The main steps with alternative processing techniques of the biobutanol production process are illustrated in Fig. 1. As the cost of raw material has a significant influence on price [14], the main interest has recently been in low-cost substrates such as agricultural residues or industrial by-products and waste materials. Upstream processing before the fermentation includes the pretreatment of feedstock biomass, hydrolysis, and in some cases detoxification of inhibitors formed during the pretreatment. The fermentation step is commonly called as Acetone-Butanol-Ethanol (ABE) fermentation, based on the main products. This anaerobic fermentation consists of two stages: first the acidogenic phase where Clostridial bacteria produce acetic and butyric acids, carbon dioxide (CO₂) and hydrogen (H₂) from sugars, followed by the solventogenic phase where acids are converted into acetone, butanol and ethanol, typically in the ratio of 3:6:1. After the fermentation, final products are recovered and purified in downstream processing [15]. Adsorption, gas stripping, liquid-liquid extraction, pervaporation, perstraction and reverse osmosis are the most used separation methods integrated with the ABE fermentation [16].

Figure 1.

Steps in the biobutanol production process (modified from [10])

The main shortcoming of this biochemical production pathway is the low yield of the fermentation process, caused mainly by butanol inhibiting the growth and metabolism of Clostridia. Thus, there is a need to find improvements for the fermentation process. Research has been done e.g. by modifying the bacterium strains to stand out inhibitors better, by enhanced fermentation techniques, and by combining the downstream part into the fermentation step and removing butanol continuously from the system [11]. Novel feedstocks and optimized production processes for utilization of the raw materials are also essential development areas for more sustainable and efficient production of butanol. In this paper, the focus is especially on the selection of different feedstocks and their influence on the sustainability for the process.

Biobutanol can be produced biochemically from a range of feedstocks. When selecting the raw material for an industrial scale process, plentiful supply, low price, reasonable transportation costs and the ease of feedstock bioconversion should be taken into consideration. Molasses, potato, corn and other starch materials as well as cassava have been the principal raw materials used in industrial scale production of biobutanol [6]. Despite the fact that fossil fuel based chemical production process of butanol took over the fermentation process, research and development of biochemical process continued during the 1980s and 1990s, and new raw materials were tested. Interest in the process arose again in the beginning of the 21st century, focusing mainly on non-food residues and wastes. Use of by-products and waste materials is desirable also in terms of resource efficiency and waste minimization. Table 2 illustrates the advantages of different biobutanol feedstocks.

The EC is not demanding but encouraging industry, governments and non-governmental organizations (NGOs) to set up voluntary certification schemes for biofuels. The Certification guarantees that biofuels produced under the certified label are sustainable and production is done according to criteria given in RED. [57]

At present, the following European Sustainability criteria for the production of biofuels and bioliquids for energy applications - Principles, criteria, indicators and verifiers - EN 16214 standards have already been approved and published; Part 1: Terminology, Part 3: Biodiversity and environmental aspects related to nature protection purposes and Part 4: Calculation methods of the greenhouse gas emission balance using a life cycle analysis approach. Further, approval of Technical report prCEN/TR 16214-5, Part 5: Guidance to the conformity assessment and the use of the chain of custody and mass balance will be voted on summer 2013. FprEN 16214-2 Standard proposal Part 2: Conformity assessment including chain of custody and mass balance is not yet approved and will go to another voting. [58]

There is also an international standard draft ISO/CD 13065 ‘Sustainability criteria for bioenergy’ under preparation, but most probably it will be available only after few years [59]. So at the moment, all standards made by CEN Technical Committee (TC) 383: Sustainability produced biomass for energy applications are not yet approved. It seems also that at least the standard parts approved at the moment include neither GHG emission and fossil fuel balances, biodiversity, environmental, economic and social aspects nor indirect effects.

There are only a few assessments of biobutanol production available in the literature and most of them evaluate only economic sustainability. In 1980, Lenz & Morelra [60] found that high-quality molasses was an unattractive alternative at that time, whereas liquid whey waste material was found to have better economic potential. Gapes [61] concluded in 2000 that Acetone-Butanol fermentation may be economic when cheap, low-grade substrates are processed on a relatively small industrial scale. In contrast, when Pfromm et al. [62] compared the technical and economic aspects of fermentative biobutanol and bioethanol production processes using corn and switchgrass, they concluded the ABE process to be disadvantageous at the level of technology in 2010. The reason quoted was the low yield and productivity per time and volume of fermenter and the lower heating value per gram of processed biomass, as compared to bioethanol production by yeast fermentation. Recently, Kumar et al. [63] compared cellulosic (bagasse, barley straw, wheat straw, and switchgrass) and non-cellulosic (glucose, sugarcane, corn, and sago) feedstocks. Sago and glucose were too costly as feedstock biomass, while sugar cane and cellulosic materials were economically feasible with production costs of 0.59–0.75 $/kg of butanol. Further, fermenter size, plant capacity and production yield were noticed as crucial design and process parameters. In 2004, Ramey & Yang [64] reported production costs of 1.07 $/gallon (0.33 $/kg) for their process starting from corn and cost of 0.54 $/gallon (0.17 $/kg) for whey permeate used as a feedstock. With the current butanol price in Europe, about 1.50 $/kg, the biochemical production can be feasible already in this stage of the art. As comparison, production costs for petroleum-based butanol are about 1.35$/gallon (0.44 $/kg) [65]. However, the process is using propylene as starting material and thereby production costs of the chemical process are very sensitive to the crude oil price.

Different recovery methods have been evaluated together with the economics of the butanol production process [16],[66]–[70]. For instance, pervaporation (a membrane-based separation technique) may reduce production costs, especially if combined with the fermenter as a hybrid process [6],[71],[72]. Recently, conceptual designs of alternative process routes for biobutanol production from sugarcane molasses were done using Aspen Plus and Aspen Icarus modeling tools [73]. Different fermentation modes (batch or fed-batch), bacterium strains (Clostridium acetobutylicum ATCC824 or PCSIR-10, Clostridium beijerinckii BA101) and downstream processing techniques (steam stripping distillation, liquid-liquid extraction or gas stripping with CO₂) were investigated. Fed-batch fermentation with gas stripping was evaluated to be the only viable design in present economic conditions in South Africa, but the technology is still unproven on the industrial scale.

Natural Resources Canada has developed a model called GHGenius for the estimation of life cycle energy balances and emissions of the primary GHGs and of many other contaminants in connection with the production and use of existing and potential transportation fuels. Emissions can be predicted for the past, present and the future as far as 2050. This model was used for the analysis of the corn-to-butanol pathway [74]. Another calculation model, GREET (the Greenhouse Gases, Regulated Emissions and Energy Use in Transportation), was developed by Argonne National Laboratory. This well-to-wheels (WTW) analysis tool is based on the evaluation of different vehicle-fuel combinations on the basis of full fuel-cycle or vehicle-cycle. The model consists of Microsoft Excel multidimensional spreadsheets and calculates the consumption of energy, emissions of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and other pollutants, i.e. volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen oxides (NO_x), sulfur oxides (SO_x) as well as particulate matter measuring 10 micrometers or less (PM₁₀) or 2.5 micrometers or less (PM_2.5) [75]. Using the GREET model and the Aspen Plus simulation tool, Wu et al. [76] estimated the potential life-cycle energy and emissions effects associated with the use of biobutanol. They concluded that vehicles fueled with biobutanol produced by the examined process could result in fossil energy savings of 39–56%, and reduction of 32–48% of GHG emissions compared to conventional gasoline.

Recently, Swana et al. [77] evaluated the net energy production and feedstock availability for transportation biofuels by life-cycle assessment. Studied feedstocks included switchgrass, hybrid poplar, corn stover and wheat straw, all assumed to be produced domestically for biofuel production in the US. They concluded that by sustainable harvest based on current yields in the US, these biomasses can be converted to 8.27 billion gallon (31.31 billion liters) of biobutanol vs. 10.31 billion gallons (39.03 billion liters) of bioethanol. Since butanol has a better energy density, the replaced amount of gasoline is higher: 7.55 vs. 6.97 billion gallons with ethanol. It is to be mentioned that, with 2010 consumption levels of 378 million gallons/day, this is sufficient for merely 20 days of automotive transport without efficiency measures to reduce the fuel consumption. Ultimately, it will be crucial to restrain present consumption levels.

Directives do not specify the standard conversion values or the input numbers used to obtain the default values for each economic operator’s calculations. To harmonize the GHG calculation system for biofuel emissions, specialists from several EU countries have been united within the BioGrace project during 2010–2012 [78]. They listed all standard conversion values needed for GHG emission calculations and provided an Excel-based software for performing the calculations. Development and implementation of the software was done together with governmental policy makers, economic operators, auditors, advisors and certifiers. In addition, the Roundtable on Sustainable Biofuels (RSB) [79] is an international intent to bring together players from all fields for ensuring the sustainability of the production and processing of biofuels. This initiative has also developed a third-party certification system.

In terms of economic impact, the price of feedstock and the costs of biofuel production are the most defining impacts. The cost of biofuel production can be calculated mainly based on the capital and operational costs of the process. According to Demirbas [82], operational costs (e.g. feedstock, chemicals, labor, maintenance, insurances, taxes) represent about one third of the total cost per liter of fuel, while the share of capital costs is about one-sixth of the total cost per liter. There may be great variations in the production costs of the chosen raw materials, process techniques, and the scale, capacity and location of the plant. In debates about the positive effects of renewable and bioenergy projects, the aspect of generating regional added value is discussed [83], [84]. Heck [83] suggests regional added value to be defined as “the sum of all additional values originating in a region in a given time period”. Further, “particularly social, ethical and environmental issues should be considered in addition to the purely monetary aspects such as cost reduction, increase of purchasing power, higher tax revenues, and retention and generation of jobs” [83].

Environmental impacts in relation to biofuel feedstocks include direct and indirect land-use changes (LUC and ILUC), water footprint and other natural distraction. LUC and ILUC can have significant impacts on greenhouse gas balances and eutrophication [85]–[87]. The risks of nutrients removal, soil erosion and water run-off, as well as loss of natural biota, habitats and wildlife also have to be considered [88]. The expected positive impacts by using biomass based raw materials are reduced need of fossil fuels, potential GHG savings and improved carbon economy [82]. In addition, the biofuel production process has direct environmental impacts in terms of energy consumption and waste generation. In our assessment, we have evaluated the different challenges which each feedstock presents in terms of energy and water consumption during its pre-processing as well as the amounts of wastes/by-products they entail. Special attention is given to the use of toxics in pre-processing, as it will also impact on the safety of employees.

Regarding social impacts, biofuels in general are credited with enhanced energy security due to reduced dependency on imported crude oil, as well as increased employment. In our assessment of different biomass feedstock, providing jobs and development of rural areas are considered as positive impacts, especially in the case of raw materials from the agricultural and forest sectors. A key element in comparing different feedstocks is competing demand of feedstock, especially in the case of food-based feedstocks. For this reason, the ethical considerations are also assessed. This will also impact on customer acceptance, which may be subjective and is a theme of actual social dialogue. Some of the feedstocks assessed are currently still under research as potential raw material of biobutanol production. In terms of wider societal impact, the innovation and knowledge potential research efforts also need to be considered. As well, education and training for new processes will positively impact on societal capital.

To understand the combined impacts of feedstock supply and the production process, all three aspects of sustainability should be taken into consideration simultaneously. Currently, there is no unified triple bottom line assessment method available. In addition, while economic and environmental impacts can be quantified, most social impact categories can only be described in qualitative terms. In order to provide comparable data, we used a methodology described in Saavalainen et al. [89] in the assessment. A numerical value between +2 and -2 was assigned to all impact categories, illustrating the level of influence; positive impact was valued by a positive amount up to +2, depending on the potency of positive influence. For negative impacts the values were negative, with -2 for the most severe impact. In the case that the selected feedstock had no influence towards either direction, the value of 0 was assigned. This method allows for the comparison of all impact categories at the same time and visualizing with a spider diagram.

As a boundary condition for this sustainability study, it was assumed that the biobutanol is produced in a western European country, Finland if feasible. The assessment is limited to cultivation, harvest/collection and upstream processing (pretreatment) of feedstocks, excluding the rest of the process chain. Four main biomass sources were compared: food crops represented by corn, non-food crops illustrated by straw, food industry by-product with whey permeate as an example and sawdust as a representative of wood-based biomass. The assumptions for all four feedstocks used in the assessment are summarized in Table 3.

The first challenge in the process and product evaluation is defining system boundaries, the accurate description of all process steps and selecting relevant and measurable indicators for the evaluation. In many cases, it is demanding to find sufficient and reliable information on all process steps and, thereby, some assumptions or simplifications are required. The production processes for biofuels as well as their direct and indirect environmental and social impacts are very complex and, if evaluation is not based on the same definitions and boundaries for the process, the evaluations are not comparable.

When a sustainability assessment is done in a research and design phase, the target is to drive innovation for sustainability. This would require decision-support tools for the sustainability innovation process. With such tools, it would be possible to affect the environmental performance at the early design phase. It would, undoubtedly, provide a competitive advantage to the company if the consumption of energy, material, and water, as well as the emissions to air and water and waste releases of new products and processes would be known in advance. To gain information on all the above mentioned indicators is very important in the phase of adjusting the value-chain management. In the case of process development of biofuel production, legislative requirements such as the RED Directive will also have to be taken into consideration. It has been pointed out that the RED methodology excludes many critical issues such as indirect land use impacts and does not adequately consider allocation problems and uncertainty of individual parameters [90]. Evidently, there is a need to define common criteria for sustainability for biofuels. It is expected that the standards defined by technical committee of CEN/TC 383 will provide a solution to this problem and it would offer tools to be used also in the early design phase.

Meanwhile, mandatory blending targets, tax exemptions and subsidies have been set to increase the production and use of biofuels, although the EC is regulating the use of tax exemptions and incentives in Member States in order to avoid overcompensation. Increased oil prices will probably make production and use of biofuels more attractive in the future, but it is yet unclear how much and for how long a time will government support be needed before the biofuel economy can become profitable for industry and consumers. [2]