Relative Cost Polyester 1.0 Polypropylene 1.2 Acrylic 1.6 Fiberglass Aramid 4.0 PPS 4.4 Polyimide 4.6 PTFE Table 1 Fabric Relative Cost Chart In India, till now the most common and successful technology for kiln baghouse de-dusting has been Reverse air. Pulse-Jet is not considered a successful technology yet. However, in many other countries, pulse-jet technology has been accepted. Table 2 summarizes the high temperature filter media options along with their chemical, mechanical and temperature resistance properties. 3

4 Type of filter media Filter Media Pulse Jet (PJ) Baghouse Conventional Nomex bags (without PTFE membrane) PTFE Membrane laminated P-84 Fiberglass Ryton Nomex P-84 Fiberglass Maximum operating temperature 204 deg.c Dry heat Varies (Depends on fabric composition & temperature) 260 deg.c 191 deg.c 204 deg.c Varies (Depends on fabric composition & temperature) 260 deg.c Superflex 260 deg.c Ryton Reverse Air Baghouse (RABH) 191 deg.c Conventional Fiberglass 260 deg.c Membrane Fiberglass 260 deg.c Chemical Resistance Poor resistance to Acid and water resistance to Acid & water good resistance to Acid and water Excellent resistance to all chemicals except Oxygen Poor resistance to Acid and water resistance to Acid & water good resistance to Acid and water good resistance to Acid and water Excellent resistance to all chemicals except Oxygen good resistance to Acid and water Resistance to Flexing (Pulsing) Poor Poor Poor Table - 2 : Types of filter Media for Kiln Baghouse Remarks Not recommended for Kiln BH Can be used if moisture & temp is low. High fabric cost Can be used but has low bag life due to poor flex resistance Can not be used for >10% oxygen level, also more expensive, hence not commonly used Not recommended for Kiln BH Can be used if moisture & temp is low. High fabric cost Can provide 2 4 year bag life with PTFE membrane lamination Best filter media for PJ kiln BH, high cost Not commonly used in Kiln BH 3-5 year Bag life can be achieved 7-11 year Bag life can be achieved with good quality membrane To achieve long bag life with fiberglass fabric type filter bags in a pulse-jet technology, following care has to be taken; a) The dust has to be kept out of the fabric By laminating with a good quality PTFE membrane. b) The baghouse design has to be good to minimize high velocity abrasion of the fabric. c) The cleaning system should be designed for softer and gentle cleaning to reduce flex failures 4

5 From the above Tables-1 and 2, it is evident that the best options for baghouse are : 1. RABH with conventional or PTFE membrane/fiberglass filter bags 2. PJ baghouse with PTFE membrane laminated Fiberglass and Superflex TM Fabric filter bags Selection of bags and baghouse technology should also consider the following points which are important to the plant operator. 1. Long bag life, so that the maintenance and the shutdown frequency can be minimized 2. Low and stable pressure drop, so that the power cost is minimized and the kiln can operate at stable draft. 3. Predictable performance so that the bag change outs can be planned. 4. Low emissions, so that the pollution load is minimized Now, let us look at the Capital and Operating cost comparison for these 3 options. Capital Cost comparison : In the following Table 3, four different options are compared for their capital cost. It should be noted that the filter design data for baghouse with membrane bags is based on actual operating data collected from installations with W. L. Gore s membrane filter bags. It should be noted that there is a very broad range and quality of membrane filter bags available in the market, therefore one should design the baghouse based on the actual operating data from such membrane bag supplier. Capital Cost of the Baghouse Option - 1 Option - 2 Option - 3 Option - 4 Type of Baghouse Reverse Air Pulse - Jet Type of Filter bags Woven PTFE Membrane Fiberglass laminated to Fiberglass Superflex Air Flow, M 3 /hr RA Fan volume, m 3 /hr Net Air to Cloth ratio, m/min Estimated Baghouse Installed Cost, Rs in Lacs Operating Cost Comparison Savings in Capital Investment over Option- 1, Rs in Lacs 46 (190) (267) Interest Rs in Lacs/Yr % of Premature bag failures in 4.5 years Cost of Pre-mature bag failures, Rs in Lacs/Yr Expected Bag Life, Years Bag Replacement Cost, Rs in Lacs/Yr Power savings due to Lower BH DP, Rs in Lacs/yr 0 (8) Power cost for bag cleaning, Rs. in Lacs/Yr Total Operating Cost, Rs/Yr Net Savings compared to option -1, Rs/Yr 48 (94) (76) Specific Filtration Cost (Rs/m 3 of gas filtered per year) Table 3 : Capital and Operating Cost Comparison 5

6 The above analysis is based on an estimated cost and may vary from case to case based on steel cost, baghouse design and specifications. As can be seen from the above analysis, the Reverse Air baghouse technology with good quality PTFE membrane filter bags has low Capital cost, Operating cost and emission rate. It should be noted that the above comparison is based on 40% higher filtration velocity (smaller baghouse) for membrane bags compared to conventional fiberglass filter bags. Therefore, the selected PTFE membrane filter bags must have the capacity to provide 40% higher airflow/bag compared to conventional filter bags. We have come across membrane bags which are only as good as conventional bags to ones that provide 1.5 x airflow and 2 to 3 times the life of a conventional bag. Thus, it is very important to use the data from the same membrane bag supplier. VI. Case studies : Now let us look at some case studies for Kiln baghouses in India. Case Study 1 : New Reverse Air Baghouse to replace an ESP A cement plant in India, modernizing and expanding the plant capacity. New design capacity 3400 TPD. Existing ESP replaced with a new Fuller design reverse air baghouse. Design Conditions Airflow (including RA) = m 3 /hr Bag size = 292 mm x 9144 mm long No. of bags = 1920, No. of Compartments = 16 Net air to cloth ratio = 0.89 m/min. Results The plant restarted in December 1995 and and was operating at 3700 to 3800 TPD capacity. The baghouse operated at an average pressure drop of 80 to 150 mmwg pressure drop across filter bags with raw mill down (Direct mode). The airflow is high in the direct mode due to addition of dilution air for cooling. The individual compartments clean down to mmwg pressure drop. The air to cloth ratio is about 10-20% above design. Several times, the baghouse has recovered from a total loss of cleaning system failure, without any manual bag cleaning during the initial plant start-up. The plant capacity was further increased to 4500 TPD by water spray in the down comer duct. The compartments near the BH Fan are having higher airflow compared to other compartments and hence had higher wear and tear, requiring faster bag replacement. Three compartments were replaced with new Gore bags after 7.5 years and the remaining bags were replaced after 8 years. Case study 2 : Retrofitting an existing baghouse with membrane bags 16 compartment reverse air type Thermax bag house for Cement Kiln/Raw mill venting in India. Problem : Customer decided to increase the production capacity by increasing the pre-heater fan capacity. For this, the bag house had to handle higher airflow maintaining the same pressure drop across the bag house. 6

7 Solution : Installed GORE-TEX Membrane/ Acid resistant fiber glass filter Bags in 4 compartments (25%) in July Results : The operating data with raw mill down (Direct operation) are as under : Before installing Gore-Tex Parameter After installing 25% Gore-Tex Membrane Filter Bags Membrane Filter Bags % Kiln feed rate, TPH Airflow, m 3 /hr Pressure drop, mm wg The first lot of bags have completed more than 8.5 years without loss of any bag. The customer carried out plant upgradation from 3300 TPD to 4500 TPD in phases. The customer has replaced additional 21 modules in two kiln baghouses in year The second lot of bags have also completed 4 years trouble-free operation, without loss of any bag. Case study 3 : Retrofitting an existing baghouse with membrane bags A 10 compartment reverse air type, Kirloskar AAF make bag house for Cement Kiln/Raw mill venting in India. Problem : The baghouse was operating at mmwg pressure drop and this was affecting the productivity of the plant especially in the direct mode. The bag life was about 3 to 3.5 years and this needed to be improved. Solution : Installed GORE-TEX Membrane/ Acid resistant fiber glass filter Bags in 4 compartments in April 2003 and another 2 compartments in July Results : The results are better than the expected and with both raw mill and coal mill running, the operating data are as under : Before installing After installing 60 % Parameter Gore-Tex Membrane Gore-Tex Membrane % Change Filter Bags Filter Bags Kiln feed rate, TPH Airflow, m 3 /hr 729, , Air to cloth ratio, Net m/min Pressure drop, mmwg Filter Drag, mmwg/m/min Total (Airflow + DP) benefit 49 % bags changed 60 The plant has not reached full production level due to problems in the pre-heater section but the baghouse DP has reduced substantially. The filter drag (resistance) has reduced by about 47% with just 60% of the bags. There has been zero bag failures since installation. 7

8 Conclusions : Baghouse is preferred technology over Electrostatic Precipitator for de-dusting of cement kiln in India. In the present Indian conditions, the Reverse air baghouse design seems to offer more economical solution over pulse-jet baghouse. quality, PTFE Membrane filter bags provide lower capital and operating cost for reverse air baghouse in a cement kiln compared to conventional fiberglass filter bags. PTFE membrane laminated Fiberglass and composites of fiberglass fibers offers the best filter media option for pulse-jet baghouse design. The impact of lower import duty and exchange rate has lead to more cement plants switching to membrane filter bags in last one year compared to all previous years combined. REFERENCES : 1. Fabrics For High Temperature Gas Filtration - Mr. Dilip Sakhpara, W.L. Gore & Associates, (Presented in the 6 th NCB Seminar on Cement And Building Materials, November New Delhi, India) 8