=plants =farming =biomass =energy =chemicals
I've previously said that:
- growing
plants for non-food purposes has some of the lowest CO2 mitigation costs
- various uses of biomass are much cheaper than (non-biological) direct air
capture of CO2
- specially-bred Miscanthus sinensis is often my preferred
source of on-purpose non-food biomass
Some agricultural byproducts are currently cheaper than growing biomass on-purpose and available in fairly large quantities:
- wheat straw
- corn stover
- sugarcane bagasse
- oil palm empty fruit bunches
Currently, the only reason to grow on-purpose biomass for chemical processing is those byproducts not being available in sufficient quantities.
The US DoE made
this
long report on options for growing on-purpose non-food biomass. In 2016,
they estimated $60 to $80 / ton dry mass at the farmgate for growing
on-purpose biomass. Adjusted for inflation, that would be $78 to $104 in
2023. I believe $75/ton is feasible, but techno-economic analysis of novel
processing plants should generally be conservative.
Methane has >3x
the energy/mass of dry biomass, so natural gas is of course cheaper per
joule in the USA today. It's also easier to transport and process. For
biomass to be competitive, it needs to provide extra value and/or people
need to be willing to pay extra for CO2 mitigation.
plant evaluation criteria
Here are some criteria I considered when thinking about potential sources of
biomass for various uses:
yield (tons dry mass / hectare / year)
cellulose
hemicellulose
lignin
protein
inorganics
digestibility (by cows/etc)
inputs
water
nitrogen
phosphorus
potassium
planting difficulty (seeds /
rhizomes / etc)
vulnerabilities
weeds
insects
microbes
climate
biomass uses
The relative
importance of those qualities depends on the application. What are some
current applications for inedible plants?
fuel
Plants can be burned. Wood has been used as fuel since prehistoric times,
and is used in some power plants in Europe today. High lignin content is
good for this because it has higher energy density and is more hydrophobic
than other plant constituents. The main desirable characteristics are low
water content, low ash content, and high ash melting point. Biomass tends to
have a lower ash melting point than coal, which can lead to ash sticking to
heat exchangers. That's one reason it's sometimes been burned together with
coal in power plants.
animal feed (for eg cows)
Cows can eat grass - but not all kinds of grass. High digestibility and high
protein are good.
structural materials
Wood can
be used to make buildings. Generally, high cellulose content is good for
structural wood.
chemical processing
Furfural is
a chemical made from biomass today. It comes from pentoses, so high
hemicellulose content is good. Sometimes corn cobs are used.
upcoming applications
What are some
applications for inedible plants that might be more important in the future?
chemical processing
Hemicellulose can be converted to furfural. Cellulose can be
converted to levulinic acid. Hydrochar can be produced as a byproduct. I
think that plants converting sugarcane bagasse or specially-bred Miscanthus
sinensis to levulinic acid + furfural + hydrochar are economically viable on
a large scale. This process involves several chemical steps and optimizing
it has a lot of subtle issues.
structural materials
Some bamboo is strong. One reason it's used less than wood is because
it's highly susceptible to rotting. However, engineered bamboos can prevent
that. I think using furfurylated bamboo as a structural material is
practical. That would require large amounts of furfural production.
Plant fibers can be used as a reinforcement for some plastics. This has a
similar effect to glass fibers, but they decompose at a much lower
temperature which limits the plastics they can be used with.
sequestration
If you bury biomass, it will eventually decompose, and much of it will
be converted to CO2 and volatile acids, but there are ways to prevent that.
The cheapest way to prevent decomposition is probably keeping biomass
sufficiently dry, by drying it then adding CaCl2. This is one of the
cheapest ways to sequester CO2 from the atmosphere.
questionable applications
Here are some applications that have had a significant amount of research
but that I don't think are very practical.
hydrolysis and
fermentation
Cellulose can be hydrolyzed with dilute
sulfuric acid. In theory, the resulting sugars can be fermented to ethanol.
In practice, this is much too expensive, and I don't expect that to change.
on-purpose biogas
Fermentation of biomass
can produce methane and volatile carboxylic acids. Those can be used as
fuels. I think capturing methane from landfills makes sense, but growing
on-purpose biomass just to get methane from it decomposing seems much too
expensive to me.
some common and candidate plants
corn
This
is the most-grown crop in the USA. The main source of ethanol in the US is
fermentation of corn starch. Corn cobs are relatively good for burning as
fuel (low ash content) or conversion to furfural.
yield ~= 15-20 tons
- high yield
of starch
- corn cobs have high hemicellulose content
digestibility = high and low
inputs = high
planting = easy
vulnerabilities = high
switchgrass
This is a common grass in the USA, and for that reason has been a major
focus of US biomass usage research programs. It uses fewer nutrients than
most faster-growing grasses, because it transfers some nutrients to its
roots for winter.
yield ~= 5-20 tons
digestibility = low
inputs
= low
planting = easy (seeds)
vulnerabilities = low
climate = wide
range, varies by species
tall fescue grass
This is commonly grown as feed for cows.
yield ~= 10-15 tons
digestibility = medium-high
inputs = medium-low
planting = easy
(seeds)
vulnerabilities = low
climate = wide range
Miscanthus x giganteus
Miscanthus x giganteus has been a major focus of European biomass usage
research programs because it grows quickly.
yield ~= 20-40 tons
digestibility = low
inputs = medium
planting = medium (rhizomes)
vulnerabilities = low
climate = cold temperate to subtropical
energy cane
This is a hybrid of sugarcane and a related grass that produces less
sugar but more biomass. It grows very quickly but probably requires too much
fertilizer.
yield ~= 50-150 tons (depending on inputs and subspecies)
digestibility = low
inputs = high
planting = medium (cuttings)
vulnerabilities = medium
climate = semitropical to tropical
water hyacinth
This grows on open freshwater, not on land. It would be an excellent
crop, but its water requirements are generally much too large. See also
this post.
yield ~= 40 tons
- protein =
very high
- digestibility = high
inputs
- water =
very high
- nutrients = high but can be supplied by agriculture runoff
planting = easy
vulnerabilities = low
miscanthus sinensis
Miscanthus
sinensis is a common species of grass with wide variation. Its widely
varying characteristics are a positive, because they mean it can be bred for
various goals effectively. For chemical processing, it would probably be
bred for growth rate and high hemicellulose content.
yield = 5-40
tons (varies greatly with subspecies)
- cellulose =
45-60% of insoluble mass
- hemicellulose = 22-40%
- lignin = 8-15%
digestibility = low to medium
inputs = medium-low
planting = easy (seeds)
vulnerabilities = low
climate = wide range, varies by subspecies