Biomass Gasification - American Chemical Society

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Steam−O2 Blown Circulating Fluidized-Bed (CFB) Biomass Gasification: Characterization of Different Residual Chars and Comparison of Their Gasification Behavior to Thermogravimetric (TG)-Derived Pyrolysis Chars Xiangmei Meng,*,† Patricia Benito,‡ Wiebren de Jong,† Francesco Basile,‡ Adrian H. M. Verkooijen,† Giuseppe Fornasari,‡ and Angelo Vaccari‡ †

Process and Energy Laboratory, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands Department of Industrial and Materials Chemistry, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy

‡

ABSTRACT: Studies of the pyrolysis of biomass fuels and their residual char gasification are important to optimize and model gasification processes. In this paper, the characterization of circulating fluidized-bed chars (CFB-chars) was primarily performed. They were sampled from Agrol, willow, and dried distiller’s grains with solubles (DDGS) gasification using an atmospheric pressure 100 kWth steam−O2 blown CFB gasifier. The physical and chemical properties of char samples were studied by powder X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 adsorption/desorption at −196 °C, and scanning electron microscopy (SEM) coupled with energy-dispersive scattering (EDS). Sequentially, the pyrolysis behavior of the Agrol, willow, and DDGS chars was investigated using thermogravimetric analysis (TGA) coupled with a Fourier transform infrared (FTIR) spectrometer. Gasification reactivities of CFB-char and char obtained after pyrolysis (PYR-char) were further investigated. The influences of the pyrolysis temperature (750 and 850 °C), heating rate (10, 30, 50, and 70 °C/min), CO2 concentration (10, 20, and 30 vol %), and gasification temperature (900, 1000, and 1100 °C) on the reaction rate of the char−CO2 reaction were studied. The volumetric reaction model (VRM) and the shrinking core model (SCM) were applied to determine kinetic parameters. SEM images showed that Agrol chars were very porous, with different superficial cavities and thin walls. Willow chars had a more compact agglomerated structure, while DDGS chars had a macroporous structure with rounded pores of different sizes and some particulates on the surface. The results observed from EDS analysis revealed that the composition of the chars was not completely homogeneous. XRD patterns showed that char samples had a disordered graphite-like structure. Agrol and willow showed similar pyrolysis behavior, and their related weight loss rates increased with an increasing heating rate. The XRF-analyzed results showed that the inorganic elements of the Agrol char were formed by Ca, Fe, K, Mg, and Si. The willow char was mainly composed of Ca and K, with minor amounts of Fe, Mg, P, and Si. However, DDGS char was mainly dominated by K and P, with a lesser amount of Ca, Mg, and Na. The char gasification rate increased with an increasing gasification temperature, CO2 concentration, and heating rate, while it decreased with an increasing pyrolysis temperature. Generally, the calculated activation energy (Ea) values using the SCM were slightly lower than those using the VRM. The calculated Ea value for PYR-char using both models was in the range of 90−210 kJ/mol, while the calculated Ea values for CFB-char were in the range of 55−120 kJ/mol. The predicted results using both models showed a reasonably good agreement with experimental results, in particular, with those obtained at a lower gasification temperature and lower CO2 concentration.

1. INTRODUCTION Biomass fuels are gaining attention as a potential source of alternative energy sources to increase energy independence on fossil fuels and reduce environmental pollution. Biomass can be converted into more valuable energy carriers via either biochemical or thermochemical conversion technologies.1,2 The thermochemical biomass gasification has received the highest interest because of the increased options for combining with various power generation systems using gas engines, gas turbines, and fuel cells.3 The biomass gasification process consists of sequential steps, such as drying, pyrolysis, gasification or partial combustion of the residual char, and homogeneous reactions among produced gases. The char reaction is generally the slowest step, thus being the rate-limiting step during biomass gasification. For example, Peters and Bruch4 studied the thermal decomposition of wood particles with a diameter of 4 cm and found that the drying process was © 2011 American Chemical Society

complete at approximately 140 s with the evaporation temperature of 100 °C. Maschio et al.5 studied the influence of kinetic and diffusion phenomena on the pyrolysis of biomass particles using thermogravimetric (TG) techniques and other apparatuses. They found that the pyrolysis process of biomass particles could be either kinetics or both heat-transfer- and kinetics-reaction-controlled depending upon particle sizes, and the pyrolysis process lasted about 250 s. Chen and Gunkel6 reported that char gasification occurred generally at high temperatures and could take around 3000 s. Under similar operational conditions, char combustion is normally much faster than its gasification, which is also slower than drying and pyrolysis processes.7 To effectively model and Received: September 28, 2011 Revised: December 2, 2011 Published: December 2, 2011 722

dx.doi.org/10.1021/ef201465w | Energy Fuels 2012, 26, 722−739

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Table 1. Investigation of Char Characterization and Char Reactivity Reported in the Literaturea pyrolysis conditions reference

biomass

8

Miscanthus, corn, wood, wheat, DDG, DDGS, etc. olive cake and cacao shells

9

11

Pinus radiata, Eucalyptus maculata, sugar cane

temperature (°C)

heating rate (°C/s)

1000

fixed bed

combustion

TGA

combustion

TGA

isothermal and nonisothermal 800

PTGA

850b

TGA

850

SEM, XRD, N2 and CO2 adsorption

combustion

fixed-bed reactor

900

SEM, XRD, RM

combustion

TGA

850−900

gasification combustion gasification combustion

TGA

900−1100 400−600 750−910 150−700

SEM, mercury porosimetry, N2 adsorption/desorption, He pycnometry SEM

DTR

950b 950

500 20

WMR tubular reactor DTR fixed bed fixed bed DTR

rice husk

23

willow

750, 850

0.17−1.17

14

pine seed shells, olive husk, and wood chips

850

74

cellulose and lignin

15

olive waste and straw

13

rice husk, eucalyptus

char reaction

1.2−3.3 × 104

104−105 0.17

400, 500, 700, 1100 800−1000

1 × 10

4

TGA BFB

0.04 500

temperature (°C)

reactor

400−800

1000 600−900 800−900 1200

12

char reactivity

retort box furnace free-fall reactor

gasification

gasification

reactor

tubular furnace TGA thermobalance reactor

850

char analysis SEM, oil immersion microscopy SEM

mercury porosimetry, N2 and CO2 adsorption N2 adsorption mercury porosimetry, N2 adsorption/desorption

a

DTR, drop-tube reactor; WMR, heated wire-mesh reactor; PTGA, pressured TGA; BFB, bubbling fluidized bed. bUnder pressure (up to 20 bar), with all others at atmospheric pressure.

DDG, DDGS, and rapeseed, that had the widest intrinsic reactivity profiles produced the least reactive chars with the thickest walls and the solid contents. The aforementioned survey of the literature shows that less research has been focused on studying the morphology and reactivity of char produced from biomass gasification in a fluidized bed, especially in a steam−O2 blown circulating fluidized-bed (CFB) gasifier. Subsequently, the combined study of the pyrolysis process and char reactions is also less reported in the literature. Moreover, the gasification behavior of chars produced from some special agriculture residues, such as DDGS, has been seldom studied, which may lead to some difficulties for performing the modeling of DDGS gasification in the gasifier. Therefore, the target of this research paper is to solve these problems with an emphasis on the following: (i) chemico-physical characterization of several char samples, which were obtained after gasification of three different fuels in a 100 kWth steam−O2 blown CFB gasifier, (ii) investigation of the pyrolysis behavior of three different fuels at different heating rates in TGA coupled with a Fourier transform infrared (FTIR) spectrometer, and (iii) study toward the gasification behavior of different char samples under different conditions (e.g., different temperatures and CO2 concentrations) in TGA.

optimize gasification processes, a good understanding of char reaction kinetics is important. Furthermore, reduction and conversion of char can also increase the product gas yield from biomass gasification and the overall system efficiency, because there is always a certain amount of biomass converted to char instead of syngas during gasification. Therefore, to better understand the char conversion step, many researchers have applied different analytical techniques to achieve more information about the char morphology development. Table 1 presents a brief overview of some recent existing studies about biomass char characterization. Results presented in Table 1 indicate that scanning electron microscopy (SEM) analysis is one of the most popular analytical techniques, which has been widely used to observe char morphologies,8−12 followed by N2 and CO2 adsorption and desorption.10,13−17 Thermogravimetric analysis (TGA) has been frequently used to investigate the reactivity of the char produced under different operation conditions (e.g., different heating rates, pressures, temperatures, and fuel particle types). For example, Guerrero et al.12 reported that chars obtained from rice husk pyrolysis with a more ordered structure were more reactive than those produced from eucalyptus pyrolysis probably because of its higher surface area as well as the higher values of its H/C and O/C ratios. Cetin et al.10,11 found that chars produced at high heating rates underwent plastic deformation (i.e., melted), which resulted in a different structure compared to their parent fuels. Senneca14 and Zanzi et al.15 reported that the content of mineral matter in the ash could have pronounced catalytic activity to enhance char reactivity. Avila et al.8 studied morphology and reactivity characteristics of 10 different biomasses, including energy crops (Miscanthus and corn), typical agricultural feedstock (wheat and short cereal), industrial waste sources [sunflower, rapeseed, and distiller’s dried grains (DDG), and dried distiller’s grains with solubles (DDGS)], and forestation wastes (olive and Swedish softwood). For all biomass fuels, their intrinsic reactivity during burnout was measured using a non-isothermal TG method, while the morphology of the products was characterized using SEM and oil immersion microscopy. They found that biomass fuels, such as

2. EXPERIMENTAL SECTION 2.1. Char Sample. Agrol, willow, and DDGS gasification has been carried out using an atmospheric pressure 100 kWth steam−O2 blown CFB gasifier. These three fuels were obtained from the company Lantmännen (Sweden). Willow is a common woody biomass. Agrol is a commercial solid as a kind of wood pellet, which is made from pure sawdust and shavings from sawmills, while DDGS is a byproduct from ethanol production. During gasification, different operation conditions [e.g., temperature, oxygen/biomass equivalence ratio (ER), and steam/ biomass mass ratio (SBR)] were used, and their influences on product gas distribution and tar formation were investigated. A detailed description of the gasification process is available from Meng et al.18 ER was calculated as the ratio of oxygen supplied to the oxygen required for the complete stoichiometric combustion of the biomass on a dry and ash-free basis, 723


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Table 2. Process Parameter Settings for CFB-Char Obtained from Agrol, Willow, and DDGS Gasification char type

char Agrol A4−15

char Agrol A11−23

char willow W4−19

char willow W12−1

char DDGS D10−9

temperature (°C) ER SBR

700−830 0.35−0.38 1.0−1.45

830−850 0.35 1.12

700−830 0.38−0.39 0.9−1.2

830−850 0.38 1.0

780−830 0.38 0.81−0.83

while SBR was calculated as the ratio of steam supplied to biomass supplied on an as-received basis. The CFB gasifier has a riser length of 5.5 m with an inner diameter of 83 mm and a downcomer inner diameter of 54 mm. It consists of flow meters, thermocouples, differential pressure meters, and weighing devices. There are two high-temperature filters [ceramic tissue candle filter (BWF), Germany] and a Si−SiC ceramic candle filter (Pall Filtersystems, Werk Schumacher, Germany), which can be switched during operation. The detailed characteristics of this CFB gasifier are available in several published papers.19,20 After gasification, some residual char samples (CFB-char), produced under different operational conditions, have been collected from the downcomer (see Table 2). In Table 2, A, W, and D represent the fuel type of Agrol, willow, and DDGS, while the number represents the “month−day” when char samples were collected. For example, A4−15 means Agrol char obtained after Agrol gasification on April 15, 2010,18 with similar settings for other char samples. In this work, the properties of these CFB-char samples were characterized using different techniques. Moreover, the gasification behavior of some selected CFB-char samples with CO2 was investigated (see below). Additionally, because it is fairly inconvenient to study the devolatilization process during Agrol, willow, and DDGS gasification in a CFB gasifier, the pyrolysis behavior of these three fuels was investigated using a TGA Q600 apparatus coupled with a FTIR spectrometer (Nicolet 5700). Some description about the FTIR spectrometer is reported by Giuntoli et al.21 The TGA apparatus is capable of providing a simultaneous measurement of heat flow and weight change of the same sample from ambient temperature (∼20 °C) to 1500 °C. A separate Inconel 600 tube permits introduction of reactive gases into the sample chamber. The FTIR spectrometer can be used to identify and quantify gases, such as H2O, CO, CO2, CH4, and NH3, released from pyrolysis. A small, heated stainless-steel line was used as a connection between these two apparatuses, allowing for purge gas N2 and released gaseous products to flow from TGA to the FTIR spectrometer. The sampling line and the gas cell of the FTIR spectrometer were kept at 150 °C to avoid the condensation of wax and tar produced during pyrolysis. A simplified schematic drawing of the TGA−FTIR setup is shown in Figure 1. Pyrolysis experiments were

temperature was increased at different heating rates (HR = 2, 5, 10, 30, 50, and 70 °C/min) to a desired pyrolysis temperatures (T_Pyr = 750 and 850 °C), the temperature was equilibrated at 35 °C for around 20 min to completely flush the gas cell of the FTIR spectrometer to obtain a good background for the gas analysis. The gasification behavior of char samples (PYR-char) obtained after pyrolysis with CO2 was further studied (see below). 2.2. Char and Fuel Characterization. The physical and chemical properties of the chars were studied by powder X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 adsorption/desorption at −196 °C, and SEM coupled with energy-dispersive scattering (EDS). SEM/EDS analyses were performed using an EVO 50 Series Instrument (LEO ZEISS) equipped with an INCAEnergy 350 EDS microanalysis system and INCASmartMap for imaging the spatial variation of elements in a sample (Oxford Instruments Analytical). The accelerating voltage was 25 kV, and the spectra collection time was 100 s. XRD powder analyses were carried out using a Philips PW1050/81 diffractometer equipped with a graphite monochromator in the diffracted beam and controlled by a PW1710 unit (Cu Kα, Ni filtered, λ = 0.154 18 nm). A 2θ range from 5° to 80° was investigated at a scanning speed of 70° h−1. XRF analyses were performed in an XRF spectrometer wavelength dispersion (XRF−WD) Panalytical Axios Advanced equipped with a Rh target X-ray tube and a 4 kW generator. A total of 2.5 g of char sample and 2 g of wax were milled for 10 min and then pressed at 200 kN to obtain a 40 mm diameter pellet. Standardless analyses with a collimation mask of 37 mm, were performed. The uncertainty in the measurements was approximately 5% of the given values. Specific surface area measurements were carried out using a Micromeritics ASAP 2020 instrument. Samples were previously degassed under vacuum, heated to 200 °C, and maintained for 200 min at a pressure below 30 μmHg. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method over the 0.005−0.1 p/p0 range (where p/p0 is the relative pressure, with p being the absolute pressure and p0 being the saturation pressure). The micropore area and external surface area were calculated from the t plot in the 3.5−5.0 Å t value range. The total pore volume was obtained at p/p0 = 0.995. The contribution of the micropores was obtained from the t plot, whereas the volume of the mesopores was obtained from the Barrett−Joyner−Halenda (BJH) method using the adsorption branch.22 In addition, inorganic main components in the Agrol, willow, and DDGS three fuels were analyzed by inductively coupled plasma− optical emission spectroscopy (ICP−OES) [model TJA-IRIS-Advantage with echelle optics and a charge injection device (CID) semiconductor detector, with an axial and radial observation wavelength of 165−900 nm]. The detection limit is ≤0.005 wt %. O and S were analyzed by the CHNS analyzer (System LECO). S was measured by the infrared (IR) absorption of the combustion gas SO2, while O2 was separately analyzed. 2.3. Char Reactivity. The gasification behavior of three CFBchars (A4−15, W4−19, and D10−9) and PYR-chars with CO2 was studied using TGA. The experimental procedures for char gasification have been described somewhere else.23,24 Before the experiments were performed, willow and DDGS chars were ground to small particles. The particle size distribution of char samples was performed using a Microtrac S3500 series particle size analyzer, and the particle size distribution was determined as well as proper images of very small particles to be seen. In general, around 90% of chars had a diameter below 0.9 mm. Char gasification was performed at different isothermal temperatures (T_Ga = 900, 1000, and 1100 °C) using different CO2

Figure 1. Schematic diagram of the TG setup modified from ref 24. performed in a N2 environment: N2 at a flow rate of ±100 mL/min supplied via a primary gas supply line (see Figure 1). Before the 724


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concentrations (CO2 = 10, 20, and 30 vol %). To achieve different CO2 concentrations, a certain amount of pure CO2 supplied via a second gas supply line was introduced and further mixed with N2 from the primary gas supply line. The CO2 flow rate was controlled by an external mass flow controller (see Figure 1). The volumetric reaction model (VRM) and the shrinking core model (SCM) were applied to determine Arrhenius kinetic parameters for char gasification. The VRM assumes that the char particle reacts homogeneously with CO2 and that the particle size remains constant, while the density decreases during the reaction.25 The SCM assumes that the reaction initially occurs at the external surface of char, and gradually, CO2 diffuses through the gas film and the ash layer and reacts on the unreacted core surface, which keeps on shrinking but always exists during the reaction progress.26,27 The overall reaction rates for VRM and SCM are expressed in eqs 1 and 2, where X, KVRM, KSCM, n, and CCO2 represent the char conversion ratio, the reaction rate constants of VRM and SCM, the reaction order, and the concentration of CO2 (vol %), respectively. KVRM/KSCM and X were calculated using eqs 3 and 4, where k0, Ea, Rg, and T represent the pre-exponential factor (min−1), the activation energy (J/mol), the universal gas constant (8.314 J mol−1 K−1), and the reaction temperature (K), and m0, mt, and mf represent the initial char weight, the char weight at time t, and the residue char weight, respectively.

dX = KVRM(1 − X )CCO2n dt

(1)

dX = 3KSCM(1 − X )2/3 CCO2n dt

(2)

⎛ −E ⎞ KVRM (or KSCM) = k 0 exp⎜⎜ a ⎟⎟ ⎝ R gT ⎠

(3)

X=

m 0 − mt m0 − m f

K and Ca on the surface, with smaller amounts of Mg, Fe, Al, Si, and P, while DDGS char had high contents of K and P, with some amounts of Na, Ca, and Mg observed on the SEM images. The main composition of the char samples obtained by XRF analysis and their parent fuels is summarized in Table 4. The results showed that the inorganic content of the Agrol char was mainly formed by Ca, Fe, K, Mg, and Si. The willow char was mainly composed of Ca and K, with minor amounts of Fe, Mg, P, and Si, whereas DDGS char was mainly dominated by K and P, with a lesser amount of Ca, Mg, and Na. When a comparison among the inorganic elements in the chars and the original biomass fuels is made,18 in general, there is a good agreement between the values, which means that the most abundant elements in their parent fuels were also those present in the chars in a higher percentage. However, some deviations from this behavior were observed. For instance, for Agrol, a quite larger Fe content was measured in the char together with an increased amount of Mg and Si. This behavior may be due to the deposition of some olivine bed material. O measured by XRF is related to the presence of oxygen-containing compounds, such as oxides or phosphates in the chars; therefore, the largest O content in DDGS and willow chars would suggest a higher ash content in the chars. In fact, from XRF analyses, it was determined that DDGS char had the highest ash content, followed by willow and Agrol chars, which were in good agreement with the ash contents of the original biomass fuels.18 It should be remarked that XRF analysis gives accurate relative concentrations of the elements. However, the overall mineral content in the char may be overestimated because of the nature of the samples.28 A comparison of the XRD patterns from different char samples is shown in Figure 5. It can be seen from XRD patterns that, for all char samples, two broad diffraction lines at approximately 23° and 44° 2θ were observed over the examined 2θ range (5−80° 2θ), which were attributed to the (002) and (101) diffraction lines of a graphite-like structure. Therefore, it may be stated that the gasification temperature does not largely modify the structure of the chars. Unlike for graphite, aromatic rings forming layers of chars are irregularly stacked and randomly arranged. The background observed in the diffraction patterns is related to the presence of amorphous carbon, whereas the low-angle (002) diffraction peak was attributed to the existence of a γ band on its left side associated with the packing of a saturated structure, such as aliphatic side chains.12,29,30 Moreover, the shift of the (002) peak from 25° 2θ, as generally observed for graphite carbons,31,32 to 23° 2θ indicated a highly disordered structure of biomass chars.12,16 The increase of the intensity of the diffraction lines for the Agrol chars appointed that a more regularly ordered carbon lattice structure was achieved, not only in the stacking of the layer but also in a single atomic plane. Because the (101) peak of 44° 2θ was attributed to graphite-like atomic order within a single plane, a sharper diffraction line for willow and Agrol chars indicated a higher crystallite diameter in these solids.12 Furthermore, the sharp peaks observed at around 31° and 44° 2θ in the DDGS char were related to the presence of a potassium calcium phosphate, whereas in willow chars, the reflection lines of SiO2 (quartz) were observed at 21°, 26°, and 42° 2θ. N2 adsorption/desorption isotherms of three char samples are shown in Figure 6. It is remarked that, because the diffusion of N2 into the micropore network was very slow, the

(4)

3. RESULTS AND DISCUSSION 3.1. Characterization of CFB-Char. The microstructure and qualitative chemical composition of the chars was studied by SEM coupled with EDS. The SEM images of Agrol, willow, and DDGS chars are shown in Figures 2, 3, and 4, respectively. It can be observed from these SEM images that Agrol chars were very porous with different superficial cavities and thin walls, which indicates that the fibrous structure of the parent biomass was practically retained. The macropores observed were probably related to the evolution of the volatile matter during the gasification. There were no significant differences between A11−23 and A4−15 samples. However, surprisingly, from Table 2, it can be seen that these two Agrol chars were obtained from Agrol gasification experiments when different operation conditions (e.g., SBR, ER, and temperature) were applied. Willow chars also had a fibrous morphology, while the structure was more compact and agglomerated, which points toward some plastic deformation that might have taken place. Moreover, small particles deposited on the surface were observed, which may be attributed to some ashes. DDGS chars had a macroporous structure with rounded pores of different sizes and some particulate matter on the surface. Some slit-shaped pores were also observed in the DDGS char. In these samples, plastic deformation seemed to take place to a greater extent. The results observed from EDS analysis revealed that the composition of the chars was not completely homogeneous, the inorganic elements were mainly present in the small particles, and their amounts varied depending upon the zone analyzed. Agrol and willow chars had high contents of 725


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Figure 2. SEM and EDS pictures of Agrol chars.

equilibration of N2 took a relatively long time and, in some cases, could not be reached during the measurement.10 The isotherms were classified according to the International Union of Pure and Applied Chemistry (IUPAC) classification.33 The shape of the isotherms of the char samples depended upon the biomass fuel. Agrol char isotherms were classified as type I, characteristic for microporous materials (500 Å). The hysteresis loops were characteristic for slitshaped pores. Lastly, the analyzed results showed that the DDGS char was mainly composed of micropores. The differences in the adsorption and desorption branches at low pressure may be related to the condensation of N2 in the micropores that could not be evacuated during the desorption part of the analysis; this behavior was previously reported by

Lee et al. in biochars produced from cornstover under fast pyrolysis conditions.34 BJH pore size distributions obtained from the adsorption branch (see Figure 7) indicated that Agrol char contained mesopores with sizes between 2 and 200 Å, with a maximum at ca. 30 Å. The average pore diameter was around 40 Å. Willow chars showed slightly larger mesopores, between 20 and 400 Å, with an average pore diameter of around 62 Å. On the other hand, no mesopores were observed in the DDGS 727


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Figure 4. SEM and EDS pictures of DDGS chars.

char. The average pore diameter obtained by the Horvarth− Kavazoe (HK) method for the microporous samples was in the 5−10 Å range, with a maximum at around 6.8, 6.9, and 5.9 Å for A4−15, W12−1, and W4−19, respectively. Specific surface area values obtained using the BET method are shown in Table 3. It can be seen that the Agrol chars had the largest specific surface area (SBET), with values up to 521 m2/g (A11−23), followed by willow chars, with values up to 439 m2/g (W12−1), and DDGS char, with values of 22.5 m2/g. In all cases, a large part of the surface area corresponded to the micropore area (Sm). The porosity of the inorganic matter (ashes) in the samples was considered negligible. No significant difference was observed in SBET of two Agrol char samples (504 m2/g for A4−15), regardless of the gasification temperature applied, while they did differ for two willow char samples (296 and 439 m2/g for W4−19 and W12−1, respectively). The higher the temperature, the larger the surface area values. The significant difference in SBET of different fuel chars could be related to the ash content in parent fuels, because DDGS had a much higher ash content (4.82 wt %) than those of willow (2.52 wt %) and Agrol (0.14 wt %).

The large amount of ash in the DDGS biomass may melt, leading to plastic transformations of the char and blocking of the pores; moreover, the presence of dead-end pores in the char prevented any access to the adsorbing gas.35 However, the presence of micropores not measured by N2 adsorption cannot be ruled out. The total pore volume (Vp) values, because of both micropores (Vm) and mesopores (Vmeso), were in agreement with the specific surface area values; i.e., the larger the surface area, the larger the pore volume. 3.2. Fuel Pyrolysis Behavior. The weight loss (TG, %) and derivative weight loss (DTG, %/min) for Agrol, willow, and DDGS at different heating rates (HR = 2, 5, 10, 30, 50, and 70 °C/min) are shown in Figure 8. In Figure 8, H2-TG and H2-DTG represents TG and DTG curves for three fuels at HR = 2 °C/min, respectively, with similar settings for others. The results presented in Figure 8 show that heating rates largely affect the TG/DTG profiles of Agrol pyrolysis in all stages. In general, the TG/DTG curves shifted toward higher temperatures as the heating rate increased. The weight loss started from the starting of the experiment to approximately 150−200 °C, which can be seen from the observable peak in 728


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Figure 5. Comparison of the XRD patterns from different char samples.

the DTG curves. This first-stage weight loss mainly corresponded to the release of the moisture and some light-volatile compounds in the biomass sample.36,37 Within the temperature range of 200− 800 °C, the volatiles in Agrol were gradually released, which can be seen from another three peaks observed in the DTG curves: a less pronounced shoulder peak, a remarkable main peak, and a long tail zone. This pyrolysis behavior is well-known and identified for lignocellulosic materials, where the shoulder, the main peak, and the long tail are mainly attributed to the decomposition of hemicellulose, cellulose, lignin, and extractives.38−41 According to Yang et al.,41 the decomposition of hemicellulose and cellulose, respectively, occurred within the temperature ranges of 220− 315 and 315−400 °C, while that of lignin happened slowly and covered a broad temperature range from 150 to 900 °C. The hemicellulose- and cellulose-related peaks are normally overlapped largely because of the mineral matter present in the biomass sample acting as a catalyst for their thermal decomposition, as reported by Lapuerta et al.42 and Varhegyi et al.43 A similar pyrolysis behavior was observed for willow at different heating rates. However, in general, willow had a better separation between the hemicellulose- and cellulose-related peaks. The shoulder peak occurred at a lower temperature and became more evident. These observations generally agreed with the reported results from other researchers.44,45 Biagini et al.44 studied the devolatilization of different biomass residues and found that the DTG curves of olive cake and rice husks were fairly similar at various HR, whereas the shoulder that occurred at earlier temperatures was more evident for olive cake than rice husks because of their different properties. Grønli et al.45 reported that hardwoods generally exhibited a more clear-cut separation than softwoods between the first and second reaction zones, which were related to the decomposition of hemicellulose and cellulose. Kastanaki et al.46 reported that the less pronounced and well-pronounced shoulders observed in the DTG curve of the cotton residue and forest residue, respectively, indicated less amount of hemicellulose in the cotton residue than in the forest residue. The pyrolysis behavior of DDGS was fairly different from Agrol and willow fuels. The results presented in Figure 8 indicate that the overall decomposition temperature range of DDGS was definitely broader than those of Agrol and willow. The hemicelluloserelated peak became a well-defined peak instead of a shoulder peak. There was also no clear separation between the drying and pyrolysis steps. Furthermore, another additional peak was

Figure 6. Comparison of the N2 adsorption/desorption isotherms from different chars.

observed in the tail zone of DDGS pyrolysis, which was probably due to some residual compounds remaining from the ethanol fermentation process, as reported by Giuntoli et al.21 To better quantify effects of different heating rates on pyrolysis characteristics of these three fuels, several characteristic devolatilization temperatures and their related rate parameters were introduced here, which were calculated as the method suggested by Grønli et al.45 Figures 9 and 10 show the change trends of several characteristic devolatilization temperatures and their related weight loss rates at various heating rates. Tonset and Toffset represent the temperatures when the pyrolysis started and ended, and Tshoulder and Tmax represent the temperatures when the hemicellulose-related shoulder peak and cellulose-related main peaks occurred, respectively, while Ttail represents the temperature when the peak occurred in the tail zone of DDGS. Rshoulder 729


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Figure 8. TG and DTG curves for Agrol, willow, and DDGS at different heating rates.

From Figure 9, it can be clearly seen that characteristic devolatilization temperatures and their related weight loss rates increased with increasing the heating rate, which may be due to heat-transfer limitations, as reported by Kumar et al.36 and Aqsha et al.47 According to Aqsha et al.,47 the heat transfer between the crucible and the sample was more efficient at a lower heating rate, which resulted in a proper drying and pyrolysis process. Contrarily, less efficient heat transfer may occur at a higher heating rate, which led to a faster increase in the devolatilization rate, thus shifting the peak of the weight loss rate. As seen in Figure 10, with increasing the heating rate from 2 to 70 °C/min, Rmax observed from Agrol pyrolysis sharply increased from 2.4 to 61%/min; meanwhile, Tmax shifted from approximately 337 to 405 °C. For DDGS, it was difficult to determine Tonset because the variations in DTG curves are hardly detectable. Because of high fluctuation and vibration of derived DTG, all characterized temperatures (except for Tmax) at HR of 2 and 5 °C/min were less accurate and may have ±0−10 °C difference. Agrol fuel had the highest Tonset, Tshoulder, and Tmax values, followed by willow and DDGS fuels, while DDGS fuel had the highest Toffset value,

Figure 7. Comparison of the BJH pore size distributions of different char samples.

Table 3. Specific Surface Area Values (SBET), Micropore Surface Area (Sm), External Surface Area (SEXT), Total Pore Volume (Vp), Micropore Volume (Vm), and Mesopore Volume (Vmeso) of Different Char Samples Obtained by N2 Adsorption/Desorption sample

SBET (m2/g)

Sm (m2/g)

SEXT (m2/g)

Vp (cm3/g)

Vm (cm3/g)

Vmeso (cm3/g)

DDGS W4−19 W12−1 A4−15 A11−23

22.5 296 439 521 504

22 194 348 404 433

a 102 90 117 71

a 0.222 0.259 0.272 0.314

a 0.082 0.143 0.166 0.218

a 0.121 0.178 0.092 0.052

a

Not available.

represents the weight loss rate occurring at the temperature of Tshoulder, and other settings are similar. 730


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HR = 10 °C/min were measured and quantified using the FTIR spectrometer, and the results are shown in Figures 11 and 12,

Figure 9. Effects of heating rates on different characteristic devolatilization temperatures for Agrol, willow, and DDGS.

Figure 11. Light volatiles released from for Agrol and willow pyrolysis.

Figure 10. Effects of heating rates on different maximum rates for Agrol, willow, and DDGS.

Figure 12. Light volatiles released from for DDGS pyrolysis.

followed by willow and Agrol fuels, which meant that DDGS fuel had a wider decomposition range (Toffset − Tonset) and Agrol and willow fuels had a similar decomposition range. These observations generally agreed well with the results reported by other researchers.21,44,45 For example, Biagini et al.44 reported that the higher the heating rate, the higher the values of Tonset, Tmax, and Toffset. With increasing the heating from 5 to 100 °C/min, the observed Tonset in the DTG curves of rice husks increased from around 257 to 344 °C. Aqsha et al.47 found that, with increasing the heating rate from 5 to 50 °C/min, Rmax observed in the DTG curves of sawdust devolatilization sharply increased from approximately 4 to 26%/min. 3.3. Light Volatiles from Pyrolysis. The light volatiles released from Agrol and willow and DDGS pyrolysis at

respectively. From these figures, it can be seen that the products released from Agrol, willow, and DDGS are mainly CO, CO2, and H2O, followed by small amounts of CH4. For three fuels, it seemed that their physically absorbed moisture was evolved during the drying process below a temperature of approximately 150 °C and then the pyrolytic water was released continuously up to a temperature range of 500−700 °C. These general observations agreed with the results reported by other researchers.21,48 It can be seen in Figure 11 that the emissions of CO and CO2 from Agrol and willow pyrolysis showed remarkable peaks within the temperature range of 340−440 °C, and the shapes of their curves were fairly similar to their DTG curves. The release of CO and CO2 could be largely attributed to the decomposition of the two macro components (hemicellulose 731


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around 395 and 664 °C, respectively. The different emissions of N compounds observed between Agrol, willow, and DDGS pyrolysis could be due to their different structural properties. As wellmentioned in a previous study,18 the N amount (wt %, dry) present in Agrol, willow, and DDGS fuels was around 0.15, 0.69, and 5.52 wt %, respectively. These values indicate that DDGS fuel contains much more amounts of N than Agrol and willow. A similar conclusion has also been drawn by Giuntoli et al.,21 who reported that almost no N compounds were detected from olive residue and peach stone pyrolysis because of their relatively low N content (0.8 and 1.4 wt % N in peach stones and olive residues, respectively). 3.4. Char Gasification Results. 3.4.1. Char Gasification Behavior. The gasification behavior of PYR-char and CFB-char under different operational conditions was examined. Figures 13,

and cellulose), which are normally present in biomass fuels. However, there are some controversies about which component decomposition chiefly contributes to the release of CO and CO2. Jeguirim et al.49 studied the devolatilization kinetics of Miscanthus straw and reported that the decomposition of hemicellulose and cellulose components led essentially to CO and CO2 emissions. Yang et al.41 and Wang et al.50 found that the cellulose decomposition contributed limitedly to the release of CO and CO2 compared to the release because of hemicellulose decomposition. Their reported results appeared a bit contrarily to the results obtained in this work, because the peak related to the hemicellulose decomposition normally occurred below 315 °C, while the released peaks of CO and CO2 observed in this work occurred at around 360 °C, which are within the decomposition temperature range of cellulose. This behavior has been well-identified and explained by Giuntoli et al.39 They reported that, because the decomposition of the cellulose component was highly affected by the presence of inorganic catalysts in the fuel samples,40,43 its related peak was usually at a lower temperature than expected from the single cellulose experiments, which is why some researchers found that the contribution of cellulose to the global release of CO and CO2 was limited in comparison to those from hemicellulose decomposition.41 The release of CH4 occurred within a widely higher temperature range of 440−640 °C, which was because CH4 was normally derived by the cracking of methoxyl groups in the lignin part of biomass.39,41 It can be seen in Figure 12 that the emission behavior of CO2 and CH4 from DDGS pyrolysis was fairly similar to those from Agrol and willow pyrolysis. However, in comparison to the released behavior of CO from Agrol and willow pyrolysis, the released CO from DDGS pyrolysis occurred in a wider temperature range, even up to 840 °C, and the decomposition of cellulose and hemicellulose components appeared to show a less prominent contribution to CO emission. Moreover, a released CO tail peak was observed at a temperature as high as 840 °C. This behavior has also been observed by other researchers. For example, Giuntoli et al.21 and Jiang et al.51 observed a released CO tail peak at a temperature of around 890 °C. The release of CO up to high temperatures was largely attributed to the secondary reaction of the residues, which condensed in the char.21,41,44 The aforementioned SEM analysis results also indicated that the surface of DDGS chars contained some condensed particulate matter compared to Agrol and willow chars, which might explain the different CO emission behaviors observed between the pyrolysis of these three fuels. Besides H2O, CO, CO2, and CH4, some additional amount of N compounds, such as NH3, HCN, and HNCO, were detected from DDGS pyrolysis. However, the amount of these N compounds released from Agrol and willow pyrolysis seemed to be negligible. Among the three N compounds, NH3 was the main N compound released at low temperatures ( Na−char > Ca−char > Fe−char > Mg− char > raw char. Zhang et al.56 studied the gasification reactivity of biomass chars derived from a wide range of plant origins and concluded that the maximum rate at a high conversion range was mainly attributed to the catalytic effect of K. Thus, it is easily understandable that DDGS and willow chars were more reactive than Agrol char because of the enhanced catalytic effects of inorganic elements in their ashes. Moreover, XRD analysis results also indicated that Agrol char had a better carbon crystalline order, which could also lower its reactivity, as reported by Kumar et al.,57 Cetin et al.,11 and Lu et al.,29 probably by lowering the concentration of reaction sites. However, the high ash content present in the char sample could decrease the active surface area of chars,58 because the internal structure becomes less accessible to the gaseous reagents for the heterogeneous reactions.59 Indeed, SEM and EDS analysis results indicated that DDGS char had more condensed particles on its surface, and it also had a much lower SBET (22.5 m2/g) compared to willow char (296 m2/g), which may be the reason why DDGS char turned out less reactive than willow char. DeGroot and Shafizadeh60 reported that the inorganic content of the lignite char was more than 5 times greater than that of cottonwood char, but their reactivities were similar. Cetin et al.11 reported that a higher global char gasification reactivity was

enhanced with increasing either the gasification temperature from 900 to 1100 °C or the CO2 concentration from 10 to 30 vol %. This observation is fairly reasonable because, at a higher temperature, the reaction rate is increased as more energy is supplied to overcome the Ea barrier, which is well-described by the Arrhenius equation.53 The incomplete char reaction at a lower CO2 concentration was probably due to the reduction of active site density by N2 in a high concentration surrounding the surface.54 Similar results were observed for the gasification of willow and DDGS chars, which can be clearly observed in Figures 14 and 15. However, willow and DDGS PYR-char obtained at HR = 10 °C/min and CFB-char had a much higher conversion rate when they were under the above-mentioned unfavorable gasification conditions. According to Di Blasi,7 an enhancement in the char conversion rate is ultimately due to an improvement of several important factors: surface area and accessibility, carbon active sites and catalytic active sites created by indigenous or added inorganic matter, and the local gaseous reactant concentration. Consequently, the reactivity is determined by the chemical structure, inorganic components, and porosity of the samples. When the results obtained during the characterization of the chars are taken into account and the porosity of the samples is considered (see Table 3), it was expected that Agrol char, which showed the largest surface area and highest porosity, should be the most reactive char sample; however, the opposite behavior was observed. The differences among the reactivities of char samples from different fuels under unfavorable gasification conditions are probably due to catalytic effects of the ashes in the fuels, as reported by other researchers.55,56 The aforementioned XRF 733


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Table 4. Main Composition (Mass %, db) of the Char Samples Obtained by XRF and Their Parent Fuels char type

char Agrol A4−15

char Agrol A11−23

char willow W4−19

char willow W12−1

char DDGS D10−9

Agrol

willow

DDGS

Al Ca Fe K Mg Mn Na O P S Si

0.22 3.74 2.03 2.40 0.97 0.61 0.02 5.51 0.08 0.02 1.13

0.17 3.26 2.70 2.08 1.16 0.65 0.03 5.42 0.10 0.02 0.88

0.44 10.44 1.04 4.60 0.89 0.22 0.08 9.89 0.93 0.11 1.47

0.31 11.60 1.12 4.61 0.56 0.21 0.06 10.29 0.99 0.10 1.62

0.02 1.37 0.16 11.75 1.58 0.09 1.06 15.13 6.02 0.02 0.28

Biomass Gasification - American Chemical Society

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