Calculating Kilowatt-hour output of firewood, anyone?

[luke_mclachlan]

The reason it changes with moisture content is the huge energy boost needed to boil off the water. The specific heat capacity of water is 4.187 KJ per degree kelvin. So stored at 20c room temp, then to heat the water in the log to 100 is roughly 400kJ. To boil that water off so that it can go up the chimney, the latent heat of evaporation is 2270 kJ per kg. So that same Kg of water will take a further 2270Kj to boil. so roughly 6/7 of heat to boil and 1/7 of heat to get to 100c. Hope that helps.

 

ok so is this why, when I read about calorific value, some people quote "net calorific value", because in fact a lot of the intrinsic energy in firewood is wasted?

 

I have a lot more to learn about biomass!

[daltontrees]

ok so is this why, when I read about calorific value, some people quote "net calorific value", because in fact a lot of the intrinsic energy in firewood is wasted?

 

I have a lot more to learn about biomass!

 

I expect pellets have a very high surface area to volumen ration compared to logs, allowing rapid drying and possibly escape of moisture during burning before it has to be boiled off. All good for calorific value.

[difflock]

The moisture is driven off either before compressing or during the compression/extrusion of the pellets, not after.

Work on 4Kw per Kg for "dry" firewood, i.e. at 15 to 20%

[openspaceman]

ok so is this why, when I read about calorific value, some people quote "net calorific value", because in fact a lot of the intrinsic energy in firewood is wasted?

 

I have a lot more to learn about biomass!

 

Some losses are inevitable in burning any hydrocarbon or carbohydrate simply because a product of the combustion is steam, and as has been pointed out steam takes away a lot of the heat needed to change it's state from the liquid. So fuels containing hydrogen have two different heating values, the higher one is where all the heat in the steam is allowed for (HHV) the lower ones just accepts the losses in the steam cannot be recouped (LHV).

 

With natural gas condensing boilers the flue gas is cooled ( below 56C) by the heat exchanger to below the dew point of the water vapour and some of this heat is recovered from the flue gas into the heat exchanger, droplets of water are formed and run out the drain or are seen as mist. In general with wood this is not practical so the flue gases exit at over 100C. Burn wood that is wet and these losses are greater.

 

The sums are fairly elementary but they should track the values in the link to the tables that were posted. The concept is simple, the LHV is in the oven dry wood (ash is fairly inconsequential with wood but not grasses) and any moisture from the fuel must leave as steam at the flue exit temperature and thus subtracts 0.75kWh for every litre of steam produced. So if the LHV of as sample of dry wood is 5.17kWh but it is burned at 50% mc wwb then each kg burned will contain 0.5kg of dry wood with an LHV of 2.585kWh but the steam will carry away 0.5 times 0.75kWh of this leaving you with 2.21kWh of heat in the stove.

 

The difference between LHV and HHV is academic but as every kilo of dry wood burns to produce 0.55kg of steam the HHV will be 0.75 time 2.3 ( about 0.4kWh) higher than the LHV

[openspaceman]

The moisture is driven off either before compressing or during the compression/extrusion of the pellets, not after.

 

Quite

 

The very fact that pellets hold together is testament to their dryness, leave them in a damp environment and they turn back to sawdust.

 

The Sprout Matador ring dies had provision to add steam as a lubricant and prevented heat in the die, from friction, charring the pellets.

[luke_mclachlan]

Hi all. I just thought a little update would fit nicely, especially as you were all kind enough with your replies. I contacted a university and was forwarded to a doctor of engineering. Funnily enough the university has just started a commercial venture whereby they rent our their lab space for a (hopefully not too exorbitant) fee. I'm now waiting to hear about those fees. Should be interesting.

 

I have two very kind suppliers who have offered to give me feedback on a price model that I have developed and coded for the internet. I am looking for 2-3 more biomass suppliers, preferably UK based. Any takers by all means PM me. Unfortunately I'll be away for the next two weeks as from this evening, in case you do PM me and you don't receive a immediate reply.

 

Thanks all!