7.1 Introduction
7.2 Biomass production potential and efficiencies
7.3 Fuel alcohol production from biomass
7.4 Methane fermentations
7.5 Fuels derived from microalgae
7.6 Conclusions
References
Biological energy production begins with the photosynthetic fixation of CO2 into biomass (starches, lignocellulosics, etc.) and is followed by conversion of biomass via various microbial processes to fuels (ethanol, methane, hydrogen, oils), as discussed in previous chapters. In the case of algal production of hydrogen and vegetable oils, both processes are conducted by a single organism. Even in these cases there is a clear differentiation between the photosynthetic processes of CO2 fixation (and oxygen production) and the subsequent conversion of the fixation products to renewable fuels. The production of the waste biomass is of little concern in the conversion of waste materials, such as in methane fermentations. Still, ultimately, the source of all biological fuels is photosynthesis, carried out by plants and algae. The efficiency of photosynthesis (Chapter 2) is, thus, a central issue in the future development of these renewable biological energy sources.
Other overriding issues in the future of biological energy systems are the overall efficiencies of converting biomass to useful fuels, the economics of such processes, their environmental impacts, their competitiveness with thermochemical conversion processes for biomass (combustion, gasification), their resource potential, and, perhaps most important, their compatibility with evolving economic and political structures. Biofuels would for example, complement solar electricity in the renewable energy mix of the future. This chapter presents some projections on the development of renewable biological energy systems in the 21st Century.
The human race is already, directly and indirectly, exploiting a large fraction, almost half (1) of the total primary production of the planet - through agriculture, fisheries, forestry, and other activities. Indeed, few productive ecosystems around the world remain in their natural state, not diminished by the actions of mankind. As global ecosystem exploitation and destruction reaches its end-point sometime in the first half of the next century, and natural environments cease to exist outside a few more-or-less protected enclaves, our reliance on the bounties of nature will end. We will then depend entirely on how well we can manage the remaining biological and physical assets of our planet, to sustain the existence of a human population whose current growth rate is only exceeded by its accelerating consumption of natural resources. In addition, we will need to manage these resources within the context of a rapidly changing global environment, with unpredictable climate, ever scarcer raw materials, and diminishing productive land, which is being covered by settlements or devastated by exploitative practices. To mention just a few of the challenges we are leaving for the next generations.
If catastrophe - economic and population collapse - is to be avoided, we must not only curb populations and consumption, but must also develop and implement more efficient and environmentally benign technologies, more available to the large populations currently not enjoying the benefits of our technological economy. Biological energy systems can play an important role in this transformation of the human economy and condition, necessary for our survival through the 21st Century.
Photosynthetic efficiencies are the ultimate limiting factor in biological energy production. Currently agricultural and forestry efficiencies seldom exceed 1% of total solar energy input converted to a recoverable product. However, even with current productivity's, the potential for photosynthesis to produce food, fiber, fuel, and forest products is truly enormous. Total primary productivity exceeds current fossil fuel consumption by an order of magnitude. Recently a group of experts projected the potential of improved forest preservation and management and agricultural practices to mitigate the greenhouse effect by sequestering CO2 into long-term biomass and soil carbon and by replacing fossil fuels with biofuels. They concluded that the entire fossil fuel CO2 emissions could be balanced with biological systems (2). In addition, this could be accomplished without diminishing the production of food, fiber, and forest products. Even conservative projections by this group indicated that biological processes could generate greenhouse gas off-sets equivalent to almost half the current annual atmospheric CO2 rise, and at relatively low costs. These actions do not even require the heroic measures of "planetary management" suggested by some, such as increasing primary productivity through trace nutrient fertilization of oceanic and terrestrial environments (3).
Such global projections must of course be tempered with more detailed assessments of specific local and regional possibilities of biomass energy systems. For example, in most projections of biomass systems for mitigation of greenhouse gases, it is assumed that photosynthetically fixed CO; can be sequestered for long periods (> 100 years) into above ground biomass, soil carbon, and other long-term storage forms. This is problematic, and would become a limiting factor within a few decades, as forests reach their maximum (climax) biomass levels. Thus, the major emphasis must be the development of biological energy systems that use biomass fuels to replace fossil fuels, and thereby reduce emissions of CO2 and other greenhouse gases. Fundamental to such systems is maximizing the efficiency of the first stage of the processes + photosynthesis.
As reviewed in Chapter 2, maximal photosynthetic efficiencies are currently projected at 10% of total solar energy conversion, based on laboratory studies. In practice less than half of this has been documented in even the most productive outdoor experiments, and in plant cultivation only a 1 to 3% conversion efficiency is maximally obtained, of which less than half is typically recovered in the actual product (grain, fiber, etc.). For trees much lower productivity's are noted, in large part due to the minimal inputs (fertilizers, pest control, management) into forestry.
For microalgae systems, which are of very high productivity's, results from experimental and commercial systems are not significantly different from the more productive higher plants in similar climates. However, if laboratory results with microalgae cultures could be achieved in outdoor ponds, a 10% solar conversion efficiency should be possible, i.e. about a three-fold increase over currently achieved productivity, over 200 mt/ha/yr (metric tons/hectare/year). Why have such high solar efficiencies and productivity's not been achieved to-date?
The answer is the light saturation effect: the rate of photosynthesis does not increase with increasing light intensities. This is because at high light intensities, e.g. sunlight, the number of chlorophyll molecules absorbing a photon greatly exceeds the capacity of the photosynthetic apparatus to process this trapped energy bounty. The excess photons arc lost as heat or fluorescence, with the result that light is wasted by algae near the surface, while those below are shaded, resulting in decreased productivity's. Although this problem was recognized decades ago (4), and even the possible solutions proposed at that time, little progress has been made in this area in several decades.
One proposed solution is the use of prisms or optical fibers that transmit light to the deeper layers of the ponds. Although feasible in laboratory settings, the use of such devices is economically prohibitive. Another approach, is rapid mixing of the culture, so that, on average, after a photon has been absorbed by the photosynthetic apparatus, the algae are transferred to the dark, where they can process the photon energy at relative leisure. However, this requires such rapid mixing as to also be only a laboratory curiosity (though of great fundamental interest).
This leaves the third approach + to reduce the chlorophyll content of the algae so as to avoid, or at least minimize, photon absorption, and, thus light wastage and self-shading. A reduced chlorophyll content could be accomplished by the modem techniques of genetic engineering, and this approach presents the clearest method for achieving high photosynthetic efficiencies, approaching the theoretical maximum, with outdoor (full sunlight) algal cultures (5).
It must be recognized, however, that this would only be a first, though critically important, step in the overall effort to develop high productivity, low-cost microalgal production systems. Another step is the development of algal strains capable of producing a high yield of a desirable product, whether it be starches for fermentation to ethanol, vegetable oils, hydrocarbons, or hydrogen. Again we can invoke the powers of modem molecular biology to accomplish these goals, in particular the rapidly evolving technology of metabolic engineering, which allows manipulation and control of entire enzyme pathways. Metabolic engineering could allow for the simultaneous optimization of photosynthetic efficiencies and product formation. A two stage process would likely be optimal, similar to higher plant agriculture, where vegetative biomass production is followed by grain formation.
Low chlorophyll algal strains, and more generally, metabolically engineered strains that produce high levels of desirable products, could not compete with wild-type algae, just as crop plants cannot compete with weed species. Like agricultural systems, algal ponds suffer from invasions by grazers and diseases. A new technology of "microecological engineering" will need to be developed to prevent, if not at least minimize, such contamination and allow open pond mass culture of selected species, particularly of genetically/metabolically engineered strains. The alternative approach, closed photobioreactors, is not practical, except as a small inoculum production stage. As argued below, microecological engineering, the integration of molecular and fermentation biotechnologies with microbial ecology, will also be required in the future development of the other biological energy conversion processes addressed in this report + ethanol and methane fermentations.
Recently, Greenbaum et al. (6) reported the surprising finding that mutants of the green alga Chlamydomonas missing half the photosynthetic apparatus normally associated with CO2 fixation and hydrogen production, still carried out both reactions at near normal rates. This suggests that algal cultures may actually require less than the 8 to 10 quanta of photons needed for reducing one CO2 molecule to carbohydrates suggested by the accepted mechanisms of photosynthesis. Although such increased photosynthetic efficiencies remain to be demonstrated, this remind us that surprises are still occurring even at the most fundamental levels of our understanding biological processes. This also exemplifies the difficulty of projecting any future technology. In any event, the goal of achieving much higher than current efficiencies with both microalgae and higher plant, outdoors in sunlight, does appear plausible, and justifies continued and focused R&D. The insights and techniques developed for maximizing microalgae mass culture productivity's could also be applied to higher plant photosynthesis. Although the limitations of light saturation are generally not as severe as in an algal ponds, considerable scope is available for improving the productivity of higher plant production systems. Of greatest potential, in terms of both yield improvements and increasing global biomass resources, would be the application of such techniques to forestry. Thus, microalgae systems, whose R&D can be accelerated due to their short generation times, can in the future not only contribute to the specific goals of microalgal energy systems (Chapters 5 and 6, see also below) but, perhaps most important, could also lead to more efficient, productive, and economical exploitation of higher plant photosynthesis and biological energy systems generally.
One of the most immediate and important applications of biological energy systems could be in the production of ethanol from biomass. In the U.S. in excess of 5 billion liters per year of ethanol fuel is currently produced by fermentations, using corn starch as the substrate. This is a conventional technology, which is economically competitive only because of the large government subsidies to encourage corn production and its conversion to fuel ethanol. Although some process improvements are possible, no major breakthroughs are foreseen, nor is the cost of corn starch likely to become more attractive, in light of the increasing food needs of the world's population. Energy and greenhouse gas emissions balances are also not particularly favorable for com-to-ethanol systems, being at best only slightly positive, and then only if more expensive, capital intensive, technology is used. Lower cost fermentation substrates and processes are required.
As reviewed in Chapter 3, lignocellulosic biomass is a potential substrate for ethanol fermentations. This requires first pre-treating the biomass to open up the lignocellulosic polymers, to allow their hydrolysis by either enzymes or acids, followed by fermentation of the hexose and pentose sugars derived from the cellulose and hemicellulose fractions, respectively. The residual lignin can be used as a boiler fuel, or as a starting material for higher value products (phenolics, polymers, etc.). Extensive research has been carried out in this field for decades (7), but practical processes have yet to be demonstrated. Indeed several problems hamper such a processes: pretreatment is cumbersome; enzyme hydrolysis is slow and requires large amount of enzymes, while acid hydrolysis is fast, but results in poor yields and many degradation products that inhibit fermentations; and the fermentations themselves are complicated by the two types of sugars present, as well as the potential for contamination. As regards contamination, the sterilizable fermenters used in laboratory research, and even most demonstration projects, are not practical for large-scale commercial operations. In conventional corn starch fermentations, contamination is minimized by the relatively rapid batch fermentation process, selective conditions (e.g. low pH), inoculation and clean (though not sterile) start-up and operating conditions. The key problem for ethanol fermentation of lignocellulosics is the long times required for the enzymatic saccharification, which is preferred over acid hydrolysis as it results in higher yields. The exponential increase of contamination problems with cycle time, is a major limitation of the enzyme hydrolysis process.
Recently research has concentrated on the development of improved processes for enzymatic hydrolysis and fermentation of the pentose sugars using genetically engineered microbes (8). As in the case of microalgae culture in open ponds, microecological engineering techniques will need to be developed to maintain such strains in large systems, subject to invasion and contamination by potentially much faster growing wild microbes. Such microecological techniques would relieve the constraints of having to maximize the amounts and activities of the enzymes used in this process, or maintain strictly (expensive) aseptic conditions.
Projections of the future cost of producing ethanol from lignocellulosic biomass are as low as $0.13/liter ($0.50/gallon) of ethanol (9), compared to almost thrice this for current processes. Such low-cost projections are based on numerous favorable assumptions; one being that the same microbial culture would simultaneously provide the cellulolytic enzymes and ferment the whole suite of carbohydrates generated by enzymatic hydrolysis. This eliminates the costly enzyme production and separate fermentation stages of the process. Although such projections are very optimistic, they point out the critical R&D objectives required to achieve the very low cost processes that would make this technology viable, not only in competition with fossil fuels but also with alternative, lower cost, biomass conversion processes, such as gasification and combustion.
For the near-term future, the technology for producing ethanol from cellulosic biomass will likely begin with utilization of the most readily fermentable and lowest cost resources available, such as paper mill and food processing wastes. In the longer-term, agricultural residues, less suitable for thermochemical conversion processes because of their high ash content, are very large potential resources for ethanol production. However, this still requires a major R&D effort. Planned demonstration projects in the U.S. should allow an assessment of how far the technology has progressed, and how much it may still need to be advanced. It is difficult to predict' the relative market shares of the various technologies for lignocellulosic biomass energy conversion: combustion, gasification, pyrolysis, ethanol fermentation, or, considered next, methane fermentations. Whatever the conversion technology, in the future lignocellulosic biomass will certainly continue to be a, if not the, major renewable energy source, as it already is for the majority of people on earth.
Methane fermentations, reviewed in Chapter 4, are currently widely used in the conversion of municipal sludge and some industrial (e.g. food processing) wastes to biogas, a mixture of methane and CO2. Some animal manures are also converted to biogas in developed countries (in particular Denmark) and low-cost farm-scale systems fed agricultural and animal wastes are in widespread use in some less-developed countries, in particular China and India. In addition, over the past decade several advanced processes were commercialized for the utilization of low-strength wastes (Chapter 4). Despite these applications, only a small percentage of the potential resources suitable for methane fermentations is currently utilized.
One reason for this, being that the technologies for biogas utilization have lagged behind the processes for methane fermentations (also called anaerobic digestion). Biogas is produced in relatively small amounts, so replacement of natural gas is generally not feasible, after considering the costs of gas clean-up (to remove CO2 and other impurities) and compression. Biogas storage is generally not practical, at least not more than about one day's worth of biogas. Thus, biogas must be used essentially as and where produced. For moderate to relatively large facilities electricity generation from biogas is the best option (10). However, this requires generators, which, after capital and operating costs are considered, add about $0.025/kWhr to the price of the electricity, in addition to the cost of the biogas. Alternatively, biogas can be used as a vehicular fuel, after removing most of the CO2 and moderate compression. Such systems have been used at several municipalities in the U.S., and can provide cost competitive transportation fuels for specialized applications (e.g. local service fleets). However, such approaches are only suitable where sufficiently large and sustained waste resources are available, and local costs of conventional power or transportation fuels are high. For smaller-scale systems, the utilization of the biogas has to be coupled with domestic or small commercial/industrial uses, in cooking, water heating, and similar applications.
Apart from biogas utilization, the technology for its production still requires improvement, particularly for high solid, high lignocellulosic wastes and residues. Agricultural residues, just as in the case of ethanol, could provide a very large potential resource for biogas production. However high solid fermentation processes must still be developed. The highly mechanical systems currently in use in Europe for anaerobic digestion of municipal solid waste are generally not practical, except where cost is (almost) no object. Low-cost, high solids methane fermentations using plastic liner-enclosed batch pits, can be considered. However, their practicality remains to be demonstrated. The management of such systems, including addition of moisture, nutrients, and inoculation with adapted microbial cultures, to prevent volatile acid overproduction (which would inhibit the process) and rate limitations, remains to be developed, as is the control of gas production to better match biogas production with utilization.
Biogas production is certainly the simplest, potentially lowest cost, biological energy production process, compared with the other technologies highlighted in this report. The large greenhouse effect of methane gas, some ten times that of CO2, makes any mitigation (capture, use) of anthropogenic sources of methane, such as from animal wastes, even more valuable than other biological energy sources, which mitigate CO2 emissions by replacing fossil fuels. Thus, biogas technologies, including recovery of gas from landfills and from animal manure, should receive additional consideration based on this benefit.
Methane fermentations require the development of microecological engineering techniques geared toward predictable, controlled, maximal gas production, better fitting production with utilization. Such process control techniques could initially best be applied to stirred tank fermenters and the innovative and multistage continuous hydraulic processes reviewed in Chapter 4. Additionally, hydrogen fermentations should be included in this management concept, either for the production of methane-hydrogen mixtures for vehicular fuels (a volumetric mixture of about 70:30 CH4:H2 significantly reduces tail pipe emissions) or even production of pure hydrogen (11). The greater challenge will be the development of process control techniques for high solids batch processes or covered lagoons, systems which provide both the largest future potential and greatest technological challenges for methane fermentations.
Microalgae mass cultures have been considered for almost fifty years as potential biofuel sources, with the first conceptual engineering analysis presented in the late 1950's (12). Initially proposals were for the production of biomass suitable for methane fermentation in combination with waste water treatment (13). More recently, emphasis has been on higher value fuels, particularly hydrogen (11, 14, see Chapter 5) and vegetable oils (15, see Chapter 6), in stand-alone energy production systems, preferably coupled to utilization of CO2 from power plants. Compared to other biological energy systems, either combined with waste conversion or as stand-alone systems (growing of energy crops followed by utilization as, or conversion to, fuels), microalgae systems are relatively undeveloped, requiring longer-term R&D.
In the case of hydrogen production by microalgae, two fundamentally different processes can be considered: those involving direct electron transfer from water to hydrogen, or indirect processes, in which first CO2 is fixed into a fermentable substrate, which then is converted to hydrogen via algal metabolism (16). The former approach suffers from the oxygen sensitivity of the hydrogen evolution reaction, among other limitations, while the latter is more complex, requiring multiple stages and more challenging metabolic control. A recent economic analysis of a large-scale (over 100 hectares) two-stage indirect process, concluded that this concept had economic potential, with costs projected at $12 to 15 per million BTU (million British thermal units) (17). Direct biophotolysis systems would be more expensive, as the entire surface area would need to be covered in closed photobioreactors, vs. only one tenth of this for two-stage process. The above cost estimate was based on a 10% solar conversion efficiency in the first (CO2 fixation) stage and a 100% solar conversion efficiency in the second (H2 evolution) stage; ambitious, but not unreasonable long-term R&D goals. (In the second stage most of the energy comes from stored starch, allowing for such high solar efficiencies). Of course, this is a largely conceptual process at present, although based on known metabolic processes in microalgae.
Apart from photosynthesis, the hydrogen evolution reactions themselves must also be efficient. This eliminates, almost a priori, the enzyme nitrogenase in hydrogen production, as this enzyme has a high ATP requirement + reducing efficiencies by about half. Although the reversible hydrogenase enzyme has a much higher inherent efficiency, high yields with this enzyme remain to be demonstrated. In dark anaerobic fermentations of organic substrates to hydrogen, yields are low, typically 20% or less of stoichiometric (18,19).
By contrast, photosynthetic bacteria can convert many organic substrates, including waste materials, stoichiometrically to hydrogen (and CO2), assisted by light energy. These high yields have made this process a subject of much R&D. Unfortunately these bacteria use the nitrogenase enzyme, lowering potentially achievable solar conversion efficiencies. Similarly, most research on indirect hydrogen production by cyanobacteria has used nitrogen-fixing species. In the future, microalgae that produce hydrogen via a reversible hydrogenase should be favored. In any event, photosynthetic hydrogen production will require long-term R&D. Among the many challenges the design of closed, low-cost, photobioreactors, in which the light-driven hydrogen evolution reactions would take place, is of importance.
Similarly, the production of lipids, that is vegetable oils and hydrocarbons, by microalgae is a long-term R&D objective. Essentially a similar two-stage process to that conceived for hydrogen production could be envisaged: in the first of which algal biomass would be produced, followed by a lipid induction stage. Both stages would however be open ponds, of much lower cost than the closed photobioreactors envisioned for the hydrogen production stage discussed above. Lipids would be used as vehicular fuels, either directly or after processing to "biodiesel" (methyl or ethyl esters). The CO2 required for growing the algae would be obtained from power plant flue-gases, or similar sources. Thus, this process would be a method for directly reducing greenhouse gas emissions from power plants. Although likely more expensive than most other biomass energy systems, microalgae ponds provide one of the lower cost methods for directly utilizing and mitigating power plant CO2 emissions, compared to other options such as ocean disposal (15).
Microalgae biotechnology has advanced considerably over the past few decades, from a laboratory curiosity to relatively large-scale (> 10 hectares) commercial systems based on open ponds and, in total, producing several thousand tons of biomass. However, this technology is only in its infancy. Only two algal species, Spirulina platensis and Dunaliella salina, can be cultivated outdoors with assurance, by providing chemically selective growth media (high bicarbonate and salinity, respectively). Cultivation of high-lipid microalgae in open ponds has been successful at the 0.2 hectare demonstration-scale (20), but remains to be developed as a practical process. By combining such processes with other objectives, in particular waste water treatment and the production of higher value products, including foods and chemicals, nearer-term development and applications of microalgae processes for renewable energy production are likely.
The biofuels discussed above and in prior chapters are only a few of the renewable energy products that can be produced with higher plants and microbes. Plant and algal photosynthesis is the engine that drives the global biological carbon cycle, which greatly exceeds the fossil fuel carbon cycle, while providing us with all our food and forest products, and, even today, a significant fraction of our fiber and fuel needs. There is great potential for a major expansion of biomass systems, both for "green energy" and for other renewable resources. Such expansion must primarily come from increases in productivity, i.e solar conversion efficiencies, without, however, as has been the case in the "agricultural revolution", increased fertilizers, chemicals, and other non-renewable inputs into the cultivation systems.
Microalgae can help guide the way to these objectives, by demonstrating the fundamental approaches to increasing photosynthetic efficiencies. Microalgae production systems can also lead in the development of microecological engineering techniques, required to achieve the process stability and low costs in "open" microbial processes for production of ethanol, methane, and hydrogen from conventional biomass sources. Microalgae processes, combining waste treatment with the production of higher value products and renewable fuels, can demonstrate how multipurpose/multiproduct systems maximize environmental and economic benefits, compared to dedicated, single purpose, energy farming systems.
The future success of biomass energy products will depend on their economics, relative to the competitive costs of other sources. Currently biomass energy sources are generally not economically competitive, due to the low prices of fossil fuels. However, low fossil fuel prices are due not only to the low cost of extracting fossil fuels, but, and perhaps most important, to the fact that their prices do not reflect their true costs: to the environment, society, and our future. These "externality" costs of fossil fuel resources are paid by all, in particular future generations, in environmental degradation, climate change and unsustainability of our economic systems. Although biomass energy systems are already competitive with fossil fuels in some specific cases, major expansion of renewable bioenergy will require not only advances in technology but also in economic accounting of their lower environmental and societal costs, compared to fossil fuels.
Of course, bioenergy systems have actual and potential environmental impacts of their own. Agriculture and managed forestry represent the largest-scale human intervention, often wholesale destruction, of natural ecosystems, leading to loss of their benefits, from biodiversity to management of the hydrological cycles (1). Adding to this environmental insult the injury of large-scale, intensive biomass systems, could make their environmental impacts greater than even those of fossil fuels. However, there is another way: the development of sustainable, multi-purpose, integrated biomass systems, based on highly efficient photosynthesis and microbial processes, generating a variety of products, including fuels. Such systems have the potential of reducing the environmental impacts of agriculture and forestry, while providing the food, fiber and even much of the fuel, required for an acceptable standard of living for all of mankind in the 21st Century.
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