Petroleum-based fuels have been the primary type of transportation fuel in the United States for decades. Until the 1960s, domestic production of these fuels met the vast majority of the nation’s demand. U.S. oil production peaked in the 1970s, but demand continued to grow. The desire to reduce reliance on foreign oil imports and to improve energy security sparked interests in research and development (R&D) of alternative fuels. In 1978, the U.S. Department of Energy’s (DOE) Office of Fuels Development initiated the Aquatic Species Program whose goal is to produce renewable transportation fuels from algae (Sheehan et al., 1998). That program furthered the understanding of algae’s potential as a feedstock for fuel through its development and characterization of a large collection of oil-producing algae, its research to improve understanding of the biological triggers for enhancing oil production in algae, and its work on demonstrating open-pond systems for large-scale algae cultivation (Sheehan et al., 1998). Biofuels derived from algae and cyanobacteria1 were considered a promising alternative fuel for improving energy security for the following reasons:
• Microalgae, macroalgae,2 and cyanobacteria convert solar energy to chemical energy for their growth and development through the process of photosynthesis. Some species also can be grown in heterotrophic conditions, where an exogenous source of organic carbon is provided.
1 Cyanobacteria, also called cyanoprokaryotes, were historically known as blue-green algae. For simplicity, biofuels derived from macroalgae, microalgae, and cyanobacteria grown under photosynthetic conditions all are referred to as algal biofuels.
2 Macroalgae are multicellular algae that lack true roots and leaves. Macroalgae are found in fresh water and marine water, soil, and growing on other organisms.
• Unicellular algae and cyanobacteria have the advantage of being able to complete a reproductive cycle in a matter of hours or a few days. Therefore, they can be harvested on a daily or weekly basis.
• The oil productivity of many species of algae exceeds that of oil crops (Patil et al., 2008).
During its two decades of operation, the Aquatic Species Program built a collection of more than 3,000 species of oil-producing microalgae (Sheehan et al., 1998). Program research shed light on algal physiology and biochemistry and the relationship between oil content in cells and algal productivity. Efforts also were made to demonstrate the feasibility of large-scale cultivation of algae in open ponds (Sheehan et al., 1998). The Aquatic Species Program was terminated in 1996 when DOE was under budget pressure. At that time, the price of oil was less than $20 per barrel (EIA, 1999). In contrast, a technoeconomic analysis conducted in 1982 estimated algal biofuels would cost about $60 per barrel of oil equivalent under an optimistic scenario and about $120 per barrel of oil equivalent under a conservative scenario (Benemann et al., 1982).
Volatile oil prices observed from 2000 to the present renewed interests in alternative fuels. In addition, mounting evidence of global climate change raised concern over the carbon footprint of using fossil fuels. Greenhouse gases (GHG)—such as carbon dioxide (CO2), nitrous oxide, and methane—are heat-trapping gases that produce a warming effect on the Earth’s atmosphere. CO2 emissions from burning of fossil fuels account for a large portion of GHG emissions (NRC, 2011a). In the United States, the use of petroleum-based fuel in the transportation sector accounted for 30 percent of the nation’s CO2 emissions in 2009. Although using algal biofuels for transportation would produce tailpipe emissions comparable to those from using petroleum-based fuels, algae and cyanobacteria take up CO2 during growth and thereby offset some of the CO2 emissions (Brune et al., 2009). In addition, the net impact on CO2 emissions also depends on the quantity of fossil fuels used throughout the algal biofuel production pathway. Life-cycle assessment (LCA), discussed later in this chapter, attempts to account for and aggregate the energy requirements and CO2 impacts over the whole production pathway.
Domestic production of renewable fuels including algal biofuels has the potential to meet the dual goals of improving energy security and decreasing GHG emissions from the transportation sector. However, a dramatic decrease in foreign oil importation and reduction in GHG emissions in the United States will require the production and use of multiple alternative transportation fuels. Biofuels produced from algal feedstock could be one of the alternatives. The number of startup companies working on the development of algal biofuels has been increasing, and some oil companies are investing in algal biofuels (Mascarelli, 2009; Mouawad, 2009). The U.S. military is interested in substituting part of its fuel use with renewable energy sources including algal biofuel (Physorg.com, 2010), and the Defense Advanced Research Projects Agency funded projects for developing technologies to produce affordable algal biofuels (Lundquist et al., 2010). Given the interest in algal biofuels, the DOE Energy Efficiency and Renewable Energy’s (DOE-EERE) Office of Biomass Program (OBP) held a workshop in 2008 “to discuss and identify the critical barriers currently preventing the economical production of algal biofuels at a commercial scale” (DOE, 2010). DOE and private companies are actively investing in R&D for algal biofuels to resolve technical barriers, improve feasibility of large-scale production, and reduce costs. In addition to developing production technologies, any developing industry also needs to consider sustainability. Addressing sustainability concerns and challenges as the industry develops can help ensure its success well into the future. Ignoring sustainability at the
Statement of Task
The committee is tasked to examine the promise of sustainable development of algal biofuels, identify potential concerns and unforeseen sustainability challenges and unintended consequences for a range of approaches to algal biofuel production, explore ways to address those challenges, and suggest appropriate indicators and metrics that can inform future assessments of environmental performance and social acceptance associated with sustainability. Although economics is an important aspect of sustainability, the study will not assess costs of algal biofuels. Algal biofuel production approaches and technical systems are still emerging, and facilities have not reached commercial scale. Public data on the economics of algal biofuel production is sparse. Therefore, it is premature for the committee to conduct generalized economic analyses of algal biofuels.
The committee will:
• Identify the potential sustainability concerns for commercial production (including larger centralized and smaller distributed facilities) of algal biofuels associated with a selected number of different pathways of biomass production and conversion. Potential concerns to be addressed could include the availability and use of land, water, and nutrient resources; human health and safety associated with feedstock cultivation and processing; potential toxicity associated with algal metabolites and their adverse impacts on downstream coproducts; and other impacts that are of social and environmental concern.
• Identify information or data gaps related to the impacts of algal biofuel production.
• Suggest indicators and metrics to be used to assess sustainability concerns across the algal biofuel supply chain and data to be collected now to establish baseline and to assess sustainability. Identify indicators that are most critical to address or have the greatest potential for improvement through DOE intervention. This input will inform DOE-EERE OBP’s broader analysis of biofuels and bioenergy sustainability.
• Using selected approaches as illustrations, discuss whether any, or combinations of, the identified challenges could present major sustainability concerns. Are there preferred cost and benefit analyses that could best aid in the decision-making process, and could those decisions be performance based and technology neutral?
outset might exacerbate the sustainability issues for future generations and make it difficult for an industry to successfully scale-up (Azapagic and Perdan, 2000).
At the request of DOE-EERE’s OBP, the National Research Council (NRC) appointed an independent committee to examine the sustainable development of algal biofuels. (See Appendix A for committee membership.) The purpose of this study is to identify and anticipate sustainability concerns associated with large-scale deployment of algal biofuels, discuss potential mitigation strategies, and suggest indicators and metrics that could be used and data that could be collected to evaluate sustainability across the biofuel supply chain to monitor progress as the industry develops (Box 1-1).
1.2.1 Defining Sustainable Development
“Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs” (United Nations, 1987). Most definitions of sustainability include and integrate an economic, an
environmental, and a social dimension (Hammond, 2000; IISD, 2011; United Nations, 2011). A recent NRC report identified four key societal sustainability goals for agriculture. Those goals are:
• “Satisfy human food, feed, and fiber needs, and contribute to biofuel needs.
• Enhance environmental quality and resource base.
• Sustain the economic viability of agriculture.
• Enhance the quality of life for farmers, farm workers, and society as a whole.” (NRC, 2010b; p.23)
In the context of algal biofuels, the goals of sustainable development can be framed as follows:
• Contribute to energy security, particularly the domestic supply of transportation fuel.
• Maintain and enhance the natural resource base and environmental quality.
• Produce fuel that is economically viable.
• Enhance the quality of life for society as a whole.
The four aspects of sustainability are interconnected in many ways, some of which are synergistic or mutually reinforcing, others of which might involve tradeoffs among goals. An example of synergy could be technological improvements in algae and cyanobacteria production and in processing the biomass to fuels. Those improvements would enhance fuel yield, contribute to energy security, increase resource use efficiency, and reduce cost of production, and therefore contribute to transportation fuel needs and improve environmental, economic, and social sustainability. An example of a tradeoff could be pollutant management, which would contribute to maintaining environmental quality and minimizing human-health impacts but could add to the cost of production.
1.2.2 Components of Sustainable Biofuel Development
As in the case of plant-based biofuels (NRC, 2011b), algal biofuels could provide opportunities to improve energy security, reduce GHG emissions, and maintain and enhance the resource base and environmental quality, but their production also could raise sustainability concerns. Whether those opportunities will be realized depends on how the industry develops. It is prudent to consider potential sustainability concerns that might arise and to avoid or mitigate them as the industry develops. Sustainability of plant-based biofuels has been discussed, and criteria for assessments have been developed by various entities over the past decade (ESA, 2008; Markevicius et al., 2010; NRC, 2010a,c). Examples of sustainability criteria are shown in Table 1-1. Many of the sustainability criteria apply to algal biofuels.
220.127.116.11 Energy Security
Whether and how much algal biofuels would contribute to energy security depends in part on the resources (for example, land and water) available for algal biofuel production, the productivity of algae cultivation, the yield of the processing of algae to fuel, and the ability to integrate the various components of algal biofuel production into one functional system and to scale it up. Resource limitations bound how much algal biofuel could be
TABLE 1-1 Examples of Sustainability Criteria Used to Evaluate Plant-Based Biofuels.
• Cost of production
Cost competitiveness with respect to other fuel alternatives.
• Economic development
Effects on the standard of living and economic health.
• Fiscal effects
Effects on fiscal balances.
Resource Use and Environmental
• Energy balance
Energy output in fuel per unit of energy input to make the fuel over its life cycle.
• Resource use including land and water
Land and water requirements to produce one unit of fuel.
• Pollutant emissions including GHG and criteria pollutants
Emissions (for example, CO2 and sulfur oxides) over the life cycle of one unit of fuel.
Effects on ecological species and communities (for example, habitat destruction or enhancement).
• Competition for resources being used for other human activities
Effects of resource use (for example, water and nutrients) for biofuel production on other activities (for example, farming food crops and animals).
• Cultural acceptability
Acceptability of the effects of biofuel production.
• Visual impacts
Perception of landscape aesthetics.
• Health effects
Effects of emissions (for example, air-quality emissions) on human health.
produced, but technological progress could enhance the productivity of algal feedstock and fuel yield.
Cost of production is an important aspect of sustainable development that applies to all nonpetroleum-based alternative fuels including algal biofuels. Alternative fuels are not likely to penetrate the fuel markets if they are much more expensive for consumers than other fuel alternatives (NRC, 2008, 2011b). Although government policies and subsidies can facilitate and accelerate the market penetration of biofuels, the biofuels eventually would have to become economically viable without subsidy. Brazilian ethanol was heavily subsidized when Brazil’s National Alcohol Program was initiated, but the government subsidies gradually were phased out in the 1990s. Sugar-cane ethanol has been economically viable in Brazil since 2003 (Solomon, 2010).
As discussed in Box 1-1, the committee was not asked to analyze costs of algal biofuels. Published estimates for costs of algal oil and algal biofuels span a wide range of about $1-$25 per gallon (Williams and Laurens, 2010; Gallagher, 2011; and references cited therein). The wide range reflects a number of factors including when the estimate was made. The cost estimates reported in the literature were not in constant dollars and therefore are not directly comparable. Some cost estimates were for algal oil before upgrading to fuels. A wide range of technologies could be used in an algal biofuel production system resulting in varying costs. This range in estimated costs reflects the immaturity of algal biofuel production and the uncertainties associated with a developing industry (Williams and Laurens, 2010). It is still premature to analyze and draw any conclusions about the economic sustainability of algal biofuels, particularly when costs likely will decrease with ongoing
technological developments. Although this report does not address costs of algal biofuels, it makes occasional reference to economics if there are known critical synergies or tradeoffs between economics, productivity, resource use, and environmental effects.
18.104.22.168 Resource Use and Environmental Effects
Land, water, and nutrients are required for cultivating plants and algae. Cropland acreage in the United States has been decreasing in the past few decades (Nickerson et al., 2011), and the water levels of some aquifers used for irrigated agriculture have been declining (NRC, 2001). Nutrient runoff from row-crop agriculture into surface water and its environmental effects has raised concerns (NRC, 2009). Some of these concerns for resource use and availability and for the environment might be alleviated by developing algal biofuels because the production of algae and cyanobacteria biomass does not require high-quality land resources, as in the case of the production of sugar cane or corn for ethanol, and soybean or other oilseeds for biodiesel (Schenk et al., 2008). Algae and cyanobacteria can be grown in saline waters or nutrient-rich wastewater that is not suitable for agriculture or human consumption (Woertz et al., 2009; Bhatnagar et al., 2010; Chinnasamy et al., 2010; Craggs et al., 2011). In addition, enriching algae and cyanobacteria cultures with CO2 and other nutrients helps maximize photosynthetic algal biomass production on a large scale. One suggestion is to co-locate algal biomass production sites with stationary industrial CO2 emission sources like fossil fuel-fired power plants to integrate the plant CO2 emissions with the algal cultivation system. Another suggestion is to locate algal biomass production facilities near wastewater sources, such as municipal wastewater treatment plants. Algae cultivation systems can use the nutrients present in wastewater that has undergone primary or secondary treatment thereby serving as a nutrient removal component of wastewater treatment. An important issue then to assess is the number of potential sites for algae cultivation that are near both a source of CO2, such as fossil-fired power plants, and a source of nonpotable water, such as wastewater or saline water. Resource use and maintaining the quality of the natural resource base necessary for developing algal biofuels will play a role in the sustainable development of algal biofuel. This report focuses on the sustainable development of algal biofuels with respect to resource use and effects on the environment.
22.214.171.124 Social Well Being
Although biomass production of algae and cyanobacteria is not likely to compete for high-quality arable land with crops, there could be social concerns about land use that need to be considered in the development of algal biofuels. For example, situating algae and cyanobacteria biomass production in the U.S. desert Southwest could be perceived as a good use of low-value land by some, but as an intrusion into pristine land by others. Similarly, the use of genetically engineered organisms in production systems could affect social acceptability. This report discusses how the resource use and environmental effects of large-scale algal biofuel production could affect the social acceptability of algal biofuels.
1.2.3 Sustainability of Transportation Fuel
The preceding section mentioned some potential sustainability concerns for large-scale development of algal biofuels (which will be discussed in detail in later chapters along with opportunities to mitigate them), but the sustainability of algal biofuels cannot be viewed in isolation and needs to be put into the broader context of the transportation-fuel sector
for two reasons. First, there is not one alternative fuel that can replace all the petroleumbased fuels used in U.S. transportation. Few options are available to reduce petroleum use (NAS-NAE-NRC, 2009), and algal biofuels could become a future option for reducing petroleum use and GHG emissions from the transportation sector. Second, every fuel source has its positive and negative effects on the resource base or other aspects of the environment. Therefore, the overall sustainabilities of different fuels have to be compared to assess whether replacing one fuel with another would contribute to improving sustainability. Therefore, the committee cautions that the report is not to be read as a mere list of sustainability concerns, but as a discussion of resource use and environmental effects that need to be compared with those of other fuels to see which fuel option is more sustainable or better balances the various sustainability objectives.
This section presents a brief overview of the tools and methodologies used for assessing the sustainability of algal biofuels in this report. The objective here is not to provide results from the application of these methodologies to algal biofuels, but to provide a brief description of the approaches used in this report and how they help meet the overall objectives of providing indicators and approaches to measuring the sustainability of algal biofuels. It focuses on several basic concepts: the systems analysis framework, indicators of sustainability, LCA, and futures or scenario analysis. Indicators are repeated measurements, observations, or model results that “are used to represent or serve as proxies for impacts of outcomes of concerns” (NRC 2010b, p.32). LCA and futures analysis are methodologies for estimating resource use and environmental effects. Systems analysis is an integrating conceptual approach for evaluating impacts of algal biofuels.
1.3.1 Systems Analysis Framework
As Holmes and Wolman (2001) have pointed out, the systems analysis approach emphasizes the development of comprehensive strategies and impact assessments by integrating all “critical physical, biological, socioeconomic, and engineering processes and constraints into a unified framework” (Figure 1-1). Typically quantitative models are used to define the most effective outcome or tradeoffs among multiple outcomes for a given set of system inputs. Historically, the application of this methodology involved “elucidating the objective(s) in the solution, developing a comprehensive description [of the system], formulating alternative solutions, and [quantitatively] analyzing the alternatives with respect to the magnitude and distribution of their consequences” (Holmes and Wolman, 2001, p. 177). The systems analysis framework is particularly applicable to algal biofuels. Of all of the current renewable energy alternatives, biofuels derived from algae and plant-based resources represent one of the most complex systems integration challenges. Part of the complexity is due to the diverse set of feedstocks, and logistical and conversion technologies that designers of bioenergy systems can select from as major components of a biofuel industrial ecology. In addition, many of these technologies are at different evolutionary stages of development ranging from an intriguing possibility to large-scale pilot demonstrations. Further adding to this complexity is the diverse way that these technologies can be integrated to design and implement advanced biofuel systems. This diversity in the mixing of technologies and the possible integration schemes is a driver for innovation as currently seen in the diverse commercial approaches to algal biofuel development. At the
FIGURE 1-1 Schematic representation of a production system, including system inputs and outputs.
same time, this diversity creates challenges for documenting critical material, energy, and monetary flows needed to assess performance.
Understanding the performance of alternative designs for producing liquid fuels from algae requires the adoption of a systems framework for assessing alternative designs. The systems framework illustrates the interdependent nature of the individual supply chain components and the system inputs and outputs. The understanding developed from such a representation is fundamental for applying a wide array of sustainability tools such as LCA, engineering process modeling, and cost-benefit analysis.
Biofuel sustainability indicators are metrics of defined aspects of sustainability that represent system status or progress toward sustainability goals. Some researchers and institutions distinguish between definitions of indicators and metrics, while others see substantial overlap in the concepts. The definition of an indicator used in this report is “a measure that is somehow indicative of some unmeasurable environmental goal such as environmental integrity, ecosystem health, or sustainable resources” (Suter, 2001). Indication of sustainable development of algal biofuels is indirect, through the union of metrics of resource use, other environmental impacts, social acceptance (all considered in this report), and economics and energy security (not considered in this report). Specific metrics of water quality or quantity or GHG emissions, for example, are viewed as indicators of sustainability or sustainable development.
Because sustainability includes environmental, economic, and social dimensions (in addition to energy and energy security, which may be classified separately), indicators also typically are divided among these categories. Categories of resource requirement indicators that have been discussed for biofuels include total and consumptive water use,
nutrient use, total land use, and net energy return, and categories of environmental indicators include net GHG emissions, water quality, and biodiversity. This report emphasizes the sustainability of the broad environment and thus presents categories of indicators of aspects of the environment that are pertinent to algal biofuels. This report also emphasizes the sustainability of resource use that determines the viability of the biofuel system. Indicators at this interface between environmental and economic sustainability also are presented and discussed in this report. Specific sustainability indicators pertaining to other aspects of the economy (for example, international trade, profitability, employment) are beyond the scope of this study, though clearly these will influence and be influenced by indicators of environmental sustainability. Social indicators of biofuel sustainability often are not derived or considered, but such potential indicators for algal biofuels could be developed. However, the focus of this study is on environmental sustainability and indicators related to environmental impacts and natural resource requirements.
Because sustainable development implies progress toward sustainability goals, it is important to understand baselines for indicators of sustainability. Moreover, the attribution of particular environmental and social effects to algal biofuel production requires an understanding of baseline and reference conditions. An appropriate definition of a baseline is conditions that would have prevailed in the absence of algal biofuel production. In principle, the baseline incorporates dynamic land-use and associated environmental changes in the region, but in practice it is often simpler and more certain to consider the conditions that prevailed prior to biofuel production.
The use of particular units can influence the way that sustainability indicators are interpreted (Turnhout et al., 2007; Corbière-Nicollier et al., 2011; Efroymson et al., 2012). Units may include volume or mass of resources required; concentrations, emissions, or loadings of chemicals to environmental media; and abundance of organisms or habitat area. The units may have denominators of land area, energy produced, or volume of fuel. Choosing a denominator such as land area or volume of fuel can facilitate comparisons between alternative land uses or fuels but also can add to the uncertainty associated with an indicator. For example, land area may include the area for infrastructure or the area for infrastructure plus a buffer. Including time as a factor in an indicator allows the duration of an environmental effect to be considered. Including coproduct quantities in the divisor of an indicator can imply that decision makers have determined that part of an effect should be formally attributed to the coproduct.3
Sustainability and sustainable development encompass diverse goals and targets that relate to dynamic human values. Movement toward sustainability cannot be assessed unless the specific goals are defined and targets and metrics for aspects of sustainability are selected. Many international organizations are developing sustainability indicators for biofuels. These include the Roundtable on Sustainable Biofuels (RSB), the G-8-endorsed Global Bioenergy Partnership (GBEP), and others (van Dam et al., 2008). Additional organizations have been created to promote sustainable biofuel industries, such as the Council on Sustainable Biomass Production in the United States (CSBP, 2010). The International Sustainability and Carbon Certification system for biomass and bioenergy has been implemented globally by 750 stakeholders from 45 countries (ISCC, 2012). Some organizations recommend a large number of sustainability indicators. For example, RSB (2011) recommends more than 200
3 Coproducts are commercial products such as nutrient supplements, animal feedstuff, or chemical feedstocks that can be coproduced from the pathways that produce algal biofuels and marketed. For example, after lipids have been extracted from algal biomass, the lipid-extracted biomass might be processed to become animal feedstuff or animal feed supplements.
indicators and measures of biofuel sustainability. A challenge is to winnow generic lists of biofuel sustainability indicators to a suite that is appropriate for a particular assessment problem and is technically and economically practical (McBride et al., 2011; Efroymson et al., 2012). Many of the efforts to develop generic biofuel sustainability indicators have focused on plant-based biofuels—corn ethanol, cellulosic biofuels, and agricultural biodiesel. Therefore, some recommended indicators may not be pertinent to algal biofuels, and some important potential indicators may not appear on previously published lists.
Turnhout et al. (2007) suggested that the successful application of indicators is specific to each situation. What typically leads to a sustainability assessment is a decision or other purpose, combined with sustainability goals. Sustainability goals may include concepts such as efficient use of resources, maintenance of water quality, maintenance of biodiversity, and minimization of waste (Sydorovych and Wossink, 2008). Indicators would have to be selected to reflect goals. Moreover, the context of a biofuel sustainability assessment is important for selecting, measuring, and interpreting sustainability indicators (Efroymson et al., 2012). The context for the application of sustainability indicators includes the purpose of the assessment, the region, the scale of analysis, the relevant policies context, the decision context (including stakeholders), and available data on baselines and reference scenarios (Efroymson et al., 2012).
A sustainability assessment for algal biofuel production may entail comparing algal biofuels with business-as-usual scenarios for energy use (that is, using mostly petroleumbased gasoline in transportation as is done today), alternative energy sources (for example, other biofuels or other algal biofuel pathways), previous land uses or land uses that would have occurred in the absence of biofuel production, or alternative sites for the facility. These comparisons may lead assessors to prioritize various sustainability indicators differently and may lead to different measurement or modeling methods and units.
1.3.3 Life-Cycle Assessment
LCA is a set of methods, databases, and tools that aims to characterize the environmental impacts over a life cycle of a product or service. LCA is defined as “a systematic set of procedures for compiling and examining the inputs and outputs of materials and energy and the associated environmental impacts directly attributable to the functioning of a product or service system throughout its life cycle” (ISO, 2006). The life cycle in the context of algal biofuel production refers to a chain of activities that includes extraction of raw resources, producing materials, manufacturing, transportation, use, and disposal (Baumann and Tillman, 2004; EPA, 2006). Figure 1-1 shows a schematic of the chain of activities involved in a production system, and LCA attempts to account for and aggregate a resource requirement or an environmental impact over the whole pathway. However, it is generally infeasible to analyze every process in a life cycle. Data and knowledge limitations imply that LCA entails selection of a “system boundary” that delineates processes included in the analysis versus those excluded.
One approach to LCA involves numerical modeling of material flows in supply chains. The idea is to map a target product to a set of activities or processes (or sectors) and use input-output tables to estimate cumulative material flows per unit product. An input-output table delineates requirements of inputs to generate a set of outputs (for example, iron ore, coal, and electricity as inputs for crude steel as an output). This input-output approach has the advantage of being able to rely on economy sector level data to quantify the relationship between energy, resources, and the final products (Miller and Blair, 2009). However, given the nascent nature of algal biofuel production, the LCAs discussed in this study will focus
primarily on the process analysis approach to LCA that uses specific information on the energy, nutrients, and emissions associated with each component of the process, which is combined to get a complete LCA of resource requirements. The process approach to LCA is a bottom-up approach that builds a full supply-chain estimate through the examination of the individual components. In contrast, the economic input-output approach to LCA (EIOLCA) (Bullard and Herendeen, 1975; Hendrickson et al., 2006) is a top-down approach that uses a holistic model of an economy divided into sectors, with the input-output table describing economic transactions between sectors (Leontief, 1970). To briefly address uncertainty in LCA models, the bottom-up process method suffers from variations in defining the system boundary when data on part of the supply chain are unavailable, while the EIOLCA has error associated with the aggregation of processes into economic sectors (Williams et al., 2009). Hybrid LCA is a set of methods that aims to combine process and EIOLCA methods to reduce uncertainty (for example, Bullard et al., 1978).
A second component of LCA, impact assessment, interprets life-cycle material flows in terms of environmental impacts. A major thrust of impact assessment is mapping flows to multiple types of impacts (for example, climate change, resource availability, and human toxicity) and developing ways to inform decision-making tools to navigate these multiple impacts (Baumann and Tillman, 2004; EPA, 2006). Many of the LCAs done for other biofuels are reviewed in the NRC report Renewable Fuel Standard: Potential Economic and Environmental Effects of U.S. Biofuel Policy (NRC, 2011b).
LCA can provide important insights into the sustainability of algal biofuels. Algal biofuels potentially have lower GHG emissions compared to petroleum-based fuels and they might not generate significant new negative impacts. Estimates of life-cycle GHG emissions for other biofuels span a wide range depending on the feedstock type, management practices used to grow feedstock, and whether any land-use changes were incurred. Algal biofuels thus need to be vetted with LCA and other approaches. Also, mass-scale agricultural systems induce significant material inputs of water and nutrients and emit various pollutants. LCA can help characterize material flows associated with such requirements for an algal biofuel industry.
There are challenges to using LCA to assess sustainability of algal biofuels. These challenges, including the issues associated with defining the system boundary for LCA analysis, are discussed in other publications (NAS-NAE-NRC, 2010; NRC, 2011b). LCA primarily is formulated as a retrospective description of existing supply chains. Algal biofuel production is in early development and there are limited historical data. In addition, technological progress and scale-up will affect future material flows but are challenging to forecast.
In addition to LCAs for assessing environmental variables, social LCAs are being developed to compare social impacts of products, processes, or companies, and to identify potential areas of improvements (Jorgensen et al., 2008). Social LCA is in early development stage, and consensus has yet to be reached on the impact categories to be included and how they would be measured (Dreyer et al., 2006; Jorgensen et al., 2008).
1.3.4 Scenario Analysis
A scenario is a characterization of a possible future. Scenarios take many different forms and can be constructed in many different ways (Chermack et al., 2001). Generally, scenarios “provide conceptual and quantitative frameworks to describe and assess” an activity or technology (NAS-NAE-NRC, 2010, p.292). Scenarios typically “use qualitative analysis and quantitative assumptions to integrate the environmental, technologic, economic, and deployment-related elements” into a framework to compare alternative possible outcomes
(NAS-NAE-NRC, 2010, p.292). Scenarios do not simply extrapolate historical data but try to develop internally consistent sets of conditions that are needed to occur to attain a given set of outcomes. For example, scenarios for algal biofuels might look at potential system design, resource requirements, and infrastructure needs required to reach a given percentage of the liquid fluids market. Scenarios can help define the environmental and resource sustainability issues that might accompany a greatly expanded algal biofuel production system.
Scenarios are a part of a more general future analysis. Elements of future analysis include trend projections, systems modeling, and scenarios; the analysis can combine these elements in different ways. Trend projection involves extrapolation of retrospective information to the future. A central element of the trend projection process is simply deciding on a functional form for the trend, such as linear, exponential, or some other relationship (Craig et al., 2002). Further, trend analysis is best for known systems where there is a large quantity of historical data, which is not true for algal biofuels. Systems modeling consists of identifying relationships between variables of interest (Ibid). For example, an econometric model finds the optimal statistical fit between variables that are assumed to be related by a predefined functional form. A systems dynamics model develops causal relationships between quantities of interest and evolves the future from some initial condition using these relationships. Future issues relevant to the sustainability of algal biofuels include: how individual technology elements will develop (for example, algae cultivation), how technology elements will combine to yield a fuel production system, and how the production system will link to natural systems (for example, salt versus fresh water).
Algal biofuels can be produced from a variety of feedstocks (autotrophic microalgae and cyanobacteria, heterotrophic microalgae, and macroalgae) using different processing technologies (for example, transesterification of algal oil, thermochemical conversion of algal biomass such as gasification and pyrolysis, or direct synthesis of alcohol). Examining the promise of different combinations of feedstocks and processing technologies to sustainably develop algal biofuels within the timeframe of this study was not feasible. Therefore, the committee limited the scope of the report in three ways following the guidance of the study sponsor and the committee’s expert judgment.
First, this study focuses on biofuel production systems that use autotrophic microalgae as a feedstock in the United States. Heterotrophic approaches for algae cultivation are excluded because DOE-EERE considers production of biofuel using heterotrophic algae as a biochemical pathway to convert another feedstock (a sugar source such as cellulosic biomass) rather than a pathway that directly produces fuels from algae (Pate, 2011). The exclusion of the heterotrophic pathways is not a judgment on the validity of these approaches. Second, the study sponsor indicated macroalgae as a feedstock was of lower priority for this study than microalgae and cyanobacteria and suggested that the committee could consider macroalgae if time and budget allowed. The committee could not fully address the sustainability of using macroalgae as a feedstock not only because of time and budget constraints, but also because of the sparse literature on this topic. The focus on microalgae also is consistent with the research and investment patterns in algal biofuels. Third, the study relies on published literature so that the well-studied topics are emphasized more often than less well-studied topics relative to others in the report.
The committee developed its report based on members’ expertise and information gathered from the public record. In its examination of publicly available information, the
committee relied on peer-reviewed papers; reports produced by government agencies and other interested parties; and documents filed as part of regulatory activities, including patent applications and environmental-impact assessments. In addition, the committee gathered information through presentations at open committee meetings from government agencies, companies, and others involved in the algal biofuel supply chain, researchers from academia, and other groups. The information gathered at these public meetings was augmented by public webinars and solicitation of information from algal biofuel companies. The information gathered during these activities helped form the basis for the description of the algal biofuel supply chain, resource requirements, and impacts discussed in subsequent chapters. In analyzing this information, the committee relied on the methods described earlier.
The report addresses the statement of task in the following ways. Chapter 2 provides an overview of algal biofuel supply chain and examples of different cultivation, harvesting, dewatering, processing, and coproduction methods that could be used in producing algal biofuels. Chapter 3 introduces selected algal biofuel production systems as examples to illustrate challenges and sustainability concerns of algal biofuel production and possible tradeoffs among sustainability goals. Chapters 4 and 5 discuss potential concerns related to resource use (for example, availability of land, water, and nutrient resources) and environmental effects and how some of those concerns might affect social acceptability of algal biofuels, respectively. For each category of resource use and environmental effect, indicators and metrics to be employed and data to be collected to assess sustainability are suggested. Chapter 6 summarizes the sustainability challenges for each of the selected algal biofuel production systems introduced in Chapter 3 and uses them to illustrate benefits and tradeoffs of each system.
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