[Gasification] RPS experience with linear hearth

doug.williams Doug.Williams at orcon.net.nz
Fri Feb 3 04:30:57 CST 2012


Hi Peter and Colleagues,

Appreciate this level of information, and know the work it takes to record all these details:
 
> In this message I will only focus on the gas out of the RPS linear 
> hearth, it is already long winded but am attempting to provide as much 
> detail as possible without compromising proprietary bits.

For the reason of retaining your propriety bits, my comments have to be restricted, so that what you have and how you think it works, is retained. 
 
> Doug has mentioned our gas analysis as being "very unusual" for an air 
> drawn system so for the benefit of the list I provide it here so people 
> can see for themselves:
> 
> Major Gasses,
> Hydrogen: approx  36.00%
> Carbon Monoxide : approx  28.00%
> Nitrogen:    29.00%
> Carbon Dioxide:      6.81%
> Low level Gasses,
> Oxygen:         Less than 0.02%
> Methane:     0.96%
> Argon:     0.37%
> There were also a range of minor gases in the less than 50ppm range.

The high H2 is unusual in a downdraft gasifier, but it's creation results from how the char bed behaves in "certain" conditions of design. This design also sees the CO and other combustible gases, form in a way that while "different", can be explained, and is confirmed by the waste char, and flare colours.
 
> The flow rate measured at the time of the test  was 130m3/hr, though 
> this was not recorded on the lab certificate.

We learn as we go, but testing times should begin after the system has reached it's heat operating soaked temperature, and then measured over a range of flows. Depending on the intention of the tests, not just gas analysis, the test time should be extensive enough to show that the gas making is sustainable with the selected fuel.
 
> This is not full flow and the same system has been measured up to 
> 400m3/hr without apparent over aspiration for a 20 minute run before 
> overheating of the fan motor caused it to trip.  The sustained upper 
> limit has not been determined and may well be lower or higher (it has 
> taken a while to get a suitable high temperature fan of adequate 
> capacity, but will have one within the next few weeks, like most 
> components we have ended up building this ourselves), but the system is 
> quite comfortable at 200m3/hr with similar gas quality observed in the 
> flare and can be turned back to 40m3/hr without losing this

Hot gas suction fans can be a high maintenance component. The gas outputs can be estimated given sufficient information, but, in this case, you have proprietary knowledge protection and I will not breach that in order to explain for the benefit of others.
 
> snip<
 
> We have made numerous attempts to engage university researchers to 
> formally measure system performance without any real success.

Not easy to do any where, unless you find one set up to do gas analysis, then make a department donation for a test to be done. Better know as bribe  with cash.

>snip<
 
> Condensate test results RPS first development unit
> 
> Percentage    Compound
> 40.625    Pyridine    C5H5N
> 2.298    Column Bleed
> 7.509    Phenol    C6H6O1
> 0.750    Methyl Phenol    C7H5O1
> 1.305    Methyl Phenol    C7H5O1
> 
> 3.340    Naphthalene    C10H3
> 0.529    Dodlecene    C12H24
> 1.163    1 Methyl Naphthalene    C11H10
> 1.097    2 Methyl Naphthelene    C11H18
> 0.566    Tetradecine    C14H28
> 5.092    Biphenylene    C12H8
> 0.562    2, 3 Dimethyl 1 Naphthelene    C12H12
> 0.869    Dibenzofuran    C12H8O1
> 0.674    ?
> 1.125    Fluorene    C13H10
> 0.818    ?
> 0.552    ?
> 9.291    Anthracene    C14H10
> 2.077    Anthracene    C14H10
> 0.777    Anthracene    C14H10
> 1.626    4HCyclopentaphenathracene    C15H10
> 0.806    1methylAnthracene    C15H12
> 0.951    2Pheny1Naphthalene    C16H12
> 6.725    Fluroanthene    C16H10
> 1.762    Pyrene    C16H10
> 7.112    Fluroanthene    C16H10
> 100.001

This incorrectly done condensate analysis taken from the gas bag walls, are a few of over 200 chemical compounds found in the unstable chemistry of pyrolysis gas in the fuel hopper. When found in the final gas supply after the gas cleaning,etc, indicate that the tar cracking is less than opimised for either the output, or the type of fuel being tested.
 
>  Doug also indicated that if we are only getting low overall condensate 
> levels then the water is probably going out with the gas as steam, and 
> ordinarily I would agree, except none of our observations of our linear 
> system whilst operating on optimal fuels support this.

The key word here is optimal fuels which create the right gas making for all gasifiers. It is however agreed already, that more moisture can be turned to H2 "if" in beds are performing a certain way.
 
> The condensate analysis above does not show any free water at all (we 
> did query this at the time and asked whether this result was after water 
> had been excluded, ie reporting only the percentages of the non water 
> component, but were told no, if water had been present it would have 
> been reported).

The material collected from the bag wall would have the appearance of light oil, or thin grease. They float on water, but do not take it up, and if left out in the open, they literally evaporate.

> Before I go on I would add that yes we have seen wet gas out of the 
> system, but only when running truly excessively high mc feed stocks in 
> the range of 30-50%. At 40% mc H2 drops to as little as 5% and CO to 11% 
> with a corresponding increase in CO2 & N2 (lab analysis result during 
> testing of mixed wood chip/ sewerage sludge blends). Stretching a length 
> of paper towel over the (un-ignited!) gas stream under these conditions 
> results in it getting rather damp quite quickly, and a brown condensate 
> dripping off the outer rim of the flare head can be observed (no funny 
> comments about the possible relationship to sewerage sludge please...). 
> Under other much less extreme gasification conditions though no moisture 
> collecting in the paper or free liquids on nearby metal surfaces are 
> readily apparent.

Environmental conditions are also a factor affecting condensate formation, as is altitude. The dew points of producer gas drop accordingly with dryer fuel. The fact that sewage sludge is a hazardous waste for biological reasons, would seem to be ignoring the problems of heavy metal emissions, specifically mercury. Testing with an incorrectly operating  gasifier, is no test at all, unless it is operating in a very thorough tar cracking mode. This first ensures that the pyrolysis gases are fully disassociated, and then the mercury is taken up by the activated carbon that fills the reduction zone. If you do this again, then ensure you are doing emission testing as well as the gas analysis.
 
> The following additional observations are for "chunky" wood fuels below 
> 25% mc (the fuel spec at the time of the formal gas analysis, piece 
> sizes ranging from 25mm to 50mm on a side).

Width and thickness also affect char formation behaviour.

> * Yes we do have gas cooling (of our own design like the rest of it), 
> and gas exit temperature immediately prior to the flare head are between 
> 40oC and 70oC, depending on flow rate, and the current system also 
> includes mesh mist filters on the exit from the coolers, which we 
> thought might also be an efficient way of trapping the sub 10 micron 
> particulates, assuming that these would be wetted by condensate. 

This is probably why you do not see much condensate. I have seen dew points of producer gas go down to around 30C, but not made any real study of all the permutations possible. One thing for sure though, is that if you have submicron carbon blacks in your output gas, then they are there because of moisture still in the gas. Dry gas cannot carry particulates in suspension.
>snip

 > * The system "chuffs" when the mc is below 25%, a resonance coming from 
> the intakes sounding a little like a fast revving steam engine and the 
> upper hopper vibrating like a long, low drum roll.... Hand held digital 
> anemometer also records this as a regular, fast pulsing of the air 
> intake flows.

You are seeing a phenomena  created in the oxidation char, where the interstitial space opens up channels or caverns usually beginning in front of the air inlets.The normal process of oxygen connecting with carbon in a continuous ribbon to where the gas exits the char is disrupted. On entering the hole in the bed, the CO on the surface of the char flashed creating a mini explosive pulse. This combustion flash results in CO2, and that quenches the flare, but makes more CO ready for the next air flow, by reduction, reducing the temperature 

 > This seems consistent with a rapid cycling of water cracking and the 
> free oxygen made available displacing that from the incoming air.  This 
> cracking uses up thermal energy which then drops below the threshold 
> required to support this water shift reaction, and the process pauses, 
> reverting to pulling in outside air to satisfy the oxygen demand, 

I cannot separate this possibility from my previous comment, but the bed channelling and holes in the bed do provide more dwell time for the formation of H2, because the gas velocity is lower than in a packed carbon bed.
>snip<


> *Charcoal from the ash bin has a very high fixed carbon in the 85-93% 
> range (reported by a NATA certified lab) consistent with high 
> temperature. 

High fixed carbon char is great for burning and carbon sinks, and it will have other uses as well. It is not however activated carbon, which is light and fluffy, and is the result of the gas being made by reduction, an important requirement if you are going to play with contaminated fuels. The char you show and have tested, may be made by high temperature, but when found in a gasification system, originates in the slow moving char that has no gas flows through it. (no further comment)


Soot taken from the particulate collection system below the 
> cyclones has been examined with a microscope and we are told it had a 
> crystalline structure normally also only found when forming under high 
> (>1000oC) temperatures.

Crystalline soot's are reformed from the pyrolysis gases, not the char.( I don't wish to comment further on this, as it affects what Peter might call proprietary knowledge.)
 
> My wife Kerry, equal co developer, has asked that I also point out when 
> we designed the original system we made allowance for running it in 
> either downdraft, or updraft, mode and have used this facility to break 
> up bridging on occasion when working with difficult fuels (though on the 
> early units this does interrupt the gas flow to the flare). The current 
> (Mark 3.2?) under construction allows for this without stalling the 
> system. This was done as part of improving the overall material handling 
> side on the path to a more easily automated commercial model.

Whenever you see bridging of the fuel, you can create bed conditions and gas analysis similar to the RPS gasifier, which Peter has honed to his liking. Round gasifiers behave exactly the same where they have deep beds, and how they work will depend on how good your suction fan performs. I assume that the current system Peter, has no fuel feeding, or waste clean out, and is all manual?  If so, then automation may very well completely change the way the it all behaves.

In closing, it is necessary to appreciate that gas can be made from any combustible materials, and multi testing of fuels has been done many times over. It cannot prove that a gasifier is different from others, just by making gas, that's the easy part. Remember that if the gas making is not working correctly, then you have to start adding components to make it do so. It's a good reality check to review how much effort it takes to keep the gas flowing. 

My spare time is up folks, and I have to drop out of sight again, but will watch ongoing discussion.

Doug Williams,
Fluidyne Gasification.











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