The Burning of fossil fuels makes one of the biggest contributions to the accumulation of CO2 in the atmosphere and because of their depleting supplies are unsustainable. An alternative renewable source of fuel is required. Already well known methods to replace fossil fuel burning power stations include the use of solar, wind, hydro and geothermal power. It is reasonable to assume that with advances in technology fossil fuel power stations can be eventually replaced. However the burning of fossil fuels in combustion engine automobiles and machinery is a different matter. Efforts have been made to create electric and hydrogen cell powered automobiles but their success has been limited. An alternative to this is another form of hydrocarbon fuel termed biodiesel which has originally been produced from oil seed crops such as oil palms or even from animal fats. But because the demand is so big for automobile fuel these methods of biodiesel production are unfeasible as they cannot meet the demand either because there is simply not enough or that they will have a serious impact on world food production if viable amounts of biodiesel were produced. However this does not spell the end for biodiesel as it can also be made from microalgae. Microalgae have similar or greater oil content then oil plants but require much less room to produce the large amounts of oil needed to be converted to biodiesel. They can also be cultured in areas were oil crops cannot be cultivated meaning no crop land needs to taken up to replace food crops with oil producing organisms.
In this paper we will discuss the benefits of producing biodiesel from microalgae giving the preliminary design of a biodiesel plant to understand the feasibility of producing biodiesel from microalgae. The method of culture, separation, oil extraction and process of turning algae oils to a useable form of biodiesel are discussed.
- Project Background
Although the predictions for when fossil fuels will eventually be depleted are contentious it is a fact that they will run out. Without them the entire world will be in trouble. Therefore sustainable or renewable forms of fuel are required. Researchers have been looking into this for decades with differing degrees of success. Typically when the subject of alternative fuel sources is discussed it usually concerns electricity production and the replacement of coal or gas burning power stations with hydro, solar, geothermal and wind power etc. However although the power industry is a large user of vast amounts of fossil fuels so are automobiles and machinery with combustion engines. Finding an alternative source of fuel to power automobiles and such machinery will mean we will lessen the demand on our finite stocks of fossil fuels as well as lowering the rate of accumulation of carbon dioxide (CO2) in the atmosphere.
One alternative source of fuel is biodiesel for which the technology to produce and use already exists and has existed for many years. At the moment biodiesel is mainly produced from either, oil crops, waste cooking oil or animal fats. But due to the excessive need for automobile fuel it soon becomes apparent that these sources will not be able satisfy a significant amount of this demand. Resources of cooking oil and animal fats are far too small to make any real difference while oil crops would require to large an area for cultivation before they can have a significant effect and this would also have a negative impact on food production which would not be acceptable.
Biodiesel can also be produced using microalgae and much of the technology already used in biodiesel production/use can also be used when pertaining to fuel from this source. The major advantage using microalgae as a source over oil crops is that the area required for cultivation that would produce significant amounts of biodiesel is a great deal less. Chisti (2007) states that in the US to replace 50% of their automobile fuel 24% of their total cropland would need to be used too grow oil crops where as with microalgae this figure is reduced to 3% or less. Chisti (2007) also shows that even the best oil producing crop, the oil palm, produces only ~5950 litres per hectare while microalgae can produce up to about 136,900 litres per hectare. This works out at around 23 times more oil per hectare.
This advantage comes from the fact that many microalgae’s can double their biomass in a 24 hour period and microalgae with oil levels of 20-50% (% weight of dry biomass) are fairly frequent (Chisti, 2007). Some microalgae under certain conditions can have oil levels in excess of 50% (Spolaore et al., 2006). The microalgae Botryococcus, is capable of having oil levels of up to 90% (Metting, 1996). This makes microalgae more oil rich then oil crops and produces much better yields of oil for their biomass when compared with oil crops.
Microalgae are also very hardy and are capable of growing in many different types of environment with many species capable of growing in water with a high salt content (Sheehan et al., 1998). This ability to survive even in saline waters means that they could be cultivated in coastal areas were no crops are typically suited to grow. This means that in reality no cropland would need to taken as microalgae can be grown in places which are unsuitable for oil and other kinds of crop.
But how do you actually produce biodiesel from microalgae? Much the same way as you would do from oil crops. You cultivate your algae so that there is an increase in biomass. This can be done using two methods, raceway ponds and photo-bioreactors (Chisti, 2007). Photobioreactors offer a more controlled environment for algae growth and produce larger yields of biomass but are much more complicated and expensive to run. Raceway ponds are usually open to the environment meaning there is a chance of contamination from foreign algae but they have however been used to produce large amounts of biomass themselves. For this project we will use a raceway pond.
During cultivation microalgae require nutrients such as phosphorus and nitrates to be added to the growth media while CO2 is also required for growth. The nutrients can be supplied by the addition of commercial fertilizers to the growth media. It is possible to obtain CO2 produced by power plants burning fossil fuels at very little expense which microalgae are happy to grow with (Yun et al., 1997). This is also good for the environment as CO2 which would normally be released into the atmosphere can now be fixed due to microalgae growth.
After cultivation you then remove the oil from your algae. This oil includes fats, sterols and triglycerides etc. Therefore microalgae with high oil content and a high growth rate would be desired. After removal of the oil this can now be called the parent oil and this is used to recover the biodiesel by a process called transesterification. This process involves the addition of methanol and a catalyst and these are involved in a reaction with the triglycerides in the parent oil. The reaction requires 3 moles of methanol for every mole of triglyceride and produces 1 mole of glycerol and 3 moles of methyl esters which is the biodiesel (Fig1).
Fig1. Figure showing the transesterification of oil to biodiesel were R1-3 are hydrocarbon groups.
For industrial processes 6 moles of methanol are typically used for each mole of triglyceride to make sure that the reaction is driven towards biodiesel and results in yields in excess of 98% on a weight basis (Fukuda et al., 2001). Transesterification can be carried out using various types of catalyst including acids, alkalis and lipases. Transesterification using alkali-catalysts is typically 4000 times faster than using acid-catalysts and although lipase-catalysts have important advantages over alkalis such as, less energy intensive and glycerol is removed more easily, they are much more expensive (Fukuda et al., 2001). Alkali-catalysts are used at a concentration of around 1% by weight of oil and transesterification is carried out at 60°C with the reaction taking about 90 minutes to complete with the biodiesel being recovered by repeated washing with water (Chisti, 2007).
As well as producing biodiesel from microalgae other products can be taken out that will make it more economically and environmentally sustainable. Production of biodiesel could be completely carbon neutral if all the power for the process came either from the biodiesel itself or from anaerobic respiration of waste microalgae biomass which produces methane gas (Chisti, 2007). The waste biomass also contains high quantities of proteins, carbohydrates and other nutrients and could be potentially used for animal feed (Sanchez Miron et al., 2003). Also depending on the specific microalgae used the extraction of other high-value products may be possible (Chisti, 2007).
Another major advantage of using microalgae would be the possibilities of genetic and metabolic engineering that could be carried out on them to improve the yields of biodiesel (Dunahay et al., 1996). Genetically manipulating microalgae would cause less ethical problems than modifying plants even if they are not grown for human consumption. As of now very little work has been done on understanding and improving microalgae biology but if this method of producing biodiesel is to reach its potential then these avenues need urgently exploring.
Other microbes such as yeast could be potentially used to produce biodiesel (Ratledge & Wynn, 2002). Yeast is a heterotrophic microorganism and requires to be grown on an organic source of carbon such as sugar. This organic source of carbon would be more expensive then the gaseous source of carbon microalgae can grow with. Production of biodiesel with photosynthetic microalgae is more efficient then heterotrophic microorganisms because the organic source of carbon required by such microorganisms would probably come from plants and was ultimately produced by photosynthesis in the first place (Chisti, 2007).
As of now microalgae are the only potential source of biodiesel which can produce a significant amount of fuel that it can lower the use of fossil fuels used to power automobiles and combustion engines. This proposed plant aims to produce 100,000 kg of microalgae biomass per year.
- Plant Location
The location of the biodiesel plant will be on suitable land near the town of Brigalow (Queensland, Australia, latitude – 26° 51′ 0 S, longitude – 150° 46′ 60 E) which is approximately 140 km inland of Brisbane. It should be placed in a location with close access to the Kogan Creek Coal-Powered Power Station for easy supply of CO2. It should also be nearby the Condamine River or one of its tributaries to be used as a source of water.
This place has been selected because of its close proximity to the Kogan Creek power station which will act as a supply of CO2 for the plant. The power station is modern and opened in 2007 and has helped bring new jobs to Brigalow and the surrounding area as will the introduction of a biodiesel plant. As part of the Australian governments Carbon Pollution Reduction Scheme they are looking to reduce the carbon emissions of all power stations across the country. By linking the Kogan Creek power station with the biodiesel plant this will help with reducing the harmful emissions being given off into the atmosphere.
Other reasons for selecting this place are because of the type of microalgae which will be cultured at the biodiesel plant. Dunaliella salina (D. salina) is native to Australia and has been collected both at Alice Springs and Rottnest Island and should be suitable for the environment and climate of Queensland. This microalga prefers a temperature of around 25°C and the subtropical environment of Brisbane and the surrounding areas should be suitable. The average maximum temperature of Brisbane varies from around 20-30°C during the year. There are also on average about 8.5 hours of daylight throughout the year. It is also rather humid and this should reduce the amount of evaporation from ponds. The level of rain, although not that high maybe a slight problem, due to how heavy it rains when it actually does. This could be dealt with by covers for ponds.
Dunaliella has also been mass cultured in Australia previously (Chaumont, 1993). Also the land around Brigalow seems to be mostly farmland or unused with very little of it marked as National Parks. Australia should also be less expensive and more suitable then countries such as the UK or the US.
- Process Overview
The total process shall involve 4 unit operations: ponds, cell separation, oil extraction and transesterfication. Each unit will be briefly discussed but a more detailed description will be done in section 5. The reason for choosing the type microalgae we have will also be discussed.
We have chosen to use ponds rather than photobioreactors due their less complex design and cost. We shall use a basic design such as that seen in Fig. 1 Chisti (2007) where the flow is driven by a paddlewheel near the feed in stream. The pond will consist of loops with baffles on the corners guiding and adding turbulence to the flow. The toughest challenge will be mixing of the culture medium for good gas transfer and good growth of biomass. Further devices could be added to help aid mixing and these may include airlift and other types of pumps. It will also be a good idea to have several medium sized ponds rather than one large pond in case problems are encountered. They will have several feeds including feeds for water plus microalgae, CO2 and for other nutrients. Delivering the CO2 to the plant will be challenging but it would best off being piped to the plant but we will also have to consider storage because it is likely the power station will produce in excess of what we need.
Cell separation will be carried out via a method of filtration. Unfortunately this maybe one of the more expensive unit operations due to the low yield of biomass produced from ponds and therefore the high volume of culture medium that needs to be filtered. During this part of the process the cells need to be reduced to a paste with as little water present as possible. This is to allow efficient extraction of oils in the next part of the process (Banerjee et al., 2002).
The oil will be extracted from the cells using pressing + solvent extraction which has proved a more successful method then pressing alone (Banerjee et al., 2002). A good solvent to use would be hexane because it is immiscible and has a different density to water, is easily removed and is readily available and inexpensive (Banerjee et al., 2002). One major problem that maybe encountered is the propensity of cells to clump together when carrying out this process on a large scale. To overcome this good mixing will be required during extraction (Banerjee et al., 2002). 5
Transesterfication will be carried using methanol and similar to how it was discussed earlier in section 1. The catalyst to be used will be an alkali because the reaction occurs faster than with an acid catalyst and is more economically feasible then with a lipid catalyst (Fukuda et al., 2001). The major problem concerned with this part of the process is loss of biodiesel yield due to saponification and this can be overcome by keeping the oil and alcohol dry and the oil should have a minimum of free fatty acids (Chisti, 2007).
We have decided to use Dunaliella salina (D. salina) as our microalgae of choice and this was for a number of reasons. Firstly it was because this strain of microalgae has been mass cultured in ponds before and also because it has been mass cultured in similar climates to our chosen location (Chaumont, 1993). D. salina has a doubling time of roughly 22 hours. It is also native to the country our plant will be situated and should be easily available and capable of growing well in this environment. It also has the ability to survive in brackish water. It is also capable of producing lipids at up to 44% of its own dry weight (Weldy & Huesemann). Although this is not as high as other strains, other strains have their own problems. Dunaliella has previously been used for the production of β-carotene for many years (Chaumont, 1993). Although this is not our target product you would still think research would have been carried out on the biology of this strain with the purpose of strain improvement which may still be amenable for what we would want.
Two other strains where considered when choosing our microalgae and these where Botryococcus brunii which seemed an ideal candidate with researchers reporting oil levels of up to 90% dry weight of cells. However the oil produced by Botryococcus brunii is not capable of undergoing transesterfication because it lacks the free oxygen atom required and it has a long generation time of about 6 days (Banerjee et al., 2002). The other type of microalgae considered was Chlorella which has a similar level of lipid content to Dunaliella and has been used in mass culture before. However these mass cultures suffer from problems of instability, require inoculations and production runs are short, and this makes their use more expensive (Sheehan et al., 1998).
Choosing the type of strain to use is a very difficult decision. There are more than 30000 strains of microalgae. For most very little is known and even in the cases where there is information there still is not a great deal. Even the claims of the lipid contents in different strains seems contentious as they can vary quite drastically and what was found at the bench may not be the case when it is used in a scaled up process. Because so little is known about the different microalgae strains it may be worth considering the advice of Sheehan et al., (1998). They suggest setting up small scale pond systems at your chosen location initially and letting them become invaded with the local strains. These strains can then be assessed for their suitability for your chosen goal.
We aim to produce 350 kg of microalgae biomass per day and propose the plant will be capable of running for 300 days producing 105,000 kg of biomass annually. The remaining 65 days will be used for essential maintenance and downtime due to mechanical malfunctions and adverse weather conditions. Our alga is D. salina and will we assume it has lipid/oil content of 30% by dry weight of cells.
Rotary-drum Vacuum Filter
After mass culture of microalgae they need to be separated from their culture medium before the algae lipid contents can be extracted. This can be done via three common methods filtration, centrifugation and sedimentation (Richmond et al., 2005). This part of the biodiesel production process has been suggested to make up at 20-30% of the total cost (Gudin & Therpenier, 1986). This is especially true if ponds are used to culture microalgae because of the large volumes of culture broth needing to be separated due to the low concentration of algae.
Sedimentation is by far the easiest process and uses gravity to separate solids in water. This method is useful when dealing with very large volumes of water with low concentrations of solids and is sometimes used with another separation process as a primary concentration step. The use of flocculants can aid sedimentation. Flocculation is the process of solids in a solution becoming aggregated due to the addition of a polymer. The aggregation of solids facilitates separation.
Centrifugation is basically sedimentation using enhanced gravity. Centrifuges range from small worktop centrifuges to large industrial ones and are thought to be the most common method of cell separation for large scale microalgae culture. There are three main types of centrifuges used for such purposes: disc-stack bowl, scroll discharge decanter and tubular bowl. Initially the use of centrifuges may be expensive because of the need to carry out pilot scale evaluations. Although they are expensive centrifuge technology has moved on further then any of the other separation techniques.
A large scale biodiesel production plant would probably involve a two step separation process probably involving sedimentation and centrifugation. However because our plant is only a pilot plant much smaller volumes of culture broth are being dealt with and thusly a cheaper and faster method of separation can be used. Sedimentation even with flocculation is relatively slow and with the improvements in centrifuge technology, although they are very efficient, they are also very expensive. Separation via filtration seems the more sensible option.
The most common machine used to filter microalgae is the continuous rotary-drum filter using either a vacuum or pressure. We are going to use a vacuum. Our continuous rotary-drum vacuum filter (fig3) is composed of 16 radial chambers were vacuums are applied (250-500 mmHg). Surrounding the vacuum chambers is the filter cloth which in our case is covered by diatomaceous earth forming a porous filter cake. The drum rotates at 1 rpm and is partly submerged in the culture broth. The culture broth is drawn through the filter by vacuum and the microalgae are deposited on the filter cake. The filter cake is eventually scrapped off and the scrapings sent for further processing. Although the filter cake is composed of both algae and diatomaceous earth this does not effect the next process of solvent extraction (Ruane, 1977). Rotary-drum filters are less expensive then centrifuges and are capable of being run continuously and can handle large throughputs.
If you assume that the microalgae culture broth has a filtrate rate of 5 m3 hr-1 m-2 then three continuous rotary-drum vacuum filters each with a filtration area of 20 m2 (60 m2) will be able to filter 350 kg of biomass (2428 m3 of water) in approximately 8.09 hours.
The chosen method of lipid extraction will be pressing followed by solvent extraction using hexane. These two methods are used in combination to disturb the algae cellular membrane and to produce a lipid yield in excess of 95%. Hexane is used rather then other solvents because it is easily available, relatively inexpensive, is reusable and has been used before for this purpose (Banerjee et al., 2002). As with other solvents hexane is also dangerous because of its volatile explosive properties and possible carcinogenic effects. Hexane is an environmental and explosive hazard and must be stored and used at least 50 ft away from other facilities. After the microalgae have been put through the oil press all products are then sent for solvent extraction. Extraction occurs in an enclosed band conveyor were the cellular material are passed slowly along the band while being sprayed with hexane. The band itself is lined with porous stainless steel clothes. Extracted lipids move through the stainless steel cloth and are collected below in a counter current direction. The amount of solvent used for 350 kg of algae biomass is shown below
|(Table 1). Mass in||(kg)||Mass out||(kg)|
|Pressed algae||350.00||Extracted algae + solution||420.00|
After the oil has been extracted from the microalgae it can be converted to biodiesel and this is done by a reaction called transesterfication. Transesterfication involves the use of an alcohol (methanol) and a catalyst (NaOH) to convert the triglycerides in the algae oil to glycerol and methyl esters. Glycerol and the methyl esters will separate themselves into to phases with the methyl esters on top of the glycerol. This makes there removal easier but the methyl esters still need washing with water to remove any trace glycerol. The methyl esters are separated from the glycerol and can now be called biodiesel.
This process will take place in a 50 litre stirred tank reactor made from carbon steel and is heated by a steam jacket. The reaction is carried out at 60°C at atmospheric pressure. Under these conditions and using an alkali catalyst the reaction takes about 90 minutes to complete (Chisti, 2007). The reaction is shown in Fig1 with one mole of triglyceride and three moles of methanol producing one mole of glycerol and three moles of methyl esters, while in the presence of the alkali catalyst. In reality up to six moles of methanol are used to make sure the reaction is driven towards the production of methyl esters whose yield can be greater than 98% on a weight basis (Fukuda et al., 2001). The concentration of catalyst used is normally 1% by weight of oil (Chisti, 2007). So under these conditions 99.75 kg of triglycerides from algae oil will produce.
Transesterfication seems to be the only viable method to produce biodiesel but there are still considerations that have to be taken into account. The choice of catalyst as mentioned 12
earlier is important with alkali catalysed reactions being 4000 times faster than acid catalysed reaction; they are also considerably cheaper than using lipase enzymes (Fukuda et al., 2001). Higher temperatures and pressures can also be used to decrease the time taken for the reaction to occur but this increases the cost of the process (Chisti, 2007).
- Safety and Environmental Considerations
Safety is always a major concern in the design of a process, both for the safety of people working on the plant and the surrounding environment. It is in the interest of the plant to either use processes that are inherently safe or make processes which are inherently unsafe as safe as they possibly can be. Through using good engineering principles a designer should be more than capable of doing this. Failure to do so will lead to problems which will only cost the plant money such as replacing unreliable/unsafe equipment, downtime, loss of sales and also third party claims.
Hazop studies would be carried out in a more detailed design of the individual processes. This considers the mechanical failures which could occur in each unit operation and also methods to prevent them such as safety devices and good operating practices. Along with this the chemical hazards should also be studied with appropriate action being taken for the individual chemicals. This can be done by looking at the materials safety data sheets (MSDS) for each chemical.
Considering each of our processes individually the raceway ponds are inherently safe. However considerations should be made for the prevention of our strain of microalgae from contaminating local water sources where it may interfere with the indigenous strains of algae. Our strain is particularly hardy and may out compete other strains and this will affect the biodiversity of the surrounding area. Fertilizer used to feed the ponds should be securely stored as well. CO2 is a known asphyxiant but should not cause too many problems as it is not being used in an enclosed area.
The continuous rotary-drum filters are also inherently safe and with no chemicals being added in this process there are no chemical dangers. The only real dangers are mechanical and this can be prevented via appropriate safety guards and good operational practice.
The extraction process is more inherently unsafe then safe but this can be overcome. The main safety issues are in regards to the use of hexane. Hexane is extremely flammable as both a liquid and vapour and can be explosive and therefore must be treated with caution. To account for this, extraction will be carried out at least 50 feet away from the rest of the plant. Hexane should also be stored appropriately to prevent leaks and damage to the surrounding environment. Hexane is a commonly used chemical in industrial processes and existing information for the safe use and storage of it should be sort.
The transesterfication process should be somewhat inherently safe although the methanol used is flammable but accidents can be prevented through good operational practices. NaOH can be harmful but is not a massive danger if used properly.
Storage of the biodiesel should be in a place that is well contained to prevent leakage into the environment and also prevent ignition by fire.
On the whole the plant should be inherently safe as many aspects which are unsafe can be overcome and have been overcome in industry before by the use of proper safety equipment and good operational practice. (Sinnot, 1999, was used for this section)
- Estimated Costs
Estimations were made with the aid of Molina Grima et al. (2003)
Estimated costs of plant design and build:
|Hexane extraction band conveyor||£80,000|
|Hexane storage tanks||£46,000|
|Methanol storage tanks||£2,000|
|Carbon dioxide supply station||£6,200|
|Water pump station||£14,000|
|Construction and design of site||£2,000,000|
Estimated operating costs for the production of 350 kg of biomass or 100.21 kg/111.3 litres of biodiesel per day x 300 days:
|Biomass production cost||£1,680,000|
|Production of oil||£420,000|
Cost of biodiesel per litre:
300 days x 350 kg biomass = 105,000 kg biomass annually
300 days x 100.21 kg biodiesel = 30,063 kg biodiesel annually
300 days x 111.3 litres biodiesel = 33,390 litres biodiesel annually
33,390 litres / £2,100,000 = £62.89 cost of biodiesel per litre
- Selected aspect of plant design or operation for detailed analysis
To make the whole process of producing biodiesel from microalgae more economically viable making use of the by-products left over after the algae oil has been extracted is essential. Although this deoiled algae cake no longer contains lipids it still contains carbohydrates, proteins and other useful vitamins and minerals that can be isolated and made into a viable product that can be sold. The dry deoiled algae cake on its own maybe used as a nutrient rich animal feed.
One of these products is the production of nutrients for animal or human consumption. This includes removing vitamins from the left over algae which can then be made into health food supplements which there is a very large market for. In 1981 the US market for nutritional supplements was worth around $1.1 billion (US Department of Commerce, 1982) and had risen to $4.8 billion by 2003 (The U.S. Market for Nutritional Supplements, 2004). This market will continue to grow and although the US represents the largest market these supplements are also popular across the globe. Many types of algae excrete their vitamins so recovery of these will be from their culture broth. This would not be that difficult if it was not for the low concentration caused by the high culture volume. Little is really known about how much of the algae vitamin content is actually excreted but it has been reported that Anabaena hassali can excrete up to 94% of its biotin content (Borowitzka & Borowitzka, 1988). However due to the enormous numbers of species of algae and their differences it is clear that not all species will have such a high excretion rate.
Considering the proposed algae for use in this plant, D. salina, it is known that it contains tocopherol and the B-series vitamins including biotin (Borowitzka & Borowitzka, 1988). How culture for biodiesel production affects these algae constituents is unknown and will need studying. The major product produced from D. salina at this time is the secondary metabolite β-carotene. It is an orange coloured accessory pigment that can be used in food and drink production as a colouring agent and is most abundant in our microalgae D. salina (Borowitzka et al., 1984). β-carotene can also be used as a dietary supplement and is converted to vitamin A by animals (Borowitzka & Borowitzka, 1988). Because of this D. salina are already mass cultured in Australia for the production of this product with an existing viable market. β-carotene and biodiesel production may fit in quite together as both can be recovered via solvent extraction (Molina Grima et al., 2003). How well biodiesel production fits with production of other nutrients is not really known but needs to be studied to make the process as viable as possible.
One problem that could exist is that deoiled algae that have come into contact with hexane during the lipid extraction may no longer be suitable for use of its by-products for animal or human consumption. Either away round this problem should be sort or an alternative method that makes the whole process economically viable should be sort. Because the production of biodiesel is still not as cheap as fuel production from fossil fuels biodiesel production may benefit from being supplemented from the profits of other viable algae products that already exist. This way a biodiesel production plant could produce both biodiesel and for example β-carotene. Production of β-carotene is profitable and these profits could be put into the production of biodiesel. This is something that is unlikely to be carried out by private companies but could be put in place by government so to keep the price of biodiesel affordable and competitive.
As well as nutritional supplements the left over deoiled algae could also be used for the production of methane gas. The left over algae can be left to be digested anaerobically and this in turn produces both methane gas and CO2 (Gavrilescu & Chisti, 2005). This combustible gas can then be either sold on or used within the plant to make its own electricity. Using it within the plant is very attractive idea because by doing this the plant can become completely self sufficient and make biodiesel production a completely carbon neutral process.
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