Ideally, you should aim at increasing the combustion air temperature rather than injecting more air.  Higher moisture in the fuel and more combustion air reduces your boiler efficiency.  I could go on about the combustion process and why hot air is better than more air, but I think you get the idea.

Mark .. Thank you and I agree. But I had a co-worker argue with me ..that by evaporating the moisture, more oxygen was liberated and less air would be needed. I was asking this question to make sure I wasn't off base

How do you liberate oxygen by evaporating moisture?????  I dont think your buddy knows what he is talking about.

Can one envision the idea that evaporating moisture may expose more "fuel" surface to the flame reducing the requirements of excess air to complete the combustion process ?

I don't understand your point.  The first stage of the combustion process for biomass fuels, is to evap. moisture.  The excess air requirements are a function of flame stability and unburnt carbon loss.  Excess air also affects the production of CO and NOx.
Experiance tells us that the excess air for wood firing is in the order of 30% at BMCR.  
If you reduce this, the amount of CO will increase, the unburnt carbon loss will increase, the NOx production will decrease and the flame stability will be adversly affected.  That is, you may get an increase in furnace puff, depending on your moisture content and how variable it is.

Like most arguements I see an element of truth in all this.

In an ideal world the oxygen from the moisture (H2O) in the fuel would be released for reaction with the carbon in the fuel. This would however mean the hydrogen from the moisture would be left to react with oxygen to form, guess what, H2O.In the real world moisture is only heated up in the boiler and represents an efficiency loss, particularly where it enters as a liquid and leaves as a vapour. The efficiency loss due to moisture in the flue gas and the lower net calorific value of the fuel will mean that more fuel has to be fired in the boiler for the same thermal output where higher moisture fuels are used. More fuel fired will mean a higher air flow rate is required. From a combustion perspective the presence of moisture inhibits combustion in the early stages and operation at low oxygen levels may lead to the increased formation of CO and possibly carbon residues (depending on the firing system) in relation to drier fuel. Putting it another way slightly more excess air may be required to offset the combustion inhibiting effect of the moisture.

Increasing air pre-heat is certainly a good idea but may be limited by the plant design and may also impact on overall boiler efficiency depending how it is achieved.

In summary:

1. No additional oxygen is liberated by moisture in the fuel.

2. Increasing fuel moisture will require higher fuel flow and correspondingly higher air flow for the same thermal output due to moisture heat losses (both sensible and latent).

3. An small increase in excess air may be required with increasing moisture to prevent the formation of CO and carbon residues compared to drier fuel.

Is everyone happy with this?

This is a little bit off line but talking about combustion reactions, i felt it appropriate to ask my question.
How could one account for the reduction of CO emission from charcoal on addition of oil palm fibre briquettes. The briquettes have a moisture content of about 7%. Could the moisture play a role, perhaps acting as a catalyst in the combustion of CO to CO2?

idd233:

As far as I know the subject of whether water vapor "as is" catalyses the reaction you mentioned, or whether radicals or hydrogen from water decomposition do it, is still not clear. It is evident that humidity, up to a certain %, helps in reducing the ratio CO/CO₂, and that the reactions taking place are numerous.

Quoting Robert D. Reed in his Furnace Operations edited by Gulf, in the section titled Possible Carbon Oxidation Chemistries: "The fireman who hand stoked a coal-fired boiler knew that when he sprayed his coal with water prior to scooping it into the furnace, his fire was hotter, made less smoke and he burned less coal per 1,000 lb of steam."



 

Water spraying may cause chem reaction in coal and actually helps reduce emissions and not run hotter but burn more efficient thus reducing the quantity burned per load of steam, but I doubt it will do the same on wood, It may add a little oxygen fron the moisture as well, but then it will be runoff by the moisture which will require more heat to dryout.
ER

Mike wrote:
In an ideal world the oxygen from the moisture (H2O) in the fuel would be released for reaction with the carbon in the fuel.

Maybe so,  but not in THIS world,  not to any appreciable degree anyway.
What WILL happen is that higher moisture levels in green wood (say) will require more heat to evaporate, thus cooling the combustion process and slowing it.

A better idea would be to have a heat exhanger in the stack,  and use stack (waste) heat to pre-heat the combustion air.

I have seen it both ways, wood fuel burned without preheated combustion air, and with.  There is no comparison.  The preheated combustion air helps drive off the moisture so that the carbon and gaseous material in the wood can ignite and burn.  Without the preheat, the incoming air has to be heated first, and then the moisture has to be driven off, all of which has produced no useful work yet from the fuel.

Regarding the release of hydrogen from the wood moisture, you better hope not.  It is called water gas, and it takes a pretty hot furnace to do it, out of the range of most wood fired furnaces, but not to say that it hasn't happened when water leaked into a very hot furnace.

But when it does, it is explosive.  When it has been suspected in cases that I know of (can't be proven) the results were catastrophic.  Not the thing one wants to try to do on purpose.

In normal wood combustion, any H2 in the combustion calcs is a product of hydrogen in the wood, not in the moisture the wood brings into the furnace.

The fuel moisture just gets heated up and carried through the boiler or whatever, and is a loss.

rmw