Dreaming wild and crazy dreams again. Visited with a couple of farmer/ranchers talking about
biochar. The key point both were making is that if they do it with
wood that it is either going to be labor intensive or to involve lots of machinery to harvest the wood and get it in a good size for doing char and then to crush or grind it smaller after so it doesn't leave bigger sharp chunks that are a danger to livestock or tires.
Now the ideal would be to change to wood in the form of small chips to sawdust or other bio-matter that would work here like
hay or straw etc so no grinding or crushing necessary. The trouble with these is they are basically an insulator and charring them is going to require a bunch of equipment too or a lot of labor to get the heat into the middle. Or is it? I just went by a post on Permies discussing doing it with sawdust a few days and got to thinking “could we add a bunch of heat pipes into the sawdust to reduce the insulation distance?”.
Heat pipes are a passive element that moves large quantities of heat. If they are in a near vertical orientation with the heat at the bottom they can be a simple pipe with no fancy interior wicking material need. Basically it is a sealed pipe filled with some sort of refrigerant at the bottom with the top initially filled with vacuum. As the bottom is heated the liquid boils making gas and it carries the heat up until it condenses on the wall of the pipe and runs back down as a liquid. Now what refrigerant? How about just
water? We need to reach roughly a 450 to 550 degrees F temperature to do char. Since the ideal pore size is created by low and slow we want to stay probably below the 550. Lets add a bit of margin for error and call it 600 degrees. At that temperature that is about a 1500 psi charge in the pipe. Lets say we picked 1 inch schedule 80 pipe. In steel it has a roughly 4900 psi max allowable pressure and if we had to go to stainless it has a bit over 2400 psi max pressure. Would like steel for expense but what about corrosion safety? Now as a sealed tube this is a potential steam bomb so how do we moderate the risk? We add temperature and safety limits. What if you added a screw on top point in each pipe to make reloading it viable. In that point put a soldiered shut hole with a melting point matching the 600 degrees roughly so if the pipe over heated it would blow with some sort of shield so the blast was deflected down Add in some sort of pressure fit relief valve and you could over pressure too. Cap the bottom end with the point on the top. Build a capper that let you put one end or the other on the pipe while pulling vacuum. Place your pre-measured distilled water volume in the pipe and pull to vacuum and screw the cap on to seal.
Make
enough pipes so you could fill your container with say one every foot or every 6 inches as nearly vertical spikes. You want off vertical slightly as the heat pipe works faster if the condensate inside all runs to one side of the pipe to run back down. And you want the water fill line inside the pipe basically on the heat side and the steam part of the pipe in the char side For build cost and easy emptying and loading you want as few pipes as possible. Design right and you can keep adding pipes till it works as needed.
Now you want a big metal barrel or box to put your material in. The pipes only need to extend 1/3 to ¼ of the height of the box as that will be your final char level left if you are not in a hurry and can allow time for it to settle as it chars. Put a heavy follower on top of the material to push stuff down as it chars that is set up to miss the spikes. This will make it slower but
should improve ability to dump and reduce materials cost to build. If the pipes were made out of 20-22 foot lengths of pipe chopped in thirds that would give you say a bit over 2 feet as the heat absorber below the box and a bit over 4 feet as the surface doing the charing.
Put your container with the pipes in as the roof of a heat bell and
feed that bell from your fire or burner. The heat that comes out of the first bell at the bottom edges goes into another bell for drying and possibly starting the char in another box or barrel. Then onto a third where the char is cooling down for removal.
Since most farms are heating shops or sheds can we use that same heat to make the char? We will have some loss in the system but the char should add flamable gas and distallate out to add to the fuel. Guessing in the end in a well designed system you should do a little better than break even on
energy needed.
Now with a properly designed set of spikes could you do the same thing with say round bales? The barrel to hold the bale and the char goes over over the spikes. Then put the bale on top of the spikes and push down gently with the loader to get it down as far as possible. Might only be 6 inches to a foot on the spikes. After all we are doing a large scale bed of nails. There again a heavy follower set on top. As the spikes get hot, they should start charing with both air heat by the pipes and heated pipes in direct contact and thus the bale opening a path for them to go farther in and the bale should settle on them over time shouldn't it? If not, it might be necessary to push the spikes into the bale like nails a few at a time. We don't have any trouble backing even a double bale spike into a bale for pickup so this should not be any worse.
Now we want to add some other safeties and control systems. In the hot one of the heat bells lets add 2 chimneys. The first one controlled by a bimetal spring that opens it at say 575 so hot air rises and it starts cooling the bell while wasting some heat. The second chimney, which should be bigger, put a spring loaded soldier link door like is used in building ducts for fire control with the link set to melt at roughly 600 or just below the melting point of the spike heat relief. If it gets too hot the link melts the door springs open wasting the heat but limiting how hot it can get. Between those 2 passive systems the spikes should hopefully never get hot enough to go off if you have a power failure. Then in the bell add some ducting as a heat exchanger with a fan controlling it so you can control how much heat you rob to heat the building. A simple set of snap switches to control how much heat you rob. Add a second and possibly a third fan that simply blow cold air into the bell to cool it as a back up plan. This would give an active multi-layer system backed by multiple layer redundancy in passive systems for safety.
Final major piece would be 3 removable insulated covered to keep the air out and the heat in the char chamber. 2 large steel container with
rock wool in between maybe, to make each cover? Then some sort of easily removed seal ring to connect the cover with the top of the heat bell to make it mostly air tight.
Some sort of rotary race track moving the assemblies around the circle would probably be ideal. But might be to complicated too. A simple diverter aiming the heat at a given bell first and some adjustable dividers between the bells might be simpler.
So what am I missing in the thinking? Why wouldn't it work? Could it be done cost effectively? Labor effectively? A loader to lift covers on and off and load and empty the chambers should be all of the needed big machinery. So far I see 2 major flaws in this thinking and one questionable. 1. the heat pipes need to stay above freezing or the water going to ice, will break them. 2. The small amount of char you get out of a round bale covers a very small area. Roughly 42 cubic feet of char and at 3% soil
carbon from it into the top 2 feet of soil that is roughly 3/4” that is roughly 56 square feet of soil covered in 3/4” of char. Great for gardens and small spaces but to make a noticable change in 100 acres the change will take a very long time. Now the one gain is that because of its its long life expectancy the char added can slowly build over decades or centuries and still make gain. The one questionable one is would 1” schedule 80 pipe be big enough and rigid enough. The pressure rating of the pipe falls rapidly as diameter increases so for reasons of cost and ease of use keeping the pipe smaller is far better.
Farmers and ranch who feed bales nearly always have weedy or bad bales so the char source is here and would be very little expense. For people with sawdust and wood chips there again it is mostly free. If either group is heating a building then potentially the needed heat to do this is free and might actually gain them a little bit energy wise.
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