jack vegas

Use paper plates...

Yikes!  I visited this thread a couple weeks ago and decided not to post since the thread seemed dead.  Now, BOOM!  It's alive again!

The original post wanted to assemble a minimum cost 3 month "just in case" food supply to tide one over in an emergency.  In a later post the author mentioned that he hoped to keep the cost under $200.  Pretty easy in those days since 3 months of rice and beans would have cost less than $50.  It can still be done for under $100.

That post was made in 2011 and in the intervening years, food costs have risen by over 50% with a good fraction of that happening over the last year or so.  By comparison, the price of gold at the end of 2011 was about $1,550/ounce (admittedly a bad year to buy gold).  Today it is selling for about $1,800/ounce.  Only a 16% increase.   Rice and beans as an investment outperformed gold by a margin of nearly 3 to 1!

Food inflation is projected to continue rising steeply over the next year or two so whatever you can buy now will end up saving money over time.

I suggest that food storage be considered an investment, not a cost.  If you want a 3 month emergency food cushion, dip into your savings NOW and buy 3 months worth of items that you are already eating with a 3 month or greater shelf life, then rotate as you buy more to keep them fresh.  These days food inflation pays better dividends than most standard investments.

Matthew - Oh boy does your last response to Hank Fletcher have my head reeling.  Lots of new stuff to think about!  The Johnson Su Bioreactor paper is a real find.  I haven't read it in depth yet but its bound to keep me awake tonight.  Its particularly interesting to me because after getting into this conversation I'm thinking more and more about how to build a larger scale system needed to accommodate larger chunk material like yard waste and wood chips.

Matthew - the way the original system was set up the hair dryer was used primarily as a heat source to heat the box and its contents to 140F while the computer fan was used to draw warm oxygen rich box air through the drum continuously.  Maybe a reptile bulb suggested in the last post could be used in place of the hair dryer.  This should work as well and emulate your bifurcated system I think.   It would also eliminate the noisy fan from the hair dryer which probably has a poor service life over the long run.  BTW - I used Chinese temperature controllers that are (were) prolifically available on ebay for $15-$20.  They come with instructions that, even in fractured English translation, allow simple setup of temperature controlled systems.  Just search ebay for "temperature controller".  I use panel mount types that look similar to this.

Matthew - I like your central wand.  One reason is that it would allow the fan to blow air rather than suck.  My sucking fan had to work in a humid vapor saturated environment which is not great for an electrical device.  There should probably be a dam in the the middle of the wand however to divert all flow through the drum.  Without it, air could preferentially just travel in one end and out the other, without much flow through the perforations.  Need to think about how to handle the rubbing seal.

Bigger hair dryer - A bigger hair dryer may not help much since its important not to overheat the bed beyond about 140 deg-F, otherwise it kills rather than nurtures the bacteria.  I picked the one I did since it was powerful enough and also it was DC.  Another option might be to use an infrared "reptile heater" bulb installed inside the box.  These can be had for about $25 including a a remote thermostat.  In this case the computer fan would still be needed to push or pull air through the drum.

Clean-out residue and estimating heat rate - Since I was mostly experimenting, I never ran the thing more than 4-5 days without shutting it down.  I cleaned it out every time, though I'm guessing it could easily have run a month before clean-out would be needed.  I estimated heat by taking the total dry material added over a run and dividing by the hours that were run, estimating 8,000-9,000 Btu/lb of dry material.

I think your two barrel approach is a good one since if actually used to heat a space, it would be nice to keep one running while the other is down for clean-out.  Also, since these things can't be easily throttled its probably a good idea just to run them full blast for base heat load and have another source (I used my electric heat) on a thermostat to trim the final few degrees of heat desired.

Your last post came through just before posted this so I'll need to think it through before responding.

Corrado de Cesare - Wow!  I love northern Italy!  Wish I could spend every summer there.  Unfortunately it has been maybe 20 years since I was there last.  Yes I have seen this site and it has a wealth of info for anyone interested in generating heat from compost.  The site is a little hard to navigate since a lot of the links seem corrupted.  I've learned these can generally be fixed by eliminating one of the "public3" parts from the web addresses.  For some reason they are duplicated which makes the links fail.  He started his experimenting with a "giant bag" approach that I still find intriguing.  Perhaps difficult to manage but a great SHTF idea.

Hank Fletcher - I think your concept has some merit.  First, I think reactor volumes can be fairly small if they are well insulated.  The key to how small is the size of the particles being digested and the ability to add more.  The smaller the particle (thinking saw dust or lawn clippings) the more surface area and the faster the digestion.  The other key to small volumes is how to draw the heat crated out effectively.  Small volumes have small surface areas.  From a practical standpoint, I suspect 20-30 gallons volume is about the minimum.  Below that and the power output is probably too small to be useful for anything more than heating a single room or very small cabin.  I've thought of using 5 gallon stainless steel soup pots for reactors but never pursued them.  Also, more in keeping with your concept, I thought of a rectangular reactor enclosing a stack of large baking sheets, each filled with a few inches of fuel.  A problem I see with your concept is how to reload it.  My baking sheet concept allowed removal of each layer for refill while the others continued to "bake".

As long as digestion is strictly aerobic there is little problem with dumping the CO2 and water vapor directly into a living space assuming normal ventilation.  However, the humidity could be a problem for you.  It is for me, so I dumped the gasses to outside the house.  I love the idea of using aluminum cans for ducting!

To all - I finally broke down and calculated heat soak through the barrel.  Can't believe I never did before.  Perhaps I did and promptly forgot about it.  Anyway, a 55 gallon drum has about 21.6 sqft of surface area.  The maximum surface temperature I can imagine is about 140 deg-F and at that point the steel can only radiate about 145 Btu/hr-sqft.  So without convection, pure radiation can only dump about 3,130 Btu/hr.  Convection with air (or water) is needed to draw more heat out of the system.  This is both good and bad.  The good news is an un-insulated drum could reach full operating temperature so insulation isn't needed.  On the other hand, a fan is needed to be to push enough air over the drum to draw heat away through convection.  Both to prevent over-heating and to draw more heat out of the system.  It seems my insulated box served as a duct to channel that air.  I may have been operating very close to the limits of my system and not have known it!  So a good design would probably use the hair dryer to start the system and to protect against sudden cooling and the "box" should be configured to provide a good air cooling jacket to maximize convective heat transfer.  Just about any material could be used for this air cooling jacket since it doesn't need to provide insulation.  Cardboard and duct tape might be used to build a well formed cooling jacket and a relatively quiet and inexpensive box fan might be used as the convective air source!

Good question Lif Strand, and boy do I admire you for sleeping in an un-insulated horse trailer sealed with manure!  I started with the insulated box because I wasn't using the hair dryer to preheat the system and needed to hold the heat till the system came up to a reasonable operating temperature.  With the hair dryer one could probably dispense with the insulation.  I'd still use the box though since it contains the heat to a large extent which results in more uniform heating overall.  It also serves as a heat exchange duct to guide the air flow over the drum.  What I describe is a high performance system that generates uniform heat throughout the compost so it all decomposes uniformly at high temperature.  It also digests nearly everything loaded.  There is little compost left once it is finished running.  Just a bit along with what looks like (and probably is) undigested mineral waste.  Very much like ash in a stove once all the wood and coals have been consumed.

William Bronson - I've played a lot with concepts for vertical barrels but have never built one.  I've always focused on how to provide the absolute optimum conditions for the bacteria so  I could maintain continuous high operating temperature.  That said I remain curious about systems that might be less than optimum.  How bad do they have to be before they don't work?  Everything is worth a try as long as the conditions needed for the bacteria to thrive are delivered.

Let me suggest a slight variation to your bottom water sump concept.  Inject air into the gap between the compost and water surface using a hair dryer like blower.  Initially the heat element would be on, heating the bed rapidly to operating temperature.  It would cycle on and off to maintain optimum bed temperature.  Once the bed generates enough of its own heat the heating element would no longer come on and just the fan component of the hair dryer would continue operation feeding fresh oxygen to the bed.  The computer fan from the horizontal concept would no longer be needed.  I think mixing would still be needed, but perhaps a simple paddle system could be used that is hand cranked periodically.

Combine this with Lif Strand's question regarding the need for the insulated box.  Don't use one.  The hair dryer will certainly allow the compost to reach optimum temperature.  The question is can it remain there once the heater is turned off.  If it does, then the insulation is not needed.  To prevent the compost from getting too hot, a simple box fan might be aimed at the outside of the reactor.  It would cycle on, or speed up, whenever the compost starts getting too hot.  The only missing element I see in this is a way to preheat hair dryer air when the heater is off so cold air is not introduced into the compost.  It might look something like the attached image.

Matthew - You managed to identify one of the big problems with this design on the first pass.  Congratulations!  The illustration didn't allow for describing the feed approach I used.  As previously mentioned I did the initial loading and final clean-out through the removable drum head.  For loading during operation I had a small hatch in the box with a matching resealable port in the drum that allowed feeding the drum without removing the box.  If you face the left hand side on the drawing, the ports were at the 9 o'clock position.  I'd stop the rotisserie and fans, open the ports and pass fresh material into the drum.  It was tedious and it meant leaking warm air from the foam cocoon and feeding a lot of fresh cold material into the drum all at once which was definitely not good for the bacteria.  To make feeding a bit easier and faster I made a long tool from a piece of 6" PVC that was split in two to make a long trough-like scoop.  I'd fill the scoop, stick it through the ports, then rotate the scoop so it dumped it contents into the drum.

I'm pretty proud of the design from the standpoint of automating temperature maintenance for continuous high performance digestion, but from an operability standpoint it was disappointing.

Some problems and thoughts on possible solutions:
Rotating drum - many desirable mechanical adaptations were not possible due to the drum rotation and particularly due to the PVC pipe "wand" down its core.  At first I thought the wand was a pretty clever way of solving the rotational seal problem, but it prevented filling the drum more than about 40 % full due to leakage through the gap between the drum and pipe.  I'm fixed on the rotisserie approach because I think it does the best job of maintaining moist material and breaking clumps.  I've considered internal paddles but power requirements to push material with paddles could be excessive.  In my ideal design I'd use a stationary vertical drum, but I've never been able to adequately address all the issues to my satisfaction with that configuration.

Feeding - As described above, it was tedious, required breaching the thermal cocoon, and it meant dumping a lot of cold material into the drum all at once.  Ideally there should be a way of feeding small quantities of fresh material frequently or even continuously without breaching the thermal cocoon.  Since a lot of my experimentation used wood pellets I considered a pellet feed setup similar to a pellet stove.  That could probably be achieved but it would preclude using wood chips or yard waste.  Wood chips are very inexpensive where I live and I can get large quantities free in the fall from tree trimming services.  I also have a surprisingly large amount of yard waste on my property that I'd really like to use since I put so much effort into cutting it back in the fall.  A note of feed material.  Early on I scored an 800 lb load of wood pellets that had gotten wet because the owner had stored it outside.  Entirely useless to him but perfect for me and free for the taking!  Also, we use wood pellets for cat litter.  Urine soaked pellets... the perfect feed material and a great way of disposing of them!

Clean out - Clean-out at the end of a run was a pain.  I'd remove the box, withdraw the wand, then tilt the drum up onto its sealed end.  After that it was a simple job of rolling the drum outside on a hand truck, dumping the residue in the garden and hosing it out.  The pain was disassembling the unit to get to the drum.  I'm older now (72) and it would be neat to find a way of only handling the residue and not the whole drum.  I keep noodling on ways of cleaning it out like a wood stove... except in this case the ashes would be moist soil that doesn't blow around!

Heat transfer - though I've never run calculations, I think the drum could be a limiting factor.  Its volume is such that if full and running at high digestion rates, more heat might be created than can be transferred through the drum's steel walls.  I need to look into this.  It turns out that with continuous feed of small particles like sawdust or grass clippings, a small volume can generate a lot of heat when running in high performance thermophilic mode.  It would be nice to make sure adequate heat transfer is available to be able to use that capability.

Regarding your question about feed rate - Using wood pellets I could load 40-50 lb of pellets and once running well they were consumed at a rate of about 1 lb/hour.  This works out to an equivalent of about 8,000-9,000 Btu/hr depending on fuel quality.  If I could have filled it more than 40% full I feel I could have pushed output to 15,000 Btu/hr.

I put together a more complete description of my "Barrel in Box" compost reactor to help people understand how it was built and how it worked.

Physical description with reference to the attached drawing:
The core of the unit is a 55 gallon steel drum with a removable head to facilitate loading and clean out.  A steel drum is used to maximize heat transfer and the inside and outside are epoxy coated to prevent rust.  The drum is mounted horizontally on four rollers allowing it to rotate rotisserie style.  The rotisserie motion is driven by a 12 volt automotive electric window motor (not shown).  Since I doubt the motor is rated for continuous operation it is controlled using an interval timer so that it runs for 1 minute every 30 minutes.  Periodic rotation is desired to keep the compost material evenly moist (a small water puddle is always present in the bottom of the drum) and also to break up any clumps so anaerobic regions don't form.

The drum assembly is enclosed in a removable box made of rigid 2" foam insulation with a 6 inch air gap between the drum and box walls.  The box limits heat loss from the system so that the compost can come up to and maintain operating temperature.  During operation a 12 volt 6 inch boat bilge duct fan pushes air into the box where it is heated and then exits the other side of the box through an 8 inch duct.  Not shown are thin plastic film flaps over the downstream side of the inlet and outlet ducts to minimize air entry into the box when the duct fan is not operating.  These flaps are lightly weighted with wood strips (tongue depressors) to minimize air entry into the box when the fan is off, but also allow the flaps to open when the fan is on.

A PVC pipe is rigidly mounted to one end of the box and penetrates a hole in the drum head.  The hole is slightly larger than the PVC pipe creating a small air gap so the drum can rotate without the pipe rotating and also to facilitate air flow into the drum to feed oxygen to the compost.  On the exterior end of this pipe a 2 inch 12 volt computer fan is mounted to pull fresh air through the drum head gap and then exhaust CO2 and water vapor laden gas overboard.  The PVC pipe also provides a convenient place to attach a thermocouple for remote monitoring of compost temperature.

Finally a small 12 volt 200 watt hair dryer is included to feed warm air into the box during startup and to help maintain temperature if the compost inadvertently chills too quickly during operation.

Operation:
Operation begins by turning on the rotisserie and hair dryer circuits.  The fan circuits are off during startup.  The hair dryer heats the air inside the box to about 125 deg-F.  Without the hair dryer the compost will slowly self-heat but the warm hair dryer air really kick starts the process and the compost is up to temperature in a matter of hours, not days.  There is enough oxygen in the air inside the system to allow the compost to reach about 100 deg-F at which point the fan circuits are turned on manually.  After that the computer fan stays on continuously.  There is little risk of it pulling cold air into the reactor since it draws from the larger heated box surrounding the drum. The computer fan is on a rheostat which provides some degree of throttling.  The main duct fan uses a thermal switch that turns the fan on whenever the temperature at the box exit exceeds 135 deg-F.  At first it cycles fairly often but as the compost heating builds it runs for longer and longer periods.  I adjusted the fan discharge using a rheostat so that even when fully operational the fan cycles off briefly after 15-20 minutes.  This is to ensure that it will still cycle for even longer periods if heat output increases unexpectedly.  Eventually I planned to insert a rheostat override circuit if temperature exceeded 150 deg-F so the fan would throttle up to maximum speed to cool runaway heating which could kill the bacteria.  The hair dryer circuit uses a thermal switch to turn it off whenever temperature exceeds 125 deg-F.  Once this temperature is reached during startup the pile continues self-heating rapidly.   Should the pile cool unexpectedly, the hair dryer will cycle on at 125 deg-F to prevent excessive cooling that might also kill the thermophilic bacteria.

I tried to make the system as bulletproof as possible so that it would be hard to inadvertently kill the bacteria.  It could be simplified considerably but that would require more attention and careful monitoring.  With the exception of the rotisserie interval timer, everything ran on 12 volts DC so that it could be battery powered during a power outage.  I never found a 12 volt interval timer I liked.

Can't say with certainty, but two things make me think that pound for pound, aerobic digestion will produce more heat.  First, a large portion of the elements in the feed material remain as methane after anaerobic digestion, so what ever energy value it has is still locked up and has not yet been released.  Second, after anaerobic digestion is complete there is a relatively large fraction of the original material remaining as sludge that can be further broken down by aerobic digestion.

Yikes!  This really took off!  First, my sincere thanks to Pearl for making this a thread!  I probably would never have done it on my own and I really appreciate having a place that this can be discussed.  I'll try to catch up with some random responses, but let me reiterate, I'm no expert.  Just a curious experimenter so I'm looking for input just like everyone else.

1) Yes, Jay Angler, sawdust and urine works!  My original intent for the barrel in a box was to make a pellet stove without combustion to circumvent code and insurance problems.  I thought it would be cool if a renter or apartment dwellers could use one as a portable appliance that wouldn't jeopardize their lease.  Wood pellets are basically sawdust and once you get them wet so bacteria can digest them, they revert to wet sawdust.  I used urine as a handy source of nitrogen to get things going... so to speak.  The smaller the particle size the faster it digests, so sawdust gets things hot quickly (like hours not days) and require feeding the unit more frequently.  The result is a lot of heat quickly from a small unit.  For this reason I figured wood pellets would be an ideal material for a small high performance unit.

2) Matthew - to clarify, I used a small fan to circulate air through the barrel to both to introduce oxygen and to exhaust CO2 and water vapor.  The flow rate is low and there is no reason it can't be dumped into the room if things are running properly.  If things don't run right and the little critters don't get enough oxygen, things can become anaerobic and smelly.  I vented outdoors primarily because I have a humidity problem.  The bacteria doesn't need trace elements to survive.  They eat glucose from the organic matter and water and poop out CO2 and heat.  Most trace elements in organic matter are fine, but some of course are toxic.  Most common yard and forest materials are fine.  Sorry, sadly no pictures.  Victims of the great back-up drive crash in 2018.

Some general notes:

Bacteria are tough little critters but they are living creatures and need care and feeding to prosper.  Too hot, too cold, not enough oxygen or water, and they die.  If we make sure these factors stay within proper bounds, they live and make heat.  Stray outside the limits because they are suddenly chilled because a lot of cold fresh material is dumped into their cozy little lair too quickly, and they die... quickly.

Digesting one pound of material releases about 8,000-9,000 Btu.  To digest 1 lb of material per hour, air flow should be about 7 CFM.  Only about 1.2 CFM in needed to provide adequate O2, but exhausting CO2 and water vapor required the full 7 CFM.  Hence, by making sure that adequate flow is available to exhaust byproducts, plenty of O2 is available.

If fresh wet material is used very little water needs to be added.  If dry material is used, about 1/2 lb of water needs to be added to every pound of dry matter.

Mesophilic bacteria will bring temperatures up to about 120 deg-F.  After that they begin to die and thermophilic bacteria take over and generate heat at a much higher rate.  Somewhere beyond 150 deg-F they die and if they get colder than about 120 deg-F they also die.  Its important to keep them in the sweet zone of about 125 to 150 deg-F.

Such devices are nearly 100% efficient.  A couple percent is lost in the biological processes, and a couple percent is lost if it is vented outside.  If it is located indoors, all the rest goes into the space being heated.

Digesters aren't easily throttled.  The oxygen supply can be restricted somehat to slow things down, but generally they are either on or off, and if they are off, they usually need a couple days to get back up to temperature.

These things are basically wood stoves that use biological decomposition to make heat and CO2.  This means lugging many pounds of material into the house to keep it warm, be it fire wood, wood pellets, or yard waste.  Plan accordingly.