How to make low ash biochar

I am considering making biochar from my large pile of prunings from our acre in the city.  I saw this video on youtube "https://www.youtube.com/watch?v=NrTaISI9fm4" where he makes it outside using a brick chimney kiln.  I want to build kiln on a concrete patio we aren't really using.  

I want to use the biochar in our garden beds and also fruit tree guilds but our soil is ALKALINE!   According to Joe Gardener,  "There are good and poor quality biochars available, so purchase with caution. Many biochar products contain ash. The higher the content of ash, the more the biochar material will affect your soil pH. A good quality biochar will keep your soil within the neutral zone, while a biochar high in ash can significantly raise your pH."


How do I determine how much ash will be in my biochar with this method?  I want low ash to not raise PH any higher.  

I also want to make a bit of biochar in our Quattro wood insert inside during winter using this method "https://www.youtube.com/watch?v=jxBUqk2M3Y8.  I bought the hotel pan to put the small pieces of wood in, and the same question applies...is this going to yield a final product with low ash content?  Can you tell by looking at it if it is low ash?

Thanks for any help!

Below 450C and the bio-char is acidic and will actually hurt you garden for a year or two, even after composting and activating. The pore space are actual filled with hydrocarbon and such. Above 600C and the ph is higher and the pore space is too small for microbes, esp with biochar made above 800C. But the reality is that given a year or two the microbes will temper the bio-char and make it help you garden, giving you a bountiful harvest. More control and the bio-char is great at day one, a more laid back approach and it is good in year 2. I think long term so I am fine with year 2.

Most of the alkali compounds are water soluble. I tend to soak the biochar I make anyway, before using it, and expect that most of the alkali has washed out.

But like many things, it is possible for over thinking to get in the way. W

Will homemade biochar help your garden? Probably, yes.
Can biochar made with complicated retorts, temperature control and other things make better biochar? Also, probably yes.
Does the difference matter? Probably not.

There is a valuable saying that I like in this context "don't let perfect be the enemy of good"

Michael Cox wrote:Most of the alkali compounds are water soluble. I tend to soak the biochar I make anyway, before using it, and expect that most of the alkali has washed out.

But like many things, it is possible for over thinking to get in the way. W

Will homemade biochar help your garden? Probably, yes.
Can biochar made with complicated retorts, temperature control and other things make better biochar? Also, probably yes.
Does the difference matter? Probably not.

There is a valuable saying that I like in this context "don't let perfect be the enemy of good"

Yes, yes, yes.  The people that made terra preta didn't have thermometers measuring the temperature of their burns, retorts, PH meters, ...

They piled up trash, burned it, and buried it.  I make lots of charcoal.  Some I probably make at too low a temp to be perfect, some has some ash, some has none, some has that beautiful glass-breaking sound, some does not, some is crushed, some is not-so-crushed.  I just mix it all into my compost heaps, and use it when the compost is ready.  It seems to me it's very easy to over-think this, and very easy to make if you just make it as good as you can.

I recently noticed the place where there was a burn pile of wood from having some land cleared a few years ago had much greener and thicker weed/grass/plant growth than the surrounding area. When I looked closer, I found the topsoil is full of char of various sizes, and the soil is much more dark & rich where the burn pile was.
Since that was years before I even knew what biochar was, I know that the wood pile wasn't burnt with any specific considerations, and the leftover char wasn't treated any kind of way. My soil is already extremely alkaline, but it appears that any excess alkalinity from the ash didn't take long to level out.
This leads me to believe that, while there is an optimal way to make biochar, the less than optimal ways will still provide some benefit. Now I keep a bucket for my char, which has a mix of compost, urine, and rain water. I don't burn wood often, but when I do I will scoop the char into the bucket to soak until I use it somewhere. I just mix it all together and, when I scoop some out to use, I don't worry about which char has been in the bucket the longest. This seems to work well enough for the little effort I put into it.

I was actually just thinking about making biochar in a different way.

What if we had a slash pile we wanted to turn into biochar? What would happen if we built a rocketstove body, burn tunnel, riser, and some kind of exhaust from the bottom of the pile through the top of it, and the whole pile made, if not air-tight, then air-resistant, with wet cardboard, probably the sort you see contaminated by food, pizza boxes and such?

If the flow were restricted enough that it drew a little from the pile as well, the pile itself should become deprived of oxygen. Either that, or if, after exiting the riser at the bottom through a manifold, it exhausted through the pile itself, there should be lots of heat, but little to no oxygen, perfect pyrolisis conditions, especially if the rocketstove bit is operating above the desired temperature. I think it would be necessary to stand by with wet cardboard, in case of breakouts, but I think this could work.

Has anyone ever adapted a pit-method of making biochar in this way?

-CK

Not as you describe, but my top loaded trench method works very well for brash.

As I understand you envisage the wet cardboard etc... protecting the char from burning away by restricting oxygen. But you can do the same job without that faff by ensuring that there is always a flame front above the char layer. If the flames are consuming the oxygen then the flames are protecting the char underneath. It takes a bit or practice, but once you get the hang of it you can make a lot of great char quickly with minimal faff. Key seems to be adding fuel slwoly and steadily over a number of hours to balance flame/fuel load. Too much and it starts smoking and you get a dirty burn. Too little and the char burns away to ash. But you can tell when it needs more fuel because you can see the ash layer forming!

Here is a youtube video i found a couple days ago.  It is the record of a Biochar Workshop put on by a company that produces biochar commerically.  In it the leader of the workshop, Bob Wells demonstrates how to make biochar in a do-it-yourself retort made from two barrels and a chimney and he goes through the whole process including how to make the retort.  I found it fascinating, but it is a long video.  I hope you all enjoy it and learn as much from it as I did.

Here is the link :  

Sincerely,

Ralph

OOPs. I forgot to also link in Part 2 of the video.

Link :  

FYI the first video is about 40 minutes and the second just under an hour

Also this video is very relevant to this particular thread as the biochar he creates with his DIY retort is almost no ash.

Sincerely,

Ralph

Here is a link to my thread where I explain my trench method.

Biochar - Trench method

I haven't had such large amounts to burn again since then, but will at some point soon. I like this approach because it needs nothing more than a shovel - no complex faffing with a retort, and I can easily scale it to the size of the project at hand. More, or larger brash = dig a longer trench. The only requirement is water to quench at the right time.

I'd upgrade my kit with a long handled shovel, as digging the still hot char out of the pit can be a bit on the warm side.