Hemp and Carbon - The HGS view

Our basic premise is that hemp is far more productive than typical agro-forestry projects, producing annual, versatile biomass alongside more rapid CO2 uptake. It can produce a vast range of sustainable raw materials with an overall low environmental impact, as well as improving soil structure, using low fertiliser and no other chemical inputs (i.e. reduced agrochemical residues).

Hemp can be grown on existing agricultural land (unlike most forestry projects), and can be included as part of a farm's crop rotation with positive effects on overall yields of follow on crops. This, along with super versatility in diverse soil conditions and climates, makes hemp cultivation a viable and genuine potential large scale contributor to GHG mitigation.

Replacing Unsustainable Raw Materials

The vast quantities of hemp derived products and raw materials created by large scale cultivation could replace many oil-based unsustainable products and materials, particularly in construction, locking in captured CO2 and creating secondary benefits to the global environment. In particular, hemp could be used to replace significant quantities of tree-derived products, allowing reduced use of existing tree populations, thus maintaining their CO2 uptake.

Hemp also produces much higher quantities of stronger and more versatile fibre than cotton, and many other fibre crops, which often have very high chemical residue and water footprints. Extra processing required by hemp is also at least partially offset by its recycling potential.

Carbon Absorption of Hemp - HGS Preliminary Conclusions

Our carbon uptake estimates are calculated by the examining the carbon content of the molecules that make up the fibres of the hemp stem. Industrial hemp stem consists primarily of Cellulose, Hemicellulose and Lignin, whose chemical structure, carbon content, (and therefore absorbed CO2) are shown in the following section:

• Cellulose, 70% of stem dry weight:

 

 

 

 

 

 

 

Fig 1: Chemical structure of Cellulose (Hon, 1996).
        
Cellulose is a homogeneous linear polymer constructed of repeating glucose units. The carbon content of cellulose accounts for 45% of its molecular mass.

• Hemicellulose, 22% of stem dry weight:

 

        

 

 

 

Fig 2 : Chemical structure of Hemicellulose (Puls and Schuseil, 1993).

 

Hemicellulose provides a linkage between cellulose & lignin. It has a branched structure consisting of various pentose sugars.  Based on an example of hemicellulose structure like the acetylated xylan chain with ? - 1, 2 bond to 4 - O - methyl glucuronic acid & an ? - 1, 3 bond to L - arabinofuranose pictured above the carbon content of hemicellulose accounts for 48% of its molecular mass.

• Lignin, 6% of stem dry weight:

 

 

 

 

 

 

Fig 3. Chemical structure of Lignin (Hon, 1996).

Lignin is a strengthening material usually located between the cellulose microfibrils. The lignin molecule has a complex structure that is probably always is variable (3). Using the example above, the carbon content is calculated to be 40% of the molecular mass.

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 CO2 absorption 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).

Using Hemcore's UK yield estimates (5.5 - 8 T/ha) this represents 0.46 to 0.67 tonnes of Carbon per hectare (UK) 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

Hemp 'Self Offsetting.'

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 zero pesticide/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. 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. (5)

         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.

         5. Roger M Gifford (2000) Carbon Content of Woody Roots, Technical Report N.7, Australian Greenhouse Office.