Sorghum cultivar sweetens prospects for biofuels



Biofuels are an energy source which can be considered renewable and carbon neutral in the annual to decadic time scale. Until recently, the biofuels industry in South Africa was based on maize and jatropha as feedstock. The situation changed considerably and biofuels are limited to non-food and non-toxic crops. A cultivar of sorghum that has been around since the 1800s shows great promise as a non-food feedstock.

Initial expectations for the biofuels industry in South Africa were based heavily on specific crops as feedstock, but the main contenders, maize and jatropha, have been removed from the sector. Maize is banned as an energy source as it is a staple food. Jatropha, which was once a leading contender, is also banned because it leaves behind a toxic seedcake. Poisoning from jatropha has been reported in several countries. The global maize price increases and the shortage of basic food stuffs in Mexico, which were directly linked to biofuels investments, has influenced South Africa to exclude the use of maize in its initial stages of biofuels development [3].

Fig. 1: Sweet sorghum is a multipurpose crop [7].

Biofuel production

Biofuels are produced from biomass. The biofuels may be in solid (vegetable wastes, and a fraction of the urban and industrial wastes) liquid (bio-alcohols and biodiesel) or gaseous (biogas and hydrogen) form [1].

First-generation biofuels are produced directly from cereal crops (e.g. wheat, maize), oil crops (e.g. rape seed, palm oil, sunflower and soya oil) and sugar crops. Biodiesel is a first generation biofuel. Other first-generation biofuels are bioethanol, biogas and vegetable oils.

Second-generation biofuels are produced from lignocellulosic materials such as crop residues. The syngas produced by gasification of biomass is used as precursor of second generation biofuels like biomass-to-liquid (BTL), bio-dimethylether/methanol, biosynthetic natural gas and biohydrogen.

Bio-oil, produced by pyrolysis of biomass, and cellulosic ethanol are also second-generation biofuels. Some studies suggest that the manufacture of cellulosic biofuels from crop residue uses more energy than what is obtained from the residue, and that removal of crop residue from lands depletes the soil. The large scale production of biofuels is currently based on first generation fuels, and second generation is largely centred on small plant for own use by farmers and industry.

Industrial strategy

The biofuel industry is driven by the “Biofuels industrial strategy of South Africa” (BIS) developed by the Department of Energy. The original document favoured sugar cane and sugar beet in order to produce bioethanol and sunflower, canola and soya beans for purposes of producing biodiesel. The draft “position paper on the South African biofuels regulatory frame work” published in January 2014, recommended that, in view of the need to expedite the first phase of the implementation of the BIS, grain sorghum and soya beans should be the only feedstocks used for the manufacture of bioethanol and biodiesel respectively, and the industry has followed this lead.

Grain sorghum used to be cultivated extensively, but production declined as the local market demand for sorghum decreased. However, large parts of South Africa are well-suited to sorghum cultivation (more than for maize) and its drought-resistant properties make it an attractive crop, for example, as a feedstock for local ethanol manufacture.

The maize farmers’ project in Bothaville was abandoned when the government banned the use of maize and wheat as feedstocks for biofuels because of a possible threat to food security. Thereafter, sorghum gained favour, because it is a drought-resistant, non-water-intensive crop and because it has previously been a crop produced extensively by black farmers, and the project was resurrected as a sorghum-based plant. Ten years ago, South Africa produced about 700 000 t of sorghum a year, but because of the continuous decline in sorghum-beer consumption, it now produces about 80 000 t .

Table 1: Comparison of the utilisable yield (sugar and ethanol) for sweet sorghum at Hatfield and Ukulinga experimental farms obtained from the 2010/11 and 2011/12 growing seasons [6].
Sugar yield t/ha Ethanol yield l/ha
Hatfield (UPE)
2010/11 4,2 2059
2011/12 5,8 2828
Ukulinga (UKZN)
2010/11 2,2 1075
2011/12 5,3 2606

Sweet sorghum

Grain sorghum is the major sorghum crop grown in many countries including South Africa. Sweet sorghum, a variant, has been recognised globally as a potential biofuel crop for ethanol production, and has the same or better growing characteristics.

Sweet sorghum (also called Sorgo) is an indigenous African plant belonging to the family Sorghum bicolor. Sweet sorghum produces grain, which is harvested for human consumption and contains sweet juice in the stalk. After harvest, the stalks are squeezed for the sweet juice, which can be turned into sugar or fermented into ethanol. The remaining stalk material can be used as animal feed or pre-treated, hydrolysed and fermented to produce ethanol. Sweet sorghum is considered as one of the most promising crops for the production of ethanol at low cost [5]. Ripe sweet sorghum typically consists of approximately 75% cane, 10% leaves, 5% seeds and 10% roots by weight [5].

Unlike other bioenergy feedstocks, sweet sorghum produces food products and other valuable by-products (from its grain, stalks and leaves), thus eliminating the “food vs. fuel” issue so often raised by biofuel critics. Sweet sorghum can be used for food, fuel, fodder, fibre and fertiliser production. Hence, sweet sorghum is a multi-purpose crop which can be cultivated for the simultaneous production of grain from its panicle (for food, mainly flat breads and porridges); sugary juice from its stalk (for making syrup or ethanol); and bagasse and green leaves (as an excellent fodder for animals, or as organic fertiliser, or for paper.

As a C4 crop with a high level of directly fermentable stem sugars and the ability to produce high biomass under adverse conditions, sweet sorghum is considered an ideal biofuel crop for the first and second-generation bioethanol production [6].

Crop yield compared to grain sorghum

Grain sorghum is grown primarily for grain production, so a comparison is difficult. Sweet sorghum may grow up to 6 m tall and produce significantly higher biomass yields compared to grain sorghum. Stems of sweet sorghum are thicker and fleshier than the grain varieties. Usually, grain yield in sweet sorghum is very low and grains are not suitable for use as human food.

Ethanol production potential

The sugar concentration in sweet sorghum stalks is measured in Brix units, which represents the percentage of soluble sugars. One-degree Brix is equal to 1 g of sugar per 100 g of juice. The Brix content varies in different varieties and also depends on the environmental conditions, internode position, time of the year, and stage of harvesting. Sweet sorghum can accumulate juice up to 78% of the total biomass, whereas the Brix content of sweet sorghum has been estimated to range from 14 to 23%. The sugars in sweet sorghum stalks mainly comprise sucrose (~75%) with some amount (~2.6%) of fructose and glucose. Soluble sugars in the form of glucose, fructose, and sucrose in sweet sorghum are readily fermentable [7].

Ethanol yields vary depending on seasonal variations. In a study conducted at UKZN and UPE, yields ranged from 1075 l/ha to 2828 l /ha over two seasons (see Table 2) [6].

Energy balance

Producing crops requires energy, and converting biomass into biofuel is also a factor. Crops produced for food do not consider energy as an input, there is no bottom line. Crops for biofuels however do have an energy balance to consider. The crop should produce a significant net gain in energy. Concerns have been expressed that certain crops and processes would consume more energy than the biofuel would produce. According to US Department of Agriculture, the ratio of energy invested to energy obtained during biofuel extraction from sweet sorghum is estimated to be 1:8 [4].

Water usage efficiency (WUE)

The results from a study carried out by Mengistu et al, [6] confirm that sweet sorghum has high WUE under a range of climatic and water supply conditions, and is therefore a suitable candidate feedstock for biofuel production. South Africa is a water-scarce country and, in terms of the National Water Act of 1998, it is necessary to assess the potential water use and likely impact of potential biofuel feedstock production on water resources. The South African Department of Water Affairs (now the Department of Water and Sanitation) has also decided not to support the production of biofuels under irrigation. Biofuel feedstocks which use less water, such as sweet sorghum, are likely to have a much lower impact on water resources. It is likely that sweet sorghum can be grown in rainfed areas and on marginal lands due to its adaptability to drought and other environmental conditions and can thus open up opportunities for small-scale farmers as it exhibits a higher rural development potential.

Development

As with any commercial crops, cultivars have been developed which maximise sugar production, and minimise other aspects, (non-flowering for instance). There is however a limited availability of certified sweet sorghum seed worldwide [6], but this should improve as more biofuel crops are planted.

Conclusion

The biofuel industry strategy had the aim of development of a modest biofuels sector, which targets under-utilised land in former homelands, and should have a minimal impact on both food security and prices. Currently, 14% of arable land is under-utilised and is in areas which lack the market access biofuels plants will provide and infrastructure that agricultural and infrastructural support programmes should provide [2].

Sweet sorghum grows under the same conditions as grain sorghum, and the substitution of crops or expansion of lands currently under grain sorghum to sweet sorghum for ethanol production should be possible, enabling the goals of the strategy to be met.

References

[1] A Sales: “Production of biodiesel from sunflower oil and ethanol by base catalysed transesterification”, http://www.diva-portal.org/smash/get/diva2:443295/fulltext01.pdf
[2] M Mamkeli: “Biofuels in South Africa”, Environment Ensight, 20 March 2014.
[3] DoE: “Biofuels Industrial Strategy of the Republic of South Africa”, Department of Minerals and Energy, December 2007.
[5] R Mutepe: “Ethanol production from sweet sorghum”, http://www.crses.sun.ac.za/files/services/conferences/annual-student-symposium/daphney-mutepe-paper.pdf
[6] M Mengistu: “A preliminary investigation of the water use efficiency of sweet sorghum for biofuel in South Africa”, Water SA, Vol. 42 No. 1, January 2016.
[7] S Matur, et al: “Sweet sorghum as biofuel feedstock: recent advances and available resources”, Biotechnol biofuels, 8 June 2017.

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