It's been a while since I last did a blog, now I have been playing with something new so I thought I'd share it here.
Whilst I am still a novice on the subject, I can offer some advice and that may inspire you to look further into this concept. It isn't new, it isn't hard and there are many benefits. 

Bio-char. What is it?
It isn't exactly charcoal - in essence it is "cooked wood". Charcoal is burned wood.
What do I do with it?
You can cook with it and you can bury it in your garden or mix it into your compost. There would be many other uses but the "bury it" is the one we are concentrating on.
Why would I bury it?
It is going to turbo charge your garden - but there is a small catch (not really a catch but you should do this), if it isn't charged/activated it can actually hinder your plants growth until it fills with it's goodies. I am being deliberately vague here "scientifically speaking" because as I said I am a novice and although I understand the concept and the end results, I am no scientist and so I am using "Dave lingo" to explain what I do know.

How do I activate it?
I understand steeping it in manure & water is one way, I actually soak it in worm wee which we collect from the worm farms. Once again there would be a myriad of ways to achieve this. I'd leave it to soak/steep for at least two days but I choose to leave it a week.

How do I make it?
There are many ways - one is to use an old cast iron pot with a lid. You put the wood you are going to turn into char into the pot, put on the lid and put the pot in the open fire. The wood will get to a certain temperature and begin to "Gas". Google will tell you all about this process if you want to know - for me I'm more interested in the end result.
Another way is to use/make a retort. Once again, there are a few ways I am aware of to do this. You could spend hours looking at the different models. I'm going to concentrate on the way I did it.

How did I do it?
Lets begin by saying it took a few goes and tweaks to get better results and whilst I'm not going to say I have it perfect, I can make bio-char with this method and pretty much no smoke so that is a great start. I'm going to be playing with a few things to refine the process even more so will elaborate when I feel I have it close to perfect, but for now this works and I am happy with the results.
You need an outer drum (obviously we make them out of corrugated iron but you can salvage/use anything to use) that has an open top and a sealable lid. It doesn't have to be a water tight seal but the better it seals, the better the result. You require an inner drum that is slightly smaller and shorter than the outer or "burn" drum. This also needs to be eventually sealed once it is full of timber/wood. My drums are 510mm X 840 high for the outer one with a slip on galvanised lid 200mm deep and a 150mm flue 900mm long. The inner drum is 410mm X 600 high and a galvanised slip over lid that is 570mm long (so just shorter than the drum). Both drums have bottoms. The outer drum needs several holes drilled around the base for air intake and smaller holes around the top, just below where the 200mm lid rests when in place.
You place the inner drum inside the burn drum and fill it with the wood you intend to turn into char. It really needs to be dry, the drier the better. You don't have to be fussy, I don't load it to heavily or pack it in, I just randomly toss in whatever is available. Dry pruning's cut into 100mm to 400mm lengths are great, as are wood chips or off cuts of palings. Try to avoid treated timber though. Once it is full, secure the slip over lid, then I centralise it using long wooden lengths that are about 50mm sq. and 600 long. It doesn't have to be perfect, these will be burned. I wedge the lengths down at 12 O'clock, 3, 6, & 9 then slip more wood in the open gaps until it is as full as you can get it. The less gaps the better. Now I use cardboard, paper and bark, then small twigs and ramp up until you have a reasonable fire. Light it and let it burn down. At this stage you will obviously get smoke (like any conventional fire) but it is for a short while. Once it has well caught alight, I poke the fire so that it is towards the outside of the drum, I then put more wood on it but not overload it, you have to be able to secure the lid. With this step done, I secure the lid with the chimney. Virtually no smoke comes out (if it is Smokey, you may need more breather holes or your fire might not have been hot enough before you put the lid on). I just played around with this until I got it right. 
Now the fun begins!!!
As the inner drum heats up, the wood inside begins to "gas off". With my process, I lit the drum at about 2 pm. It burned "conventionally" for about an hour and a half, then the gas took over. By 4 pm it was rocketing - the gas was coming off the wood, pushing down the inner sleeve and out the bottom and firing up the gap between the drums.
As there is no oxygen in the inner drum, the wood inside doesn't burn. I have seen examples of this process lasting for hours. Mine burned for about 3 hours on the gas.
The next day when I opened it up, bio-char!!!

During the burn there is so much heat produced, you can cook on it, boil water, I haven't actually looked properly into using that energy yet. I have cooked some snags & steak on it, but there are many more options to use the heat if you think about it.
The gas produced is a fuel, if you could harvest it safely, store it then use it later, there are examples of running engines on it too. Purifying water with evaporation is a possibility, heating of course, many possibilities. 

I will end this here for now, I hope to add to it as I get more/better results and keep an eye out to see the results of using the char in the garden. I already have been using charcoal, including as a growing medium in my aquaponics system.
Look at the facebook page to see progressive videos of the retort in action.

Cheers
-Dave