Characterization of Biochars Derived from Pyrolysis of Biomass and

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Characterization of biochars derived from pyrolysis of biomass and calcium oxide mixtures Frances Wilson, Priscilla Tremain, and Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03221 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Characterization of biochars derived from pyrolysis of biomass and calcium oxide mixtures Frances Wilson, Priscilla Tremain, Behdad Moghtaderi* Priority Research Centre for Frontier Energy Technologies and Utilization, Department of Chemical Engineering, The University of Newcastle, Callaghan, Australia Keywords: biochar, lime, pyrolysis, biomass Abstract This study forms the fundamental foundation for the development of a novel carbon arrestor process to produce a functionalized biochar. In this study, an experimental investigation was carried out on the production and characterization of biochar produced utilizing a novel carbon arrestor process which operates under the principle of in-situ pyrolysis of biomass with lime (CaO) to produce a functionalized biochar product for use as an agronomic soil amendment. Two biomass sources were utilized, a woody biomass, eucalyptus pilularis (or blackbutt) sawdust and a herbaceous biomass, wheat stem. Characterization of the biochars produced as well as the gaseous products was completed via thermogravimetric analysis (TGA), micro-gas chromatography (GC), solid state Fourier transform infrared (FTIR) spectroscopy, nitrogen adsorption and scanning electron microscopy (SEM). The addition of CaO to the pyrolysis process resulted in a significant reduction in CO2 evolution via the carbonation reaction of CaO, and the formation of H2 and CH4 at lower temperatures in significantly higher quantities in

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comparison to biomass pyrolysis alone. This was achieved via carbonation and hydration reactions of CaO with pyrolysis gases and catalytic effects of CaO. Biochar produced with no CaO from blackbutt had the highest surface area (201 m2/g), while the wheat stem was considerably lower (6.7 m2/g), and both had morphology resembling the parent biomass. The addition of CaO resulted in a drop in surface area (37 m2/g) for blackbutt biochars, with wheat stem biochar presenting similar surface areas to the respective blackbutt biochars. Greater swelling of char particles, a significant reduction in particle size and considerable fracturing of the CaO particles was evident. Biochars produced with the addition of CaO resulted in a reduction in oxygenated functional groups on the surface, determined via FTIR and to a lesser extent, elemental analysis, which may be beneficial for char stability and longevity in soil.

1. Introduction World population growth, resource shortages and global warming have increased the need for cleaner energy production, more effective waste management, minimization of greenhouse gas (GHG) emissions and increased food production efficiency. Food security and hence access to environmentally sustainable food resources is one of the key global challenges in the 21st century. In this context and from an agronomic perspective, the role of soil amendments (e.g. fertilizers) for enhanced food production cannot be overstated. Soil amendments are defined as any substance applied to soil with the intent of improving physical and chemical properties to ultimately improve crop yields. The application of amendments to soil is a widespread practice in modern agriculture due to increased food demand from the growing population and decreased land productivity from overuse. Therefore, the positive effects of amendments on both plant and soil are of extreme importance in achieving sustainable food production. One such soil ameliorant is biochar; a carbon rich, organic substance derived from the pyrolysis of biomass. Biochar has been increasingly studied of late due to its carbon sequestration potential [1-3] and ACS Paragon Plus Environment

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its ability to improve crop yield [4] and improve soil health by improving nutrient and moisture retention particularly in coarsely textured soils [5-8]. However, there has been a general lack of biochar use in agronomic practice. This stems from a number of key bottlenecks: the energy intensive and expensive production process, availability of biomass feedstocks to produce biochar and the costs related to applying biochar to soil. Hence, a step change in the biochar field is necessary to facilitate the sustainable production and application of biochar. A potential solution is the functionalization of biochar by optimizing the biochar production process. In the biomass gasification field, carbon capture and catalytic tar cracking has been reported in the literature by harnessing the carbonation/calcination cycles of alkali metal and alkali earth metal oxides/carbonates [9, 10]. The CO2 capture mechanism involves the carbonation of the metal oxide, which is exothermic: + → ℎ This process is reversible, with calcination of the metal carbonate resulting in a pure CO2 stream when the metal oxide is reformed, which is endothermic: → + ℎ These mechanisms can be used in the production of biochar. Prior to pyrolysis, the metal carbonate requires calcining to its oxide form. The metal oxide can then be utilized in-situ to the pyrolysis process to capture carbon dioxide release during the devolatilization of biomass, which occurs at temperatures of 300-500 ° C, to form the metal carbonate, and simultaneously release heat. Calcium carbonate (CaCO3) has been identified as a suitable CO2 capture material for biomass pyrolysis processes up to 500°C, as it retains its carbonate at these temperatures [9, 10].


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Hence in this study, a novel carbon arrestor process is proposed, utilizing lime (CaO) in-situ with the pyrolysis of biomass to produce a functionalized biochar product for use as an agronomic soil amendment. Lime was chosen as it may capture carbon dioxide during biochar production[11-18]; have catalytic properties to produce cleaner gaseous products of higher calorific value [11, 12, 18-20], that may be used to improve the energy efficiency of biochar production; and offer both fertilizing and soil acidity management properties to the final functionalized biochar product. That is, a single application product for farmers, as opposed to multiple applications of different soil amendments i.e. liming agents and biochar at separate times. Previous studies have investigated slow pyrolysis of biomass and CaO for the purpose of improving understanding of gasification or fast pyrolysis processes [11, 15, 16, 20-22]. However, there is a lack of systematic characterization of biochar derived from pyrolysis of biomass and CaO mixtures, for example Veses et al. [12] reported higher oxygen content and lower carbon in CaO catalyzed woody based biochar; Florin & Harris [20] reported particle swelling as a result of cellulose pyrolysis in the presence of CaO; and Perander et al. [15] reported char showing the formation of CaCO3 and K2CO3 on surfaces in a study of CO2 gasification of spruce wood with Ca and K catalyst addition. In this study, the pyrolysis of a woody and herbaceous biomass was investigated at different biomass to CaO ratios for in-situ pyrolysis. The effect of increasing CaO addition on the characteristics of the functionalized biochar product was assessed by a comprehensive characterization study, in addition to the biochar production process being studied in detail. 2. Materials and Methods 2.1. Feedstock


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The feedstocks utilized for biochar production were wheat stem, being an herbaceous crop residue, and sawdust of eucalyptus pilularis, commonly known as blackbutt. Proximate and micro-elemental analysis of the biomass is presented in Table 1. The biomass was sieved to 75150 µm. Table 1. Proximate and ultimate analysis of biomass samples.

Sample Blackbutt Wheat stem

Proximate (dry basis) Moist. Vol. FC Ash (%) (%) (%) (%) 5.3 79.5 18.9 1.6 2.4 71.4 17.7 10.9

C (%) 48.9 47.4

H (%) 6.6 6.7

Ultimate N S (%) (%) 0 0.3 0.2 0.2

O* (%) 44.2 45.5

* O by difference

During the pyrolysis process, calcium oxide (CaO) was combined with biomass in different mass ratios. The CaO was sieved to 75-150 µm and was prepared via the calcination of limestone (CaCO3). The calcination procedure entailed a heating rate of 20 °C/min to 800 °C and holding for 30 minutes under nitrogen flow of 60 mL/min. XRF analysis of the CaO is presented in Table 2. Table 2. XRF analysis of calcium oxide on an oxygen free basis.

Ca 97.56

Elemental Composition (w.t.%) Fe Mg Al Si Mn 0.23 0.43 0.15 1.21 0.38

K 0.04

2.2. Biochar production methods A TA Instruments Q50 thermogravimetric analyzer (TGA) was used to analyze the samples mass loss over the biochar production process. In the TGA setup, biochar was produced at heating rates of 5, 10 and 20°C/min and biomass to CaO ratios of 1:0, 4:1, 2:1, 1:1 and 1:2. For all TGA experiments, 5 mg of biomass, a peak heating temperature of 650°C, 10 minute hold


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time and nitrogen sweep gas flow of 100mL/min was used. A 5 mg sample size was used, so that only a thin layer of particles were present on the TGA crucible to ensure uniform heat distribution through the sample and uniform flow of the nitrogen carrier gas over the sample. To quantify diatomic gases, as well as other light hydrocarbons and gases, biochar was produced in larger quantities using a horizontal furnace coupled with a micro-gas chromatograph (microGC). Biomass to CaO ratios of 1:0, 4:1, 2:1, 1:1 and 1:2 were examined using 2.5 g of blackbutt and 1.0 g of wheat stem and the respective amount of CaO, which were physically mixed before being loaded into a quartz tube reactor (I.D. 15 mm). The temperature-heating program entailed a heating rate of 10°C/min to the peak heating temperature of 600°C before holding for 30 minutes under a nitrogen flow of 100 mL/min. Gases from pyrolysis process were passed through a condenser before entering an Agilent 490 Micro-GC, which was equipped with two columns (molecular sieve (MS)-5A and Porapak Q) and two thermal conductivity detectors (TCD). 2.3. Biochar characterization methods Proximate analysis was conducted via TGA using the method described by Garcia et al. [23]. Elemental analysis was carried out via a PerkinElmer PE2400 CHNS/O elemental analyzer and a PerkinElmer AD-6 ultra-microbalance at the Chemical Analysis Facility at Macquarie University, Australia. For the 1:0 and 1:1 biochars, a series of intermediary chars were produced at peak heating temperatures of 300 to 650 °C in 50°C increments. These samples were analyzed using solid state FTIR analysis on a PerkinElmer Spectrum Two spectrometer, with a universal ATR sampling accessory and a scan range of 400 – 4000 cm-1. Biochar specific surface area was determined by nitrogen adsorption (77 K) employing BET methods via a Micrometrics Gemini 2375 and VacPrep 061 system. Morphologic and elemental characterization of the raw biomass and biochars was carried out via scanning electron microscopy (SEM) on a Zeiss Sigma VP ACS Paragon Plus Environment

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FESEM with a Bruker light element silicon drift detector (SSD) energy-dispersive x-ray spectroscopy (EDS) detector located at the EMXRAY unit at the University of Newcastle. 3. Results and Discussion 3.1. Biochar production analysis A series of experiments were conducted using TGA. The derivative thermogravimetric (DTG) curves for the pyrolysis of biomass with CaO are presented in Figure 1 and Figure 2. Figure 1 illustrates pyrolysis with and without the addition of CaO, for both blackbutt and wheat stem, at heating rates of 5, 10 and 20°C/min to a peak heating temperature of 650°C. For both pure biomass pyrolysis and that with CaO added, a lower heating rate resulted in a lower peak devolatilization temperature. At temperatures above 120°C thermal decomposition begins with removal of chemically bound moisture; from 200-260°C hemicellulose decomposition occurs, 240-350°C cellulose decomposition and from 280-500°C lignin [24, 25]. Pyrolysis of blackbutt biomass (Figure 1A and 1B) resulted in two devolatilization stages; 200-288°C and 288-338°C for a heating rate of 10°C/min. These stages are characteristic of hemicellulose and cellulose decomposition respectively. Wheat stem pyrolysis resulted in only one obvious devolatilization stage of 200313°C, but due to its broad temperature range is indicative of a combination of hemicellulose, cellulose and lignin decomposition. The overlap of the typically distinct constituent peaks was attributed to the similar ratio of hemicellulose to cellulose in the wheat stem sample which was in the order of 30 wt% for both components [26]. In addition, the inherently different sugar composition of hemicellulose between herbaceous and woody biomass may result in slight variations in stability, leading to differences in decomposition temperature. Pyrolysis with CaO and biomass, similarly had two initial devolatilization stages for blackbutt and one for wheat


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stem, however an additional stage was apparent from 375-405°C and 366-405°C for blackbutt and wheat stem, respectively. This additional stage was attributed to tar cracking reactions between the CaO and liquid intermediate tars formed during pyrolysis to produce lighter gases, and decomposition of Ca(OH)2 [27-29]. Increasing the heating rate resulted in a peak shift to higher temperatures for this additional devolatilization stage.

1.4

1.4

A

1

oC/min 2020C/min

1

oC/min 1010C/min

0.8

B

1.2

oC/min 5 5C/min

0.6 0.4

DTG (%/°C)

DTG (%/°C)

1.2

0.8 0.6 0.4 0.2

0.2

0

0

100 200 300 400 500 600

100 200 300 400 500 600 Temperature (°C)

Temperature (°C)

1.4

1.4

C

1.2

oC/min 2020C/min

1

oC/min 1010C/min

0.8 oC/min 5 5C/min

0.6 0.4

D

1.2 DTG (%/°C)

DTG (%/°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 0.8 0.6 0.4 0.2

0.2

0

0 100

200 300 400 500 Temperature (°C)

600

100 200 300 400 500 600 Temperature (°C)

Figure 1. Differential thermogravimetric (DTG) curves at three heating rates. (A) blackbutt 1:0 biomass to CaO ratio; (B) blackbutt 1:1 biomass to CaO ratio; (C) wheat stem 1:0 biomass to CaO ratio; (D) wheat stem 1:1 biomass to CaO ratio. Figure 2 presents the effect of biomass to CaO ratio on the thermogravimetric curve. As the amount of CaO added to the pyrolysis process increased, a slight shift to higher temperatures was


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observed for the peak devolatilization temperature for both blackbutt and wheat stem biochar. Further, less mass loss was observed in the initial stage (wheat stem) or two stages (blackbutt) of devolatilization for higher CaO ratios, likely due to carbonation and hydration of CaO. However significantly more mass loss was observed in the final devolatilization stage likely due to a combination of the decomposition of Ca(OH)2 as suggested by Florin et al. [30] and catalysis of secondary tars formed. From the TGA studies, a heating rate of 10°C/min was seen as optimum and has been used for all other analyses, while the optimum peak heating temperature was 500°C to allow for complete devolatilization, completion of secondary reactions and porosity development [31]. 1.4

1.4

A

1 0.8

1:0 4:1 2:1 1:1 1:2

1.2 DTG (%/°C)

1.2 DTG (%/°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2

B

1

1:0 4:1 2:1 1:1 1:2

0.8 0.6 0.4 0.2

0

0 100

200 300 400 500 Temperature (°C)

600

100

200 300 400 500 Temperature (°C)

600

Figure 2. Differential thermogravimetry (DTG) at five biomass to CaO ratios and a heating rate of 10°C/min for; (A) blackbutt and (B) wheat stem. Gas evolution from pyrolysis at different biomass to CaO ratios was analyzed using a micro GC. The quantitative evolution of H2, CH4 and CO2 for both blackbutt and wheat stem pyrolysis in terms of volume percent per gram of biomass over five biomass to CaO ratios are illustrated in Figure 3. For biomass pyrolysis without the addition of CaO (1:0), there is no H2 production until 450°C for both blackbutt and wheat stem. The addition of CaO, across all ratios, resulted in H2 release at a significantly lower temperature of 250°C and 300°C for blackbutt and


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wheat stem respectively and a simultaneous increase in H2 production occurred in comparison to other treatments as the CaO ratio increased. The CO2 released during pyrolysis followed a decreasing trend for increasing CaO ratio. The evolution of CH4, generally increased as the CaO ratio increased, however, this was not a consistent pattern. A number of reactions may be attributed to the gas evolution patterns with CaO addition. The CaO reacts in a carbonation reaction to form CaCO3 at temperatures of 300-600°C, effectively capturing CO2 evolved during pyrolysis. Hence as the amount of CaO in the pyrolysis process is increased, there is more capacity for CO2 to be captured, resulting in lower CO2 concentrations in the product stream. Carbonation of CaO will preferentially occur over the hydration of CaO to Ca(OH)2 according to thermodynamic theory [20], meaning in the absence of CO2 evolution, water vapor will hydrate the CaO particles to form Ca(OH)2. Blackbutt pyrolysis with CaO consistently presented a carbon capture effect across all biomass to CaO ratios, fully capturing all CO2 under the 1:2 ratio. In contrast, wheat stem and CaO biochars, at the ratios tested, consistently increased carbon capture across all biomass to CaO ratios above 450°C and did not reach a point where all of the CO2 evolved was fully captured. A second reaction, in addition to CO2 capture, is apparent in Figure 3. This is highlighted by the two-staged evolution of H2 and CH4 for CaO based biochar. The first stage corresponds to the initial stage of decomposition of biomass from 220°Cand 250°C for blackbutt and wheat stem respectively. In this stage, methoxyl–O–CH3, carboxyl (C=O) and carbonyl (C-O-C) functional groups present in hemicellulose and cellulose undergo cracking to form CH4, CO2 and CO [32]. The presence of CaO may act as a catalyst for the cracking of the aforementioned functional groups [11]. The second stage of evolution commenced from 380°C for CH4 and from 380-480°C for H2, with higher molecular weight gases and vapors, i.e. C2 and higher


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hydrocarbons, also evolved throughout the pyrolysis process. The presence of CaO in the pyrolysis process acts as a catalyst to catalytically crack the higher molecular weight pyrolysis products, resulting in the evolution lighter molecular weight gases such as H2 and CH4. The second stage may also be thermodynamically driven, resulting in H2 and CH4 production with the removal of CO2 from the pyrolysis atmosphere, driving the water gas shift reaction to H2 production [11, 20]. A fourth factor driving the first and second reaction stage is the potential impact of the increase in thermal energy produced in pyrolysis with CaO in situ with the exothermic carbonation reaction. This may partially explain further cracking of higher molecular weight hydrocarbon vapors and gases across all temperature ranges. Although blackbutt and wheat stem have a similar elemental composition in terms of C, H, O, N and S, there are two differentiating compositional characteristics of blackbutt and wheat stem biomass. Wheat stem possesses a higher level of minerals, evident in an ash level of 10.9%, compared to blackbutt of 1.6% (Table 1); and blackbutt has a distinct hemicellulose component, evidenced in the DTG curves (Figure 1 and Figure 2), while wheat stem did not. In explanation of the difference between gas evolution for wheat stem and blackbutt pyrolysis presented in Figure 3, variations in the hemicellulose, cellulose and lignin contents of the two biomass samples results in different distributions of higher molecular weight hydrocarbons being formed during pyrolysis. Hence, the catalytic performance of CaO in the pyrolysis process results in a different product distribution of non-condensable gases including CH4 and H2 [33]. Further, as both biomasses have similar C, H and O compositions (Table 1), the higher volume of H2, CH4 and CO2 for blackbutt may be attributed to the lower mineral content of blackbutt at 1.6% compared to 10.9% for wheat stem (Table 1).


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Blackbutt

Wheat stem 7 1:0 4:1 2:1 1:1 1:2

H2

6 5 4

Gas Composition (vol% / g biomass)


7

3 2 1 0

5 4 3 2 1

250

350

450

550

650

150

250

Temperature (°C)

350

450

550

650

Temperature (°C)

4

4

CH4



1:0 4:1 2:1 1:1 1:2

H2

6

0 150

3

2

1

CH4 3

2

1

0

0 150

250

350

450

550

150

650

250

350

450

550

650

Temperature (°C)

Temperature (°C) 4

4

CO2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

2

1

0

CO2 3

2

1

0 150

250

350

450

550

Temperature (°C)

650

150

250

350

450

550

650

Temperature (°C)

Figure 3. Quantitative product gas evolution of H2, CH4 and CO2 via micro-GC for biochar production at five different biomass to CaO ratios. Blackbutt presented on the left and wheat stem on the right.


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3.2.Biochar characterization 3.2.1. Proximate and Elemental Analysis According to the thermogravimetric and gas analysis, a peak heating temperature of 500°C was found to be optimal. A series of chemical and physical characterization studies were conducted on the biochars produced at a peak heating temperature of 500°C. Table 3 presents the proximate and elemental analysis for biomass and biochars as well as the normalized yield of biochar at a peak heating temperature of 500°C. The calculation method for the normalized yield is presented in Equation 1. The normalized yield was calculated in order to attain a better representation of the amount of char in the samples and was determined by removing the amount of calcium oxide added to the pyrolysis process from the yield calculation. % =

!"#$ % &

(1)

where mf is the final mass of char and calcium at the completion of pyrolysis, mb is the initial mass of biomass and mCaO is the initial mass of calcium oxide added to the pyrolysis process. The proximate analysis in Table 3 illustrates that pyrolysis of biomass removed volatile matter and increased the fixed carbon content in the char. As the amount of calcium added to the pyrolysis process increased (i.e. from 4:1 to 1:1 biomass to CaO ratios) a simultaneous increase in ash content occurred. The normalized yields were similar for the 1:0, 4:1 and 2:1 blackbutt chars, however a lower yield was observed for the 1:1 biochar. In contrast, normalized yields for wheat stem presented a general downward shift in yields from 31.5% to 25%, except for the 1:1 char, which presented higher at 33.8%. Elemental analysis revealed for both biomass types, 4:1 biochar had the greatest carbon content on a dry ash free basis, while 1:2 biochar had the lowest. The H/C atomic ratio decreased


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greatly when biochar was produced in comparison to raw biomass. This was attributed to the loss of moisture and aliphatic compounds. Table 3. Proximate and ultimate analysis and yields of biomass and biochars produced in this study. Blackbutt

Wheat stem

Biomass

1:0

4:1

2:1

1:1

1:2

Biomass

1:0

4:1

2:1

1:1

1:2

Moist. (a.d.%)

5.3

4.1

3.8

2.3

1.8

0.7

2.4

4.9

1.1

1.0

0.3

0.2

Vol. (d.b.%)

79.5

17.8

30.2

31.2

29.8

26.9

71.4

12.1

25.4

25.0

21.6

16.5

FC (d.b.%)

18.9

81.9

36.3

17.9

10.4

5.8

17.7

62.9

22.6

18.6

10.2

6.4

Ash (d.b.%)

1.6

0.4

33.5

50.8

59.7

67.3

10.9

24.9

52.0

56.4

68.1

77.1

C (d.a.f.%) H (d.a.f.%) N (d.a.f.%) S (d.a.f.%) O (d.a.f.%)* H/C# O/C#

48.9

83.2

77.1

62.9

52.0

39.9

47.4

89.0

71.7

54.9

63.3

51.8

6.6

3.1

3.4

3.2

2.6

1.8

6.7

3.3

3.2

3.4

4.3

4.1

0.0

0.1

0.1

0.0

0.0

0.0

0.2

0.4

0.3

0.1

0.3

0.0

0.3

0

0.3

0.1

0.1

0.0

0.2

0.0

0.0

0.0

0.0

0.0

44.2

13.6

19.1

33.7

45.3

58.3

45.4

7.3

24.9

41.5

32.1

44.1

1.6

0.4

0.5

0.6

0.6

0.5

1.7

0.4

0.5

0.7

0.8

0.9

0.7

0.1

0.2

0.4

0.7

1.1

0.7

0.1

0.3

0.6

0.4

0.6

-

18.1

17.0

17.4

14.4

17.7

-

30.0

31.5

28.6

33.8

25.0

Normalized Yield (%)

a.d. – air dried; d.b. - dry basis; d.a.f. – dry ash free basis; * - oxygen by difference; # - molar basis

For both blackbutt and wheat stem biochar, the O/C atomic ratio for 4:1 increased from an already low 1:0 char value and steadily increased with increasing CaO addition. The increase in O/C ratio at high CaO levels was attributed to a greater amount of CO2 being captured by the CaO in the pyrolysis process which was then released during the elemental testing procedure. It is proposed that the addition of CaO to the pyrolysis process significantly increased the release of oxygenated compounds for blackbutt pyrolysis, evidenced by the low O/C ratio for 4:1 blackbutt biochar, however as the amount of CaO present in the pyrolysis process increased, so too did the amount of CO2 that was able to be captured, skewing the O/C ratios for high CaO contents.


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Wheat stem biochar had a similar elemental C content to blackbutt, at 89.0% compared to 83.2%. Biochar produced from wheat stem in the absence of CaO was characterized by lower O/C ratios than that of blackbutt-based biochar. This difference may be explained through the differences in composition of the two biomasses, namely the difference in hemicellulose and cellulose content which for wheat stem is in the order of 30 wt% and 30wt%[26] respectively and for blackbutt is in the order of 15 wt% and 55wt%[34] respectively. Wheat stem 1:0 char had a considerably low O/C ratio at 0.1 compared to 0.7 for raw biomass, whereas the H/C ratio dropped from 1.7 to 0.4. The increase in O/C ratio as CaO is added to the pyrolysis process is because of a lower C content in the product char, illustrated through the decline from 89% C for 1:0 char to 71.7% for 4:1 char, and a corresponding increase in O. This contrasts with the notable initial increase in C content of the blackbutt chars when CaO is added. At temperatures below 350°C, the lack of CO2 production in wheat stem pyrolysis would result in hydration of CaO to Ca(OH)2 whereas for blackbutt, over the same temperatures, carbonation and CaCO3 formation would have occurred. These reactions may explain the initial jump in O/C atomic ratio for the pyrolysis of wheat stem upon addition of CaO. Similarly, this may explain the increasing H/C ratio for wheat stem pyrolysis with incremental increases in CaO. 3.2.2. Nitrogen Adsorption BET surface areas are presented in Table 4. A maximum surface area of 201 m2/g was observed for 1:0 blackbutt biochar (i.e. no CaO addition) and as the CaO increased, a drop in the surface area was observed. The decline in surface area was partially due to the low surface area of the CaO added and CaCO3 (0.4 m2/g) formed in the pyrolysis process. The surface area of wheat stem derived biochars followed a dissimilar trend. For 1:0 wheat stem biochar, a low surface area of 6.7 m2/g was found in comparison to the values obtained for


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biochars with CaO addition at 24.5 - 33.9 m2/g. The relatively low surface area reading for the 1:0 biochar may suggest micro-porosity of the sample such that the pores were too small for effective nitrogen adsorption, re-solidification of tars on the char surface blocking pores or melting on the surface of the char eliminating porosity. However, the higher surface area obtained with CaO addition suggests a change in the physical properties of the char. This indicates a change in pore size or pore size distribution through the addition of CaO in the pyrolysis process. The change may be attributed to CaO catalyzing the pyrolysis reaction cracking of heavy tars and reducing pore blockage in addition the changing the pore size distribution. Table 4. BET surface area of biomass, CaCO3 and biochars produced in this study. Blackbutt BET SA (m2/g)

CaCO3 0.4

Wheat stem

1:0

4:1

2:1

1:0

1:2

1:0

4:1

2:1

1:1

1:2

201

78.1

37.9

37.5

22.3

6.7

24.5

33.9

31.2

32.9

3.2.3. SEM SEM micrographs are presented in Figure 4 for the feedstock materials and the different biochars produced at magnification of 200x. Figure 4A and 4F are images of raw biomass, blackbutt and wheat stem respectively. Figure 4A, 4B, 4F, and 4G show the initial particle size of the raw feedstocks and 1:0 biochars were relatively similar as they were sieved to 75-150 µm. For the biochars produced without CaO addition (1:0), Figure 4B and 4G, the char particle size remained similar to the parent material. Figure 4 demonstrates the progressive effect of increasing the presence of CaO in pyrolysis on the final biochar product. It is apparent for both blackbutt and wheat stem biochars that char size reduces as the amount CaO/CaCO3/Ca(OH)2 present in pyrolysis increases. This is apparent with a progressive domination in the images of larger particle sized CaO/CaCO3/Ca(OH)2. The reduction in biochar particle size was coupled with a reduction in normalized yield for wheat stem, but not so for blackbutt, where yields


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Energy & Fuels

remained relatively constant. This suggests the occurrence of fragmentation of the biochar particles as opposed to increased thermal decomposition. The influx of thermal energy upon carbonation of the CaO, potentially affecting heating rates in the 300-400°C range, may be one contributing factor for this effect.


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A

F

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Figure 4. SEM micrographs of blackbutt and wheat stem biomass at 200x magnification: (A) raw blackbutt; (B) blackbutt biochar 1:0 biomass to CaO ratio; (C) 4:1 ratio; (D) 1:1 ratio; (E) 1:2 ratio; (F) raw wheat stem; (G) wheat stem biochar 1:0 biomass to CaO ratio; (H) 4:1 ratio; (I) 1:1 ratio; (J) 1:2 ratio. ACS Paragon Plus Environment

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The morphology of the calcium minerals and the effect of pyrolysis on these are demonstrated in the SEM images in Figure 5. Non-pyrolyzed CaCO3 in Figure 5A is characterized by smooth sharp surfaces. Cracking and fragmentation of the CaO/CaCO3 occurs with pyrolysis, as demonstrated for both blackbutt and wheat stem pyrolysis at a biomass to CaO ratio of 1:1, in Figure 5C and 5E respectively. This was attributed to surface structures swelling and fracturing upon calcination to allow the release of CO2 from the CaCO3 and the formation of larger sized CaCO3/Ca(OH)2 upon carbonation and hydration during pyrolysis. The formation of CaCO3 and Ca(OH)2 from CaO represents a higher molar volume resulting in swelling and cracking of the CaO [30]. A

B

C

D

E

F

Figure 5. SEM micrographs at 2000x and 5000x magnification respectively of: (A) & (B) CaCO3; (C) & (D) CaCO3/Ca(OH)2 in biochar of 1:1 blackbutt to CaO ratio; and (E) & (F) CaCO3/Ca(OH)2 in biochar of 1:1 wheat stem to CaO ratio. ACS Paragon Plus Environment

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SEM micrographs for both blackbutt and wheat stem are presented in Figure 6 and Figure 7 respectively. Magnifications of 800x and 5000x allow examination of the morphologies of the raw biomass and biochars produced at varying biomass to CaO ratios. For the biochar produced without CaO addition (1:0), Figure 6B and Figure 7B, the char particle size remained similar to the parent material as did the morphology of the char produced. There was a degree of swelling and melting evident on the surface of the char particles. With the introduction of CaO to the biochar production process, a significant reduction in both char and calcium particle size was evident. There were observable mineral particles on the surface of the char particles which were approximately 1-5 µm in size, evidenced by their lighter hue under from the back scattered electrons (BSE) detector on the SEM. There was also a significant amount of cracking evident for the calcium rich particles, attributable to particle swelling as a result of carbonation and hydration reactions, as discussed earlier. There also appeared to be a greater amount of char particle swelling and melting for the CaO treatments, this may result in the closure of pores and coincides with the decrease in surface area with increasing CaO ratio. From the magnifications presented it is not possible to determine the pore size of the biochars or confirm the presence and to confirm the prevalence of micro-porosity in the 1:0 wheat stem biochar as previously discussed in relation to the measured surface area.


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Energy & Fuels

A

F

B

G

C

H

D

I

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J

Figure 6. SEM micrographs of blackbutt biomass and biochars at 800x and 5000x magnification respectively for; (A) & (F) biomass; (B) & (G) biochar 1:0 biomass to CaO ratio; (C) & (H) biochar 4:1 biomass to CaO ratio; (D) & (I) biochar 1:1 biomass to CaO ratio; and (E) & (J) biochar 1:2 biomass to CaO ratio. ACS Paragon Plus Environment

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A

F

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Figure 7. SEM micrographs of wheat stem biomass and biochars at 800xand 5000x magnification respectively for; (A) & (F) biomass; (B) & (G) biochar 1:0 biomass to CaO ratio; (C) & (H) biochar 4:1 biomass to CaO ratio; (D) & (I) biochar 1:1 biomass to CaO ratio; and (E) & (J) biochar 1:2 biomass to CaO ratio. ACS Paragon Plus Environment

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Energy & Fuels

3.2.4. Solid State FTIR Figure 8 and Figure 9 present solid-state FTIR spectra for 1:0 and 1:1 blackbutt biochars, respectively, produced at peak heating temperatures of 300 to 650°C. For the 1:0 biochar (Figure 8), as the final pyrolysis temperature increased, a significant change in the functional group composition was observed. In the raw biomass spectrum, the broad peak from 3100 – 3400 cm-1 was assigned to OH stretching. For all the biochars produced at 300 to 650°C, the OH stretch peak is non-existent due to the removal of residual water in the pyrolysis process. The peak at approximately 1700 cm-1 was assigned to carboxyl groups. The carboxyl group reached a maximum at 400°C, but is no longer visible in 1:0 biochar produced at 650°C, suggesting volatilization. Aromatic C=C groups were assigned to the 1600 cm-1 peak. This peak was persistent in all spectra, reaching a maximum at 400°C and decreasing afterwards, with a slight peak shift in the 600°C and 650°C spectra. C-O stretching and C-O-C asymmetric stretching were assigned to 1035 cm-1 and 1165 cm-1 respectively. The C-O stretch peak was clearly discernible in the raw biomass and 300°C char however disappeared at higher temperatures, likely due to devolatilization of oxygenated functional groups which is beneficial for char stability and longevity upon application to soil [35]. The same was apparent for the C-O-C asymmetric stretching peak.


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Figure 8. Solid state FTIR for blackbutt biochar with 1:0 biomass to CaO ratio. Solid state FTIR was also completed on 1:1 biochar as presented in Figure 9, however it was impossible to determine the organic functional group composition as the spectra were dominated by CaO and CaCO3 peaks. The peaks at 730, 881 and 1446 cm-1 were attributed to CaCO3 [36], while the peak at 3659 cm-1 was attributed to CaO [37]. From Figure 9, a clear change in the CaCO3 peak intensity can be seen between 400 and 600°C. The CaCO3 peak increased to a maximum at 500°C, where the maximum amount of CO2 capture occurred, before reducing again as the temperature reached 600°C as CO2 starts to be driven off. This was confirmed by the CaO peak intensity increasing to its maximum at 600°C.


24

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Energy & Fuels

Figure 9. Solid state FTIR for blackbutt biochar with 1:1 biomass to CaO ratio. Figure 10 presents the solid-state FTIR spectra for 1:0 wheat stem biochars, produced at peak heating temperatures of 300 to 650°C. As for the 1:0 blackbutt biochar (Figure 8), as the final pyrolysis temperature increased, a significant change in the functional group composition was observed. Aromatic C=C groups were assigned to the 1600 cm-1 peak. This peak reached a maximum at 400°C with a slight peak shift in the 550°C and 600°C spectra. C-O stretching was assigned to 1035 cm-1. The C-O stretch peak being present in the raw biomass developed into a strong peak, being a notable functional group in the 500°C biochar, in contrast to the blackbutt biochar, the presence of which may reduce the stability of the biochar and its longevity in soil [35].


25

Energy & Fuels

300 350

Transmittance (arbitrary units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

400 450 500 550 600 650

4000

3500

3000 2500 2000 1500 Wavenumber (cm-1)

1000

500

Figure 10. Solid state FTIR for wheat stem biochar with 1:0 biomass to CaO ratio. The 1:1 wheat stem to CaO biochar FTIR spectra is depicted in Figure 11, and as per the blackbutt spectra, is dominated by the calcium based spectra, making it difficult to determine other features of the char. However, C-O functional groups at 1035 cm-1 are presented as a peak, suggesting oxygenated C-O groups remain present in wheat stem char with CaO present. With the addition of CaO however, this peak reaches a maximum at 450°C, reducing for subsequent higher temperature biochars as CaCO3 formation intensifies. Hence, the pyrolysis of wheat stem to 500°C, in the presence of CaO, reduces C-O oxygenated functional groups in the product biochar, potentially improving the stability of the char and its longevity when applied to soil.


26

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300

Transmittance (arbitrary units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

350 400 450 500 550 600 650

4000

3500

3000 2500 2000 1500 Wavenumber (cm-1)

1000

500

Figure 11. Solid state FTIR for wheat stem biochar with 1:1 biomass to CaO ratio.

3.3.Potential applications In this study, a novel biochar, rich in calcium, has been produced and characterized by a carbon arrestor process. There are significant advantages to producing a char with CaO in-situ. Lime (CaO) is already utilized broadly in agriculture for pH management of soils. By producing a biochar that is rich in lime, application in real world agricultural scenarios is lower cost and risk as the consumer is still applying a product that is already utilized. However, they can also reap the added benefit of the lime containing biochar. A further benefit is that the presence of CaO in the pyrolysis process greatly improves the quality and calorific value of the gaseous products. This is because CO2 is captured in-situ driving the pyrolysis reactions to produce more H2. This allows for a more economical biochar production process through utilization of the higher quality product gases for heating requirements and concurrently reducing carbon emissions.


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4. Conclusions A novel carbon arrestor process was proposed for in-situ pyrolysis of biomass and CaO. The production of biochars derived from woody and herbaceous biomasses with CaO was analyzed using a combination of TGA and micro-GC, and the biochars were characterized via FTIR, nitrogen adsorption and SEM. It was determined that in-situ pyrolysis of biomass and CaO greatly favored the formation of H2 and CH4, while significant reductions in CO2 emissions were observed. The addition of CaO to the pyrolysis process resulted in chars of similar surface area even though biochar produced from the biomass alone had significantly different values at 201 m2/g and 6.7 m2/g for blackbutt and wheat stem respectively. SEM revealed a significant reduction in char and CaO particle size with higher levels of CaO present in the pyrolysis process. Further, CaO particles dispersed across the char particles and significant cracking of CaO was apparent. This was attributed to swelling associated with the formation of CaCO3 and Ca(OH)2 during the pyrolysis process, which have larger molar volumes than the original CaO. The presence of CaO in the pyrolysis process saw a reduction in oxygenated functional groups on the surface of the biochars when pyrolysis temperatures exceeded 450°C. This property is potentially beneficial in supporting improved char stability and longevity in soil. Hence, the results presented in this study have shown that biochars produced with CaO in-situ to the pyrolysis process have improved product gas composition as well as favorable properties for soil application. Future work for the development of a carbon arrestor process for the production of biochar will investigate the application of the novel functionalized biochars to soils. Corresponding Author *Corresponding Author, email: [email protected]


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Energy & Fuels

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgments The authors wish to acknowledge the financial support provided by The University of Newcastle. References (1)

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Characterization of Biochars Derived from Pyrolysis of Biomass and

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