First Year Results for Marc's Compost Furnace

This is an email from Marc detailing the results he achieved and lessons learned from operating his compost pile furnace over the first full season.


The details on his compost furnace are listed here...  -- this is a link to Marc's blog entry on the project.  
The heat from compost section of Build-It-Solar is here...


Here is the email from Marc with the first year results:


Hi Gary,

Here's the good and the bad to date:

The pile generated ~25,000btu/hr as per the design specification and did so without turning  from the end of Sept. through the end of Dec.  maintaining a 40 deg. F temperature differential between the recirculating water and the ambient outside temp.   For the first month or so of operation, the water temp was much higher in the 130~140 deg F range.  Using Nitrogen fertilizer granules (urea) to combine with the wood chips worked rather well, but reapplication was necessary by the end of Dec to maintain the pile.  In Jan. the differential temperature dropped to 30 deg. F and by Feb. dropped another 5 deg.  Further supplement of Nitrogen and/or water or air was pointless as the pile had gone ostensibly dead by the end of Feb.  Total cost for the Nitrogen granules was ~$300.

Using inexpensive garden hose for the heat exchanger in the pile worked pretty well (there are 2 300' layers of loops at differing elevations in the pile independently selectable).  The cheap hose is actually better suited to the purpose due to its thin walls for easy heat exchange.  Having hose traversing the pile makes turning it over impractical.  This year I've placed concentrated reservoirs into the pile for heat exchange to facilitate turning it over in the spaces around the tanks.  To a similar point, the insulation blanket needs to be in a protective jacket both to avoid fouling (insects and debris) as well as to keep the moisture out and again to facilitate easy removal for gaining access to the pile for turning.  Pile access was further impeded by the fencing itself as well as the plastic sheathing and cover tarp.  I incorporated a gate in the sidewall of the pen for my own access, but of no value when access for heavy equipment is required.  Initially I kept the end wall removed for that purpose.

Regarding aeration, regardless of some disinformation on the web to the contrary, aeration of the pile is insufficient to sustain combustion - the pile still needs to be turned over periodically.    For my purpose, the point of forced air pile aeration  was to obviate the need for mechanical intervention to turn the pile.  There's no such thing as a free lunch. While it's possible that the flow rates were insufficient for that purpose, theoretically the blower, size and number of holes were all calculated to give the correct cfm through the pile to sustain efficient combustion (according to those who have studied and quantified the problem).  Since the pile wasn't instrumented I can't actually verify the impact of running the aeration blower other than by empirical means which seemed to show that running it without preheating the influent air cooled the pile down and lowered the water temps during the winter by as much as 10deg F.  If I were to undertake serious development, I would definitely try to preheat the air.

The sprinkler hose used for maintaining moisture wasn't optimal.  What would typically happen is that the top layers of the pile would dry out and the bottom of the pile would gain excessive moisture.  While I tried various duty cycles for the sprinklers and aeration blower, I was never able to maintain a good moisture balance - again illustrating the need for mechanical intervention to mix the pile.  I would probably use either reaction force driven sprinklers in another incarnation or rows of nozzles around the perimeter of the pile.

It was fun to enter the side gate under the tarp cover from the cold into a greenhouse hot moist environment  in the snowy winter.  Where there was a small break in the sidewall insulation, you could see steam pouring out like a miniature chimney.  To do this right, a proper insulated structure should be constructed to contain the pile and it's working environment and adequate instrumentation procedures and computer control employed to optimize the process.  Once optimal operating parameters and algorithms were established, the function could obviously be assigned to a purpose specific dedicated controller/plc etc.

Like so many other promising technologies, this too isn't quite "there" yet. That said, if I were to run it "as is" without further investment this year other than for a like quantity of Nitrogen fertilizer, considering initial investment in materials I would be operating "in the black" by the end of this heating season assuming the pile dies again early in March.  The main issues other than those stated are that it takes a good deal more time to construct and handle all that material than originally imagined.  In particular, moving the material around was a total pita.  That said, this year should be much easier since I have a tree service that has been delivering wood chips to my property all summer long, making the task so much easier since I won't have to truck them over from the local utility where they have a free pile :)   When I get the system "dialed in" I'll post more detailed information for those who may wish to replicate it.    Thanks very much for the link Gary, and feel free to include my email for those who may wish to contact me.

Marc Silverman


You can email Marc questions at:  longeze AT wi DOT rr DOT com  (replace AT with @ and DOT with periods)





Gary September 29, 2009