The science behind hemp as a carbon sink

One hectare of industrial hemp can absorb 22 tonnes of CO2 per hectare. It is possible to grow to 2 crops per year so absorption is doubled. Hemp’s rapid growth (grows to 4 metres in 100 days) makes it one of the fastest CO2-to-biomass conversion tools available, more efficient than agro-forestry.

Biomass is produced by the photosynthetic conversion of atmospheric carbon. The carbon uptake of hemp can be accurately validated annually by calculations derived from dry weight yield. This yield is checked at the weighbridge for commercial reasons prior to processing. Highly accurate figures for total biomass yield and carbon uptake can then be made, giving a level of certainty not available through any other natural carbon absorption process. The following carbon uptake estimates are calculated by the examining the carbon contentof the molecules that make up the fibres of the hemp stem. Industrial hemp stem consistsprimarily of Cellulose, Hemicellulose and Lignin, whose chemical structure, carbon content,(and therefore absorbed CO2).

  • Cellulose is 70% of stem dry weight. Cellulose is a homogeneous linear polymer constructed of repeating glucose units. The carbon content of cellulose accounts for 45% of its molecular mass.
  • Hemicellulose is 22% of stem dry weight. Hemicellulose provides a linkage between cellulose & lignin. It has a branched structure consisting of various pentose sugars.
  • Lignin is 6% of stem dry weight. Lignin is a strengthening material usually located between the cellulose microfibrils. The lignin molecule has a complex structure that is probably always is variable .

To summarise the above, one tonne of harvested stem contains:

  • 0.7 tonnes of cellulose (45% Carbon)
  • 0.22 tonnes of hemicellulose (48% Carbon
  • 0.06 tonnes of lignin (40% Carbon)

It follows that every tonne of industrial hemp stems contains 0.445 tonnes Carbon absorbed from the atmosphere (44.46% of stem dry weight). Converting Carbon to CO2 (12T of C equals 44T of CO2(IPCC)), that represents 1.63 tonnes of CO2 absorption per tonne of UK Hemp stem harvested. On a land use basis, using Hemcore’s yield averages (5.5 to 8 T/ha), this represents 8.9 to 13.4 tonnes of CO2absorption per hectare of UK Hemp Cultivation.

For the purposes estimation, we use an average figure of 10T/ha of CO2 absorption, a figure we hold to be a reasonably conservative estimate. This is used to predict carbon yields, but CO2 offsets will be based on dry weight yields as measured at the weighbridge.

The roots and leaf mulch (not including the hard to measure fibrous root material) left in situ represented approximately 20% of the mass of the harvested material in HGS’ initial field trials. The resulting Carbon content absorbed but remaining in the soil, will therefore be approximately 0.084 tonnes per tonne of harvested material. (42% w/w) (5).

Yield estimates are (5.5 – 8 T/ha) this represents 0.46 to 0.67 tonnes of Carbon per hectare (based on UK statistics) absorbed but left in situ after Hemp cultivation.

That represents 1.67 to 2.46 T/ha of CO2 absorbed but left in situ per hectare of UK Hemp Cultivation. Final figures after allowing 16% moisture (Atmospheric ‘dry’ weight) are as follows:-

CO2 Absorbed per tonne of hemp stem 1.37t
CO2 Absorbed per hectare (stem) (UK) 7.47 to 11.25t
CO2 Absorbed per hectare (root and leaf) UK) 1.40 to 2.06t

Industrial hemp is a self offsetting crop

According to Defra, UK Farming emits a total CO2 equivalent of 57 millions tonnes in GHG’s. UK agricultural land use is 18.5 million hectares. This amounts to an average of around 3.1 tonnes of CO2 per hectare total embodied emissions. As a low fertiliser and zeropesticide/herbicide crop, with little management input, the carbon emissions of hemp cultivation is well below the average. Therefore we can assume the matter remaining in soils roughly offsets the cultivation and management emissions.

References

  1. Hon, D.N.S. (1996) A new dimensional creativity in lignocellulosic chemistry. Chemical modification of lignocellulosic materials. Marcel Dekker. Inc. New York.(5)
  2. Puls,J., J. Schuseil (1993). Chemistry of hemicelluloses: Relationship between hemicellulose structure and enzymes required for hydrolysis. In: Coughlan M.P., Hazlewood G.P. editors. Hemicellulose and Hemicellulases. Portland Press Research Monograph, 1993. (5).
  3. 3. Bjerre, A.B., A.S. Schmidt (1997). Development of chemical and biological processes for production of bioethanol: Optimization of the wet oxidation process and characterization of products, Riso-R-967(EN), Riso National Laboratory, Roskilde, Denmark.4. Anne Belinda Thomsen, Soren Rasmussen, Vibeke Bohn, Kristina Vad Nielsen and Anders Thygese (2005) Hemp raw materials: The effect of cultivar, growth conditions and pretreatment on the chemical composition of the fibres. Riso National Laboratory Roskilde Denmark March 2005. ISBN 87-550-3419-5.
  4. 5. Roger M Gifford (2000) Carbon Content of Woody Roots, Technical Report N.7, Australian Greenhouse Office.

 

Author: James Vosper BSCHons, FRGS

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