In 1994 a four year research program to evaluate the feasability of the cultivation of hemp (Cannabis sativa L., Cannabinaceae) as a raw material source for the paper industry was finalized. Three aspects, primary production, pulp processing, and paper production, based on an integrated chain model from farm to factory were studied (See Figure 1).
This research program was part of a broader business concept study as a basis for starting a commercial hemp pulp processing unit. In this paper a summary of the research results will be presented. For more detailed information see van Berlo (1993).
Since 1986, a collection of commercial available germplasm including old cultivars and wild material was initiated. The research objectives which culivar was best suited for paperpulp production under Dutch condition. The stem composition and content of cannabinoids were used as the selection criteria.
Based on earlier studies it was concluded that a late flowering resulted in a higher yield. In field trials carried out in successive years, several cultivars (Kompolti Hybrid TC, Kompolti Sárgaszáru, Kompolti Hyper Elite, Kozuhara, and Fedrina 74) were evaluated. “Kompolti Sárgaszáru” and “Kompolti Hyper Elite” proved to be the best suited in regards to yield and fiber quality. Yield in dry matter varied between 11 t/ha for peat soil and 17 t/ha for clay soils. The bast fiber content varied between 12% and 35%, the woody core varied between 50% and 75%. The total of the two fractions remained more or less constant at 85% of the stem dry substance. Cannabinoids content of the different accessions varied between 0.06% and 0.22% for THC, in comparison with 10% THC for drug types.
Growth trials were carried out at three different locations (peat soil, sandy clay, and heavy clay) in a three year period (See Figure 2). Hemp gives the best results when cultivated on light soil at a soil pH of at least 5. Fertilization of 12 kg N, 15 kg K²O and 4 kg P²O5 is needed per ton of dry matter. Seeding should start from mid Apr., as soon as soil conditions make it possible. On heavy clay soils compaction must be prevented as this severely hinders the growth. Hemp can be grown as a row crop with optimum distance 12.5 cm between the rows and a plant density of 900,000 plants/ha. At a seed weight of 19 mg and an emergence of 85% a quantity of 20.1 kg seed/ha is required for planting.
No herbicides are necessary as the crop suppresses weeds. On weed infested soils, plant density should be relatively higher and the row distance less. No major diseases have been identified, however during wet seasons, Sclerotinia and Botrytis, result in lower yields. Spraying with fungicides had no effect.
Harvesting / Postharvest Treatment
The best harvest time is from the beginning to mid Sept. depending on the cultivar.
Harvesting and postharvest treatment with existing equipment is a decisive factor in the production of hemp pulp. If the farmer can carry out most of the needed activities with already available equipment, the direct costs will be low thereby promoting a succesful adoption of this crop. The following harvesting techniques were studied: topping, cutting, windrowing, chopping, press baling, and ensilage. In topping, most of the leaves and flowers from the top of the plant are removed prior to harvesting. The cutting has to be done in such a way that as little sand as possible is picked up, while maintaining optimum yield. It is best carried out with a so called row distance independant cutter bar in front of a forage harvester.
After chopping the hemp can be ensiled at the farm. To optimize the processing of hemp into pulp over the year drying and ensilage was studied as a conservation storage technique. Ensilage is appreciable cheaper than drying and more reliable. The paper quality of ensiled hemp is slightly less than of the dried hemp, but still of acceptable quality.
The hemp stem consist of about 65% corefibers and 35% bast. The corefiber consist of 40% cellulose, 24% hemicellulose and 22% lignin, the length is 5.5 mm. The bastfiber is 75% cellulose, 9% hemicellulose and 5% lignin with a length of average 20 mm. As the differences between these two fractions merit a separation into two separate raw materials, research was carried out into this aspect.
Separation can be carried out before of after storage at farm level or at the pulping facility. As harvest takes place in Sept., a rather wet period, dry separation was discarded. Two options were selected, sieving and flotation. Flotation proved to be a feasible technique with a selectivity of more than 90% for bastfiber.
Four pulping processes were investigated for hemp (See Table 1). Depending on the end application of the pulp, thermomechanical pulping (TMP) of the whole stem is possible. A more optimal result was reached when the core fraction from the earlier in core and bast fraction separated stem was thermomechanically pulped and milled in a refiner and the bast fraction, together with an alkaline (pre)treatment, was extruded in a double screw extruder (See Figure 3, Figure 4). Extrusion has the advantage of 30% less energy consumption, while the length of the fiber was better controlled by fine tuning of the extrusion conditions. A patent was filed for this process by ATO-DLO.
Table 1. Comparison of four pulping processes investigated for hemp.
|Pulping process||Temp. (°C)||Time (h)||
|Akaline TMP (CTMP)z||120||Extruder <4% NaOH|
|Organosolv||195||0.5||9:1||Ethanol/water = 1.2|
zCTMP = chemithermomechanical pulping.
In most cases, especially for short lived applications, there is no lignin removal. The desired color of the paper can be adjusted by bleaching with peroxide (See Figure 5); when pure cellulose is the end product, an ozone treatment is possible.
An important factor to be considered in pulping (See Table 1) is the cost of waste water treatment. In waste water treatment, biological oxygen demand, aquatic toxicity, and the color of the water are important factors. In principle the waste water contains easily degradable components such as sugars, fatty acids, and alcohols while lignin an almost non-degradable component is responsible for the color. A combined anaerobic-aerobic treatment with an upfront dilution process to remove the resins, effectively solves the problems.
Different types of paper were produced from the pulped hemp. The traditional applications for hemp papers vary from cigarette paper to art papers and represent a total volume of about 120,000 t/year worldwide, making up about 0.05% of the world pulp volume. However, for these applications a chemical pulping is necessary. For wider bulk applications, such as printing and writing papers and corrugated board, cost is the decisive factor (See Table 2).
Table 2. Cost of chemical and chemomechanical processes for hemp vs. chemical processing for softwoods.
The properties of chemomechanical core and bast pulp compare very favorable with the traditional wood pulps (See Table 3, Table 4).
Table 3. Chemomechanical characteristics of bast pulp vs wood pulps for printing and writing grade blends.
|Fiber||Beating degree (SR)z||Density (kg/m3)||Tensile strength (Nm/g)||Burst strength (kPa.m2/g)||Tear strength (mN.m2/g)||Brightness (ISO)z||Opacity (ISO)|
zSR = standard reflection, ISO = international standardization organization.
Table 4. Chemomechanical characteristics of core pulp vs wood pulps for printing and writing grade blends.
|Fiber||Beating degree (SR)||Density (kg/m3)||Tensile strength (Nm/g)||Burst strength (kPa.m2/g)||Tear strength (mN.m2/g)||Brightness (ISO)||Opacity (ISO)|
|Hemp core refiner||59||714||59||2.7||2.8||78||75|
We conclude that hemp pulp is very competetive with the usual pulps when printing and writing paper application is taken into consideration (See Table 5).
Table 5. Economics of hemp assuming high yield, small module scale, and no recovery system. Investment is calculated for a 5,000 ton/year pilotplant.
|Operation||Annual cost/tonne (US$)|
zNon-bleached standard kraftpulp, price is used as a reference.
The building of a pilot unit with the capacity of about 5,000 t/year for upscaling studies is now under consideration.
van Berlo, J.M. Dec. 1993. Papier uit hennep van Nederlandse grond. ATO/DLO, Wageningen.
*Special thanks to Mrs. M.J.J.M. van Kemenade for ATO-DLO at Wageningen, the Netherlands for making available all relevant information.