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!