Preventing Boiler Explosions with Combustion Analysis
Hello everyone. This is Doug Simmers, product manager for combustion analyzers, and today I’m talking about analytical methods for protecting electrostatic precipitators. While I’m using an example from paper mills, many boilers and industrial furnaces utilize electrostatic precipitators for removing fly ash and other particulate from flue gases.
Bark is burned in a bark boiler along with other waste wood products to produce steam that is used in the pulping process, to drive the paper machine, and for many other uses. After combustion, the flue gases from the boilers are often passed through an electrostatic precipitator that uses static electricity to gather the fly ash. An electrostatic charge is induced in the flowing particles and then the particles are collected onto the energized plates with a negative voltage through electrostatic attraction. The negative voltage on the collector plates can be several thousand volts and there is some potential for electrical arcing inside the precipitator. If a combustible gas mixture is allowed to flow through the unit, the result can be an explosion. Combustion analyzers can tell boiler operators when explosive gases begin rising in the boiler flue gases, so they can take action to modify the fuel/air ratios, bypass the flue gases around the precipitator, or power down the precipitator. Good flue gas analysis of O2 and CO also helps the operator to optimize efficiency, and balance the combustion inside of large furnaces.
If an operator sees the O2 going down and the CO going up, that indicates there is a problem developing. Most engineers prefer that the analyzers be placed just ahead of the electrostatic precipitator in order to ensure that flue gases flowing through the precipitator have a low level of combustibles (CO) and a sufficiently high level of O2. 
In some cases, extremely high particulate levels can negatively affect the optical measurement of the CO. If the IR source energy is blocked by the fly ash, the performance is degraded. In these cases, the CO instrument can be mounted downstream of the precipitator after the fly ash is removed. While the location prior to the precipitator provides the fastest speed of response, high levels of CO in the flue gases typically take many minutes to develop, and the downstream measurement can still provide timely indication of increasing CO.
The initial response to a situation of falling O2 and increasing CO is to correct the fuel/air ratio (more air, less fuel), since bypassing or unpowering the precipitator may result in opacity exceedences. A reliable set of O2 and CO analyzers is the key to assist the operator to make the proper decisions surrounding the operation of an electrostatic precipitator.
You can check out more details on this application in my article in Pulp and Paper International which can be accessed HERE.
Faizal Jaafar | April 4th, 2012 at 7:44 pm #
From maintenance point of view, what are the common breakdown of Emerson’s Oxymitter & CCO 5500 analyzers, & how is reactive maintenance done to rectify them?
Bonnie Brown for Doug Simmers | April 10th, 2012 at 8:21 am #
Faizal Jaafar,
Doug had this answer for you:
Oxymitter – Uses ZrO2 fuel cell technology that is very robust and long-lived. Calibrations will uncover most problems, and should be done once every 3 months in a clean-fuel process, more if there is much acid in the flue gases (SO2 or HCL).
CCO 5500 – Is an optical across-duct measurement. Most problems are application-based:
high vibration of the duct
high particulate levels (IR energy cannot get through)
mis-alignment of source and reciever (most ducts shift as they warm up).
Purge air is a must to keep the optics clean. If the purge is lost, the analyzers should be dismounted to prevent overheating.