Fluidyne Gasification Archive

Condensate from Producer Gas Cooling

Condensate that forms in the piping and components of any raw biomass gasifier when the gas is cooled, contains considerable observable information. While in some cases the project may call for these condensates to be analysed in a laboratory, a "visual and sniff" test is sufficient as a screen test, to identify potential problems with the gas making process. Any expensive testing, including gas analysis, can be conducted after the gas making behaviour is optimized, allowing the gas cleaning/cooling components to perform their functions without contaminations from uncracked hydrocarbons.
 
For many, first time experiences related to the condensate from cooling of producer gas, take many by surprise in it's variation. In the following photos collected from numerous projects, I have attempted to explain some of the reasons for these variations. Because gasification covers a wide range of gas making principles, it is not possible to duplicate all the types of condensate likely to be encountered by researchers, but the basic interpretation remains much the same.
 
1. As a rule of thumb, any black floating tar with or without water, tells you the gas is not suitable for reliable operation of engines.
 
2. Strong smelling condensate usually has rainbows on it's surface, and most likely show a yellow stain on the filter paper test. You can expect light pyrolysis oils, to condense out in the engine manifold under the throttle butterfly.
 
3. Hot gas filtration, strips out both soot and ash if used before the cooling/condensing phase, resulting in a condensate around pH 7 or slightly less. It may contain sub-micron carbon blacks, but these should drop out of suspension leaving a clear aqueous condensate, if free from tar or pyrolysis oils.
 
4. If purple condensate forms, you will be seeing dissolved ash. It is a sign that a lot of the oxidation zone reactions are more combustion than oxidized gas making.
 
5.  Green condensate has a less than 6 pH making it acid, which will leach out copper from any components that are in contact with the condensing phase. It may originate from pump housings, bushes, or copper type heat exchangers. You can expect short life of these assemblies.
 
6. When soap like bubbles form on top of the condensate while draining, the colour can be clear or black, due to the higher alkalinity of the sample over pH 7. It should be noted that high alkalinity condensate has a high surface tension, and will refuse to drain out of some configurations of packed bed filters, blocking gas flows.
 
7. Disposal of condensate should always be done in a responsible way, rather than tipping it out if less than obviously clean. Some gasifiers do make activated charcoal as a waste clean-out, and this may be used to make a simple filter for cleaning up the condensate.
 
8. Black condensate is often no more than soot in suspension, and if left over night, will settle to the bottom, leaving reasonably clear condensate. It is common to find in the simple systems fitted with condensers before the gas filtration.
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This information should only be used as guide to obtain a better understanding of condensate observation. It can become a valuable field tool for those more practically involved with evaluation of gasification systems, providing clear signs about the process that may be in contradiction to claimed operating specifications.
 
August, 2010
 

Many condensate samples drain out black, but overnight, solids in suspension settle to the bottom as shown in these two sample jars. The yellow tinge indicates the presence of light pyrolysis oils in the gas, but the ease by which solids are held in suspension, indicates that the pH will be over pH7.
This shows a condensate tank level indicator where the condensate has settled overnight.
Draining the clear condensate shown in the previous photo, creates soap like bubbles that hold together. This indicates a pH of over pH7, causing an increase of surface tension as the alkalinity increases. The darker colour is only a trick of reflecting light through the wet concrete.
This shows how clear the condensate can become when the solids are filtered out before the condensing phase. There is just a hint of yellow through the depth of the liquid, but care is required to ensure that the colour does not come from reflected surfaces.
This is the same sample taken inside on a lighter surface. It is as clean as you can get from a wood gasifier if the whole system is operating with stable parameters.
This condensate provided a very unusual sample, as the very black liquid had a highly reflective skin floating on the top. This was comprised of floating particulate that was not tar or pyrolysis oils. It was not practical to pay for it to be analysed, as it could be separated with a charcoal filter. It did however provide some interesting discussion as we did some basic testing.
After disturbing the surface, the skin separated into platelets exposing the liquid surface. It would not flow back over to it's original mirror surface.
We knew that the sample would be pH7 or less, but the large amount of carbon blacks being flushed out of the condenser being tested, provided proof of soot reversion caused by slower quenching of gas temperatures.
Here you can see the pH and conductivity test of this unusual condensate.
Shown before and after overnight settling, the yellow tinge tells us the condensate needs clean-up before disposal through a charcoal filter.
This simple filter is made from a length of 2" plastic pipe with a gauze mesh in the bottom, then filled with waste charcoal out of the gasifier. The activated charcoal will clean up the condensate to allow safe disposal of the aqueous liquid, and the char can be burnt when it dries out.
Stripped of suspended solids and trace pyrolysis oils, this filtered condensate was allowed to be tipped down the drain after meeting approval of local health inspector.
Most simple system gasifiers with condensers, then filters will have a condensate similar to this sample. Carbon blacks and soot in suspension, dissolved ash to give it a purple colour, and soap like bubbles caused by the alkalinity of around pH 8.2.
This handy gadget allows you to suck or blow gas through a filter paper to see if soot or pyrolysis oils are present in the gas as it happens. Made out of four squares 0.25" plastic, the outer two have threaded holes for  tube tails, and the inner two have a large hole to form a chamber on each side if the clamped filter paper. Glue them together as pairs to prevent leaks, and the filter paper can form it's own seal.
This is not the type of condensate you would like to see if you are operating engines. The uncracked pyrolysis gases have reformed into a sticky grease type floating deposit in the condensate. A sure sign that key parameters are needing adjustment.