Comparison of Rice Husk and Wheat Straw: From Slow and Fast

Article pubs.acs.org/EF

Comparison of Rice Husk and Wheat Straw: From Slow and Fast Pyrolysis to Char Combustion Daphiny Pottmaier,*,†,§ Mário Costa,‡ Timipere Farrow,† Amir A. M. Oliveira,§ Orestes Alarcon,§ and Colin Snape† §

Department of Mechanical and Materials Engineering, Federal University of Santa Catarina, Florianopolis, SC, 88040900, Brazil Department of Mechanical Engineering, Instituto Superior Técnico, University of Lisbon, Lisbon, 1049-001, Portugal † Department of Environmental and Chemical Engineering, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, United Kingdom ‡

ABSTRACT: Thermochemical conversion of world top crops (rice and wheat) has been extensively investigated (TGA, DTF, SEM, XRD, BET, EA), and main insights are discussed in light of materials and process kinetics. Overall, the results show that the rice husk presents lower reactivity than the wheat straw for all thermal processes regardless of the final temperatures (300 °C− 1300 °C), residence times (0.6 s−300 min), and atmospheres (100−340 mL·min−1 N2/air). The higher reactivity of wheat straw is attributed not only to higher alkali and ash contents but also to differences in both silica morphology and graphitic structure after pyrolysis. Chars produced from slow pyrolysis present more homogeneous characteristics than those produced from fast pyrolysis. Combustion of the chars from slow pyrolysis (up to 900 °C) show similar kinetic parameters with activation energies, Ea, of 101.8 and 101.0 kJ·mol−1 with pre-exponential factor, A, of 4.3 × 107 and 9.6 × 107 min−1 for rice husk and wheat straw, respectively; while chars from fast pyrolysis (up to 1300 °C) show a range of values. Reaction times at 90 wt % loss (min) and rate constants ko (min−1) gives a more clear difference in values even for chars from slow pyrolysis with 12.4 and 0.221 for rice husk and 4.3 and 0.499 for wheat straw, correspondingly. These results are discussed herein according to changes in the physical and chemical characteristics of the nascent chars and, consequently, on their reactivity.

1. INTRODUCTION Biomass is a fuel readily and widely available in most parts of the world, being a renewable and carbon neutral source of energy. It is a natural composite made of organic (cellulosic-based) fibers and inorganic (mineral-based) phases,1−6 which can be used as solid, liquid, or gaseous fuel depending on the physical, chemical, or biological conversion.7−11 In general, thermal conversion is the most suitable technical option, since the dry mass basis heating value for biomass presents a narrower range when compared to their bioethanol potential for chemical and enzymatic digestibility for biological conversion.12 Regardless the wide diversity of bioenergy feedstock, biofuels are usually grouped in woody, agricultural, and other types of organic wastes.13−15 Agricultural residues hold an important potential as bioenergy source, with large quantities of crop residues and leftovers, among which maize (Zea mays), wheat (Triticum aestivum), and rice (Oryza sativa) are the world top three.16 In many countries, a common practice is open field burning after the harvesting season, which means misused energy and environmental and health hazards.17 Thus, rice residues are currently a major problem for highly populated countries such as Brazil, India, and China, where rice is heavily present in the daily diets; but these biomass wastes could also be sought as a strategic source of renewable energy. Thermal conversion of biomass materials (Figure 1) involves a complexity of chemical reactions and physical mechanisms, which are influenced by the variability of the feedstock characteristics.18 Usually, the three main processes involved are drying/moisture evaporation, devolatilization/pyrolysis, and © 2013 American Chemical Society

Figure 1. Scheme of biomass thermal conversion (materials and processes).

char burnout/gasification. During the drying/moisture evaporation process at around 100 °C, the moisture changes to vapor and diffuses along the pores to the surface and out, which at certain conditions may originate cracks.19 Received: August 30, 2013 Revised: October 2, 2013 Published: October 4, 2013 7115

dx.doi.org/10.1021/ef401748e | Energy Fuels 2013, 27, 7115−7125

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Table 1. Kinetic Values of Rice Husk and Straw from the Most Relevant Works sample a

rice straw

charDTF800 °C charDTF1000 °C charDTF1200 °C

rice husk rice straw rice straw wheat straw rice husk

a

Tiso (°C)

gas

Ea (kJ·mol‑1)

Ao (min‑1)

850−1050

CO2

2.3 × 109 5.1 × 107 3.6 × 107 3.6 × 108 5.1 × 106 9.7 × 106 1.5 × 1010

N2

238.3 202.9 203.4 100.92 83.72 80.83 115.59 112.5

air

73.40

4.9 × 104

Maiti et al., 200650

CO2

200 180

2.4 × 1011 8.1 × 1010

Bhat et al., 200147

5 900 30 677 5−100 627 10 1000

rice husk rice graina rice powder

β (°C·min‑1) Tfinal (°C)

He air

char 600−700 °C 750−900

ref Yuan et al., 201151

Huang et al., 201149 Zhaosheng et al., 200852 Biagini et al., 200848

Chars from DTF at °C temperature.

extrapolate almost any other set of conditions. The accuracy of the predictions will surely depend on the experimental approach and kinetics model. Thus, the collection of the data and understanding of the mechanisms during thermal conversion represent a crucial stage toward the optimum design of biomass energy systems. These studies are also important for ignition, flame, combustion stability, and pollutants formation.45 Experimentally, and commonly, kinetics measurements are obtained using a Fluidized Bed (FB) or a Drop Tube Furnace (DTF) as pilot-scale devices and using Differential Thermal Analysis (DTA) or Thermo-Gravimetric Analysis (TGA) as laboratoryscale techniques. Both approaches have known advantages and disadvantages regarding their performances for the study of biomass kinetics. The DTA/TGA provides an easy, fast, and cheap way to describe thermal reactions and their parameters, particularly in regime I or chemical controlled, while the FB/ DTF creates an environment closer to industrial reality with larger amount of material and higher temperatures, shorter residence times, and extremely fast heating rates.46 In this context, the thermal behavior of these two main residues of world top agricultural crops (rice husk and wheat straw) is investigated using both TGA and DTF emulating thermal processes (slow and fast pyrolysis, gasification, and combustion). For comparison, and used herein for discussing the results, kinetics parameters of these two biomasses from the most relevant works up-to-date are listed in Table 1.47−52 Moreover, the chemical and physical phenomena related to their thermoconversion are based on the results obtained with the characterization of the materials (biomass, char, ash) by Elemental Analysis (EA), Brunauer−Emmett−Teller (BET) Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD).

The biomass devolatilization/pyrolysis process is characterized by the release of volatile matter, which, in most of the biomass fuels, initiates at about 200 °C, slightly dependent on their chemical and physical characteristics. The quantity of released volatiles is mainly related to particle heating rate, final temperature, residence time, and particles size.20−23 Combustible volatiles mainly consist of H2O, CO2, CO, H2, and light hydrocarbons (mainly CH4), while incombustible volatiles consist of heavy hydrocarbons (typically tars). Overall, the behavior of biomass fuels during pyrolysis has often been referred to the behavior of its natural components (cellulose, hemicellulose, and lignin),24 which can consist of the slow conversion of lignin between 160 °C−900 °C and a faster decomposition of hemicellulose 220 °C−315 °C followed by cellulose 315 °C−400 °C.25 However, more recently results showed that these natural compounds may interact in different and complex manners.26 It is also known that quantity and nature of the present mineral matter influence pyrolysis rates and subsequent reactions.27 In any case, operational conditions such as final temperature,28−30 residence time,31 and environment pressure32,33 show more significant impact in the yields of charcoal (for wood, theoretically 36.7 wt %)34 and in its carbon structure (from graphene sheets to turbostratic crystallites).35−37 Reaching char combustion/gasification process by increasing temperatures and oxidant environment, it will occur the partial or total oxidation/reduction of the charcoal/char (carbon and ashes). The morphology of the chars is generally analogous to that of the parental biomass,38 except for small cracks and internal cavities/porous resulting from the volatiles decomposition and their gases escape.19 Ratio of graphite and soot in these carbonaceous residues has shown to significantly influence their reactivity for both CO2 and O2 enriched atmosphere.39 However, reaction rates in an enriched-O2 environment usually present higher values than in CO2 or steam;40 and regarding the ash formation, agricultural biomass is rather different in comparison to woody biomass due to its higher phosphorus content.41 Another distinct feature is their high silica content, so that alkali silicates can be formed up to 900 °C,42 which may result in less fouling and slagging problems.43 However, the washing of wheat straw, removing alkali compounds plus chlorine and sulfur, has been shown to increasing ash melting temperature.44 Finally, by parametrizing these reactions as a function of state variables (temperature, pressure, concentration), one can

2. EXPERIMENTAL APPROACH 2.1. Green Biomass and Chars. Samples of rice husk (RH) and wheat straw (WS) were collected from agricultural residues, grounded, sieved into the 90−125 μm range, and prepared in 3 g samples for the DTF and about 10 mg samples for the TGA. Proximate and ultimate analyses were performed using TGA and EA practices. The BET model was applied to obtain the specific surface area of the samples from the nitrogen physisorption under isothermal conditions (with a Micromeritics ASAP 2000). XRD patterns (with a Siemens D500) and SEM (with a FEI Quanta 600) were collected and studied for better understanding of the samples structural and morphological characteristics such as phases nature, composition, shape, and distribution. 7116


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Table 2. Chemical and Physical Characteristics of the Biomass Fuels (wt %, air-dried basis) rice husk DTF 900 °C DTF 1100 °C DTF 1300 °C wheat straw DTF 900 °C DTF 1100 °C DTF 1300 °C

rice husk wheat straw a

moisture

volatile matter

fixed carbon

ash

particles yielda

C

6.7 1.3 1.2 0.6 6.3 2.3 1.5 1.1

66.0 12.4 9.8 2.5 63.9 22.2 16.4 10.8

14.3 23.0 21.3 12.8 13.3 23.9 22.3 11.3

13.0 61.9 67.7 84.1 16.5 52.9 60.4 76.8

19.4 15.8 13.2 25.4 21.9 18.0

39.3 32.4 27.6 19.9 36.7 43.1 36.7 27.4

H

N

surface area (m2·g‑1)

5.7 0.9 2.18 0.8 0.2 79.6 0.5 0 25.4 0.2 0 13.5 5.1 0.7 1.86 0.9 0.5 6.9 0.7 0.4 19.0 0.2 0 30.7 ash analysis (wt %, air-dried basis)

SiO2

CaO

K2O

Al2O3

MgO

P2O5

Fe2O3

SO3

others

88.2 34.3

2.8 24.1

3.7 15.1

0.3 7.7

1.3 3.7

1.6 3.4

0.2 2.9

0.8 2.6

1.1 5.9

Remaining particulate matter after DTF runs.

Figure 2. SEM images of rice husk: green biomass (a) and chars after fast pyrolysis in the DTF at 900 °C (b), 1100 °C (c), 1300 °C (d). mL·min−1. The method run is similar to BS - proximate analysis (PA), except that the final temperature here is 850 °C.53 2.3. Fast Pyrolysis. The DTF is constituted of a feeding system, furnace reactor, and a sample probe. The feeding system at the top has a screw-feeder with a volumetric flow rate of 2 cm3.min−1 and a gas flow system. The furnace is an electrically heated vertical ceramic tube (length of 2 m, inner diameter of 50 mm, and 5 mm thick walls). At the bottom of the DTF, the samples are collected by the probe using a

2.2. Slow Pyrolysis, Char Gasification, and Direct Combustion. A TGA Q500 instrument was utilized for both slow pyrolysis and direct combustion with measurements under isothermal conditions and at different heating rates. Char gasification profiles were obtained with the following schedule: Heat up to 110 °C, stabilization during 5 min, then heating up to 700 °C at 50 °C·min−1, followed by stabilization during 15 min, under 100 mL·min−1 of nitrogen, and finally equilibrating at 350 °C−500 °C, switching to air under a volumetric flow rate of 100 7117


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Figure 3. SEM images of wheat straw: green biomass (a) and chars after fast pyrolysis in the DTF at 900 °C (b), 1100 °C (c), 1300 °C (d).

Figure 4. XRD patterns: green rice husk and chars (left) and wheat straw and chars (right). pumping system.54 The DTF was operated under a nitrogen gas flow of 370 mL·min−1 for a residence time of 600 ms at temperatures of 900 °C, 1100 °C, and 1300 °C.

It is interesting to compare the present data with that in the databases PHYLLIS213 and BIOBIB.14 In these databases there are 94 results of wheat (Triticum aestivum) straw and 6 of rice (Oryza sativa) husk. The values for WS, as received basis, vary between 55.3 wt % and 71.4 wt % for the volatile matter content, between 12.9 wt % and 23.5 wt % for the fixed carbon content, between 38.1 wt % and 42.1 wt % for the carbon content, and between 3.6 wt % and 5.5 wt % for the hydrogen content; the corresponding values for RH are 48.3 wt %−69.3 wt %, 14.9 wt

3. RESULTS AND DISCUSSION 3.1. Materials (Biomass > Char > Ash). Table 2 presents the main chemical and physical characteristics (air-dried basis) of the biomass fuels, including the analysis of their ashes and chars, after fast pyrolysis in the DTF. 7118


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Figure 5. Carbon content of biomass fuels during slow pyrolysis: at 50 °C·min−1 as a function of final temperature (left) and up to 900 °C as a function of heating rate (right).

Figure 6. Thermal profiles under isothermal conditions (insets with isotherm peak/total area): pyrolysis in nitrogen flow (left) and direct combustion in air flow (right).

Figure 7. (left) Slow pyrolysis (N2) and direct combustion (O2) profiles at 50 °C·min−1 for rice husk and wheat straw, (right) Arrhenius plot of the rate constant for devolatilization.

%−22.2 wt %, 27.9 wt %−44.2 wt %, and 4.2 wt %−5.5 wt %, respectively. It is concluded that the obtained results (Table 2) fall inside the database range for both WS and RH, except for the lower carbon content (36.7 wt %) of the present WS and lower fixed carbon content (14.3 wt %) of the RH. There are several sources of biomass variability, as those inherent to species or biomass traits,55 but also due to harvest, collection, and storage practices.18

In the case of the surface area values, which is not included in the databases, the available literature indicates values between 1 m2·g−1 and 50 m2·g−1 for green biomasses as received basis, between 100 m2·g−1 and 1000 m2·g−1 for biomass chars, and of about 1600 m2·g−1 for activated chars.56,57 It is known that the reaction rate is proportional to the available surface area, referred to also as active sites, and which decreases after a certain point as the reacted regions start to overlap. The present green biomass fuels present a very low surface area (2.18 m2·g−1 for RH and 1.86 7119


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indexed as (002) stacking of the graphitic basal plans in chars.35,59,60 The peak shape is characterized by both the crystallinity degree and crystal size of the material; as crystal size is reduced the respective diffraction peaks become increasingly broader. Thus, using the Scherrer’s equation, crystallite sizes of graphitic phase in the green biomass fuels were calculated accordingly to be about 70 nm for WS and 50 nm for RH. Returning to Figure 4, it is seen that the WS chars present very distinct XRD patterns compared to that of their parental WS but analogous XRD among themselves. Differences within the presence and quantity of phases in the resulting chars are noted with respect to newly formed calcium carbonate (CaCO3) and crystalline silica or quartz (SiO2). While the presence of calcium carbonate decreases with pyrolysis temperature, crystalline silica increases together with an increase in the amorphous silica (silica glass). Note that the equilibrium temperature of the reaction CaCO3 = CaO + CO2 occurs at 848 °C, while the transformation of silica glass into quartz takes place at ∼1000 °C. Rice husk chars have XRD patterns (Figure 4) analogous to those of the green biomass and also among them despite the pyrolysis temperature. In the case of RH, both green biomass and biomass chars present similar and predominantly amorphous structure, with large crystallites (75 nm for the DTF900 char >70 nm for the DTF1100 char >80 nm for the DTF1300 char) as assigned to the graphitic phase (main peak at 25°). Minor peaks have been also observed, especially for the DTF1300 char, which are assigned to crystalline silica (main peak at 26.7°). Moreover, the Si-rich beads and their spherical nature on the surface of RH chars, as evident from the SEM images, reveal a thermal conversion in the presence of a molten phase. Although the filament nature is generally preserved, the particles aspect ratios decrease, and the structure becomes more lacelike as degradation proceeds.38 Overall, there is an increase in porosity with temperature accompanied by an increase of the specific surface area such as that observed for the WS, but not for the RH chars. This anomaly may be related to the high silica content of RH, which is transformed into amorphous silica over the chars specific surface area. 3.2. Processes (Biomass Pyrolysis − Char Gasification − Direct Combustion). The literature presents large discrepancies in regard to kinetic data for biomass fuels, where the heterogeneity of these materials plays an important role. It should be stressed that systematic errors related to the experimental analysis also contribute to the scattering of the kinetic data. These have spawned different reassessments by

Table 3. Kinetics Parameters for Gasification, under Isothermal Conditions, of Chars Resulting from Slow (TGA) and Fast (DTF) Pyrolysis at 425 °C

Rice Husk slow pyrolysis direct combustiona DTF 900 °C DTF 1100 °C DTF 1300 °C Wheat Straw slow pyrolysis direct combustiona DTF 900 °C DTF 1100 °C DTF 1300 °C a

90%

t (min)

K95−5% (min‑1)

Ea (kJ·mol‑1)

A (min‑1)

101.8 85.4 224.9 101.4 98.4

4.3 × 107 3.1 × 106 7.8 × 1016 1.0 × 108 2.0 × 107

12.4 5.8 4.6 12.7

0.221 0.241 0.583 0.188

101.0 92.2 104.7 123.2 125.7

9.6 × 107 9.3 × 106 5.7 × 108 7.9 × 109 4.0 × 109

4.3 1.5 2.5 8.0

0.499 1.798 1.224 0.364

Nonisothermal conditions.

m2·g−1 for WS), such as their chars after fast pyrolysis (Table 2). Note, however, that a recent study in rapid pyrolysis of pine sawdust under similar conditions (800 to 1200 °C) reported a similar range of BET results (47.5 to 8.9 m2·g−1).51 Nonetheless, the differences in the BET values may be related to the melting of cell structures and of pore structure disappearance, as observed in the SEM images (Figures 2b-d and Figures 3b-d), where char particles have a slightly more rounded shape than the particles of green biomass (Figures 2a and Figure 3a). Figures 2a−d reveal that the morphology of the WS particles resembles leave-like and flat shape, while Figures 3a−d disclose that RH is characterized by an edged shell-like outer cover together with more rounded particles. Despite treatment temperatures and heating rates, resulting chars retain most of the features of parental fuels according to the SEM analysis, and this tendency was recently observed with in situ microscopic analysis.58 The XRD patterns displayed in Figure 4, however, show very distinct materials structures between green biomass fuels and their resulting chars, especially for WS. The XRD patterns of the green biomass fuels, as expected, are very similar and indicate a long-range amorphous material, although the green RH also presents a crystalline phase, Whewellite, an organic mineral of calcium oxalate monohydrate (CaC2O4·H2O). This phase has been previously identified in RH samples and other biomass fuels.6 The main broad peak in the low angle region (3°−35°) is

Figure 8. Gasification of chars (TGA 900 °C) at isotherms of 350 °C up to 500 °C: (left) for rice husk and (right) for wheat straw. 7120


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Figure 9. Gasification rates for rice husk chars from TGA - slow pyrolysis at (a) 900 °C, DTF - fast pyrolysis at (b) 900 °C, (c) 1100 °C, (d) 1300 °C.

Figure 10. Gasification rates for wheat straw chars from TGA - slow pyrolysis at (a) 900 °C, DTF - fast pyrolysis at (b) 900 °C, (c) 1100 °C, (d) 1300 °C.

equation, which is conveniently expressed by the canonical expression

using extravagant mathematical kinetic models, which were recently reviewed and critically discussed.61 However, they will

⎛ −E ⎞ dα = k(T )f (α) = A exp⎜ a ⎟f (α) ⎝ RT ⎠ dt

serve for nothing without the elucidation of the more fundamental physical-chemistry aspects associated with the biomass thermal reactions. The majority of the kinetic models

(1)

where t is the time, A is the pre-exponential factor, Ea is the activation energy, and R is the gas constant (= 8.314 J·mol−1·

proposed essentially employs a rate law that obeys the Arrhenius 7121


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K−1). The extent of the reaction, or conversion, is denoted by α as the mass fraction of biomass or volatiles evolved (α = ((wo − wt)/ (wo − wash))). Thus, the reaction rate r is calculated by the derivative of the conversion, r = (dα/dt). Most significant reaction parameters of biomass reactions are taken by a combination of A, Ea, and f(α), the so-called kinetic triplet.61−64 Nonisothermal kinetics functions represent reaction rates ((dα/dT)) at a constant heating rate (β) as ⎛ −E ⎞ k(T ) dα A = f (α ) = exp⎜ a ⎟f (α) ⎝ RT ⎠ β β dT

insights on the mechanisms and possible effects in the successive char combustion. The carbon content of the chars presents a similar pattern for changes in both final temperature and heating rate. All samples show a slight increase in the carbon content from 350 °C up to 700 °C and a more pronounced decrease from 700 °C up to 850 °C. While the hydrogen content decreases continuously to zero in both biomass fuels, the nitrogen remains detectable for all samples at the highest temperature and heating rate. These findings, similar to switch grass and corn stover chars,69 suggest that the types of carbon in the chars are weakly related to the final temperature and the residence time/heating rate. The more pronounced volatiles release is verified in the first stage of the pyrolysis at temperatures below 300 °C, especially for WS, which is an indication of its higher content of xylan and cellulose and its higher reactivity (Figure 6). It is also observed that there is no distinct crossing point between active (volatiles evolution from xylan and cellulose) and passive zones (lignin conversion) as predicted to happen at about 500 °C. Direct combustion at the same conditions, in isothermal (Figure 6) and nonisothermal (Figure 7) modes, was conducted for both RH and WS. This kind of comparison has been already used by other investigators to examine how direct combustion at low heating rates is favored with respect to the devolatilization/ char burnout schemes.21 From both profiles of slow pyrolysis and direct combustion, it can be concluded that WS presents a higher reactivity than RH already at lower temperatures regardless atmosphere, as indicated by the steeper weight losses and their respective derivatives already lower than 300 °C. Above this temperature, and up to 450 °C, reactions assigned mostly to lignin transformation into char become then more important in the thermoconversion of RH. Thus, these profiles suggest a higher content of lignin in the RH (compared to WS), and it is in agreement with the previous observations indicating the indirect relation between the lignin content and the biomass reactivity.70 Indeed, it is interesting to refer here that the PHYLLIS2 database13 presents the following values for xylan (XY), cellulose (CEL), and lignin (LI): - 19.7−24.3 wt % XY, 34.3−36.1 wt % CEL, 14.3−34.8 wt % LI (for RH), - 10.5−39.1 wt % XY, 28.8−51.5 wt % CEL, 8.0−30.0 wt % LI (for WS). Another factor affecting the biomass reactivity is the carbonates transformation, found as a major alkaline based compound,71 and observed for WS as the transformation happens in the last weight change at 600 °C in both atmospheres. It is well accepted that alkali and alkaline earth metals also play a role in biomass reactivity, and, from the oxides (SiO2, CaO, Al2O3, MgO) present in these two biomass fuels, alkali K2O (WS≫RH) crosses the stability regions of CO and CO2 at a temperature as low as 427 °C.41 To further elucidate these differences in the thermal conversion of WS and RH, scan profiles from 5 °C·min−1 to 100 °C·min−1 were also taken under nitrogen and air flows of 100 mL·min−1 and up to a final temperature of 900 °C. The kinetics parameters, under nonisothermal conditions, were calculated separately for the devolatilization process under both atmospheres as the first main reaction and char burnout as the second reaction for the direct combustion runs. Thermal conversions for both biomass fuels as expected and confirmed (Figure 7) start with a first reaction (devolatilization) characterized by a semiplateau at about 300 °C, with a small

(2)

Assuming as first-order-based models (n = 1), the reaction rate becomes proportional to the unreacted material (1 − α) and the rate constant (k): dα k = (1 − α)n ; dT β

dα = k(1 − α)n dt

(3)

Considering the initial conditions, α = 0 and t = 0, the rate constants are calculated as the slope of −ln(1 − α) = kt for isothermal conditions and of −ln(1 − α) = (k/β)t for nonisothermal conditions. In this work, reference values for reaction conversions were taken between 0.05 and 0.95 (k5−95%) and the conversion time for the reaction at 0.9 (t90%).65 Maximum reaction temperatures were those from the first derivative peaks of the thermogravimetric curve (−dw/dt). Slow Pyrolysis and Direct Combustion. The first solid state reaction of biomass thermal processes comprises the formation of a solid carbonaceous residue (char and ash). This also consists of a step with the release of volatile species (combustible and incombustible), reason for the term devolatilization regardless of the type of atmosphere (reductive or oxidant). Slow pyrolysis, in 100 mL·min−1 nitrogen, for RH and WS, at a heating rate of 50 °C·min−1, shows an increase of volatiles release (wt %) as a function of final pyrolysis temperature for 250 °C, 350, 500, 600, 700, 900 °C as follows: 31.5 < 56.7 < 64.1 < 62.6 < 65.9 < 68.1 (for wheat straw), 21.3 < 53.4 < 64.6 < 60.9 < 63.9 < 63.4 (for rice husk). As expected from 350 to 500 °C is observed a pronounced release of volatiles related to the main decomposition of the natural polymers (hemicellulose/xylan, cellulose, lignin). From 700 to 900 °C a further decrease is observed for the WS, assigned to the carbonates transformation as indicated by the XRD patterns of the nascent chars (Figure 4). This carbonates reaction was also observed during decomposition of paper sludge and characterized by a larger release of CO2.66 For the present WS, a steep weight loss is verified at 600 °C in both isotherm and scan modes regardless atmosphere. Such production of carbon dioxide may promote self-gasification of biomass, as already reported,67 and second reactions may occur depending on the biomass composition, promoting further carbonization and structural change of the char.68 The wt. % of volatiles released during slow pyrolysis with final temperature of 900 °C, as a function of the heating rates 10, 20, 50, and 100 °C·min−1 resulted in the following steady profiles: 62.0 < 62.0 < 64.1−62.4 (for wheat straw), 63.9 < 64.4 < 65.1−64.3 (for rice husk). The correspondent carbon content of the resulting chars after slow pyrolysis (Figure 5), in both scan and isothermal modes, was measured to clarify and compare the differences in the release of volatiles for the biomass fuels. This characterization at different reaction stages during slow pyrolysis is intended to give 7122


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temperature lag between the biomasses of about 10 °C for the nitrogen curves, and about 20 °C for the air curves. In the oxidant atmosphere, the devolatilization is followed by a second reaction at about 450 °C related to char burnout and also characterized as a semiplateau in the temperature profiles of both biomasses. Additionally, only for WS there is a third conversion at about 650 °C, and as already discussed for the isothermal conditions, this may be related to the formation of alkaline carbonates. During pyrolysis a temperature gradient occurs because of the endotherm reactions taken place, already reported for decomposition of pure cellulose.72 As mentioned, despite the biomass nature and the atmosphere, the devolatilization process is characterized by a single plateau at about 300 °C (Figure 7 left) with a small temperature lag. However, the evolution of the reaction (i.e., rate as a function of conversion) varies significantly with differences in the ratio of the natural components (xylan, cellulose, and lignin) and the overall biomass kinetics. Additionally, the catalytic effect of the alkali elements enhances the diffusion paths for the volatiles release from xylan and cellulose, such as catalyzes the transformation of lignin into char. Recent findings on the iron and zinc impregnation (of cellulose, xylan, and lignin) suggest simultaneous reaction improvement in the volatiles release and char formation by either rearrangement or depolymerization of these natural compounds.73 In the oxidant atmosphere, the devolatilization development of both biomass fuels presents similar maximum rates (0.58 min−1 for WS and 0.56 min−1 for RH) at different reaction stages at an early stage (25%) for WS and closer to the middle of the conversion (40%) for RH. Differently, in the reductive atmosphere, the maximum rates for devolatilization reaction are less alike (0.52 min−1 for WS and 0.48 min−1 for RH) but occur at the same stage, above half of the conversion (60%). This behavior may be the result of more interaction during decomposition among the natural components under an inert atmosphere, while there is bigger overlap for char formation and decomposition under an oxidant atmosphere. Finally, char burnout (second and smaller peak, sharper for WS and broader for RH) takes place between 400 and 500 °C, as indicated by their temperature profiles (Figure 7 left). Therefore, this happens at lower temperatures than a known proposed direct combustion scheme: evolution of volatiles (up to 300 °C), ignition of volatiles (500 °C−650 °C), burning of volatiles (650 °C−800 °C), and then burning of char (700 °C−850 °C).74 Moreover, the maximum reaction rate at 435 °C for WS is three times higher than that at 445 °C for RH, indicating as well by the curves shapes. The WS reactivity is higher for direct combustion, during both steps of devolatilization and char burnout reactions, when compared to the RH profiles under the same conditions (Figure 7 right). The larger the distance between the N2 and Air curves in the Arrhenius plots for the same biomass, the higher the oxygen effect in the reaction. Indeed, in a previous study it was suggested that the inflection in the Ln K vs 1/T plot (Figure 7 right) is related to changes even in the reaction mechanisms.75 Finally, biomass chars formed from slow pyrolysis for both biomass fuels (WS and RH) resulted in a similar set of kinetics parameters (Figure 7 right). Fast Pyrolysis and Char Gasification. For the fast pyrolysis in the DTF (Table 2), it was found that char/particles yields decrease from 19.4 wt % to 13.2 wt % for RH and from 25.4 wt % to 18 wt % for WS. These results are comparable to those reported in the literature for rice straw and other biomass fuels.51 As expected, the resulting chars present higher ash contents at

the expense of carbon and hydrogen contents and as a function of their pyrolysis temperature (900 °C−1300 °C). Thus, as in the case of slow pyrolysis, the increase of the final temperature is likely to promote the formation of different types of carbonaceous materials/chars, and this is indicated by their BET surface areas (Table 2). The gasification of the biomass chars obtained in the DTF was conducted at temperatures from 350 °C up to 500 °C under isothermal conditions and profiles taken to obtain their set of kinetics parameters for RH and WS (Table 3). An example (Figure 8) of gasification profiles is given for the chars produced from slow pyrolysis (TGA) at 900 °C. The set of kinetic parameters for direct combustion (Table 3) was taken from thermal profiles, under nonisothermal conditions, using heating rates from 5 °C·min−1 to 100 °C·min−1 up to a temperature of 900 °C (Figure 7 right). The reactivity of the biomass chars from fast pyrolysis (DTF), as shown by the kinetics parameters (Table 3) and thermal profiles (Figures 9 and Figure 10), varies with the pyrolysis final temperature for both RH and WS. For RH chars, the reactivity of the slow pyrolysis (TGA 900 °C) is comparable to that of the fast pyrolysis at the highest temperature (DTF 1300 °C). Differently, the reactivity of WS chars from slow pyrolysis (TGA 900 °C) is higher than that of WS chars from fast pyrolysis at the highest temperature (DTF 1300 °C). This behavior is mainly attributed to the carbon content available into the chars produced at these temperatures. Gasification profiles of both fuels (Figure 9 and Figure 10) indicate a more resilient carbon material as the pyrolysis temperature increases such as lower carbon and hydrogen contents in the DTF char. A more ordered and graphitic structure of the remaining carbon material is expected to decrease the char reactivity; e.g. pure soot is still less reactive under both than wood and WS chars produced at high temperatures (1000 to 1400 °C).76 Additionally, RH chars present high amounts of silica in the amorphous phase, as shown by their XRD patterns, which may indicate melting of oxides and deactivation of the available surface, as also suggested by the decrease in the BET values. The reactivity profiles for the WS chars have a more pronounced decay with conversion rate, as a consequence of decrease in the chemical gradient, available reactants, and unreacted surface. These are the main reasons for the decrease in the reactivity with temperature for the DTF chars, such as evidenced by their lower reaction rates and their respective less sharp curves. The more pronounced tale of the WS curve is related to the presence of alkaline compounds in its ashes. Indeed, higher contents such as those found in woody biomass77 may produce even fluctuated profiles and indicate different reaction domains in the char. Thus, a smoother curve is connected to a more homogeneous and less reactive char, such as observed in the gasification profiles of chars produced in the DTF at 1300 °C.

4. CONCLUSIONS The main conclusions of this study are as follows: • Rice husk (RH) presents lower reactivity than wheat straw (WS) for the studied thermal processes despite final temperatures (300 °C−1300 °C), residence times (0.6 s−300 min), and atmospheres (N2/air). • The higher reactivity of WS over RH is attributed, in addition to known features such as higher alkali and ash contents, to differences in both silica structure and morphology (RH: glassy 7123


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(10) McKendry, P. Energy production from biomass (part 3): Gasification technologies. Bioresour. Technol. 2002, 83 (1), 55−63. (11) Panwara, N. L.; Kotharib, R.; Tyagic, V. V. Thermo chemicalconversion of biomass − eco friendly energy routes. Renewable Sustainable Energy Rev. 2012, 16, 1801−1816. (12) Godin, B.; Lamaudière, S.; Agneessens, R.; Schmit, T.; Goffart, J.P.; Stilmant, D.; Gerin, P. A.; Delcarte, J. Chemical composition and biofuel potentials of a wide diversity of plant biomasses. Energy Fuels 2013, 27 (5), 2588−2598. (13) ECN, Phyllis2: Database for biomass and waste. In http://www. ecn.nl/phyllis2/Browse/Standard/ECN-Phyllis (accessed October 16, 2013). (14) TUWien, BIOBIB - A Database for biofuels. In http://www.vt. tuwien.ac.at/biobib/EN/ (accessed October 16, 2013). (15) CNE, Fuel specifications and classes - Part 1: General requirements. In Solid biofuels; 2010; Vol. EN 14961-1:2010. (16) FAOSTAT, Food and Agriculture Organization of the United Nations. In http://faostat.fao.org/site/291/default.aspx (accessed October 16, 2013). (17) Lim, J. S.; Abdul Manan, Z.; Wan Alwi, S. R.; Hashim, H. A review on utilisation of biomass from rice industry as a source of renewable energy. Renewable Sustainable Energy Rev. 2012, 16 (5), 3084−3094. (18) Kenney, K. L.; Smith, W. A.; Gresham, G. L.; Westover, T. L. Understanding biomass feedstock variability. Biofuels 2013, 4 (1), 111− 127. (19) Benfell, K. E.; Liu, G.-S.; Roberts, D. G.; Harris, D. J.; Lucas, J. A.; Bailey, J. G.; Wall, T. F. Modeling char combustion: The influence of parent coal petrography and pyrolysis pressure on the structure and intrinsic reactivity of its char. Proc. Combust. Inst. 2000, 28 (2), 2233− 2241. (20) Munir, S.; Daood, S. S.; Nimmo, W.; Cunliffe, A. M.; Gibbs, B. M. Thermal analysis and devolatilization kinetics of cotton stalk, sugar cane bagasse and shea meal under nitrogen and air atmospheres. Bioresour. Technol. 2009, 100 (3), 1413−1418. (21) Biagini, E.; Tognotti, L. Comparison of devolatilization/char oxidation and direct oxidation of solid fuels at low heating rate. Energy Fuels 2006, 20 (3), 986−992. (22) Butterman, H. C.; Castaldi, M. J. CO2 as a carbon neutral fuel source via enhanced biomass gasification. Environ. Sci. Technol. 2009, 43 (23), 9030−9037. (23) Senneca, O. Kinetics of pyrolysis, combustion and gasification of three biomass fuels. Fuel Process. Technol. 2007, 88 (1), 87−97. (24) Van de Velden, M.; Baeyens, J.; Brems, A.; Janssens, B.; Dewil, R. Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction. Renewable Energy 2010, 35 (1), 232−242. (25) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86, 1781− 1788. (26) Couhert, C.; Commandre, J.-M.; Salvador, S. Is it possible to predict gas yields of any biomass after rapid pyrolysis at high temperature from its composition in cellulose, hemicellulose and lignin? Fuel 2009, 88 (3), 408−417. (27) Raveendran, K.; Ganesh, A.; Khilar, K. C. Influence of mineral matter on biomass pyrolysis characteristics. Fuel 1995, 74 (12), 1812− 1822. (28) Antal, M. J.; Croiset, E.; Dai, X.; DeAlmeida, C.; Mok, W. S.-L.; Norberg, N.; Richard, J.-R.; Al Majthoub, M. High-yield biomass charcoal. Energy Fuels 1996, 10 (3), 652−658. (29) Cetin, E.; Moghtaderi, B.; Gupta, R.; Wall, T. F. Influence of pyrolysis conditions on the structure and gasification reactivity of biomass chars. Fuel 2004, 83 (16), 2139−2150. (30) Dall’Ora, M.; Jensen, P. A.; Jensen, A. D. Suspension combustion of wood: Influence of pyrolysis conditions on char yield, morphology, and reactivity. Energy Fuels 2008, 22 (5), 2955−2962. (31) Hasan Khan Tushar, M. S.; Mahinpey, N.; Khan, A.; Ibrahim, H.; Kumar, P.; Idem, R. Production, characterization and reactivity studies of chars produced by the isothermal pyrolysis of flax straw. Biomass Bioenergy 2012, 37 (0), 97−105.

plates, WS: crystalline beads) and to graphitic structure (RH: ordered, WS: disordered) of the carbon content in the char. • Slow pyrolysis kinetics of RH (t90%= 12.8 min) was lower than that of WS (t90%= 8.5 min) under certain heating rate conditions (5−100 C·min−1). These values under nonisothermal conditions resulted in the following: an activation energy, Ea (kJ· mol−1)/a pre-exponential factor, A (min−1) = 47.6/2.9 × 104 (for RH) and = 72.2/4.6 × 106 (for WS). • Overall, chars produced from slow pyrolysis (TGA) have more homogeneous characteristics than those from fast pyrolysis (DTF). Thus, the later evidently imposes significant changes in the physicochemical properties of the nascent chars by enhancing their reactivity, except for DTF 1300 °C chars. • Gasification/combustion of homogeneous TGA chars has the following set of kinetics parameters: 101.8 kJ·mol−1 and 4.3 × 107 min−1 for rice husk, 101.0 kJ·mol−1 and 9.6 × 107 min−1 for wheat straw. The heterogeneous DTF chars (up to 1300 °C) show a range of values between 98−225 kJ·mol−1 and 107−1016 min−1 for rice husk, 105−126 kJ·mol−1 and 109−108 min−1 for wheat straw. • Direct combustion, under slow heating conditions, resulted in kinetics values (Ea, A) of 29.8/6.1 × 102 for RH and 43.6/1.4 × 104 for WS; moreover, reaction times were similar with t90% of 6.9 min for RH and of 6.5 min for WS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.P. thanks the University of Nottingham and CNPq/CAPES for financial support and fellows for helping with the experimental analysis.

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REFERENCES

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Comparison of Rice Husk and Wheat Straw: From Slow and Fast

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