Biofuels of the Third Generation. Do Microalgae Solve the Energy Problem?

Transcription

1 5 Biofuels of the Third Generation Do Microalgae Solve the Energy Problem? 10 Term Paper in Major Course Biogeochemistry and Pollutant Dynamics 15 Department of Environmental Science, ETH Zürich Author: Simon Tanner 25 Tutor: Martin Ackermann 30 35

2 Abstract With respect to the CO 2 problem on climate change, the dependence of the global economy on fossil fuels and its limited source, different energy sources than fossil fuels are needed. A possible energy source for transport fuels is biodiesel made out of microalgae. To establish this biodiesel, a lot of hurdles have to be taken; technical, biological, ecological and economical hurdles. Other alternative fuels like bioethanol have already taken some of these hurdles. These hurdles or aspects have one summarised criterion which they have to fulfil: sustainability. Behind this shibboleth there are concrete criteria for each aspect which are not all fulfilled from the already applied biofuels. Can microalgal biodiesel beat (bio-)fuels from other sources? To answer this question, a lot of aspects and their related criteria which are relevant to produce and apply biodiesel from microalgae were described and assessed; mainly the aspects of energy, technique and economy. On some criteria different results and opinions exists. But overall a big potential of microalgal biodiesel as the main (sustainable) transport fuel appeared. 55 Content Introduction State of Science Biofuels of first and second generation Third generation Cultivation of Microalgae Organisms Energy balance Technical appliance of Biodiesel Economic affairs Possible scenario to produce biodiesel from microalgae Different opinions on quantities Open questions and discussion Required Area Quantities Economics Improvements Conclusion and Assessment References Introduction 80 The global economy is massive dependent on fossil fuels. 80, 9 % of the total energy production in 2006 was from fossil fuels. The amount of used fossil fuel for energy production in 2006 was in the range of 9'000'000 k tonnes of oil equivalents [1]. The using of fossil fuels has many disadvantages. The source of it is finite. By burning fossil fuels, 2

3 CO 2 is released, which is among other things responsible for the greenhouse effect which is after all responsible for the global warming. For these reasons since a couple of years there is a lot researches in progress for alternative energy sources. This Paper will focus on the state of research in biofuels particular on the biofuels of the third generation 1. The first generation of biofuels is ethanol made out of crops. This kind of fuels has, like fossil fuels, also many disadvantages. First of all it needs agricultural space for its cultivation. This means that it is in competition with the arable land for human nutrition. The consequences of this competition are the scarcity of food and the increasing of the world food prices. In the last few years the increasing demand on biofuels had a big impact on the raising grain prices [2]. Beside this economical and world food affair aspect, there is also an ecological aspect. One of the goals of producing and using crop biofuels was the reducing of greenhouse gas emissions due to burning fossil fuels. The idea behind is that the released CO 2 due to burning biofuels is rebounding by crop growth through the mechanism of photosynthesis. But the hole gaining process of ethanol from crops consumes a lot of energy (mostly from fossil fuels). First of all the industrial cultivation and harvesting needs machines operating by fossil fuels. Then the transformation from plants to ethanol is also energyintensive. Another aspect is the cultivation of crops for biofuels in regions like Amazons. In such regions the forest clearing for gaining arable land looses the state of a sink for CO 2. In conclusion some biofuels (depending on the crop from is made of) has the worse eco-balance than fossil fuels themselves [3]. The second generation of biofuels was made out of residues from crops, animals, timber and food. This application reduces the disadvantage competition with human food. But the crop residues are an essential source of nutrients for plants. Burning these crop residues means decreasing of organic matter in agricultural soils and using more mineral fertilizer like ammonium which is made under high energy use. Supplemental the biofuels of the first and the second generation are not economical competitive. Now the third generation of biofuels/biodiesel is developing. In a very short abstraction it is biodiesel made directly by microorganisms, mainly by microalgae. This paper documents the state of science of producing biodiesel from microalgae and compares it with the first and second generation biofuels. Especially it wills asses this new generation of biodiesel in the aspects biological and technical feasibility, potential disadvantages and the economical competitiveness. The question for guiding this paper was: is the using of biodiesel from microalgae instead of fossil fuels in the near future a real alternative? The focus lies on the technical and biological applying in a significant range of amount. 2. State of Science Biofuels of first and second generation The biotechnological background of first and second generation biofuels is quite the same. Simplified it is i) carbohydrate transformation to alcohol (methanol and ethanol) and/or methane gas and ii) transformation of fatty acids with methanol to long chain alkyl esters (diesel). The difference between first and second generation biofuels is, as written above, the feedstock. Not in a chemical way, but what it can be potentially used for (human food, fertilizer etc.). For the first 1 The descriptions of first, second and third generation biofuels are not scientific consistent. This paper uses first generation biofuel as biofuel made of crops, second generation biofuel as biofuel made of residues and third generation biofuel as biofuel made by microalgae. 3

4 125 generation the feedstock consists of crops which were cultivated for this reason. The feedstock of the second generation biofuels consists of plant and animal residues. The principally production way of the first generation biofuel is fermentation of sugars to ethanol. Either the feedstock is sugar-rich plants like sugar cane or sugar beets containing a lot of sucrose Figure 1: Overview of the producing steps of the first (1.), second (2.) and third (3.) generation biofuels (C 12 H 22 O 11 ) or starch-rich plants like maize and potatoes. In this case the production must undergo a prior step to hydrolyse starch to sugars. Yeast with the enzymes Invertase and Zymase will be added to the sucrose. Invertase converts sucrose with water into Fructose and Glucose. From these products Zymase produces ethanol and carbon dioxide. The chemical reactions are: C 12 H 22 O 11 (Sucrose) + H 2 O C 6 H 12 O 6 (Glucose) + C 6 H 12 O 6 (Fructose) Invertase C 6 H 12 O 6 (Glucose/Fructose) 2 * C 2 H 5 OH (Ethanol) + 2 * CO 2 Zymase At a temperature of C the fermentation process requires about 70 hours. The waste product is dried distillers grains with soluble and can be used as livestock feed. [4] For biodiesel production oil-rich crops are needed. The crops are usually rape seeds and soy beans. The chemical reaction for the biodiesel production is called transesterification. The triglyceride molecules (fat or oil) react like a hydrolysis with methanol instead of water. The products are glycerine and biodiesel (ethyl or methyl esters). For this reaction is also a catalyst in form of sodium hydroxide or potassium hydroxide needed. Figure 2: Transesterification [6] 4

5 As mentioned above, biofuel of the second generation can be produced out of plant residues, meaning mainly lignocelluloses rich material. This means that the lignocelluloses have to be pretreated. The pre-treatment is hydrolysis of cellulose and pyrolysis of lignin. The mostly applied pyrolysis for producing biofuels is the so called flash pyrolysis. That means quickly heat the feedstock between 350 and 500 C for less than 2 seconds [5]. This destroys the crystalline structure and makes the lignin vulnerable for further hydrolysis. The hydrolysis of cellulose is mostly an enzymatic one due cellulase occurring in decomposers. The products are sugars, which can be fermented like the feedstock in the first generation biofuels producing ethanol. For the production of biodiesel of the second generation the feedstock is animal or plant fats from residues Third generation The third generation biofuels has bigger differences to the first and second generation biofuels than they have to each other. The biggest difference is the feedstock. In contradiction with the first and second generation biofuels the third generation is made out of microorganisms, primary microalgae. Table 1: Comparison of crop-dependent biodiesel production efficiencies from plant oils (modified from [7]) Plant source Biodiesel (L/ha/year) Area to produce global oil demand (hectares 10 6 ) Area required as percent global land mass Cotton , Soybean , Mustard seed 572 8, Sunflower 952 5, Rapeseed/canola 1,190 4, Jatropha 1,892 2, (0 a ) Oil palm 5, Algae (10 g m 2 day 1 at 30% TAG) Algae (50 g m 2 day 1 at 50% TAG) Area as percent global arable land 12, (0 a ) 98, (0 a ) a If algal ponds and bioreactors are situated on non-arable land; jatropha is mainly grown on marginal land. TAG = Triacylglycerids But the biotechnological pathway is the same (see transesterification above). The focus of this paper lies on the biodiesel production from microalgae. Other potentially produced fuels from microalgae are ethanol, methane and hydrogen. The cultivation of algae has many differences compared to cultivation of plants or animals for producing biofuels. To produce biodiesel from algae, it is not necessary to use arable land. And the area which is needed per entity biodiesel is much smaller than if it is made from plan oils (see Table 1). On the other hand an infrastructure like bioreactors has to be built. 5

6 Cultivation of Microalgae Today there are two known kinds of reactors which are practical to produce large scale of microalgae: The raceway (open) pond system and the Photobioreactor. Raceway ponds are commonly used and relatively simple. Since the 1950s raceway ponds are used for producing cyan bacterial biomass for food. The light feeding during daylight happens only on top which is open. With a paddle the broth is mixed and circulates through a loop to prevent sedimentation. At the end of the circulation loop behind the paddle the biomass will be harvested. The costs for raceway ponds are relatively low, but the productivity is it too. Due to the easy handling and maintain the operating of raceway ponds is cheaper than operating of photobioreactors. Also the building costs are relatively low, because it is no transparent material and a wide range of material can be used. The productivity of a raceway pond with a typically water depth of circulation stream of cm is about mg L -1 day -1 (dry weight) which means an areal productivity on an average raceway pond of about g m -2 day -1 [6][7]. Another disadvantage is the open top. Through this interaction with the atmosphere the evaporation is a big factor and water has to be refilled. Because of the gas exchange due to the open pond the use of carbon dioxide is less efficient than in photobioreactors. Another factor of the low productivity is the contamination with other microorganisms. The most applied photobioreactor is the tubular system. In difference to the open systems closed photobioreactors are constructed of transparent materials like glass or plastic. For optimising the productivity it is necessary to maximise the surface to volume proportion. For this reason the broth is in tubes of a diameter of 0.1m or less. Often these tubes are arranged in fences like solar collector. Mixing of the broth is necessary for prevent sedimentation, supply carbon dioxide and conduct oxygen. To prevent kinetic limitation a CO 2 partial pressure of 0.15 kpa, and a stochiometric demand of 1.7 g CO 2 per gram biomass has to be maintained. This makes it necessary to contribute CO 2 from an external source like coal or gas power plants. The produced oxygen by photosynthesis has to be removed out of the system. Because a high dissolved oxygen concentration can inhibit photosynthesis. And in combination with a high solar radiance it can produce photooxidants which damages algae cells. Figure 3: A tubular photobioreactor with fence-like solar collectors. [13] 210 What else than sunlight do microalgae consume? The medium within the algae grow has to consist of water, dissolved CO 2 and nutrients. There are a few reasons to use waste water or seawater instead of freshwater. First of all it is the scarcity of drinking water (on a global view). Another advantage of using waste water is the possibility of containing valuable nutrients like nitrogen, 6

7 phosphorus and trace elements. This makes a smaller amount of fertilizer necessary and the production cheaper. Seawater can also contain these nutrients but normally in a lower concentration. The problems of using waste water are the unstable concentrations of the contaminants and too high concentrations of elements and molecules which can inhibit the algal growth or kill the cells. The concentrations in waste water have a big fluctuation depending on the location and the time. From the harvested algae primary the contained fatty acids (triglyceride) are used. The residues of the algae contain also a lot of carbohydrates, proteins and other nutrients. These residues could potentially used for animal feed and/or methane production. But the methane production of algae residues is at the moment not economical competitive with methane production out of other biological compounds. Another possibility is to use the produced methane for gaining energy to run the reactor (see figure 4) 2.4 Organisms In the chapter before the cultivation of the microalgae was explained. But what kind of organisms are they or should they be? The used organisms in the already existing and running reactors are microalgae. Microalgae belong, like the land plants, to the clade of the Viridiplantae, also known as green plants [11]. In difference to the land plants, microalgae or microphytes are single cell organisms. As a consequence of this they do not have branches, leaves or roots. But they can live in chains or groups and they practise photosynthesis as well as land plants do. The microalgae for cultivating and further making biodiesel out of them, should be oil-rich and rich on saturated fatty acids (see technical appliance below), should have a high grow rate and should have a high net primary production with respect to the oil production. The oil content of microalgae, which can be used for making biodiesel are commonly in the range of 20-50% by the weight of dry biomass. But the content of other microalgae can exceed to 80%. In comparison to oil crops, microalgae can double their biomass within 24 hours [6], [7]. A possible way to maximise the biomass production is to engineer the metabolism of the organisms. For instance increasing the photosynthetic efficiency or minimise the respiration are imaginable possibilities. The optimised net primary oil production is at the end a mixture of a high content of oil consisting of saturated fatty acids and a high biomass production per time which needs not that much broth [6]. Even the energy balance would not be that good as them for first or second generation biofuels the very high grow rate is a big advantage of the third generation biofuels. 2.5 Energy balance In fact that no power plant is driven by biodiesel from microalgae yet, the comparison with other technologies is difficult. However there is an existing Life-cycle Assessment (LCA) [8] which among other things has calculated the energy balance from combusted biodiesel from microalgae. The culture consisted of Chlorella vulgaris, grew in an open raceway pond system. One culture was with normal nutrients, one with low nitrogen. Further there were two extraction methods tested, dry and wet extraction. The main result is shown in table 2. The net energy balance (NEB) is in these four treatment in a range between 0.98 and This is the ratio of the output and the input energy. A ratio below one means higher energy consumption than the energy gain. Compared with NEB from ethanol and methane made out of crops, these data are in a similar range [9]. 7

8 260 Table 2: Most Impacting Flows Generated by the Production of 1 kg of Biodiesel (modified from [8]) - normal nutrient content - dry lipid extraction - normal nutrient content - wet lipid extraction - low nitrogen content - dry lipid extraction Energy consumption (input in MJ) Energy production (output in MJ) Energy balance (difference in MJ) Energy balance (output/input) - low nitrogen content - wet lipid extraction Technical appliance of Biodiesel Biodiesel should be a one to one substitute for fossil diesel or biodiesel made of crops. Transportation vehicle or heaters should operate with biodiesel from algae instead of fossil diesel or plant biodiesel. In fact that biodiesel from crops is already used in relatively broad part, technical applications for using biodiesel exist. The problem is that biodiesel is chemically not exactly the same as diesel from oil seeds. Or in other words, there are some standards for using biodiesel. For example in European Union there are two standards, one for biodiesel in vehicles and one for biodiesel as heating oil. The main problem of microalgal oil is the high content of unsaturated fatty acids. Unsaturated fatty acids and fatty acids methyl esters have more double bounds than saturated ones. Typically alga oils content four or more double bonds. This high content of double bonds make the oil less oxidative stable than oil with a high content of saturated fatty acids. This is a problem in storage. If the oil starts to oxidise the quality of it is getting lower. The most existing standards request a lower content of the total unsaturation of the oil than microalgal oil contents. But with the technique of partial catalytic hydrogenation of the oil, the part of double bond fatty acids can be reduced. This technique is commonly used for making margarine from vegetable oils [6] Economic affairs For being economical competitive the coasts for microalgal oil may not be higher than the price of crude fossil oil. Not till then microalgal oil can replace fossil oil as a source of hydrocarbon feedstock for the petroleum industry except microalgal biodiesel will be subsidised. For this reason Chisti, 2007[6] made an equation to calculate which production costs of algal oil are economical competitive by given petroleum price: C algal oil = 6, 9 * 10 3 C petroleum with C algal oil in $ per litre microalgal oil and C petroleum $ per barrel crude oil. So this equation converts the price per barrel for crude oil into price per litre for algal oil. For example with the crude oil 8

9 290 price on November 16 th 2009 of 77, 40 $ per barrel [10], the microalgal oil should be 0.53 $ per litre. The calculated coasts for different bioreactors are between 1.40$ and 1.81$ per litre oil from microalgae [6]. This means that the current costs of microalgal oil are 2.5 to 3.5 times too high to be economical competitive Possible scenario to produce biodiesel from microalgae After all these information one big question remains: Is it feasible in foreseeable future to displace a big part of the fossil transport fuels with biodiesel from microalgae? And is the production of it ecological and economical better than the production and use of crop-derived bioethanol and biodiesel? In an opinion letter in Trends in Biotechnology (2007), Chisti postulated yes and sketched (figure 4) such a competitive production cycle [13]: Figure 4: A conceptual process for producing microalgal oil for biodiesel [13] First of all it requires some feedstock compartments for producing microalgal biomass like light, carbon dioxide, water and inorganic nutrients. The light is sunlight. For optimising the sunlight yield the surface-to-volume ratio of the reactor, respective of the broth, have to be as big as possible. In this concept a tubular photobioreactor with fence-like solar collectors has been chosen (see figure 3). A CO 2 fertilisation is required to enhance the photosynthesis. This CO 2 may come from fossil fuel power plants for low or no costs. The ideal solution would be getting the CO 2 from a power plant operated by biogas produced from the carbon-rich biomass residues. These residues were generated after gaining oil out of the microalgae. After that, the residues would be processed by anaerobic digestion, which produces biogas. If the CO 2 cycle can get closed, the use of biodiesel from microalgae is CO 2 neutral. Otherwise (in case that the CO 2 fertiliser has to come from power plant operated by fossil fuels) it is an atmospheric CO 2 source, because the production of microalgal biodiesel dependents on burning fossil fuels. 9

10 The water which can be used here is seawater like commonly used for growing microalgae, freshwater or brackish water from lakes, rivers or aquifers. The required nutrients are nitrates, phosphates, iron and some trace elements. The costs of these nutrients can be minimized by using brackish water, where some nutrients are already available. Also increasing the scale of the producing amount can decline the costs per ton significantly. After harvesting the algal biomass the broth can be recycled. That means water and the still available nutrients in it can be recovered for further algal biomass production. The residues which remain after the oil extraction are rich in carbon and nutrients. As a result of this, these residues are suitable for anaerobic digestion to produce biogas. This produced biogas generates power for running the photobioreactor. And the released CO 2 from this process is the feedstock of the photosynthesis for the algal biomass production. The solid residues after the anaerobic digestion is still nutrient rich and can be used as fertilizer or as animal food Different opinions on quantities In a facility as shown in figure 4 standing in a tropical zone, Chisti [13] postulates a microalgal biomass productivity of about 1.5 kg m -3 d -1. Assuming oil content of 30% dry weight, 100 m 3 biodiesel can be produced per hectare and year. Compared with the 0.53 billion m 3 transport fuel needs of U.S., such microalgal biodiesel plant would require an area of environ 5.4 million hectares or 3% of the U.S. cropping area. Including the assumption that at least 9360 MJ of energy per metric ton can be gained through anaerobic digestion from the microalgal biomass after the oil harvesting, the microalgal production can run independent from external energy sources. For these reasons Chisti [13] says that producing and using biodiesel from microalgae is better than biofuels from terrestrial plants. The biggest advantage is the almost closed production cycle and due this fact the CO 2 neutral production. The production of biofuels from terrestrial plants needs more fossil fuel input for cultivation and harvesting the crops. Reijnders remarks in a letter [14] that Chisti did not include the fossil fuel consumption for building and operating the bioreactor and the production of supplying nutrients like nitrogen. So he calculated the net energy yield for different biofuels production methods and for photovoltaic modules. The conclusion he found was a negative net energy yield for producing oil from the algae Dunaliella tertioltica and for a microalgae biofuel production in a state-of-the-art bioreactor (table 3). Table 3. Net energy yields in GJ has1 years1 for biofuels and photovoltaic modules [15] The topic net energy yield did not occur in the paper from Chisti [13]. So he answered Reijnders with a letters response [15] in which he calculated the energy balance of algal oil production. First he criticised the two too simple studies of the large-scale algae culture which shows little understanding of the topic. He mentioned that these studies from Hirano and Sawayama (see Table 3) overestimate the fossil energy need. Further he criticises the assumptions of the biomass productivity. In his opinion a well operated photobioreactor has doubled biomass productivity than 10

11 in the calculations from Reijnders. All in all Chisti calculated a total energy yield for algal oil production of 87, 9 MJ kg -1 where an energy input of 31, 24 MJ kg -1 oil is required (Table 4). The result is an energy ratio (NEB) of 2.8. Compared with the energy ratio of bioethanol of sugarcane which is in the range of 8 microalgal oil is not competitive. But Chisti remarks, that his calculations assume microalgal oil content of only 20% in dry weight. With possible much higher oil content, the energy ratio could exceed 7. And the most energy (about 45%) is used through fertilisation. As shown in figure 4, a big fraction of the fertiliser can be reused after harvesting and anaerobic digestion. At the end the main question is if one of these biofuels in foreseeable future can replace almost completely the fossil fuels. The energy ratio is not the only crucial criterion. For sure it has to be at least bigger than one. Another criterion is the required (arable) land for the production. In this issue Chisti [15] calculated it for bioethanol from sugarcane and for biodiesel from microalgae. If both of these fuels each should accommodate the demand in U.S. of transport fuels, bioethanol from sugarcane will require 70%, and biodiesel from microalgae will require less than 11% of the U.S. cropping area. But this is only a hypothetical value and a comparison of the required area because photobioeactors could be built on non-arable land. Table 4: Energy account of algal oil production [15] a Estimated as MJ kg1 of urea and 2.94 MJ kg1 of diammonium phosphate. b Using sedimentation followed by continuous vacuum belt filters. c Approximate only in view of the developmental nature of algal oil recovery processes. d Estimated as 80.4 MJ m2 of facility area divided by a 20 year working life and the mass of oil produced annually. e Estimated as fossil energy requirement of 27.2 MJ t1 of machinery (including equipment for biogas production) divided by the 20 year working life of equipment and the mass of oil produced annually. f Assuming the same energy content in algal oil as in rapeseed oil, or 37.9 MJ kg1. g Assuming a biogas yield and energy content of 0.5 m3 kg1 of spent biomass and 25 MJ m3 of biogas, respectively. Later Reijnders [16] compared different transport biofuels with respect to their impact on climate relevant issues and food prices. He argued that is feasible to produce biodiesel from microalgae with a positive net energy gain but only in open pond reactors. For the closed bioreactors he estimated still a bigger energy input than energy gain. And the closed bioreactors are only running under extreme conditions (i.e. high salt concentrations or ph) for suppressing unwanted organisms 11

12 400 that it is not possible to cultivate microalgae for an efficient oil yield. In such bioreactors also the possible emission of N 2 O could avoid the net release of greenhouse gases. Reijnders postulated that only biofuels from sugarcane or oil palm could help limiting climate change without an impact on food prices Open questions and discussion Simplified the main open question is: Who is right? Reijnders [14] or Chisti [13], [15]? Or in other words: Is biodiesel from microalgae a real alternative to fossil diesel? Is it feasible to produce fuels from microalgae in sufficient amounts, with almost none competition to human food, carbon neutral and economic competitive in not more than a few years? 3.1 Required Area One of the strongest and most frequent argument from Chisti [6], [13], [15] is the small consumption of (arable) land for producing microalgal biodiesel. It is a major criterion considering the impact of energy crop cultivation on food prices [2]. And it is right that reactors for microalgal cultivation can build on marginal land. But Chisti assumed always for his required-landcalculations that these bioreactors were located in tropical zones. In my opinion is this not very realistic. For producing all fuel needs of the U.S. it would need about 20 M ha [6], [15]. It may be possible to find such areas in tropical zones. But the world consumption of transport fuels is about four to five times higher as the U.S. consumption [1]. Anyway, let us assume that there is enough marginal land (about 100 M ha or 6 % of the tropic landmass) available in tropical zones for producing algal biodiesel the distribution is not calculated yet in the energy consumption of the algal oil production. The distribution of microalgal oil from the lower latitude to all over the world would need a lot of energy. Compared with fossil fuel distribution, I think it would not differ that much, because the crude oil sources are relatively local concentrated similar to the assumed microalgal oil production. But in competition with the biofuels from terrestrial plants this topic will have an influence. Hypothetically the cultivation of these crops can be evenly distributed all over the world. In this case the energy inputs per energy output for distribution to the consumers will be smaller than for the microalgal oil production scenario as described above. In my opinion the calculation of the required area for producing biodiesel of microalgae to satisfy the world demand on transport fuels has to be different as them of Chisti. For logistics reasons it is not very realistic to build all of such plants in tropic zones. So for a realistic assumption it will be better to do a mixed calculation of the biodiesel production in different climate zones Quantities The most controversial issues are the questions about the net energy yield and the impact on greenhouse gases. To think about a possible answer it is necessary to compare data which have the same background. In the question about the net energy yield in this chapter we will talk about open raceway pond reactors. The truth of the net energy yield is somewhere between the positions of Chisti [6], [13], [15] and Reijnders [14], [16]. Both calculated with their own boundary conditions and each only for one case. The newest research on this topic discussed in this paper [9] calculated the energy input and output in open raceway ponds for four different conditions. And the results 12

13 show a more differential picture than from Chisti and Reijnders. One treatment (with N fertilisation and dry harvesting) resulted in a negative net energy yield. The other three treatments were positive with respect to the energy yield. The climatic conditions of the analysed cultures were Mediterranean, thus close to the assumed tropical conditions from Chisti [6], [13], [15]. In my opinion the results from Chisti [6], [13], [15] and Reijnders [14], [16] are a bit ideological coloured and calculated with their own assumed conditions. Based on the results from Lardon et al. [8] further big improvements in microalgal cultivation are probable possible and could make the energy gain bigger. 3.3 Economics Independent from the technical details how to run a facility for cultivate microalgae, the produced biodiesel has to be economical competitive to replace fossil fuels. So the economical competitiveness is dependent on the petroleum price as shown in the economic chapter above. As we have seen the technical details are manic fold and often uncertain. So it is idle to calculate a hypothetical price for one litre of biodiesel from microalgae. I think the only way to know how high the crude oil price has to be is to run a prototype reactor in realistic industrial boundary conditions. This means an annual biomass production of at least 100'000 kg and a closed cycle as shown in figure 3. Then the costs and the gains could be easily evaluated and the price level of crude oil from where on the biodiesel is economical competitive could be calculated. Producer subsidies could also be an economical improvement. For example bioethanol from sugarcane is monetary supported by state subsidies too. On this point of view there is no reason to deny such subsidies for biodiesel of microalgae Improvements Beside all the uncertainties as discussed below, in the production chain of biodiesel from microalgae are some parts on which research is going on to improve the production efficiency primary in biological issues. These issues were not that much discussed in this paper neither were these a topic in the dispute between Reijnders and Chisti. A big part of theses issues is the enhancing of the photosynthesis efficiency. With reducing the number of the chlorophyll-binding LHC proteins the efficiency of the light conversion into starch through photosynthesis can be increased. This strategy has been found in nature [7]. These algal cells have a lighter density which results in a deeper habitat (they are living not on the water surface). And this means overall a lower efficiency in the culture. The challenge is now to engineer an algal cell which lives on the water surface and has a reduced LHC antenna. Another big issue with a high potential to enhance the yield and quality of the microalgal biomass is the improvement of the algal metabolism. Studying transcriptomics and proteomics will help to identify and understand the expressing genes and expressed proteins which are involved in the lipid synthesis way. Engineering these genes and the synthesis steps will probably increase the energy gain and the quality and/or amount of the oil (see also technical appliance ). The increasing of the energy gain should raise the NEB due higher oil content in the algae. Also the improvement of the bioreactors could increase the competitiveness of biodiesel from microalgae. The bioreactor has to have at least a biomass production of about 100'000 kg per year to make it economical competitive. To make the cultivation more efficient the surface-volume-ratio has to be maximised. Lardon et al. [8] demonstrated that the oil extraction consumes a big part of the required energy to produce biodiesel from microalgae, namely 70% for the wet extraction and 90% for the dry 13

14 extraction. So it seems to be clear that the improvement of the efficiency of these techniques could make the production much more sustainable. 3.5 Conclusion and Assessment The production of biodiesel of microalgae seems to have a big potential. Some aspects of the whole production chain are already fit to apply the biodiesel production effective compared to fossil diesel and biodiesel from crops. The technical aspect is the less contentious point. It is feasible and already proved and applied to run reactors to cultivate and harvest high oil containing microalgae. The question of the net energy gain (see Different opinions on quantities above) is here not included. Uncertainties exist on the feedstock, mainly the CO 2 fertilisation, at least at the commissioning of a bioreactor. For an efficient biomass production the algal broth has to be fed with CO 2 from power plants running by fossil fuels. In the result this means an equal net CO 2 release to the atmosphere than without such a bioreactor but more energy gain. And the goal should be to replace the fossil fuels by biodiesel. A part of the CO 2 to fertilise the algal broth could come from the produced and burned methane which is made out of the algal biomass residue. But it needs more CO 2 because a part of the sequestrated CO 2 is bound in the harvested oil. Also the required nutrients are not exactly calculated. This could be a problem, mainly the phosphorus. Phosphorus is a worldwide limited resource. A big demand would compete with the fertilisation of crops for human nutrition because the cultivation of crops requires phosphorus fertilisers. But a big part of the phosphorus, in the form of biomass residues after anaerobic digestion (figure 4), could be reused for fertilise arable land. A big advantage of cultivation microalgae is the use of brackish water. The limitation of safe freshwater is a big global problem. So biodiesel is in this topic much better than fossil fuels or fuels from crops. The conveying and refining of crude oil creates a lot of contaminated water, depending on the technique. And in lot regions crops have to be irrigated With all the (partial contradictory) information above I would say that biodiesel from microalgae has the highest potential of the alternative biofuels to replace almost all the fossil transport fuels in a sustainable way. In fact that the NEB calculated in table 2 comes from newer research than the data which Chisti and Reijnders used in their dispute, the assumption of a feasible and energetic rentable biodiesel production is right. The high potential is originated in the fact that is known where the technical process steps can be improved. Even if the NEB of third generation biodiesel is lower than the NEB from biofuels of crops, microalgal biofuels could gain more energy in the same time period than first or second generation biofuels. That is because the NEB is always calculated for one production cycle. In fact that microalgae can double their biomass within about 24 hours (much faster than any cultivated land plant), the production cycle for biodiesel from microalgae is much shorter and the net energy gain per time is much bigger than them for biofuels out of crops. Under the biological and technical conditions as described above it is already feasible to produce biodiesel from microalgae. But at the moment the costs of the production are still too high or the crude oil price is too low. Assuming that the price of fossil fuel in foreseeable future will increase depending on the shortage of those resources, the biodiesel from microalgae will become automatically economical competitive. The possibility that a lot of countries will introduce a carbon tax in the next few years is relative high. This would also raise the crude oil price and make the diesel from microalgae more economic competitive. But there are also ways to make the production cheaper or more efficient for make the production more economical independent of the crude oil price. For example the implementation of state subsidies for microalgal biodiesel is realistic. Another big question is if this biodiesel has the better eco-balance than biodiesel made out of other 14

15 feedstock. Exactly these eco-balances are a big problem because of their complexity. An ecobalance for making different biodiesels comparable has to include many compartments. First of all the net energy balance has to be calculated, indeed from cradle to grave. This means that also the so called grey energy (hidden energy like energy from transport, from building and running the facilities or energy to produce fertiliser) has to be included. If this balance is negative (NEB < 1), it is senseless to produce such a fuel. Another important factor for the comparison between biofuels from microalgae and biofuels from crops is the impact on environment and climate, particularly the net CO 2 balance has to be estimated and/or calculated. For calculating the CO 2 balance, aspects like the decreasing of the carbon pool in soils due cultivation of energy-rich crops have to be considered. Other environmental aspects like water consumption and pollution or heavy metal emissions have to comprise into the life cycle assessment. In a comprehensive LCA also aspects on issues which could have impacts on social peace have to be a part in the calculation. For example the raised food prices because of the high demand on biofuels from crops have also to be considered. At the end all these very different aspects has to be standardised on an equal entity and then weighted. It seems clear that such an assessment is not an exact science and that the weighting and standardising is depending on the used calculation technique and on the person who does it. For these reasons the results of different life cycle assessments for the same product never correspond with each other. In such a case you have to assess the assumptions, data and calculations of each LCA to form an own opinion about the different results. After all I mean that politics and industry should support the production and use of biodiesel from microalgae. I think that such a biofuel has at least the potential to have a better eco-balance than the eco-balance of fossil fuel or of biofuel of the first and second generation. To asses this eco-balance or make comparable LCAs of these fuels it is necessary to produce and apply microalgal biodiesel in an industrial magnitude. States have to guarantee a price for microalgal biodiesel as an incentive for potential investors to build and operate such plants. The other option is that states take on the role of investors and owners of microalgal biodiesel plants References [1] International Energy Agency (2009); [2] Rosegrant, M.W. (2008); Biofuels and Grain Prices: Impacts and Policy Responses; International Food Policy Research Institute, Washington, USA [3] Gnansounou, E. et al. (2009); Life cycle assessment of biofuels: Energy and greenhouse gas balances; Bioresource Technology, 100 (2009), p [4] Zinoviev, S. et al. (2007); Background Paper on Biofuel Production Technologies; Area of Chemistry, ICS-UNIDO, International Centre for Science and High Technology United Nations Industrial Development Organization [5] Wikipedia (2009); 15

16 [6] Chisti, Y. (2007); Biodiesel from Microalgae; Biotechnology Advances, 25 (2007), p [7] Schenk, P.M. Et al. (2008); Second Generation Biofuels: High-Efficiency Microalgae for Biodiesel Production; Bioenergy Research, Nr. 1, 2008, p [8] Lardon L. et al. (2009); Life-Cycle Assessment of Biodiesel Production from Microalgae; Environmental Science & Technology, Vol. 43, Nr. 17, [9] Power N. M., Murphy J. D. (2009); Which is the preferable transport fuel on a greenhouse gas basis; biomethane or ethanol?; Biomass and Bioenergy, Vol. 33, Nr. 10; 2009 [10] Tecson (2009); [11] Wikipedia (2009); [12] Wikipedia (2009); [13] Chisti Y. (2008); Biodiesel from microalgae beats bioethanol; Trends in Biotechnology, Vol. 26, No. 3 [14] Reijnders L. (2008); Do biofuels from microalgae beat biofuels from terrestrial plants?; Trends in Biotechnology, Vol. 26, No [15] Chisti Y. (2008); Response to Reijnders: Do biofuels from microalgae beat biofuels from terrestrial plants?; Trends in Biotechnology, Vol. 26, No. 7 [16] Reijnders L. (2009); Acute View Transport biofuels: Can they help limiting climate change without an upward impact on food prices?; Journal of Consumer Protection and Food Safety, Nr. 4,