From the time we became involved in gasification back in the 70s, down here in New Zealand, our isolation from the centres responsible for evolving tar measurement and the lack of measuring equipment required us to adopt a practical approach to this issue.

As the only reason to measure tar levels is to determine its acceptability for use in engines, turbines, compressors, etc. the first question to ask is "can this gas do the job", not "how much and what sort of tar is in the gas". Even the smallest amounts of condensible hydro carbons cause problems in a commercial application, and I haven't found an answer to my question of how much measurable hydrocarbons exist when they no longer condense in the high vacuum of the engine manifolds.

Rather than waste money measuring a gas too dirty in the first place, a simple screen test can be applied, that will justify any further tests.

There are several options actually depending on your situation.

A: Gas analysis to determine CH4; a distillation gas which almost disappears if thermal cracking is efficient. Over 3% and light pyrolisis oils and heavier hydrocarbons begin to condense in various places.

B: Using a preferably cold gas stream, (-65 degrees C) pass it through a Watman 5 filter paper. There should be condensate present that wets the paper. If just clear when dry O.K., but if brown stain appears, condensable hydrocarbon present. Reject!

C: The filter paper is also useful to collect particle samples that can be classified if a high resolution or even better an electron microscope if available. Engine quality gas has carbon black present, and although we haven't had need to explore the possibility, believe them to include C60 and C70 carbons. Their size will take them through any collector if moisture is present in the gas.

D: A test flame of engine quality gas has little radiant heat although there is a small amount from the carbon blacks. Flame temperatures are usually around 1,050 0C with 1,100 OC the absolute maximum temperature.

E: Failing all else, use a small single cylinder engine and see if tar forms in its manifold.

While this whole issue of tar measurement tries to establish what is acceptable, the onus is on gasifier manufacturers to deliver appropriate equipment and prove what is possible.

It is possible to make producer gas without condensable hydrocarbons, and the term low tar gas cannot be applied to these systems if you have no condensate to measure. Having said this, I should mention water. In the absence of hydrocarbons it will be slightly mauve or clear, contain particles that settle out, and be about pH 8.2.

So to sum this all up, nothing should condense out of the gas other than water and carbon particles in a vacuum situation similar to an engine manifold. This should be the type of test to develop with zero acceptance in the presence of hydrocarbon condensate. If we don't allow gas with these distillations into an engine or turbine, we don't have to figure out which or what is causing the problems. We must aim for a zero standard as I am sure that our technology will not reach the degree of reliability demanded of commercial technologies without this incentive in place.

The screen test method was evolved with the assistance of Dr Jim Cousins, New Zealand's top scientist in gasification and biomass, who worked in our old DSIR laboratory in Wellington. It was the only method available and we resolved our design problems of condensable hydrocarbons.

I did see a mention of Benzine in the gas and yes this is another indication that the gasifier is not cracking the distillation gases. You should only have CO and H2 with traces (maybe) of CH4 with the rest being inert.

Our newest project in Canada has just taken delivery of the gas and emission equipment for the EPA testing programme. It is all state of the art stuff and we will settle once and for all whether down draught gasifiers can be no tar at the same time!

Graeme has put up a carbon black photo on the Fluidyne Archive and the simple holder for the filter paper. I found some of the original test filters which came out of my International van during the time of developing granular bed filters. You can see the colour of the paper.

Front view	Back view
Click image above for a full size version [1692 x 1188]	Click image above for a full size version [1005 x 664]