ENHANCING CAPACITY OF REVERSE AIR BAGHOUSE IN A CEMENT PLANT USING HIGH TECHNOLOGY PTFE MEMBRANE FILTER BAGS IN ORDER TO OVERCOME THE BOTTLENECK FOR ENHANCING PRODUCTIVITY OF KILN : A CASE STUDY N K Hapani.* S Ramarao*, Dilip Sakhpara** * Gujarat Ambuja Cements, India ** W L Gore & Associates, India Presented at the Seventh NCB International Seminar on Cement And Building Materials, November 1998 - New Delhi, India ABSTRACT This paper details the users actual experience with a new type of filter bags. Gujarat Ambuja cement has installed a reverse air baghouse for extraction of kiln and raw mill dust in their first line at Ambujanagar, Kodinar, Gujarat. It was found that as the production in the kiln was increased beyond the rated capacity, the airflow and pressure drop across the baghouse increased. This lead to production bottleneck when the raw mill was off, requiring a reduction in the kiln feed rate. W L Gore, having extensive experience in retrofitting of existing baghouse for enhanced airflow, was contacted. With partial replacement of filter bags in the baghouse and minor modifications in the entire baghouse system, GACL was able to achieve the desired production rate and also achieved substantial reduction in power consumption and maintenance time/cost with the use of GORE-TEX Membrane filter bags. INTRODUCTION Gujarat Ambuja Cement Ltd (GACL), located at Ambujanagar, near Kodinar, Gujarat, The Ambuja unit has a dry process 5 stage precalciner kiln supplied by M/s Krupp Polysius, Germany. The Plant was commissioned in the year 1986. At the time of installation, the plant was rated for a production of 2500 TPD. The plant is equipped with a Reverse air bag house supplied by M/s Thermax Ltd, for dedusting kiln and raw mill gases. The capacity of the Ambuja kiln was increased from its initial capacity of 2500 TPD to 3500 TPD over the last ten years through various improvement and modernisation program s. The latest was being enhancing the capacity of Preheater fan by changing the size of the fan impeller. This was expected to raise the capacity of the kiln by around 10%. The detailed analysis of all the upstream and down stream equipment revealed that, except for the reverse air bag house all other equipment have margin to handle the additional capacity. Especially the reverse air baghouse was becoming a bottleneck during the direct operation of the kiln due to its limited filtering area. It was expected that during the direct operation of the kiln, the differential pressure across the baghouse may increase beyond 200 mmwg, and we may have to reduce the feed to kiln by 4-5%. 1
Various options were considered to over come the above problem and the issue was discussed extensively internally within the organisation and with equipment suppliers, some of the options are : 1. Addition of some more modules to bag house to increase filtering area 2. Reducing volume and temperature of preheater gases by spraying water in downcomer duct, during direct operation of kiln 3. Partial/Full replacement of conventional fiberglass filter bags with PTFE membrane filter bags to increase the air to cloth ratio of baghouse Out of the various options available, option No. 3 was considered for following reasons: Addition of some more modules to baghouse is capital intensive and also there are space constraints due to very compact layout of the plant. Water spray in Preheater downcomer even though seemed feasible, had problems like lump and coating formation on preheater fan and also availability of water was also a problem. PARTIAL REPLACEMENT OF CONVENTIONAL BAGS WITH PTFE MEMBRANE BAGS The option was carefully analysed and detailed calculations were made. Different membrane filter bag suppliers were contacted to carry out the modernisation program. Based on the discussions with filter bag suppliers M/s W L Gore was selected to carry out the above modification due to following reasons M/s W L Gore was having enough experience of carrying out replacement of filter bags in existing reverse air baghouses and also has more than 20 years of experience in manufacturing membrane filter bags. M/s W L Gore had engineers locally available who were thoroughly knowledgeable and trained to analyze the baghouse design for such modernisation activity Both M/s GACL and M/s W L Gore took the task of partial replacement of conventional bags with PTFE membrane bags as a challenge as it was first time such a replacement was being done in India. The operating data of the existing baghouse was collected extensively in order to decide no of filter bags to be replaced for the required capacity enhancement. It was decided to replace filter bags in four modules out of 16 modules with Gore bags based on the following criteria Required reduction in pressure drop and increase in airflow to achieve the desired kiln production The data collected should be conclusive technically and statistically. That means the reduction in pressure drop and increase in airflow should be more than the margin of error in testing equipment. Base levels for both airflow through bag house and pressure drop across bag house were fixed jointly to assess the performance of the PTFE membrane bags 2
MEMBRANE SURFACE FILTRATION CONCEPT In membrane surface filtration, a laminate consisting of an eptfe membrane and a backing substrate is employed to remove particulate matter from the gas stream. The eptfe membrane is a microporous node and fiber structure that provides an effective pore size much smaller than that of conventional in-depth filter media. (See Figure. 1) This membrane is laminated to the surface of a backing material. The backing material is typically manufactured from a traditional filtration fiber and is selected based on the thermal and chemical requirements of the gas stream. The eptfe membrane is chemically inert to most compounds and can withstand continuous operating temperatures of 260 C. Figure 1: SCANNING ELECTRON MICROGRAPH SURFACE COMPARISON 200x 600x GORE-TEX membrane 200 x 600x Acid Resistant Fiberglass Fabric The eptfe surface stops all particulate matter at the surface of the filtration media. The membrane can be thought of as a permanent, factory applied primary dust cake. From various porosity membranes, a membrane is selected such that the particle size, particle shape, bulk density, agglomerativeness, cleaning cycle parameters, or inlet dust loading does not affect filtration efficiency of the membrane, for the given application. An external cake forms on the surface of the media during process operation. The eptfe membrane provides a smooth surface from which the dust cake is released during a cleaning cycle. Dust release requires less energy since particulate releases easier from the membrane than from individual fabric fibers as is the case with conventional filters. Since no internal dust cake is ever developed, the permeability recovers to almost original levels after each cleaning cycle with very little decrease in permeability over time. The end result is lower pressure differential and higher airflow. Filter life is enhanced since there is no gradual buildup of an internal primary cake to slowly blind the media or abrade the fibers. Figure 1* is a scanning electron micrograph comparing the eptfe membrane to conventional Teflon-B Fiberglass Fabric. The first view of each material is at a magnification of 200x. The second view is each material at 600x. Corresponding incremental micronsized particles are included for reference. This comparison reveals why the eptfe membrane is capable of capturing even submicron size particulate matter, whereas the conventional filter media requires the formation of a primary dust cake to achieve acceptable filtration performance. 3
CONVENTIONAL DEPTH FILTRATION A conventional depth filtration process relies on the formation of an internal ('primary') dust cake within the cross-section of the filtration material to filter particulate from the gas stream. The formation of this cake is sometimes referred to as seasoning; or conditioning. Limestone or other materials are added as a precoat to new conventional filter bags to help develop this primary cake. The cleaning system (shaking, pulsing, or reverse air) may be delayed upon initial start-up to facilitate the development of the primary cake. Particulate capture efficiency is dependent on the maintenance of this dust cake. During the cleaning cycle, the primary cake may be disturbed and must be re-established to provide good efficiency. Sometimes a puffing phenomenon of particulate emissions occurs during each cleaning cycle as the primary cake is disturbed and rendered less effective. During filtration, an external ('secondary') dust cake accumulates on the outside filtration surface of the media. Ideally, it is this secondary cake that is removed during a cleaning cycle. The permeability recovered during each cleaning cycle (reflected in a decrease in the pressure differential across the media) is dependent on how much of the secondary cake is released. The more cleaning energy imparted to the media, the more secondary cake is released. Over a period of time, the internal, primary cake becomes denser, eventually decreasing the media permeability. This is reflected in a rising differential pressure that can not be recovered during normal cleaning cycles. At this stage, the filter media may be removed for washing in an attempt to remove the primary cake or the filter media may be replaced with new filtration media. Factors such as particle size, particle shape, bulk density, agglomerativeness, cleaning cycle parameters, and dust loading affect the development of the primary and secondary dust cakes. These factors also affect the maintenance of the primary cake responsible for particulate capture efficiency. For these reasons, similar cement processes may exhibit different characteristics in regard to filtration media operation. BAGHOUSE DESIGN DETAILS Parameter Value Gas Volume, M 3 /hr Rawmill Running 320000 Raw Mill Not Running 546000 No Of Modules 16 Total No Of Bags 1728 Net air to cloth ratio, m/min Rawmill Running 0.327 Raw Mill Not Running 0.56 Bag size, Length x Diameter mm Type 1 10339 x 292 Type 2 7800 x 292 Type 3 10339 x 200 4
COMMISSIONING Location of four modules for replacing with membrane bags were selected in such a way that the additional air flow and dust load in the baghouse is well balanced. The bags were replaced in May 1997. M/s W L Gore has provided all the necessary pre installation, installation and post installation support services on timely basis M/s W L Gore has also extended its support in optimizing the cleaning cycle after installation of membrane bags. RESULTS ACHIEVED After the initial three months of stabilization period, the performance of baghouse was monitored jointly by GACL and W L Gore and following are the results achieved before and after installation of membrane bags DATE : Measured 5-May-97 All conventional Bags 14-Oct-97 4 modules with Gore bags % Change Kiln Feed tph 208 217 Clinker tph 127.57 133.09 BH Fan Step No 14.00 14.00 BH Fan Damper % 98.00 98.00 RA Fan Speed % 90.00 96.00 BH Inlet Temp Deg C 230 231 BH Inlet Draft mmwg -60-70 BH DP mmwg 190 170-10.5 Flow Measurement At Baghouse Stack Temperature Deg C 180 180 Static Pr mmwg -15 15 Velocity Pr mmwg 14 18 Density Kg/M 3 0.78 0.78 Velocity M/Sec 15.22 17.46 Volume M 3 /Sec 146.39 167.94 Volume M 3 /Hr 527003 604567 15 Volume Nm 3 /Hr 318060 363805 Based on the flow measurements, the baghouse flow was increased by 15 % compared to measurements taken on 5 May 97 and DP has reduced by 10.5% 5
BENEFITS ACHIEVED Pressure Drop across the baghouse reduced by 10% during raw mill off condition Air flow through bag house simultaneously increased by more than 15% through the baghouse. The kiln is able to operate at expected capacity during raw mill stop condition Till date there are no premature bag failures. The bags are in clean condition and are expected to give better life. CONCLUSION It was clear from the above that well-engineered project combined with a good support from the equipment supplier can provide good results to the end user. 6