August 8, 2010, Jim Welch joined me to teach a biochar workshop at Eagle's Flight Farm in Orwell VT. Jim's task was to set up his 5-gallon retort portable pyrolysis equipment, and do a demonstration burn—to once again put the sight and sound of "rocket" in rocket stove. However, Jim added some sizzle to the rocket. He built a new piece of equipment to add an extra thrill to his usual show: a second burner to the rear of the rocket stove firebox, fed by pyrolysis gas coming out of the retort.
Jim's early versions of this hybrid rocket stove + biochar burner had a pipe to vent pyrolysis gas downward and out of the retort. This pipe has a "T" joint to allow gas to be flared off inside the rocket stove, or to be vented out the back.
At the start of a burn, Jim opens the back tap. After about 15 minutes of intense heating of the retort, pyrolysis phase 1 begins as white smoke (steam) vents out of the retort. The first few minutes, mostly water vapor is expelled from the rear
of the retort. This "wet" smoke is not flammable, and retort temperature is held near 100 degrees C.
pyrolysis phase 1: steam
Gradually, white shifts to yellow (or blue) as other vapors join water exiting the retort. After a few minutes, pyrolysis gas includes enough hydrogen, methane and other combustible vapors to ignite if in contact with a flame. But too much water is still present to sustain a flame. The smoke is still too wet to burn.
Eventually, enough water distills out of biomass in the retort, and retort temperature climbs up beyond 100 degrees C. The volume of volatile gases—especially hydrogen, methane and carbon monoxide, all flammable—jumps higher quickly. At this point, pyrolysis gas exiting the retort can easily ignite
and sustain combustion without a "pilot light" in the firebox. This pyrolytic ignition point varies vastly with the type of biomass and its condition, but is in the range 200 to 250 degrees C. This second phase of pyrolysis is self-sustaining, and a huge surplus of energy is released from the biomass.
So, a small amount of fuel burned in the rocket stove firebox triggers stage 2 pyrolysis, and the release of many times more energy from 5 gallons of biomass in the retort. One beautiful feature of this controlled combustion by gasification is virtually smokeless operation.
Everyone who witnesses what happens next is big-eyed astonished at the amount of energy released from 5 gallons of biomass. Depending on the rate of biomass gasification and size of the retort's exit pipe, pyrolysis gas can build up significant pressure trying to jet out of the retort. With Jim's 5-gallon retort and small diameter pipe, internal pressure is enough to emit a long, loud, sustained "whoooshhh"—and bulge outward to deform his 5-gallon retort.
In recent burns, Jim found the volume of pyrolysis gas leaving the retort in phase 2 of gasification is so great, the rocket stove can't suck enough air in to burn all these hydrocarbons in the tiny firebox. Black smoke expelled up his chimney was
evidence of unburned carbon and incomplete combustion—lost energy and lower efficiency.
For effective greenhouse heating, this rapid release of heat and gas needs to be slowed down and extended in time. We need ways to tame this fiery dragon to take longer, gentler outbreaths.
At this point, we have no way to control the pyrolysis process and dial down the rate of gasification. Three control strategies are possible:
restrict fuel—remove gas
restrict air—limited effectiveness
lower temperature—water heat exchanger
Pyrolysis phase 2 releases a tremendous outburst of energy, much of it as heat and combustible gas. Only 10 to 20% of this energy is needed to sustain the pyrolysis process. The rest can be removed. Since Jim's rocket stove is overwhelmed to contatin and combust this rapid energy release, the sensible first step toward process control is to draw off surplus gas, and divert it to other purposes besides heating the retort.
So, Jim went with #1: tap off the fuel, instead of flaring all the gas off in the rocket stove firebox. His new equipment is a large Bunsen-style burner attached to the back tap. Lacking a simple, easy, portable way to collect and store pyrolysis gas, Jim built an external burner to flare the surplus outside the rocket stove, and thus reduce retort heating.
Jim's backtap burner now flares off the huge majority of pyrolysis gas—and puts on an entrancing show. Although Jim increased his exit pipe diameter, significant pressure still drives gas out of the retort. The "whoosh" of the
rocket stove is now more muted—drowned out by an insistent sizzle as a 2-to-3-foot flame whips and snaps out of Jim's "Bunsen" burner. The sound reminds me of cooking brown rice in a pressure cooker. Jim suggests he can roast a chicken in a pyrolysis flare, although it may be too hot. This time of year, I'm very interested to heat a hot tub.
Cooking rice or roasting chicken isn't idle talk. Jim's backtap burner gives visible and audible reality to a key question: how to extract energy from biochar prodution. After mastering making charcoal, step 2 is to harvest surplus energy released by biomass carbonization. Jim's ability to tap surplus gas out the back of his burner invites us to divert the gas to useful purposes. This potentially huge harvest of energy is available in three forms: heat, gas & liquids.
Heat released by pyrolytic combustion at over 250 degrees C is the most immediate energy to harness. In a greenhouse, process heat can be harnessed to heat air, but a more useful strategy is to use hot water to heat soil under growing beds. We will begin exploring tapping this heat with a simple copper coil heat exchanger.
Combustible gas is the second energy product to harvest. Jim's backtap burner is a solid step toward capturing this gas. But instead of burning the gas immediately, equipment must be devised and built to store the gas. This add-on equipment must also scrub out corrosive and non-combustible components of the gas.
Third, volatile vapors can be condensed and reprocessed into liquid biofuels and useful chemicals. Liquid recovery requires more add-on equipment in the form of a water-jacket condenser, which means more heat recovery with water.
After 10/10/10, Jim Welch and I began discussions with Saratoga Apple owner Nate Darrow to build a pyrolytic burner to heat the newest greenhouse—half the size of the others, in a convenient, accessible location by the warehouse and store. The greenhouse will have a radiant heating system of underground pipes to heat soil under growing beds. Our kiln, retort and chimney must have heat exchangers to supply hot water to a reservoir to feed these soil-heat pipes.
A further possibility beyond heat, biochar & biofuels is to power a gas turbine or combustion engine to drive an electric generator—a potential demonstrated by Frank Jeffers from Georgia at the Biochar Field Day at NESFI Nov. 14, 2009.
This fall, in California, for the 350.org Oct. 10 Global Work Party for Climate Change, All Power Labs tested their first "Power Pallet"—a compact, tightly designed unit that sits on a pallet-size footprint and produces useful heat, gas, biochar, and 5 kilowatts of continuous electric power.