Article pubs.acs.org/est
Synergetic Sustainability Enhancement via Current Biofuel Infrastructure: Waste-to-Energy Concept for Biodiesel Production Eilhann Kwon, Haakrho Yi, and Young Jae Jeon* Bio-Energy Research Team, Research Institute of Industrial Science and Technology (RIST), Kwang-Yang-City, Cholla-Nam-Do, South Korea ABSTRACT: The concept of waste-to-energy (WtE) with regards to the utilization of byproducts from the bioethanol industry (e.g., distiller’s dried grain with solubles: DDGS) was employed to enhance the renewability of biodiesel, which would be an initiative stage of a biorefinery due to the conjunction between bioethanol and biodiesel. For example, DDGS is a strong candidate for use as a biodiesel feedstock due to the tremendous amount that is regularly generated. On the basis of an estimation of possible lipid recovery from DDGS, ∼30% of the biodiesel feedstock demand in 2010 could be supported by the total DDGS generation in the same year. Considering the future expansion of the bioethanol industry up to 2020, the possible lipid recovery from DDGS would provide more than 6 times the biodiesel feedstock demand in 2010. In order to enhance the renewability of biodiesel, the transformation of lipid extracted from DDGS into fatty acid ethyl ester (FAEE) via a noncatalytic transesterification reaction under ambient pressure was investigated in this work. The newly introduced method reported here enables the combination of the esterification of free fatty acids (FFAs) and the transesterification of triglycerides into a single step. This was achieved in the presence of a porous material (i.e., charcoal), and the optimal conditions for transformation into biodiesel via this noncatalytic method were assessed at the fundamental level.
1. INTRODUCTION Approximately 85% of all petroleum-derived oil produced is consumed in the transportation sector.1 The simultaneous problems of fossil fuel depletion and environmental degradation mean that there is an increasing urgency for the development of renewable energy alternatives with diversification of sources for securing future energy supplies and increasing awareness of environmental impact.2−4 Thus, biofuels (e.g., bioethanol or biodiesel) are now regarded as feasible alternative options for transportation fuel due to their compatibilities with current internal combustion engine technology and distribution networks.5,6 However, the high cost of biodiesel produced from lipid feedstock (i.e., ∼70% of biodiesel production cost) is one of the major obstacles to obtaining a sustainable supply for worldwide communities.7,8 An analogous situation is present in the bioethanol industry. Thus, the extensive use of biomass to produce biofuel still remains controversial in terms of ecological perspectives, and these issues clearly need to be resolved in a fully transparent manner.8−10 Moreover, the ethical dilemma of using biofuels that compete with food resources has to be considered when applying this technology as an alternative energy source.11−13 These considerations have promoted the development of second- and third-generation biofuels.14−17 As a desirable scenario for the sustainability of these transportation fuels, the concept of a biorefinery for the production of fuels and chemicals has been proposed.13 In this regard, it is imperative to find a way to incorporate the production of biofuels at an initial stage of a biorefinery. The possibility of producing biodiesel using the byproducts from the bioethanol industry, such as distiller’s dried grain with © 2013 American Chemical Society
solubles, was investigated herein by assessing the noncatalytic transesterification process. A tremendous amount of ethanol sludge is inevitably generated by the bioethanol industry (i.e., the residue separated from the ethanol fermentation broth). This sludge consists of ∼25 wt % (i.e., wet basis) of solid containing yeast cells.12 This has traditionally been used for the supplementation of animal mixed-feed and fertilizer.12 Most biodiesel consists of methyl esters of vegetable oils or animal fats and has been produced by the transesterification reaction of triglycerides with short-chain alcohols, mainly methanol (MeOH).18−23 Thus, exploiting distiller’s dried grain with solubles (DDGS) for biofuel production would be a desirable end use. This work describes the optimal conditions for transforming lipid extracted from DDGS into fatty acid methyl ester (FAME) and fatty acid ethyl ester (FAEE) via a noncatalytic method. In addition, the feasibility of utilizing DDGS as the biodiesel feedstock is validated and evaluated.
2. MATERIALS AND METHODS 2.1. Sample Preparation. DDGS was obtained from local brewers located in Jeonju and Kwangyang-city, South Korea, and contained Saccharomyces cerevisiae, which is one of the most common species of yeast. For lipid content assessment, DDGS was dried in an oven at 95 °C for 3 days. Solvent extraction using Soxhlet apparatus (Cole-Palmer, USA) was carried out at Received: Revised: Accepted: Published: 2817
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Table 1. List of 2nd Generation Bioethanol Producers, Their Cell Biomass, and Possible Lipid Recovery organism
cell biomass (g/g substrates)
glycerol (g/g substrates)
Saccharomyces cerevisiae
0.2
0.17
lipid contents (wt %, dry basis)
Pichia stipitis
0.1
Zymomonas mobiliz
0.03
7.0
Escherichia coli
0.05
6.5
6.5−15
6−10
references Dyer et al.25 Sedlak and Ho26 Matsushika et al. 27 Bellido et al.28 Carnicer et al.29 Hermans et al.30 Jeon et al.31 Lawford and Rousseau32
controlled using a mass flow controller (Brooks SLA 5800 series, USA). A water-circulating condenser was connected to the TR. The CO2 gas used in the experimental work was immediately released to the fume hood, meaning that the biodiesel transformation was carried out at ambient pressure. A computer-aided control system by LabVIEW (National Instrument, USA) was employed. GC/MS (HP-7890A/5975C MSD) and GC/FID were used for measuring FAME and FAEE. The analysis of biodiesel was carried out using a method previously reported by the authors.
65 °C for 1 day, using n-hexane (Sigma-Aldrich, St. Louis, USA). 2.2. Characterization of Lipid Extracted from DDGS. Fatty acid profiles of the lipid extracted from the DDGS were measured by means of acid esterification at 60 °C for 48 h using H2SO4 (Sigma-Aldrich, St. Louis, USA). Determination of the acid value (AV) of the extracted lipid was carried out according to the KS H ISO 1242 method. The AV was calculated using the following equation: AV = V × c × 56.11/m (V: volume of KOH in mL; c: concentration of KOH in M; m: mass of lipid of DDGS in g). The characterization of the DDGS and lipid extracted from the DDGS was carried out using a NETZSCH STA 449 F3 Jupiter thermo-gravimetric analysis (TGA) unit capable of both TGA and differential temperature analysis (DTA) measurements. The TGA unit used enabled a temperature increase from ambient to 1250 °C. Three embedded mass flow controllers controlled the flow ranges of the purge gas and protective gases in the TGA unit. The gases used for the experiments were ultra high purity (UHP) gases purchased through AirTech Korea. The TGA unit had two inlet ports. One was for the protective N2 gas or an inert gas (i.e., He or Ar), which protected the balance mechanism from any heat or effluent gases, and was set at a flow rate of 20 mL min−1. The second port was used for the introduction of the gas mixture of interest to provide the desired atmosphere during the experiment. The flow rate of the atmosphere inlet was 100 mL min−1 which, combined with the protective gas flow of 20 mL min−1, yielded a total flow past the test sample of 120 mL min−1. This flow rate was maintained throughout all of the experiments performed. 2.3. Experimental Setup for Biodiesel Conversion. A tubular reactor (TR), consisting of a 25.4 mm outer diameter (od) quartz tubing (Chemglass CGQ-0800T-13) and 25.4 mm stainless Ultra-Torr Vacuum Fitting (Swagelok SS-4-UT-6400), was used for maintaining airtight conditions; the maximum possible pressure inside the reactor was 1.5 bar. Charcoal was packed into the reactor, and the required experimental temperature was achieved using a split-hinged tubular furnace (AsOne, Japan) and was simultaneously monitored by an S-type thermocouple. The charcoal used in the TR was characterized in terms of surface area (0.2527 m2g−1) and pore distribution (average pore diameter: 31.7249 mm) using a BELSORP II (BEL, Japan). An insulation collar (high-temperature Duraboard insulation) at the end of the furnace was used to block heat transfer during operation and to protect the quartz tubing. The extracted DDGS lipid and primary alcohol (MeOH/EtOH) were continuously fed into the TR using a gear pump (microannular gear pump MZR2904, Germany) and a HPLC pump (Lab Alliance PN#F40SFX01, USA). The flow rate of CO2 was set and
3. RESULTS AND DISCUSSION 3.1. Estimation of Supplying Capacity of the Lipid Feedstocks from DDGS. Utilizing lipid extracted from DDGS as a biodiesel feedstock is well suited to the concept of wasteto-energy (WtE), as it concurrently reduces the waste from the bioethanol industry. On the basis of the United States Department of Agriculture (USDA) report in 2007, total annual production of bioethanol is expected to reach approximately 1.5 × 109 kL by 2020, which includes second generation bioethanol (i.e., bioethanol from lignocellulosic biomass: 6.1 × 108 kL). So far, commercialized production of bioethanol from lignocellulosic biomass has not been reported, meaning that only possible candidate microorganisms for the fermentation process have been identified. Reference values from the literature for estimating the possible lipid recovery from DDGS are summarized in Table 1. For example, S. cerevisiae, P. stipitis, Z. mobiliz, and E. coli have been considered as candidate microorganisms for the fermentation process to produce second generation bioethanol. However, E. coli was subsequently excluded because of its low tolerance to EtOH during the fermentation process.24 Three scenarios for estimating possible lipid recovery from DDGS were considered. The first was the sole use of S. cerevisiae for the fermentation process. The others consisted of the first generation of bioethanol being produced using S. cerevisiae and the second generation being produced using either Z. mobiliz or P. stipitis. The possible lipid recovery from DDGS based on these three different scenarios is illustrated in Figure 1. Comparing the lipid recovery in 2000 and 2010, there is a significant increase by a factor of ∼5 that can be attributed to the increase in production of first generation bioethanol. On the basis of the positive future perspectives for the successful technical development of second generation bioethanol, the maximum achievable lipid recovery from DDGS in 2020 is estimated to be 9 × 107 ton, which would be approximately 6 times that of the current biodiesel consumption (i.e., 294.69 thousand barrels per day in 2010; source: United States Energy Information Administration). In addition, using the 2010 data, the maximum achievable lipid recovery from DDGS could 2818
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initiated in the relatively high temperature region, rather than the DDGS itself. One interesting observation was that the mass decay of lipid extracted from the DDGS correlated well with that of the DDGS itself in the temperature region of 200 to 300 °C. This mass decay was attributed to the devolatilization of free fatty acids (FFAs) in the lipid of DDGS as the possibility of mass decay from residual n-hexane could be excluded because of its low boiling point (68 °C). Measuring the AV is one way of validating this hypothesis. The AV of the extracted lipid was found to be 29.49. As half of the AV is roughly equivalent to the FFA content, it was calculated that the extracted lipid contained 14.7% FFAs, which is in agreement with the DTG curve in Figure 2. The slope change of the DTG curve at ∼265 °C indicated that most FFAs were devolatilized at this point. Similarly, a slope change can be observed at ∼480 °C in the DTG curve, which is explained by thermal cracking and was validated by the detection of hydrogen with micro-GC. In general, hydrogen was observed during the pyrolysis process because of thermal cracking. The high content of FFA would arise from the long drying process that DDGS undergoes. Thus, converting lipid from DDGS with an alkali base catalyst (i.e., KOH or NaOH) into biodiesel is not recommended because of the likely saponification reaction. The lipid contents of DDGS used in this study were found to be fairly similar, with total contents of ∼12 and ∼13 wt % with respect to DDGS. The profiles of fatty acids in the lipid extracted from DDGS were carried out via acid esterification with sulphuric acid at 60 °C for 48 h. The major fatty acids were in the range of C16−18, which was similar to the forms found in edible vegetable oils. Thus, utilizing the lipid from DDGS is a feasible option and would be practical to implement in the biodiesel industry as the lipid feedstock. Furthermore, the residue of the DDGS after extracting the lipid could be used for the supplementation of animal mixed feedstock or fertilizer, among other things. 3.3. Transforming Lipid from DDGS into FAME and FAEE. As discussed in Sections 3.1 and 3.2, lipid extracted from DDGS is a strong candidate for use as a biodiesel feedstock. However, utilization of lipid containing a high amount of FFAs is a big challenge because the current commercialized alkalicatalyzed biodiesel conversion system would cause the saponification reaction.33 Thus, transforming lipid from DDGS into biodiesel via a noncatalytic method is necessary. Moreover, the environmental benefits would be increased if FAEE was produced rather than FAME as MeOH is mostly derived from the petroleum industry. However, most reported noncatalytic transesterification reactions are carried out under supercritical conditions (i.e., pressure higher than 10 MPa), which directly leads to high operational and equipment costs. Thus, a noncatalytic transesterification reaction under ambient pressure is highly desirable. Previous work carried out by the authors34,35 reported that noncatalytic biodiesel conversion under ambient pressure via a continuous feeding system could be achieved in the presence of porous materials. In addition, the main driving force for this reaction was identified as being temperature rather than pressure.34,35 This means that the activation energy of the noncatalytic transesterification reaction could be overcome by providing adequate thermal energy. A porous material would increase the contact time between the triglycerides and the primary alcohol due to its intrinsic tortuosity and absorption/ adsorption capability. For example, a porous material, such as
Figure 1. Estimation of world bioethanol production and possible lipid recovery from DDGS based on three different scenarios.
support ∼30% of biodiesel lipid feedstocks (i.e., 16.97 million ton), which is clearly a promising prospect in the sector of renewable energy. Biodiesel inevitably generates byproducts, and so as more is consumed, a surplus of this byproduct would become problematic. However, one of the main byproducts, glycerin, could be used as a carbon source for culturing microorganisms such as S. cerevisiae, as shown in Table 1. This is another example of a virtuous cycle in which waste is reduced. Microorganisms with a high lipid content and rapid growth rate could be utilized as oleaginous cells for supplying lipid for biodiesel feedstock. 3.2. Characterization of DDGS and Lipid Extracted from DDGS. As a case study, DDGS containing S. cerevisiae and lipid extracted from DDGS were characterized using TGA in N2, at a heating rate of 20 °C min−1 from ambient to 1000 °C. However, the subsequent mass change was not identified in the temperature region above 680 °C, and so, the TG and DTG curves in Figure 2 do not show the data above this point. The
Figure 2. Representative TG and DTG curves of DDGS containing S. cerevisiae and lipid extracted from DDGS.
mass decay of DDGS corresponding to the temperature region of the thermal decomposition of lipid of DDGS is shown in blue in Figure 2 for convenience. As shown by the DTG curve in Figure 2, the DDGS contained a significant amount of volatile matter (∼70 wt % of the initial sample mass) and the maximum rate of the DTG occurred at ∼300 °C. Compared to the value for the lipid extracted from DDGS (i.e., ∼380 °C), the DTG curve of DDGS was much sharper. However, the onset and end temperatures of the thermal degradation seen in both the TG and DTG curves were almost identical for both samples, which indicated that it was the devolatilization of the lipid that was 2819
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Figure 3. Representative chromatogram of FAEE from extracted lipid at 390 °C.
Figure 4. Representative chromatogram of the mixture of FAME (black) and FAEE (red) at 370 °C.
representative chromatogram of FAEE of extracted lipid at 390 °C is illustrated in Figure 3. Major peaks of FAEE were labeled, but unreacted fatty acids and mono-, di-, and triglycerides were not. The chromatogram illustrates the conversion of FAEE from lipid of DDGS via a noncatalytic method under ambient pressure. The profiles of the major FAEEs were derived from the C16−18 range of fatty acids, which is consistent with the previous discussion in Section 3.2. One interesting observation from Figure 3 was the identification of squalene. This is a hydrocarbon and a triterpene and is a nutrient and vital part of the synthesis of cholesterol, steroids, hormones, and vitamin D in the human body. The origin of squalene in Figure 4 is derived from the DDGS. The raw material used by the local brewer that provided the DDGS was rice, and it is known that rice barn oil contains squalene. Thus, the presence of the squalene peak in the chromatogram suggests that squalene and lipid were simultaneously extracted from the DDGS. The optimal conditions for FAME conversion via a noncatalytic method were reported in the previous work carried out by the authors. However, the optimal conditions for FAEE conversion have not yet been established. In this regard, it was imperative to investigate the reaction rate of noncatalytic transesterification with different primary alcohols (i.e., MeOH
charcoal, can trap triglycerides and primary alcohol, such as MeOH, in its pores by means of absorption/adsorption. The gas phase of MeOH (bp = 65 °C) behaves like a mobile phase, and the liquid phase of triglycerides acts like a stationary phase. Thus, heterogeneous transesterification mainly occurs in the pores. The converted biodiesel and glycerin are then eluted from the pores as a result of their relatively lower boiling points compared to that of the triglycerides. Considering the average molecular size of triglycerides (∼2 nm), a material containing meso- and macropores was deemed suitable for the noncatalytic transesterification reaction.34,35 These observations allowed us to combine the esterification of FFAs and transesterification of triglycerides into a single process. Furthermore, the previous work carried out by the authors reported that the noncatalytic biodiesel conversion could be enhanced by the presence of CO2. On the basis of the previous findings,34,35 transforming lipid from DDGS (source: brewer in Kwangyang-City, South Korea) into FAEE was carried out with the TR as described in Section 2.3. Quartz tubing was employed to avoid any catalytic effects on the transesterification reaction due to metals contained in stainless steel. The volumetric flow rates of extracted lipid and EtOH were 10 and 3 mL min−1, respectively. In addition, 100 mL min−1 of CO2 was continuously fed into the TR. The 2820
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4. CONCLUSIONS This work demonstrates that DDGS is a strong candidate for use as a biodiesel lipid feedstock by means of assessing the possible recovery of lipid from DDGS. Lipid extracted from DDGS was thermally characterized, and its basic properties including AV and the profiles of fatty acids are reported. In addition, the noncatalytic transformation of extracted lipid into FAME and FAEE was investigated at the fundamental level, and the optimal conditions for conversion to biodiesel were provided. This work showed that the conversion efficiency to FAME and FAEE reached ∼98 and ∼96%, respectively.
and EtOH) at the fundamental level. Theoretically, transesterification with MeOH performs better than that with EtOH, which can be explained by steric factors. In order to obtain more detail on this, the noncatalytic transformation of extracted lipid into biodiesel with various ratios of MeOH to EtOH was performed. The experimental conditions, including temperature and feeding rate of lipid, primary alcohol, and CO2 were the same as those in Figure 3. The representative chromatogram of the mixture of FAME and FAEE is shown in Figure 4. The molar ratio of MeOH to EtOH used for the biodiesel conversion was 1.5. An interesting observation from Figure 4 was that the areas of the FAME peaks were bigger than those of the FAEE peaks, even though a higher molar ratio of EtOH was used. This is clearly demonstrated by the peak of C16-range fatty acid in Figure 4. Thus, this observation suggests that noncatalytic transformation of extracted lipid into FAEE is more difficult than into FAME. Transformation into FAEE would need a higher thermal energy or a longer retention time than for FAME. However, Figure 4 shows that the noncatalytic FAEE conversion with lipid extracted from DDGS is possible. Thus, it was imperative to establish the optimal experimental/operational conditions for producing FAME and FAEE with regards to temperature and the ratio of lipid to primary alcohol. The conversion to FAME was almost identical to the previously reported work. The conversion to FAEE was lower than that of FAME over the entire experimental temperature range, which was consistent with the observations in Figure 4. Figure 5 provides more evidence that the conversion to FAEE requires more thermal energy compared to FAME.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 82-61-799-2738; fax: 82-61-799-0768; e-mail: yjjeon@ unsw.edu.au. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 5. FAME and FAEE conversion under various temperatures and molar ratios of extracted lipid to primary alcohol (MeOH and EtOH). 2821
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