Biomass gasification. How does it really work?

[()blivion]

Hello all.

I started this thread because me and lurkepus have a fundamental disagreement on how the typical biomass gasification system works. The original debate can be found in this thread. https://www.electro-tech-online.com/threads/pic-mcu-ice-speed-governor.132904/ I brought the debate here because it is highly off topic for that thread.



A condensed summery of our positions on the matter.

I am fairly sure, in accordance with the Wiki article on biomass gasification, that the majority of the combustible gas's produced by the process are carbon monoxide, and hydrogen. And that this would indicate that one wants to use just the right amount of oxygen, as carbon monoxide can not be made without oxygen.

Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide. This is achieved by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen and/or steam.

lurkepus, on the other hand seems to be convinced that he can make more/better gas, by just heating the biomass in a totally oxygen starved environment. He fears adding oxygen will consume more fuel, rather than produce it. And that excess oxygen could cause an explosion. He believes the main process works instead through the science of pyrolysis.

I don't believe he is entirely wrong, excess oxygen will cause more fuel consumption, and could lead to an explosion. And the article for biomass gasification does mention pyrolysis as a contributing factor to the whole process.

2. The pyrolysis (or devolatilization) process occurs at around 200-300°C. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.

However, I am convinced that the majority of the reactions that produce combustible gas DO NOT come from pyrolysis. Rather they come from light oxygenation of the carbon content of the biomass. As such, I believe that his plan for an oxygen void pyrolysis chamber will ultimately fail because he will not be able to convert the solid carbon into carbon monoxide or methane gas.

I will of course allow lurkepus to speak for himself on the matter, before I go into detail. But, I believe the heart of this debate is caused by the little known fact that carbon monoxide is, in fact, a combustible gas. If anyone would like to chime in on our positions, please feel free.

 

[lurkepus]

This is one of many witch explain a few types of units .. I have been reading alot about the subject but ok i might have mixed up a few thing anyway here is the video ..

https://www.youtube.com/watch?v=9ftJyphJnYk

 

[()blivion]

Yup, notice at 1:30 into that video, all the compounds on the left hand side are fairly complex hydrocarbons? They all are C[SUB]6[/SUB]H[SUB]10[/SUB]0[SUB]5[/SUB] (Cellulose). All bio matter is going to have some complex hydrocarbons, so pyrolysis will always make some usable fuel gas. But pyrolysis alone will not turn all of the solid mass into fuel gas. Edit: "Carbohydrates", not "hydrocarbons". (the mistake was one of words, not science)

Pyrolysis reactions will "denature" the most complex chemicals down to methane and hydrogen, and a few other compounds. But in the end, a good portion of the carbon is NOT going to be gasified this way as methane. This left over carbon ends up as char after Pyrolysis.

The only way to make carbon a combustible gas, (Edit: in what is commonly called a biomass gasifier) is to either mix it with one oxygen atom, as in carbon monoxide. Or to mix it with four hydrogen atoms, making methane gas.

The methane reaction is more up hill (endothermic), and requires "more parts" to make.

The carbon monoxide reaction is downhill (exothermic), and only requires a few parts.

 

[tracidfish]

Well all I can add is in the many bio digetsers that I have worked on we do not introduce oxygen. That sai we take no steps to remove it. Just heat it, stir it and collect the gas (a simplified version).

 

[()blivion]

Yes, it *IS* very important not to add TOO MUCH oxygen. If you do, you will cause excess combustion and carbon DIOXIDE production, which obviously isn't good.

Most gasifiers make the choice between using air, or pure oxygen, based on performance verses investment. They do this because with just air, all the extra gases dilute your fuel, and your end system loses power. With pure oxygen, you get more fuel per volume of producer gas, or better fuel density. That is, air contains by percentage 20/70/9/1 (oxygen/nitrogen/argon/trace) gasses, but only oxygen plays a particular important role in the process. The rest of the gasses just take up space and screws with your engines effective displacement.

 

[lurkepus]

As they said in the video ther will always be a little percentage og air/oxygen either to leaky seals and air in the feedstock .. ther will also be some water witch wil be made into hydrogen and OXYGEN in the prosess ..

Another thing is that most of the energy will be left behind in the filter system of an gassifier .. if you collect and destill that tar you wil get more energy out of the gassification ..

 

[()blivion]

As they said in the video ther will always be a little percentage og air/oxygen either to leaky seals and air in the feedstock .. ther will also be some water witch wil be made into hydrogen and OXYGEN in the prosess ..

The oxygen that comes from water generally only becomes useful in a reaction with already existing CO, and it gets used in the reaction, not made. So you have to have oxygen present from other sources to make CO first, then use the water for steam reforming.

Honestly, this is really only a way to deal with the water in your fuel in a cleaver and useful way. Ideally you would just have absolutely dry fuel, but that is impractical for realistic syngas production.

The reaction with water, requires CO, and the oxygen being made gets used up...
CO + H2O → CO2 + H2

You can also use water in a reaction with methane. But you have to be making a massive amount of methane, which would be just as burnable by it's self and would require a catalyst and much larger temperatures to even get the reaction to take place anyway.

This reaction requires methane...
CH4 + H2O ⇌ CO + 3 H2

Finally, if there is enough oxygen leaking into your system to be of use, then by definition it is not an "oxygen void" system, like what you are wanting to make. It either has enough oxygen to be useful as a reactant, or it does not. You only get to pick one, you can't have it both ways.

EDIT: Both the RED fuels above would produce more energy if the water was replaced with oxygen, and both of them are gases, so there is no desire to make these reactions take place OTHER THAN to deal with water.

Another thing is that most of the energy will be left behind in the filter system of an gassifier .. if you collect and destill that tar you wil get more energy out of the gassification ..

Most gasifiers use a combustible filter at some point, like straw or sawdust or some other cheap natural fiber like burlap/hemp. This collects the tars and debris, then you can later use that as more feed stock if you want. Though... the whole point of a gasifier is that you already have a plentiful cheap fuel to feed them with. So having some fuel loss is not very detrimental to the overall system anyway.

It's not gold we are talking about, it's wood, it LITERALLY "grows on trees". If you have a problem getting primary fuel, you probably shouldn't be making a gasifier in the first place.

 

[lurkepus]

I am fairly sure, in accordance with the Wiki article on biomass gasification, that the majority of the combustible gas's produced by the process are carbon monoxide, and hydrogen

Witch means that you really don't know so besides the wiki page ,what are you experience with gasification and the various types of gasifiers ?
I have spent 2 years reading and looking at various types of units . I'm currently working on a design based on the knowledge i have obtained on the net and by watching videos of actually working gasifiers .

 

[()blivion]

Witch means that you really don't know so besides the wiki page.

Not really. It means I was trying to be modest so as not to seem like an arrogant know it all jackass passing his knowledge off as fact to a community of very intelligent people.

Truth is, I do actually know for sure that carbon monoxide is a combustible gas and that it is almost certainly (85-90% likely) a major fuel produced by a gasifier. It's pretty straightforward, one carbon monoxide molecule combines with one oxygen atom to become one carbon dioxide molecule. Carbon monoxide is a flammable gaseous form of carbon used in gasifiers because solid carbon is not a gas. The wiki article, and many other resources, just confirm my understanding of the underlying process. You are of course not obligated to believe me though.

Also, if you will notice, the title of this thread is ended with a question mark. That is because it was a question to hopefully pull in people that do know the answer with 100% certainty, or maybe someone with a working gasifier that can do testing. I don't actually know the answer to every question every time, so having community input could have been very useful.

But this space is probably not the best place for looking for such people. A chemistry or alternative energy forum would probably be better. But I live in ETO, not in one of those other places. And here we do have a sub forum for such things. So that's where we are.

what are you experience with gasification and the various types of gasifiers ?

The answer your fishing for is "I have never built one myself". And I don't need to. The problem is fundamental, not practical. I don't need a device or test to tell me the the straightforward parts I already know. It's science.

Much like the telescope has little or nothing to do with astronomical debates, and the computer has little or nothing to do with computer science debates, the gasifier it's self is has little or nothing to do with this debate. It is the science that drives their functionality that is under question, and I have been studying that science for a very long time.

To put the case of apparatus vs science directly into point with an example, Einstein never built an atom bomb himself, but you can be sure he understood more about the core science than the people that actually did build them.

Anyone can follow online plans and make something that works, but only after the engineers have came up with and refined the details. One generally need to understand what is really happening when making new theory's and coming up with design improvements. Knowing nothing is just guessing. Trial and error may work to some degree, but that usually only leads to something that functions, and the builder not necessarily knowing why. And possibly coming to the wrong conclusions because of it.

I have spent 2 years reading and looking at various types of units

Cool, I have spent most of my life researching science, engineering, and technology, in a part of the world that excels at just those things. I could also wallpaper my house with high level degrees if I wanted too. But who's experience and intelligence is larger is not up for debate here. it's the science that is.

Hearsay and claims of experience are no substitute for provable science and mathematics. If you have valid scientific points to back up your research, I am listening.

I'm currently working on a design based on the knowledge i have obtained on the net and by watching videos of actually working gasifiers.

Many people successfully build and use things without understanding the underlying science. Even if everyone that ever successfully built and used a gassifier tells you that they run in some way or another, that doesn't mean they actually know or are correct.

Case in point, many people erroneously believe that microwave ovens heat objects from the inside out. But ask any physicist and they will tell you a different (correct) story. And there are plenty of other situations and examples just like this one.


My whole stance on the issue as of now.
I was only trying to straighten out the underlying science of the device for the sake of anyone who wanted to know. But now this debate is looking like it is going to devolve into a flame war, and I don't want that. So I am probably just going to unsubscribe and leave. I know what I need to.

I am not trying to pick a fight with you, or irritate you, or anything even remotely like that. I genuinely believe my points on the debate and genuinely believe that you have the wrong idea.

I firmly believe that your idea of making a super heated chamber that is devoid of oxygen, in the hopes of creating more/better gas through just pyrolysis is flawed. Many of the most important chemical reactions in the apparatus need a steady supply of oxygen to be carried out. And virtually all of the documentation and real devices that have been built reflect this.

Otherwise, show me a gasifier that makes syngas and doesn't use a small/steady supply of oxygen?

 

[jpanhalt]

I really don't see what the fuss is about. Pyrolysis and gasification are closely related, at least in the mind of a chemist. Wikipedia defines pyrolysis as:

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen.

However, other definitions and common usage among chemists are more general. Pyrolysis often involves some degree of decomposition, as implied by the "lysis" part of the word. However, pyrolytic rearrangements with no loss of material (i.e., no change in molecular weight) are still considered pyrolysis. Pyrolysis can also involve participation of other compounds in the reaction.

Perhaps Wikipedia is trying to distinguish combustion -- an oxidative process -- from pyrolysis and so excludes oxygen. That is a poor way to make that distinction. Oxidation can occur in the presence of oxygen, but oxygen is not required for oxidation. Oxidation reactions, including those that involve oxygen, frequently are not combustions.

So, if we get past the nomenclature for pyrolysis, I don't see why oxygen or water cannot be either included or excluded in gasification.

When I first saw the following comments, I wondered where this thread would lead.

Yup, notice at 1:30 into that video, all the compounds on the left hand side are fairly complex hydrocarbons? They all are C[SUB]6[/SUB]H[SUB]10[/SUB]0[SUB]5[/SUB] (Cellulose). All bio matter is going to have some complex hydrocarbons, so pyrolysis will always make some usable fuel gas. But pyrolysis alone will not turn all of the solid mass into fuel gas.

None of the compounds referenced on the left side are hydrocarbons.

Pyrolysis reactions will "denature" the most complex chemicals down to methane and hydrogen, and a few other compounds. But in the end, a good portion of the carbon is NOT going to be gasified this way as methane. This left over carbon ends up as char after Pyrolysis.

Clearly, the reaction conditions need to be defined and some supportive data given. What percentage of available carbon is converted to methane vs. the percentage available as CO? Why not get the methane first, then convert the rest to CO? (Methane produces 3X the heat energy compared to CO.)

The only way to make carbon a combustible gas, is to either mix it with one oxygen atom, as in carbon monoxide. Or to mix it with four hydrogen atoms, making methane gas.

Wrong. Methyl chloride is one example of a flammable gas that doesn't meet that definition. Acetylene (C2H2) is another.

For a productive dialog on gasification, some simplification is necessary for communication, but careless oversimplification will lead to misunderstandings. The meat of the issue for me is not whether it should be called pyrolysis (with or without water and/or oxygen), but what is the output efficiency, what are the reactants, and what are the products. By output efficiency, I mean the heat value of the gases produced balanced against the heat needed to produce them. Here's a useful table that may help in that assessment: https://www.engineeringtoolbox.com/heating-values-fuel-gases-d_823.html

Finally, I came across this publication: https://www.electro-tech-online.com/custompdfs/2013/02/Vol-18_3-0001.pdf It has some interesting analyses. I think a discussion of the conversion of waste materials or coal to more easily transported and stored fuels should include liquification too. Why leave out methanol, if it can be produced efficiently by pyrolysis?

John

 

[tomizett]

This is really interesting... wish I'd listened harder in my chemistry lessons!

Thanks to all the contributors for an intelligent, informed and good-natured discussion though.

 

[heydonms]

I have no experience with gasification as such, however I have had a bit of experience making charcoal.

The goal in making charcoal is almost the opposite to gasification, you want to drive off just the volatiles while leaving as much of the carbon behind as possible. The way we achieve that is by heating timber in an oxygen starved environment.

I can confidently say:
1. If a oxygen starved environment is maintained throughout, very little of the material is consumed (shape and patterns on the charcoal when it comes out looks a lot like the timber that went in (other than being black and shiny))
2. If you let air in, quite a bit of the timber is consumed (the charcoal is noticeably smaller, cracked, and smoother (i.e. surface features have disappeared))
3. If you burn the volatiles that are driven off, you do get a reasonable amount of heat but not as much as you get from burning the charcoal later.

I can't say for sure, but I think this would tend to support Oblivion's position. There is a significant amount of fuel contained in the carbon left over when organics are burnt in an oxygen poor environment, if some of that material can be converted to a flammable gas, then that fuel can be collected by the gasifier. Adding a limited amount of oxygen and ensuring the other relevant conditions are suitable for CO production, seems like it would achieve that.

I can see that adding oxygen also risks consuming some of the volatiles that are being released, so it seems like the best option might be to run it oxygen poor to begin with, collect the volatiles, and then when there is only charcoal left, slowly introduce oxygen.

 

[Just for the fun of it]

This topic made me join this site mainly because i have been looking into making a gassifer, Now to point out that i have never made one and that im not very chemically informed but from the research i have been doing and small tests i have done i think that yes you could make a gassifer without introducing oxygen (this is actually a debate ive been having with another person.. well not that it can be done but which would be best) But i think that it wouldn't be as efficient or effective, mainly because you would have to heat the material anyways with something and then you would have to filter it way more due to none of the tars being burned. also you wouldnt get as much energy out of the material and be left with bio char? Then a bigger problem would be that you wouldnt have a easy way to control the production of gas like you would if you had a internal combustion engine supplying the vacuum to keep the process going... unless you where to produce more gas then you need in which case you would have a positive pressure system where leaks would be a worse factor then with the neg system..


Like I stated im no expert or am I going to claim that i understand it fully and my reply may be full of errors, If you see some please point them out as i am going to be constructing one in the near future to run a small (1-5kw) gen and would like all the input possible.. Thanks.

Also just to test if the theory worked we took a hot plate and a paint can filled the can with saw dust put lid on put a nail hole in it and let it get hot a lit the escaping gasses..

 

[cowboybob]

Welcome, Just for the fun of it!

Since you're embarking on a pretty thoroughly investigated alternate energy source (AES), remember that it is not a completely investigated effort.

I await the reports of your efforts.

I remember my first AES system using a wind energy trap (savonius rotor/generator) and electrolysis to generate O2 and H. It worked fine. Except I was unaware, at that time, that H is extremely difficult to contain (re-absorption or leakage through the walls of a variety of storage materials). Soup cans and mayonnaise bottles did not work. That snag brought me to my knees.

So, proof of concept, but nothing for me to add to the mix of previous experiments.

But it was a lot of fun doing it...