Breaking char down into smaller pieces

I have a moderate amount of char that I'd like to break down into small pieces.  

Any suggestions on good methods?  I don't have a large press of any form, so this will likely be a manual process.

Cujo Liva wrote:I have a moderate amount of char that I'd like to break down into small pieces.  

Any suggestions on good methods?  I don't have a large press of any form, so this will likely be a manual process.

The simplest method is to spread it out on a hard surface and then drive over it repeatedly. You could even stomp it with heavy boots...I "paved" a muddy pathway with large chunks last winter and a couple of months of foot traffic broke it up nicely (it also pushed it into the soil, which was my main goal).

I use the method that Phil is talking about.  I just take a piece of wood  and put the char under it, evenly spaced but in a few inches from the absolute edges.  Usually, I stomp on it first.  Then I just drive over it any time I drive anywhere or come back.  I take a few passes at first.  It's very effective and sustainable, because you were already going to drive your car anyway. I find that mine is done in a week.

John S
PDX OR

I might be remembering this that is really about something else, anyway ...

I remember someone putting something in a bag ( lets use biochar) then running over it with a vehicle, lawnmower, etc.

Would this work?

Hi Cujo,
I have not tried, but I heard a guy who swears by using a wood chipper... as long as the char is wet. If its dry, you get a cloud. If its wet, you get a pile of small sized char.

I have driven over it with my truck. Just on the dirt where it was burned (this was a big burn pile for fire mitigation that i quenched early) and it worked great. Got about 125 gallons of crushed char with minimal effort.

I have also put the char in a tote box and used a hand tamper. Worked good for about 10-15 gallons but took longer and was more labor-intensive, obviously. Results were maybe 5-10% better by hand just because I could return unsmashed pieces to the tote for a second go around.

Good comments Dan. Char is low-tech, high-tech. And so are our methods.

Some of what I see is boutique char in pretty packages at the local greenhouse, from a closed kiln and carefully inoculated with all the right stuff. Pretty cool.

But I think most of it mimics nature's messy processes, dirty woodland wildfires, prairie grassfires for 10,000 years since the glaciers retreated.

The weather is breaking now, from a -39C windchill, and I am about to do a giant burn, because it is now safe to burn. I think I will stabilize a lot of atmospheric carbon, and put it to work, building soil and growing food for people. Guess I'm a nut, but this feels right.

Thanks Doug!

I don't think you are nuts at all. Like they say, when do you want to smell smoke, in the winter or in the summer? How do you want to see fire? Piles burning on the side of a road or an entire hillside? I know burns have gotten away and caused damage and destruction and its horrible but I truly believe that it is the only way out of the mess we have created for ourselves thinking we could "protect" the forests that have burned, like you said, for thousands of years. I think we need to now do the dirty work and put the forest back into its ancestral state. It's a monumental task.

But you do get some of the greatest soil amendment there is, biochar, out of it!

Phil Stevens recommended the book "Fire-A History". I read it and thought it was great. It really laid out how important fire has been to human civilization.  I could see how we have been suppressing fires and how they have led to forest conflagrations.  Sometimes it feels like we can accept that fire has been an important part of civilization, and use biochar to help us get out of our problems or we are in for some planetary chaos. We are already experiencing some of this.

I think that Phil also had a unique device for crushing char that was really effective on a large scale.  Not remembering it exactly though, because I have a small property-small scale.
John S
PDX OR

Cujo Liva wrote:I have a moderate amount of char that I'd like to break down into small pieces.  

Any suggestions on good methods?  I don't have a large press of any form, so this will likely be a manual process.

I just fill the bottom of a 5 gallon bucket with charcoal and crush it with a 4-5 foot long 2×4. Wear a dust mask.

I tried that. It wore me out, and I still couldn't get it crushed.   I still have my 2 x 4, and I nailed on another piece of wood to one end,  so it wouldn't wear my hand out too much.  I make too much biochar to do that method, so I drive over it instead.  I generally completely fill a 5 gallon bucket with each burn.  If it works for you, then great.

John S
PDX OR

John Suavecito wrote:Phil Stevens recommended the book "Fire-A History". I read it and thought it was great. It really laid out how important fire has been to human civilization.  I could see how we have been suppressing fires and how they have led to forest conflagrations.  Sometimes it feels like we can accept that fire has been an important part of civilization, and use biochar to help us get out of our problems or we are in for some planetary chaos. We are already experiencing some of this.

Glad to hear you enjoyed that book. I recently re-read it for third time and am out there trying to get as many converts as possible to burn stuff for the better. Now I want to read all his stuff.

John Suavecito wrote:I think that Phil also had a unique device for crushing char that was really effective on a large scale.  Not remembering it exactly though, because I have a small property-small scale.

This is the post where I talked about it initially: crushing contraption. I want to weld some big thick beads along the surface of one of the drums to help "encourage" harder pieces into the pinch point.

One thing I really encourage is that anyone crushing or grading biochar work with it damp if at all possible. I'm currently making a couple of dry batches using a metal lid to end the burn, and the screening process is a horrible chore with all the dust flying about.

Yes, during our very dry and hot summers, I will sometimes take the upper piece of wood off and spray water on the char before driving over it again. Partly to decrease the dust. Partly so it doesn't go everywhere.  And it might make it easier to crush.

John S
PDX OR

I typically don’t bother crushing.

The char either goes into the chicken run, added to their deep litter. Periodically I dig out the run and spread the mulch on the garden. Usually after a month or so the char is fully incorporated and bigger bits have shrunk.

Alternatively, I make the char near where it will be used and after quenching I apply it as a top dressing. Having been doing this for a few years now, I can see that the char gets fully incorporated within a few years just through regular action. Again, I periodically top dress with well rotted woodchip mulch/chicken bedding/compost.

None of these applications have been negatively affected by larger pieces of char.

I also found that the method I use to make char means I don’t tend to get strong big lumps. I use an open trench method to burn fairly thin brash - any heavier wood goes into the woodshed. I rarely find pieces more than half an inch across. Burning heavier larger diameter pieces is what causes big chunks of char.

Crushing the biochar gives it more surface area. It's like getting much more biochar for free.  It's that much more effective.  Different people crush it down to different levels. You really don't need to crush it into powder. Then you have to worry about inhaling the dust, which is exactly the point of watering it, as Phil said.   When mine comes out of the barrel, there are many pieces that are as big as a softball-way too big to be helpful.  You are really limiting the amount of microbial and nutrient interaction then.    The one time where you don't want to crush it as much is if you live in an area with extended droughts. It stores water better underground when it's larger.  I am guessing that only crushing it down to the size of a large cherry might be optimal then.  Most people crush it more than that.

John S
PDX OR

I've seen this argument before - I'm not convinced how true it is. The surface area for adsorbtion and release of nutrients is due to the incredibly high porosity of the biochar. That porosity is a feature of the pyrolysis process.  Grinding the large pieces down will increase this, but only a tiny fraction compared to the internal porosity.

In my experience the roots and fungal systems happily extend through the pieces anyway, and they get mechanically broken down through these biological process, albeit over time.

So 1) I'm not convinced that the increased surface area is meaningful and 2) even if it is meaningful, the char gets broken down considerably in the soil anyway over a few years.

In the area I used biochar first, about 8 years ago, very few pieces larger than 2mm are still visible, but the soil is generally noticeably darker in that area.

I looked through Google Scholar and read many articles. I literally could not find a single article in which they advocated or even considered not crushing it. This is a basic science concept that my materials science Phd friend immediately mentioned, even though he is not a gardener.  Likewise, there are people who don't want to inoculate/charge their biochar, perhaps because it smells bad or is inconvenient.  Again, every single article advocated charging the biochar before installing it.  The evidence is overwhelming.  There are many people today who have concluded that climate change isn't real because it would be inconvenient if it were true. Again, not very scientific.   Yes, the soil after placing biochar should be darker, because biochar is black.  A tiny root can grow through pieces of biochar.  The benefits of biochar aren't limited to tiny roots.  Access to the microbiology is important, as well as its use as a potential shelter for them.  It creates the diverse soil food web, which creates the nutrition in the soil, per Elaine Ingham.   Not accessible if not broken down.  The original Terra Preta people didn't have to crush their biochar, because they didn't have Western property regulations and weren't working on a Western timetable. Believe what you want to, but the evidence is readily available.

https://onlinelibrary.wiley.com/doi/full/10.1111/gcbb.12885

https://avrdc.org/download/project-support/v4pp/training-trainers/1-2-1-general-soil-and-water-management/1-2-2-biochar/Guidelines-on-Practical-Aspects-of-Biochar-Application-to-Field-Soil-in-Various-Soil-Management-Systems-1-T-1-2-2.pdf

https://www.sciencedirect.com/science/article/abs/pii/S0959652622041932

John S
PDX OR

My degree was in chemical engineering and we had a whole course on porous structures, as they are used as catalysts in a whole host of reaction systems. We were taught, pretty consistently that the internal surface area, due to the porosity, was so much greater than the external surface area that it could essentially be neglected in calculations.

This website indicates high porosities due to the pyrolysis conditions - not due to later processing such as grinding.

https://www.celignis.com/biochar-surface-area.php

500m^2/g - as suggested for the "best" biochar is a huge figure.
____

Anyway, my point is generally that the claimed key benefit of biochar - that of adsorbtion and release of nutrients for plants to use.

Some back of the envelope maths:

a cube of biochar 1cm x 1cm x 1cm has: Surface area 6cm^2, volume 1cm^3, internal surface area 250m^2

Crushing it down to cubes 1mm x 1mmx 1mm: each has volume 1/1000 cm^3, there are 1000 of them, and each has surface area of 6/100cm^2... so for a 1000 mini cubes the total surface area is 60cm^2.

So crushing from 1cm to 1mm increases the external surface area by x10, but does not affect the surface area due to porosity.

60cm^2 is vanishingly small compared to the 250m^2. Even if you crushed it further, down to 0.1mm size, the surface area of 600cm^2 is still tiny in comparison.

________

There are other benefits to having a fine texture, rather than coarse lumps, but it seems like the argument that it is due to increased surface area is likely not the key reason for crushing.

Michael Cox wrote:There are other benefits to having a fine texture, rather than coarse lumps, but it seems like the argument that it is due to increased surface area is likely not the key reason for crushing.

But the other benefits are exactly what I am looking for.  It's not about increasing the total surface area.  I want my biochar to be well distributed in the soil.  Larger lumps don't do that until many years later if ever.  Terra Preta utilizes highly weathered and broken down charcoal as a key ingredient.  I want my plants (particularly young/small ones) to be able to access the enhanced nutrients and biology, not have to find it.

Until someone does actual and repeatable tests with broken down char vs. larger lumps and come up with a clear advantage, I'll continue trying to emulate the original "black soil".

I fully agree with this - the value of even incorporation does seem meaningful when applying to larger areas. It also makes it easier to work with in some ways.

As I said, my personal observation has been that char does get broken down and distributed over time, even from a simple top dressing while mulching. Digging it in - unless you are disturbing the soil for other purposes - seems like unnecessary effort. Does top dressing slightly larger chunks make optimal use of the char in year 1? Probably not. But they do get incorporated and broken up over a few years.

But for a soil amendment that persists for potentially hundreds of years I don't see that as a problem. It is not like a fertiliser or compost where the most value is in the next growing season, and diminishes with time.

I guess that my personal experience is also coloured by type of char I work with. Pretty consistently the char I produce comes out of the burn with fragments of between 1" and 1/2". I don't have great big chunks of charcoal that need smashing up. If your process is producing big pieces of char - apple sized, for example - I would look at both your process and your feedstock.

Smaller diameter brash automatically makes fine, friable, and porous char. Thicker pieces of feedstock - like you would use in your fireplace - char more slowly, make bigger chunks, and don't tend to generate the highly porous texture we are aiming for.

EDIT: I wandered away from my point. The specific issue I was trying to tackle was whether crushing char was meaningful from the specific perspective of surface area. I'm personally convinced the surface area issue is a red-herring, despite getting frequently repeated.

Crush your char if it makes it easier for what you plan to use it for. But also recognise that grinding it superfine is likely not beneficial.

Yup.  Most of my starting charcoal is 2"-3" (~5-7cm).  I want to get most of them down to 1/2" (~1cm) or smaller.  Any resulting powder will just help with the dispersion in the soil.

I don't have figures at hand, but intuitively I would expect smaller pieces of biochar to have faster adsoprtion rates as the external surface area increases relative to volume. The porosity doesn't change, but the distance traveled to the interior of a grain will be shorter. This is why larger pieces have more drought-proofing value...the water held in the middle will be slower to move out via capillary action but still available to plant root hairs and fungal hyphae.

I don't bother crushing the stuff that goes onto the paddocks, but instead let the hooves of the sheep do the work.

Cujo Liva wrote:But the other benefits are exactly what I am looking for.  It's not about increasing the total surface area.  I want my biochar to be well distributed in the soil.  Larger lumps don't do that until many years later if ever.  Terra Preta utilizes highly weathered and broken down charcoal as a key ingredient.  I want my plants (particularly young/small ones) to be able to access the enhanced nutrients and biology, not have to find it.

Is the Terra Preta biochar well weathered and broken down because of time, or do we suspect/know that those creating it ground/crushed the char?  I'm asking because I don't know the answer...I always assumed the creation was always more "accidental, but then more on purpose once they saw some benefits" but I have no clue what type of preparations they may have done.

I have never seen nor heard of char that wasn't porous.  That kind of goes against what char is.   I also haven't seen anyone on this site advocating for super fine char.  I have argued against it here in this thread.  

I doubt very much that most people are working with materials that are 1" in size or smaller before burning.  Even if surface area isn't the reason, I still haven't found anyone on Google Scholar advocating against crushing biochar and I've only seen it discussed as a crushed medium.  If your material is already small, you may not need to crush it, but advocating against crushing it seems like an unhelpful position to take if most people have larger feedstock.

Smaller sized char helps the soil drain and retain a certain amount of moisture. It moves more readily through the soil.   I want the mycelium to travel freely through the soil, using the biochar.

Digging the spade in vertically and jimmying it back and forth to fill the crevasse with biochar is not a lot of work.  It feeds into the area of the soil where the roots will access the life in the biochar and prevents the biochar from drying out in the summer when it is too close to the surface.

John S
PDX OR

Burlap sack + driving over it is the easiest way I've heard so far. I don't have one though so I just found a flat rock and fist sized rock that I used to smash bio char down with after it's gone through the slow compost pile, it isn't all that fast but it's satisfying and I usually just smash as much as I need to fill up some new pots or amend the holes for new plantings.

The bags are easily available at coffee stores, which are everywhere.

John S
PDX OR

Just to add a note about this, one thing I've done to increase the amount of smaller particles isn't to crush it so much as is it to just make more of it and to start with smaller particles in the beginning.

I use a heck of a lot of wood chips, and diverting some to the biochar retort is no big deal.

So I still end up with a couple of baseball sized chunks, not many though, the biggest are golf ball sized. I don't really fret it and just try to increase production of smaller stuff.

Tony, I'm surprised that you're burning wood chips.  Can you make a clean burn with them? Is it in a retort?

I get a much wider variety of sizes of char after crushing with bags.  I consider this to be an advantage.  We have very dry summers here. A bigger chunk will hold more moisture during the summer and that can make the difference for a plant that sends its roots in desperate searches for water.  The smaller chunks are more efficient in terms of mixing with the soil.  Every so often, I will need to manually break the big chunks into smaller pieces. THat's ok.

JohN S
PDX OR

John Suavecito wrote:Tony, I'm surprised that you're burning wood chips.  Can you make a clean burn with them?
JohN S
PDX OR

JohN, I use a barrel with a saddle cut to create BioChar, the last couple of burns I dropped wood chips on the coal base, about a half inch thick at a time... as soon as I started seeing ash form on the top of the chips I added another layer. After a few layers, I mixed the chip charcoal down in the barrel, added a layer of larger material. Once that layer had converted to charcoal I did the chip thing again. Those 2 burns yielded a lot more processed BioChar than any other burns I have done, approx 43 gallons of 1/4 inch and smaller char with about 2 gallons of material that did not pass thru my 1/4 inch screen.

Hal,
This is intriguing. I don't know what a saddle cut is.
I am trying to visualize how that works but I just don't quite get it yet. Probably someone else doesn't understand it either, so I would appreciate it if you could show it or explain it further. It sounds great.

John S
PDX OR

John, I've made a bunch of these by cutting an oval out of the side of a drum. The burn is done with the drum in horizontal orientation and you can control the flame cap with minor rotation from the vertical.

John Suavecito wrote:Tony, I'm surprised that you're burning wood chips.  Can you make a clean burn with them? Is it in a retort?

I get a much wider variety of sizes of char after crushing with bags.  I consider this to be an advantage.  We have very dry summers here. A bigger chunk will hold more moisture during the summer and that can make the difference for a plant that sends its roots in desperate searches for water.  The smaller chunks are more efficient in terms of mixing with the soil.  Every so often, I will need to manually break the big chunks into smaller pieces. THat's ok.

JohN S
PDX OR

John,

I built a biochar retort, a sealed drum with no oxygen going in and just heat from a pipe. It's a 6" steel pipe going into the bottom of the barrel, with a weld and 90 degree cut, and the pipe sticks out of the top of the barrel lid, cracks sealed up with rock wool. So yeah it all just gets pyrolized or whatever it is.

The wood gas gets shot back into the fire chamber once the pressure starts to happen.

Matt McSpadden wrote:I have not tried, but I heard a guy who swears by using a wood chipper... as long as the char is wet. If its dry, you get a cloud. If its wet, you get a pile of small sized char.

I have tried chipping wet charcoal myself and I would not recommend. I ended up with black sludge and a gummed up chipper. The charcoal had been extinguished with water and left there to drain for a week or so. I do have a modified chute on my chipper so that might not have helped for the sludge but it might also have been a worse mess without the chute. The next time I used the chipper the black dust cloud that came out of it was impressive. I could imagine the trees going "cough cough, wtf, man! cough cough" So I'm not going to try chipping dried charcoal. It's a small Champion brand chipper, maybe other types would do but not mine.

John Suavecito wrote:I looked through Google Scholar and read many articles. I literally could not find a single article in which they advocated or even considered not crushing it.

I admit I've never researched this like you so this is just an opinion. I read through the links provided in your post and they talk about the effects of char on heavy metals, water retention, crop yields, etc. Interesting reductionist research. But who here makes char from sewer sludge or apply tons per acre?  If viewing char as an amendment within a conventional agriculture system then yeah, crush it I suppose. My feedstock is a mix of sizes so the char I get goes from 2-3" to powder. I sometimes top-dress with it and I've seen larger pieces with holes plugged up by silks so things live in there. I like that. Maybe nobody talks about larger pieces because nobody tried it?  Just saying.

There is an advantage to not crushing it down to powder.  I live in a climate where it gets very hot and dry in the summer. No rain for a month or two, literally.  More plants die in the summer due to dry heat than in the winter due to our mild cold.  Depending on how hot and how dry it gets, having those chunks is a good way to store some moisture inside the chunks.  I crush it in the driveway in large bags of burlap like products.  I have to remove them from between the tires after a couple of days because it will get too crushed and lose its ability to store much moisture.  I try to stop it when it goes from powder to about 1-2 inches in diameter, mostly falling between. For my climate, that's optimal IMO.  I also have a suburban garden, so I'm not working on a farm-scale project, so it works for me, but might not for everyone else on a different scale.

John S
PDX OR

I am only dealing with a small amount of biochar that I produce but perhaps the mechanics might be scalable somehow?

I take my new biochar and scoop it into a bucket. From there, I just use a 2x2 stick or a broom handle and just poke it down to size. I think of it as a large scale mortal and pestle except you don't have to put much effort behind breaking the biochar up into pieces.



I like the simplicity because there is minimal prep, minimal tools, and the main cost is your time.

I am planning on trying putting it in a burlap sack and seeing if smacking it with a wood mallet might be another technique.

Sounds like a system that works on your scale.  
If you wanted to, you could still drive over it with one tire in the burlap bag.

John S
PDX OR

First I want to say I love the responses by Michael Cox, and I agree we need not stress over crushing.  That said there is some real value so here is what I do.  Also like Micheal since I do almost open top burns (cone kilns) my pieces most start pretty small anyway.  The quench alone fractures things a lot.  So I recommend start with heavy quenching alone will save a lot of work.

When my burn is done I quench, I leave it over night in the kiln and move it to buckets the next day.  

I have two screens, one is half inch and one is quarter inch.  I chop a bit at the char in the buckets with a sharp shooter shovel (if you don't know this is what I mean, https://amzn.to/4eSWAbQ ).  I put it though the 1/2 inch screen and push a bit with a hoe when I do this breaks up more.  

I then put what went though the 1/2 inch screen though a 1/4 inch screen and separate into three grades.  

1/2
1/4
Large

I get about 2/3drs of a batch into one of the smaller grades.  Now if you are going to crush further you only deal with what needs crushing based on your goals.

You can toss the big stuff into a bag and do the drive over it with a vehicle thing now MUCH more effectively as you are dealing with less and well trust me it crushes better.  Put that back though the screens.  You could also do that with the half inch but I don't.  All up to you but again you are only processing the part needing it.  

What I do with each "grade"

1/4 and finer - Goes in my feed for Ducks/Geese/Chickens, twice a week a tbs goes in the dogs food.  Also what goes in my worm bins and the big thing is it is what goes in my potting soil/starter mix.  Any I don't use for that goes into animal bedding in the coop.

1/2 if I need more 1/4 it goes in a bag and gets a crush, otherwise into the animal bedding.  That is a 1-2 year process before it gets to a garden.  Because once a year the deep litter goes into bioreactors.  By then a lot of it is smaller anyway.  If not who cares, it works wonderfully.  

Large - most of this is about the size we used to call "nut coal" in the coal industry (about a nickel to a quarter in size).  None is ever very big.  So if I need more it gets the bag and drive over trick and a couple screenings.  And sorted again, into the three classes.  But what if i don't want to crush more.  First it is also fine in bedding but I think we are missing a reality here, biochar is CHARCOAL and damn good charcoal.  

So gasp I end up with may be 20% of a run as large, I keep it separate and when I want to cook with charcoal I use it for that purpose.  It is fantastic for it.  I use the "side baskets" (like this in a weber kettle https://amzn.to/3B1BIBn ) and it works way better than briquettes and you get a lot more in due to more space efficiency.  

I can hear it now (OMG it is for muh soils and my CO2 sequestrations).  Hold on, you cook all the time the heat comes from somewhere.  I am using huge amounts of waste that would otherwise all go back to the atmosphere in break down if I didn't make the char.  I get bad ass soil amendments, but I also get fuel from scraps.  

My reasoning is despite it sounding like a ton of work, it isn't, super easy and fast.  So I spend more time making char then crushing it.  So I make a LOT more.  So more goes into the soil in the end.  

I will also say I have used a wood chipper and it works wonderfully.  WHEN you get the moisture just right which can be tricky.  But doing the big stuff after screening usually can do that.  But this all seems VERY hard on the chipper and when it is too wet it is a real tar looking mess.  One hack is after you do it put a few buckets of wood chips in to help clean it out and toss those in the compost or bedding but I have honestly quit this for a simpler model.  

Last it is really cool to make char in a grill then cook in that grill with some of the char you made.  Do you own a Weber Kettle Grill or one of the hundreds of clones of it, then you have a kiln.  Don't have one, check Craigslist or FB Marketplace you can likely get one in good shape for 50ish bucks.  Hell get two at that price.  A kettle is a PERFECT cone kiln, dare I say it works better then some you can buy marketed as kilns.  

Here is a video of me using one to make biochar, I just added a rotisserie ring, so I can up my yields by about 25%.  Any questions ask, I don't post here often but I always answer questions when I do so.