gasifier efficiency/ relation to moisture content

[pybyr]

Hello all--

Can someone please give me a substantiated, factual analysis & explanation [see footnote 1, below] of how relative moisture content of firewood quantitatively affects efficiency/ heat output in a wood gasification boiler?

I realize that water content in the wood needs to be boiled off before or during combustion, and that this takes away from net heat output.

What I would like to better understand is the degree/ extent of difference in net heat output, assuming the same mass of wood (which will equalize between different species, as a pound of one species should have nearly the same fuel value as a pound of a different species), at, sat, 30%, 25%, 20% and 15% moisture content.

Thanks in advance- my inquiring mind eagerly awaits... I am trying to find specific reasons to look forward to things getting better as I get further ahead in my wood harvesting in future years.

Footnote 1: Please no broad brush generalities along the lines of "gotta season it for two years"-- I'm interested in the physics and chemistry of how this quantitatively plays out.

 

[jebatty]

Energy in Wood may get close to what you are looking for. I also recall Nofossil some time ago posted an excellent analysis of this topic. Maybe he will chime in too.

 

[Nofossil]

I'm way too busy at the moment to reopen this - I have a job to deliver seven NoFossil controllers for a hotel energy management system. Basically, you can do the number crunching for the latent heat of vaporization for the weight of water (970 BTU/lb) - that energy is gone up the chimney. I think it's around 8% of the wood's energy content at 20% MC. Converting water to steam takes a pretty amazing amount of energy.

 

[Chris Hoskin]

I think our blog post will help at: http://blog.woodboilers.com/2009/06/european-union-to-north-american-wood.html

 

[heaterman]

Simplified..........

Nofossil is correct on the vaporization issue. 970 btu's to change 1 pound of liquid H2O @212*F into vapor at the same temp. Phase change is what it is called. It's how steam heating systems work as well as refrigeration systems also.

Without getting into actual heat contained in a 10# piece of wood just consider the heat lost at different moisture content.........10# of wood at 35% moisture means that 3.5 # is water that has to be turned into vapor in order for the actual Carbon content of the wood to be released. So 3.5 x 970 = 3395 btu's expended to effect the phase change and get the water out of the wood/carbon. In this scenario, for every 10% you drop the MC you have 970btu's available for your house rather than boiling the water out of the wood. That adds up very quickly.
Not a lot of thought required to see that wood seasoned to 20% as opposed to 35% can make a huge difference in the amount of fuel used to transfer XX number of btu's into your heating system.

Forget the "generalities" ....just get a moisture meter and season to 20-25%. 1 year, 2 years isn't the point.

 

[mwk1000]

I just had a very nice graph of the situation, I'll do it again and post the picture. I was lazy last week and reloaded my eko with not well seasoned aka 30% moisture wood that was in the shop to dry. The chart of my HX performance says it all. One load of seasoned wood drove the HX at 65-55 degree transfer rates for many hours, the reload with damp wood was lucky to get up to 20 - and that was into an already hot boiler.

All those BTU's wasted in changing water to steam so I could humidify the great outdoors.

 

[DBoon]

Hi NoFo, I've always enjoyed your posts and wondered what you were up to nowadays. Glad to hear that the small project you started for yourself may be turning into something more. Good luck with it.