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Life cycle sustainability assessment of biofuels
Abstract
Sustainable bioenergy production can be considered a key issue in the global effort for the mitigation of greenhouse gas (GHG) emissions and effects of climate change. A holistic investigation of the supply chain and the production routes of biofuels is required in order to accomplish sustainable processes. This analysis should consider technical, economic, and environmental issues regarding the production of raw biomass, the applied conversion technologies, as well as end-product distribution and use. While the technical and economic aspects can be quantified by means of well-established physics and finance techniques, the quantification of the environmental impact has always been a complex task. The numerous environmental aspects, as well as the different methodologies of their analysis, have resulted, in previous years, in a scientific Babel Tower. Life-Cycle Assessment (LCA), a technique introduced in the 1960s and established since the 1990s, presents a comprehensive methodology for the quantification of the environmental impact of processes, including biofuel production, as well as being as a reliable decision-making tool.
Keywords
Biofuels; Cost–benefit analysis; Energy crops; Global warming potential (GWP); Greenhouse gases (GHGs); Life Cycle Impact Assessment (LCIA; )3.1. Introduction
Sustainable bioenergy production can be considered a key issue in the global effort for the mitigation of greenhouse gas (GHG) emissions and effects of climate change. A holistic investigation of the supply chain and the production routes of biofuels is required in order to accomplish sustainable processes. This analysis should consider technical, economic, and environmental issues regarding the production of raw biomass, the applied conversion technologies, as well as end-product distribution and use. While the technical and economic aspects can be quantified by means of well-established physics and finance techniques, the quantification of the environmental impact has always been a complex task. The numerous environmental aspects, as well as the different methodologies of their analysis, have resulted, in previous years, in a scientific Babel Tower. Life-Cycle Assessment (LCA), a technique introduced in the 1960s and established since the 1990s, presents a comprehensive methodology for the quantification of the environmental impact of processes, including biofuel production, as well as being as a reliable decision-making tool.
The goal of this chapter is to present the key issues of LCA biofuel production. Section 3.1 focuses on the main challenges of sustainable biofuel production and addresses the key issues regarding the environmental-friendly processes for producing biofuels. Section 3.2 introduces the methodological framework of LCA and highlights its role in the promotion of biofuels. The key issues that should be considered when conducting an LCA of a biofuel production system are presented. The importance of each step of the complete life cycle of the biofuels, including raw material production and extraction, processing, transportation, manufacturing, storage, distribution, and utilization of the biofuel, is discussed. Section 3.3 aims to present important sustainability aspects of the major biomass to biofuel conversion routes, including first-, second-, third-, and fourth-generation-derived biofuels. Section 3.4 provides an overview of recent LCA studies in the field of biofuel production, the main assumptions applied in those studies, and the challenges raised during the investigation of alternative biofuel production systems.
3.2. Main challenges for biofuel sustainability
3.2.1. The necessity for “green biofuels”
The global need to mitigate the green house gase (GHG) emissions and the consequent negative environmental impact of the extensive use of fossil fuels highlight the need for the promotion and utilization of more sustainable and carbon-neutral fuels. The production of biofuels has gained significant attention at various levels, including academia, industry, and policy makers. Significant effort is also given in order to identify the “best” pathways toward the production of biofuels. This effort includes the selection of the most appropriate feedstock, the definition of the most efficient conversion technologies, and finally focuses on the properties of the end product, which is the biofuel. Another aspect that has recently gained interest is the environmental impact of the production process of biofuels. Greenpeace published a report in 2011 (Mainville, 2011) stating that given the limited amount of forest biomass that can sustainably and effectively be used to provide low-carbon energy, governments need to scale up other energy options such as energy conservation and wind, solar, and geothermal energy. Additionally, it is justifiable that full life cycle and forest carbon-balance analyses on biomass projects are required to ensure that they are indeed climate friendly.
A challenging issue regarding the promotion of biofuels over fossil fuels is the achievement of sustainability by considering the three interrelated pillars of sustainability, namely economic, environmental, and social (Fig. 3.1). The term “sustainable development” was defined in 1987 by the Brundtland Commission as “development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs.” The aforementioned definition is generally recognized as a standard and a starting point for most who aim to define the concept of sustainability (Buytaert et al., 2011). In order to achieve sustainability, the environmental impacts of each phase of the biofuel supply chain (i.e., production or collection of biomass feedstock, feedstock processing, conversion to biofuel, and end-product distribution) must be evaluated using well-defined criteria. The evaluation is always difficult due to the great number of factors that are weighted differently by the involved stakeholders (i.e., farmers, manufacturers, policy makers, economic development agencies, local communities, etc.) (McBride et al., 2011; Seay and Badurdeen, 2014).
Figure 3.1 Interrelated pillars of sustainability (Amigun et al., 2011).
The key question is how to measure biofuel sustainability in such a complex system with a diversity of feedstock, a large number of biofuel pathways, and variations on specific interests of the stakeholders. The answer lies within the establishment of environmental and other indicators, which enable the assessing of the sustainability of different types of bioenergy systems. The indicators should, however, apply to both large installations and local sites, and also should be useful to diverse stakeholders (McBride et al., 2011).
3.2.2. Effective sustainability schemes for biofuels
In an attempt to measure the sustainability of biofuels at different levels, the European Commission endorsed a set of 133 indicators regarding various categories/themes, such as socioeconomic development, demographic changes, public health, climate change and energy, sustainable transport, natural resources, global partnership, and good governance (Gnansounou, 2011). Various certification schemes were also developed in the past aiming to respond to the aforementioned concerns regarding sustainability assessment. However, the existing schemes vary considerably in scope since they are developed to provide answers and solutions to different concerns/questions. At the European Union level, three main initiatives toward the regulation of biofuels sustainability have existed since the mid-2000s, namely:
• the Cramer commission for “Sustainable Production of Biomass” (in the Netherlands, in 2005/2006) (Cramer, 2006);
• the Renewable Transport Fuel Obligation (RTFO) (in the United Kingdom, in 2007); and the International Sustainability; and
• Carbon Certification (ISCC) sustainability ordinances (in Germany, in 2008).
ISCC requires compliance with sustainability requirements for biomass production and cultivation, GHG emission savings, and traceability and mass balance (Scarlat and Dallemand, 2011). Also, in 2008, CEN (European Committee for Standardisation) established the CEN/TC 383 Committee for “sustainably produced biomass for energy applications” to elaborate a European standard of sustainable biomass for energy applications, such as transport, heating, cooling, and electricity (Ponte, 2014). EN 16214 standard series (i.e., Parts 1–4) has already been published from 2012 to 2014.
In June 2010, the European Commission adopted a scheme for certifying sustainable biofuels under the Renewable Energy Directive (RED) (2009/28/EC). Under this scheme, all biofuels used in the European Union have to comply with sustainability criteria, including:
• land use;
• a minimum reduction of GHG emissions over the whole value chain; and
• a system monitoring the whole value chain from feedstock to the pump.
However, no social or food security aspects were included in the sustainability criteria.
On an international level, other certification initiatives include:
• the Roundtable on Sustainable Biofuels (RSB) initiative of the Swiss “École Polytechnique Fédérale de Lausanne” (EPFL) and other partners;
• the Renewable Fuels Standard (RFS); and
• The California Air Resources Board (CARB) Low Carbon Fuel Standard (LCFS), adopted in 2007.
The RSB standard includes 12 principles, criteria, and requirements differentiated in minimum and progress requirements, as well as guidelines on best practices in the production, processing, and use of biofuels for transport, while the RFS and LCFS require the blending of biofuels in transport fuels and reduction of GHG emissions by decreasing the carbon content of transportation fuels, respectively (Scarlat and Dallemand, 2011).
A detailed comparison of the existing biofuel certification schemes based on their principles, criteria, and indicators regarding key socioeconomic and environmental issues was conducted by Van Dam et al. (2010). Some of the systems cover certain selective areas in biomass production (i.e., agriculture, forest, and fair trade). Various initiatives propose or are developing methodologies and default values to calculate the reduction of GHG emissions for bioenergy chains.
3.2.3. Scientific studies for biofuel sustainability certification
Several studies have been recently conducted with efforts to further improve the current status of biofuel sustainability certification. Gnansounou (2011) proposed a logic-based model for the sustainability assessment of biofuels. The proposed model uses a hierarchical structure to link multiple factors from the more specific variables to the most general one, sustainability performance. The study proposed 7 general and 20 specific indicators for assessing the social, economic, and environmental performance of a biofuel supply chain.
According to Lora et al. (2011), a sustainable biofuel shall meet at least the following requirements:
• to be carbon neutral;
• not to affect the quality, quantity, and rational use of available natural resources;
• not to have undesirable social consequences; and
• to contribute to the society economically.
The authors stated that the sustainability assessment of biofuels shall be conducted through a multicriterial approach, and indicated a list of important issues to be investigated, such as: the contribution to GHG emissions reduction; land use and carbon stock changes induced by the growing demand for biofuels; the efficiency of produced fertilizers; the utilization of coproducts and residues; the impact of biofuel production on water resources; and soil, social, (e.g., food prices) and economic (i.e., production costs) issues.
A number of sustainability issues and relevant indicators for bioenergy systems were presented by Buytaert et al. (2011). The main issues affecting the social and economic performance of lignocellulosic biofuels and the key questions to be answered in an effort to assess their sustainability were highlighted by Raman et al. (2015). In another study, Markevicius et al. (2010) identified 35 sustainability criteria that can be used in the field of bioenergy. The criteria were grouped into categories such as social, economic, and environmental. Mangoyana et al. (2013) focused on the integration of social, economic, and environmental issues to establish holistically the sustainability of biofuel systems. Amigun et al. (2011) focused their study on the development of biofuels in Africa and highlighted the most important sustainability issues (i.e., economic, environmental, and social) in this specific geographic region. Jansen and Rutz (2011) examined the main challenges in biofuel production and investigated the available sustainability tools and initiatives, which can be utilized to ensure sustainability of biofuel production in Latin America. In another study, Mukherjee and Sovacool (2014) focused on palm-oil biofuel production in Indonesia, Malaysia, and Thailand in order to identify the current state of knowledge surrounding the associated sustainability implications. Based on the results of their investigation, specific measures are suggested aiming to improve sustainability in each country.
3.3. Life cycle sustainability assessment methodology
A wide range of sustainability-assessment methods have been developed in recent years. Some well-known and commonly used tools for sustainability assessment are Criteria and Indicators (C&I), Life-Cycle Assessment (LCA), Environmental Impact Assessment (EIA), and Cost–Benefit Analysis (CBA) (Buytaert et al., 2011). This section focuses on LCA, aiming to present its basic stages.
LCA is a useful, effective, and widely applied technique that can be used to quantify the environmental impact of a product or service by taking into consideration all the inputs and outputs of a system, starting from the raw material supply to the final disposal of the product/service. LCA considers the consumption of resources, the relevant emission and impact of each stage of the life cycle, and estimates the impact in various phases of the life cycle. According to the purpose of the study, the LCA can be carried out using different methods. A usual classification of LCA studies is that of attributional and consequential LCAs. The former describes the environmentally relevant flows to and from a life cycle and its subsystems, while the latter describes how environmentally relevant flows will change in response to possible decisions in the future (Finnveden et al., 2009). With regard to LCA of bioenergy systems, the consequential method appears as the most broadly applied (Cherubini and Strømman, 2011).
The LCA methodology is standardized according to ISO 14040 (ISO, 2006a,b), which defines the main stages of LCA as follows (Fig. 3.2):
1. Goal and scope definition
2. Life Cycle Inventory (LCI)
3. Life Cycle Impact Assessment (LCIA)
4. Interpretation
3.3.1. Goal and scope definition
The first step of an LCA aims to define the purpose of the study. The definition of the goal of the LCA study is very important, since it can affect the results of the study as well as the relevant conclusions. The goal shall clearly state the intended application, the reason(s) for carrying out the study, as well as the intended application and audience of the results. The definition of the scope of the LCA is also important and must be well-defined in order to ensure that the goal of the study will sufficiently be addressed. The scope of LCA must consider the functions of the product system and identify the system boundaries and the investigated product system to be studied. The selected functional unit (FU), the allocation procedures, the types of impact assessment methodology, and impact categories should also be included in the scope of the study. Furthermore, data requirements and subsequent interpretations, assumptions, limitations, data-quality requirements, type of critical review, if any, and the type and format of the final results must be included (ISO, 2006a,b).
Figure 3.2 Life-Cycle Assessment: four stages (ISO 14040:2006).
According to ISO 14040, FU is used to provide a reference to which all the inputs and outputs of the investigated product system are related. The functional unit is essential in order to ensure the comparability of the results of the study on a common basis.
According to the findings of Cherubini and Strømman (2011), four types of functional units can be identified in LCA of bioenergy systems:
1. Input-unit related, where the functional unit is the unit of input biomass, either in mass or energy unit. This unit can be selected by studies that aim at comparing the best uses for a given biomass feedstock.
2. Output-unit related, where the functional unit is the unit of output, like unit of heat, power produced, or kilometer of transportation service.
3. Unit of agricultural land referring to the hectare of agricultural land needed to produce the biomass feedstock.
4. Year: results of the assessment may be even reported on a yearly basis.
The definition of the system boundary identifies the number of processes that are included in the investigated system. The analysis may focus on specific stages of the life cycle and include (or exclude specific stages) accordingly. For example, a “cradle to grave” analysis examines the full life cycle of a product system from resource extraction (“cradle”) to use phase and disposal phase (“grave”). It includes the material and energy production chain and all processes from the raw material extraction through the production, transportation, and use phase up to the product's end-of-life treatment. On the other hand, “cradle-to-cradle” implies production, use, and recycling, whereas a “cradle-to-gate” analysis assesses a partial-product life cycle from resource extraction (cradle) to the factory gate (i.e., before it is transported to the consumer) where the use phase and disposal phase of the product are omitted in this case. Finally, a “gate-to-grave” analysis includes the processes from the use and end-of-life phases of a product once it leaves the factory (see Fig. 3.3).
Figure 3.3 LCA: System boundaries (GaBi, 2015).
In LCA studies, a different definition is usually given to express the system boundaries. “Well to Tank” (WtT) boundaries, for example, include all steps from the production of biomass feedstock to the transport and distribution of fuel, while “Well to Wheel” (WtW) boundaries also include, in addition to WtT steps, the fuel use (end-of-life). Infrastructures may or may not be included within the system boundaries (Menten et al., 2013). Cherubini and Strømman (2011) indicated that the majority of LCA studies limit their scope to the assessment to GHG and energy balances without considering any possible contribution of bioenergy to other impact categories. The authors attribute this scientific attitude to the fact that the mitigation of climate change and reduction of fossil fuel consumption are the main driving factors for worldwide bioenergy development.
3.3.2. Life Cycle Inventory
The second stage of an LCA study is the inventory analysis, in which the product system is defined. In this phase, the system boundaries are set and the designing of the flow diagrams, including unit diagrams, is implemented. The collection of necessary data regarding the processes enclosed in the predefined system boundaries, the performing of allocation steps in cases of multifunctional processes, as well as the final calculations, are within this stage of LCA (ISO, 2006a,b). The main output of LCI is a list of inputs to and outputs to the environment from the investigated product system associated with the defined/selected functional unit (Guinee, 2002).
Data collection regards every single process included within the system boundaries. Thus, data regarding raw material and energy inputs, products (or coproducts), and waste streams of the investigated system, and emissions to the environment must be classified accordingly. It is worth noting that during the data collection phase, new requirements and system limitation may be identified, which will indicate a necessary change of the data collection procedure in order to ensure that the goals of the study will be successfully addressed. Data collection may be time-intensive and complex when there is a lack of readily available data (Curran, 2013), especially when site-specific systems and processes are investigated. However, data collection process can potentially be simplified, since commercial and software-based databases exist where general and site-specific data can be retrieved from such as Ecoinvent database, ELCD, SimaPro, GaBi, etc.
The collection of data is followed by the data calculation step, where validation of data and allocation procedures are applied. The validation of data can be used for the improvement of the quality of the collected data and thus to improve the outcome of the study, or it may indicate the need for supplementary data (Jensen et al., 1997). Allocation in LCA is carried out to attribute shares of the total environmental impact to the different products of a system. The allocation concept is extremely important for bioenergy systems, which are usually characterized by multiple products, and have a large influence on final results (Cherubini and Strømman, 2011).
3.3.3. Life Cycle Impact Assessment (LCIA)
The objective of Life Cycle Impact Assessment (LCIA) is to further process and interpret the results of LCI in terms of their potential impact on the environment and the society. LCIA consists of three mandatory stages:
• selection and identification of impact categories,
• classification, and
• characterization,
and three optional stages:
• normalization,
• grouping, and
• weighting (ISO, 2006a,b).
The first step deals with the identification of relevant environmental impacts (e.g., global warming, acidification, terrestrial toxicity, etc.). The classification step aims to sort and combine the LCI results into classes or impact categories according to the respective impact on the environment, human health, and resource use. According to Cherubini and Strømman (2011), the majority of bioenergy-focused LCA studies (i.e., around 90%) includes GHG emissions within their scope, and consequently examines the effect on global warming potential (GWP). Other impact categories, like acidification, eutrophication, etc., are estimated by 20–40% of the studies.
In the characterization step, all the LCI results are multiplied by characterization factors in order to be converted and combined into representative indicators of impacts that reflects their relative contribution to the various impact categories. The result is expressed as an impact score in a unit common to all contributions within the impact category. The normalization step aims to compare the quantified impact of a certain flow to a reference value, for example, in a worldwide or regional total. In the grouping step, impact categories are assigned into sets to better allow the interpretation of the results into specific areas of concern. Finally, in the weighting step, the category indicator results are grouped and weighted to include societal preferences of the various impact categories (ISO, 2006a,b; Guinee, 2002). LCIA can be performed by using different methodologies. In each methodology, the environmental impact is classified and characterized using two main approaches, namely, the problem-oriented approach (midpoint) and the damage-oriented approach (end-point). The former translates impacts into environmental themes such as climate change, acidification, human toxicity, etc., while the latter translates environmental impacts into issues of concern such as human health, natural environment, and natural resources (GaBi, 2015).
Midpoint-oriented LCIA methodologies include the CML (2001), EDIP (Potting and Hauschild, 2004), TRACI (Bare et al., 2003), USEtox (Rosenbaum et al., 2008), and the Method of Ecological Scarcity, also known as the UBP Method (Brand et al., 1998). EDIP (Potting and Hauschild, 2004) investigates the possibilities for inclusion of exposure in the LCIA of nonglobal impact categories (i.e., acidification, photochemical ozone formation, ecotoxicity, nutrient enrichment, human toxicity, noise). The TRACI method developed by the US Environmental Protection Agency is a midpoint approach that defines 11 specific impact categories, while the UBP method permits impact assessment of life cycle inventories according to the “distance to target” principle (GaBi, 2015).
Various LCIA methods operate with both midpoint and end-point indicators. Eco-indicator 99 works with three damage-oriented categories, namely human health, ecosystem quality, and resources. These categories are subdivided into midpoint indicators falling under human health impact from climate change that here is considered equivalent to global warming. The Impact 2002+ and ReCiPe methodologies operate with the same three damage-oriented impact categories as Eco-indicator 99. Additionally, ReCiPe methodology operates with 18 midpoint indicators that are similar to what is used in the CML methodology (GaBi, 2015; Dreyer et al., 2003).
3.3.4. Interpretation
An LCA study completes with the interpretation phase, where the overall process is evaluated, the results obtained from previously completed phases (i.e., LCI and LCIA) are reported, and overall conclusions and recommendations are drawn and made, respectively. According to ISO 14040 (ISO, 2006a,b), the interpretation shall clarify that the results of the LCIA indicate potential environmental effects, but they do not predict actual impact on specific impact categories.
A consistency check is necessary at the beginning of the interpretation phase in order to determine whether the assumptions, methods, models, and data applied in the analysis are consistent with the goal and scope of the study. A completeness check is also deemed necessary in this phase to ensure that all relevant information and data needed for the interpretation phase are available and complete. The contribution analysis that follows aims to indicate the contribution of specific flows, processes, or impacts to the results. The interpretation phase may include the perturbation analysis, where the effects of small changes within the system on the results of an LCA are evaluated. In the last step of the interpretation phase, conclusions and recommendations based on the results obtained from previous phases of the LCA are documented (Guinee, 2002).
The comparison of LCA studies in the field of biofuels and bioenergy systems indicated a variation of the results, even in those cases where similar bioenergy chains were examined. Cherubini and Strømman (2011) stated that this variance can be attributable to differing data sources and ages, key input-parameter values, agricultural managements, and other methodological differences regarding definition of system boundaries, the allocation procedure, the applied reference systems, and other indirect effects.
3.4. LCA considerations of biomass to biofuel conversion routes
Biomass can be utilized to produce different forms of biofuels, namely, solid, gaseous, and liquid fuels. It can be upgraded into high-energy density fuels such as charcoal, liquid fuels (biodiesel, bioethanol), and gaseous fuels (such as hydrogen, producer gas, or biogas). The biofuels are categorized in general into four generations; the major LCA consideration of each is presented in this section.
3.4.1. First-generation biofuels
The first-generation biofuels (or conventional biofuels) are derived from various food crops such as wheat, corn, sugar, beet, etc. Based on different technologies, three main types of the first-generation biofuels used commercially are biodiesel, bioethanol, and biogas (Baskar et al., 2012). Biodiesel is produced through transesterification of vegetable oils, and residual oils and fats can be used as a substitute of diesel, with minor modifications, in diesel-fueled engines. In the transesterification process, triglycerides react chemically with alcohol (e.g., biomethanol), in the presence of catalyst or enzyme, and generate biodiesel and glycerol (Singh et al., 2014). Bioethanol is produced through the fermentation of sugar or starch and as a substitute for gasoline or as feedstock for ethyl tert-butyl ether (ETBE), which blends more easily with gasoline. Biogas, which is a mixture of methane and carbon dioxide, is produced through anaerobic digestion of organic materials (Naik et al., 2010).
Despite many advantages of the first-generation biofuels, various issues have hindered their widespread adoption. The fact that the production of the first-generation biofuels mainly depends on agricultural food crops that could otherwise be used for food and feed purposes, initiated a societal debate on food-versus-fuel, since one of the reasons for rising food prices is due to the increase in the production of these fuels. Especially in arid and insular regions, the production of the first-generation biofuels is not environmentally and technically feasible. Fokaides et al. (2015) justified that due to the high production costs in arid and insular environments, the price of energy crop seed is significantly higher, compared to the price that would be determined as feasible, providing biofuel prices considered as noncompetitive to conventional fuels. Also, through the implementation of GIS analysis, it was concluded that the possibility of promotion of energy crops under semiarid and subtropical environment is extremely limited and in any case insufficient to satisfy the expected contribution of biofuels.
3.4.2. Second-generation biofuels
The second-generation biofuels (or advanced biofuels) are derived from lignocellulosic biomass, nonfood crop feedstocks, agricultural and forest residues, and industrial wastes. They are mainly produced through the utilization of physical, thermochemical, and biochemical technologies, usually after a pretreatment stage of the biomass feedstock (Liew et al., 2014). The pretreatment step is a very important step to prepare the biomass properties (e.g., size, moisture, density, etc.) in order to facilitate the conversion processes (Agbor et al., 2011).
The most common physical conversion processes are briquetting, pelletizing, and fiber extraction. Briquetting is the method used to convert loose biomass into high-density solid blocks, while during pelletization, the fine-particle raw material is compacted to pellet under pressure. Fiber extraction regards the extracting process of fibers from biomass residues, which can potentially be utilized as burning fuel (Liew et al., 2014).
Pyrolysis, gasification, liquefactions, and direct combustion are the main thermochemical processes available for the production of second-generation biofuels. Pyrolysis regards the conversion of biomass to liquid, solid, and gaseous fractions by heating the biomass in the absence of air (Mc Kendry, 2002). Pyrolysis can be classified as slow or fast, according to the operating conditions. Slow pyrolysis favors the production of solid biofuels, whereas fast pyrolysis is used for liquid (bio-oil) and gaseous biofuel production (Fokaides and Polycarpou, 2013). Gasification is the conversion process of biomass into a combustible, gaseous fuel mixture known as synthetic gas or syngas, by the partial oxidation of biomass at high temperatures (e.g., 800–900°C) in a gasification medium such as air, oxygen, or steam (Mc Kendry, 2002b). The produced syngas is primarily used to produce fuels and intermediate chemicals (Liew et al., 2014). During liquefaction, biomass with high lignin content is depolymerized or broken down into a small, unstable, and reactive molecule, which can be repolymerized into a liquid product with various ranges of molecular weights. The aid of solvent, syngas, and catalysts is necessary for the conversion to heavy fuel-oil product (Rowlands et al., 2008).
Direct combustion regards the burning of biomass in excess air to produce heat. Volatilization of combustible vapors from the biomass occurs, which then burns as flames. Biomass combustion is applied for the conversion of biofuel to heat; however, power production can also be achieved using secondary-conversion technologies (Bridgwater, 2003).
Common biochemical pathways for second-generation biofuels include fermentation for the production of ethanol and other alcohols. Fermentation is an anaerobic process where the glucose (or carbohydrates) of organic wastes are converted to ethanol through a series of chemical reactions. A pretreatment step is necessary to increase the yield of sugar, followed by enzymatic hydrolysis and the subsequent fermentation, or by the simultaneous saccharification and fermentation (SSF) process (Romero-Garcia et al., 2014).
3.4.3. Third- and fourth-generation biofuels
The third-generation biofuels refer to fuels derived from algae. It is currently considered to be a feasible-alternative, renewable-energy resource for biofuel production, overcoming the disadvantages of the first- and second-generation biofuels. Algae shows high efficiency in converting solar to chemical energy; hence, it has a much better perspective of producing biofuel compared to the first- and second-generation biofuels (Liew et al., 2014). This fact, together with their ability to accumulate lipids, the ability to be cultivated in controlled environments, and the exploitation of CO2 directly from industrial emissions for their growth, can potentially give biofuels, which have low competition with food crops, limited environmental impacts, and a significant contribution to the mitigation of GHG (Collet et al., 2013). The production of the third-generation biofuel is achieved through the conversion of algae oil to biodiesel by means of the transesterification process.
Although biofuel production using microalgae seems to obviate land use and food conflict, which are disadvantages that appeared in the first- and second-generation biofuels, further research is required in order to improve this biofuel production method in terms of the amount of produced energy and its viability (Liew et al., 2014). The identification of the most promising algae species, and the improvement of the production technologies and harvesting processes, are some of the challenges that need to be faced in the future (Alam et al., 2015).
The fourth-generation biofuels are produced by technologies that are able to successfully convert optimized biomass feedstock. The key characteristic of those fuels is the capture and sequestration of CO2, since a higher amount of CO2 is consumed in their generation than that produced during their use (Baskar et al., 2012).
3.5. Overview of major findings of selected LCA studies in biofuel production
This section provides an overview of recent LCA studies in the field of biofuel production, the main assumptions applied in those studies, and the challenges raised during the investigation of alternative biofuel-production systems.
3.5.1. Selected LCA studies on energy crops
A significant parameter that defines the performance of LCA studies regarding energy crops lies in the boundaries considered. The majority of the energy crops that LCA studies focus on the agricultural production systems and on the energy crop–cultivation processes. Kim et al. (2014) compared a number of LCA studies for maize production, highlighting the differences in the obtained numerical results, due to different assumptions and boundaries. Nonrenewable energy consumption in maize production ranges from 1.44 to 3.50 MJ/kg, which represents up to 10% of the energy content of the end-product (bioethanol). Goglio et al. (2012) evaluated the environmental impacts and energy benefits of sunflower and maize crops, both in rotation with wheat crop cultivation. It was proven that from the environmental point of view, low-input cropping systems are the most suited to exploit the environmental advantages of agricultural production of biomass feedstock. Bacenetti et al. (2014) examined the environmental impact of a single (maize) and a double (maize plus wheat) crop system, showing a worse environmental performance for the double crop system.
Several LCA studies were implemented for the production of maize as energy crops (Brentrup et al., 2004a,b; Charles et al., 2006; Biswas et al., 2010). Zaher et al. (2013) developed an LCA-based methodology for the evaluation of potential carbon credits resulting from tillage intensity reduction in winter wheat (WW) cultivation, concluding that reduced-tillage agricultural practices lead to reduced emissions from fuel usage and increased carbon sequestration. A study conducted by Biswas et al. (2008), assessing GHG emission for the prefarming, on farm and postfarming stages of wheat production, showed that 1 kg of wheat to port was equivalent to 0.304 kg CO2. The relation of the prefarm, onfarm, and postfarm stages accounted for 45%, 44%, and 11% of the total global warming potential, respectively. Fallahpour et al. (2012) studied the environmental impact of wheat and barley, under rain-fed and irrigated farming systems, concluding that agricultural production systems with a high level of yield do not always contradict environmental safety, and suggested organic amendments instead of chemical fertilizers as a method for “low input agriculture.” Sørensen et al. (2014) considered the LCA of spring barley, winter barley, winter wheat, and winter rape seed. The impact of different tillage and machinery systems was justified.
LCA was also applied in other types of crops such as sugar beets, sweet sorghum, and potato. A study conducted by Brentrup et al. (2001) investigated the environmental impact of the production of sugar beets, while Foteinis et al. (2011) examined cultivation, transportation, and processing of sugar beets for sugar and bioethanol production purposes. Machinery and fertilizing were the two main processes considered in the stage of cultivation of sugar beet crops. The energy efficiency and impacts from environmental perspective, of sweet potato–based bioethanol production were also evaluated by Wang et al. (2013). Buratti and Fantozzi (2010) developed a new LCA methodology, applied in biomass production processes. Validated with two different agricultural techniques regarding fiber sorghum crop production.
3.5.2. Selected LCA studies on solid biofuels upgrade
Several studies were conducted in the recent past for the analysis of the environmental impact of upgraded solid biofuels. Hanandeh (2013) compared pelleting, briquetting, pyrolysis, and composting of olive solid waste using LCA, concluding that pelleting for domestic water heating was the alternative with the lesser environmental impact. Porso and Hansson (2014) calculated the energy consumption required for the pretreatment of willow and poplar pellets, concluding that the pretreatment energy was about 11 times the energy value of the delivered biofuel. Benetto et al. (2015) investigated the properties of grape marc pellets, concluding that the weighting given to ecosystem quality, as adopted by the impact assessment method, is higher than 30%. This study also indicated that the drying of pellets is the main contributors to the environmental impacts. Cherubini and Ulgiati (2010) proved that the changes in the soil carbon pools and the production of pellets contribute the most in determining the final GHG balance. The impacts of using wood pellets to replace traditional firewood for domestic heating was investigated by Pa et al. (2011). Harvesting, transportation of harvested material to sawmill, sawmill processing, transportation of sawmill by-products to pellet mill, pellet mill operations, packaging, and finally pellet transportation were considered in the LCA analysis. According to the results of this study, switching from firewood to wood pellets holds great potential in lowering the impacts on human health, ecosystem quality, climate change, and primary energy consumption. Fantozzi and Buratti (2010) included in their analysis regarding the combustion of Short Rotation Coppice wood pellets for domestic heating purposes, the environmental impact of machinery and infrastructure. They concluded that these contributed only to 2% of the overall impact.
3.5.3. Selected LCA studies on biofuel thermochemical pretreatment
The environmental analysis of thermochemical biomass–processing routes is also highly considered in the literature. In Hanandeh (2015), the utilization of olive husk in a mobile pyrolysis unit, under different pyrolysis conditions, was investigated. The results indicated a significant GHG emission–saving potential for all the investigated scenarios. The impacts of wood waste gasification for district heating were investigated by Pa et al. (2013). The results of this study indicated that wood pellet gasification is superior to wood waste gasification in terms of primary energy consumption and health impact. Adams et al. (2015) analyzed the associated environmental impacts with production and delivery of conventional wood pellets and torrefied wood pellets by using cradle-to-gate LCA. The study concluded that on an MJ- delivered-basis, torrefied pellets reduced fossil fuel consumption and GHGs emissions compared to conventional wood pellets, when a low drying energy (3.0 MJ/kg water removed) is assumed. Tsalidis et al. (2014) evaluated the environmental impacts of torrefied and pelletized biomass direct co-firing with coal on a 20% energy-input basis. The stages that contribute the most to the environmental impact of the said processes are the co-firing and the transportation.
3.5.4. Selected LCA studies on the overall impact of biofuel production
Fazio and Monti (2011) conducted cradle-to-grave impact assessments of alternative scenarios, including annual and perennial energy crops for electricity/heat or the first- and second-generation transport fuels. Perennial grasses resulted in reduced environmental loads compared to annual crops. Regarding the first-generation biodiesel from sunflower, it was found to have less impact compared to rapeseed on energy base, while similar results were retrieved on land base. As for the first-generation bioethanol, land-based impacts of wheat were found much lower compared to maize, while energy-based impacts were similar in two crops. For the second-generation biofuels, as well as for thermochemical conversion, switchgrass showed the lowest impacts on hectare basis, while similar impacts to giant reed and miscanthus were observed on energy basis. The study also indicated that the first-generation biodiesel was less impacting than the first-generation bioethanol. Bioelectricity/bioheat productions showed lower impacts than the first-generation biofuels and the second-generation bioethanol based on thermochemical conversion.
Menten et al. (2013) investigated the main factors impacting GHG emissions variation in the results obtained from LCA studies of second- (G2) and third-generation (G3) biofuels. A total of 47 studies were analyzed, and the results provided interesting facts. GHG emissions of G3 biofuels were higher than those of ethanol. European estimates were found lower than North American estimates for all types of biofuels. The LCA results were also found to be affected by technical variables and the type of LCA approach (i.e., attributional vs consequential). Regarding the third-generation fuels, the productivity of algae as well as its oil content were found to negatively and nonlinearly affect the results.
The objective of the study conducted by Weinberg and Kaltschmitt (2013) was to determine the overall GHG emissions for the first-generation ethanol derived from sugar beet and wheat, with a special focus on the use of by-products. According to the results, ethanol production from sugar beet has lower GHG emissions compared to wheat-derived ethanol. The highest GHG reduction was achieved through the biogas production from sugar beet pulp and vinasse; the latter being a by-product of ethanol production from sugar beet.
Silalertruksa and Gheewala (2009) evaluated the energy efficiency and renewability of bioethanol systems in Thailand and identified the current significant environmental risks and availability of feedstocks by means of an LCA and net energy balance. The LCA results indicated three major factors that directly affect the GWP of cassava and molasses ethanol, i.e., types of fuel used in ethanol plant, trash burning during harvesting of sugarcane, and credits from surplus electricity from bagasse at the sugar mill. Regarding the negative effects of trash burning, the authors proposed the utilization of trash as fuel in sugar milling as a measure to benefit from energy credits of the cane trash.
3.6. Conclusions
Sustainable production of biofuels represents a major challenge of the biomass scientific community. It requires a holistic approach into the entire supply chain, starting from the feedstock per se and ending with the last stage, which is the delivery of the produced biofuel. This chain involves numerous stages, each of which has its own impact on the environment. LCA constitutes a reliable, standardized, comprehensive methodology for the quantification of the environmental impact of biofuel production. In this chapter, the main aspects of LCA tailored for biofuels are presented. LCA considerations for the first-, second-, and third-generation biofuels are discussed. Finally, the selected LCA studies for four specific biomass fields are described.
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