This is about using exhaust as fuel oxidiser, since I read that CO₂ is part of the process in a gasifier.
I don’t know if this will support a wood gassifier’s heat, since CO2 is the final product in regular fire, so may need a little air added to the wood above the place the exhaust is added or a pilot flame in the gassifier (will need to be operated by gas produced in a separate gasifier partition). in UNWGG p7 it says there is a pyrolysis stage in the wood before it gets to the reaction zone, so the wood also needs heating (so the exhaust idea may only be suitable for charcoal-only units, which adds value to the idea of such pre-processing. With exhaust added, self-created charcoal would be consumed quickly). The CO2 would be inert unless heated to a high temperature, so perhaps add coils (glow plugs?) around inside to increase the temperature before inducted into the bottommost area, or may merit a new design.
The gassifier will only need gasify half the carbon (since half is volitant in the gas) so unit can be smaller and use half the charcoal (maybe the fire extinguisher shell may be enough for a car!), insulation should be more critical. Document says the CO2 must contact the charcoals. Diesel exhaust and petrol exhaust could be different temperatures. Wood gassifiers should be able to accept a percentage of exhaust too. Applying the rest of the exhaust around the input charcoal (or wood) container to pre-heat it could help also (provided it does not start pyrolysing). the intake must be passive: there is twice the exhaust than required, so ensure the exhaust is not pressurising the chamber. Maybe you can use the exhaust around the gasifier case to complement the insulation and dissipate any leaked CO.
Your thinking is on the right track and what you describe is what we call a EGR, exhaust gas return.
Reduction of CO2 and H2O in any gasifier is key to its operation. Unfortunaly, both molecules require an enormous amount of energy in order for them to get back in to a “burnable state”. And if wood is used as a fuel, it doesent even have enaugh energy and chemical balance to sustain reducing all of its own CO2 and H2O, thats why good gasifiers extract the extra “dead weight” via hopper gas condensation.
Charcoal, however, has a net exess of energy and it is beneficial to add those gases in the reactor.
In case exhaust (CO2) is added, indeed what you say will happen. But the problem is, all you can inject is about 17% vol, or the extremely energy hungry CO2 will cool the reaction too much. Now, same will happen when you inject steam/water BUT in this case, you also get Hydrogen wich will drasticly increase flame speed (power, efficiancy) without adding any aditional nitrogen (wich exhaust gas has many). Pure chargas (or EGR injected chargas) burns wery slowly compared to a hydrogen rich gas.This is why most tend to avoid the EGR.
You shared why you thought it shouldn’t work on wood: I have already predicted charcoal-only because charcoal I decided is in a ripe state for reaction with CO₂, wood has matter in it that will cool the process too much - water evaporation. And in a charcoal gasifier there actually shouldn’t be much H₂O unless there’s a pilot fuel(edit:or old charcoal), but it would already be dry vapour in the exhaust.
The “enormous amount of energy” for heating of the CO₂ back to pyrolysis temperatures could mostly come from a heat exchanger against the exiting gas. And unit will need much insulation since its capacity to keep warm is reduced (i presume the second oxidisation reaction with CO₂ would yield less heat than the first with O₂).
You mean when reinjecting exhaust? Correct. But there is no need to do that. Simplest way is to use moist charcoal as fuel (in a downdraft). About 20% moisture will give you dry gas, cool the temps a bit, loads of hidrogen, no dust when refueling and is self metering.
As you know, I am building a downdraft gasifier. I’m considering how to add water and how much to add to my charcoal. I’ve seen several suggestions for 5%- 10% and now your 20%.
First, the method. Different feedstocks produce different charcoal density. My charcoal is stored dry. I assume that I should weigh my dry charcoal and determine what % of that weight in water should be added. I’m thinking about using a garden watering can to sprinkle water as I am stirring the charcoal. Perhaps watering the charcoal in layers would be easier? Maybe using a tuyere water drip would be easiest?
Second, what percentage of water (5, 10, 15 or 20%) is optimum for what kind of running (constant speed generator vs stop and go equipment)?
Any suggestions are appreciated.
Bruce, each gasifier reguires its own special twerks. But as a rule of thumb, l add just enaugh water so that the charcoal stops emiting dust when mixing and shuffeling around. Im only guessing that thats at about 20%.
Do keep in mind thugh that over time charcoal will absorb about 10% moisture on its own. From the air. If you add 20% to this you are alredy in the 30s… so probably best way to judge is by feal, just like best bread is baked and kneaded by feel.
Edit: yeah, probably the best way is analitic, determine the exact ideal amount of water. But who has time for that? Woodgas is supposed to be practical. Just this year l run out of charcoal a few times, helping friends. We lit up a oil drum full of twigs, went to lunch, doused with water and when cool just “season to taste” with water. We produced 5 hours worth of fuel on a lunch break.
Sounds like a quick easy recipe for charcoal in a barrel to a charcoal Gasifier. Add dry twigs and light with a lighter. Fire it how long? Go eat lunch, come back quenched with water for moisture seasoning to touch and put into the gasifier. It’s ready to go, all preheated charcoal. I like it.
I want to try this.
You all are suggesting water for dust mitigation, but that could cool it too much for exhaust reuse, and i am anticipating the design may even need reworking.
No the water is to cool the reaction just like adding the EGR. However, instead of adding weaker CO gas it produces much more powerful water gas instead. This is why no one is using EGR anymore as using water is much more benificial and its simply easier.
When the water is shifted to CO and H2 this reaction process is exothermic absorbing heat and preventing your nozzle from melt down. Listen to these guys they been doing this a long time and have tons of opperation hours under thier belt. They know what they are doing and what they are talking about.
EGR is nothing new and we’ve all known about this for a long time.
EGR also adds Nitrogen back into the process. Where direct water injection removes it adding more energy density to the gas output. This is partly why adding the water adds that extra power its not all just the added H2 but also the elimination of the nytrogen.
Its the other way around. When charcoal stops emiting dust thats just a sign that l added enaugh water for a cool reaction and strong, dry gas. Primary goal is to get the water in there, the fact that it then doesent dust is just a side benefit. Other thain that, what Matt sayd exactly.
Hi Bruce , if it was me i would start off by treating the charcoal the same way as you do for your updraft unit , i would then use one of the many ways to meter in a said amount to the nozzle .
Dosing pump can be very accurate
A drip line , same type used in hospitals would work good , make sure u have a way to break surface tension ,maybe do what Matt does small amount of engine crankcase pressure into the tank .
Or just a plain old fashioned piece of copper pipe with a oiler type valve on it to limit the water to so many drips per Sec/Min
I have used all and they work great .
Benefit of adding water in afterwards is you may be able to tune your system on the fly ,i noticed my first attempt of downdraft , my water needs were all over the place once you wet the charcoal your sort of stuck only being able to add more and not being able to take out .
Dave
I think you maybe overcomplicating it. Adding water injection is not complex; you need a water tank, a valve, hose and an injector tube that feeds directly into the nozzle. The water will imediatly flash to steam and the gasifier will crack it post nozzle.
Oxygen is not a byproduct of this reaction. Thermal carbon water cracking is where the carbon reacts and tears out the oxygen from the hydrogen and combines with it. CO is the only thing created and the Hydrogen is then the by-product of the new creation.
