WO2008029173A2

WO2008029173A2 - Cultivation of sweet sorghum for bioethanol production

Info

Publication number: WO2008029173A2
Application number: PCT/GB2007/003411
Authority: WO
Inventors: Arsen Badalov
Original assignee: Nasamax Limited
Priority date: 2006-09-08
Filing date: 2007-09-10
Publication date: 2008-03-13
Also published as: WO2008029173A3; GB2441593A; GB0617763D0

Abstract

A method of producing bioethanol from sweet sorghum plants is disclosed. The method comprises planting sweet sorghum plants sequentially on a plurality of plots of land 4a-4h such that when a plot of land 4h is being planted the plants in another plot of land 4a are ready for harvesting. The method of the present invention ensures that there is a continual fresh supply of plant material to the bioethanol production plant (5) throughout the year.

Description

Cultivation of Sweet Sorghum for Bioethanol Production

The present invention describes an efficient and productive method of cultivating sweet sorghum (sorghum bicolor) to provide the sugar and lignocellulosic feedstock required by a fully integrated bioethanol production plant. The method of cultivation ensures that there is a continual supply of fresh juice and lignocellulose material available each day to meet the feedstock requirements of the ethanol production plant, and conversion processes are used in the ethanol plant which produce effluent water rich in plant nutrients so that wastewater from the plant can be used to promote the growth of more sweet sorghum.

Natural ethanol is traditionally produced by either the fermentation of sugar derived from sugar bearing plants br the fermentation of glucose produced by the hydrolysis of starch extracted from grain bearing plants. Ethanol is a clean burning fuel and the bioethanol derived from cultivated sugar or grain bearing plants is becoming increasingly important as a source of renewable energy. Bioethanol has potential as a liquid fuel for both road transport and power generation applications, providing the bioethanol can be produced cost effectively and on a large industrial scale.

When bioethanol is used as transport fuel it can either be as neat alcohol or more likely it will be a blend of ethanol and a conventional petroleum based road fuel. Ethanol is a stable, nontoxic material that is reasonably safe to handle, and blended fuels containing ethanol can be stored and distributed in much the same way as petroleum fuels.

Sugar bearing plants grown to produce bioethanol include sugarcane and sugarbeet, and corn (or maize) is the main grain bearing plant currently used for bioethanol production. These particular plant species are primarily grown for food production and the sugar and cereal produced from the plants are traded commodities, and as such their price is subject to market demands, subsidies and quotas. Fluctuations in the availability and market price of sugar and cereals can significantly affect the price of the bioethanol produced from these materials. In addition, the use of valuable arable land to grow crops for energy instead of food can be a sensitive political issue.

The restricted yields from established energy plant species is currently a major constraint to the growth of bioethanol; for example sugarcane, sugarbeet and corn only produce one crop a year and this limits the amount of raw material feedstock that is available for conversion into bioethanol. Consequently to ensure that there is always a supply of feedstock readily available for the production of ethanol either large stocks of sugar or cereal have to be stored at the ethanol plant or feedstock has to be imported from different sources at various times of the year. Bioethanol can also be produced from the residual biomass of a plant by using an enzymatic hydrolysis process to convert the cellulosic material in the biomass into fermentable sugars. However, at the present time only the sugar or starch from sugarcane, sugarbeet and corn tends to be used for ethanol production because the residual baggasse from sugarcane and the residual pulp from sugarbeet is generally used as fuel to produce the energy requirements of the ethanol plant. The residual biomass from corn, which is known as stover, is normally ploughed back into the land to provide nutrients for the soil.

To produce the large volumes of bioethanol needed to meet future demands, it will become increasingly important that plant species containing a large amount of lignocellulose material as well as plentiful sugar or starch are grown to manufacture bioethanol. Preferably the plant should also be a species that is not normally used for food production so that the ethanol feedstock will be less prone to market price fluctuations. A plant species capable of growing in poor soils or on marginal land under various climatic conditions would also be beneficial because the plant could be grown as a dedicated energy crop on land not normally used for food production.

In this respect, sweet sorghum (sorghum bicolor) is a sugar bearing plant species that has significant advantages compared to the sugarcane or sugarbeet currently used for ethanol production. For example, sweet sorghum makes an ideal energy crop because it is not widely grown for food purposes; sweet sorghum is an unusual sugar bearing plant in that it can be grown in different climates; sweet sorghum is drought resistant and with adequate fertilisation it can be grown in various types of soil; sweet sorghum has a prolific growth rate and the cane contains large amounts of sugar and lignocellulosic material; finally the cellulosic material available in sweet sorghum is very suitable for enzymatic hydrolysis.

Sweet sorghum requires a minimum germination temperature of 7⁰C; a minimum growing temperature of 15⁰C; and an optimum average growing temperature of about 26°C. Sweet sorghum is drought resistant because the plant has a large fibrous root system that is very effective in absorbing water from the soil. Sweet sorghum is therefore a water efficient plant that responds well to controlled irrigation; for example field trials have shown that sweet sorghum requires less than 200 litres of water to produce one kilogram of dry matter, whereas some energy plant species need as much as 1000 litres of water. Consequently sweet sorghum is a particularly useful energy plant for climates that do not have regular or sustained rainfall during the normal growing cycle, providing local supplies of stored or aquifer water are readily available for irrigation purposes.

High growth rates can also be achieved with relatively low inputs of nitrogen and phosphate fertilisers, although sweet sorghum does need large amounts of potassium. The approximate amount of nutrients needed to grow sweet sorghum effectively and prolifically are given in Table 1.

Table 1 Nutrients Required to Effectively Grow Sweet Sorghum

Sweet sorghum is a unique sugar bearing plant in that it can be grown in tropical, subtropical, Mediterranean and temperate climates. For example, under tropical conditions fresh biomass yields of over 160 tonnes/hectare have been recorded; 150 tonnes/hectare or more under subtropical conditions; 120 tonnes/hectare under Mediterranean conditions; and up to 100 tonnes /hectare under temperate conditions.

Sweet sorghum also grows so quickly that under the right climatic conditions it is feasible to grow more than one crop a year. For example, cultivation trials carried out in subtropical Paraguay established that certain varieties of sweet sorghum were able to reach maturity in less than 120 days. As the climate in subtropical Paraguay is reasonably consistent throughout the year it is feasible to grow between two and three crops of sweet sorghum per annum, providing the plants are supplied with an adequate amount of water and fertiliser. Complex inter-related chemical processes are involved in an integrated bioethanol production plant and efficient plant operation is very dependent on a constant supply of sugar and lignocellulose feedstocks being readily available all year round so that the various processes are able to operate continually. This is difficult to achieve with traditional sugar bearing plant species because the plants are cultivated on an annual basis only and the plants ripen at specific times of the year. Consequently either very large stocks of locally produced cane, sugar or sugar juice have to be held at the ethanol production plant or raw materials have to be brought in from other sources at different times of the year to supplement supplies. Furthermore, although the baggasse from sugarcane and the pulp from sugarbeet could be used to produce ethanol by enzymatic hydrolysis, at the present time the residual biomass tends to be used as fuel to produce the heat and power requirements of the ethanol plant. In an integrated ethanol plant based on sugarcane or sugarbeet it might therefore be necessary to supplement the residual biomass feedstock with other sources of lignocellulose such as straw or wood. For example, both corn stover and wheat straw are being considered as a source of lignocellulose for ethanol production; however, because these materials are produced from annual plants, very large stocks of stover or straw would have to be stored at the ethanol plant. Sweet sorghum therefore has appreciable benefits as a bioethanol feedstock because under the right climatic conditions it is possible to grow between two and three crops a year, and sweet sorghum also produces high yields of sugar juice and lignocellulose material. For example under tropical or subtropical conditions each crop of sweet sorghum should produce about 160 tonnes of fresh matter, mainly as cane, from a hectare of land.

The present invention describes a method of cultivating sweet sorghum that is able to provide a continual supply of fresh sweet sorghum all year round to meet the feedstock requirements of a fully integrated ethanol plant. Fresh sweet sorghum would be delivered each day to the ethanol plant and there would be enough sugar juice and lignocellulose material available to meet all the feedstock requirements of the plant. Although the cultivation scheme could be used in various climates, it is particularly suitable for tropical and subtropical climates where the climatic conditions favour two or three crops of sweet sorghum a year. A typical subtropical climate is described in Table 2. The method of cultivation described in the invention would probably require irrigation as well as regular rainfall to promote rapid growth of the sweet sorghum, and the estimated water requirements are also given in Table 2.

Table 2 Typical Subtropical Weather Conditions

The method of cultivation is based on growing sweet sorghum on individual plots of land preferably measuring about 100 metres x 110 metres, i.e. an area of 1.1 hectares, which is an appropriate amount of land for planting and harvesting in a day. A number of plots of land would be combined together to form a cultivation unit; by way of example, under warm subtropical conditions a cultivation unit would probably comprise 128 plots of land.

If one plot of land in the cultivation unit is then sequentially planted each day with sweet sorghum, when the last plot of land is planted with sweet sorghum after 128 days, the sweet sorghum on the first plot of land will have reached maturity and be ready for harvesting. From then on there will be a fresh supply of sweet sorghum available every day for the ethanol plant as the sweet sorghum in the next plot of land reaches maturity.

After being harvested, the first plot of land would be replanted, and because in theory up to three crops a year of sweet sorghum could be cultivated under warm subtropical conditions, there would then be a continual supply of fresh sweet sorghum available all year round from the cultivation unit.

After allowing for the slightly lower temperatures and reduced daylight during winter months, and the time needed to harvest the ripe sweet sorghum and then replant a new crop at the end of each growing cycle, it is more likely that about 2.5 crops a year, rather than 3 crops, would be achieved under warm subtropical conditions.

Once the cane of the sweet sorghum has been harvested, the sugars in the juice inside the cane tend to break down fairly quickly, and a further advantage of the proposed sweet sorghum cultivation system is that the cane delivered to the ethanol plant will always be fresh and the sugar content in the juice will be near its peak level.

There would be enough units under cultivation to ensure that there was sufficient fresh sweet sorghum available every day to meet the sugar juice and lignocellulose requirements of the integrated ethanol production plant. Because the cultivation system is modular in concept, the amount of sweet sorghum under cultivation and the capacity of the ethanol plant can easily be changed to meet market demands. For example, if the ethanol production plant has to be expanded to meet increased demand, it is relatively simple to increase the number of cultivation units to provide the extra feedstock required by the ethanol plant.

Because sweet sorghum will be cultivated continually on the same land, each cultivation unit will be equipped with an irrigation system that will be able to supply appropriate amounts of fertiliser and extra water to promote the growth of the next crop of sweet sorghum. Harvesting equipment is available that can harvest the sweet sorghum and simultaneously cut the cane into small 13 mm pieces ready for processing at the ethanol production plant. The harvesting equipment can also pull out the roots of the harvested plants, which helps to break up the soil and prepare the ground for the next planting of sweet sorghum. A cultivation scheme based on 2.5 crops a year of sweet sorghum grown under typical warm subtropical conditions is described more fully in Table 3.

Table 3 Cultivation System for Sweet Sorghum

The principles of the cultivation system could also be applied to tropical conditions, where it should be possible to grow three crops of sweet sorghum a year, and to cooler subtropical conditions, where it might only be possible to grow two crops a year. For example, a smaller cultivation unit, i.e. a unit with a smaller number of plots of land, would be used for tropical conditions and a larger cultivation unit, i.e. a unit with a larger number of plots of land, would be used for cool subtropical conditions.

In temperate climates, where it would only be possible to grow one crop a year of sweet sorghum outdoors, a sequential cultivation system could still be used to extend the growing season of the sweet sorghum so that a growing campaign could be devised which would allow fresh material to be readily available to the nearby ethanol plant over a longer period of time. Alternatively, more than one crop a year of sweet sorghum could be achieved in temperate climates by using a combination of outdoor cultivation units during the warm months and indoor cultivation units under glass or polythene during the cold months. From a first broad aspect therefore the invention provides a method of cultivating sweet sorghum that can produce large volumes of sweet sorghum throughout the year so that there is a continual supply of fresh juice and lignocellulose material available every day to meet the feedstock requirements of an integrated ethanol production plant located near the cultivated sweet sorghum. At the ethanol production plant the cut pieces of fresh sweet sorghum cane would initially be crushed to extract the sugar juice, and the juice would then be fermented with an active yeast to convert the sugar into bioethanol.

The residual baggasse from the cane crushing process, which is primarily a mixture of cellulose, hemicellulose and lignin, is initially subjected to prehydrolysis with alkali or acid to separate the lignin and convert hemicellulose material into sugars. The remaining cellulosic material is then converted into fermentable sugars by enzymatic hydrolysis. In the present invention, alkali digestion with potassium hydroxide is the preferred method of prehydrolysis because the potassium hydroxide can be recovered by neutralising the spent potassium hydroxide with nitric acid. The effluent water from the pretreatment process will then contain potassium nitrate and the effluent water can be used as a source of valuable potassium and nitrogen nutrients to promote the growth of more sweet sorghum. Nitric acid could be used as the reactant in the prehydrolysis process and the nitric acid could then be neutralised with potassium hydroxide to form potassium nitrate. However, potassium hydroxide is the preferred digestion reagent because it is milder and less aggressive than nitric acid and it is less likely to degrade useful hydrolysis products.

After filtering the neutralised effluent water to remove residual solids, filtered wastewater from other processes at the ethanol plant would be added to the effluent water.^' Water and chemicals used in the ethanol production plant will therefore be fully recovered by using the wastewater to promote the growth of sweet sorghum in the nearby plantations. Distillate wastewater from the distillation processes at the plant will be depleted in dissolved oxygen, and the effluent water will therefore be reoxygenated to above normal levels before it is used as plant feed. The additional oxygen in the wastewater will also help to promote plant growth. By directly combining the cultivation of sweet sorghum with a nearby ethanol production plant, an appropriate amount of fresh feedstock can be delivered daily to the ethanol plant and nutrient rich effluent water from the ethanol plant can be used to feed more sweet sorghum. The wastewater from the ethanol plant will even include the moisture and nutrient elements present in the sweet sorghum that was originally delivered as feedstock to the plant. The combined system will therefore help to conserve valuable water resources and reduce the usage of manufactured fertilisers.

The relative amounts of potassium and nitrogen in the effluent water from the ethanol plant would be adjusted to suit the requirements of sweet sorghum, and an appropriate amount of phosphorus would be added to provide a balanced fertiliser. The balanced fertiliser and additional irrigation water would be supplied to the sweet sorghum at appropriate times during its natural growing cycle either by a conventional spray or drip- feed surface irrigation system or by a sub-surface irrigation system that would feed the water and fertiliser directly to the roots of the plants.

A fully integrated bioethanol plant with a capacity of 60 million litres of ethanol a year would require about 500,000 tonnes of fresh sweet sorghum per annum to provide the sugar and lignocellulose feedstock needed by the plant. This amount of sweet sorghum would need around 10 million tonnes of irrigation water and a similar amount of rainwater to grow effectively. The ethanol plant would produce about 2 million tonnes of nutrient rich effluent water, which would provide a significant proportion of the irrigation water needed by the sweet sorghum as well as a substantial part of the fertiliser required by the sweet sorghum. From a further aspect therefore the wastewater from the prehydrolysis process and other production processes in the ethanol plant will preferably contain potassium and nitrogen in the form of potassium nitrate. After the wastewater has been reoxygenated to above normal levels, the resulting effluent water will provide a valuable source of water and nutrients to promote the growth of more sweet sorghum on the nearby plantations.

Regular irrigation of the sweet sorghum with the nutrient rich wastewater will encourage the growth of prolific volumes of fresh sweet sorghum throughout the year. Because fresh cane will be delivered daily to the ethanol production plant, the cane will always contain significant amounts of fresh sugar rich juice and lignocellulose material, which in turn helps to produce high yields of bioethanol from the ethanol plant.

The cultivation system is extremely productive; for example, under subtropical conditions it should be feasible to grow 2.5 crops per annum and the yield of sweet sorghum would then be about 400 tonnes/ha/year. An efficient fully integrated sugar and lignocellulose ethanol plant should be able to produce 50,000 litres of ethanol from this amount of raw material, and this yield of ethanol is about eight times greater than the yield of ethanol, 6000 litres/ha/year, normally expected from a conventional sugar fermentation plant.

This is illustrated more fully in Table 4, which indicates the predicted ethanol yields that would be expected from combining the proposed method of sweet sorghum cultivation with a nearby integrated ethanol production plant. The estimates are based on growing 2.5 crops of sweet sorghum a year under warm subtropical conditions. Table 4 Predicted Ethanol Yield

By way of example, if an ethanol production plant was surrounded by eight cultivation units each with a cultivatable area of 141 hectares, there would be enough fresh sweet sorghum available every day, 1280 tonnes, for the ethanol plant to produce about 160,000 litres of bioethanol a day. This output is equivalent to 56 million litres of ethanol per annum, and a production capacity of 56 million litres per annum is probably close to the optimum size of plant required for efficient and economic ethanol production.

Less than 12 km² of cultivatable land would be needed to produce the sweet sorghum required to manufacture this amount of bioethanol, and after allowing for the land taken up by the ethanol production plant and access roads, the total amount of land required for a combined cultivation and ethanol production system with an output of 56 million litres of bioethanol per annum would still be less than 20 km².

Obviously sweet sorghum grows at different rates under different climatic conditions, and this affects the amount of land required to produce sweet sorghum continually all year round. For example, under tropical conditions it should be possible to grow three crops of sweet sorghum a year, whereas in certain cooler subtropical climates it may only be possible to grow two crops a year. The area of land in a cultivation unit would therefore have to be varied to allow for the different growth rates of sweet sorghum under various climatic conditions. Table 5 provides details of the cultivation units that would probably be required under tropical, warm subtropical and cool subtropical conditions to produce a continual supply of fresh sweet sorghum to an integrated ethanol production plant with nominal capacity of 60 million litres of bioethanol a year.

The estimates in Table 5 are based on the assumption that sweet sorghum would reach maturity after 100 days, 120 days and 140 days under tropical, warm subtropical and cool subtropical conditions respectively. The cultivatable plots of land in each cultivation unit would still be 100 x 110 metres, and it has been assumed that the yield of sweet sorghum would be constant at 160 tonnes/ha/crop under all climatic conditions. In practise it is likely that the yield of sweet sorghum would be more than 160 tonnes/ha/crop under tropical conditions and the yield might be less than 160 tonnes/ha/crop under cool subtropical conditions.

Table 5 Cultivation Requirements for Different Climatic Conditions

The principles of the invention will now be described with reference to Figures 1 , 2 and 3 where:

Figure 1 is a plan illustration of a single cultivation unit for growing sweet sorghum under warm subtropical conditions;

Figure 2 is a plan illustration showing the cultivation of sweet sorghum combined with a bioethanol production plant;

Figure 3 is a schematic side view of a surface irrigation system for sweet sorghum.

Figure 1 shows the layout of a cultivation unit 1 for a warm subtropical climate that has 128 plots of cultivatable land 2 arranged in a grid formation with access roads 3 to allow farm equipment to reach the individual plots of land. For illustration purposes the plots of land are numbered sequentially from [1] to [128].

Each plot of land 2 is 100 metres x 110 metres and has an area of 1.1 hectares. The cultivation unit 1 therefore has a cultivatable land area of 141 hectares and a total land area, including the access roads, of 178 hectares.

A plot of land would be sequentially planted with sweet sorghum each day, starting at plot [1] followed by plot [2] and so on until the cultivation unit is completely planted after 128 days. By the time the last plot of land [128] has been planted, the sweet sorghum in the first plot of land [1] will have reached maturity and be ready for harvesting.

From then on there will be a supply of fresh sweet sorghum available each day for the nearby ethanol production plant as the sweet sorghum in the next plot of land reaches maturity.

After being harvested, each plot of land will be prepared for immediate replanting so that the cultivation process is repeated and there will be a continuous supply of fresh sweet sorghum available all year round from the cultivation unit.

Figure 2 is an illustration of a typical subtropical sweet sorghum cultivation system directly combined with an integrated ethanol production plant. The ethanol plant 5 is surrounded by eight cultivation units 4a to 4h inclusive, and the ethanol plant is connected to each cultivation unit by a series of access roads 6.

All of the sweet sorghum growing in the cultivation units 4a to 4h is within 2 km of the ethanol plant 5. Consequently harvested sweet sorghum can readily be shipped to the ethanol plant and nutrient rich effluent water from the ethanol plant can easily be piped to each cultivation unit.

On reaching maturity, each plot of land in a cultivation unit would yield about 160 tonnes of sweet sorghum, and on any particular day of the year sweet sorghum should be reaching maturity on one plot of land in each cultivation unit. Up to 1280 tonnes of fresh sweet sorghum could therefore be available each day from the eight cultivation units surrounding the ethanol plant, which is enough raw material to produce 160,000 litres of bioethanol from an integrated ethanol production plant. Based on a 350 day working year this would equate to

56 million litres of bioethanol per annum.

The sweet sorghum cane will be harvested by equipment that will cut the cane into 13 mm pieces and then load the pieces of cane into a trailer towed by a tractor.. When the trailer is fully loaded, the tractor will take the trailer straight to the delivery bay at the ethanol production plant for immediate off-loading. All tractor units at the site will be designed to run on bioethanol to avoid the use of fossil based petroleum fuels.

Because sweet sorghum will be grown continually on the same plot of land, the soil will require careful preparation before each new planting and solid residues from the ethanol plant will be used wherever possible as a mulch and soil conditioner.

It is also essential that each crop of sweet sorghum is regularly fed with the requisite amount of water and fertilisers to promote healthy and rapid plant growth. Either a conventional spray or drip-feed surface irrigation system could be used to irrigate and feed the sweet sorghum, or alternatively a sub-surface irrigation system that feeds water direct to the plant roots could be used. A typical drip-feed surface irrigation system is described with reference to Figure 3.

Irrigation water from an aquifer or storage pond is supplied to storage tank 7, and effluent water from the nearby ethanol production plant is supplied to tank 8.

The wastewater produced by the ethanol plant will be rich in potassium and nitrogen, and before the effluent water is discharged from the ethanol plant to tank 8, the relative amounts of potassium and nitrogen in the water will be adjusted to suit the nutrient needs of sweet sorghum. An appropriate amount of phosphorus will be added to the wastewater and the wastewater will be re-oxygenated to above normal levels so that the effluent water in tank 8 is able to provide all of the nutrients required by sweet sorghum.

Pump 11 supplies either irrigation water from tank 7 or liquid fertiliser from tank 8 to the main water supply pipe 12, which transfers water to individual plots of land within each cultivation unit.

The supply of irrigation water from tank 7 or liquid fertiliser from tank 8 to pump 11 is controlled by valve 9, and is dependent on whether the sweet sorghum under cultivation requires water or feed at any particular time. The amount of water or liquid fertiliser supplied to the main water supply pipe 12 is measured by gauge 10.

Pipe 12 runs above ground at a height of at least 25 cm above normal ground level. Pipe 12 is connected to a series of irrigation tubes or hoses 13 located in each plot of land.

The tubes 13 run just above ground level and are positioned so that a tube will run along each row of sweet sorghum plants 14.

The rows of sweet sorghum will be planted 90 cm apart, and each plot of land will have 109 rows of sweet sorghum. Consequently each plot of land will have 109 lengths of irrigation tube 13. The irrigation tubes 13 will be fitted with simple water spray emitters, which will face downwards so that the water falls directly onto the roots of the sweet sorghum plants.

The sweet sorghum plants are planted 30 cm apart in each row, and the water emitters will be spaced 30 cm apart in tube 13 so that a water emitter will be positioned directly above the root system 16 of each sweet sorghum plant.

At appropriate times during the growing cycle of the sweet sorghum either irrigation water from tank 7 or liquid fertiliser from tank 8 will be supplied to pipe 12 and then to each individual tube 13. The water or liquid fertiliser 15 emitted from tube 13 will then be fed directly onto the root system 16 of each individual sweet sorghum plant.

Unconverted biomass, residual baggasse and other solid residues from the conversion and distillation processes at the ethanol plant are rich in mineral nutrients, and because of the close proximity of the cultivation units the process residues will always be used as a mulch top dressing to improve the soil and help promote the growth of more sweet sorghum. Plant roots, residual biomass, lignin and combustible residues from the ethanol conversion processes will be dried and the dry material will be used as boiler fuel to produce the heat and power requirements of the ethanol plant. The boiler ash will also be rich in mineral nutrients and the ash will be used as a soil conditioner.

The present invention therefore describes an efficient and productive method of cultivating sweet sorghum that is combined with an integrated ethanol production plant located close to the sweet sorghum plantations. The cultivation system provides fresh sweet sorghum all year round and in sufficient quantities to ensure that there is enough fresh juice and lignocellulose material available each day to meet the feedstock requirements of the integrated ethanol plant. Conversion processes are used in the ethanol plant that produce effluent water rich in plant nutrients so that the wastewater can be used as fertiliser to promote the growth of more sweet sorghum in the nearby plantations.

The method of crop cultivation described in the invention is primarily aimed at productively growing sweet sorghum under tropical or subtropical conditions so that fresh feedstock is continually available for a nearby ethanol production plant. However, the principles of the invention could be applied to any sugar or grain bearing energy plant species that is able to produce more than one crop a year, either by natural growth outdoors or more likely by a combination of natural outdoor growth during the warm months and forced cultivation during cold months. Methods of forced growth could include heated indoor cultivation under artificial light; heated cultivation under glass or polytubes; and indoor hydroponic cultivation under controlled climatic conditions.

Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.

Claims

Claims:

1. A method of producing bioethanol from sugar or grain bearing plants, wherein the plants are grown on a plurality of plots of land, wherein the plants are planted sequentially so that when a plot of land is being planted the plants in another plot of land are ready for harvesting, and wherein material from the harvested plants is supplied to a bioethanol production plant.

2. A method as claimed in claim 1 , wherein said plants are sweet sorghum plants.

3. A method as claimed in claim 1 or 2, wherein a first plot of land is harvested and replanted, and then another plot of land is harvested and replanted in sequence so that there is a substantially continual supply of freshly harvested plants available for the bio-ethanol production plant.

4. A method as claimed in any preceding claim, wherein the plots of land are harvested and re-planted sequentially every day.

5. A method as claimed in any preceding claim, wherein said plots of land are arranged together in a cultivation unit.

6. A method as claimed in any preceding claim, wherein said plurality of plots of land or a plurality of cultivation units are provided to produce enough fresh plants to meet the feedstock requirements of the bioethanol production plant.

7. A method as claimed in claim 6, wherein the number of plots of land or cultivation units are selected depending on the local climatic conditions and the number of crops of plants produced by each plot of land per year.

8. A method as claimed in claim 7, wherein 3 crops per annum of said plants are grown in at least some of said plots of land under tropical conditions; wherein 2.5 crops per annum of said plants are grown in at least some of the plots of land under subtropical conditions; or wherein 2 crops per annum of said plants are grown in at least some of said plots of land under cool subtropical conditions.

9. A method as claimed in any preceding claim, wherein said plots of land each have the same predetermined area.

10. A method as claimed in claim 9, wherein said predetermined area is about 1.1 hectares.

11. A method as claimed in any preceding claim, wherein a cultivation unit in tropical conditions has 112 plots of land and a total cultivatable area of 123 hectares; a cultivation unit in warm subtropical conditions has 128 plots of land and a total cultivatable area of 141 hectares; and a cultivation unit in cool subtropical conditions has 144 plots of land and a total cultivatable area of 158 hectares.

12. A method as claimed in claim 11, wherein under tropical conditions the plants take approximately 100 days to reach maturity; under warm subtropical conditions the plants take about 120 days to reach maturity; and under cool subtropical conditions the plants take about 140 days to reach maturity.

13. A method as claimed in claim 12, wherein the approximate yield of fresh plant material from a tropical cultivation unit is about 59000 tonnes/year; the yield of plant material from a warm subtropical unit is about 56400 tonnes/year; and the yield of plant material from a cool subtropical unit is about 50600 tonnes/year.

14. A method as claimed in claim 13, wherein the yield of ethanol from the plant material is approximately 125 litres/tonne of fresh material.

15. A method as claimed in claim 14, wherein the yield of ethanol produced from a tropical cultivation unit is about 7.4 million litres/year; the yield of ethanol from a warm subtropical cultivation unit is about 7 million litres/year; and the yield of ethanol from a cool subtropical cultivation unit is about 6.3 million litres/year.

16. A method as claimed in claim 6, wherein eight warm subtropical cultivation units supply 1280 tonnes of fresh plant material a day to the bioethanol production plant so as to provide enough feedstock to produce about 160,000 litres of ethanol/day or about 56 million litres/year.

17. A method as claimed in claim 6, wherein 8.1 tropical cultivation units, 8.6 warm subtropical units or 9.5 cool subtropical units are provided so as to meet the feedstock requirements of a 60 million litres/year bioethanol production plant.

18. A method as claimed in any preceding claim, wherein at least one access road is provided to allow farm equipment to reach each individual plot of land and to ship harvested plants to the bio-ethanol production plant.

19. A method as claimed in any preceding claim, wherein at least some of said plants are grown under artificially controlled light and/or temperature-conditions.

20. A method as claimed in claim 19, wherein more than one crop a year of said plants are grown in temperate regions by growing at least some of said plants outdoors during warm months of the year and growing at least some of said plants indoors under glass or polythene during cold months.

21. A method as claimed in any preceding claim, wherein effluent water rich in plant nutrients from the bio-ethanol production plant is used to promote the growth of said plants.

22. A method as claimed in claim 21, wherein lignocellulose material from said plant material is subjected to prehydrolysis with potassium hydroxide or nitric acid in the bioethanol production process.

23. A method as claimed in claim 22, wherein waste liquor from the prehydrolysis process is neutralised with nitric acid to produce effluent water containing potassium nitrate.

24. A method as claimed in claim 22, wherein waste liquor from the prehydrolysis process is neutralised with potassium hydroxide to produce effluent water containing potassium nitrate.

25. A method as claimed in claims 23 and 24, wherein wastewater from the bio-ethanol production process is added to the neutralised liquor, and wherein the relative amounts of potassium and nitrogen in the effluent water are adjusted to suit the nutrient needs of said plants, and the effluent water is then used as liquid fertiliser to promote the growth of said plants.

26. A method as claimed in any of claims 21-25, wherein phosphorus is added to the effluent water in an amount to suit the nutrient needs of said plants.

27. A method as claimed in any of claims 21-26, wherein the effluent water is re- oxygenated to above normal levels.

28. A method as claimed in any of claims 21-27, wherein the effluent water is supplied to said plants by either a spray or drip-feed surface irrigation system, or by a sub-surface irrigation system where the water is supplied to the roots of said plants.

29. A method as claimed in any preceding claim, wherein said plants are grown in the same country as that in which the bioethanol production plant is located.

30. A method as claimed in any preceding claim, wherein said plants are grown on plantations located close to the bioethanol production plant.

31. A method of producing bio-ethanol from sugar or grain bearing plants, said method comprising staggering the planting and harvesting of said plants so that said plants are grown and harvested substantially throughout the year, and supplying material from the harvested plants to a bio-ethanol production plant.

32. A method as claimed in claim 31, wherein said planting is staggered so that different plants are ready for harvesting at different points throughout the year, and wherein the plants that are ready for harvesting have reached maturity and are harvested so as to provide a fresh supply of plant material for the bioethanol production plant.

33. A method of cultivating sweet sorghum, wherein under certain climatic conditions sweet sorghum is grown all year round in a manner whereby there is enough fresh sweet sorghum available every day to meet the sugar and lignocellulose feedstock requirements of a bioethanol production process located adjacent to the sweet sorghum plantations, and effluent water rich in plant nutrients from the bioethanol process is used to promote the growth of more sweet sorghum in the plantations.

34. A method of producing bioethanol comprising growing sugar or grain bearing plants in plots of land adjacent to a bioethanol production plant, providing freshly harvested plant material to said bioethanol production plant, and subjecting said freshly harvested plant material to a bioethanol production process.

35. A method as claimed in claim 34, wherein said sugar or grain bearing plants are grown in plots of land around said bioethanol production plant.

36. A method of irrigating or fertilising plants comprising supplying said plants with effluent water from a bioethanol production process.

37. A system for producing bioethanol, said system comprising a bioethanol production plant and a plurality of plots of land which have been planted sequentially with sugar or grain bearing plants so as to include a newly planted plot of land and a plot of land which is ready for harvesting.

38. A method as claimed in claim 37, wherein said plants are sweet sorghum plants.

39. A method as claimed in claim 37 or 38, wherein said plots of land are arranged together in a cultivation unit. „

40. A system as claimed in claim 37, 38 or 39, wherein said plots of land are located adjacent one another.

41. A system as claimed in any of claims 37 to 40, further comprising an irrigation system for supplying said plots of land with nutrient rich wastewater from the bioethanol production plant.

42. A system for producing bioethanol, said system comprising a bioethanol production plant and a plurality of plots of land located in the vicinity of said production plant, wherein said plots of land have been planted at different times with sugar or grain bearing plants so that some of said plants are ready for harvesting substantially all year around.

43. A combined system of cultivating sweet sorghum and producing bioethanol comprising a number of cultivation units containing a multiplicity of cultivatable plots of land planted sequentially with sweet sorghum so that the sweet sorghum on one plot of land in each cultivation unit will reach maturity every day and provide a source of fresh sugar and lignocellulose material for the bioethanol production process.

44. A system for producing bioethanol, said system comprising a bioethanol production plant and a plurality of plots of land comprising sugar or grain bearing plants located adjacent to the production plant.

45. A system as claimed in claim 44, wherein said plurality of plots of land are arranged around said production plant.

46. A system for irrigating plants comprising a bioethanol production plant, at least one plot of land for growing said plants, and an irrigation system for supplying said plot of land with effluent water from the bioethanol production plant.

47. A system for fertilising plants comprising a bioethanol production plant, at least one plot of land for growing said plants, and means for supplying said plot of land with effluent water rich in plant nutrients from the bioethanol production plant.