grass for biomass

=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

 




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