Pyrolysis Characteristics and Char Reactivity of Oedogonium sp. and

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Pyrolysis Characteristics and Char Reactivity of Oedogonium sp. and Loy Yang Coal Youjian Zhu,† Chi Wai Kwong,‡ Philip J. van Eyk,‡ Rocky de Nys,§ Dingbiao Wang,*,∥ and Peter J. Ashman*,‡ †

School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, People’s Republic of China ‡ School of Chemical Engineering, University of Adelaide, Adelaide, South Australia 5005, Australia § Centre for Macroalgal Resources and Biotechnology (MACRO), James Cook University, Townsville, Queensland 4811, Australia ∥ School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, Henan 450001, People’s Republic of China ABSTRACT: The proposition of combining the application of algae in wastewater treatment, CO2 fixation, and energy production maximizes the opportunity of algal bioremediation. In this paper, the freshwater macroalga Oedogonium, a key target species for the bioremediation of waste waters, was investigated for its utilization in energy production via gasification or cogasification with coal. The pyrolysis characteristics of Oedogonium and the effects of pyrolysis conditions on char reactivity were investigated in this paper, and an Australian lignite was also studied for comparison purposes. The pyrolysis process of Oedogonium and coal can be divided into three stages: moisture evaporation, volatile release, and decomposition of the remaining carbonaceous compounds. However, the devolatilization of Loy Yang coal was slower and occurred over a wider temperature range compared to Oedogonium. As the heating rate increased, the pyrolysis curves for both Oedogonium and Loy Yang coal shifted to higher temperatures. To investigate the effects of pyrolysis conditions on char reactivity, a tube reactor was employed to generate char samples under a range of pyrolysis conditions. The produced char samples were characterized using a CHN analyzer and scanning electron microscopy technique. It was found that char samples almost maintain their original shape following pyrolysis at a low heating rate. However, evidence of char swelling was observed in the structure of chars prepared at a high heating rate. Pyrolysis conditions also had a significant influence on the reactivity of the derived chars. Char reactivity for both fuels was enhanced at a higher heating rate and lower pyrolysis temperature. Oedogonium char reactivity was around 2−5 times that of Loy Yang char prepared under the same pyrolysis conditions. However, the differences in reactivity became less significant as the pyrolysis temperature increased. Oedogonium has a higher devolatilization rate and higher char reactivity than Loy Yang lignite. It implies that it is easier to be gasified, and the co-gasification with lignite is advantageous.

1. INTRODUCTION Biomass utilization is gaining increased focus because of its renewable and carbon-neutral characteristics. Macroalgae, as aquatic biomass, has a long history of use in the pharmaceutical, chemical, and food industries.1 More recently, new applications in wastewater treatment and energy production1,2 have received attention because of the short growth cycles, high production yield, and high fixation rates of CO2 compared to terrestrial biomass. In addition, its cultivation can be effective using wastewater on non-arable land.3 Most of the current research on algae-to-energy has focused on fermentation3−5 and anaerobic digestion6−9 to produce biogas or bio-oil. Nevertheless, these methods commonly have a lower conversion rate, and the reaction rates are remarkably lower than thermochemical conversion methods.10 Of the thermochemical conversion methods, gasification could convert the solid fuel into combustible gases with high efficiency and low emissions.11,12 This process primarily consists of two stages:13 (1) pyrolysis or devolatilization and (2) char reaction with gasifying agents. The pyrolysis characteristics of various macroalgae have been investigated10,14−18 in recent years. Wang et al.14 investigated the pyrolysis characteristics of Enteromorpha clathrata at © 2015 American Chemical Society

different heating rates and also analyzed the compositions of gaseous products using thermogravimetry−mass spectrometry (TG−MS). The pyrolysis characteristics of three red marine macroalgae were investigated by Li et al.17 They proposed that the pyrolysis process can be divided into three stages, and the kinetic parameters were calculated using three different methods. Ross et al.10 characterized five types of macroalgae by proximate and ultimate analyses, inorganic content, and calorific value. Thermal treatment and combustion behaviors were investigated using a thermogravimetric analyser (TGA) and pyrolysis−gas chromatography−mass spectrometry (GC− MS) to explore its suitability of thermochemical utilization. It is well-known that the char reaction process is generally much slower than the pyrolysis process and is, therefore, the rate-determining step in combustion and gasification processes.19,20 Previous research has shown that pyrolysis conditions have a significant influence on the formation of biomass char and its reactivity. Cetin et al.13,20 found that the biomass char particles had a smooth surface and large open Received: March 28, 2015 Revised: June 30, 2015 Published: July 6, 2015 5047

DOI: 10.1021/acs.energyfuels.5b00642 Energy Fuels 2015, 29, 5047−5055

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Energy & Fuels

2. EXPERIMENTAL SECTION

structure under a high heating rate. The porous structure of char samples produced under lower heating rates mainly consisted of micropores, while char samples produced under a high heating rate had a higher total surface area and contained a large amount of macropores. The apparent char gasification reactivity also increased as the heating rate increased. Guerrero et al.21 found that the pyrolysis heating rate and temperature had an obvious effect on the eucalyptus char elemental composition, surface area, and morphology. The high heating rate char also had a higher reactivity in oxygen because of the higher surface area and higher hydrogen and oxygen contents of the char sample. However, research on the reactivity of algae-derived char is limited. Kirtania et al.22 investigated the char reactivity of a microalga in both CO2 and steam environments and compared this to the reactivity of woodchips. Char samples were prepared in both a TGA and an entrained-flow reactor to investigate the effects of the heating rate on char reactivity. At a high temperature (1100 °C), the woody char was more reactive than the algae char, but the reactivity of these two fuels was similar at a lower temperature (below 950 °C). Furthermore, the algal char prepared in the entrained flow reactor was less reactive than the char generated from a TGA. This conflicts with the previous results13,20 for terrestrial biomass, where biomass char reactivity increased as the heating rate increased.13 Char reactivity of one microalga,23 Nannochloropsis gasitana, was measured using a TGA in a steam atmosphere, and the effects of the char reaction temperature, particle size, sweeping gas flow rate, and steam concentration were investigated. Char reactivity increased as expected when the reaction temperature, sweeping gas flow rate, and steam concentration increased. However, the char reactivity also increased as the particle sizes increased, which is contrary to the finding of other biomass fuels. This was proposed to be due to the high porosity of the large algae particles. Currently, the new proposition of combined applications of algae in wastewater treatment and energy production has attracted increasing attention. However, because of the conditions under which the wastewater macroalgae are cultured, they have a different fuel composition, in particular ash, compared to those cultivated in clean water. This could affect the reactivity of the algae. Lane et al.24 found that the char reactivity of microalgae was much lower than that of macroalgae and proposed that this could be due to the high ash content of the employed microalgae. Consequently, a clearer understanding of the char reactivity of wastewater-cultivated algae is necessary to the design and operation of the corresponding algae gasification/co-gasification process. The aim of this research was therefore to explore the use of one type of macroalgae via gasification or co-gasification with coal. The green macroalga, Oedogonium sp., was chosen because it is not only a target species for treatment of wastewater25 but also a potential biomass fuel for bioenergy production.26 A widely used Australian lignite, Loy Yang, was used for comparison. The specific objectives were to (1) investigate the pyrolysis characteristics of the chosen fuels, (2) characterize the char samples generated under different pyrolysis conditions, and (3) investigate the effects of pyrolysis conditions on char reactivity. These results could lay a foundation for the understanding of the gasification/co-gasification characteristics of the employed macroalgae.

Two fuels were used in this research: one is a green macroalga, Oedogonium sp. (hereafter referred to as OD), and the other is an Australian lignite, Loy Yang (hereafter referred to as LY). Oedogonium sp. was chosen because it is not only a target species for treatment of wastewater25 but also a potential biomass fuel for bioenergy production.26 LY lignite, as one type of Victorian lignite, is a significant resource for power generation in Australia.27 Detailed information about its growing conditions (high nitrogen environment) and harvest methods are provided in a previous work.28 After harvest, excess water was removed first from the biomass using a domestic washing machine as a centrifuge, and the biomass was then dried in a solar kiln (50 °C). The as-received coal has a high moisture content (∼60 wt %), and it was dried in air to a constant weight. The dried samples were milled using a knife mill to reduce the particle size and then sieved to a particle size of less than 106 μm. The proximate and ultimate analyses are presented in Table 1.

Table 1. Proximate and Ultimate Analyses and Calculated Heating Values for OD and LY Coala OD moisture content (wt %) 7 Proximate Analysis (wt %db) fixed carbon 14.7 volatile 77.3 ash 8 Ultimate Analysis (wt %daf) C 46.00 H 6.00 O (by difference) 35.40 N 3.93 S 0.20 Cl 0.36 HHV (MJ/kg)db 19.1 a

LY 6 48.80 46.50 4.80 65.40 4.50 24.40 0.47 0.44 0.04 25.7

db, dry basis; daf, dry and ash-free basis.

2.1. Pyrolysis Experiments. Pyrolysis experiments were carried out using a Setaram Labsys TGA/differential scanning calorimeter (DSC). Samples of approximately 5−6 mg were loaded into an alumina crucible, and the experiments were conducted under nonisothermal conditions with N2 as the sweeping gas (80 mL/min) to provide an inert environment. Samples were heated from ambient temperature to 900 °C at 10, 20, 40 K/min, respectively. Prior to the experiment, baseline experiments without samples were carried out to account for changes in the apparent weight because of buoyancy effects.29 The weight loss curves were later corrected by subtraction of the baseline obtained under the same conditions. Two replicates were performed for each experiment to assess the reproducibility of the results. 2.2. Char Preparation. The char samples were prepared using a fixed-bed reactor at both low and high heating rates. The schematic diagram of the experimental setup is shown in Figure 1. N2 (500 mL/

Figure 1. Schematic diagram of the fixed-bed reactor. 5048


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Energy & Fuels min) was used as the sweeping gas during the pyrolysis process. Before the experiment, samples (3 g) was placed in a steel-mesh container and degassed for 0.5 h in a N2 atmosphere. At a low heating rate, the sample was placed in the center of the furnace and heated at 10 K/min to the desired temperature. At a high heating rate, the sample was initially placed in the inlet end of the reactor tube and located outside the furnace to ensure that the sample temperature was lower than 100 °C. After the furnace reached the desired reaction temperature, a steel rod was used to push the mesh container into the center of the furnace. The sample was thus heated at a rate that was estimated to be approximately 20 K/s. The sample was maintained at the reaction temperature for 0.5 h to complete pyrolysis; then the furnace was switched off; and the sample was cooled in a N2 atmosphere to 100 °C. The char samples were ground and sieved to a particle size in the range of 53−150 μm for the following experiment. Char samples collected from all of the pyrolysis experiments were also weighed using an electronic balance to determine the char yield. 2.3. Char Gasification Reactivity. The char gasification experiment was performed under isothermal conditions using the same TGA. During the experiment, a small amount of sample (around 6−8 mg) was placed in the crucible and N2 (80 mL/min) was used as the sweeping gas while heating the furnace to the desired temperature. The furnace was first heated to 105 °C and then held for 10 min to dry the char. The furnace was then heated to 800 °C at a heating rate of 10 K/min and held for 10 min to stabilize the weight signal before the gasification reaction was initiated by switching the flow of carbon dioxide (80 mL/min). The reaction was allowed to proceed for 5 h to ensure that all of the char had reacted. The final mass of the sample was recorded as the mass of ash. The conversion rate (Xi) was calculated by eq 1. The instantaneous overall reactivity (Ri) was determined using eq 2, which is generally used to express the overall char reactivity.30,31 To compare quantitatively the char reactivity difference, an average overall reactivity (Rave) was calculated using eq 3

Xi =

m0 − m i m0 − m f

(1)

Ri =

1 dX i 1 − X i dt

(2)

Figure 2. TG−DTG curves of (a) OD and (b) LY, at a heating rate of 5 K/min.

identified as occurring between the starting temperature and the initial temperature of the main pyrolysis process, Ti, which is taken as the temperature at the inflection point in the DTG curve between the two peaks. Stage II occurs from Ti to the end temperature of the main pyrolysis process, Te, which is taken as the temperature at the inflection point in the DTG curve between the largest peak and the shoulder to the right of the largest peak. Stage III occurs from Te until the final pyrolysis temperature. The characteristic temperatures used to characterize the pyrolysis process, Ti, Te, and Tmax, are shown in Table 2. For OD, the weight loss in stage I corresponds to the evaporation of the moisture. The decomposition of proteins, lipids,

n

R ave =

∑i = 1 R i n

(3)

where m0 is the initial mass of the char sample, mi is the mass of the sample at reaction time t, mf is the mass of the residual ash at the end of the experiment, t is the time, and n is the number of Ri. The average overall reactivity, Rave, is usually calculated within a conversion level range where experimental errors are acceptable. In our study, Rave is determined in the conversion rate ranges from 10 to 80% (main reaction zone); this range is selected to minimize weight measurement uncertainties because of the very small weight loss in the early stage of the gasification process and to avoid high reactivity values as the fractional conversion approaches 1 in the final stages of reaction.32 2.4. Char Characterization. The elemental composition of the char samples was measured using a CHN analyzer (LECO TruSpec). Surface morphology of the raw fuel and char samples was investigated using scanning electron microscopy (SEM) (Philips, model XL30). During SEM analysis, the particle samples were dispersed on a carbon tape and coated with carbon to avoid charging by electron interaction. Secondary electron (SE) mode was chosen to investigate the surface morphology of the char sample.

Table 2. Parameters Describing the Pyrolysis Characteristics of OD and LY Coal for Different Heating Rates

OD

LY

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Characteristics. The weight loss (TG) and the derivative weight loss (DTG) curves during pyrolysis of OD at a heating rate of 5 K/min are presented in Figure 2a. Three distinct stages can be observed during the pyrolysis process, in accordance with previous studies.14,17,33−35 Stage I can be

heating rate (K/min)

Ti (°C)a

Tmax (°C)b

Te (°C)c

(dw/dt)maxd

residuese (%)

5 10 20 5 10 20

137 150 152 159 177 200

299 311 316 412 424 438

349 368 377 518 531 549

3.3 6.4 13.7 0.9 1.7 3.4

24.0 26.7 27.7 51.5 53.0 54.2

a

Ti is the initial temperature of the main pyrolysis (°C). bTe is the end temperature of the main pyrolysis (°C). cTmax is the temperature of the maximum reaction rate (°C). d(dw/dt)max is the maximum reaction rate (%/min). eResidues mean the remaining char and the ash. 5049


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Energy & Fuels carbohydrates, and other hydrocarbons mainly occurs in stage II to form tars and gaseous products.33 During stage III, a shoulder exists at the right of the main peak, suggesting a slow and gradual loss of weight (from Te to approximately 600 °C), resulting from the decomposition of the remaining carbonaceous products.34 No major weight change was observed after around 600 °C. A similar phenomenon for LY can be seen from Figure 2b. As shown in Figure 2b, the pyrolysis process of LY could also be divided into three stages. The first two stages are the same as OD, and in the third stage, the sample weight decreases progressively until the end of the pyrolysis process. In comparison to OD, the devolatilization of LY is somewhat slower and occurs over a wider temperature range. It is wellknown that the pyrolysis characteristics of solid fuels mainly depend upon their composition. Macroalgae are mainly composed of proteins, lipids, and carbohydrates, which are decomposed at a low temperature. Meanwhile, macroalgae have a high alkali content, and these alkali metals generally have catalytic effects on the thermal decomposition process and increase the decomposition reaction rate.17,36 Therefore, the weight loss mainly occurs in a narrow temperature range. The volatile matter in coal is more complex, including mainly alkane, aromatic hydrocarbon, and aliphatic hydrocarbons,37 and these different components are decomposed in a wide temperature range; this leads to the weight loss occurring over a wider temperature zone. The residue mass of LY, shown in Table 2, is much higher than that of OD, which is attributed to its higher fixed carbon content (Table 1). The effects of the heating rate on the TG and DTG curves for the pyrolysis of OD are shown in Figures 3 and 4,

Figure 4. Plot of the DTG curve for OD at heating rates of 5, 10, and 20 K/min.

Figure 5. Plot of the TG curve for LY at heating rates of 5, 10, and 20 K/min.

Figure 3. Plot of the TG curve for OD at heating rates of 5, 10, and 20 K/min. Figure 6. Plot of the DTG curve for LY at heating rates of 5, 10, and 20 K/min.

respectively. Similarly, the effects of the heating rate on the TG and DTG curves for the pyrolysis of LY coal are shown in Figures 5 and 6, respectively. It can be seen from Table 2 that, as the heating rate increases, there is a lateral shift to a higher temperature for Ti, Te, and Tmax. Kinetic analysis of the pyrolysis processes at different heating rates has been calculated using the Coats−Redfern method,30 and the results can be seen from Table 3. From the table, it can be seen that the kinetic parameters vary with the heating rate. However, the kinetic parameters for 5 and 10 K/min do not vary much compared to the results for 20 K/min. The possible reason is that, at a heating rate of 20 K/min, the sample temperature is not equal to the TGA temperature and there is a temperature gradient between the center and surface of the sample. Hence, the lateral

shift of the pyrolysis curve to a higher temperature with the increase of the heating rate could be mainly attributed to (1) the temperature gradient between the center and surface of the sample under a high heating rate and (2) the change of the apparent pyrolysis kinetics at a higher heating rate because of the heat-transfer resistance. This lateral shift has been reported in the literature for different types of biomass38,39 and other types of algae.17,34,35 It can also be observed from the DTG curve that (dw/dt)max increases with the increase of the heating rate. This could be due to the heterogeneous structure of the biomass with many different constituents, each with their own characteristic evolution peaking at different temperature ranges 5050


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can be clearly seen that char samples produced under a higher heating rate and lower pyrolysis temperature have a higher content of hydrogen and oxygen and a higher H/C ratio (molar basis). A similar trend is also observed for LY. From the literature,38,40 hydrogen and oxygen in the char structure may act as adsorption sites to react with the gasifying agent. Thus, the content of hydrogen and oxygen in the char could be related to the availability of the active sites in the char and, therefore, has an influence on the char reactivity with different gases. On the other hand, a low H/C ratio implies that the char samples are more aromatic,42,44 which suggests a lower reactivity. 3.3. Char Morphology. The morphological characteristics of the parent sample and the derived chars are described in this section. This provides an understanding of the effects of pyrolysis conditions on changes to the morphology of char and explains the differences in reactivity under various pyrolysis conditions. The SEM images of the raw OD and the derived char samples are presented in Figure 7. A typical particle of raw OD fuel shows a non-porous and fiber shape, with some wrinkles and tiny spots on the surface, and the ends of the particles are rough. This could be due to attrition during the cutting and grinding process. At a low heating rate, the char samples almost keep their original shape, with a slight opening on the surface. As the temperature increases, the surface of the char particles becomes smoother and the particles fuse together. The char surface morphology changes significantly under a high heating rate. From Figure 7e, it can be observed that the char exhibits an elliptical shape and smoothed surface with large internal cavities at 800 °C. There is a large opening and breakage as well as obvious shrinkage when the temperature increased to 900 °C. At 1000 °C, the there are clear signs of melting and fusion. The SEM images of the raw LY and the char samples are shown in Figure 8. The external shape of the char samples is similar to the raw LY sample, and no distinct differences can be seen (images not shown). However, clear differences occur at high magnifications (10000×). The raw LY sample has an irregular shape and a rough surface with small pores. At a low heating rate, more pores and large cavities can be observed at a low pyrolysis temperature. The particle surface becomes smoothed as the pyrolysis temperature increases. At a high heating rate, cracks and fractures occur because of the rapid

Table 3. Kinetic Analysis Results of the Pyrolysis under Different Heating Rates heating rate (K/min) OD

5 10 20

LY

5 10 20

stage II III II III II III II III II III II III

E (kJ/mol)

A (min−1)

n

R2

55.4 96.9 56.1 87.5 59.7 118.6 35.0 135.1 36.4 167.4 40.8 162.6

× × × × × × × × × × × ×

0.37 4 0.3 2.5 0.4 3.8 0.55 2.4 0.6 3.2 0.7 2.8

0.995 0.966 0.998 0.945 0.999 0.959 0.997 0.977 0.998 0.983 0.998 0.974

8.08 2.81 1.41 8.94 5.57 3.18 1.44 8.36 3.49 1.63 1.48 9.27

3

10 106 104 103 104 108 101 106 101 109 102 108

during the pyrolysis process. At high heating rates, these different constituents decompose simultaneously and the adjacent peaks overlapped to form wider peaks.40,41 3.2. Char Yield and Elemental Composition. Table 4 shows the results of the char yields produced at different heating rates and temperatures. It can be observed for both fuels that the char yield decreases as the pyrolysis temperature increases; however, as the heating rate increases, the char yield increases. For OD, as the temperature increased from 800 to 1000 °C, the char yield decreased from 24.6 to 20.3% at a low heating rate and decreased from 23.0 to 18.8% at a high heating rate. For LY, as the temperature increased from 800 to 1000 °C, the char yield decreased from 54.5 to 47.4% at a low heating rate and decreased from 48.2 to 44.0% at a high heating rate. A higher temperature resulting in a lower char yield could be due to secondary decomposition reactions continuing to occur in the solid matrix at a high temperature.42 On the other hand, the reason for the enhanced char yield under a lower heating rate is that a lower heating rate results in longer residence times inside the reactor and favors secondary reactions, such as repolymerization and recondensation, which ultimately lead to the formation of the solid char.34,43 In all of the cases, the char yield of LY is much higher than of OD, which corresponds to their fixed carbon content (Table 1). Table 4 also shows the elemental compositions of the produced char samples under different pyrolysis conditions. It

Table 4. Char Yield and Elemental Composition (wt %, Dry Basis) OD final temperature (°C)

800

LY

900

1000

800

900

1000

54.5 89.4 1.0 0.9 8.7 0.140

50.3 88.9 0.7 1.0 9.4 0.059

47.4 90.3 0.4 1.1 8.2 0.054

48.2 86.9 1.0 0.9 11.2 0.142

46.9 88.4 0.6 0.9 10.1 0.082

44.0 89.4 0.7 1.1 8.9 0.089

LHR Char char yield (%) C H N O (by difference) H/C ratio (molar basis)

24.6 65.1 0.6 4.2 30.1 0.114

23.8 66.5 0.6 3.5 29.4 0.103

char yield (%) C H N O (by difference) H/C ratio (molar basis)

23.0 60.5 0.8 4.3 34.4 0.155

19.4 61.3 0.7 3.1 35.0 0.129

20.9 69.0 0.5 1.9 28.6 0.094 HHR Char 18.8 62.8 0.6 2.0 34.6 0.113 5051


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Figure 7. SEM images of (a) raw OD algae, char samples produced under a low heating rate at (b) 800 °C, (c) 900 °C, and (d) 1000 °C, and char samples produced under a high heating rate at (e) 800 °C, (f) 900 °C, and (g) 1000 °C.

Figure 8. SEM images of (a) raw LY coal, char samples produced under a low heating rate at (b) 800 °C, (c) 900 °C, and (d) 1000 °C, and char samples produced under a high heating rate at (e) 800 °C, (f) 900 °C, and (g) 1000 °C.

thermal expansion, and as the pyrolysis temperature increases, the particle surface becomes smoothed. This same phenomenon has been observed across the literature of the pyrolysis of coal.45,46 It is well-known that,13,20,42 under a low heating rate, the natural pores of the fuel allow for the gradual release of volatile components without significant morphological change, and therefore, the particle shape is fundamentally maintained. However, particles start to fuse together at a high temperature, and this is attributed to the melting of the cell structure and the occurrence of plastic transformation at a high temperature.13,47 Under a high heating rate, volatiles are released over a very short time (usually in a few seconds), which produces substantial internal overpressure inside the cell and causes breakage/cracks as well as coalescence of small pores. The large internal cavity and a more open structure are eventually formed.20,21 3.4. Char Reactivity. Figure 9 shows the relationship between the reaction time and carbon conversion. For OD chars, as the pyrolysis temperature increases, the reaction time increases, and as the heating rate increases, the reaction time decreases. LY chars exhibit the same trend; however, they take a longer time for complete reaction compared to OD chars.

The instantaneous overall reactivity of OD and LY chars is presented in panels a and b of Figure 10, respectively. There is a slight increase of instantaneous overall reactivity with char conversion up to approximately 80%, and then the instantaneous overall reactivity increases sharply from 80 to 100% conversion. Char reactivity increasing with increasing conversion has been widely observed for biomass in the literature.24,47−49 According to Laurendeau,50 the overall reactivity is proportional to the intrinsic reactivity. However, it is also related to the char structure, the inorganic constituents, and the porosity.50 As the reaction progresses, the char porosity and inorganic/C ratio increase accordingly.51 Hence, the phenomenon of increasing overall reactivity with conversion can be explained by the increasing inorganic/C ratio and increasing char porosity as the char−CO2 reaction progresses.48 Together with the average overall reactivity results displayed in Figure 11, it can be concluded that, for both OD and LY, low-heating-rate char is less reactive in CO2 than high-heatingrate char. The enhanced char reactivity at a high heating rate has been shown in previous studies.20,21,52,53 From the SEM analysis, it can be seen that the rapid release of volitiles at a high heating rate usually generates char samples with large openings and carivities. This will increase the char porosity and, epecially, 5052


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Figure 9. Relationship between time and char conversion: (a) OD results and (b) LY results.

Figure 10. Instantaneous reactivity of the char produced at different pyrolysis conditions: (a) OD results and (b) LY results.

the fraction of meso- and macropores, and these parameters are usually related to the quantity of active sites,54 which is considered to have a direct relationship to char reactivity.20,21 On the other hand, both high-heating-rate (HHR) and lowheating-rate (LHR) chars show a decreasing trend of reactivity as the pyrolysis temperature increases, which is consistent with other studies on coals52,53 and some types of biomass.47,55 The aforementioned CHN analysis showed that the char samples produced under a high temperature have a smaller H/C ratio than the low-temperature char samples; this means that a high temperature favors a more ordered carbon structure and, therefore, reduces the accessibility of the active sites to the gases.52,53 Min et al.47 also found that chars produced at high pyrolysis temperatures were mainly composed of benzene rings or major backbone chains, which are less reactive than branched chains and, therefore, had a low reactivity. It is also found that, under the same pyrolysis conditions, OD char is more reactive than LY char and its Rave is 2−5 times that of LY char. It is well-accepted56 that alkali and alkaline earth metallic species have catalytic effects during char combustion and gasification and OD contains more alkali and alkaline earth metallic species than LY.28 The higher biomass char reactivity could also be due to the high surface area and porosity of the biomass-derived char structure.57 However, it is noteworthy that, as the pyrolysis temperature increases, the average overall reactivity difference between these two chars decreases. The possible reason might be that, at a high pyrolysis temperature, the structure of both OD and LY chars is highly ordered and

Figure 11. Averaged reactivity of the char samples produced at different pyrolysis conditions.

the difference in surface area and porosity between these two fuels is insignificant.

4. CONCLUSION The pyrolysis characteristics and effects of pyrolysis conditions on char reactivity of a freshwater macroalga Oedogonium and an Australian lignite were investigated to explore the potential of using wastewater-cultivated algae as an alternative solid fuel in gasification/co-gasification. The pyrolysis process of OD can be divided into three stages: moisture evaporation, volatile release, and decomposition of the remaining carbonaceous compounds. The pyrolysis of LY coal 5053


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(8) Migliore, G.; Alisi, C.; Sprocati, A. R.; Massi, E.; Ciccoli, R.; Lenzi, M.; Wang, A.; Cremisini, C. Biomass Bioenergy 2012, 42, 69−77. (9) Vivekanand, V.; Eijsink, V. G. H.; Horn, S. J. J. Appl. Phycol. 2012, 24, 1295−1301. (10) Ross, A. B.; Jones, J. M.; Kubacki, M. L.; Bridgeman, T. Bioresour. Technol. 2008, 99 (14), 6494−504. (11) McKendry, P. Bioresour. Technol. 2002, 83, 47−54. (12) McKendry, P. Bioresour. Technol. 2002, 83, 55−63. (13) Cetin, E.; Moghtaderi, B.; Gupta, R.; Wall, T. F. Fuel 2004, 83, 2139−2150. (14) Wang, S.; Jiang, X. M.; Wang, N.; Yu, L. J.; Li, Z.; He, P. M. Energy Fuels 2007, 21, 3723−3729. (15) Ross, A. B.; Anastasakis, K.; Kubacki, M.; Jones, J. M. J. Anal. Appl. Pyrolysis 2009, 85 (1−2), 3−10. (16) Anastasakis, K.; Ross, A. B.; Jones, J. M. Fuel 2011, 90 (2), 598− 607. (17) Li, D.; Chen, L.; Zhang, X.; Ye, N.; Xing, F. Biomass Bioenergy 2011, 35 (5), 1765−1772. (18) Wu, K.; Liu, J.; Wu, Y.; Chen, Y.; Li, Q.; Xiao, X.; Yang, M. Bioresour. Technol. 2014, 163, 18−25. (19) Bridgwater, A. V. Fuel 1995, 74, 631−653. (20) Cetin, E.; Gupta, R.; Moghtaderi, B. Fuel 2005, 84, 1328−1334. (21) Guerrero, M.; Ruiz, M. P.; Alzueta, M. U.; Bilbao, R.; Millera, A. J. Anal. Appl. Pyrolysis 2005, 74, 307−314. (22) Kirtania, K.; Joshua, J.; Kassim, M. A.; Bhattacharya, S. Fuel Process. Technol. 2014, 117, 44−52. (23) Sanchez-Silva, L.; Lopez-Gonzalez, D.; Garcia-Minguillan, A. M.; Valverde, J. L. Bioresour. Technol. 2013, 130, 321−31. (24) Lane, D. J.; Ashman, P. J.; Zevenhoven, M.; Hupa, M.; van Eyk, P. J.; de Nys, R.; Karlström, O.; Lewis, D. M. Energy Fuels 2014, 28, 41−51. (25) Lawton, R. J.; de Nys, R.; Paul, N. A. PLoS One 2013, 8 (5), e64168. (26) Neveux, N.; Magnusson, M.; Maschmeyer, T.; de Nys, R.; Paul, N. GCB Bioenergy 2015, 7 (4), 673−689. (27) Zeng, C.; Favas, G.; Wu, H.; Chaffee, A. L.; Hayashi, J.-i.; Li, C.Z. Energy Fuels 2006, 20, 281−286. (28) Zhu, Y.; Piotrowska, P.; van Eyk, P. J.; Boström, D.; Kwong, C. W.; Wang, D.; Cole, A. J.; de Nys, R.; Gentili, F. G.; Ashman, P. J. Energy Fuels 2015, 29 (3), 1686−1700. (29) Kwon, E. E.; Jeon, Y. J.; Yi, H. Bioresour. Technol. 2012, 123, 673−677. (30) Coats, A. W.; Redfern, J. P. Nature 1964, 201, 68−69. (31) Di Blasi, C. Prog. Energy Combust. Sci. 2009, 35, 121−140. (32) Guizani, C.; Escudero Sanz, F. J.; Salvador, S. Fuel 2013, 108, 812−823. (33) Peng, W.; Wu, Q.; Tu, P.; Zhao, N. Bioresour. Technol. 2001, 80 (1), 1−7. (34) Zhou, S.; Wu, Y.; Yang, M. Bioresour. Technol. 2010, 101 (1), 359−365. (35) Sanchez-Silva, L.; Lopez-Gonzalez, D.; Villasenor, J. Bioresour. Technol. 2012, 109, 163−172. (36) Jones, J. M.; Darvell, L. I.; Bridgeman, T. G. Proc. Combust. Inst. 2007, 31, 1955−1963. (37) Jones, J. M.; Kubacki, M.; Kubica, K.; Ross, A. B.; Williams, A. J. Anal. Appl. Pyrolysis 2005, 74, 502−511. (38) Williams, P. T.; Besler, S. Fuel 1993, 72, 151−159. (39) Williams, P. T.; Besler, S. Renewable Energy 1996, 7, 233−250. (40) Haykiri-Acma, H.; Yaman, S.; Kucukbayrak, S. Renewable Energy 2006, 31, 803−810. (41) Ounas, A.; Aboulkas, A.; El Harfi, K.; Bacaoui, A.; Yaacoubi, A. Bioresour. Technol. 2011, 102 (24), 11234−11238. (42) Guerrero, M.; Ruiz, M. P.; Millera, Á .; Alzueta, M. U.; Bilbao, R. Energy Fuels 2008, 22, 1275−1284. (43) Maiti, S.; Purakayastha, S.; Ghosh, B. Fuel 2007, 86, 1513−1518. (44) Lu, L.; Kong, C.; Sahajwalla, V.; Harris, D. Fuel 2002, 81, 1215− 1225. (45) Xu, S.; Zhou, Z.; Yu, G.; Wang, F. Energy Fuels 2010, 24, 1114− 1123.

presented the same phenomenon. However, the devolatilization of LY was slower and occurred over a wider temperature range compared to that of OD. As the heating rate increased, pyrolysis curves for both OD and LY shifted to higher temperatures and the characteristic parameters of Ti, Te, Tmax, and (dw/dt)max all increased. For both fuels, the elemental composition and surface morphology of the produced char varied with the pyrolysis conditions. SEM analysis showed that the char samples mostly keep their original morphology under low heating rates. However, under high heating rates, a swelling structure was observed. The pyrolysis conditions also had a significant influence on the char reactivity. For both fuels, the char reactivity increased with the heating rate and decreased with the pyrolysis temperature. Under the same pyrolysis conditions, the average overall reactivity, Rave, of OD char was 2−5 times that of LY char. However, the differences in Rave became less as the pyrolysis temperature increased. In conclusion, OD has a higher devolatilization rate and higher char reactivity than LY lignite. It implies that it is easier to be gasified, and the co-gasification with lignite is advantageous.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-371-6778-0113. E-mail: wangdb@zzu. edu.cn. *Telephone: +61-8-8313-5072. E-mail: peter.ashman@ adelaide.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Youjian Zhu is grateful for the financial support of the China Scholarship Council (CSC) and the Project of Excellent Scientist Fund in Henan (Project 12410051002). This research was support by Australian Research Council’s Linkage Projects Funding Scheme (Project LP100200616) with our industry partner SQC Pty Ltd and the Australian Government through the Australian Renewable Energy Agency (ARENA) as well as the Advanced Manufacturing Cooperative Research Centre (AMCRC), funded through the Australian Government’s Cooperative Research Centre Scheme. The authors also acknowledge the support of Muradel Pty Ltd and MBD Energy. Drs. David Roberts, Andrew Cole, and Marie Magnusson at James Cook University are acknowledged for the culture and supply of alga.

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REFERENCES

(1) Gao, K.; McKinley, K. R. J. Appl. Phycol. 1994, 6, 45−60. (2) Roesijadi, G.; Jones, S. B.; Snowden-Swan, L. J.; Zhu, Y. Macroalgae as a Biomass Feedstock: A Preliminary Analysis; Pacific Northwest National Laboratory: Richland, WA, 2010. (3) Wei, N.; Quarterman, J.; Jin, Y.-S. Trends Biotechnol. 2013, 31 (2), 70−77. (4) Adams, J. M.; Gallagher, J. A.; Donnison, I. S. J. Appl. Phycol. 2009, 21 (5), 569−574. (5) Huesemann, M. H.; Kuo, L.-J.; Urquhart, L.; Gill, G. A.; Roesijadi, G. Bioresour. Technol. 2012, 108, 305−309. (6) Filipkowska, A.; Lubecki, L.; Szymczak-Ż yła, M.; Kowalewska, G.; Ż bikowski, R.; Szefer, P. Oceanologia 2008, 50 (2), 255−273. (7) Costa, J. C.; Gonçalves, P. R.; Nobre, A.; Alves, M. M. Bioresour. Technol. 2012, 114, 320−326. 5054


Article

Energy & Fuels (46) Meng, F.; Yu, J.; Tahmasebi, A.; Han, Y.; Zhao, H.; Lucas, J.; Wall, T. Energy Fuels 2014, 28, 275−284. (47) Min, F.; Zhang, M.; Zhang, Y. J. Anal. Appl. Pyrolysis 2011, 92, 250−257. (48) Moilanen, A. Thermogravimetric characterisations of biomass and waste for gasification processes. Academic Dissertation, Åbo Akademi University, Åbo, Finland, 2006. (49) Ollero, P.; Serrera, A.; Arjona, R.; Alcantarilla, S. Biomass Bioenergy 2003, 24, 151−161. (50) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221−270. (51) Lu, L.; Sahajwalla, V.; Harris, D. Metall. Mater. Trans. B 2001, 32 (5), 811−820. (52) Chitsora, C. T.; Muhlen, H.-J.; van Heek, K. H.; Juntgen, H. Fuel Process. Technol. 1987, 15, 17−29. (53) Cai, H.-Y.; Guell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 15−24. (54) Mermoud, F.; Salvador, S.; Van de Steene, L.; Golfier, F. Fuel 2006, 85, 1473−1482. (55) Burhenne, L.; Damiani, M.; Aicher, T. Fuel 2013, 107, 836−847. (56) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23−30. (57) Zhu, W.; Song, W.; Lin, W. Fuel Process. Technol. 2008, 89, 890−896.

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Pyrolysis Characteristics and Char Reactivity of Oedogonium sp. and

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