Advantages of sweet sorghum for bioethanol production

by   Alessia Vecchiet

July 14, 2010

The growing request of renewable sources for the substitution of fossil fuels in the transport sector have primed the necessity of widening the energy crops for ethanol production view.

1. Introduction

The growing request of renewable sources for the substitution of fossil fuels in the transport sector, against the 10% objective introduced by the Climate-Energy Package and the concomitant European indication in favour of the environmental sustainability of biofuels, have primed the necessity of widening the energy crops for ethanol production view, waiting for the availability of the second generation bioethanol which will allow a valorization of the residues.

In this scenario sweet sorghum appears as very promising, thanks to some its own peculiarities that suggest a high sustainability for the produced bioethanol:

  • good productive yields vs moderate cultivation inputs: the bioethanol yield from the sugars contained in the stalks is comparable with those deriving from the use of cereal grain (corn, wheat) but with the use of lower cultivation inputs. In fact the biomass yield is 15-18 t/ha expressed as dry matter, with modest water requirements and fertilization;
  • positive energy balance: wide bagasse and stillage availability (i.e. by-products from sugar extraction and ethanol distillation respectively), recoverable as direct energetic products (e.g. biofuel) or indirect energy products (e.g. conversion into biogas), corresponds to a positive output/input balance.

The interests on sweet sorghum results increased, moreover, by the opportunity of carving a role in the bioethanol pathway that this crop offers to the agricultural sector – generally reserved just to the industrial sector – thanks to some aspects bound to the biomass processing:

  • high perishability during the transport:the high water and sugar contents (75-80% and 400-500 kg/t d.m. respectively) at the harvesting time impose a reduction of the distance between the sites destined to the agronomic production and the plant site; so its planning requires a contextualization that should be consider also the supplying capacity at local level. In this hypothesis farmers become suppliers of the raw material;
  • reduced technological content of the processing: the sweet sorghum sugars conversion requests just their extraction before fermentation; the simplicity of the plants, higher than the plants which process cereal grain, allows a reduction of the scale and the proposal of a decentralized bioethanol production. In this hypothesis the opportunity for the farmer’s cooperatives of controlling the primary production transformation phase and for the farmers (as members of the same associations) of participating in the increment of the added value of the products, is offered.

For all these reasons, the 1st generation ethanol produced from sweet sorghum presents a high environmental, economic and energetic sustainability: the ascribed GHGs saving is 70-71%, the technical simplicity of the processing and the exploitation of by-products “EU ethanol market is controlled by big industrial groups and large agricultural cooperatives of the sugar” guarantee the economic viability also for decentralised small-medium plants (max 15,000 t/y) and its energy ratio is 1.7-7.3 depending on the by-products recovery way which is adopted.

In the current situation the EU ethanol market is controlled by big industrial groups and large agricultural cooperatives of the sugar and alcohol industries and mainly cereals are processed in big plants (100,000-200,000 t/y). This situation is due to some relevant barriers: economic, logistical, ecological, environmental, social and dissemination barrier.

It is very important a contribution to change the current situation concerning the raw material diversification, decentralisation and sustainability of 1st generation ethanol processing from sweet sorghum, which can be grown in the southern regions of the EU. At the moment the ethanol chain is not taken in account because of the absence of know-how about its potentialities.

2. Increased awareness about sweet sorghum as ethanol crop

Sorghum is an annual crop with a lot of varieties (at the present estimated in 4,600 approximately), and some of theme have a very high sugar content in the stems. Its bioenergy applications are numerous: sweet sorghum can be used to produce ethanol, but alternatively also biogas through anaerobic digestion, fiber sorghum (pelletized or not) can supply CHP plants, grain sorghum can be employed for food, feed and energy needs of small isolated communities. Grain sorghum is not so suitable for the ethanol production, because in these varieties the sugar are prevalently polymerised to starch, which then requires hydrolysis before the alcoholic fermentation.

This plant can be grown in wide world regions, as tropical, subtropical and temperate areas: it requires total accumulated daily temperature during the growing cycle of 2,600-4,600°C and its minimal germination temperature is 8-10°C. It is suitable for poor soils, which are characterised by different kind of texture (heavy clay or high sand content), pH range (4.5-8.5) and salinity; consequently it can be grown in marginal lands and allows the reduction of the land competition with the food chain. For all of these reasons many seeds companies and agricultural research institutes in EU, but especially in third countries (i.e. India, US, Philippines, Costa Rica), are selecting and testing the most interesting varieties, in order to propose a product more suitable for different regions and for various applications.

The technological content of the sweet sorghum processing plant is quite simple because only the sugar extraction is required before the alcoholic fermentation, unlike the cereal grains (e.g. corn, wheat, barley) which requires additional stages for the conversion to ethanol. For this reason, sweet sorghum is suitable for the processing in decentralised small-medium plants (max 15,000 t/y as capacity).

3. Sweet sorghum characteristics and advantages

Sweet sorghum belongs to the Sorghum bicolor L. Moench species (Andropogoneae of the Poaceae family). It has an ethanol productivity of 3.9-4.7 t/ha, higher than the other sorghum varieties.

The world’s largest ethanol producers are Brazil and USA, which together are accountable for more than 65 % of global ethanol production. Fuel ethanol is produced in Brazil mainly from sugar cane and in the USA from corn. Table 1 shows the conversion yield of sweet sorghum compared with sugar cane, sugar beet, corn and wheat.

Sweet sorghum has a high fermentable sugar content, in fact the fermentable sugars yield corresponds to 8-10 tons/ha. The total sugar production from sweet sorghum is about 31.4 % of dry matter and the sugars consist of 85% sucrose, 9% glucose and 6% fructose on average.

It easily adapts to various climatic conditions and it has the ability of growing throughout the whole year and requires less fertilizers and water if compared with other conventional crops, and the total amount of energy required for production and harvesting is lower too. Table 2 shows that sugar cane is the best source for bioethanol production because it has an environmental advantage through its good energy balance; sweet sorghum is on the second place, but it can be cultivated in EU because of low water and low fertilizer requirements (the cultivation is possible on marginal lands not used for food cultivations and also in dry areas, without necessitating of irrigation).

The bioethanol production costs rely on the raw materials themselves, on the prices of those materials, on the production process, on the extent of undertaken refining and the supplementary utilization of by-products; the costs also depend on the production scale and the produced wastes.

Fermentation technology for sugar juice is well known from the sugar cane utilization and it is a cheaper technology than the technology which employs starch material.

Sweet sorghum can be grown also in many kinds of soils (as texture, pH, salinity), included marginal areas, and requires low inputs: the referential biomass yield (15-20t/ha d.m.) is obtained without irrigation (250-310 kg/kg d.m. considering rainfall and eventual irrigation reinforcement), and with a low fertilisation (N 119 kg/ha; P2O5 24 kg/ha; K2O 10.8 kg/ha). For the cultivation, the machines used for corn can be applied, so that the re-conversion is not expensive for the farmers; the harvesting could be carried out with the mower-shredder-loader machine.

Sweet sorghum is a C-4 crop with a very high photosynthetic rate represented by the high capacity of absorbing the CO2 (~45 t CO2/ha per cycle).

Sugars from sweet sorghum can be converted to ethanol directly because they do not require any pre-treatment before fermentation

Sugars from sweet sorghum can be converted to ethanol directly because they do not require any pre-treatment before fermentation. The pH is adjusted to about 4–5 with inorganic acid and the sweet juice is inoculated with yeast: the fermentation is carried out at 20–32 °C for about 1–3 days. The fermented matter, which typically contains about 6–10 % w/w ethanol, is distilled in the purification section of the plant. Ethanol is then separated in the distillations columns and the use of plants with multiple effect allows the reduction of the thermal consumptions. Finally it is dehydrated (azeotropic dehydration): ethanol which comes out the distillation process contains a residual water of 4,5% v/v that can be removed through molecular sieves made of zeolite. The ethanol yield is 87 l/ton of processed sweet juice. The efficiency (expressed as the ratio between the process yield and the theoretical yield) is about 94 %.

Sweet sorghum is a seasonal crop, therefore a biomass storage system must be considered for an all year round production. Sweet sorghum juice and stalks could be both stored: the first one foresees the seasonal harvesting and processing of the stalks, storing then the juice to process it during all the rest of the year, the second one foresees the storage of the harvested biomass with an ensiling process.

To preserve the sugar juice an evaporation performed with an external energy source is required in order to reach a high concentrated juice.

Another solution is the ensiling of the biomass. This process allows a more complete hydrolysis (with a considerable energy saving) if compared to the common pretreatments which require extensive heating before and during hydrolysis. More importantly, the ensilage technique prolongs the harvested sorghum availability for the fermentation process. The ensiled sweet sorghum conserves its fermentable sugar component and the process itself works as a pretreatment for the lignocellulose conversion in case of decision of utilize bagasse for second generation ethanol.

4. By-products exploitation

The possibility of destine for energy recovery the high amount of by-products of the sweet sorghum processing differentiate this crop from those actually employed in the bioethanol chain and allows to bioethanol of assuming widely positive values for the energy balance with already mature technologies. Among the options studied till now, the combustion in cogenerative units of the exsiccated bagasse and of biogas obtained by the anaerobic digestion are the most promising solutions. These valorization ways allow the covering of thermal and electrical internal requirements (i.e. solid-liquid fermentation, fermentation, distillation, dehydration, exsiccation of bagasse, anaerobic digestion in mesophilic conditions of the stillage, alimentation of cogenerative units) giving a surplus most of all of electric energy (80% of the produced energy) but also of heating (40% of the produced one) that can be sold, contributing to the profits.

5. Energy ratio and environmental sustainability

The available data about the EU model for the ethanol produced from sweet sorghum suggest an output/input ratio of 1.7 -7.3, depending on the strategy chosen for the by-products exploitation, a high GHGs saving in accordance with the RES Directive and a low water footprint, if all the crop components will be used.

The main environmental advantage relative to the use of bioethanol in substitution of gasoline and/or the use as bio-ETBE instead of fossil antiknocks is the abatement of the transport sector contribution to the GHG emissions. For bioethanol produced by sweet sorghum this assertion assumes an absolute validity because the saving calculated with the methodology indicated by EU is 71%, one of the most virtuous values among the attainable ones.

The use of sweet sorghum for bioethanol production conciliates the production of sustainable bioethanol applying already mature technologies with a higher involvement of the agricultural sector in the pathway.

6. Small-medium scale bioethanol production plant from sweet sorghum

Sweet sorghum could favour the diffusion of the bioethanol pathway in the local agricultural sector, because the creation of small-medium plants involves the plant building near the fields where the biomass is harvested (max 50 km approximately). This choice allows an increase of sustainability of the bioethanol production, because the impacts of the transport phases are reduced.

The size of small-medium plants does not exceed the 15,000 t/y of ethanol.

There are many criteria to select a suitable bioethanol production technology, such as plant scale, investment and operation cost, management operations, and conversion yield, which have a reflecting effect on the production cost. Another important criteria is the energy balance that shows the efficiency of processing energy used.

For the concrete expression of these results in real events that can act as incentive for their wider diffusion some steps are necessary.

First of all a discussion of this opportunity with all chain actors that can be translated in a desirable perfecting of it, is required.

Secondly a higher investment on this crop by some actors (seed suppliers and farmers) whose role is that of contributing to give the platform to start this initiatives, is important too.


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