Cloggie52

Hi all,

On Jim's  latest mail I saw the TU/e was one of the proud PP owners. As it is near where Iive  I like to get in contact. If you are on the forum, please let me know if you are interested to collaborate,

Hugo

Hi all,

In the following page: http://wiki.gekgasif... Energy Balance
is written:

Heat of vaporization of water
   100C liquid to 100C gas = 2260 kj/kg
(this is the killer.  note it takes more than 7x the energy to vaporize water at 100C than it does to heat the water from 25C to 100C)

One of the fundamentals of Thermodynamics is that the way you move between two states does not influence the outcome. The description of the process here is one at constant pressure (100 kPa). The end situation is superheated steam.at 1000C at 100kPa.


let's do the following thought experiment: Heat the water from 25C to 1000C, but keep the pressure high enough to supress evaporation. At 375 C, just above the critical point we release the pressure adiabatically and. we contue our heating process to 1000C. End situation the same, but no evaporation heat added. The total energy added is therefore the same in the 2 processes.

This makes that the evaporating the water at 100C (i.e. add a lot of heat at 100C) is a blessing in disguise. It enables the use of low value heat, rather than the use of heat at higher temepratures, which tend to be more "costly".

The reason I want to explore this route is to see if the literature is not exaggeratin gthe effect of water on the thermal efficiency.

using the data on the same page, one sees the following outcomes.
Energy in Wood 18MJ/kg
bone dry wood delivers in the gasifier: 15.5 MJ/kg
20% moisture (on dry wood): 14.6 MJ/kg
35% moisture : 13.9 MJ/kg
50% moisture:  13.2 MJ/kg
100% moisture : 11,9 MJ/kg ( as much water as wood in the process!!!).

As different types of wood vary between 19.2 and 18.4MJ/kg and even more for other biomass,the water content is from a thermal point of view not an overriding consideration.

It is an overriding consideration from a commercial point of view (you don't want to pay a wood price for water).

I am interested to know if there are other technical considerations to reduce the amount of water.

Hugo