Off-gas collection

Roof mounted hoods A hood is fixed on the furnace roof and collects fumes through a gap between die furnace shell and the roof. In addition, fumes arising from the working door and the tap-hole are also captured. Roof mounted hoods are heavy and subject to distortion from heat. If fitted to an existing EAF, problems may arise due to increased stmctural load of the roof lifting and rotating mechanism. Typical extraction rates for this system are 7500 to 15000 Nm /tonne 

Partial furnace enclosure Mobile or fixed enclosiu es are moimted around the furnace and tapping area. The former are retracted in order to accommodate eharging and tapping, while the latter are equipped with a mobile roof and side-walls for the same purpose. Exhaust gases are collected through a main exhaust duct at the top of the enclosiu e, assisted by fans inside the duct to guide the fumes to the exhaust. With these types of system collection, efficiencies of up to 98 % are possible 

Flue-gas capture allows the controlled evacuation and treatment of the flue-gas stream, and results in a minimisation of both fugitive and guided emissions. 

Capturing the exhaust increases the energy consumption. As it allows the cleaning of the exhaust gas, it will also generate dust for disposal or re-use. 

In a Polish foundry, the installation of a total enclosure over two EAFs with a capacity of 8.5 tonnes each, resulted in a reduction of the noise level from melting from 91 dBA to less than 85 dBA. 

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Wet—open combustion. Excess air is admitted to the off-gas collection system, allowing combustion of carbon monoxide prior to high-energy wet scrubbing for air pollution control. 

According to the developer, the WRS TDU differs from other systems in that it utilizes an off-gas collection system rather than an off-gas incineration system. The off-gas collection system eliminates problems associated with incomplete combustion. 

Copolymer IB-MA (l.Og) was heated under nitrogen for 6 hr. at 235-240°C, with off gas collection in a mineral oil trap. Isobutyr-aldehyde was confirmed in the mineral oil by IR, with 1740 cm-1 absorption and odor. The recovered polymer (0.73g), which was no longer soluble in water, EA, MEK and THF, was soluble in DMF, DMSO and y-butyrolactone. Heating 1.5g of IA-MA copolymer under the same conditions produced similar results, with 1.39g polymer recovered. 

Fig. 2.7-1 Off-gas collection system (a) with two alternative methods of disposal (b) flare or catalytic combustion.

Finishing/post-casting operations off-gas collection and treatment... 

As indicated in Table 4.53, some finishing techniques do not generate dust and consequently do not necessitate an off-gas collection system. 

For abrasive cutting, shot blasting and fettling, BAT is to collect and treat the finishing off-gas using a wet or dry system. The BAT associated emission level for dust is 5 - 20 mg/Nm The techniques for off-gas collection and exhaust air cleaning are discussed in Section 4.5.10.1 and 4.5.10.2. 

For the operation of induction furnaces, BAT is to melt clean scrap use good practice measures for the charging and operation use medium frequency power and, when installing a new furnace, to change any mains frequency furnace to medium frequency to evaluate the possibility of waste heat recuperation and under specific conditions to implement a heat recovery system. For exhaust capture and treatment fi om induction furnaces, BAT is to use a hood, lip extraction or cover extraction on each induction furnace to capture the furnace off-gas and maximise the off-gas collection during the full working cycle to use dry flue-gas cleaning and to keep dust emissions below 0.2 kg/toime molten iron. 

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