[Gasification] Candle filter experiences

Doug doug.williams.nz at gmail.com
Tue Oct 31 13:22:36 CDT 2017

Hi James and Bjorn,

Bjorn's comments to you reminded me of more issues that we experienced 
with candles, which in hind sight looked simple to resolve at the time 
(just throw in a few more $$$ at the black hole). Pre-coating involved 
obtaining talcum dust, and using that to reduce the pore size of the 
ceramic pore structure. We also had a preference for removing all larger 
dust particles using cyclones, so only the finest dust reached the 
candle surfaces. This in turn created very rapid pressure drop across 
the ceramic wall, requiring more frequent pulsing. In the end, we let 
all the solids in gas suspension go to the candles so that a more mixed 
filter cake formed on the surface. Yes I said filter cake, because the 
candle surface only provides a platform for the impacting mixed 
particles to collect, because if they enter the ceramic pores, then they 
will block rapidly. From memory, we worked with our flow rate at 7-9" 
inches W.G. with the pulse initiating at around 14" W.G.  If you are 
tempted to work at hight pressure differentials, the ceramic wall can 
collapse, because the gas temperatures through the ceramic pores in 
localized spots is squeezed releasing more heat, weakening the ceramic 
fibres which collapse.. If you plan to operate at higher temperatures to 
start with, then watch the rates of pulsing to keep the pressure drop low.

By accident during maintenance, grinder sparks ignited the soot cake on 
the candles and residual dust in the exit soot auger. Rather than try to 
put it out because it was a very low level smolder, we closed it up and 
let it burn out overnight. It left only white ash, but it turned out 
positive as a way to clean the candles after a bad run, but care must be 
taken not to allow moving air entry during this smoldering combustion.

We tried high temperature filter bags and quickly found they block in a 
blink, and are destroyed if a spark gets in. There is also an issue of 
the back pulse not working with the system supplied that worked  
perfectly for dry dust. The size of the filter boxes also required them 
to be insulated, but condensation issues could not be over come 
compounding their operational  and maintenance problems.

One last comment to remember, is that carbon dust caking in elevated 
gasification temperatures, will bond together, making it very different 
from those free of carbon free dust. This of course brings conflicting 
views from those presented by the candle manufactures. We found that 
suggestions from the manufacturer (a really dedicated team) related to 
just the filtration principles on which the technology was founded, 
having no personal expertise in the black arts of gasification.

Metal screen bio-oil filtration works at a cost, but you then have to 
dispose of the toxic waste oil. It was promoted as a fuel for combustion 
in boilers etc, but this I think was as attempt to clean up the 
gasification image to attract funding. I doubt if appropriate studies 
were made on the resulting combustion emissions(:-)

So much enthusiasm and still we struggle with filtration issues.

Must finish to start my day!

Doug Williams,


On 01/11/17 00:54, Björn Kuntze wrote:
> Dear James,
> I can´t say anything to the flue gas fitering system, but I may be able to
> contribute to the filtering of product gas from gasification processes:
> In Germany nowadays most projects use bag filters with working temperatures
> of 120 - 160°C. Others use hot gas filtration at around 400°C using
> stainless steel mesh filter materials. Wet gas cleaning with oil scrubbers
> or electrostatic filters is used in older projects but not common at all for
> new projects.
> For fifteen years there were (and there still are) people advertising their
> high temperature filter systems (candles like Herding, or BWF and metal
> mesh). They claim that these filters can handle tar load. But, according to
> my experience, with tar in the gas you will never get happy at all. It is
> not even an issue of the filter concept. Due to harmful substances in the
> tar you cannot even organize maintenance of such system when following
> health and safety regulations. So such filter won´t help you either.
> Furthermore, as already stated, once condensing conditions appear, the whole
> thing is gone.
> The candles (no matter who is the manufacturer) typically do have a problem
> when the dust in the gas is very fine (soot...). In the past the
> "overdesigned" filters typically turned out to be "underdesigned" in the
> real project with the filter manufacturer saying: "we did not expect that
> there is so much fine dust...". So they start with precoating. At the end
> some projects had to order a second filter of same size to run in parallel.
> I know a few projects where this was the recommended solution.
> So, if you go for that, increase the filter surface right from the beginning
> no matter what the manufacturer tells you.
> You might also consider the German company Calida Cleantech GmbH
> (http://www.calida-cleantech.de/en/) (formerly LignoGen GmbH). They also
> provide hot gas candle filter systems. I know one of their projects in
> Germany where they filter product gas quite well. I know that they are
> interested in projects filtering pyrolysis gas. You might contact them.
> Mit freundlichen Grüßen / with best regards
> Björn Kuntze
> MasterGas UG (haftungsbeschränkt)
> Bloherfelder Anger 7
> 26129 Oldenburg
> Germany
> Geschäftsführer: Björn Kuntze
> Mobil:  +49 (0) 177 345 1557
> Email:   kuntze at mastergas.de
> Web:    www.mastergas.de
> Handelsregister:  HRB 211272 – Amtsgericht Oldenburg
> USt-ID-Nr:  DE307863882
> St.-Nr:         64/213/02649
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> -----Ursprüngliche Nachricht-----
> Von: Gasification [mailto:gasification-bounces at lists.bioenergylists.org] Im
> Auftrag von James Joyce
> Gesendet: Dienstag, 31. Oktober 2017 00:48
> An: gasification at lists.bioenergylists.org
> Betreff: [Gasification] Candle filter experiences
> We are just about to order candles for a flue gas filtration application at
> 350 deg C. We have also been asked to design a 400 deg C candle filter
> system for pyrolysis gas, for installation ahead of a condenser.
> This is one example (not necessarily the supplier we will use)
> http://www.herding.de/files/hGlobal/Downloads/Technical_product_descriptions
> /ALPHA/UK/Herding_ALPHA_Biomass_Filterunit_uk.pdf
> I have read some of the case studies for larger gasifiers. I am interested
> to know what experiences, if any, have been had with candle filters at the
> smaller scale (say 2-20 tonnes per day of biomass gasified). Reliability is
> my main interest, number of hours between replacement ... and performance as
> far as downstream equipment such as condensers and engines are concerned.
> Regards,
> James
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