The valorization of glycerol: Economic assessment of an innovative process for the bioconversion of crude glycerol into ethanol and hydrogen
Highlights
► Innovative process for highly efficient joint ethanol and H2 production from crude glycerol. ► No need for pretreatment, nutrient supplementation, nor sterilization. ► Energetic and economic assessment to evaluate the possibility to reach economic viability. ► Bioethanol production cost as low as 0.21 €/L. ► Major bottleneck is not the downstream processing but the cost of large-sized reactor.
Introduction
The dependence on fossil fuels as our primary energy source has led to a number of problems, such as energy crisis, environmental pollution, also contributing to global climate change and health problems; still fossil fuels represent the great majority of total energy supplies in the world today [1], [2], [3], [4], [5], [6].
Therefore, renewable resources and biofuels, such as hydrogen, bioethanol and biodiesel, are gaining more importance as an alternative pollution-free solution for the future [7]. Clearly, the increasing use of biofuels, both for energy generation and transportation purposes, is of particular interest, not only because they allow mitigation of greenhouse gases, but they also help diversifying and improving the security of energy supply, and may even offer new employment possibilities [8], [9]. Therefore, the EU, as well as the United States, Brazil and China have all launched ambitious programs promoting biofuels in the world [8], [10]. In 2008 the European Union took the lead position in the production of biofuels, with a production of over 500,000 tons/year [9], but the sector is highly dynamic and in continuous evolution. According to the Strategic Energy Technology Plan (SET-Plan), the EU is committed to increase the share of biofuels in transportation to the level of 10% by 2020 (while the share of renewable energies should increase to 20% of total EU energy consumption), with a view to reducing its dependency on oil and contributing to reduce the effects of climate change.
However it is important to bear in mind that this target should be reached under the condition that (1) the biofuels produced are sustainable, and (2) that so-called 2nd-generation biofuels become commercially viable, due to increasing concerns about the sustainability of those first-generation biofuels currently available, which are made from agricultural crops (such as corn, sugar beet, palm oil and rapeseed). In fact the EU “parliament’s industry and energy committee” asked for at least 40% of this goal to be met by non-food and feed-competing second-generation biofuels, or by cars running on green electricity and hydrogen [11].
To reach this goal, innovative technologies such as the conversion of lignocellulosic biomass to ethanol [12] and the use of oil accumulating algae in the production of biodiesel are being investigated [13], [14], [15]. These approaches are very promising and might provide abundant non-food feedstocks for the production of biofuels, with environmental benefits and large net energy gains [7]. However, it is clear that no single renewable-energy strategy will be able to provide energy security, and only a combination of integrated strategies has the potential to significantly decrease our dependence on fossil fuel.
An important contribution to the reduction of fossil fuel consumption and GHG emission can come from bio-ethanol, by far the most widely used bio-fuel for transportation worldwide, with a global output, in 2007, of 49.6 billion liters [16]. However, despite the large use of this biofuel, only recently there has been a growing interest to find out new non-food and cheap carbohydrate sources for production of bio-ethanol [8].
In the short-term, the production of bioethanol as a vehicular fuel is almost entirely dependent on starch and sugars from existing food crops [8], [17], [18]. Lignocelluloses need first to be broken down by a combination of physical, chemical or enzymatic steps to sugars (pre-treatment), subsequently the resulting glucose and other sugars are fermented to produce ethanol [9], [19]. In any case, pretreatments of these kind of biomasses can be very expensive [20], even though a large contribution to the production cost seems also to derive from biomass feedstock [21]. Therefore, finding out an abundant low-cost biomass than can be fermented with only very limited pre-treatment is essential. In fact, the production process itself also requires a large amount of thermal energy (especially the concentration and dewatering of ethanol by distillation [22], [23], which might partly explain why bioethanol has not played a larger role in comparison to cheaper oil derived fuels [24]. The development of second-generation biofuels from lignocellulosic biomass resources will represent the future technology for ethanol conversion [10] and can be regarded as an integral part of the emerging bio-economy, which will also have effects on food security, climate change, and rural development [8], [19].
Although most attention has been focusing on ethanol (especially in US and Brazil, and more recently China and EU), interest in biodiesel is also constantly increasing [4]. It is obtained through the transesterification of animal fats or vegetable oils, using methanol (or eventually ethanol) as a substrate; in the presence of strong alkaline catalyst the triglycerides are converted into a mixture of methyl esters and glycerol [25], [26]. With a ton of biodiesel produced, the process theoretically yields 100 kg of glycerol as a byproduct [13]. Clearly, despite this increasing interest, biodiesel is not a new fuel and it has been used in diesel engines and heating systems for over 25 years now. With its 254 factories, the European Union has chosen biodiesel as its main renewable liquid fuel, using rapeseed as the primary oil source. Thus, EU biodiesel production has shown an exponential growth in the last decade, accelerating from about 500,000 tons in 1998 up to 9 million tons in 2009, with a production capacity of 22.1 million tons in 2011 [27]. Since the EU is committed to increase the share of biofuels in transportation to the level of 10%, this trend is not likely to change significantly in the next years. However, in the last years biodiesel production costs have been increasing, due to the accumulation of crude glycerol as a byproduct. The continuous increase of biodiesel production will generate large amount of waste glycerol surplus, therefore, the valorization of this by-product in value-added products seems to be a very promising strategy [28].
Ethanol production from glycerol could reduce production costs by almost 40% (no steam-explosion needed) when compared with production from conventional corn-derived bioethanol, which is already considered an advantageous substrate [29]. Moreover, if fuels and reduced chemicals are targeted together, there are many advantages for using glycerol over sugars (the most commonly used fermentation substrate), which might translate into higher yields and lower capital and operational costs [7]. In fact, converting glycerol to pyruvate generates twice the amount of reducing equivalents produced from glucose or i.e. xylose. Nevertheless, while high purity glycerol is a very important industrial feedstock, crude glycerol derived from biodiesel production possesses very low value, due to the impurities [30], and the purification of glycerol (with a view to being sold), is not a viable option for the (especially small/medium sized) biodiesel industry anymore [31]. In fact, crude glycerol is usually contaminated with water, methanol, soap, oil, and other compounds deriving from the transesterification process. These contaminants cause high purification costs, when converting crude glycerol by traditional chemistry methods [32]. Biological conversion shows interesting potentialities, since it might help circumvent the disadvantages of chemical catalysis (e.g. low product specificity, use of high pressure and/or temperatures, inability to use crude glycerol with high levels of contaminants, etc.), while offering the opportunity to synthesize a large array of products and functionalities [13]. Thus the setting up of biorefineries that co-produce compounds of higher economic value, along with biofuels, has been proposed as a solution for the economic viability of this product [33].
Aim of this work was therefore to evaluate the economic viability of an innovative process, which is under development in our lab, for the bioconversion of crude glycerol into ethanol and hydrogen.
Section snippets
Resume of glycerol fermentation tests on which the economic evaluation is based on
All glycerol fermentation experiments were performed using a highly specific microbial mixed culture (MMC) able to grow on crude glycerol as the only carbon source, without the need of adding yeast extract, tryptone, mineral – or vitamin solution, which was previously selected and optimized [34]. Crude glycerol was supplied by ItalBiOil srl., a biodiesel factory in south Italy, and contained up to 90% of glycerol, while the impurities were mainly composed of salts (∼7%), ashes (∼2%), methanol
Results and discussion
As previously reported, the plant configuration was based on the best results obtained so far, with 26 g/L of ethanol and 9 L of hydrogen (per liter of fermenter) in a fed-batch fermentation reactor. Clearly, the possibility to achieve a high and joint production of ethanol and hydrogen, in non-sterile conditions and without any pre-treatment nor nutrient supplementation [34], [35], [36], makes this process already very attractive, in comparison to non-food lignocellulosic ethanol production,
Conclusions
In this study we synthesized the results of an innovative process that uses microbial a mixed culture to efficiently convert crude glycerol into biofuels, with a joint production of ethanol and hydrogen (both with high production and yields), without the use of nutrient supplementation, in non-sterile conditions. Since the kinetics of the process and the final ethanol concentration still need to be further improved, we performed an energetic and economic assessment of the bioconversion process,
Acknowledgements
This research has been financially supported by the STF-Science and Technology Fellowship Programme China (EuropeAid/127024/L/ACT/CN, STF China), and conducted in cooperation with HIT and ENEA.
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- 1
Co-corresponding author. Address: Joint Center for Microbial Ecology in Engineered System, Harbin Institute of Technology, P.O. Box 2614, 202 Haihe Road, Harbin 150090, China. Tel./fax: +86 451 86282195.