We recently did work for a Client who occasionally fires sunflower hulls with his fire tube boiler; at other times he uses pea coal. We used the opportunity to provide a simple change-over capability with his boiler control system so that either coal or hulls firing can be selected with the push of a button on the operator panel.
At the time of commissioning the control system alterations I noticed that the operators operated the boiler at deep (negative) furnace pressure (-45 Pa) and extremely high stoker speeds to prevent the fire from burning back into the fuel hopper. To me it seemed that the deep negative pressure in fact promoted burn back, rather than preventing it, especially at lower stoker speeds.
So our first step was to increase furnace pressure to -10 Pa and, lo and behold, the burn back problem improved dramatically. However, a new phenomenon surfaced – with the FD fan and stoker running at the same speeds as before (ID fan now running slower), the length of the fire increased significantly, with some burning fuel dropping with the ash into the ash trolley. The only reasonable explanation for this new problem was that less (tramp) air entered the furnace now that the furnace pressure (draught) was more positive than before, resulting in less available oxygen and slower combustion of the fuel.
Going back some 10 years into the past I remember an incident where a client complained about his boiler smoking excessively under normal operating conditions. I happened to be on site at the time and suggested he lowers the furnace pressure from -5 Pa to -20 Pa. The shade of the smoke from the stack turned significantly lighter, much to the delight of the user who was desperate to do something about his smoking stack. I later realized that most of the tramp air introduced in this way probably acted as over-fire air, mixing with the volatiles, increasing turbulence and providing for more complete combustion of the combustible gases.
With this in mind I conducted an experiment on a brand new coal fire tube boiler for which we supplied the control system. The client was under immense pressure from the local community to reduce smoke emissions from the stack (coal volatiles exceeded 30%) and hence the boiler was operated at elevated levels of excess air and deep draught as a means to alleviate excessive smoke and protecting the guillotine door from fire burning back under it.
With a portable gas analyzer in hand I conducted a simple experiment to establish the influence of draught on flue gas oxygen level and hence combustion efficiency. With the boiler operating under stable conditions the draught was set at -5 Pa and the flue gas O2 level was measured at 10,6%. Thereafter only the speed of the ID fan was increased to lower the draught to -25 Pa and the flue gas O2 level increased to 11,5%. No adjustment of excess air, or anything else that could influence the air-fuel ratio. But even so the combustion air increased by almost 10%! And even with a brand new boiler tramp air ingress seems to be a draught induced reality.
So what is the significance of all of this? Probably more than one may realize at first glance. Firstly my combustion calculator indicated an additional 2% fuel energy loss because of the lower furnace pressure and resulting increased flue gas oxygen percentage.
Secondly one can only speculate at the response of the control system if a flue gas analyzer has been incorporated to trim the air-fuel ratio – most tramp air bypasses the fire, or fails to take part in the combustion process. Read more about that in Boiler Bits 8.
It is not unusual to see boilers being operated at -20 to -30 Pa furnace pressure in this day and age, especially those boilers still equipped with photohelic type draught controllers.
Want to save fuel? Consider the furnace pressure at which the boiler is being operated. Quite often some of the low hanging fruit is not so easy to spot, but easy to find if one knows where to look. And don’t underestimate the benefits of investing in a high tech combustion control system – it may reveal even more low hanging fruit!
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.