Incinerator Start-up

[laver]

A question possed to me by operations after reviewing the current start-up procedure. I guess this could be considered safety more then environmental, but none the less it all ties together in one way or the other... We currently have a natural draft incinerator. Our procedure is fairly generalized in that once the natural gas is added to the front end, operations is to light the flame. But if that flame is not to ignite (for whatever reason) then how much time should be aloughted before the incinerator is completely purged of all fuel gas and the procedure restarted? I know I will have to find the gas flow rate as well as the air flow rate but is there a way of determining when an 'explosive atmosphere' becomes 'too explosive'? How is this normally determined?

 

[Fizzhead]

Laver:

Any explosive atmosphere is "too explosive". I believe what you are trying to determine is the purge time necessary to achieve an non-explosive atmosphere (defined as a maximum of 25% of the Lower Explosive Limit (LEL))after terminating the flow of fuel and exhaust gas to this unit. Check out

http://www.cheresources.com.

You might be able to find something there to help you. Otherwise, any good chemical processing handbook should include purge calculations. As a rule of thumb in my work I have always held to a minimum of 10 complete air changes under any circumstances and have often actually ended up using 10 minutes of purge air at no less than 50% of maximum flow just to be sure, despite what the calculations said! Good luck!

 

[Fizzhead]

Laver:

Just checking back in I see that I really didn't answer your first question. Explosive concentration varies for each gas in air and again for mixtures. The are characterized by a Lower Explosive Limit (LEL, the minimum concentration where the gas/air mixture is explosive in a laboratory and the Upper Explosive Limit (UEL). The level canbe correlated to measured concentration using any number of commercially available units, or can be measured with an "LEL Meter". The LEL meter is normally calibrated to a standard gas mixture that has a similar heat output when combusted over a catalyst in the unit. Be careful, it is easy to fool yourself with these units. The LEL for the standard gas is not the same as the LEL for the mixture you want to measure. If you contact someone like Scott Gasses, they can make up calibration samples for your particular mixture in the type of meter you are using. Be aware, they are not cheap!

Also, you say you have a "natural draft incinerator". By this I assume you mean that you have a fan to force combustion air into the unit, but do not also have an exhaust fan. If so, then be careful about the "exhaust rate" you use in the purge calculation. Be sure it's correct. Such units are notorious for having highly variable airflow conditions before and during startup.

 

[airheadmw]

Four (4) air pass of complete chamber area = timing for purge.

 

[davefitz]

In the USA, the minimum requried purge air requirements are defined by the NFPA ( nat'l fire prevention assoc). If I remember correctly , it is 5 volumes of air for the boiler/HRSG/incinerator.

 

[baglan]

The air purge time should be enough to ensure the gas composition in the combustion spaces or any part that gets hot (i.e. ducting and stack) is below the LEL.

The problem with a natural draught incinerator is that there may be no positive air flow through the system when it is cold and therefore it is difficult to assess how much time is needed for purging.

Before the first start up, with the air dampers are open there could be a long purge period (since the last shut-down). After a failed start attempt there could be unburned gases in the combustion chamber. Sometimes steam is used to raise the combustion chmaber temperature to start the natural draught and to create a positive air flow.

In any case it is essential to check if the gas composition in the enclosed spaces is below the LEL and preferably less than 25% of the LEL concentration before any ignition source is introduced.