Gasification Archive

Californian Char Maker August 2017

Another two years have passed since the conceptual continuous bio-char maker at CalForest Nursery in Etna, Scott Valley, California was first tested. As a really busy commercial activity that continues to invest time and $$$ outside of core business, resolving the technical challenges of developing appropriate componentry for bio-char production, steady progress can be reported.
The previously identified needs to modify the char augers to smaller diameter, now prevents jambing, and a new fuel leveling auger also functions as a refueling signal. Shut-down and air exclusion remains a manual task, but effective to prevent over night burn out of the start-up char bed. An extra water cooled core of the char chamber over the extraction grate, helps to quench the descending char, assessed a necessary when char out-put is cranked up to maximum.

Reviewing the choice of component development options from the first test programme, priority was given to returning the pyrolysis gas flares back down to ground level, and resolving blue smoke leakage of this air blown system during refueling cycles. In keeping with a policy of using existing equipment, especially the wood chip fuel system normally used for the "Shasta 2" gasifier, fuel locks were considered but discarded, having proved difficult to maintain for pyrolysis gas. Without a means to reduce internal system gas pressure by suction of the sticky pyrolysis gas, a possible solution was suggested to use a double chamber eductor venturi that creates suction using air from a blower fan.
Unable to discover prior applications of air blown eductors sucking combustible gas, design parameters were very much a guess and test process. The gas was expected to ignite at the first contact point with the air, creating a rocket engine effect. This provided the blast pressure to the second venturi gas inlet, with additional air completing the final combustion. Cast refractory creating the venturi effects were movable on their mountings, having expectations of adjusting suction or behavior of the first stage gas ignition blast. Spark ignition was installed to ensure combustible gas burnt as soon as it was present in the first wet pyrolysis gas, so that start-up smoke emission was minimal.
Final combustion is completed within the length of the refractory lined exit pipe discharging into a concrete block spark arrestor chamber. Water sprinklers were also arranged around the whole area for safety, but hand held spraying was found the most practical, as nothing exited the spark arrestor chamber except a heat shimmer.

The first cold test of this new blown/suction arrangement proved we had quite a healthy suction from the eductor. First ignition tests of the gas flare were difficult due to the large amount of moisture in the start-up gas, so a gas torch was used instead of the spark ignition. As expected the first stage suction behaved like a rocket feeding it's blast to the second venturi, accompanied by a low vibrational rumble. As all the refractories came up to temperature in the eductor, the noise level declined to a steady state, and by tuning the second venturi improved the final combustion. Emissions were monitored with CO and NoX well below limits for this stage of the test programme.

Limited char extraction trials were conducted, mainly to study the char quality evolution and best extraction rate, This also identified improvements that could be made to the extraction grate before the next series of tests later this year. Chars have been made available to researchers investigating their application, enabling wider participation of regional bio-char groups. International forestry expertise visiting this tree nursery have able to see seedling trees growing in char rooting medium, along side conventional vermiculite/peat mixes, and eventually in a year or two, visit local forestry planting sites.
As of December 2017, the charmaker is running again to establish the maximum outputs.

These char making and eductor tests were independently monitored on site by Tom Miles www.trmiles.com

This is the new component that I went to commission for the CalForest Charmaker. Called an eductor, it sucks hot pyrolysis gas lowering the internal pressure head of the updraft system.

Simple cement block spark arrester for the 1,000C+ combustion gas exhaust.

This is the quality being produced during these trials. To find one raw chip however is failure for the production process, as it limits the end applications for the char.

Without water sprays in the bag filling augers, hot char can hide in the middle of a bag, then burn it's way to the outside over-night.

Me very hot (106F) taking notes.

This is what can happen to pyrolysis gas when it is piped from one place to another.  Under about 475C it will begin to form coke and block the pipes. We expected liquid tars rather than coke, but it only built up in a non-combustion area of the design.