In our previous publications we have touched on the contribution of combustibles (smoke) to the total boiler energy loss, and although this loss is not really significant, the impact of smoke on the environment can be rather negative and downright emotional from the public’s perspective.
To add to the problem of smoke the construction of your standard horizontal fire tube boiler does not really promote the suppression of smoke, neither does high volatile coal. After all, the smoke originates from the volatile gases which are liberated from the coal when exposed to heat from the ignition arch.
So what is smoke? Smoke originates from hydrocarbons in volatile gases (C-H combinations, e.g. methane an ethylene). At high enough temperature the hydrocarbons decompose into their elements of hydrogen and carbon. The hydrogen molecules combine immediately with oxygen from the combustion air, leaving the carbon molecules in suspension, waiting for more favourable combustion conditions before combining with oxygen. In absence of sufficient oxygen or high enough temperature the carbon particles will not burn and escape through the stack as visible dark smoke. So it seems that smoke is the product of incomplete combustion, caused by either too low excess air supply, or by “cold combustion”.
Insufficient combustion air is easy to understand. It means that not all carbon atoms find a pair of oxygen atoms to combine with. This can be caused by either too low a percentage of excess air, or by insufficient mixing of combustion air and volatiles (i.e. lack of turbulence in the furnace).
The presence of cold combustion can sometimes be observed when looking through the rear peep hole. Looking up towards the roof of the furnace a blanket of dark smoke may be observed, drifting on top of the fire. A boundary layer of “cold” gases can develop along the furnace walls and roof, which are submerged by relatively cold water. In this boundary layer the temperature is too low to sustain ignition and the carbon laden combustible gas passes through the flue gas passages and the stack to the atmosphere as dark smoke.
Smoke normally shows when the boiler load is low. Combustion air flow may be low at that time, with laminar rather than turbulent flow in the furnace. Poor air and gas mixing takes place, as well as ample time for the combustibles to cool down along the roof of the furnace. Once the boiler operates at higher load the smoke may disappear to some extent, or completely.
Is there a way to suppress smoke development in a boiler? Yes, it can be done, but it may require a number of interventions.
- Under severe smoking conditions it will not be possible to eradicate all smoke, but improvement of the situation can be achieved.
- High volatile coal is the main contributor to smoke. Use lower volatile coals if possible; 28% volatiles is about the upper limit if no other suppression methods are applied.
- Segregation of coal creates a condition where a section of coal on the stoker grate, containing a higher percentage of fines, is starved of oxygen and becomes a source of smoke formation. Devices such as swinging chutes and scatter plates can successfully eliminate this phenomenon and contribute to an even distribution of coal across the width of the grate.
- Ensure adequate excess air is always supplied to the fire without invoking excessive stack losses. A well designed control system should be able to cater for this requirement.
- Improve turbulence in the furnace. With some boilers operating at a low furnace pressure (below -20 Pa) can help to some extent, but increased tramp air may inhibit combustion efficiency.
- The best way in combating smoke is to improve turbulence by introduction of over fire (secondary) air to the furnace. This over fire air enhances mixing of combustible gases with combustion air (oxygen) which is required for complete combustion thereof. Care must however be exercised that this over fire air does not contribute to cold combustion or unacceptable stack loss. (Contact us for more information regarding this intervention).
- Prevent cold combustion. The over fire air can be used to direct volatiles away from the cold zones towards the fire where temperatures are sufficiently high to cause them to ignite.
- Ensure slow and consistent release of volatiles from the coal. This can be achieved with modulating capacity control, a wide steam pressure control band width, elimination of coal segregation and sourcing coal with low duff content.
- Ensure the boiler furnace is of adequate size (volume) to successfully burn high volatile fuels, such as husk or wood. Too small a furnace may inhibit proper mixing of air and volatiles.
- Do not feed the furnace too hard. The rate of release of volatiles may exceed the capacity of the furnace to meet requirements for air and volatiles mixing, and even the capacity of fans to supply sufficient supply air for complete combustion of the fuel.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about combustion control systems and 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.