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Process Puzzler: Keep Incineration Safe

Readers suggest how to deal with a cyanide-containing fuel.

THIS MONTH'S PUZZLER
We're trying to establish the start-up procedure for burners in a natural draught incinerator. As always, there's the danger that a burner may not light in time to avoid tripping the infrared flame sensors. The standard practice after a trip is to purge any fuel present according to NFPA requirements. This would be easy if we used natural gas -- but this incinerator is in China and uses coke oven gas that contains hydrogen cyanide. How can we modify the purge procedure for this fuel? Do we need to change the burner design and other incinerator components to handle it?

DIVERT GAS DURING PURGING
I assume that the coke gas is the only source of fuel if I am reading your question correctly. If that is the case then it must be diverted during the purge cycle so that all combustibles can be removed from the furnace before introducing flame. The destination for the HCN gas can be a flare or it even can be vented to a stack if the time is short and environmental permits allow.

The burner design will probably need to be modified to handle this gas and provide stable operation. Most any burner vendor should be able to do this if you have a list of the gas components.

As an aside, you might consider providing an alternative source of fuel (propane, natural gas, etc.) for your pilot flame. That will allow you to decouple your incinerator operation from your coke furnace so that you can have the incinerator pilots lit and ready to go to "main flame" as you bring up your coke oven. This will reduce your coke gas diversion time to a minimum.
P. Hunter Vegas, senior project engineer
Avid Solutions, Winston-Salem, N.C.

SWITCH GASES
Why not use natural gas to light-off or use for the ignition and then switch to coke gas as your main fuel after the light-off?
Randy Cook, maintenance engineer/energy analyst
Yates Services, Smyrna, Tenn.

CHECK COMBUSTION CONDITIONS
Because purging is necessary, it may be better to light the burners with natural gas and then verify that the incinerator temperature will permit destruction of hydrogen cyanide. Its autoignition temperature (AIT) is 1,000 degrees F, well below the adiabatic flame temperature of natural gas at 10% excess air, which is about 3,500 degrees F. (AIT is used by the U.S. Environmental Protection Agency to rank chemicals for thermal disposal. The EPA classifies hydrogen cyanide with toluene as Class II.) According to David Lewandowski's book, "Design of Thermal Oxidation Systems for Volatile Organic Compounds" (Table 3.2,. p. 27), a temperature of 400°F above AIT is required with a residence time of at least 0.5 sec. This is for a 98% destruction set by Maximum Achievable Control Technology (MACT) standards. There is probably sufficient time and temperature to assure combustion of the cyanide but a burner specialist should confirm this. A cold spot in the incinerator could cause poor combustion. The table cited assumes good mixing and at least 3% oxygen. Injecting pure oxygen would raise the flame temperature and may improve combustion; this works in Claus burners familiar to refineries.

If natural gas or another gaseous fuel is not available, there are alternatives: A dual-service burner may permit operation with a light fuel oil. Or, it might be possible to strip the hydrogen cyanide from the fuel gas in a two-step pH-controlled process with an aqueous hydroxide. Perhaps enough gas could be purified and compressed to set aside for start-ups and shutdowns.

One sure-fire approach would be to divert the exhaust through an operating catalytic thermal oxidizer until a sufficient temperature is attained.

Fortunately, this construction is going on in China and not the U.S., where standards are far stricter.
Dirk Willard, senior process engineer
International Steel Services, Inc., New Caledonia

APRIL'S PUZZLER
We blend a partially dehydrated color paste with mineral turpentine (C9–C16 range) at a volume ratio of 15:85. The final flow is 25 gal/min. We then disperse solids, up to 20% by weight, in the solvent. The solids form clumps. The paste isn't shear sensitive. The mixture is chilled to reduce vaporization. As a result, solvent viscosity is somewhere between glue and milk after chilling. The final product is an oil-based paint that is heated before use in a sprayer. Turpentine is driven off and recycled. One engineer has proposed blending this mixture in one shot with a single hydrofoil impeller in a tank with a 1.5:1 length-to-diameter (L/D) ratio. We have baffled closed tanks available in the mix room with L/D ratios of 1:1,1.5:1, 2:1, 2:1 and 3:1 and volumes of 340 gal., 220 gal., 500 gal., 500 gal., 650 gal., respectively. Can you suggest a better way of mixing the products? How can we improve this process to make it safer, easier to operate and more environmentally friendly?

Send us your comments, suggestions or solutions for this question by March 12, 2010. We'll include as many of them as possible in the April 2010 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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