BOF primary off-gas cleaning: a new high efficiency and...
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MILLENNIUM STEEL 2008 61 BOF primary off-gas cleaning: a new high efficiency and water-saving scrubber-venturi system Spraying Systems (China) Company has developed an advanced high efficiency and water-saving scrubber-venturi system which provides enhanced dust removal and reduced water consumption compared to conventional wet systems. Plant operating results show a saving of at least 50% in water consumption compared to conventional venturi systems. Dust contents are ≤50mg/Nm 3 compared to typically 100mg/Nm 3 . The system is suitable for revamping existing wet systems as well as for new projects. A s it leaves the converter, BOF primary off-gas is at an extremely high temperature, typically 1,400- 1,600°C, and has a high dust content, typically 70- 200g/Nm 3 . The gas has to be cooled and cleaned prior to re-use or disposal. There are two basic types of BOF primary off-gas cleaning system, wet and dry, of which the majority are wet. These consume typically about 100- 1,000t/hr of circulation water depending on BOF size (30-300t) and reduce the dust content to ~100mg/Nm 3 . There are more than 600 BOFs in China, the majority of which employ the off gas (OG) system with two venturi scrubbers. Other users employ saturated scrubber-venturi systems or dry systems. ANALYSIS OF THE ISSUES Dust content and gas pressure drop In theory the final dust content in venturi-based systems depends on inlet dust content, dust particle size, water:gas ratio, design of venturi, water spraying method and gas pressure drop between the inlet and outlet. In practice, however, the dust content is strongly related to the gas pressure drop between inlet and outlet of the venturi. As an example, we can see from Figure 1 that if we want to control dust content to 100mg/Nm 3 max the gas pressure drop between inlet and outlet should be 100-120mbar. If this drop can be increased to 140-160m bar or higher, the dust content can be reduced to 50mg/Nm 3 max. The suction in a BOF gas cleaning system is provided from the downstream induced draft (ID) fan, and gas pressure drop across the venturi must be sufficient for the required dust collection efficiency. Based on experience and operational data, the total gas pressure drop is either higher than the total pressure increase – hence the dust content is often higher than desired, or the system cannot Authors: Changde Li and Morris Liu Spraying Systems (China) Co Ltd STEELMAKING cope with large amounts of gas – usually requiring a larger ID fan to rectify. This solution has been widely used in China and, although not economically beneficial, was a practical necessity. A much simpler and effective solution by some Japanese and Europe steel companies is the use of a scrubber- venturi system. These systems use the same fan but, by decreasing the gas pressure drops in other parts of the system, they maintain a higher differential pressure across the venturi for high dust collection efficiency. However, dust collection efficiency of the first venturi (of two) is not high (up to 95%), because the high gas velocity (40-70m/s). Unnecessary gas pressure drop is also high a r Fig 1 Relationship between dust content and gas pressure drop Dust content
Transcript of BOF primary off-gas cleaning: a new high efficiency and...
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BOF primary off-gas cleaning: a new high efficiency and water-saving scrubber-venturi systemSpraying Systems (China) Company has developed an advanced high efficiency and water-saving scrubber-venturi system which provides enhanced dust removal and reduced water consumption compared to conventional wet systems. Plant operating results show a saving of at least 50% in water consumption compared to conventional venturi systems. Dust contents are ≤50mg/Nm3 compared to typically 100mg/Nm3. The system is suitable for revamping existing wet systems as well as for new projects.
As it leaves the converter, BOF primary off-gas is at an extremely high temperature, typically 1,400-
1,600°C, and has a high dust content, typically 70-200g/Nm3. The gas has to be cooled and cleaned prior to re-use or disposal. There are two basic types of BOF primary off-gas cleaning system, wet and dry, of which the majority are wet. These consume typically about 100-1,000t/hr of circulation water depending on BOF size (30-300t) and reduce the dust content to ~100mg/Nm3.There are more than 600 BOFs in China, the majority of which employ the off gas (OG) system with two venturi scrubbers. Other users employ saturated scrubber-venturi systems or dry systems.
ANALYSIS OF THE ISSUESDust content and gas pressure drop In theory the final dust content in venturi-based systems depends on inlet dust content, dust particle size, water:gas ratio, design of venturi, water spraying method and gas pressure drop between the inlet and outlet. In practice, however, the dust content is strongly related to the gas pressure drop between inlet and outlet of the venturi. As an example, we can see from Figure 1 that if we want to control dust content to 100mg/Nm3 max the gas pressure drop between inlet and outlet should be 100-120mbar. If this drop can be increased to 140-160m bar or higher, the dust content can be reduced to 50mg/Nm3 max.
The suction in a BOF gas cleaning system is provided from the downstream induced draft (ID) fan, and gas pressure drop across the venturi must be sufficient for the required dust collection efficiency. Based on experience and operational data, the total gas pressure drop is either higher than the total pressure increase – hence the dust content is often higher than desired, or the system cannot
Authors: Changde Li and Morris LiuSpraying Systems (China) Co Ltd
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cope with large amounts of gas – usually requiring a larger ID fan to rectify. This solution has been widely used in China and, although not economically beneficial, was a practical necessity.
A much simpler and effective solution by some Japanese and Europe steel companies is the use of a scrubber-venturi system. These systems use the same fan but, by decreasing the gas pressure drops in other parts of the system, they maintain a higher differential pressure across the venturi for high dust collection efficiency. However, dust collection efficiency of the first venturi (of two) is not high (up to 95%), because the high gas velocity (40-70m/s). Unnecessary gas pressure drop is also high a
r Fig 1 Relationship between dust content and gas pressure drop
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reduce water consumption, Spraying Systems (China) Co Ltd, has developed an advanced high efficiency and water-saving scrubber-venturi system which provides enhanced dust removal and reduced water consumption compared to conventional wet systems.
Based on two patents the system comprises three key parts: ` A high efficiency and water-saving scrubber operating
on a semi-dry basis` An upstream and long ring slit venturi` A cyclone spray mist separator
Some features of this distinctive system are that it is very easy to manufacture from fabricated steel plate, is designed for installation in limited space, and is connected to the normal gas ducting by a non-metallic compensation adaptor or new type of water seal system (see Figure 3). Operation at 40 BOFs show that it is simple to operate and maintain, and has high and constant dust collection efficiency.
(3-5KPa). These pressure drop comparisons can be seen in Figure 2. Note that total gas pressure drop is 3,000-5,000Pa in the first venturi compared to about 300Pa with our new scrubber.Gas cooling methods Following initial gas cooling from ~1,500°C to 900-1,000°C by an indirect water cooling duct there are four main types of gas cooling systems to cool the gas further. These are: venturi, saturator, evaporator and semi-dry scrubber (see Table 1). These are based on one of two cooling principles. Saturation cooling by way of heat transfer to the liquid spray to reduce gas temperature as in systems A and B; the use of water evaporation as in C and D, which are based on one of two principles: use of latent heat of evaporation – so water needed can be decreased by 90%, and use of very high water drop velocity.
SPRAYING SYSTEMS (CHINA) CO LTDIn order to improve both dust removal efficiency and to
Cooling methods a) venturi b) saturator c) evaporator d) semi-dryWater flow rate, t/h ≥350 ≥350 450* 150Gas pressure drop, Pa 3,000-5,000 1,000 300 300Gas velocity, m/s 40-60 5 5 5Investment cost index 1 2 5 2Investment cost index of water equipment 10 12 1 3Operating cost index of water system 10 12 1 3Installation time (days) 5 5 10 5Dust removal efficiency of particles>1μm (%) 70 90 50 99 Dust removed via waste water waste water dry dust waste water
1=BOF hood inlet ; 2=cooling scrubber inlet; 3=venturi inlet; 4=venturi outlet; 5=fan inlet; 6=fan outlet; 3-4=venturi pressure drop; 5-6=total fan pressure increase
q Fig 2 Gas pressure drop comparisons between OG and new system
* including BOF gas cooling after electrostatic precipitator of 400t/hr Table 1 Comparisons of different BOF gas cooling methods for 120t BOF
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The specification parameters are shown in Table 2. The properties and characteristics of the system include:` Very fine drop sizes of 80μm Sauter mean diameter
which are evaporated quickly. Because the latent heat of water evaporation is 10 times its specific heat, the quantity of water needed for gas cooling is only ~10% of water-based systems, and cooling efficiency is very high
` The velocity of water drops produced by the spray nozzles is high (20-40m/s), thus enhancing their dust collection efficiency
` The free orifice size of nozzles is large – up to 6.3mm, compared to others at 2-3mm – so little clogging occurs and thus lower quality secondary water or even waste water can be used
` Low operation pressures: both water and N2 pressures are 0.6MPa or below
Scrubber The scrubber cools the primary OG from 900-1,000°C to ≤75°C by a semi-dry evaporation principle and collects the dust (see Figure 4). Key to its high efficiency and water saving benefits are the use of nitrogen gas/water mist-type FloMax® nozzles (similar to air mist in continuous casting), coupled with an advanced AutoJet® control system, both of which are patented products of Spraying Systems Co, USA.
A key role of a single water nozzle in the inlet area is to protect a group of FloMax lances and a non-metallic adapter from being damaged by heat and solid particles in the gas which are installed on the upstream side of the tower (see Figure 5). Several more layers of water nozzles are installed in the middle and downstream parts in order to increase dust collection efficiency and to assist dust removal into the water treatment system.
UP STREAM AND LONG RING SLIT ADjUSTAbLE vENTURIThis patented venturi (see Figure 6) has the following key characteristics: ` Gas inlet in downstream side: the pipe with higher dust
content gas is short and the gas in the piping after the transition section is much cleaner
` Adjustable throat (device in upstream side) provides better control and easy sealing
` Use of one nozzle having a large orifice resists clogging or eliminates the need for additional cleaning nozzles
Because it is a long venturi scrubber, the mixing time and distance for gas and water interaction are four times that of other types of venturi scrubbers such RD and PA. Mixing efficiency is much better, enabling fine particles to be removed with high and stable collection efficiency,
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Items parametersBOF size 30-300 tInlet temperature 900-1,000°COutlet temperature ≤75°CTotal water 50-300t/hGas pressure drop ~300PaDust collection efficiency 95-99%Water pressure 0.6MPaScrubber sizing Specific to siteN2 consumption ~300-2,000Nm3/h (pressure: 0.6MPa)Total equipment weight ~20-50t
r Table 2 Specification parameters of high efficiency and water saving scrubbers
r Fig 3a and 3b High efficiency and water saving scrubber-venturi system
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hence water consumption can be decreased considerably. Also, after the transition section of the scrubber the water:gas ratio is large. As no dust build-up occurs in the 180° elbow and duct, there is no need for cleaning by water spraying or by plant personnel.
CYCLONIC AND SPRAYING MIST SEPARATORThe gas inlet is at the bottom with cyclonic action and in a vertically upward direction. A non-moving gas-rotating plate in the middle promotes gas flow onto inside wall with cyclonic and impinging effects. FloMax® two fluid lances are installed near the outlet and may enhance mist elimination and dust collection. It may replace all other types of mist separators such as a traditional 90° elbow mist separator, and has a high mist separation efficiency of 98% or more.
CONCLUSIONS Operation at 40 BOFs indicate at least 50% water saved and lower outlet gas dust content of ≤50mg/Nm3 compared to conventional systems. Furthermore, energy savings of 1-2kWh/t steel can be achieved. The system is suitable for revamping existing wet systems, or for new projects, and can also be used as a middle stage in total dry system. The system is very low maintenance with respect to clogging and dust build-up. MS
Changde Li is BOF specialist and Morris Liu is Team Leader, at Spraying Systems (China) Co Ltd, Shanghai, China.
CONTACT: [email protected]
r Fig 4 Semi-dry high efficiency and water saving scrubber
r Fig 5 FloMax® two fluid lance
r Fig 6a and 6b Up stream and long ring slit adjustable venturi
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