Utilization of the residual glycerol from biodiesel production for renewable energy generation

Renewable and Sustainable Energy Reviews 71 (2017) 63–76

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Utilization of the residual glycerol from biodiesel production for renewable energy generation

MARK

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Quan (Sophia) He , Josiah McNutt, Jie Yang Department of Engineering, Faculty of Agriculture, Dalhousie University, Truro, Nova Scotia, Canada B2N 5E3

A R T I C L E I N F O

A BS T RAC T

Keywords: Crude glycerol Biodiesel Hydrogen Biogas Alcohols Renewable energy

A rapid growth in biodiesel production has naturally led to a surplus of crude glycerol generated. Due to the impurities present in the crude glycerol, expensive refining processes are often necessary in order for the crude glycerol to be used in the same applications as pure glycerol. As a result, the demand for crude glycerol is quite low, and biodiesel producers must find ways to dispose it. Disposal can be costly, detrimental to the environment, and wasteful. Exploration of crude glycerol utilization is of significance for not only reducing the negative impact on the environment but also for increasing the economic benefits of biodiesel production. This paper reviewed a number of valuable and practical applications of crude glycerol in the sector of renewable energy generation through processes such as fermentation, digestion, gasification, pyrolysis, liquefaction, combustion, and steam reforming. Studies indicated that an integration of crude glycerol to other systems for energy production is a promising option despite the impurities in crude glycerol, and some processes even benefit from their presence.

1. Introduction Depleting petroleum reserves and increasing concern over climate change have brought renewable energies to the forefront of the scientific community. One area of particular interest is the production of biodiesel through the transesterification of vegetable oils or animal fats. Biodiesel is renewable, biodegradable, environmentally-friendly, and can be used directly in diesel engines without major modifications. The biodiesel industry has become one of the most rapidly growing industries in the world, with global biodiesel production projected to reach 37 billion gallons by 2016 [1]. One of the co-products from biodiesel production is glycerol, which accounts for at least 10% of the volume of the resultant mixture from a transesterification process. With an increase in biodiesel production worldwide, the generation of glycerol will increase accordingly, raising a new problem: disposal of waste glycerol [2–4]. Disposing of crude glycerol can be not only costly, but also wasteful, and cause environmental problems. Therefore, it is essential to explore wiser utilization options for glycerol. Pure glycerol is used as a raw material and can be converted into various value added products in the personal care, cosmetics, pharmaceutical, and food industries. The Global Glycerol Market is expected to reach USD 2.52 billion by 2020, and biodiesel emerged as the leading source of glycerol, accounting for over 1400 kt of glycerol production in 2013 [5]. However, the glycerol produced in the making of biodiesel is

⁎

known as “crude glycerol”, which contains impurities such as methanol, fatty acid methyl esters, and salts left over from the transesterification reaction. Because of these impurities, the refining cost of crude glycerol is very high, which makes the use of it in these traditional industries infeasible despite its low market price [6,7]. Despite the low demand for the unrefined glycerol in these traditional industries, it can be an inexpensive raw material with several very practical applications in the emerging sector of renewable energy. Integration of waste glycerol into renewable energy production processes not only benefits those processes, but also helps to reduce the cost of biodiesel production, and is often times beneficial to the environment. There are a number of excellent review papers in the literature addressing the growing challenge of surplus crude glycerol, each of which has different focuses. For example, the worldwide glycerol production and market and its application in direct combustion were reviewed in articles [3,4]. Ayoub and Abdullah [7] detailed the status of glycerol production and its impact on the global market, emphasizing region-wise and demand-wise application. Gupta and Kumar [8] proposed that crude glycerol was a promising source of energy, and briefly stated the studies conducted in this field before 2012, however without technical details. Stelmachowski et al. [9] focused on the photocatalytic methods for the conversion of glycerol to hydrogen. Đurišić-Mladenović et al. [10] proposed co-gasification of glycerol with other biomass for syngas production. Potential applications of crude

Corresponding author. E-mail address: [email protected] (Q.S. He).

http://dx.doi.org/10.1016/j.rser.2016.12.110 Received 19 January 2016; Received in revised form 30 November 2016; Accepted 26 December 2016 Available online 10 January 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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Table 1 Chemical and physical properties of glycerol [4]. Property Molecular formula Molar mass Relative density Viscosity Melting point Boiling point Flash point Specific heat Heat of vaporization Heat of formation Surface tension Self-ignition

Fig. 1. Diagram outlining the various energy products that can be derived from glycerol.

glycerol in polymer technology was recently evaluated [11]. Upgrading crude glycerol to value-added products hold great promises and was mostly reviewed in the literature [2,6,8,12–18]. The aim of this paper is to provide a comprehensive review from energy perspective. An emphasis is placed on the uses of the crude glycerol as feedstock/co-feedstock for renewable energy generation as shown in Fig. 1. The authors consider this the most practical solution for the residual glycerol from biodiesel production as costly refining processes can be avoided and an addition of crude glycerol to biomass generally improves the yield of energies such as biogas, bio-oil, hydrogen and alcohols through co-digestion, co-gasification, co-pyrolysis, co-liquefaction and steam refining, etc.

C3H5(OH)3 92.09 g/mol 1260 kg/m3 1.41 Pa s 18 °C 290 °C 160 °C 2.43 kJ/kg K 82.12 kJ/kmol 667.8 kJ/mol 63.4 mN/m 393 °C

3. Energy uses 3.1. Fermentation One of the most studied approaches of using crude glycerol is fermentation. Glycerol can be used as the carbon source for a growth medium, and be fermented to produce alcohol, hydrogen, and other products via bacteria and/or yeast. Because of its biological nature, fermentation requires much lower temperatures than what is often required in many other conversion processes discussed in the following sections. However, the growth mediums required are often very specific and costly, and may offset the energy cost savings accrued due to the lower temperatures.

2. Biodiesel production and crude glycerol properties Biodiesel refers to a fatty acid alkyl esters mixture generated when vegetable oils or animal fats are transesterified with short chain alcohols. It is viewed by many as the eventual replacement for diesel oil, as it is better for environment and health, as well as engine performance [1,3]. In the transesterification process as shown in Fig. 2, one mole of vegetable oil, chemically, triglyceride reacts with three moles of alcohol in the presence of a catalyst and undergoes three sequential reactions to yield one mole of glycerol and three moles of biodiesel, chemically, fatty acid alkyl esters. The products consist of about 1 kg of glycerol for every 9 kg of biodiesel produced [19]. Further refining of the fatty acid alkyl esters leaves a mixture consisting mostly of glycerol, with catalyst salts, unreacted mono/di/ triglycerides, and water all left over from the reaction process. The composition of the crude glycerol can vary drastically depending on various reaction conditions, as well as the extent to which the crude glycerol is refined by the biodiesel plant. The percentage of glycerol present in the crude glycerol can vary from as low as 45% to upwards of 90% [20,21]. Because of the wide variation in composition of crude glycerol, it is difficult to summarize its properties. Chemical and physical properties of its main component, glycerol, are briefly summarized in Table 1.

3.1.1. Alcohol A wide variety of yeast and bacteria can be used in the fermentation of glycerol. Two common products are ethanol and/or butanol which are important biofuels. A summary of the studies conducted on the conversion of crude glycerol to alcohol via fermentation is shown in Table 2. Ethanol, due to its popularity as a fuel, is commonly the desired product of fermentation. Pachysolen tannophilus CBS4044, Klebsiella pneumoniae GEM167, Kluyvera cryocrescens, Enterobacter aerogenes, and Escherichia coli have all been used to produce ethanol [22–26]. Liu et al. [22] used the yeast P. tannophilus CBS4044 and achieved the highest total ethanol production (28.1 g/L) of any microbial process at the time, but the productivity of the process was still very low in comparison to bacteria based production. Oh et al. [23] irradiated K. pneumoniae to produce a mutated strain which greatly enhanced ethanol production to 20.5 g/L. Meyer et al. [25] found that after removing the non-glycerol organic matter from the crude glycerol, it was able to be fermented to ethanol by E. aerogenes at room temperature, reducing energy costs. Recently, significant efforts were made on genetic engineering of strains. For example, Thapa et al. [27] engineered a mutant strain, E. aerogenes SUMI014, which was able to block the formation of lactic acid and thus enhanced ethanol production. Loaces et al. [28] used E. coli with expression of heterologous genes and improved glycerol conversion, and the ethanol production rate was 0.39 g h−1 OD−1 L−1. Kata et al. [29] cloned the genes of PDC1 and ADH1 to thermotolerant yeast, Ogataea (Hansenula) polymorpha, and the fermentation was conducted at relatively high temperature (45–28 °C), resulting in increased fermentation rate. Ethanol is not the only product obtained from the fermentation of glycerol, as hydrogen is often produced simultaneously. Shams Yazdani and Gonzalez [26] engineered E. coli strains to convert glycerol to ethanol more efficiently, with additional benefits to either formate or hydrogen production. Maru et al. [30] conducted dark fermentation using co-culture of Escherichia coli and Enterobacter sp. and obtained both ethanol and hydrogen at high yields. Valle et al. [31,32] established an experimental method for screening E coli single mutants

Fig. 2. A schematic illustration of glycerol formation in the transesterification process.

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Table 2 Studies on the fermentation of crude glycerol to produce alcohols. Products

Crude Glycerol Properties

Conditions

Productivity

Production/yield

Ref.

Ethanol

82% glycerol, 10% water, 7% ash 0.5% methanol – 80–85% glycerol, 10% water, 5–6% salts, 2% MONG, 0.5% methanol 60.04% glycerol, 23.49% MONG 11.77% water, 4.70% ash Crude glycerol 78.6% glycerol 4.0% methanol 2.8% water, 4.7%ash Glycerol 0.8 g/L

P. tannophilus CBS4044, 28 °C

0.06 g/L/h

28.1 g/L

[22]

K. pneumoniae GEM167, 37 °C, 20 rpm K. cryocrescens, 500 rpm, 30 °C

0.89 g/L/h 0.61 g/L/h

20.5 g/L 27 g/L (80% yield)

[23] [24]

E. aerogenes, pH 6.8, 200 rpm, room temperature

–

–

[25]

E. aerogenes SUMI014 E-Coli with LY180 cells

– 0.39 g h−1 OD−1 L−1

34.54 g/L 75 g/L

[27] [28]

O. polymorpha with genes of PDC1 and ADH1 45–48 °C Engineered E. coli, 37 °C, 200 rpm

–

5.0 g/L

[29]

Ethanol: 1.10– 3.58 mmol/L/h Formate: 3.18 Hydrogen: 1.11

Ethanol:1.01–1.04 mol/mol glycerol Formate: 0.92 mol/mol glycerol Hydrogen: 1.02 mol/mol glycerol Ethanol: 26 g/L Hydrogen: 9 L/L fermenter

[26]

Ethanol Ethanol

Ethanol

Ethanol Ethanol

Ethanol

Ethanol and hydrogen/ formate

Ethanol and hydrogen

Ethanol and hydrogen

Ethanol and hydrogen

Butanol and 1, 3propanediol

Butanol and 1, 3propanediol Butanol and 1, 3propanediol Butanol and 1, 3propanediol

84% glycerol,

5% salts, 0.02% methanol 90% glycerol, 7% salts, 2% ash, 1% methanol, < 0.4% moisture Glycerol 47.5%, water 40.5% ash content 4.8% and nonglycerol organic matter 7.2% 37.7 g/L

85% glycerol, 8–10% moisture, 6% ash, 2.5% MONG, 0.1% methanol, 98.84% glycerol, 0.9% sodium hydroxide, 0.26% methanol Glycerol 3 M KOH/3 M H2SO4, Glycerol 40–100 g/L NH4HCO3, NaHCO3 and K2HPO4 etc

Microbial mixed culture, 37 °C, 120 rpm, pH 8

–

Mixture of Enterobacter spH1 & E. coli CECT432

–

Ethanol 1.53 Hydrogen 1.21 mol/ mol glycerol

[30]

Escherichia coli MG1655

–

[33]

C. pasteurianum DSM 525, 37 °C, pH 6.8

Butanol: 0.032–0.119 g/ L/h 1,3-PDO: 0.019–0.077

Ethanol 7.6 g/L glycerol Hydrogen 0.56 mol/mol glycerol Butanol: 0.19–0.28 g/g

C. pasteurianum MTCC 116 cells immobilized on silica, 30 °C

–

C. pasteurianum DSM525

Butanol: 0.35 g/L/h

C. pasteurianum CH4

Butanol: 0.29 g/L/h

[21]

[34]

1,3-PDO: 0.06–0.21

Butanol: 0.23 g/g crude glycerol 1,3-PDO: 0.61 g/g crude glycerol Butanol: 0.29 g/g crude glycerol Butanol: 0.39 mol/mol crude glycerol

[35]

[36] [37]

higher energy density and low volatility compared to ethanol. Gallardo et al. [34] examined the effect of crude glycerol, butyrate, and acetate concentrations on butanol yield during the fermentation of crude glycerol by C. pasteurianum DSM. It was found that higher crude glycerol concentrations favored butanol production over 1, 3-PDO, and butyrate and acetate supplementation increased butanol titre, but butyrate significantly decreased fermentation time. Khanna et al. [35] produced butanol, 1, 3-propanediol, and ethanol from glycerol using Clostridium pasteurianum MTCC 116. It was found that immobilized cells performed much better than free cells, and the use of crude glycerol had no negative effect on cellular morphology, but crude glycerol resulted in only traces amounts of ethanol. Most recently, Johnson and Rehmann [36] converted crude glycerol to butanol using Clostridium pasteurianum, and a maximum butanol yield of 0.29 g/g glycerol (or 0.36 mol/mol) was achieved. Lin et al. added butyrate as a precursor and combined with an in situ butanol removal via vacuum membrane distillation, leading to an increase in the butanol yield from 0.24 to 0.39 mol/mol glycerol, using Clostridium pasteurianum CH4

to identify the strains with enhanced ability for hydrogen/ethanol production, unfortunately, the ethanol and hydrogen yields were not provided om theses papers. Valuable results of ethanol/hydrogen production at a pilot scale were provided by the work conducted by Cofré et al. [33], indicating the promise of scaling up this process. Varrone et al. [21] assessed the economic practicality of joint ethanol and hydrogen production from crude glycerol using a highly specific microbial mixed culture which required no yeast extract, tryptone, or minerals/vitamins, which made it highly competitive with traditional carbohydrate fermentation. They demonstrated that computed energy cost was estimated to be 0.019 €/kW hth and 0.057/kE hel for 26 g/L of ethanol and retention time of 120 h. Research focusing on the production of hydrogen through fermentation is discussed in greater detail in the following Section 3.1.2. Butanol and 1, 3-propanediol (1, 3-PDO) are also commonly coproduced from fermenting glycerol with the use of Clostridium pasteurianum. Butanol is an emerging new generation of biofuel and has drawn increasing attention due to its interesting properties such as 65

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Table 3 Studies on the fermentation of crude glycerol to produce hydrogen. Method

Crude glycerol properties

Conditions

Rate of production

Yield

Ref.

Dark fermentation

Soil microbial inoculum, 35 °C, 48 h

332 mL H2/L

0.55 mol H2/mol glycerol

[39]

Dark fermentation

50% glycerol, 36% water, 4–5% sodium salts, 1.6–7.5% MONG, 1–3% methanol 50% glycerol

165.21–242.15 mmol H2/ L/d

29.00–44.27 mmol H2/g glycerol

[58]

Dark fermentation

615.4 g/L

166.0 mL/h/L

0.77 mol H2/mol glycerol

[59]

Dark fermentation

61.6% methanol, 26.5% glycerol, 5.8% KOH, 6.1% biodiesel (before purification steps) –

Immobilized Klebsiella sp. TR17, upflow anaerobic sludge blanket reactor, 40 °C, 4 h C. pasteurianum CH4, 35 °C, pH 7, 200 rpm agitation, 10 g/L glycerol Soil inocula, 350 rpm, 30 °C, pH 5.5, GCI/IGCT 2.5 h/20 h

–

0.75 mol H2/mol glycerol

[41]

T. neapolitana, 75 °C, pH 7.0–7.5, itaconic acid

620 mL H2/L

–

[62]

Activated sludge inocula, 40 °C, pH 6.5, 150 rpm Activated sludge inocula

1.1 mol H2/mol glycerol consumed 2960 mL H2/L/d

Activated sludge inocula, 37–37.3 °C, pH 7.9–8.0,

Dark fermentation

Dark fermentation Dark fermentation

– 45% glycerol, 30% methanol 84% glycerol, 10% water, 0.35% methanol, 0.25% TFM 90% glycerol, 7% salts, 2% ash, 1% methanol, < 0.4% moisture 52% methanol, 35% glycerol, 6.4% KOH 45% glycerol, 30% methanol 92% glycerol, < 6% ash 17.5 g/L glycerol 15 mg/L Tween 1 v/v glycerol Municipal sloid waste formate and glycerol acetate and glycerol

Dark fermentation Photofermentation Photofermentation

glucose and glycerol 70–75% glycerol –

Photofermentation

–

E. coli R. palustris, 50 W halogen light, 30 °C R. palustris, 150–180 µmol photons/ m2/s R. palustris, 30 °C, 200 W/m2

Photofermentation Dark fermentation +Photofermentation

10 mM glycerol –

R. palustris, 50 W halogen light, 25 °C Klebsiella sp. TR17, 40 °C

37.7 mL/g biomass/ h 64.24 mmol H2/L

Effluent from dark fermentation 85% glycerol, < 0.5% methanol

R. palustris TN1, glutamate

3.12 mmol H2/L

Dark fermentation Dark fermentation

Dark fermentation

Dark fermentation

Dark fermentation Dark fermentation Dark fermentation Dark fermentation

Photofermentation +Photofermentation

[20] 0.9 mol/mol

[63]

2191 mL/L/d

0.96 mol H2/mol glycerol 7.92 g ethanol/L

[64]

HBP, soil inocula, 30 °C, 300 rpm

–

0.17–0.18 mol H2/mol glycerol

[61]

Activated sludge inocula, 40 °C, 150 rpm Activated sludge, up-flow column reactor, 35 °C co-culture of Enterobacter aerogenes and C butyricum Eubacteria and Archaea.

–

1.42 mol H2/ mol glycerol

[65]

–

107.3 L H2/kg waste glycerol

[66]

–

1.8 mmol H2/g glycerol

[50]

–

The yield of H2 increased 1.8 times

[42]

– – –

– – – – 75% (6 H2/glycerol) NA

[45] [47]

E. coli E. coli

2

R. palustris, 30 °C, 175 W/m

– > 34 mL/g/h –

–

6.1 mol H2/mol crude glycerol (87%) – 5.74 mmol H2/g COD (80.21% glycerol conversion rate) 0.68 mmol H2/g COD (96% theoretical) 6.69 mol H2/mol glycerol

[48] [53] [54] [55] [57] [59]

[60]

in an anaerobic environment at around temperature of 30–40 °C. It has been observed that many of these strains actually perform better with crude glycerol, due to the benefits of the impurities [40,41]. Zahedi et al. [42] found that adding crude glycerol to industrial municipal solid waste nearly doubled the yield of hydrogen through a dark fermentation using Eubacteria and Archaea. Trchounian and Trchounian et al. [43] systematically reviewed the performance of E coli in the hydrogen generation and stated that metabolic engineering of E.coli was able to improve hydrogen production. A comparison between E. coli and other bacteria in terms of fermentation conditions and production efficiency was provided [44]. An interesting finding was that the fermentation of mixed carbon sources (glycerol and other agriculture/food wastes) enhanced hydrogen production. For example, they investigated the cofermentation of formate and glycerol [45,46], co-fermentation of

[37]. 3.1.2. Hydrogen Hydrogen production through the fermentation of glycerol is favorable, as hydrogen is clean, has a very high energy density, and the only by-product is water. It has received increasing attention. The conversion of crude glycerol to hydrogen can be done through two different forms of fermentation known as dark fermentation and photofermentation. Dark fermentation uses a variety of microbes, usually isolated from soil samples, such as Klebsiella sp., Clostridium pasteurianum, Clostridium butyricum, Thermotoga neapolitana, or a collection of various microbes [38,39]. The microbial culture is then added to a glycerol medium, containing waste glycerol, nutrients (compounds containing nitrogen and potassium etc.), and/or yeast, 66

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To summarize, in the microbial fermentation of crude glycerol to energy products, the key is to develop more efficient enzymes, involving the development of new strains, genetic engineering of existing strains, improvement of selectivity and durability of strains, particularly higher tolerance of VFA and impurities in glycerol. For example, the ethanol/ butanol production was improved significantly using mutant strains like metabolic engineering E coli, or stains with expression of heterologous genes. Regarding hydrogen production, genetically modified strains changed the metabolic pathways and enhanced the energy productivity. Impurities in crude glycerol do impact the yield, can be positive or negative, new technology should be developed to effectively utilize impurities. Moreover, mixing crude glycerol with other carbon sources has great potential to increase the yield of energy products. Additional benefits are reduction of logistics costs associated with biowaste transportation and better and more effective utilization/ disposal of different biowastes. More research is needed to evaluate the best combination of crude glycerol and other organic waste. Design and use of novel bioreactors which are able to facilitate in situ removal of products is another strategy to improve overall production. Optimization of operation conditions (pH value, temperature and substrate concentration etc.) is essential to improve the production efficiency. A suitable pH value has to strike a balance between bacteria growth rate and energy production rate. Another challenging issue is an accumulation of by-product such as volatile fatty acids resulting from fermentation might inhibit activity of bacteria. This can be solved by two stage fermentation, such as integrated systems (hybrid), combination of dark fermentation and photofermentation and application of combination of different bacteria strains. Although the current yield of these energy products are low, improvements on above mentioned technology would overcome these limitations, and microbial fermentation is expected to be promising for utilization of crude glycerol generated from biodiesel production.

acetate and glycerol [47], co-fermentation of glucose and glycerol [48]. This group also investigated the effect of types and concentrations of heavy metals on hydrogen yield and E coli growth rate [49]. Pachapur et al. [50] added a surfactant to a co-culture of Entero bacter aerogenes and Clostridium butyricum and enhanced hydrogen production from crude glycerol. Kumar et al. [51] used Bacillus thuringiensis EGU45H-2 on fermentation of crude glycerol and achieved a yield of 0.393 mol/mol feed. Faber and Ferreira-Leitão [52] optimized the conditions of dark fermentation of residual glycerol, and the yield and productivity of hydrogen were improved by 68% and 67% respectively compared to those in theory. Photofermentation also produces hydrogen from waste glycerol, but as the name implies, requires a light source. The most popular enzyme used in photofermentation is Rhodopseudomonas palustris with waste glycerol in a nutrient solution. Sabourin-Provost and Hallenbeck [53] used both pure glycerol and crude glycerol as feedstock to generate hydrogen and obtained 6 mol H2 /mole glycerol and no obvious inhibition observed. Pott et al. [54] found that using R. palustris, with a growth rate of 0.074 h−1, glycerol was converted into 97 mol% hydrogen at a conversion efficiency nearing 90% at a rate of 34 mL H2/gdw/h; However, inhibition of growth by impurities from crude glycerol was observed. Ghosh et al. [55,56] and Liu et al. [41] have made continuous efforts on photo fermentation of crude glycerol for hydrogen production. They applied RSM to investigate the interactive effects of process parameters such as light intensity, concentration of glycerol and level of glutamate as well as the effect of nitrogen source on hydrogen yield. Zhang et al. [57] used a modelling method to simulate the entire growth phase of R. palustris to maximize hydrogen production. A few interesting attempts have evaluated the possibility of using both dark and photofermentation in a two-stage process. Chookaew et al. [39,58] conducted on-going research on hydrogen production from crude glycerol and recently experimented with a two stage fermentation process involving dark fermentation followed by photofermentation [59]. The two stages used Klebsiella sp. TR17 and Rhodopseudomonas palustris TN1, respectively. It was found that the optimal conditions for photofermentation were when no supplementary yeast was used, glutamate was added, and the effluent was diluted. Under these conditions, the overall the total hydrogen production from the two-stage process was 6.42 mmol/g chemical oxygen demand (COD), which was 10.4% of the theoretical yield. Sarma et al. [60] also evaluated the technical and economic aspects of biohydrogen production from crude glycerol using dark fermentation followed by photofermentation. Based on maximum production and yield values found at publication of the paper, they concluded that 1 kg of crude glycerol could be converted to the equivalent of 2.56 L of fossil diesel fuel, but cost $330 to produce. 82% of the production cost was growth media components, and it is therefore necessary to reduce those costs in order to make fermentation of this kind feasible. It was also concluded that fermentation of waste glycerol was very beneficial for the environment, as the conversion of 1 kg of crude glycerol reduces 7.66 kg of greenhouse gas. These studies are summarized in Table 3. It is worthwhile to point out that microbial fuel cells are a technology commonly combined with hydrogen production. Sharma et al. [61] produced bioenergy from hydrogen producing bioreactors (HPBs) and microbial fuel cells (MFCs). It was found that the impurities in crude glycerol significantly hampered energy production in MFCs when compared to pure glycerol, and that the production of hydrogen and 1, 3-propanediol in a HPB was much more profitable. Chookaew et al. [39] experimented with a combined process consisting of dark fermentation, followed by MFC or MEC reactors. Effluent from the fermentation was fed into the MFC or MEC reactors. The MFCs had a maximum power density of 92 mW/m2 when fed 100% effluent, and removed 49.1–50.2% of COD. The MECs fed 50% diluted effluent had a yield of 106.14 ± 8.5 mL H2/g COD consumed at 1.0 V applied voltage. Detailed reviews on MFCs are not included in this paper.

3.2. Digestion Another popular method of disposal of waste glycerol is through codigestion to produce biogas [19,67–78]. Co-digestion involves the decomposition of crude glycerol along with biomass (usually sewage sludge or manure) by a microbial community at mild temperatures to produce methane and hydrogen. The waste glycerol is highly beneficial to the digestion process, as it significantly increases biogas (methane and hydrogen) production. This is because the waste glycerol improves the organic loading rate (OLR), is highly biodegradable, reduces the ammonia concentration, and improves the carbon to nitrogen ratio. The digested sludge can sometimes be suitable as a fertilizer as well. Baba et al. [67] evaluated the methane energy balance of codigestion of crude glycerol and sludge at a practical scale (5–75 L/ 30 m3/day for 1.5 years) and found that the energy output was 106% equivalent to energy input and the digested sludge applied as fertilizer increased grass field yields by 20%. Research [68] on the mesophilic anaerobic co-digestion of primary sewage sludge and glycerol was conducted, and the optimal glycerol loading was in the range of 25– 60% OLR resulting in biogas production increase by 82–280%. However, under 70% glycerol of OLR, inhibition was observed. Athanasoulia et al. [69] used a CSTR reactor and conducted mesophilic digestion. The biogas production was improved by 3.8–4.7 times when the glycerol addition was less than 4% (v/v). The effects of glyceryl addition on biogas yield at a pilot scale [70] were examined and found that a low glycerol dose (0.63% v/v) was preferable and the additional methanol yield was 1.3 m3/L crude glycerol. Rivero et al. [71] introduced a two-phase digestion process and the optimal operational conditions were identified to be OLR of 7.82 g COD/L d, with a removal of 93% CODr and hydrogen and methane yields of 0.026 LH2/gCODr and 0.29 LCH4/gCODr, respectively. Silvestre et al. [72] conducted more detailed research on the effects of the C/H ratio on biogas production under both thermophilic and mesophilic co-digestion. They 67

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in order to allow the bacterial community to adjust properly. Application of a two-stage digestion is also beneficial, using acidogenic digester and methanogenic digester in sequence, which could better control bacteria population and reduce the impact of acids on methane generation. Future efforts should be focused on the optimization of digestion parameters and digester operations, including organic loading rate, C/N ratio, retention time control on acidogenic stage and methanogenic stage and bacterial community structure etc.

found that addition of glycerol to sewage sludge could effectively balance C/H ratio, however, no improvement on biogas yield was observed when glycerol content was beyond 1% v/v in the influent. Process kinetics and microbial dynamics [73] were investigated in such a co-digestion process of sewage sludge and glycerol. Research indicated that the synergisms between glycerol and sludge did benefit the co-digestion process, 2% v/v glycerol dosing struck a good balance between increasing organic loading rate and minimizing impact on hydraulic retention time. Co-digestion of glycerol and manure also showed a lot of promise. Astals et al. [74] had early demonstrated that the specific biogas production of the co-digester was 180% higher than the monodigestion under mesophilic conditions when pig manure was codigested with 3% v/v of glycerol, on a wet basis. This increase was mainly attributed to an improved organic loading rate and a balanced C/N ratio. However, the digestate cannot be directly used as soil fertilizers or conditioner due to the presence of high levels of biodegradable matter [74,76]. Fierro et al. [75] investigated codesignation of swine manure and glycerol with an emphasis on the effects of glycerol concertation on inhibition. They found that an addition of glycerol up to 8% v/v caused a system failure due to high concentration of H2S and VFA, and thus a post-stabilization stage was necessary to achieve a complete degradation of proteins and lipids. In the research conducted by Angenent and Usak's research group [77,78], glycerol was co-digested with dairy manure for more than 900 days using a continuously stirred anaerobic reactor at mesophilic temperatures. Methane yield was 549 ± 25 mL CH4 g VS−1 at a total OLR of 3.2 g VS L−1 Day−1 and the co-digestion process was disrupted by floatation and foaming issues. Table 4 provides a summary of these studies. Using crude glycerol as a co-substrate in anaerobic digestion for biogas production is extremely attractive due to the demonstrated improvement in biogas yield and potential of transportation costs reduction. However, overloading of glycerol can cause imbalances in the digester, which can lead to inefficiencies and failures. Generally, high glycerol loadings are acceptable if glycerol additions are controlled

3.3. Gasification Gasification refers to the thermochemical decomposition of organic material under high temperatures to produce a mixture of hydrogen, carbon dioxide, carbon monoxide, methane, and light hydrocarbon gasses, termed as syngas, or producer gas. Crude glycerol has been used frequently as an additive in the gasification of biomass to improve gas yields and the hydrogen fraction of the produced gas [79–86]. Đurišić-Mladenović et al. [10] recently reviewed co-gasification of crude glycerol and olive kernel, and compared with other thermochemical methods using principal component analysis (PCA) method. They stated that the mixture of glycerol and olive kernel was a generally favorable combination for syngas production. Skoulou and Zabaniotou [79] co-gasified glycerol with olive kernel in a fixed bed reactor at 750– 850 °C with mixing levels of 24 wt%, 32 wt% and 49 wt% respectively. It was found that the syngas yield increased from 0.4 to 1.2 Nm3/kg for a mixture with 49 wt% of glycerol in biomass, and hydrogen concertation was also improved significantly. Wei et al. [80] examined the cogasification of hardwood chips and crude glycerol in a pilot scale downdraft gasifier and an addition of 20% glycerol in woodchip led to high quality fuel gas suitable for internal combustion engines. Yoon et al. [81] gasified crude glycerol in an entrained flow gasifier at 950– 1500 °C using air or oxygen air, giving carbon conversion of 92% and HHV of syngas 2500 kcal/Nm3. This group further attempted the microwave plasma gasification of glycerol for syngas production. It was observed that higher production of syngas (57% hydrogen and 35% CO) could be achieved, the fraction of syngas decreased as the oxygen-

Table 4 Studies on the co-digestion of crude glycerol to produce biogas. Sludge source

Crude glycerol properties

Conditions 3

47% glycerol

50 m plant, 1 mL glycerol/L/day, 35 °C

50% glycerol –

50 L CSTR, 35 °C, 0.63% v/v glycerol loading 4 L CSTR, 35 °C, 1% glycerol

Olive mill and slaughterhouse waste water

–

4 L CSTR, 35 °C, 1% glycerol

Sewage sludge

4 L CSTR, 37 °C, 25–60% glycerol OLR

Sewage sludge

46.5% glycerol, 5.05% methanol 50.6% glycerol, 26.5% soaps, 15.7% water, 7.1% methanol, 0.1% catalysts –

Sewage sludge

–

Pig Manure

3% v/v

mesophilic (35 °C) and thermophilic (55 °C) 5.5 L CSTR reactor, 25 rpm, 5.5 L semi-CSTR, 55 °C, 60 rpm, 3% glycerol

Swine Manure

2–8%v/v glycerol

34 °C, 25 L reactor, 80 days

Dairy manure

-

37 °C, 4.5 L reactor, 900 days

Cattle manure and noodle factory sludge Sewage sludge Sewage sludge

Sewage sludge

40 L then 60 L CSTR, 37 °C, pH 6.8–7.2, 3% glycerol

Production/yield

Product

Ref.

358 mL CH4/g COD removed 149.1 mL CH4/g COD added 1.3 m3 methane/L crude glycerol Methane production improved from 1400 to 2094 mL/d 2.9 mmol H2/g glycerol Methane production improved from 479 to 1210 mL/d 0.7 mmol H2/g glycerol 82–280% increase in methane production 4.7 times higher biogas production

Biogas

[67]

Biogas Biogas

[70] [19]

Biogas

[19]

Biogas

[68]

Biogas

[69]

Biogas

[71]

Biogas

[72]

Biogas

[74]

Biogas

[75]

Biogas

[77]

0.8 L methane/ g TVS removed

5 L acidogenic digester then 10 L methanogenic digester, OLR 7.82 g COD/L/d

68

93% COD removed 0.026 L H2/g COD removed 0.29 L CH4/g COD removed 58% COD removed CH4 66%v/v CH4 production From 0.17 to 0.47 L biogas/g VS fed (180% higher) CH4 production: 1.4 L CH4/L/d CH4 yield: 380 L/kg VSfeed CH4 production: 1.4 L CH4/L/d CH4 yield: 549 L/kg VSfeed

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Table 5 Studies on the gasification of crude glycerol to produce syngas. Method

Crude glycerol properties

Conditions

Production/yield

Ref.

Air gasification with olive kernel

85.4% glycerol, 8.4% water, 6% other

49% glycerol, 850 °C, air ratio 0.4

[79]

Hydrothermal continuous gasification by supercritical water

650 °C, 5% glycerol, continuous tubular reactor,

Gasification by supercritical water

68.53–71.18% glycerol, 2.62–4.19% methanol, 25.91–29.71% MONG, 1.49–2.51% ash, 0.01–0.04% water –

Increased from 0.4 to 1.2 Nm3/kg 19 to 33% (v/v) H2 increase Tar decrease 19.5 to 2.4% Syngas LHV: 8–10 MJ/m3 26.44–35.85 mmol/g

500 °C, 7% glycerol, 45 MPa

[85]

Gasification with hardwood chips

–

–

Steam gasification

60% glycerol, 31% methanol, 7.5% water, 1.05% KOH –

Liquid hourly space velocity of 0.77/h, 800 °C, Ni/ Al2O3 catalyst, 1:3 steam to glycerol

H2 mole fraction yield 27.9 mol%, most significant Up to 20% glycerol produce gases within ICE standards, HHV of 18.71 MJ/Kg 15% mol increase over pyrolysis 89.4% yield 62 mol% H2 1.9 L/g total gas

[84]

60% glycerol, 20% MONG, 15% methanol, 5% ash

Entrained flow gasifier, excess air ration 0.35– 0.4

K2HPO4 and K3PO4 max H2 H3PO4 and KH2PO4 max CH4 Syngas HV: Air: 1750 kcal/N m3 O2:2300–2500

Hydrothermal gasification in supercritical water Air/O2 Gasification

–

[86]

[80] [83]

[81]

Manara and Zabaniotou [87] demonstrated that co-pyrolysis of crude glycerol with lignite could be a source for H2 production. For a blend of 20 wt% glycerol with lignite, a high hydrogen yield of 65.44 v/ v% was obtained. Delgado et al. [88] co-pyrolysed glycerol and corn straw and found that suitable ratios were 3:1 or 1:1 for high oil and gas yield with great heating values. Dou et al. [89] found that water and methanol existing in crude glycerol acted as catalysts in pyrolysis and decreased the activation energy of the decomposition of glycerol, demonstrating crude glycerol a very promising additive. Xiu et al. [90] performed a co-pyrolysis of swine manure and crude glycerol in a high-pressure batch reactor under various mixing ratios to produce bio-oil. They stated that the oil yield increased dramatically by adding crude glycerol as a co-substrate and the key contributor to this improvement was free fatty acid component in the crude glycerol. The maximum oil yield of 68% was obtained at a swine manure to crude glycerol ratio of 1:3 (weight ratio). Skoulou et al. [91] conducted pyrolysis of a mixture of 25 wt% glycerol and 75 wt% olive kernel at a temperature of 720 °C and obtained the pyrolysis gas with H2 concentration of 11.6% v/v. Within the same research group, Ganesapillai et al. [92] investigated the co-pyrolysis of glycerol and olive kernel under microwave pre-treatment of feedstock. A high liquid yield of 59.53% v/v and enriched syngas (H2+CO) concentration (84.9% v/v) were obtained. The improvement in the bio-oil quality and quantity was observed in co-pyrolysis of glycerol with lignocellulosic biomass and manure in these studies as summarized in Table 6. Co-pyrolysis is undoubtedly a promising option, however, more detailed research is necessary to identify the mechanism for energy production enhancement. In addition, the operation parameters need to be optimized according to the target energy product, bio-oil or pyrolysis gas.

to-fuel ratio increased and atomization of glycerol using a nozzle improved the efficiency of the gasification process [82]. Valliyappan et al. [83] gasified glycerol in a fixed bed reactor at 800 °C in the presence of commercial catalyst Ni/Al2O3, and 68.4% syngas was produced. Cengiz et al. [84] conducted hydrothermal gasification of glycerol with a focus on the impact of acid and base catalyst on the yield of syngas, and the effectiveness of catalyst on gasification was demonstrated in the order of K3PO4 > K2HPO4 > H3PO4 > KH2PO4 for crude glycerol. Gasification under supercritical water conditions was further explored by Yang et al. [85]. The optimum reaction conditions were 500 °C, 7 wt% glycerol concentration and 2.39 mol/L KOH concentration and H2 mole fraction yield in the gaseous product was 27.9 ± 0.22 mol%. Dianninggrum et al. [86] gasified crude glycerol at in both batch and continuous apparatus under similar operational conditions and found that complete gasification was obtained under in the continuous mode. A number of gasification studies have been summarized in Table 5. Many research examined gasification under supercritical water using pure glycerol as feedstock, which is not included in this review. In this proven option for glycerol disposal and utilization, cogasification of glycerol and other feedstock was able to improve the yield of syngas or hydrogen considerably. Gasification performance is highly associated with operational parameters such as residence time, temperature and pressure etc. Particularly, the mode of reactor plays a vital role for commercial scale application. Most commonly used reactors include updraft fixed bed, downdraft fixed and fluidized bed etc. Depending on the principal components in the feedstock mixture, proper reaction conditions and gasifier would be applied.

3.4. Pyrolysis Pyrolysis is another thermochemical conversion process similar to gasification but with a focus on bio-oil production. It generates gas, bio-oil, and char, three product streams, through the decomposition of biomass at high temperatures in the absence of oxygen. The difference between the two methods is that in gasification, higher temperatures are used, so the char produced in the pyrolysis stage is further converted to syngas.

3.5. Liquefaction Liquefaction is a thermochemical process in which biomass is converted to a bio-oil with an improved heating value under moderately high temperatures but relatively high pressures. Crude glycerol can be mixed with other sources of biomass as the substrate of the coliquefaction. 69

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Table 6 Studies on the pyrolysis of crude glycerol to produce bio-oil. Method

Crude glycerol properties

Conditions

Char

Bio-oil

Gas

Ref.

Pyrolysis with corn straw

56.2% glycerol, 7.3% methanol, 2.4–4.3% water, 3.5–7.2% ash 85.4% glycerol, 8.4% water, < 6% methanol, < 51.5% MONG, 0.2% FFA 85.4% glycerol, 8.4% water 6% other 55.09% glycerol/ methanol/water, 31.09% FFA, 13.81% salts

30 °C/min to 550 °C, helium to remove air 1:1 ratio

LHV: 24.84 MJ/kg

30.52 MJ/kg

28.65 MJ/kg

[88]

850 °C, 20% glycerol

–

–

65.44 v/v% hydrogen yield 71% conversion to volatiles

[87]

720 °C, 25% glycerol

–

–

Increase in H2 production by 11.6vv%

[91]

1:3 manure: glycerol, 200 rpm, 7 °C/ min, 340 °C, 0.65 MPa

–

68% yield FFA beneficial to oil yield Yield enhanced 1.83 times by CG Lowers viscosity and density Bio-oil 59.53% v/v; syngas of 84.9% v/v

–

[90]

–

[92]

Pyrolysis with Greek lignite

Pyrolysis with olive kernel Pyrolysis with swine manure

Pyrolysis with olive kernel

–

Microwave pretreated

positive effects of methanol, glycerol, water, and some fatty acids on oil yield. Crude glycerol also lowers the pH value, decreases undesirable solid content, and produces a higher hydrogen content in gaseous stream. However, it is also found this process requires high temperature and pressure and the resulting oil has a lower carbon content. Hydrothermal liquefaction is a complex process involving many possible reactions such as hydrolysis, dehydration, dehydrogenation, esterification and re-polymerization etc., and the reaction pathways are currently not well understood. Further process optimization has to strike a good balance between yield and quality of bio-oil to maximize the economic viability.

Xiu and his co-worker conducted considerable research on coliquefaction of glycerol with manure [93–96]. In 2011, a co-liquefaction was first carried out at a temperature range of 260–360 °C and a retention time of 5–90 min [93]. Under the optimal conditions (glycerol and manure ratio of 3, temperature of 340 °C and residence time of 15 min), the oil yield was 68%, increased by 1.83 times (g/g dry matter) compared to the process without glycerol addition. This improvement was attributed to the catalytic effect of alkali salts and glycerol acting as a hydrogen-donor to stabilize the radical fragments in the liquefaction process and therefore less char was formed. Further research also found that addition of crude glycerol as a co-substrate lowered the pH value, increased hydrogen content in the bio-oil and also decreased undesirable solid content. GC-MS analysis indicated that the acidic compounds in the oil derived from manure alone were converted into methyl esters through esterification reactions occurred between methanol and FFA, implying the occurrence of cross/interactive reactions in the co-liquefaction process [94]. Possible reaction pathways of co-liquefaction were explored by performing liquefaction of manure with individual glycerol, methanol, and fatty acids respectively. They found that fatty acid, glycerol and methanol, all of those had positive impact on bio-oil generation. It was speculated that acidcatalyzed esterification reactions occurred not only because the crude glycerol had methanol, but also because methanol could be produced from hydrothermal reactions of glycerol [95]. Cheng et al. [96] further characterized the properties of bio-oil derived from co-liquefaction of swine manure and glycerol, and made a comparison with properties of bio-oil obtained from the liquefaction of swine manure alone and pyrolysis of corn stover. Generally, the crude bio-oil had a higher distillation temperature of 498.8 °C compared to a distillation temperature of 350.4 °C for diesel and 177.8 °C for gasoline. Except the first 10% fraction had a significant amount of water, HHV of the rest eight fractions were in the range of 37.16 MJ/kg to 45.38 MJ/kg. The pH value of bio-oil was close to neutral. The bio-oil contained large amounts of different esters. Pedersen et al. [97] co-liquefied crude glycerol and aspen wood. They observed that the char yield decreased significantly with increasing the ratio of glycerol and aspen wood, and the quality of bio-oil was improved with a H/C ratio of 1.6. The same group recently developed a continuous process of co-liquefaction of aspen wood and glycerol, demonstrating a significant progress made, moving to the production of bio-oil at a large scale [98]. A summary of studies done on co-liquefaction is provided in Table 7. In general, the addition of crude glycerol in to other biomass in liquefaction increases the yield of bio-oil, due to the synergistically

3.6. Combustion One of the seemingly simplest methods to dispose of crude glycerol is through combustion [3,4,99–101]. However, it proves difficult to combust on its own [99] due to glycerol's low heating value, high selfignition temperature, acrolein formation, high viscosity, high emissions, and salt content. Several researchers have developed high swirl refractory burners for use in the combustion of crude glycerol [99,100]. While combustion was possible, emissions were significantly higher when compared to other feedstocks. Use of crude glycerol in fuel briquettes made from wood cutting waste was also examined [101]. At the optimal loading of 10% crude glycerol, low CO, SO2, NOX emissions were observed. A larger concentration of glycerol in solid fuel was not detrimental to the environment, but had a negative impact on the mechanical and physical properties of solid fuels. Generally speaking, this method was not favorable from both environment and combustor performance perspectives. 3.7. Steam reforming for hydrogen generation Steam reforming involves the reaction of glycerol with steam at high temperatures in the presence of a catalyst to produce hydrogen, carbon monoxide, and carbon dioxide. The desired product of steam reforming is hydrogen gas. Most research focused on steam reforming of pure glycerol as reviewed by Schwengbe et al. [102]. Little research has been attempted using crude glycerol, most likely due to the adverse effects of impurities on catalyst behavior. Table 8 shows technical details of limited research on this topic reported in the literature [103–109]. Slinn et al. [106] examined the viability of crude glycerol reforming for hydrogen generation with platinum alumina as a catalyst. The optimum conditions were 880 °C, 0.12 mol/min glycerol per kg of 70

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Table 7 Studies on the co-liquefaction of crude glycerol to produce bio-oil. Method

Crude glycerol properties

Conditions

Production/yield

Ref.

Co-liquefaction with swine manure

55.09 glycerol/methanol, 31.09% FFA, 13.81% salts 66.95% glycerol, 26.2% methanol, 6.85% water (after being fractionated) –

340 °C, 1:3 manure to glycerol

Yield increased by 1.83 times to 68% g/g dry matter Required higher temperatures Negative impact on carbon content and heating value Improved: lower viscosity, lower pH, less solid content, higher hydrogen content

[93]

More distillable than just swine manure and pyrolysis of corn stover Distillate fraction of resulting bio-oil energy content: 36.4–45.4 MJ/kg Bio-oil yield ab 52.26% H/C of 1.19

[96]

Bio-oil yield ab 30% Bio oil energy content: 34.3 MJ/kg

[98]

Co-liquefaction with swine manure

Co-liquefaction with swine manure

Co-liquefaction with aspen wood Co-liquefaction with aspen wood

340 °C, unfractionated, 200 rpm,

1:3 manure to glycerol, 0.7 MPa, 340 °C,

45.53% of FFAs+FAMEs, 49.51% of neat glycerol and 0.37% of methanol –

400 ° C in a micro-batch reactor 3:1 manure to glycerol solid loading ratio of 20%, 1:1 wood to glycerol 400 °C, 300 bar, continuous reactor K2CO3 as catalyst.

[94]

[97]

glycerol was up to 90.9% at 650 °C. Remón et al. [108] examined the effect of acetic acid, methanol and potassium hydroxide on the catalytic steam reforming of glycerol. They later introduced a two-step process for hydrogen production. The crude glycerol was first purified with acetic acid to reduce problematical impurities, and then went through a catalytic steam reforming process. The optimum conditions for H2 production was at a temperature of 680 °C, a glycerol solution of 37 wt % and a spatial time of 3 g catalyst min/g glycerol. Under such conditions, the carbon conversion to gas was 95%, with a composition: 67 vol% H2, 22 vol% CO2, 11 vol% CO and 1 vol% CH4 [109]. Steam reforming of surplus glycerol represents a candidate method for hydrogen production. Here catalysts play a dominant role. The impurities in crude glycerol is the greatest challenge, negatively impacting the activity and durability of catalyst. In addition, glycerol decomposition formed carbon and deactivated catalysts. More research should be focused on catalyst development. In-situ CO2 sorption is a mature technology and its application in steam reforming of glycerol definitely drives the reaction in the forward direction, and improves the purity of final hydrogen product. An alternative method of reforming glycerol is through the use of supercritical water, which is gaining popularity recently. Supercritical water is water subjected to temperatures above 374 °C and pressures

catalyst and 2.5 steam/carbon ratio. Reforming catalysts lasted for several days of continuous operation with a certain degree of degradation observed, 2% of feed deposited as carbon. The yield of hydrogen was 70% of that obtained from pure glycerol reforming. The decrease in yield resulted from the chain fatty acid impurities, which were more likely to form carbon. The research conducted by Fermoso et al. [105] showed a great potential to directly convert crude glycerol into hydrogen. The process yielded H2 up to 88% with a very high purity (99.7 vol%) at atmospheric pressure, temperature of 550–600 °C and a steam/C of 3 in a fixed-bed reactor using a mixture of Ni/Co as catalyst and dolomite as a CO2 sorbent. Dou's research group conducted ongoing studies on steam reforming of glycerol [103,104,107]. For example, crude glycerol was reformed in a continuous flow fixed-bed reactor with an in situ CO2 sorption under temperature of 400–700 °C. This in situ CO2 sorption was demonstrated to remove CO2 effectively and the purity of hydrogen was 88% [103]. They also investigated the performance of different catalysts such as Ni-Cu-Al, Ni-Cu-Mg, Ni-Mg in a continuous flow fixed-bed reactor under atmospheric pressure within a temperature range from 450 to 650 °C [104]. Experimental results showed that the Ni-Cu-Al catalyst containing NiO of 29.2 wt%, CuO of 31.1 wt%, Al2O3 of 39.7 wt% performed with a high catalytic activity, the H2 selectivity was found to be 92.9% and conversion of Table 8 Studies on the steam reforming of crude glycerol to produce hydrogen. Method

Crude glycerol properties

Conditions

Yield/production

Ref.

Steam reforming using platinum alumina catalyst

880 °C, platinum alumina catalyst, 0.12 mol/min glycerol flow per kg of catalyst, 2.5 steam/carbon ratio

70% of the yield of pure glycerol (which was nearly 100%) Selectivity was the same

[106]

Atmospheric pressure, 550–600 °C, Ni/Co catalyst, dolomite as CO2 sorbent, steam/carbon=3

88% H2 yield 99.7 vol% purity

[105]

Steam reforming with in situ CO2 sorption

40% FFA, 33% glycerol, 23% methanol, 3.8% ash, 3.2% water 70–90% glycerol, < 15% water, < 15% methanol, < 5% salts, < 5% polyglycerol impurities 70–90% glycerol, < 15% water,

600 °C, with in situ CO2 sorption

Crude glycerol conversion 100% Steam conversion 11% H2 purity: 88% 5.51 mol% H2 Very little methane

[103]

Steam reforming

< 15% methanol –

450–650 °C, continuous fixed bed reactor, Ni-Cu-Al, Ni-Cu-Mg, Ni-Mg-Al catalysts 680 °C, a glycerol solution of 37 wt% and a spatial time of 3 g catalyst min/g glycerol; Ni–Co/Al–Mg catalyst fluidised bed reactor

Glycerol conversion 88–91%; H2 selectivity 78.5–92.9% 67 vol% H2, 22 vol% CO2, 11 vol% CO and 1 vol% CH4

[104]

One-stage sorption enhanced steam reforming

Two step reforming

63.17% glycerol, 1.63% water, 34.37% methanol 2.06% ash

71

[109]

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Table 9 Studies on the conversion of crude glycerol to hydrogen with supercritical water. Method

Crude glycerol properties

Conditions

Yield

Ref.

Supercritical water for hydrogen production

3.5% glycerol

600 °C, 25 MPa batch autoclaves

[111]

Hydrothermal reforming with supercritical water

42.3% glycerol,

450 °C, 31.0 MPa

570–620 °C, 255–270 bar

1 kg glycerol=10 mol H2 and light hydrocarbons Pure glycerol less hydrocarbons more hydrogen Lower calorific value: 4000 kcal/kg of crude 62.9% oil, 0.99% solids, 17.5% gas (8.89 mol% H2, 0.07 CO, 9.69 CO2, 4.76 hydrocarbons) Carbon to gas efficiency: > 87%

[112]

300–450 °C

18.9% moles of hydrogen per mole of carbon in the feed.

[115]

Supercritical water reforming

Supercritical water

33.1% FAME, 20.8% methanol > 88% glycerol, 6.5% water, 4.5% NaCl, < 0.1% MONG methanol: 20.8 wt% glycerol: 42.3 wt% fatty acid methyl esters: 33.1 wt%)

[110]

8.5–31 MPa

Despite this, there are several non-energy uses in which crude glycerol, has been found to be acceptable, and are briefly reviewed hereby. 1, 3propanediol, produced in the fermentation of crude glycerol has many non-energy uses [15,17,122–124]. It is most commonly used in the production of polyesters, but has a variety of uses in food, cosmetics, and medicines. Another popular use of crude glycerol is as an additive in animal feed [125]. Studies have found that crude glycerol is a suitable energy source for pigs and laying hens, providing additional calories to the diet, while having no noticeable negative impacts when mixed with other feeds [126,127]. Use of crude glycerol as a growth medium for fungal biomass which could be utilized as animal feed has also been examined [128]. In addition to the desired eicosapentaenoic acid, the biomass was also rich in essential amino acids and made an excellent feed. However, the yield was quite low due to soap and methanol presence. Crude glycerol can also be used in the production of polyols and polyurethane foams. The crude glycerol can be employed as a solvent in the liquefaction of lignocellulosic biomass, and performs comparably to petrochemical-based solvents [129–131]. It was found that the organic impurities, such as FFAs and FAMEs, discovered in the crude glycerol had positive influences on the foam produced [129].

above 22.1 MPa. Under these conditions, water acts both as an organic material solvent and a reactant for radical reactions. In this state, water can react with crude glycerol to produce hydrogen, carbon dioxide, carbon monoxide, and light hydrocarbons. Many different reactors and catalysts have been used in the production of hydrogen from glycerol using supercritical water, but mainly with pure glycerol [110]. As in many methods, impurities in the crude glycerol can affect catalyst behavior. However, unlike conventional steam reforming, supercritical water is capable of converting glycerol to hydrogen and other products without the use of transition metal catalyst, and the presence of salts does not impact conversion negatively, which means the use of crude glycerol is possible under the supercritical condition of water. Table 9 outlines the studies focusing on the conversion of crude glycerol to various products under supercritical water. Yu-Wu et al. [111] reformed crude glycerol with and without catalyst (K2CO3 1.5 wt%) at a temperature of 600 °C and pressure of 25 MPa, and the 10 mol hydrogen/kg glycerol was obtained. Bennekom et al. [112] looked at the suitable catalysts for glycerol reforming in supercritical water. Without a catalyst, the conversion of glycerol was 40%. All catalysts tested Pt/CeZrO2, Ni/ZrO2. Ni/CaO-6Al2O3, NiCu/ CeZrO2, and CuZn alloy significantly promoted the decomposition of glycerol, but also the water-gas shift reaction [113]. The performance of bimetallic Pt-Ni/ Al2O3 catalyst in crude glycerol reforming was tested as well [114]. The catalyst was very active, however deactivated rapidly. In the work conducted by Onwudili and Williams [115], the reaction was carried out in a batch reactor at temperatures between 300−450 °C and pressures between 8.5 MPaand 31 MPa. The gas product constituted 90 vol% of hydrogen, equivalent to 18.9% moles of hydrogen per mole of carbon in the feed. Steam reforming of crude glycerol is challenging and it is still far to a real application.

5. Conclusions and recommendations With the growing production of biodiesel in the coming years, managing the crude glycerol produced will become an increasingly difficult task. Although the development of glycerol-free biodiesel production is making progress significantly [132,133], implementation of these processes at a large scale still faces a number of challenges such as high costs, low efficiency, immaturity of relevant technologies, and involvement of sensitive enzyme etc. It is crucial to wisely utilize this waste stream generated from current biodiesel production practices. The production of value-added products/chemicals/polymers from crude glycerol holds promise, but generally are not economically viable as the residual glycerol from biodiesel production contains considerable amounts of impurities, and the refining costs offset the profits from the highly valuable products. Most of these methods are still on a small scale and/or only work well for pure glycerol, and the real costs of production are uncertain. The production of renewable energy such as hydrogen, syngas, biogas, alcohol and bio-oil offers a promising method of glycerol disposal and increasing the profitability of biodiesel production. Most of these conversion processes use biowastes or low-value biomass, and thus the impurities in the unrefined glycerol present less negative impact or very often positive impact on the production as reviewed in this paper. For example, in the processes of co-digestion, co-fermentation, co-liquefaction and co-gasification of glycerol and other biomass, the yield or/and quality of the target energy product were improved. The future efforts should focus on optimization of feedstock

3.8. Others Another potential use for the crude glycerol is in the production of fuel additives [16,116–121]. Reactions with alcohols, isobutylene, or acetone, usually in the presence of a catalyst, can produce diesel fuel additives which improve viscosity and cold flow properties, and decrease emissions [116–119]. These additives can often be added to biodiesel to help meet regulatory standards, making it an attractive method of disposing of biodiesel waste [119]. Table 10 outlines the production processes and benefits of these additives, including glycerol ethers, tertbutyl alcohol, solketal and acetyl methanol etc. 4. Non-energy uses Pure glycerol has many non-energy uses in the cosmetics and food industries, but due to the high standards required of these products, the use of crude glycerol in place of pure glycerol is often not possible. 72

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Table 10 Studies on the conversion of crude glycerol to fuel additives. Additive

Glycerol properties

Conditions

Benefits

Ref.

Glycerol ethers Glycerol tert-Butyl Ethers

Crude glycerol Pure glycerol

– Glycerol and tert-butyl alcohol, Amberlyst-15 catalyst, 5– 8 h, 70 °C

[116] [117]

Glycol-ethers mixture Solketal Acetal (2,2-dimethyl-1,3dioxolan-4-yl) methanol

99.5% glycerol Pure glycerol Crude glycerol

343 K, 1200 rpm, 28% glycerol, 69% isobutylene Glycerol and acetone (1:4), 25 °C, Amberlyst-36 catalyst Crude glycerol and acetone with p-toluenesulfonic acid monohydrate, heated to reflux, 16 h

Acetal (2,2-dimethyl1,3dioxolan-4-yl) methyl acetate Solketal

Crude glycerol

Acetyl (2,2-dimethyl-1,3-dioxolan-4-yl) methanol reacted with acetic anhydride in trimethylamine, room temperature for 4 h 3 consecutive 2-step batches, reflux, 30 min, 6/1 glycerol to acetone, are nesulfonic acid-functionalized silica catalyst

Improves cold flow properties and reduces viscosity Significant decrease in the emissions of particulate matter, hydrocarbons, carbon monoxide, and unregulated aldehydes reduces greenhouse gas emissions 94% yield 90% yield Improves viscosity Meets standards for flash point and oxidative stability Does not improve cold flow as much as triacetin Better viscosity improver, and better density properties 81% conversion High sodium content impacted catalyst activity, acidification reverses impact

[121]

85.8% glycerol

[118] [119]

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