Village Level Bioenergy System Based on Sweet Sorghum

The sustainable development of large areas of the world is today one of the greatest challenges. The living conditions of large part of the world population are greatly affected by the availability of energy, together food and water. As most developing countries have limited fossil fuel indigenous resources, import of energy fills the increasing difference between demand and production. Consequently, nowadays a large number of motivations and a growing interest exist for the development of renewable sources of energy, like energy systems based on biomass, in particular energy crops.

Following is a possible scheme called Bioenergy Village Complex. It is based on Sweet Sorghum crop and aimed at providing rural villages in developing countries sufficient energy, fuel, food and feed.

The Bioenergy Rural Village Complex is based on the consideration that bioenergy can support the basic needs of rural populations at village scale. In fact, these populations need not only energy, but also a set of other products that are essential to life, such as food, feed for the animals, etc. The Bioenergy Complex, based on sweet sorghum crop, aims at providing rural villages with the above mentioned products, essential for the sustainable development of rural areas of the world. The approach is based on the simultaneous production of a large number of biomass-based products by means of an integrated system. The technologies used will therefore be deeply interconnected: the scheme makes use of the biomass resource (sweet sorghum) in all its parts, thus reducing at the minimum the amount of produced waste. These several co-products will allow a significant improvement in the economics of the overall system, as well as the integration of food, feed and energy production.

A biomass integrated complex for a typical population of approx. 3,000 people is constituted by several subsystems, such as:
– Sweet sorghum plantation
– biomass pre-treatment unit
– cogeneration unit
– bioethanol microdistillery
– food products (cereals, milk, meat, sugar)
– village energy network connection (power, cooking fuel, bioethanol for agriculture machinery)

The bioenergy complex is based on a plantation of about 400 ha (harvested twice a year) of a dedicated herbaceous, very versatile sugar-starch-lignocellulosic crop (a specific variety of sweet-sorghum). The integrated bioenergy complex, through the production of several different co-products, will be able to provide a sufficient amount of:
– electric power for the village
– heat for biomass drying, for internal needs (sugar juice extraction and concentration, for ethanol distillation) and for other uses like: heating, green-houses, fisheries, etc.
– ethanol for numerous village uses (cooking, tractors, light) and for sale
– food (cereals, liquid sugar, milk, meat) for 3,000 inhabitants
– animal feed for cows, pigs, chickens, fishes, etc.
– other co-products (organic fertilisers, bagasse, etc.)
– other industrial biomass derived products – if needed – like: pulp for paper, charcoal, activated charcoal for drinking water, etc.

Moreover, the system will also provide numerous diversified jobs to the village inhabitants. Sweet sorghum has been selected as the main biomass resource for the following reasons:
– has a very high yield (up to 80 ton/ha of fresh matter) in terms of starch, sugar, ligno-cellulosic component, and today can be considered as the most versatile and promising energy crop.
– it can be grown in a wide range of latitudes (tropical, sub-tropical, temperate zones) as well as on poor quality soil.
– the required N2 inputs are rather low (approx. 100 kg/ha per year depending on agronomic practices), reducing the risk of water contamination. In case of land availability, rotations with leguminous crops could eliminate the need for fertilisers nitrogen inputs.
– it requires very low fertiliser and water input. The water requirement for growing Sweet Sorghum is one of the lowest : about 1/2 of the requirement for corn and about 1/3 of the requirement for sugar cane.
– sweet sorghum plantations all over the world (1995) reached 45 Mha (75 Mton/y of grains, representing the 4th world cereal). Provided that the total accumulated daily temperature during the 120-150 days cycle reaches 2,600 – 4,500 °C.

Figure 1: Scheme of the Integrated Bioenergy Complex

The integrated bioenergy complex, based on the exploitation of sweet sorghum for the production of bio-ethanol and other energy/industrial commodities, after its high economic viability has been demonstrated, offers a new sustainable path for production of bio-ethanol, which is considered a strategic fuel for the transport sector.

This integrated project, once implemented on a large-scale, or in a repetitive way, can have important economic impact in terms of reducing the production costs of ethanol. Utilisation of particular varieties of Sweet Sorghum as dedicated energy crops is, in this respect, of great importance because the process “integration” comes from the opportunity of converting the several components of the plant: starch, sugar and lignocellulosic. This very important feature distinguishes Sweet Sorghum from other grain crops (corn, wheat, barley) currently utilised for bio-ethanol production. In fact, well-known bioethanol crops (i.e. sugar beet – corn – sugar cane) are cultivated only to produce alcohol. Sweet sorghum plants, on the contrary, can provide not only sugar and grains for bioethanol production, but also large amounts of bagasse, as fuel to generate steam, electricity necessary for the bio-ethanol production process, syngas, hydrogen, etc.,

Sweet Sorghum is an “admirable high energy crop.” Among the reasons for this characteristics are:
– It has two times the photosynthetic efficiency in comparison with sugar beet, soybean, wheat etc., (its photorespiration can almost not be measured).
– It can be grown in all locations going from tropical to very northern regions (like Manchuria).
– The quantity of water needed by Sweet sorghum is only 1/3 of that needed by sugar cane (below 200 kg of water per kg of biomass).
– Very resistant to droughts (is called also “camel crop”;), to flooding, to salinity alkaline conditions.
– Growth period of Sweet sorghum is short (4-5 months) in comparison with sugar cane (8-24).
– Sweet sorghum for its high yield in grain, sugar, lignocellulosic must be considered perhaps the most promising “food – feed – energy” crop for twenty-first century.

The complex makes use of several different technologies, for biomass collection, treatment and conversion into energy and liquid biofuel. The most important are:
– Harvesting
– Biomass Pre-treatment (grain separation, cane crushing, sugar juice extraction, pelletisation)
– Cogeneration (small scale) and, in case, carbonisation
– Bio-ethanol production (fermentation and distillation)

Based on a plantation of 400 ha, to be harvested twice per year, the following productivity (average) value per year can be considered:
– Bagasse (dry): 15 Ton/ha
– Grains: 5 Ton/ha
– Sugar: 7 Ton/ha
– Leaves: 1.88 Ton/ha

Cane crushing and Sugar Juice extraction
The fresh cane will be processed in a cane crushing and sugar juice mechanical extraction unit for its conversion into bioethanol. After this operation, about 45,000 ton/year of sugar juice (5,600 tons of sugar) and about 24,000 ton/year of bagasse (50% humidity) will be available. Part of the sugar juice could be utilised as liquid sugar (high concentration), the remaining part for Ethanol production.

Pelletisation technology
New generation units have been recently developed and made available to commercialisation. Typical pellet dimensions are in the range of 6-12 mm diameter and 10-40 mm length. These machines proved to be able to treat efficiently various types of feedstock, not only sawdust and wood chips but also agricultural residues, wastes, energy crops, etc. Pelletisation represents a key technology in the above mentioned bioenergy scheme, since it allows a significant reduction in storage costs as well as a good quality of the fuel. Moreover, some interesting options are possible, as mixing the biomass with limited amounts of other feedstock as organic wastes, etc..

Cogeneration Plant
The biomass pellets are burnt in a combustion chamber to produce steam at 350 °C temperature (25 bar or higher); the steam expands in a reciprocating engine coupled with an alternator powering the electricity grid.

Carbonisation
Bagasse pellets submitted to a clean efficient carbonisation process are converted directly into charcoal pellets (for cooking/heating) without the use of any binder-agglomerating compound. Furthermore, charcoal pellets can also be eventually converted into activated charcoal (medium quality). One hectare could produce 2 tons of activated charcoal, which are sufficient to purify, all the year round, drinking water for 10,000 people.

Bioethanol production
This technology offers the fast conversion of Sweet Sorghum sucrose into ethanol. It is based on a fixed-yeast fluidised bed reactor, where gelatinous particles having uniform diameter and good mechanical properties support the yeast for the fermentation. This technology could significantly reduce the time necessary for fermenting the Sweet Sorghum juice and, therefore, the investment costs. The cogeneration unit provides the heat necessary for distillation. The bioethanol production is divided in two main stages:

Fermentation. The fermentation unit is completed with one mash buffer tank, two small yeast propagation tanks, two fermentation tanks and ancillaries. In this plant a side stream of mash is fed to the yeast propagation tank and mixed with fresh yeast available on the market, nutrients, and process water, to produce sufficient yeast cells to balance process losses. The enriched yeast suspension is then pumped to the fermentation tanks.

Distillation. The beer is fed to the distillation unit to strip and concentrate the alcohol to the required level. The distillation unit is complete with one column and with all ancillaries. This process uses a special technique to recover most of the residual ethanol remaining in the whole stillage when it leaves the column as waste, preventing valuable ethanol from being either blown into the atmosphere or discharged in the water.

Modern bioenergy complexes for rural villages based on Sweet Sorghum plantations, can supply the essential needs of a population (food, animal feed, energy) to produce extra-income from the surplus sale of several co-products for an economic sustainable activity.

Source: Giuliano Grassi, EUBIA (European Biomass Industry Association), Rond Point Schuman 6, 1040 Brussels, Belgium Tel +32-2-2828420, Fax +32-2-2828424 eMail: eubia@eubia.org