Scholarly article on topic 'Thermochemical Conversion of Microalgae: Challenges and Opportunities'

Thermochemical Conversion of Microalgae: Challenges and Opportunities Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — David Chiaramonti, Matteo Prussi, Marco Buffi, David Casini, Andrea Maria Rizzo

Abstract Research in Advanced Biofuels steadily developed during recent years. A number of highly innovative technologies have been explored at various scale: among these, lignocellulosic ethanol and CTO (Crude Tall Oil)-biofuel technologies already achieved the early-commercial status, while hydrotreating of vegetable oils (HVO, or HEFA) can be considered today fully commercial. However, despite the level of innovation in each specific technological process under consideration, the feedstock maintains a central role in making a biofuel chain really sustainable. In this context, microalgae grown in salt-water and arid areas offers a considerable opportunity for advanced biofuel production: at the same time, however, they also represent a considerable challenge. Processing microalgae in an economic way into a viable and sustainable liquid biofuel (a low-cost mass-produced product) is not trivial. So far, the main attention has been given to cultivating the microorganism, accumulating lipids, extracting the oil, valorising co-products, and treating the algae oil into biodiesel (through esterification) or HEFA (Hydrotreated Esthers and Fatty Acids), this second one representing a very high quality biofuels, almost a drop-in fuel (suitable either for road transport or for aviation), which production exceed 2 Mt y-1 today. However, extracting the algae oil at low cost and at industrial scale is not yet a full industrial mature process, and the still limited market size of algae-to-biofuels makes difficult the development of industrial-scale systems. Nevertheless, another option can be considered, i.e. processing the whole algae into dedicated thermochemical reactors, thus approaching the downstream processing of algae in a completely different way from separation. The present work examines the possible routes for thermochemical conversion of microalgae, distinguishing between dry-processes (namely pyrolysis and gasification) and wet-processes (near critical water hydrothermal liquefaction and hydrothermal gasification). Typical expected elementary composition of major products is given. Main peculiarities of batch versus continuous processing are also discussed from an engineering point of view. Major engineering advantages and challenges in thermochemically conversion of algae are identified and discussed, in view of the production of a transport biofuel. Finally, future perspectives for each route are given in terms of current and expected technological readiness level.

Academic research paper on topic "Thermochemical Conversion of Microalgae: Challenges and Opportunities"

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Energy Procedía 75 (2015) 819 - 826

The 7th International Conference on Applied Energy - ICAE2015

1 Thermochemical conversion of microalgae: challenges and

2 opportunities

David Chiaramontia*, Matteo Prussia, Marco Buffia, David Casinia, Andrea Maria

Rizzoa

3 aRenewable Energy COnsortium for R&D, RE-CORD c/o Department of Industrial Engineering, School of Engineering University

4 of Florence, Viale Morgagni 40, I-50134 Florence, Italy

5 Abstract

6 Research in Advanced Biofuels steadily developed during recent years. A number of highly innovative technologies

7 have been explored at various scale: among these, lignocellulosic ethanol and CTO (Crude Tall Oil)-biofuel

8 technologies already achieved the early-commercial status, while hydrotreating of vegetable oils (HVO, or HEFA)

9 can be considered today fully commercial. However, despite the level of innovation in each specific technological

10 process under consideration, the feedstock maintains a central role in making a biofuel chain really sustainable. In

11 this context, microalgae grown in salt-water and arid areas offers a considerable opportunity for advanced biofuel

12 production: at the same time, however, they also represent a considerable challenge. Processing microalgae in an

13 economic way into a viable and sustainable liquid biofuel (a low-cost mass-produced product) is not trivial. So far,

14 the main attention has been given to cultivating the microorganism, accumulating lipids, extracting the oil, valorising

15 co-products, and treating the algae oil into biodiesel (through esterifi cation) or HEFA (Hydrotreated Esthers and

16 Fatty Acids), this second one representing a very high quality biofuels, almost a drop-in fuel (suitable either for road

17 transport or for aviation), which production exceed 2 Mt y-1 today.

18 However, extracting the algae oil at low cost and at industrial scale is not yet a full industrial mature process, and the

19 still limited market size of algae-to-biofuels makes difficult the development of industrial-scale systems.

20 Nevertheless, another option can be considered, i.e. processing the whole algae into dedicated thermochemical

21 reactors, thus approaching the downstream processing of algae in a completely different way from separation.

22 The present work examines the possible routes for thermochemical conversion of microalgae, distinguishing between

23 dry-processes (namely pyrolysis and gasification) and wet-processes (near critical water hydrothermal liquefaction

24 and hydrothermal gasification). Typical expected elementary composition of major products is given. Main

25 peculiarities of batch versus continuous processing are also discussed from an engineering point of view.

Major engineering advantages and challenges in thermochemically conversion of algae are identified and discussed, in view of the production of a transport biofuel. Finally, future perspectives for each route are given in terms of current and expected technological readiness level. © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

26 (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Applied Energy Innovation Institute

* Corresponding author. E-mail address: david.chiramonti@re-cord.org

1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Applied Energy Innovation Institute

doi:10.1016/j.egypro.2015.07.142

David Chiaramonti et al. /Energy Procedia 75 (2015) 819 - 826 Keywords: microalgae, downstream processes, HTL, pyrolysis, biofuels, bioliquids.

28 1. Introduction

29 A large number of scientific works demonstrate that microalgal biofuels are technically feasible [1]

30 but positive economical and energetic balances have still to be demonstrated [2],[3],[4]. Microalgae

31 represent a niche technology, with currently still only limited commercial applications: nutraceuticals and

32 feed supplements, aquaculture, pigments, polyunsaturated fatty acids, diagnostic and fine chemicals;

33 among these the biofuels sector has still to express its real potential. A global turnover above 5,000

34 million US$ can be estimated for other high value products, such as: functional food, feed additive,

35 aquaculture, DHA and P-Carotene markets [5]. The main factor limiting the development of the markets,

36 and especially those of algae biofuels and food, is the production costs. The actual cost is related to the

37 complexity of the cultivation phase and the downstream processes required to extract the high-value

38 products. Despite the today costs and the real efficiency of conversion of light, although not higher than

39 that of plants [6], microalgae grown in salt-water and arid areas offers a considerable opportunity for

40 advanced biofuel production: at the same time, however, they also represent a considerable challenge.

41 The development of a commercially viable microalgae production is still representing a major challenge,

42 both from a strictly technical point of view as well as from an economic one. Despite their high

43 production potential, many research activities shown that the energy consumption within production of

44 biofuels from algae, which includes harvesting and extraction, is a limiting factor for the economics

45 balance. Sander (2010) [7] estimated that a two stages harvesting process can contribute to the 88^92% of

46 the entire energy input of the LCA and 20%^30% of the total production cost [8]. Algae downstream

47 process is strongly connected with the harvesting phase. Harvesting aims at separating these small cells

48 (1-50 ^m), at low density (0.5-3 gr l-1) from the medium. There is not yet a unique commercial solution

49 for algae harvesting, as each algae strain, downstream process and product destination can set different

50 technical specifications for this phase. Shape of algal cells, cell wall structure and oil composition vary

51 from one algal strain to another, even two different cultures of the same strain are not similar in nature.

52 Several harvesting strategies like centrifugation, sedimentation, flocculation, flotation, electrophoresis

53 and micro-filtration, and any combination of these can have been proposed to harvest microalgae. The

54 harvesting solution has thus to be coupled with the downstream process.

55 The downstream processes can be divided in two main pathways:

56 • the extraction of the lipid and/or carbohydrates and high value compounds;

57 • process the whole algae stream obtaining a bioliquid or an intermediate towards biofuels.

58 Carbohydrates are interesting for ethanol production, but currently the lipid production for biodiesel

59 has shown higher performance: according to Rodolfi et al. [9] and Studt et al. [10], the potential oil yield

60 of microalgae cultures is 5 to 20 times higher that of palm oil (ton ha-1 yr-1).

61 Specific cultivation techniques, such as starvation, can improve the oil quantity and quality toward

62 downstream transesterification to biodiesel. Removing nutrients such as nitrogen from the growth

63 medium, slows down the cell division and induces a "stress" behavior in which cell size increases and

64 neutral lipid as observed in Chlorella vulgaris [11] and Nannochloropsis. Bondioli [12] showed that

65 Nannochloropsis sp. F&M-M24 has a large potential as a renewable biofuel feedstock for: algae

66 accumulated neutral lipids up to 50% of the dry biomass, with triglycerides representing the most

67 abundant component (C16-C18), producing an oil that, with the exception of a high PUFA content,

68 fulfills biodiesel feedstock chemical requirements (results of the Italian MAMBO project). The lipids

69 contained in microalgae are intracellular, this makes the oil extraction usually more complex than the

70 extraction from terrestrial crops, such as sunflower or olive: for instance, the mechanical pressing is

71 usually not applicable to microalgae [13]. After harvesting, the biomass paste can still contains more that

72 80% (on wet basis) of moisture and this is a key factor for the definition of the downstream extraction

73 methods. Several oily fruits have similar characteristic and so wet extraction can be taken into account, in

74 order to save the biomass drying stage [14], [15].

75 Dry extraction routes are today technologically more mature and they allow for saving residues,

76 usually of high interest for the general economical balance of the plant. Chemical solvent extraction is the

77 most common method used to extract lipids from oily seeds. For algae feedstock, the real efficiency of the

78 solvent extraction is strongly related to algae strain [16].

79 Wet extraction has the big advantage of avoiding the drying. In wet pathways, cell disruption can be

80 based on mechanical approaches (microwave, ultrasonication, high pressure stresses, etc.), biological

81 approaches (use of enzyme for cell disruption, etc.) or thermochemical (Hydro Thermal Liquefaction).

82 Biological methods are based on cell degradation by means of enzymes. Although there are other

83 biological methods such as autolysis, most investigations of biological cell disruption utilize enzymes.

84 The advantages of enzymatic route are the mild reaction conditions and the high selectivity. The cell

85 envelope of microalgae, such as Chlorella, has very resistant layers, but these can be degraded by a

86 mixture of enzymes [17]. Compared with mechanical methods, the enzymatic methods exhibited very

87 competitive results [18]. The critical downfall of this method is the high cost of the enzymes.

88 Once the oil is extracted from the cells, the most common ways to produce biofuel is the

89 transesterification process. The transesterification process allows obtaining biodiesel that is a mixture of

90 fatty acid methyl esters (FAME).

91 The wide use of traditional biodiesel has highlight many limits of this product such as the not

92 complete compatibility with existing engine, low oxidation stability, poor characteristic at low

93 temperatures, etc. To overcame this limit the hydroconversion of vegetable oil is today used. The

94 hydroprocessing of triglycerides are realized by the hydrogenation of the double bonds of the oil chains

95 and the removal of oxygen by the use of a proper catalyst. This process leads to the production of a

96 mixture of C15-C18 hydrocarbons, usually commonly called HEFA, "green diesel", "renewable diesel"

97 or "bio-hydrogenated diesel", with more similar characterizes of petro-diesel than biodiesel [19] and

98 today representing a very high quality biofuels, almost a drop-in fuel (suitable either for road transport or

99 for aviation), which production exceed 2 Mt y-1 today. The hydrogenation process allows also obtaining

100 lighter fraction within the boiling point range of jet fuel or gasoline, increasing the market potential of the

101 algae oils [20], [21]. The main critical issue of this technological pathway is the implementation at very

102 large scale and competitive costs, especially considering the low specific value of the biofuels. The

103 biorefinery concept has also to tackle the issue of the differences in the market size:

104 fuels/food/feed/chemicals/energy.

105 An alternative approach can be to process the whole algae stream. Thermochemical processes are

106 available both on dry as well as on wet phase. Nevertheless the pyrolysis and HTL are largely

107 investigated of lignocellulosic materials with different technological issues than algae biomass

108 processing.

109 2. Algae pyrolysis

110 The pyrolysis process occurs in the range of 400-600 °C in absence of oxygen. A complex Bio Crude

111 Oil can be obtained in various percentage: between 30-70%, depending on the process conditions [22].

112 Dry matter is needed to feed to the reactor and this stage can require a large amount of energy. Fast

118 119

120 121

pyrolysis allows obtaining high oil yield using a heating rate (°C min-1) and short vapors residence time. The oil obtained can be considered as intermediate for a biorefinery plant, as further treatments are required to obtain a biofuel.

In figure 1 is shown a potential schema for coupling microalgae production and pyrolysis process. The gas phase produced during the pyrolysis can be used to dry the algae paste and CO2 can be recovered for algae production. Biomass can be fractionated to obtain high added value products before pyrolysis.

Figure 1: schematic of the microalgae pyrolysis process.

123 Experiences carried out in algae pyrolysis showed the feasibility of biooil production from this

124 feedstock [23]. Slow pyrolysis process has been tested in several recent works; Grierson et al. [24] tested

125 six microalgae species with slow pyrolysis. The oil yields ranges between 24 and 43%wt and a char

126 yields ranging from 34 to 63%wt. Pan et al. [25] studied the influence of temperature and catalyst on of

127 Nannochloropsis sp in slow pyrolysis conditions. In their work the oil yield ranges from 19%wt with

128 catalyst up to 31%wt in presence of HZSM-5. Advantages of the use of catalytic was found also in terms

129 of higher heating value (32.7 MJ kg-1).

130 Fast pyrolysis has been investigated in order to maximize the algae bio crude oil yield. Miao et al.

131 [26], [27] studied the fast pyrolysis of several microalgae species: C. pro-tothecoides and M. aeruginosa,

132 obtaining an oil yield ranging from 17.5 to 23.7% wt and reaching the 57.9%wt for heterotrophic

133 Chlorella protothecoides. Elliot et al. (2013) [28] processed Chlorella protothecoides and Microcystis

134 aeruginosa at 500-600°C min-1 with a residence time of 2-3 s with an oil yield 18-24%wt. Similar results

135 were obtained for Spirulina platensis. In the work the effect of the temperature was also investigated,

136 demonstrating that for this kind of feedstock a reduction of temperature leads to an increase in oil yield:

137 the maximum oil production was 57%wt with a process temperature of 450°C.

138 3. Algae HTL (Hydrothermal liquefaction)

139 Hydrothermal liquefaction (HTL) can directly convert wet biomass into a liquid biocrude oil either

140 with or without the use of a catalyst [29]. The reaction can take place on wet biomass in water at critical

141 conditions: process temperatures and pressures of 280-370 °C, 10-25 Mpa (< 2 MPa: HTC) [28]. The

142 conversion efficiency of microalgae HTL depends on various parameters including reaction temperature,

143 retention time and the composition of feedstock. Biocrude oil production from microalgae through HTL

144 has therefore received increasing attention in recent years [30]. Distinguished from the routine algae-to-

145 biodiesel approach, which largely depends on lipid contents, HTL can convert not only lipids but also

146 other organic components such as proteins and carbohydrates [31]. The chemical properties of biocrude

147 oil are highly dependent on the feedstock composition including proteins, carbohydrates, and lipids [32].

148 Biocrude oil contains the 10-20%wt of Oxygen and Nitrogen with an energy density in the range of 30149 37 MJ kg-1 [33].

150 Hydrothermal liquefaction of microalgae appears to be a very promising technology for biofuel

151 production, but still in a very early stage of development [34] and mainly batch reactors are used for

152 testing.

| iiunniinii tu (jumvaiiuii

155 Figure 2: schematic of the microalgae HTL process.

156 The typical HTL biocrude yields resulted from many studies to be close to 50-60%wt [31] depending

157 also on the use of homogeneous and heterogeneous catalysts. Minowa et al. [35] published some first

158 reports on microalgae HTL (Botryococcus braunii and Dunaliella tertiolecta) in a batch reactor fed by

159 high concentration algae mass: 50% wt.- 78.4% wt. At 300 °C the oil yield was between 37 % wt. and

160 64%wt depending on the algae processed. More recent works presented results for microalgae, such as:

161 Chlorella vulgaris and C. pyrenoidosa, Nannochloropsis occulata, Scenedesmus dimorphus,

162 Porphyridium cruentum, Desmodesmus sp. as well as Chlorogloeopsis fritschii and Spirulina

163 cyanobacteria. These works demonstrate that a wide range of microalgae can be processed in HTL

164 reactors, obtaining a mixture of oxygenated hydrocarbons with a high mass yield [36], [32], [29], [37].

165 Zhou et al. [38] investigated HTL of microalgae, combined with wastewater treatment, demonstrating that

166 low-lipid high-protein Nannochloropsis sp. (B) and high-lipid low-protein Chlorella sp. (Y) were

167 efficiently converted to biocrude oil. The highest biocrude yields were 55.0% (B) and 82.9% (Y). The

168 hydrocarbon content in biocrude was 6.7-29.8% (B) and 4.7-17.9% (Y). Elliott et al. [28] reached in their

169 study high conversions yields even with high slurry concentrations: up to 35% wt. of dry solids. Elliott et

170 al effectively applied catalytic hydrotreating for hydrodeoxygenation, hydrodenitrogenation, and

171 hydrodesulfurization of the biocrude. An important result of this study was that catalytic hydrothermal

172 gasification was effectively applied for HTL byproduct water clean-up, in order to allow nutrients recycle

173 in algae growth ponds.

174 From the studies available in literature is clear that the continuous reactors are more interesting for an

175 industrial point of view but their use tends to reduce the allowed feeding concentration and introduce

176 many technological challenges. Jazrawy et al, 2013 [39] worked with Chlorella and Spirulina with a

177 loading factor between 1-10 %wt biomass, at 250-350°C, for 3-5 min residence time and 150-200 bar.

178 The maximum biocrude yield was 41.7 %wt. The key elements for the development of microalgae HTL

179 reactors are today mainly related to the feeding stage, especially in terms of aggregation state and load

180 concentration, temperature, residence time, use of catalysts and product separation and water recirculation

181 (figure 2).

182 4. Processes comparison

183 Comparing the two processes for treating the microalgae highlight some critical issues for the

184 pyrolysis. The drying stage, required to enter in a pyrolysis reactor, is always a critical point that is

185 particularly relevant for microalgae feedstock due to the low concentration of the algae in the cultivation

186 medium: typically ranging from 10-20% even after harvesting.

187 Pyrolysis oil shows some advantages such as the lower viscosity, comparable with the vegetable oil

188 one, but in terms of yield, Nitrogen and Oxygen content and thus heating value, the HTL appears a more

189 interesting technology.

190 The specificity of the feedstock introduces critical issues for the reactor itself. The technologies

191 designed so far are based on the experience carried out in lignocellulosic materials, while algae have

192 higher inorganic and ashes content, peculiar state of aggregation depending on the harvesting and

193 pretreatment occurred, etc. The use of salty water for algae cultivation increases the problem related to

194 corrosion and solid deposition, especially for HTL due to the more critical pressure and temperature

195 operation conditions.

196 Trying to define the technology readiness level of the processes, the pyrolysis appears to be the more

197 mature one, close to demonstration scale as the prototype scale plant are already operating. HTL of

198 microalgae is moving from the applied research stage to a small scale prototyping. The upgrading of the

199 oil is at an applied scale for both pyro and HTL oils and thus it results the most critical aspect of the

200 thermochemical downstream of the microalgae.

201 6. Conclusions

202 A large number of scientific works demonstrate that microalgae based biofuels is technically feasible

203 but economical and energetic positive balances have still to be demonstrated. In terms of technological

204 pathways, the results available indicate that pyrolysis oil quality and energy yield shows minor

205 advantages with respect to HTL and thus at the present stage HTL appears a more interesting technology.

206 Nevertheless pyrolysis appears to be today more mature, close to demonstration scale, while HTL of

207 microalgae is moving from the applied research stage to a small scale prototyping. The specificity of the

208 feedstock introduces critical issues for the reactor and actual technologies are designed on the based on

209 the experience carried out in lignocellulosic materials. Critical aspects for considering pyrolysis and HTL

210 a suitable technological downstream pathways for microalgae sector are related to the oil upgrading that,

211 at the present stage, still requires to move forward from the applied research scale.

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