Some of our clients are in the fortunate position to derive boiler fuel from their production process as a waste product. They are in the edible oil business, extracting sunflower oil from the seeds and being in the enviable position to transport the waste husk over a distance of less than 100 metres to the steam plant where it is fired in standard packaged coal fired boilers to produce enough steam for the whole plant.
But this benefit of free fuel does not come without challenges and one can really write volumes about it. One of the biggest challenges in my opinion is having to burn two widely differing fuels (coal and husk) in the same furnace and expecting both to deliver the same amount of steam at acceptable levels of efficiency.
Having been designed for coal firing the boilers perform well with coal fuel and combustion optimization is fairly easy to achieve by virtue of the fact that this is the default operating mode of the boiler. Firing husk with the same boiler is the culprit – it is foreign to the design of the boiler, but with a few make-shift arrangements (like adapting stoker sprocket sizes) users succeed to some degree to use it for raising steam.
Here are a few challenges I have experienced when firing husk (biomass) in a coal (fossil fuel) boiler:
- The control system must provide for two different control programmes with different settings, so that the operator can change over from coal to husk with the push of a button, without having to enter new combustion settings, except maybe for excess air requirements.
- I have found that in most instances where husk is fired the operator sets the control to manual mode and then sets the fuel bed deep and all motor speeds or dampers to maximum. Combustion air flow may be throttled to obtain a strong draught (low furnace pressure) to prevent the fire from burning back into the hopper, or at least, this is the general perception.
- Wasting energy with husk firing is costly – when the husk is finished, the user has to revert back to coal firing which adds significantly to the cost of steam production. Firing husk efficiently can extend the productive use of this free resource significantly.
- With too much combustion air and deep suction of the ID fan, burning fragments of husk is entrained in the flue gas and end up inside the grit collector where it can set alight unburned husk particles and cause a fire. I have seen this happen, resulting in a warped grit collector and shriveled flue gas instrumentation.
- Husk firing generates significant quantities of fly ash, which has a tendency to partly settle in the reversal chamber and partly in second and third pass tubes. Regular tube cleaning is required to maintain proper heat transfer. (We are currently investigating combustion settings for husk, as well as properties of fly ash, which cause it to settle inside tubes.)
- Because husk is high in volatiles (±70%) it ignites easily and burns out rapidly. The burning back of the fire into the fuel hopper is a constant threat and suitable measures must be applied to combat this phenomenon. Husk firing is best suited to applications where the steam demand is consistently high. The moment the stoker slows down as steam pressure increases the fire starts burning back into the hopper. Operator routines, such as de-ashing, can also create conditions favouring burn back. Apart from anything else let me say at least this: a deep draught has an inclination to promote burn back, rather than preventing it.
- A deep draught also causes significant ingress of tramp air into the combustion system. This is especially true with high volatile sunflower husk, and the effects of draught are clearly visible in the volume of combustion air required from the FD fan. With a deep negative furnace pressure combustion may be influenced stronger by ingress air than by combustion air supplied by the FD fan and control of the optimum air-fuel ratio. All the more reason to employ a control system designed for the combustion requirements of both coal and husk.
- Because husk burns out rapidly the rate at which it can be fed into the furnace is rather high – we have managed a linear grate speed of some 800 mm per minute without losing ignition or running a long fire. On the other hand the minimum required stoker speed with coal firing is some 15 mm per minute. This places a huge demand on the control system and boiler hardware to provide consistently stable combustion control when either coal or husk is fired.
- The risk of overheating stoker motors always exists where the boiler is fired with both coal and husk. And quite often the stoker motor does not develop enough torque to move the grate at minimum speed (when coal is fired).
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.
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