能源微藻综述-Second Generation Biofuels-High-Efficiency Microalgaefor Biodiesel Production

Second Generation Biofuels: High-Efficiency Microalgae for Biodiesel Production Peer M. Schenk & Skye R. Thomas-Hall & Evan Stephens & Ute C. Marx & Jan H. Mussgnug & Clemens Posten & Olaf Kruse & Ben Hankamer Published online: 4 March 2008 # Springer Science + Business Media, LLC 2008 Abstract The use of fossil fuels is now widely accepted as unsustainable due to depleting resources and the accumulation of greenhouse gases in the environment that have already exceeded the “dangerously high” threshold of 450 ppm CO2-e. To achieve environmental and economic sustainability, fuel production processes are required that are not only renewable, but also capable of sequestering atmospheric CO2. Currently, nearly all renewable energy sources (e.g. hydroelectric, solar, wind, tidal, geothermal) target the electricity market, while fuels make up a much larger share of the global energy demand (∼66%). Biofuels are therefore rapidly being devel- oped. Second generation microalgal systems have the advantage that they can produce a wide range of feedstocks for the production of biodiesel, bioethanol, biomethane and biohydrogen. Biodiesel is currently produced from oil synthesized by conventional fuel crops that harvest the sun’s energy and store it as chemical energy. This presents a route for renewable and carbon-neutral fuel production. However, current supplies from oil crops and animal fats account for only approximately 0.3% of the current demand for transport fuels. Increasing biofuel production on arable land could have severe consequences for global food supply. In contrast, producing biodiesel from algae is widely regarded as one of the most efficient ways of generating biofuels and also appears to represent the only current renewable source of oil that could meet the global demand for transport fuels. The main advantages of second generation microalgal systems are that they: (1) Have a higher photon conversion efficiency (as evidenced by increased biomass yields per hectare): (2) Can be harvested batch-wise nearly all-year-round, providing a reliable and continuous supply of oil: (3) Can utilize salt and waste water streams, thereby greatly reducing freshwater use: (4) Can couple CO2-neutral fuel production with CO2 sequestration: (5) Produce non-toxic and highly biodegradable biofuels. Current limitations exist mainly in the harvesting process and in the supply of CO2 for high efficiency production. This review provides a brief overview of second generation biodiesel production systems using microalgae. Keywords Algae . Carbon sequestration . Biofuel . Biogas . Biohydrogen . Biomethane . Bioreactor . Lipid . Oil . Raceway pond . Triacylglycerides . Review Abbreviations BTL biomass to liquid CFPP cold filter plugging point CO2-e-CO2 equivalents of greenhouse gases NEB net energy balance LHC light harvesting complex OAE oceanic anoxic event PS photosystem Bioenerg. Res. (2008) 1:20–43 DOI 10.1007/s12155-008-9008-8 P. M. Schenk (*) : S. R. Thomas-Hall : E. Stephens School of Integrative Biology, University of Queensland, St. Lucia, Queensland 4072, Australia e-mail: p.schenk@uq.edu.au E. Stephens :U. C. Marx :B. Hankamer Institute for Molecular Bioscience, University of Queensland, St. Lucia, Queensland 4072, Australia J. H. Mussgnug :O. Kruse Department of Biology, AlgaeBioTech Group, University of Bielefeld, Bielefeld, Germany C. Posten Institute of Life Science Engineering, Bioprocess Engineering, University of Karlsruhe, Karlsruhe, Germany RuBP ribulose-1,5-bisphosphate Rubisco ribulose 1,5 bisphosphate carboxylase/ oxygenase TAG triacylglycerides Significance of Biodiesel Production from Algae Introduction The development of CO2-neutral fuels is one of the most urgent challenges facing our society. In the last 18 months, this fact has been brought into sharp focus by the Stern Report ‘The Economics of Climate Change’ [170] and the Intergovernmental Panel on Climate Change ‘AR4 Synthe- sis report’ [84]. Together these reports provide the most comprehensive evaluation of the causes and effects of climate change currently available. Importantly they define atmospheric CO2 levels above 450 ppm CO2-e (i.e. e = equivalent contribution of all greenhouse gases) as already being in the dangerously high range and conclude that we exceeded this threshold (currently 455 ppm CO2-e) 10 years earlier than had previously been predicted. These new and startling conclusions are leading govern- ments to establish CO2 emission reduction targets which typically lie in the range of 10–20% by 2020 (e.g. European Union). Germany recently suggested a 30% cut by 2020 if matched by other nations. Yet these proposed emission reductions are not sufficient to stabilize CO2 levels in an accepted ‘safe zone’ (i.e. below 450 ppm CO2-e; IPCC [84]). Indeed they are more likely to lead to stabilization levels above 550 ppm. At these levels much more destructive effects are predicted [84]. So, although a range of CO2 reduction scenarios to stabilize emissions have been developed, the IPCC concluded that to stabilize in a range of 445–490 ppm of atmospheric CO2-e, emissions must peak before 2015 and a total CO2 reduction of 50–85% will be required by 2050. Importantly, the IPCC indicates that this might be an underestimate due to missing carbon cycle feedback effects in the current climate change models. Consequently more stringent emission reductions (e.g. 60% by 2020) are increasingly thought to be more appropriate and such concern has led Rajendra Pachauri, the chairman of the IPCC to state, “What we do in the next 2 or 3 years will define our future” [126]. Achieving a 60% CO2 emission reduction by 2020 or even a 50–85% cut by 2050 is, however, an enormous global challenge which will require the development of a suite of renewable energies. There is now a concerted effort to develop new biofuels, of which biodiesel and bioethanol are seen as being close to market options. The areas of biomethane, biomass-to-liquid (BTL)-diesel and biohydro- gen are also developing rapidly. This review provides a brief overview of the background and recent developments in second generation microalgal systems with a specific focus on biodiesel, which have the potential to couple renewable, CO2-neutral fuel production with power plant flue and atmospheric CO2 sequestration. Specifically, it aims to define the current state of the art, and to identify major opportunities for future innovation in the biodiesel field. Biofuels and Biodiesel in the Global Context The global energy market can essentially be divided into the electricity and fuel sectors. Both sectors will have to achieve significant emission reductions to meet planned international legislated targets. Currently, the electricity sector accounts for approximately 33% of global energy and is developing a range of low CO2-emission approaches for electricity production (i.e. nuclear, solar, wind, geother- mal, hydroelectric, clean-coal technology). In contrast, fuels accounted for a much larger (∼67%) market share of the global energy consumption ∼15.5 TW (489 EJ/year) in 2005 according to the Energy Information Administration, USA. Yet, despite the obvious importance of fuels, CO2- neutral (e.g. biodiesel, bioethanol, biomethane, BTL-diesel) and CO2-free (e.g. biohydrogen) fuel production systems are far less developed. The range of biofuels available demonstrates the flexibility and potential of the biofuel industry. However, this potential was not fully realized by the first generation biofuel systems due to serious economic and environmental limitations (see “Addressing the Concerns of Biofuel Production” and “Economic Feasibility of Microalgal Biodiesel” sections). In contrast, second generation biofuel systems (such as lignocellulosic and microalgal biofuel systems) have the potential to overcome many of these limitations and target a newly emerging clean energy market which is predicted to expand rapidly to a value of US $500bn by 2050 [170], or more (current oil market value is US $2.65 Trillion). By far the largest proportion of biofuels is produced from higher plants which use photosynthesis to convert solar energy into chemical energy (Fig. 1). In nature, this chemical energy is stored in a diverse range of molecules (e.g. lignin, cellulose, starch, oils). Lignocellulose, the principle component of plant biomatter, can be processed into feedstock for ethanol production. This can be achieved by either gasification or by cellulolysis (chemical or biological enzymatic hydrolysis). These processes are currently being developed for second generation biofuel systems [41, 155] and are often referred to as ‘lignocellu- losic processes’. Similarly, starch (e.g. from corn) and sugar Bioenerg. Res. (2008) 1:20–43 21 (e.g. from sugarcane) are already being converted into bioethanol by fermentation [23, 74], while oils (e.g. from canola, soy and oil palm) are being used as a feedstock for the production of biodiesel [80, 156]. Microalgae are able to efficiently produce cellulose, starch and oils in large amounts [16, 162]. In addition, some microalgae and cyanobacteria (which produce glycogen instead of starch) can also produce biohydrogen under anaerobic conditions [20, 35, 59, 76, 111] and their fermentation can also be used to produce methane (Fig. 1; “Other Biofuels from Microalgae” section). The Flexibility of Biofuel Production Systems Central to all light-driven biofuel production is the process of photosynthesis. It is the first step in the conversion of light to chemical energy and ultimately responsible for driving the production of the feedstocks required for a wide range of fuel synthesis (Fig. 1): protons and electrons (for biohydrogen), sugars and starch (for bioethanol), oils (for biodiesel) and biomass (for BTL, biomethane). In higher plants and green algae, light is captured by specialized light harvesting complex proteins, referred to as LHCI and LHCII (Fig. 1). These are encoded by a large gene family that exhibit a high degree of homology [51] and their expression is dependent on the prevailing environmental condition (e.g. light intensity). These pro- teins bind the bulk of the chlorophyll and carotenoids in the plant and play a role both in light capture and in the dissipation of excess energy which would otherwise inhibit the photosynthetic reaction centres, in particular photosys- tem II (PSII; [81]). Excitation energy used to drive the photosynthetic reactions is funnelled to the photosynthetic reaction centres of photosystem I (PSI) and PSII via the highly coordinated network of pigments bound by the LHC, PSII and PSI subunits. In the first step PSII uses this energy to drive the photosynthetic water splitting reaction, which converts water into protons, electrons and oxygen. The electrons are passed along the photosynthetic electron transport chain via plastoquinone (PQ), cytochrome b6 f (Cyt b6 f), photosystem I (PSI), and ferredoxin (Fd) and on to NADPH (Fig. 1). Simultaneously, protons are released into the thylakoid lumen by PSII and the PQ/PQH2 cycle. This generates a proton gradient, which drives ATP production via ATP synthase. The protons and electrons are recombined by ferredoxin-NADP+ oxidoreductase (FNR) to produce NADPH. NADPH and ATP are used in the Calvin cycle and other biochemical pathways to produce the sugars, starch, oils and other bio-molecules (which collectively form biomass) that are required to produce bioethanol, biodiesel, biomethane and BTL-based biofuels. Alternatively in some photosynthetic microorganisms like the green alga Chlamy- domonas reinhardtii, the protons and electrons extracted from water (or starch) can be fed to the hydrogenase enzyme (HydA) via the electron transport chain to drive the direct photo-production of biohydrogen (Fig. 1). The production of Fig. 1 The process of photo- synthesis coverts solar energy into chemical energy and is key to all biofuel production systems in plants 22 Bioenerg. Res. (2008) 1:20–43 biohydrogen by the algae and biomethane from biomass as a downstream product are briefly discussed in the section on “Other Biofuels from Microalgae”. The Calvin cycle is an integral part of the photosynthetic process and responsible for fixing CO2, in a diverse range of organisms including primitive algae through to higher plants. The process uses ATP and NAD(P)H generated by the light reactions. In C4 and CAM plants it is coupled to ancillary processes that aid CO2 fixation, but the funda- mental photosynthetic reduction cycle reactions remain the same [175]. The Calvin cycle can be divided into three main steps that involve carboxylation, reduction and substrate (ribulose-1,5-bisphosphate (RuBP)) regeneration. The first step at which CO2 enters the cycle to react with RuBP is catalyzed by ribulose-1,5-bisphosphate carboxyl- ase/oxygenase (rubisco). The importance of rubisco is hard to overstate as essentially all carbon found in living organisms on Earth was once fixed by this enzyme from atmospheric CO2. Furthermore it is the most abundant protein on Earth, constituting some 30% of total proteins in most leaves [129]. This is partly because of its central role for photosynthesis, but also because it has a very low catalytic carboxylase performance, using as little as 2– 3 RuBP per second [104]. As its name suggests rubisco has two catalytic functions; it functions as a carboxylase as part of the photosynthetic reduction cycle, and under aerobic conditions as an oxygenase as part of photorespiration. O2 and CO2 compete for the same catalytic site, so that the efficiency of CO2 fixation can be impaired in certain aerobic environments. For example, although the specificity of the enzyme is higher for CO2 (e.g. tobacco (higher plant) 82 times, Griffithsia monilis (red alga) 167 times, Rhodospirillum rubrum (purple non-sulfur bacteria) 12 times [7]), the molecular ratio of O2/CO2 is about 540:1 in air and 24:1 in air-saturated water at 25°C. In the first step of the Calvin cycle, rubisco catalyzes the formation of two 3-phosphoglycerate molecules from RuBP, CO2 and H2O. The forward reaction is strongly favoured by the negative change in free energy of the process. In the second step, an ATP/NADPH-dependent reduction phase, these carboxylic acids are reduced to form two molecules of glyceraldehyde-3-phosphate, by the action of phosphoglyc- erate kinase and glyceraldehyde-3-phosphate dehydroge- nase. In a third step, consisting of a series of reactions a proportion of glyceraldehyde-3-phosphate is converted back to RuBP required to allow the photosynthetic reduction cycle to continue [175]. Addressing the Concerns of Biofuel Production Although biofuel processes have a great potential to provide a carbon-neutral route to fuel production, first generation production systems have considerable economic and environmental limitations and recently, the issue of biofuels has become a hot debate. While benefits of biofuels are emphasized by some [157, 167], others have criticized the economics and carbon mitigation potential of biofuel production ([19, 128, 131, 147, 190]). The most common concern related to the current first generation of biofuel systems is that as production capacities increase, so does their competition with agricul- ture for arable land used for food production. For example, current biodiesel supplies from oil-producing crops, supple- mented with small amounts of animal fat and waste cooking oil, only account for an estimated 0.3% (approx. 12 million tons in 2007) of the current global oil consumption [24, 125] and can not even come close to satisfying the existing and future demand for transport fuels. Currently approximately 8% of plant-based oil production is used as biodiesel [125] and this has already contributed to an increase of the price of oil crops over the last few years. Area Requirements for Biofuel Production It is estimated that the surface of the Earth (510,072,000 km2) receives ∼170 W m−2 of solar power on average [31, 196]. This equates to 2.735 YJ of energy per year and corresponds to ∼5,600 times the global consumption of primary energy in 2005 (488 EJ; [196]). Consequently the solar energy required to produce biofuels is available in abundance. However, even if current oil-producing crops would be grown on all arable land (assuming 29.2% of the Earth is land of which 13% are arable, energy conversion efficien- cies of 1% from sunlight to biomass, and 20% yield as oil), these would be able to cover less than half of our energy demand today. Using such conservative figures, biofuel critics have often concluded that biofuel production can not contribute in any major way to global fuel requirements. However as will be highlighted here, much higher photosynthetic efficiencies and oil yields are already achievable and so open up second generation biofuel technologies with enormous potential. The increased pressure on arable land has already resulted in considerable problems and unsustainable practises worldwide that have led to the coining of the phrase “peak soil”. For example, rainforest regions in Brazil and South East Asia are currently being cleared at an unprecedented rate to make room for soybean and oil palm plantations for the production of biodiesel. Annual biodiesel production growth rates in Indonesia, Malaysia and Thailand are currently between 70% and 250% [181]. The increased pressure on arable land currently used for food production could lead to severe food shortages, in particular for the developing world where already more than 800 million people suffer from hunger and malnutrition (figure without China; [53]). In addition, the intensive use of land with high fertilizer and pesticide Bioenerg. Res. (2008) 1:20–43 23 applications and water use can cause significant environ- mental problems. Net Energy Balance When evaluating the value and sustainability of a biofuel production process it is necessary to establish its Net Energy Balance (NEB). The NEB of first generation biofuels taking into consideration the energy required for farming, harvesting, processing, trans- port, etc. has recently been estimated to be ∼25% for corn ethanol and + ∼93% for soybean biodiesel [80], though precise values are dependent on detailed case by case life cycle analyses. While this report counters claims that energetic costs of fertilizer applications, farming machinery and processing facilities cause negative NEB values for both biofuels, it does not as yet factor in projections by the Intergovernmental Panel on Climate Change that traditional crop yields could fall up to 50% by 2020 [84]. Carbon Balance Calculating the carbon balance of a process is equally important. Biofuel production from crops is often estimated to be a near carbon-neutral process as nearly all carbon in conventional biofuel crops is derived from atmo- spheric CO2, which is released on combustion. However to be precise the overall CO2 emissions balance for biofuel production, must also be evaluated on an individual produc- tion case basis and include energy-intensive fertilizer produc- tion, the use of machinery for cultivation and refinery, and transport which are processes that currently emit fossil fuel- derived CO2. Furthermore, the production of oil from oil palm plantations established prior to the legislation of Kyoto emissions targets, is considered to lead to improved reductions of emissions compared to conventional diesel. In contrast however, if rainforest regions must first be cleared to make room for the plantation, the CO2 emissions balance is exceedingly poor [14]. CO2 Sequestration One important development which is predicted not only to greatly improve the net CO2 balance of biofuel processes, but actually contribute to atmospheric CO2 reductions, is the coupling of biofuel production to CO2 sequestration systems. These typically involve the production of Agri-char