Hi!
I've heard that, in general, fluidised bed boilers have a higher thermal efficiency compared to moving grate boilers. I've found literature that somewhat confirms this, but also literature concluding that they have equal thermal efficiency.
Anyone with proper knowledge who can elaborate this?
Br,
Andreas
Hi Andreas and all.
Mos probably this is so, for two main reasons: (a) in a typical fluidised (British spelling) boiler, say using coal as fuel, the carbon bed load is very low (around 2% of the bed weight). This is to avoid slaging of the bed mateterial (sand) and keeping bed temperatures around 800 ºC; (b) mixing effects (both longitudinal and transversal) in fluidised beds are intense due to the bubbling of air. Thus, the resistance to diffusion of oxygen towards the char particles (a very porous material) is minimized. In general, the process is controlled by both diffusion and combustion kinetics at the char surface.
Hi R. P. Peçanha
Can you please provide som e mor information.
For instance, I don't get how "the carbon bed load is very low (around 2% of the bed weight). This is to avoid slaging of the bed mateterial (sand) and keeping bed temperatures around 800 ºC" improves thermal efficiency compared to a moving grate boiler? What is the advantages of your point A) as i quote here, and why does this improve thermal efficiency?
Your point B I guess relates to a more complete combustion and complete burn-out of combustibles, more independent of size etc. compared to a moving grate. But does this actually improve efficiency?
Br,
Andreas
Hi Andreas and all.
Do we agree that thermal efficiency (fluidized beds boiler X moving grate boiler) has to do with the extent of complete combustion of carbonaceous materials (e. g., coal) to give carbon dioxide?
If so, and using kinetics terminology, we are always aiming for high conversion levels of carbon to carbon dioxide, whatever the equipment used.
To fluidise a sand bed (~ 2 % weight of coal) in the bubbling regime (thus promoting the hihghly desired bed temperature uniformity), you need much higher air flow rates as compared to moving grate boiler, for the same carbon load. Regarding the combustion reaction itself, the air is a lot in excess in fluidized bed boilers under bubbling regime. For this reason combustion tends to completion in such boilers resulting in higher thermal efficiencies.
Hi R. P. Peçanha
Thanks for your responses!
The term I was thinking when considering efficiency is more related the energy input (fuel LHV) vs. energy output (steam output), i.e., combustion gross and net calorific value. Not considering flue gas condensation. But this is perhaps a term not reflecting type of combustion very well, and rather reflects boiler configuration and downstream aspects.
When considering your term of efficiency, the difference is more clear and understandable. It least, I think I understand the term and the differences fluidised vs grate combustion, and the relation to burn-out time, which I presume is important in this aspect?
Br,
Andreas
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