Using grasses as a feedstock for biochar

[jockgill]

Using grasses as a feedstock for biochar

Of the main sources of biomass in this world, wood and grasses
dominate. Grasses are basically biomass that grows and dies in one
growing season, whereas the tree is building a structure over the
course of several years. One nice quality of grasses is that they dry
out much more rapidly as compared to wood - nobody has to "season"
their grasses prior to use. Having said that, it is still important
that the grasses be dry prior to conversion into biochar, mostly
because virtually all biochar making devices are operated on woods,
and dry wood at that.

Grass has two other properties that are worth mentioning: superior
nutrient levels, such as phosphorus and potassium, and much lower
density - compared to wood. The former means that grass makes good
biochar, and the later means that it takes a mountain of grass to make
a molehill of biochar. Most biochar making techniques yield 15 to 25
percent of the weight of dry biomass as biochar. As such, the more
biomass you put in, the more biochar you get out. In the case of
grasses, this points immediately to any and all options for
"densifying" the grass prior to conversion to biochar. Remembering
that most biochar makers were designed around wood, just strive to
form the grasses into any wood fuel shape: pellets, chips or pucks,
compressed logs, etc. Sometimes it is easier to compress the grass,
taking advantage of green grass squishiness, then dry the formed
material afterwards. After that, the next stop is biochar.

For example, field dried Grass from Ernst Seeds was compressed into
1.5 inch diameter grass tablets in a BHS Energy Slugger. The tablets
were converted to biochar in a TLUD stove using some supplemental
forced air. When the pyrolysis front reached the bottom of the TLUD,
the combustion gases changed from yellow to blue.  At that time, the
combustion was stopped by dumping the char in a quench can and sealing
the can until the char cooled. The resulting solid residue was a
easily crumbled biochar with significant adsorption properties
(roughly 5 wt % R134a uptake at 100C). The pH and TDS of the biochar
were not measured, but it is expected to be acceptable for use as a
biochar after composting with additional organic matter or mixed with
a source of plant fertilizer. The yield of biochar was not measured,
but was estimated at 15 wt % of the dry grass biomass used to fuel the
TLUD.

In summary, grasses work just fine for making biochar - but may take a
little pre-processing and practice to get grasses to behave correctly
in biochar making techniques developed around wood. But there is
nothing preventing this approach from working, except for giving it a
go.


- Hugh

Hugh McLaughlin
Director of Biocarbon Research
Alterna Biocarbon