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

[Thomas Reed]

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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>>     
>
>
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