[Gasification] "Continuous analysis of elemental emissions from abiofuel gasifier"

Sun Mar 25 14:01:54 CDT 2007

Tom,

This continued research and discussion is useful. 

In the 1970s we found that the ash fusion temperatures commonly reported
were useless for explaining what we experienced when burning straws and crop
residues in the burners and gasifiers that we were developing.  

We found that a substantial portion (20%) of the  elements responsible for
slagging and fouling - sodium, potassium, chlorine, sulfur - were lost
during ashing at 1000C. On heating potassium and chlorine will volatilize at
low temperatures and react even before combustion occurs (Dave Dayton,
Robert Evans et al, NREL, mass spec testing, 1990s). So ash fusion tests
based on using ash prepared at these temperatures were meaningless in
gasification or combustion.  There has been a recent re-examination of this
by Dr. Bryan Jenkin's group at the University of California at Davis, and
published in Energy and Fuels (American Chemical Society 2006?), which
confirms our earlier findings in more detail.

Use ASTM  D1102-84 (Reapproved 1990) - Standard Test Method for Ash in Wood.
It provides for ashing at 580 C to 600C. Loss of volatile alkali using this
method is less compared with the coal standard of 1000 C.  Most labs (SGS,
Hazen) use this wood standard for biomass. 

Whatever the mechanism 750-790 C is often a useful temperature range to
maintain when burning (or oxidizing in a gasifier) agricultural residues. It
coincides with several aspects of straw combustion. It is the typical
eutectic melting temperature for the proportion of silica to potassium often
found in straws. Above 800 C alkali salts (KCL, KOH) dominate. The salts
make sticky particles. Below 800 C alkali sulfates and carbonates dominate.
Obviously this temperature will not get you the high temperature CO2 you
need in a gasifier to reduce the char. But it can make a dirty gas that can
be combusted. The challenge with these fuels is to complete combustion of
the organic matter and cool and remove the alkali bearing inorganic ash
before it coats the tubes and creates stoppages due to plugging or
corrosion.

You can burn, or gasify, at lower temperatures and deal with tars, or burn
at higher temperatures and quench the salts to remove them. In a fluidized
bed gasifier or combustor you'll need lime or dolomite to prevent
agglomeration. In the 1970s the conventional wisdom for a fluidized bed was
that a 4:1 molar ratio of calcium to sulfur would take care of most
agglomeration issues. Commercial experience in the 1980s and 1990s showed
that high rates of bed purging were necessary to maintain a workable
balance.  Anders Nordin has done very interesting studies (published in
Energy& Fuels 2004-2005)  on the accumulation of alkali on bed media for
gasification and combustion.  

In our work (1990-1992) with ASME Research on large pilot vitrification of
ash from waste-to-energy plants we found that the heavy metals of concern
volatilized and exited the high temperature reactor (electric arc furnace)
with the off gas instead of being bound with the glass and slag as intended.
So the high temperature furnace fractionated or separated the metals. This
is consistent with our more recent commercial experience with plasma
gasification systems where the gases and fine particulates are quenched and
separated with a filter as part of the gas cleaning steps prior to use. 

The late Dick Bryers (Foster Wheeler) who was the "dean" of the ASME
Committee on Corrosion and Deposits  used to tell use that slagging and
fouling were not heating problems, but cooling problems. The temperatures
and conditions of gasification and combustion will fuse or volatilize the
troublesome elements. The problem is to figure out how to cool and remove
these elements from the process.   

Tom   Miles 

See "Alkali Deposits in Biomass Power Plants" (1995)
http://www.trmiles.com/alkali/alkali.htm and the hundreds of studies on
alkali in biomass and waste that have been published since then. 

From: Thomas Reed [mailto:tombreed at comcast.net] 
Sent: Sunday, March 25, 2007 7:06 AM
To: doug.williams
Cc: praufast at free.fr; Gasification at listserv.repp.org; Tom Miles; alia
Ghandour
Subject: Re: [Gasification] "Continuous analysis of elemental emissions from
abiofuel gasifier"

Dear Tom Miles and all:

We have become increasingly concerned that the ASTM tests for ash fusion are
particularly inappropriate for biomass gasification.  On your recent visit
you pointed our that sodium and potassium oxides were quite volatile, even
at 1000 C, and so might be vaporized during the ash fusion test.  

In my past life I grew crystals of a wide variety of materials at MIT and
wrote a book "The Free Energy of Formation of Binary Compounds".  I became
aware that many oxides do not vaporize congruently, and can exhibet a far
higher vapor pressure in a reducing atmosphere than in an oxidizing
atmosphere as in:

      METAL OXIDE + H2 ==> Metal suboxide with higher vapor pressure + H2O

I particularly know this was true for silica.  Quartz workers typically find
a ring of condensed SiO vapor around the places where they have melted
quartz in an oxy-hydrogen flame.  I suspect the same is true for Na and K
oxides.  We are having trouble with slagging of agricultural fuels at
temperatures below the reported ash fusion temperature.  

Comments?

Yours truly,

TOM REED            BEF

doug.williams wrote: 

Hi Philippe and Colleagues,

Thank you for drawing attention to this project.

 http://biopact.com/2007/03/scientists-develop-analytical-system-to.html

However, the behaviour of trace elements during gasification can be 
problematic, with environmental concerns over toxic >components, and 
process problems caused by alkali metal corrosion and fouling.

While it is appreciated that emissions need to be measured, combustion has a

history of being the worst option for waste containing contamination. 
Because these compounds were found associated with gasification, it suggests

that gasification is in itself responsible for the emission readings.

David Poole and colleagues at the University of Sheffield and SPECTRO 
Analytical Instruments, Germany have been >conducting experiments to 
continuously monitor the concentration of various trace elements in the 
raw gasification gas >from an experimental reactor (click to enlarge), in 
an effort to determine which elements are volatilised. They published 
 >their results in the Journal of Analytical Atomic Spectrometry [open 
access article].

To embark on establishing a measuring standard for gasified emissions, the 
fact that a crude updraft gasifier (experimental) is even considered 
appropriate suggests a big gap in the credibility and value of such 
projects.

 > Results of initial tests indicate that the concentration of some elements

in the gas phase are extremely high, far higher than >in combustion 
processes, and therefore are of significant concern.

The updraft gasifier drives all the very unstable distillation gases into 
the output gas, and provide a carrier for the toxic material. Combustion at 
least burns this gas close to the bed surface, and would fix a certain 
amount in the ash., so less contamination in the emission.

Owing to problems with tar formation in the gasification process, the 
analysis proved extremely challenging, and >further development of the 
sampling and pre-treatment procedure would be required to obtain more 
accurate, reliable, >and long-term continuous monitoring results.

What we see here is that emphasis is on trying to clean the tar out of the 
gas, not gasify the fuel in a way that does not make tar. If they used a 
downdraft, high temperature gasifier, that cracked the distillation gases, I

doubt if they would find enough toxic emission to measure, as tar free 
producer gas can be extremely clean. Having said that, it does depend on 
what was being gasified, but in this case, the emission problem is created 
by the crude gasification process.

In New Zealand, we had expert advise back in the 1970's about contaminated 
wood, and our advise was that in the high temperature, oxygen deficient 
atmosphere of the reduction zone, the toxic molecules detach from their 
carrier molecules, and become fixed in the waste activated charcoal of the 
reduction zone. In more recent times, emission tests of contaminated waste, 
have shown that gasification is a reliable method of disposal, so long as 
the gasifier can handle the fuel.

These tests will be expensive, and could very well be insisted upon by 
authorities who do not understand the differing types of gasification. There

is a need to keep a close eye on these types of academic studies, as it can 
be another created issue that if not challenged, be used to hold back 
implementation of start-up projects.

'Gasification is of growing interest as this can increase the energy 
efficiency of biofuel use,' said Poole, 'but the >behaviour of trace 
elements during gasification can be problematic, with environmental 
concerns over toxic constituents >such as lead, cadmium and arsenic, and 
process problems caused by others such as potassium and sodium.'

As they state biofuel specifically, and not MSW, I can only conclude that 
lead, cadmium, and arsenic, comes from demolition wood, but potassium and 
sodium is usually associated with young fresh wood like coppice willow, and 
poplar. In both cases, the correct gasification process can remove the 
problem.

High concentrations of potassium and sodium were detected, which, 
according to Poole, could result in fouling and >corrosion of gasification 
plants.

This is the same for any combustion process that burns contaminated 
distillation gases, but is a non event if the gas making is correctly 
achieved, and the moisture stripped out before the gas is combusted.

The heavy metals measured were not detected at significantly high 
concentrations. The researchers will continue to >optimise and improve 
their analytical system, and plan to develop more automated and robust 
systems.

Heavy metals are not normally found in biomass, although they can be taken 
up in certain situations. Again high temperature tar cracking gasification 
fixes heavy metals in the waste activated carbon, and do not entrain with 
the gas.

I would appreciate comment or additional information on this subject, as 
there is a need to clarify emission issues as they appear.

Doug Williams,
Fluidyne Gasification.

"

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