[Gasification] Heat Exchangers. (Was no Subject)

[doug.williams]

Hi Doug C, and Colleagues,

You clarified for me:

> The counter flow heat exchangers I am familiar with move hot and cold 
> fluids
> in opposite directions threw the exchanger to make the most of the
> temperature differential between the fluids. Before I go and gum up one on
> these with tar does is there any advantage.?

The key thing about heat exchangers and producer gas:

1.  Where they are situated in the system, before or after soot removal.

2, At what temperature the gas enters the assembly, because rapid soot 
flocculation occurs if gas is over 500C, reverting to CO2 and soot. Plus to 
remember that even if you remove the soot, and cool the gas, it will still 
make soot if it is heated again to over 500C. (Not that you would want to) 
(:-)

3. Shape of exchanger tubing, because round tubes of do not cool the water 
vapour core of the flowing gas.

4. Size of the tubing, because small bores will block quickly unless soot is 
removed first,  and any soot coating reduces the cooling efficiency.

5. Cooling flows should be turbulent on tube surfaces, because steady state 
flow will not scrub the hot boundary layer off the tube surface.

6. Vertical tube alignment, to prevent  solids settling in the tubes, and 
drain condensate.

> I've read some conflicting information about air preheating and how much 
> gas
> should be cooled before, and after filtering.

Researchers arrive at differing conclusions from their experience. IF the 
gas is destined for an engine, common sense should tell you that it affects 
the volumetric efficiency if the gas is hot. Heating input air to the 
gasifier, is easy to experiment with in practice, and is best done with the 
gasifier that you build, in the place where it is installed.

 > There seems to be many wet and dry filtering arguments and methods. Most 
of
> this is confusing so what is the most effective way.

Unfortunately it is not a question of the most effective way any more,  but 
how you dispose of the black liquors from a wet system, or soot from a dry 
system. Both are considered hazardous wastes in most places. My choice is 
dry, mainly because we do not have to remove tar, and because any water 
systems need another cooling system to get rid of the heat. Black liqueur is 
unbelievably dirty, and can foam in certain conditions of gasifier 
application. In a transport mode, and from experience, cooling the water is 
a major problem, unless in a colder climate. Then from experience again, 
freezing of the cleaning water creates another set of things to resolve, 
"usually by adding more stuff " to the system.

While it is easy to just say nothing and tip stuff down drains, or in the 
ditch, we do have a responsibility to the environment. In this respect, I 
appreciate the unseen effort and thought going into producer gas 
conditioning, and look forward to the day when a $10 gizmo is available to 
solve a very complex problem.

Doug Williams,
Fluidyne.