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Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests
ALGAL-00130; No of Pages 6 Algal Research xxx (2014) xxx–xxx
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Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests Quang-Vu Bach a,⁎, Miguel Valcuende Sillero a, Khanh-Quang Tran a, Jorunn Skjermo b a b
Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway SINTEF Fisheries and Aquaculture, P.O. Box 4762, Sluppen, NO-7465 Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 2 December 2013 Received in revised form 22 May 2014 Accepted 31 May 2014 Available online xxxx Keywords: Fast hydrothermal liquefaction Macro-algae Seaweed Direct liquefaction High-throughput screening
a b s t r a c t Hydrothermal liquefaction of sugar kelp Laminaria saccharina, a brown macro-alga harvested in Trondheim bay (Norway), was experimentally studied by a high-throughput screening technique using sealed quartz capillary reactors. Very high heating rates were achieved by the experiment technique, which significantly intensified the liquefaction and resulted in much higher bio-oil yields. The highest bio-oil yield of 79% w (dry and ash free) was obtained from the test at the highest heating rate of 585 °C/min (temperature = 350 °C, holding time = 15 min, and kelp/water ratio = 1/10 w/w). The HHV of the bio-oil was also significantly improved, being as high as of 35.97 MJ/kg (dry and ash free). In addition, a numerical prediction and modelling supported by regression analyses showed a good agreement between the present study and the literature, with respect to the effect of heating rate on the bio-oil yield. © 2014 Published by Elsevier B.V.
1. Introduction The use of algal biomass as feedstock for bio-fuel production can offer a number of advantages compared with land-based plant biomass. Being cultivated in water, algae do not compete with arable land for food crops. During the growth, algae absorb more CO2 than terrestrial biomass due to their higher photosynthetic activity [1]. In addition, growing algae can contribute to cleaning of contaminated water [2,3]. In the group of brown macro-algae, kelps are large seaweeds, harvested worldwide as a source for production of alginate, a biopolymer widely used in the food and cosmetics industry. In Norway, approximately 150,000 tonnes of the canopy-forming kelp (Laminaria hyperborea) is harvested annually, providing 5500 tonnes of alginate [4]. The use of kelp biomass for bio-energy applications has largely focused on the production of biogas by anaerobic digestion [5]. However, seaweeds have a complex composition, and complete degradation of the material requires the presence of microorganisms with a broad substrate range. Recently, some studies on production of bio-ethanol via fermentation of the sugars present in seaweed biomass (mannitol and laminarin for brown macroalgae) have been reported [6]. Thermo-chemical conversions, including pyrolysis [7,8] and direct combustion [9] of seaweed biomass are also possible, but less economically favourable. It is due to the high moisture and high alkali metal contents of seaweeds, of which the first is greatly energy sensitive and the second is highly likely to cause problems ⁎ Corresponding author. Tel.: +47 73591645; fax: +47 73593580. E-mail address: [email protected] (Q.-V. Bach).
associated with slagging, fouling and bed agglomeration in some combustion boilers. Hydrothermal liquefaction (HTL) is a conversion method capable of processing wet biomass to produce high energy-density bio-oils. The technology involves the use of water in sub-critical region conditions as reaction medium, and therefore does not require pre-drying of the feedstock. In addition, inorganic components present in the feedstock can be dissolved in the water, making the fuel products cleaner, with respect to the ash content. Due to these special capacities of HTL and the advantages of using algal biomass as feedstock, studies in the field of HTL of algal biomass have been quite active [10–18]. However, effects of process parameters on the oil yield and fuel properties of the oil are not fully investigated. Common disputations in the literature of the field include the effects of heating rate, catalyst addition, co-solvents, initial gases and even residence times on the products distribution and their fuel properties. Zhou et al. [15], for example, reported that adding 5% (w) of Na2CO3 slightly increased the oil yield from liquefaction of macro-alga Enteromorpha prolifera. However, other studies [16, 17] reported an opposite trend that the oil yield was reduced when catalysts were added. On the other hand, there is no general agreement in effects of heating rate on the oil yield. While a number of researchers reported an increasing trend of the bio-oil yield with increasing heating rates [19–21], some believed that heating rate has insignificant effects on the yield [22]. A common feature of the past studies in HTL of algal biomass is the use of lab-scale batch autoclave reactors, of which the relatively large sizes with significant heat and mass transfer limitations would probably be the main cause of these controversial observations.
http://dx.doi.org/10.1016/j.algal.2014.05.009 2211-9264/© 2014 Published by Elsevier B.V.
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
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Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
measured by means of an Orion pH/ISE Meter 920A. The pH value was 6.45 ± 0.012 and 12.41 ± 0.027 for the solutions without and with addition of catalyst (KOH/kelp ratio = 1/20, w/w), respectively.
One way to minimize the heat and mass transfer limitations in a chemical reactor is to reduce the reactor size. Indeed, in the effort of improving the bio-oil yield obtained from HTL of micro-algal biomass, Faeth et al. [20] tried to increase the heating rate by reducing the reactor size (inner diameter) to as small as of 3/8 in. (approx. 9.5 mm). They claimed that a heating rate of 230 ± 5 °C/min has been achieved, which had unprecedentedly been explored for HTL of any biomass material. Potic et al. [23] developed an experiment technique for hydrothermal treatment of biomass using sealed quartz capillary reactors with an inner diameter as small as 1 mm. Apart from the significant improvements in heat and mass transfer, this technique offers several advantages for carrying out high-temperature and high-pressure experiments in a cheaper, safer and less time-consuming manner. The quartz material is inert to the reaction media and reactants, eliminating the risk of chemical reactions to be catalysed by the reactor wall as in the case of conventional reactors made of alloy [24]. Nickel alloy reactors, for example, may cause unwanted catalytic effects during experiment [25]. In addition, since the quartz capillary tubes are transparent, visual observations of the reacting mixtures during experiment were possible, giving additional qualitative information of the process. More interestingly, due to the small size, multiple reactors can be tested at the same time, which is commonly referred to as high-throughput experiments. In this study, HTL of kelp biomass, a typical Norwegian macro-alga species was experimentally investigated by the high-throughput experimental technique, using sealed quartz capillary reactors. A special experimental setup capable of achieving very high heating rates was developed. Effects of process parameters including heating rate, holding time, and catalyst addition on the products distribution and fuel properties of bio-oil were examined. The main objective of the study was to confirm the positive effect of fast heating on the bio-oil yield, and to maximize the yield by increasing the heating rate. Therefore, comprehensive chemical and fuel characterizations of the liquid and solid products obtained from this work will be reported in a separate follow-up paper.
2.2. Reactors and preparation The HTL experiments were carried out by using quartz capillary reactors with a length of 200 mm, and an inner diameter of 2 mm. Two different wall thicknesses of 1 and 2 mm (hereafter called 1 mm-wall and 2 mm-wall reactor, respectively) were employed to support the differentiation of the actual heating rate across the reactor wall. The thicker wall requires a longer time for heat transfer and therefore reduces the actual heating rate, compared to the thinner. The quartz capillary tubes were first cleaned with distilled water, then dried and sealed at one end by an acetylene torch. Pre-prepared feed solutions (with or without catalyst) were then loaded into the reactors. Each loaded reactor was purged with nitrogen (99.99%) for 30 s and then quickly sealed at the other end. This operation was carefully optimized in order to minimize the contamination risk, considering that there might have been a chance of contaminants (combustion products from the acetylene flame) entering the capillary. 2.3. Experimental setup and procedure The HTL experimental setup consisted of a muffle furnace (Thermoconcept KC 64/13), a cylindrical sand bath made of stainless steel (of 25 cm high, and 20 cm in diameter), and two thermocouples connected with a computer via a data logger (NI 9211) for monitoring the temperature of the sand bath and/or inside the reactor. Olivine sand of an industrial type, supplied by North Cape Mineral AS, Norway was used, as received, for this study. The height of the sand column filled in the sand bath was 22 cm, making it possible to fully cover the quartz reactors of 20 cm long. The sand bath was pre-heated to a pre-set temperature and held at this temperature for 2 h to allow a steady state to be established throughout the bath. At the steady state of the furnace and the sand bath, loaded and sealed reactors were quickly placed into the sand bath (for achieving higher heating rates) or in the muffle furnace (for setting lower heating rates). After a given reaction (holding) time, the reactors were quickly taken out from the setup and immerged in a water bath at room temperature to quench further reactions. The reactors were then opened by cutting one end for products collection. Three repetitions were performed for each of the tests, from which the average values were calculated and reported as the experimental results.
2. Experimental 2.1. Feedstock characterization and preparation The feedstock used for this study was sugar kelp (Laminaria saccharina), a brown macro-alga, which was harvested in June 2011 in Trondheim, Norway. The collected sample of kelp was frozen and stored in plastic bags at −20 °C within 8 h after harvesting. The fuel characteristics including proximate, ultimate analyses and heating value (calculated from Channiwala and Parikh [26]) of the kelp biomass are given in Table 1. The frozen kelp sample was defrozen and washed with plenty of water to remove the sea salts before drying, grinding, and sieving. The grinding was carried out by using an IKA MF 10 cutting mill (from IKA®-Werke GmbH & Co. KG) with a closed bottom. Powder collected in this step was sieved in a vibrating sieving machine (Fritsch Analysette 3 Pro) to collect particles smaller than 90 μm for the HTL experiments. This selection aimed to maximize the heat and mass transfers within a single reacting particle. The feedstock in powder form was dried at 105 °C for 24 h before being balanced and mixed with distilled water to produce slurry solutions suitable for injecting into the capillary reactors. The pH of the slurry solutions (kelp/water ratio = 1/10, w/w) was
2.4. Products collection and separation Due to practical limitations and considerations, the gaseous products were not collected in this study, but calculated by mass balancing. Once a reactor was opened, a long stainless steel needle (Sigma-Aldrich Z261297) was used to inject a known amount of a dichloromethane (CH2Cl2) and water mixture into the reactor. The ratio of CH2Cl2:H2O varied from 3:1 to 5:1 (in volume) depending on the ratio of bio-oil and WSP. This procedure and operation allowed collecting the products (liquids and solids) with a minimized wall loss. The collected products mixture was then separated by means of an MSE Minor Centrifuge operated at 5000 rpm for 3 min. Three layers of products were observed
Table 1 Proximate and ultimate analyses for the feedstock (dry and ash free). Feedstock
Proximate analysis a
Sugar kelp (Laminaria saccharina)
HHVb
Ultimate analysis a
a
a
a
a
a
a
Ash
VM
Fixed C
C
H
O
N
S
16.58
69.32
14.10
39.44
5.14
52.03
2.99
0.60
14.46
VM: volatile matter, HHV: higher heating value. a w %; b MJ/kg
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
Y bio−oil ð daf; %Þ ¼
mbio−oil 100% mkelp ð100−Ash%Þ
400 350
Temperature (°C)
after centrifugation. They included a hydrophilic or aqueous phase (containing water soluble products) on the top, a hydrophobic phase (bio-oil dissolved in CH2Cl2) in the middle, and a solid phase at the bottom. Using an adjustable-volume pipette (Finnpipette™, Thermo Scientific Inc.), the top layer of aqueous products was extracted first. It was then followed by the middle layer of bio-oil, leaving the solid at the bottom. The operation of this procedure was repeated twice in order to minimize the liquid products, which otherwise may be trapped in the solid. Finally, the collected products were dried under a reduced pressure until reaching a constant weight to obtain the mass of water soluble products (WSP), bio-oil, and solid residues (SR), respectively. The product yields (Y) were calculated adopting Eqs. (1)–(4) employed by Anastasakis and Ross [16], on a dry basis:
3
300 250 200 150 100
High HR (585°C/min) Mid HR (321°C/min) Low HR (146°C/min)
50 0
ð1Þ
0
20
40
60
80
100
120
140
160
Time (s)
Y SR
mWSP 100% mkelp þ mKOH
msolid ð%Þ ¼ 100% mkelp þ mKOH
Y gaseous product ð%Þ ¼
mkelp −mbio−oil −mWSP −msolid 100% mkelp þ mKOH
ð2Þ
ð3Þ
ð4Þ
3. Results 3.1. Thermal behaviour of the quartz capillary reactors As discussed earlier in the introduction part, a key aspect of this present study is to maximize the yield of bio-oil from HTL of kelp biomass by maximizing the rate of heating. Therefore, before investigating the effect of heating rate on the bio-oil yield, it is important to understand the thermal behaviour of the reaction system with regards to heating rate and how fast the rate of heating can be achieved for the experimental setup. In order to measure and determine the heating rate (HR) it is required to know the temperature outside and inside the reactor. It is however difficult to measure the temperature inside the quartz capillary reactors, considering that the reactors are small in size and sealed when operated. Nevertheless, the small size of the reactors allows reasonable assumptions that there is no temperature gradient across the volume inside a single piece of the reactors, and that differences in internal heat transfer between the reactors filled with either water or air are neglectable. With these assumptions, it was possible to measure the temperature inside the quartz capillary reactors, using a thermocouple (1 mm in diameter) placed inside the reactors (inner diameter = 2 mm) sealed on one end and filled with air. The HR was then calculated according to Eq. (5), for which the heating time is defined as time needed to raise the reactor from starting (room) temperature (Tstart) to a preset liquefaction temperature (Tliq). T start −T liq heating time
figure. The high (or highest) heating rate of 585 °C/min approximately was achieved for the 1 mm-wall reactor heated by the pre-heated sand. The low (or lowest) heating rate of 146 °C/min approximately was recorded for the 2 mm-wall reactor heated by the pre-heated air. The 2 mm-wall reactor heated by the pre-heated sand gave the mid (or medium) heating rate in between, being 321 °C/min approximately. 3.2. Effects of heating rate on bio-oil yield
where mkelp, mbio − oil, mWSP, mSR, and mKOH are the mass of fed kelp, bio-oil, WSP, solid residue and fed catalyst, respectively.
HRðo C= minÞ ¼
Fig. 1. Profiles of temperature inside quartz reactors heated by pre-heated sand or air.
For this investigation, the optimum conditions reported by Anastasakis and Ross [16] for the HTL of a similar feedstock, brown macro-alga L. saccharina from Barnacarry Bay in Scotland, were adopted. The conditions were: liquefaction temperature = 350 °C; holding time = 15 min; and biomass/water ratio = 1/10 (w/w). The only varied parameter was the heating rate, which was controlled to be 146 °C/min, 321 °C/min, or 585 °C/min. Results from this investigation are presented in Fig. 2, where the yields of by-products including SR, WSP, and calculated gases are also plotted versus heating rate. It appears that the effects of heating rate on the products yield were very strong. The higher the heating rates, the higher the bio-oil yield was achieved. As can be seen in Fig. 2, the yield of bio-oil obtained from the HTL of kelp biomass increased from 53% to 65% and 79% when the heating rate was increased from 136 °C/min to 321 °C/min and 585 °C/min. The WSP yield exhibited a variation trend with the highest value found for the medium heating rate of 321 °C/min. In addition, the WSP yield at the highest heating rate of 585 °C/min was found to be 100
80
Yield (%)
Y WSP ð%Þ ¼
60
Bio-oil Solid residues WSP Gases
40
20
ð5Þ 0
Fig. 1 presents the curves of inside-reactor temperature plotted against time. The temperature was measured by using one-end-open reactors placed in different heating media (sand or air) preheated to 350 °C, as discussed earlier in Section 2.3. From the numerical data of the curves, the heating rates were calculated and also shown in the
100
200
300
400
500
600
Heating rate (°C/min) Fig. 2. The effect of heating rate on the products yield. (Tliq = 350 °C; holding time = 15 min; and biomass/water ratio = 1/10, w/w).
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
even lower than that of the lowest heating rate (146 °C/min). This makes it hard to draw a firm confirmation on the trend at this stage. More importantly, the overall conversion efficiency of kelp biomass in the conditions of the highest heating rate = 585 °C/min; Tliq = 350 °C; holding time = 15 min; and biomass/water ratio = 1/10 (w/w) was 85% approximately. 3.3. Effects of residence time on product yield The highest heating rate (585 °C/min) was employed for this investigation in the common conditions of 350 °C as the liquefaction temperature, and 1:10 (w/w) as the kelp/water ratio. The residence (holding) time, counted from the time the liquefaction temperature reached 350 °C to the time the reactors started being quenched, was varied within 5, 10, 15 and 20 min. The results from this investigation are shown in Fig. 3. It is interesting to see that the highest bio-oil yield is associated with the retention time of 15 min, which is in good agreement with the literature [16]. Either longer or shorter residence time led to reduced bio-oil yields. The bio-oil yield was reduced from 79% to 75% when the holding time was increased further from 15 to 20 min. Similar observations were also reported in the literature [15–17,20,22,27]. The reductions in bio-oil yield, associated with further increasing retention time, were attributed to the formation of secondary char from bio-oil. This attribution is confirmed by the present investigation, as can be seen in Fig. 3. On the other hand, the WSP yield decreased consistently with increasing retention time, but the trend for gases yield was not pronounced and seemed unchanged. 3.4. Effects of catalyst addition on the yield and fuel properties of bio-oil Effects of catalyst (KOH) addition (KOH/kelp = 1/20 w/w, dry mass) on the yield and fuel properties of the bio-oil product were investigated in the HTL conditions of Tliq = 350 °C, holding time = 15 min, heating rate = 321 °C/min, and biomass/water ratio = 1/10 (w/w). The results from this investigation are presented in Fig. 4 and Table 2, in which some data from the early investigation of the heating rate effects on bio-oil yield are also included for comparison. It can be seen from the data in Fig. 4 that the catalyst addition had a slightly positive effect on bio-oil yield, though an increase by 2% (from 65% to 67%) is hardly remarkable. It is also observed that the effect of the higher heating rate on bio-oil yield was much stronger than that of the catalyst addition. Starting from the bio-oil yield of 65% (mid-HR, no catalyst) the higher heating rate (high HR, no catalyst) resulted in a much higher bio-oil yield (79%) compared to 67% obtained by the catalyst addition (mid-HR, with catalyst). In addition, the catalyst addition seemed to favour the formation of WSP portion and reduced the gas 100
Yield (%)
80
Bio-oil
60
Solid residues WSP
40
Gases
80
Mid HR, no catalyst Mid HR, with catalyst
60
Yield (%)
4
High HR, no catalyst
40
20
0
Bio-oil
Solid residues
WSP
Gases
Fig. 4. Products distribution for HTL of kelp with and without catalyst addition in the common condition of Tliq = 350 °C, holding time = 15 min, and biomass/water ratio = 1/10 (w/w).
yield, whereas the yields of these by-products were both decreased with the increasing heating rates. The ultimate analysis of bio-oils obtained from this study is presented in Table 2. It shows that, in all cases, with or without catalyst addition, the bio-oils obtained from HTL of kelp had much higher carbon and hydrogen contents, but lower oxygen content compared with the raw kelp (Table 1). Consequently, the higher heating value of bio-oil was significantly improved, being as high as 34–36 MJ/kg. It also indicates that the fuel properties of the bio-oil products were improved more significantly by the higher heating rates than by the catalyst addition. These observations are somehow not in good agreement with past studies by Zhou et al. [15] and Anastasakis and Ross [16], which reported different effects of catalyst addition. However, it should be reminded that both of the studies were carried at much lower heating rates (25 °C/min), compared with this present study, using reactors with relatively large volumes and thus significant mass and heat transfer limitations. 4. Discussion It has been demonstrated that the use of quartz capillary reactors for HTL of seaweed biomass exhibited a number of advantages, including the possibilities to carry out multiple experiments at the same time. With the experimental setup, the technique was capable of achieving very high heating rates compared to the conventional reaction methods. Some problems were also found during the experiments. Reactor explosion was a critical issue quite sometimes due to various reasons [23]. Once the reactor exploded, the sample was lost and the experiment was repeated all over again from the beginning. Other problems were related to the operation of product collection and the mass balance determination. Because the reactor size was very small in volume, the collection of gaseous products was very difficult and their yields were normally calculated by difference in mass balance. Moreover, it was not possible to control and measure pressure inside the reactors, which can only be estimated from the reaction temperature by using steam tables (165.3 bar at 350 °C [28]). It is just because the reactors
20 Table 2 Ultimate analysis of bio-oils obtained from HTL of seaweed in different conditions.
0 0
5
10
15
20
25
Holding time (°C/min) Fig. 3. The effect of holding time on the product yield. (Tliq = 350 °C; heating rate = 585 °C/min; and biomass/water ratio = 1/10, w/w).
High HR, no catalyst Mid HR, no catalyst Mid HR, with catalyst
Ca
Ha
Na
Sa
Oa
HHVb
75.54 73.23 73.46
9.16 8.88 8.48
3.65 4.27 3.89
0.62 0.56 0.61
11.66 13.62 14.17
35.97 34.61 34.18
HHV: higher heating value. a w %; b MJ/kg
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
Y bio‐oil ð%Þ ¼ ½α ln ðHRÞ þ β 100%
ð6Þ
where α and β are independent variables to be determined from the experimental data. Other terms have been defined earlier, of which bio-oil yield, Ybio-oil, is the dependent variable of the function. From a nonlinear regression analysis (confidence interval = 0.95) using our experimental data, the independent variables α and β were determined, which allowed generating the model presented in Eq. (7) for predicting the bio-oil yield at a given heating rate. Y bio‐oil ð%Þ ¼ ½0:1859 ln ðHRÞ−0:3839 100%
ð7Þ
Applying Eq. (7), the predicted bio-oil yield curve was drawn and presented in Fig. 5, which demonstrates a very good agreement among the two studies. Indeed, the calculated curve does not only represent nicely the experimental data in this study but also fits the data reported by Anastasakis and Ross [16]. In addition, a one-sample t-test [33] was performed for every heating rate (with three replicated measurements n = 3) to elucidate
80
Bio-oil yield (%)
were sealed. The actual pressure might be different due to gases (mainly CO and CO2) produced from the process. It was estimated to be within 171.4–174.4 bar, assuming 10–15% w [16] for the gas yield, 50:50 (v/v) for the CO:CO2 ratio, and that the gases behaved ideally. Regarding the method of heating rate determination, it may be argued that the inside-reactor temperatures measured by the experiments employing one-end-open quartz capillary reactors filled with air may not represent the actual temperatures. However, the method did offer a good representation of the actual heating rates under the experimental conditions. It is because, given the assumptions discussed earlier in Section 3.1, the heating rate is a function of the thermal conductivity of the reactor wall (quartz material), the wall thickness, and the temperature gradient (between the outside and inside surfaces of the reactor wall), but not the temperatures. Since temperature at both surfaces was measured by means of two similar thermocouples placed in air on the surfaces, possible differences between the measured and the actual values of these two temperatures would have cancelled one another within the acceptable experimental variations. More importantly, it has been confirmed by this present study that higher heating rates during HTL of biomass enhanced significantly the bio-oil yield. Although the chemistry behind this effect is not yet fully understood [20], possible explanations include that rapid heating: (1) accelerates the degradation reactions of biomass materials [20,29]; (2) reduces unwanted side reactions and promotes reactions forming more bio-oil [21,30]; (3) accelerates the breaking of biomass cell and thus eases the releasing of cell contents into the hydrothermal media for subsequent reactions [20]; (4) minimizes the char formation [31] and/or the re-polymerization [32]. As presented earlier in Section 3.2, the optimum conditions (Tliq = 350 °C, holding time = 15 min, biomass/water ratio = 1/10, w/w) reported by Anastasakis and Ross [16] for the HTL of a similar feedstock, brown macro-alga L. saccharina from Barnacarry Bay in Scotland, was adopted for our present study. In the Scottish study, the influence of reactor loading, residence time, temperature and catalyst (KOH) addition was assessed. However, an assessment for the influence of heating rate was missing. Therefore, this present study can be considered as a complementary to the Scottish report. It is observed that, in similar conditions except for the heating rate, the highest bio-oil yield reported in the Scottish report was 19%, which is much lower than that (79%) from this present study. The main cause of this difference is the heating rate difference and therefore it would be interesting to include the data obtained from the two studies for a regression analysis. For this purpose, the method reported by Zhang et al. [21] was adopted, which demonstrated that the bio-oil yield from biomass liquefaction can be calculated and predicted by using a logarithm function of the heating rate with the general form in Eq. (6)
5
60
40
20
Experimental data - present work Experimental data from literature [Ref. 16] ---- Predicted curve
0 0
100
200
300
400
500
600
700
Heating rate (°C/min) Fig. 5. Modelling and prediction of bio-oil yield as a function of heating rate.
the confidence level of the experimental data, assuming that the predicted yields are the specified (true) values. The results of the t-tests for the three heating rates are presented in Table 3. It can be seen that all the tcal (calculated t) values are smaller than the tcri (critical t) value (tcri = 2.92 for a sample with a degree of freedom df = n - 1 = 2, and a cumulative probability of 95% or α = 0.05 [34]). Therefore, the null hypothesis cannot be rejected at any of the heating rates tested. It means that there was no significant difference between the experimental data and the true bio-oil yields. Finally, it is reasonable at this stage to believe that it is possible to improve further the bio-oil yield by employing heating rates higher than 585 °C/min. Such higher heating rates can be achieved in continuous flow reaction systems equipped with static mixing mechanisms, which are also suitable for upscaling, keeping in mind that the solid products or residues may cause problems. 5. Conclusion The high-throughput technique using sealed quartz capillary reactors has been applied and tested for the HTL of macro-alga (L. saccharina), aiming at maximizing the bio-oil yield and the overall conversion efficiency by increasing the heating rate. With this technique, it was possible to achieve heating rates as high as 585 ± 2 °C/min. The effects of heating rate and catalyst addition on the yield and fuel properties of bio-oils obtained from the process have been investigated and compared. It appears that the effects of heating rate are similar to, but much stronger than those of the catalyst addition. Very high bio-oil yields can be obtained from non-catalytic HTL of seaweed. In the conditions of heating rate = 585 °C/min, Tliq = 350 °C, holding time = 15 min, and kelp/water ratio = 1/10 (w/w), the oil yield was 79% w. The HHV of the oil was also significantly improved, from 14.11 MJ/kg to 35.97 MJ/kg (dry and ash free). Higher bio-oil yields and higher overall conversion efficiencies of the process would be possible to achieve by even faster heating rates. Further studies employing Table 3 T-test of null hypothesis assuming the predicted oil yields as the true values. Heating rate, °C/min
x
s
μ
tcal
Significance
585 226 146
0.79 0.65 0.53
0.048 0.043 0.044
0.80 0.62 0.54
0.27 0.97 0.64
No No No
x and s is the mean and standard deviation, respectively, of the experimental data. μ is the specified (true) value. tcal and tcri are the calculated and critical t, respectively. tcri (2) = 2.92 at α = 0.05 [34].
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
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continuous flow reaction systems equipped with static mixing mechanisms to achieve higher heating rates are recommended. Acknowledgement The financial support from the Norwegian Research Council through CenBio and STOP project is gratefully acknowledged. References [1] M. Aresta, A. Dibenedetto, G. Barberio, Utilization of macro-algae for enhanced CO2 fixation and biofuels production: development of a computing software for an LCA study, Fuel Process. Technol. 86 (14–15) (2005) 1679–1693. [2] M.F. Demirbas, Biofuels from algae for sustainable development, Appl. Energy 88 (10) (2011) 3473–3480. [3] O. Raize, Y. Argaman, S. Yannai, Mechanisms of biosorption of different heavy metals by brown marine macroalgae, Biotechnol. Bioeng. 87 (4) (2004) 451–458. [4] S.-H. Lorentsen, K. Sjøtun, D. Grémillet, Multi-trophic consequences of kelp harvest, Biol. Conserv. 143 (9) (2010) 2054–2062. [5] A. Hughes, et al., Biogas from macroalgae: is it time to revisit the idea? Biotechnol. Biofuels 5 (1) (2012) 1–7. [6] M.G. Borines, R.L. de Leon, J.L. Cuello, Bioethanol production from the macroalgae Sargassum spp. Bioresour. Technol. 138 (2013) 22–29. [7] S. Wang, et al., Research on pyrolysis characteristics of seaweed, Energy Fuel 21 (6) (2007) 3723–3729. [8] K. Anastasakis, A.B. Ross, J.M. Jones, Pyrolysis behaviour of the main carbohydrates of brown macro-algae, Fuel 90 (2) (2011) 598–607. [9] S. Wang, et al., Combustion characteristics of seaweed biomass. 1. Combustion characteristics of Enteromorpha clathrata and Sargassum natans, Energy Fuel 23 (10) (2009) 5173–5178. [10] P. Duan, P.E. Savage, Hydrothermal liquefaction of a microalga with heterogeneous catalysts, Ind. Eng. Chem. Res. 50 (1) (2010) 52–61. [11] P.J. Valdez, et al., Hydrothermal liquefaction of Nannochloropsis sp.: systematic study of process variables and analysis of the product fractions, Biomass Bioenergy 46 (2012) 317–331. [12] Jazrawi, C., et al., Pilot plant testing of continuous hydrothermal liquefaction of microalgae. Algal Res., (0). [13] L. Garcia Alba, et al., Hydrothermal treatment (HTT) of microalgae: evaluation of the process as conversion method in an algae biorefinery concept, Energy Fuel 26 (1) (2011) 642–657. [14] López Barreiro, D., et al., Hydrothermal liquefaction (HTL) of microalgae for biofuel production: State of the art review and future prospects. Biomass Bioenergy, (0).
[15] D. Zhou, et al., Hydrothermal liquefaction of macroalgae Enteromorpha prolifera to bio-oil, Energy Fuel 24 (7) (2010) 4054–4061. [16] K. Anastasakis, A.B. Ross, Hydrothermal liquefaction of the brown macro-alga Laminaria saccharina: effect of reaction conditions on product distribution and composition, Bioresour. Technol. 102 (7) (2011) 4876–4883. [17] D. Li, et al., Preparation and characteristics of bio-oil from the marine brown alga Sargassum patens C. Agardh, Bioresour. Technol. 104 (2012) 737–742. [18] B. Patel, K. Hellgardt, Hydrothermal upgrading of algae paste: application of 31P-NMR, Environ. Prog. Sust. Energ. 32 (4) (2013) 1002–1012. [19] D. Knežević, et al., High-throughput screening technique for conversion in hot compressed water: quantification and characterization of liquid and solid products, Ind. Eng. Chem. Res. 46 (6) (2007) 1810–1817. [20] J.L. Faeth, P.J. Valdez, P.E. Savage, Fast hydrothermal liquefaction of Nannochloropsis sp. to produce biocrude, Energy Fuel 27 (3) (2013) 1391–1398. [21] B. Zhang, M. Keitz, K. Valentas, Thermal effects on hydrothermal biomass liquefaction, in: W. Adney, et al., (Eds.), Biotechnology for Fuels and Chemicals, Humana Press, 2008, pp. 511–518. [22] J. Akhtar, N.A.S. Amin, A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass, Renew. Sust. Energ. Rev. 15 (3) (2011) 1615–1624. [23] B. Potic, et al., A high-throughput screening technique for conversion in hot compressed water, Ind. Eng. Chem. Res. 43 (16) (2004) 4580–4584. [24] P.R. Christensen, et al., Effects of heterogeneous catalyst in hydrothermal liquefaction of dried distillers grains with solubles, Fuel 123 (2014) 158–166. [25] T. Minowa, S. Inoue, Hydrogen production from biomass by catalytic gasification in hot compressed water, Renew. Energy 16 (1–4) (1999) 1114–1117. [26] S.A. Channiwala, P.P. Parikh, A unified correlation for estimating HHV of solid, liquid and gaseous fuels, Fuel 81 (8) (2002) 1051–1063. [27] M.K. Akalın, K. Tekin, S. Karagöz, Hydrothermal liquefaction of cornelian cherry stones for bio-oil production, Bioresour. Technol. 110 (2012) 682–687. [28] Y.A. Cengel, M.A. Boles, Thermodynamics: An Engineering Approach, 7th edition McGraw-Hill, 2011. [29] M. Watanabe, et al., Glucose reactions within the heating period and the effect of heating rate on the reactions in hot compressed water, Carbohydr. Res. 340 (12) (2005) 1931–1939. [30] B. Zhang, M. von Keitz, K. Valentas, Thermochemical liquefaction of high-diversity grassland perennials, J. Anal. Appl. Pyrolysis 84 (1) (2009) 18–24. [31] Z. Fang, et al., Liquefaction and gasification of cellulose with Na2CO3 and Ni in subcritical water at 350 °C, Ind. Eng. Chem. Res. 43 (10) (2004) 2454–2463. [32] Y. Zhang, Hydrothermal liquefaction to convert biomass into crude oil, Biofuels from Agricultural Wastes and Byproducts, Wiley-Blackwell, 2010, pp. 201–232. [33] P.D. Jacobs, et al., A novel liquid fluidized bed microreactor for coal liquefaction studies. 2. Hydrotreating results, Energy Fuel 6 (3) (1992) 249–254. [34] W. Navidi, Statistics for Engineers and Scientists, 3rd ed. McGraw-Hill, 2011.
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
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Algal Research journal homepage: www.elsevier.com/locate/algal
Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests Quang-Vu Bach a,⁎, Miguel Valcuende Sillero a, Khanh-Quang Tran a, Jorunn Skjermo b a b
Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway SINTEF Fisheries and Aquaculture, P.O. Box 4762, Sluppen, NO-7465 Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 2 December 2013 Received in revised form 22 May 2014 Accepted 31 May 2014 Available online xxxx Keywords: Fast hydrothermal liquefaction Macro-algae Seaweed Direct liquefaction High-throughput screening
a b s t r a c t Hydrothermal liquefaction of sugar kelp Laminaria saccharina, a brown macro-alga harvested in Trondheim bay (Norway), was experimentally studied by a high-throughput screening technique using sealed quartz capillary reactors. Very high heating rates were achieved by the experiment technique, which significantly intensified the liquefaction and resulted in much higher bio-oil yields. The highest bio-oil yield of 79% w (dry and ash free) was obtained from the test at the highest heating rate of 585 °C/min (temperature = 350 °C, holding time = 15 min, and kelp/water ratio = 1/10 w/w). The HHV of the bio-oil was also significantly improved, being as high as of 35.97 MJ/kg (dry and ash free). In addition, a numerical prediction and modelling supported by regression analyses showed a good agreement between the present study and the literature, with respect to the effect of heating rate on the bio-oil yield. © 2014 Published by Elsevier B.V.
1. Introduction The use of algal biomass as feedstock for bio-fuel production can offer a number of advantages compared with land-based plant biomass. Being cultivated in water, algae do not compete with arable land for food crops. During the growth, algae absorb more CO2 than terrestrial biomass due to their higher photosynthetic activity [1]. In addition, growing algae can contribute to cleaning of contaminated water [2,3]. In the group of brown macro-algae, kelps are large seaweeds, harvested worldwide as a source for production of alginate, a biopolymer widely used in the food and cosmetics industry. In Norway, approximately 150,000 tonnes of the canopy-forming kelp (Laminaria hyperborea) is harvested annually, providing 5500 tonnes of alginate [4]. The use of kelp biomass for bio-energy applications has largely focused on the production of biogas by anaerobic digestion [5]. However, seaweeds have a complex composition, and complete degradation of the material requires the presence of microorganisms with a broad substrate range. Recently, some studies on production of bio-ethanol via fermentation of the sugars present in seaweed biomass (mannitol and laminarin for brown macroalgae) have been reported [6]. Thermo-chemical conversions, including pyrolysis [7,8] and direct combustion [9] of seaweed biomass are also possible, but less economically favourable. It is due to the high moisture and high alkali metal contents of seaweeds, of which the first is greatly energy sensitive and the second is highly likely to cause problems ⁎ Corresponding author. Tel.: +47 73591645; fax: +47 73593580. E-mail address: [email protected] (Q.-V. Bach).
associated with slagging, fouling and bed agglomeration in some combustion boilers. Hydrothermal liquefaction (HTL) is a conversion method capable of processing wet biomass to produce high energy-density bio-oils. The technology involves the use of water in sub-critical region conditions as reaction medium, and therefore does not require pre-drying of the feedstock. In addition, inorganic components present in the feedstock can be dissolved in the water, making the fuel products cleaner, with respect to the ash content. Due to these special capacities of HTL and the advantages of using algal biomass as feedstock, studies in the field of HTL of algal biomass have been quite active [10–18]. However, effects of process parameters on the oil yield and fuel properties of the oil are not fully investigated. Common disputations in the literature of the field include the effects of heating rate, catalyst addition, co-solvents, initial gases and even residence times on the products distribution and their fuel properties. Zhou et al. [15], for example, reported that adding 5% (w) of Na2CO3 slightly increased the oil yield from liquefaction of macro-alga Enteromorpha prolifera. However, other studies [16, 17] reported an opposite trend that the oil yield was reduced when catalysts were added. On the other hand, there is no general agreement in effects of heating rate on the oil yield. While a number of researchers reported an increasing trend of the bio-oil yield with increasing heating rates [19–21], some believed that heating rate has insignificant effects on the yield [22]. A common feature of the past studies in HTL of algal biomass is the use of lab-scale batch autoclave reactors, of which the relatively large sizes with significant heat and mass transfer limitations would probably be the main cause of these controversial observations.
http://dx.doi.org/10.1016/j.algal.2014.05.009 2211-9264/© 2014 Published by Elsevier B.V.
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
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Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
measured by means of an Orion pH/ISE Meter 920A. The pH value was 6.45 ± 0.012 and 12.41 ± 0.027 for the solutions without and with addition of catalyst (KOH/kelp ratio = 1/20, w/w), respectively.
One way to minimize the heat and mass transfer limitations in a chemical reactor is to reduce the reactor size. Indeed, in the effort of improving the bio-oil yield obtained from HTL of micro-algal biomass, Faeth et al. [20] tried to increase the heating rate by reducing the reactor size (inner diameter) to as small as of 3/8 in. (approx. 9.5 mm). They claimed that a heating rate of 230 ± 5 °C/min has been achieved, which had unprecedentedly been explored for HTL of any biomass material. Potic et al. [23] developed an experiment technique for hydrothermal treatment of biomass using sealed quartz capillary reactors with an inner diameter as small as 1 mm. Apart from the significant improvements in heat and mass transfer, this technique offers several advantages for carrying out high-temperature and high-pressure experiments in a cheaper, safer and less time-consuming manner. The quartz material is inert to the reaction media and reactants, eliminating the risk of chemical reactions to be catalysed by the reactor wall as in the case of conventional reactors made of alloy [24]. Nickel alloy reactors, for example, may cause unwanted catalytic effects during experiment [25]. In addition, since the quartz capillary tubes are transparent, visual observations of the reacting mixtures during experiment were possible, giving additional qualitative information of the process. More interestingly, due to the small size, multiple reactors can be tested at the same time, which is commonly referred to as high-throughput experiments. In this study, HTL of kelp biomass, a typical Norwegian macro-alga species was experimentally investigated by the high-throughput experimental technique, using sealed quartz capillary reactors. A special experimental setup capable of achieving very high heating rates was developed. Effects of process parameters including heating rate, holding time, and catalyst addition on the products distribution and fuel properties of bio-oil were examined. The main objective of the study was to confirm the positive effect of fast heating on the bio-oil yield, and to maximize the yield by increasing the heating rate. Therefore, comprehensive chemical and fuel characterizations of the liquid and solid products obtained from this work will be reported in a separate follow-up paper.
2.2. Reactors and preparation The HTL experiments were carried out by using quartz capillary reactors with a length of 200 mm, and an inner diameter of 2 mm. Two different wall thicknesses of 1 and 2 mm (hereafter called 1 mm-wall and 2 mm-wall reactor, respectively) were employed to support the differentiation of the actual heating rate across the reactor wall. The thicker wall requires a longer time for heat transfer and therefore reduces the actual heating rate, compared to the thinner. The quartz capillary tubes were first cleaned with distilled water, then dried and sealed at one end by an acetylene torch. Pre-prepared feed solutions (with or without catalyst) were then loaded into the reactors. Each loaded reactor was purged with nitrogen (99.99%) for 30 s and then quickly sealed at the other end. This operation was carefully optimized in order to minimize the contamination risk, considering that there might have been a chance of contaminants (combustion products from the acetylene flame) entering the capillary. 2.3. Experimental setup and procedure The HTL experimental setup consisted of a muffle furnace (Thermoconcept KC 64/13), a cylindrical sand bath made of stainless steel (of 25 cm high, and 20 cm in diameter), and two thermocouples connected with a computer via a data logger (NI 9211) for monitoring the temperature of the sand bath and/or inside the reactor. Olivine sand of an industrial type, supplied by North Cape Mineral AS, Norway was used, as received, for this study. The height of the sand column filled in the sand bath was 22 cm, making it possible to fully cover the quartz reactors of 20 cm long. The sand bath was pre-heated to a pre-set temperature and held at this temperature for 2 h to allow a steady state to be established throughout the bath. At the steady state of the furnace and the sand bath, loaded and sealed reactors were quickly placed into the sand bath (for achieving higher heating rates) or in the muffle furnace (for setting lower heating rates). After a given reaction (holding) time, the reactors were quickly taken out from the setup and immerged in a water bath at room temperature to quench further reactions. The reactors were then opened by cutting one end for products collection. Three repetitions were performed for each of the tests, from which the average values were calculated and reported as the experimental results.
2. Experimental 2.1. Feedstock characterization and preparation The feedstock used for this study was sugar kelp (Laminaria saccharina), a brown macro-alga, which was harvested in June 2011 in Trondheim, Norway. The collected sample of kelp was frozen and stored in plastic bags at −20 °C within 8 h after harvesting. The fuel characteristics including proximate, ultimate analyses and heating value (calculated from Channiwala and Parikh [26]) of the kelp biomass are given in Table 1. The frozen kelp sample was defrozen and washed with plenty of water to remove the sea salts before drying, grinding, and sieving. The grinding was carried out by using an IKA MF 10 cutting mill (from IKA®-Werke GmbH & Co. KG) with a closed bottom. Powder collected in this step was sieved in a vibrating sieving machine (Fritsch Analysette 3 Pro) to collect particles smaller than 90 μm for the HTL experiments. This selection aimed to maximize the heat and mass transfers within a single reacting particle. The feedstock in powder form was dried at 105 °C for 24 h before being balanced and mixed with distilled water to produce slurry solutions suitable for injecting into the capillary reactors. The pH of the slurry solutions (kelp/water ratio = 1/10, w/w) was
2.4. Products collection and separation Due to practical limitations and considerations, the gaseous products were not collected in this study, but calculated by mass balancing. Once a reactor was opened, a long stainless steel needle (Sigma-Aldrich Z261297) was used to inject a known amount of a dichloromethane (CH2Cl2) and water mixture into the reactor. The ratio of CH2Cl2:H2O varied from 3:1 to 5:1 (in volume) depending on the ratio of bio-oil and WSP. This procedure and operation allowed collecting the products (liquids and solids) with a minimized wall loss. The collected products mixture was then separated by means of an MSE Minor Centrifuge operated at 5000 rpm for 3 min. Three layers of products were observed
Table 1 Proximate and ultimate analyses for the feedstock (dry and ash free). Feedstock
Proximate analysis a
Sugar kelp (Laminaria saccharina)
HHVb
Ultimate analysis a
a
a
a
a
a
a
Ash
VM
Fixed C
C
H
O
N
S
16.58
69.32
14.10
39.44
5.14
52.03
2.99
0.60
14.46
VM: volatile matter, HHV: higher heating value. a w %; b MJ/kg
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
Y bio−oil ð daf; %Þ ¼
mbio−oil 100% mkelp ð100−Ash%Þ
400 350
Temperature (°C)
after centrifugation. They included a hydrophilic or aqueous phase (containing water soluble products) on the top, a hydrophobic phase (bio-oil dissolved in CH2Cl2) in the middle, and a solid phase at the bottom. Using an adjustable-volume pipette (Finnpipette™, Thermo Scientific Inc.), the top layer of aqueous products was extracted first. It was then followed by the middle layer of bio-oil, leaving the solid at the bottom. The operation of this procedure was repeated twice in order to minimize the liquid products, which otherwise may be trapped in the solid. Finally, the collected products were dried under a reduced pressure until reaching a constant weight to obtain the mass of water soluble products (WSP), bio-oil, and solid residues (SR), respectively. The product yields (Y) were calculated adopting Eqs. (1)–(4) employed by Anastasakis and Ross [16], on a dry basis:
3
300 250 200 150 100
High HR (585°C/min) Mid HR (321°C/min) Low HR (146°C/min)
50 0
ð1Þ
0
20
40
60
80
100
120
140
160
Time (s)
Y SR
mWSP 100% mkelp þ mKOH
msolid ð%Þ ¼ 100% mkelp þ mKOH
Y gaseous product ð%Þ ¼
mkelp −mbio−oil −mWSP −msolid 100% mkelp þ mKOH
ð2Þ
ð3Þ
ð4Þ
3. Results 3.1. Thermal behaviour of the quartz capillary reactors As discussed earlier in the introduction part, a key aspect of this present study is to maximize the yield of bio-oil from HTL of kelp biomass by maximizing the rate of heating. Therefore, before investigating the effect of heating rate on the bio-oil yield, it is important to understand the thermal behaviour of the reaction system with regards to heating rate and how fast the rate of heating can be achieved for the experimental setup. In order to measure and determine the heating rate (HR) it is required to know the temperature outside and inside the reactor. It is however difficult to measure the temperature inside the quartz capillary reactors, considering that the reactors are small in size and sealed when operated. Nevertheless, the small size of the reactors allows reasonable assumptions that there is no temperature gradient across the volume inside a single piece of the reactors, and that differences in internal heat transfer between the reactors filled with either water or air are neglectable. With these assumptions, it was possible to measure the temperature inside the quartz capillary reactors, using a thermocouple (1 mm in diameter) placed inside the reactors (inner diameter = 2 mm) sealed on one end and filled with air. The HR was then calculated according to Eq. (5), for which the heating time is defined as time needed to raise the reactor from starting (room) temperature (Tstart) to a preset liquefaction temperature (Tliq). T start −T liq heating time
figure. The high (or highest) heating rate of 585 °C/min approximately was achieved for the 1 mm-wall reactor heated by the pre-heated sand. The low (or lowest) heating rate of 146 °C/min approximately was recorded for the 2 mm-wall reactor heated by the pre-heated air. The 2 mm-wall reactor heated by the pre-heated sand gave the mid (or medium) heating rate in between, being 321 °C/min approximately. 3.2. Effects of heating rate on bio-oil yield
where mkelp, mbio − oil, mWSP, mSR, and mKOH are the mass of fed kelp, bio-oil, WSP, solid residue and fed catalyst, respectively.
HRðo C= minÞ ¼
Fig. 1. Profiles of temperature inside quartz reactors heated by pre-heated sand or air.
For this investigation, the optimum conditions reported by Anastasakis and Ross [16] for the HTL of a similar feedstock, brown macro-alga L. saccharina from Barnacarry Bay in Scotland, were adopted. The conditions were: liquefaction temperature = 350 °C; holding time = 15 min; and biomass/water ratio = 1/10 (w/w). The only varied parameter was the heating rate, which was controlled to be 146 °C/min, 321 °C/min, or 585 °C/min. Results from this investigation are presented in Fig. 2, where the yields of by-products including SR, WSP, and calculated gases are also plotted versus heating rate. It appears that the effects of heating rate on the products yield were very strong. The higher the heating rates, the higher the bio-oil yield was achieved. As can be seen in Fig. 2, the yield of bio-oil obtained from the HTL of kelp biomass increased from 53% to 65% and 79% when the heating rate was increased from 136 °C/min to 321 °C/min and 585 °C/min. The WSP yield exhibited a variation trend with the highest value found for the medium heating rate of 321 °C/min. In addition, the WSP yield at the highest heating rate of 585 °C/min was found to be 100
80
Yield (%)
Y WSP ð%Þ ¼
60
Bio-oil Solid residues WSP Gases
40
20
ð5Þ 0
Fig. 1 presents the curves of inside-reactor temperature plotted against time. The temperature was measured by using one-end-open reactors placed in different heating media (sand or air) preheated to 350 °C, as discussed earlier in Section 2.3. From the numerical data of the curves, the heating rates were calculated and also shown in the
100
200
300
400
500
600
Heating rate (°C/min) Fig. 2. The effect of heating rate on the products yield. (Tliq = 350 °C; holding time = 15 min; and biomass/water ratio = 1/10, w/w).
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
even lower than that of the lowest heating rate (146 °C/min). This makes it hard to draw a firm confirmation on the trend at this stage. More importantly, the overall conversion efficiency of kelp biomass in the conditions of the highest heating rate = 585 °C/min; Tliq = 350 °C; holding time = 15 min; and biomass/water ratio = 1/10 (w/w) was 85% approximately. 3.3. Effects of residence time on product yield The highest heating rate (585 °C/min) was employed for this investigation in the common conditions of 350 °C as the liquefaction temperature, and 1:10 (w/w) as the kelp/water ratio. The residence (holding) time, counted from the time the liquefaction temperature reached 350 °C to the time the reactors started being quenched, was varied within 5, 10, 15 and 20 min. The results from this investigation are shown in Fig. 3. It is interesting to see that the highest bio-oil yield is associated with the retention time of 15 min, which is in good agreement with the literature [16]. Either longer or shorter residence time led to reduced bio-oil yields. The bio-oil yield was reduced from 79% to 75% when the holding time was increased further from 15 to 20 min. Similar observations were also reported in the literature [15–17,20,22,27]. The reductions in bio-oil yield, associated with further increasing retention time, were attributed to the formation of secondary char from bio-oil. This attribution is confirmed by the present investigation, as can be seen in Fig. 3. On the other hand, the WSP yield decreased consistently with increasing retention time, but the trend for gases yield was not pronounced and seemed unchanged. 3.4. Effects of catalyst addition on the yield and fuel properties of bio-oil Effects of catalyst (KOH) addition (KOH/kelp = 1/20 w/w, dry mass) on the yield and fuel properties of the bio-oil product were investigated in the HTL conditions of Tliq = 350 °C, holding time = 15 min, heating rate = 321 °C/min, and biomass/water ratio = 1/10 (w/w). The results from this investigation are presented in Fig. 4 and Table 2, in which some data from the early investigation of the heating rate effects on bio-oil yield are also included for comparison. It can be seen from the data in Fig. 4 that the catalyst addition had a slightly positive effect on bio-oil yield, though an increase by 2% (from 65% to 67%) is hardly remarkable. It is also observed that the effect of the higher heating rate on bio-oil yield was much stronger than that of the catalyst addition. Starting from the bio-oil yield of 65% (mid-HR, no catalyst) the higher heating rate (high HR, no catalyst) resulted in a much higher bio-oil yield (79%) compared to 67% obtained by the catalyst addition (mid-HR, with catalyst). In addition, the catalyst addition seemed to favour the formation of WSP portion and reduced the gas 100
Yield (%)
80
Bio-oil
60
Solid residues WSP
40
Gases
80
Mid HR, no catalyst Mid HR, with catalyst
60
Yield (%)
4
High HR, no catalyst
40
20
0
Bio-oil
Solid residues
WSP
Gases
Fig. 4. Products distribution for HTL of kelp with and without catalyst addition in the common condition of Tliq = 350 °C, holding time = 15 min, and biomass/water ratio = 1/10 (w/w).
yield, whereas the yields of these by-products were both decreased with the increasing heating rates. The ultimate analysis of bio-oils obtained from this study is presented in Table 2. It shows that, in all cases, with or without catalyst addition, the bio-oils obtained from HTL of kelp had much higher carbon and hydrogen contents, but lower oxygen content compared with the raw kelp (Table 1). Consequently, the higher heating value of bio-oil was significantly improved, being as high as 34–36 MJ/kg. It also indicates that the fuel properties of the bio-oil products were improved more significantly by the higher heating rates than by the catalyst addition. These observations are somehow not in good agreement with past studies by Zhou et al. [15] and Anastasakis and Ross [16], which reported different effects of catalyst addition. However, it should be reminded that both of the studies were carried at much lower heating rates (25 °C/min), compared with this present study, using reactors with relatively large volumes and thus significant mass and heat transfer limitations. 4. Discussion It has been demonstrated that the use of quartz capillary reactors for HTL of seaweed biomass exhibited a number of advantages, including the possibilities to carry out multiple experiments at the same time. With the experimental setup, the technique was capable of achieving very high heating rates compared to the conventional reaction methods. Some problems were also found during the experiments. Reactor explosion was a critical issue quite sometimes due to various reasons [23]. Once the reactor exploded, the sample was lost and the experiment was repeated all over again from the beginning. Other problems were related to the operation of product collection and the mass balance determination. Because the reactor size was very small in volume, the collection of gaseous products was very difficult and their yields were normally calculated by difference in mass balance. Moreover, it was not possible to control and measure pressure inside the reactors, which can only be estimated from the reaction temperature by using steam tables (165.3 bar at 350 °C [28]). It is just because the reactors
20 Table 2 Ultimate analysis of bio-oils obtained from HTL of seaweed in different conditions.
0 0
5
10
15
20
25
Holding time (°C/min) Fig. 3. The effect of holding time on the product yield. (Tliq = 350 °C; heating rate = 585 °C/min; and biomass/water ratio = 1/10, w/w).
High HR, no catalyst Mid HR, no catalyst Mid HR, with catalyst
Ca
Ha
Na
Sa
Oa
HHVb
75.54 73.23 73.46
9.16 8.88 8.48
3.65 4.27 3.89
0.62 0.56 0.61
11.66 13.62 14.17
35.97 34.61 34.18
HHV: higher heating value. a w %; b MJ/kg
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
Q.-V. Bach et al. / Algal Research xxx (2014) xxx–xxx
Y bio‐oil ð%Þ ¼ ½α ln ðHRÞ þ β 100%
ð6Þ
where α and β are independent variables to be determined from the experimental data. Other terms have been defined earlier, of which bio-oil yield, Ybio-oil, is the dependent variable of the function. From a nonlinear regression analysis (confidence interval = 0.95) using our experimental data, the independent variables α and β were determined, which allowed generating the model presented in Eq. (7) for predicting the bio-oil yield at a given heating rate. Y bio‐oil ð%Þ ¼ ½0:1859 ln ðHRÞ−0:3839 100%
ð7Þ
Applying Eq. (7), the predicted bio-oil yield curve was drawn and presented in Fig. 5, which demonstrates a very good agreement among the two studies. Indeed, the calculated curve does not only represent nicely the experimental data in this study but also fits the data reported by Anastasakis and Ross [16]. In addition, a one-sample t-test [33] was performed for every heating rate (with three replicated measurements n = 3) to elucidate
80
Bio-oil yield (%)
were sealed. The actual pressure might be different due to gases (mainly CO and CO2) produced from the process. It was estimated to be within 171.4–174.4 bar, assuming 10–15% w [16] for the gas yield, 50:50 (v/v) for the CO:CO2 ratio, and that the gases behaved ideally. Regarding the method of heating rate determination, it may be argued that the inside-reactor temperatures measured by the experiments employing one-end-open quartz capillary reactors filled with air may not represent the actual temperatures. However, the method did offer a good representation of the actual heating rates under the experimental conditions. It is because, given the assumptions discussed earlier in Section 3.1, the heating rate is a function of the thermal conductivity of the reactor wall (quartz material), the wall thickness, and the temperature gradient (between the outside and inside surfaces of the reactor wall), but not the temperatures. Since temperature at both surfaces was measured by means of two similar thermocouples placed in air on the surfaces, possible differences between the measured and the actual values of these two temperatures would have cancelled one another within the acceptable experimental variations. More importantly, it has been confirmed by this present study that higher heating rates during HTL of biomass enhanced significantly the bio-oil yield. Although the chemistry behind this effect is not yet fully understood [20], possible explanations include that rapid heating: (1) accelerates the degradation reactions of biomass materials [20,29]; (2) reduces unwanted side reactions and promotes reactions forming more bio-oil [21,30]; (3) accelerates the breaking of biomass cell and thus eases the releasing of cell contents into the hydrothermal media for subsequent reactions [20]; (4) minimizes the char formation [31] and/or the re-polymerization [32]. As presented earlier in Section 3.2, the optimum conditions (Tliq = 350 °C, holding time = 15 min, biomass/water ratio = 1/10, w/w) reported by Anastasakis and Ross [16] for the HTL of a similar feedstock, brown macro-alga L. saccharina from Barnacarry Bay in Scotland, was adopted for our present study. In the Scottish study, the influence of reactor loading, residence time, temperature and catalyst (KOH) addition was assessed. However, an assessment for the influence of heating rate was missing. Therefore, this present study can be considered as a complementary to the Scottish report. It is observed that, in similar conditions except for the heating rate, the highest bio-oil yield reported in the Scottish report was 19%, which is much lower than that (79%) from this present study. The main cause of this difference is the heating rate difference and therefore it would be interesting to include the data obtained from the two studies for a regression analysis. For this purpose, the method reported by Zhang et al. [21] was adopted, which demonstrated that the bio-oil yield from biomass liquefaction can be calculated and predicted by using a logarithm function of the heating rate with the general form in Eq. (6)
5
60
40
20
Experimental data - present work Experimental data from literature [Ref. 16] ---- Predicted curve
0 0
100
200
300
400
500
600
700
Heating rate (°C/min) Fig. 5. Modelling and prediction of bio-oil yield as a function of heating rate.
the confidence level of the experimental data, assuming that the predicted yields are the specified (true) values. The results of the t-tests for the three heating rates are presented in Table 3. It can be seen that all the tcal (calculated t) values are smaller than the tcri (critical t) value (tcri = 2.92 for a sample with a degree of freedom df = n - 1 = 2, and a cumulative probability of 95% or α = 0.05 [34]). Therefore, the null hypothesis cannot be rejected at any of the heating rates tested. It means that there was no significant difference between the experimental data and the true bio-oil yields. Finally, it is reasonable at this stage to believe that it is possible to improve further the bio-oil yield by employing heating rates higher than 585 °C/min. Such higher heating rates can be achieved in continuous flow reaction systems equipped with static mixing mechanisms, which are also suitable for upscaling, keeping in mind that the solid products or residues may cause problems. 5. Conclusion The high-throughput technique using sealed quartz capillary reactors has been applied and tested for the HTL of macro-alga (L. saccharina), aiming at maximizing the bio-oil yield and the overall conversion efficiency by increasing the heating rate. With this technique, it was possible to achieve heating rates as high as 585 ± 2 °C/min. The effects of heating rate and catalyst addition on the yield and fuel properties of bio-oils obtained from the process have been investigated and compared. It appears that the effects of heating rate are similar to, but much stronger than those of the catalyst addition. Very high bio-oil yields can be obtained from non-catalytic HTL of seaweed. In the conditions of heating rate = 585 °C/min, Tliq = 350 °C, holding time = 15 min, and kelp/water ratio = 1/10 (w/w), the oil yield was 79% w. The HHV of the oil was also significantly improved, from 14.11 MJ/kg to 35.97 MJ/kg (dry and ash free). Higher bio-oil yields and higher overall conversion efficiencies of the process would be possible to achieve by even faster heating rates. Further studies employing Table 3 T-test of null hypothesis assuming the predicted oil yields as the true values. Heating rate, °C/min
x
s
μ
tcal
Significance
585 226 146
0.79 0.65 0.53
0.048 0.043 0.044
0.80 0.62 0.54
0.27 0.97 0.64
No No No
x and s is the mean and standard deviation, respectively, of the experimental data. μ is the specified (true) value. tcal and tcri are the calculated and critical t, respectively. tcri (2) = 2.92 at α = 0.05 [34].
Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009
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Please cite this article as: Q.-V. Bach, et al., Fast hydrothermal liquefaction of a Norwegian macro-alga: Screening tests, Algal Res. (2014), http:// dx.doi.org/10.1016/j.algal.2014.05.009