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.

Doug Simmers here again, discussing flue gas analysis, and it’s operational value for boilers and industrial furnaces. Controlling the amount of air going into any combustion process is important in maximizing the efficiency of the furnace. 

It’s pretty easy to see why a fuel-rich mixture is inefficient, since unburned fuel goes out the stack without giving up its heat value. Besides being inefficient, the accompanying black smoke also draws the neighborhood’s attention, and operation in this mode is also unsafe.

The disadvantages of operating with too much air (lean) is not as obvious. After all — air is free, isn’t it? Since air is invisible, it’s easy to forget that it has mass, as sticking your hand out the window of a moving car demonstrates. The energy required to heat up air is called its specific heat (0.24 BTU/lb/degree F), and any air that is not used for burning the fuel merely cools off the flame. Granted, this excess heated air gives back some of its heat in to the boiler tubes, but it almost always exits the stack at a temperature significantly above the temperature it goes into the burner. This heat is lost forever, and if one considers the significant volumes passing through the furnace, this loss can be significant. Further, an excess oxygen reading of 1% is the smaller amount of gas that is being heated up, since it’s only about a fifth of the total volume of air (20.95% O₂). So a small increase in excess O₂ increases the total air going through the furnace significantly. Additionally, it costs money for the fan blowing air into the burner to move this excess air, and it also reduces the total amount of steam the boiler can produce.

In the previous blog, we discussed how the ideal O₂ setpoint is arrived at by detecting the point of CO breakthrough, but how do we determine how important running at the optimum level is? A blog is not the ideal place to run down the ASME short form calculations, but our Jim Thompson has developed a neat program that calculates this out for you (note that most utilities use more comprehensive calculations for determining heat rate). http://www2.emersonprocess.com/en-US/brands/rosemountanalytical/Gas/combustion-flue-gas-analyzers/OXT5A/Pages/O2_TrimCalculator.aspx

The procedure is to determine the “as found” operational condition of the boiler, and then determine how much lower in oxygen the boiler or furnace can operate. The payback is the final output — a great tool for justifying a project.

Next time we’ll discuss how to use the oxygen measurement to minimize the thermal NOx produced in a burner.

Until then, let me know what you think! Post any comments or questions here!