Experimental Investigation of the Oxidative Pyrolysis Mechanism of

Article pubs.acs.org/EF

Experimental Investigation of the Oxidative Pyrolysis Mechanism of Pinewood on a Fixed-Bed Reactor Shanhui Zhao, Yonghao Luo,* Yi Su, Yunliang Zhang, and Yufeng Long Institute of Thermal Engineering, Biomass Energy Research Center, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ABSTRACT: Oxidative pyrolysis of pinewood was studied on a bench-scaled fixed-bed reactor. The qualitative and quantitative analysis of oxidative pyrolysis products, including permanent gases (CO, CO2, and CH4), water, char, and tar, was conducted. Two important parameters (temperature and oxygen concentration) were taken into consideration. Results showed that oxygen improved the yields of permanent gas and water but decreased the yields of char and tar. In comparison to char and water, oxidative pyrolysis had a greater effect on permanent gas and tar yields. CO and CH4 were mostly released between 300 and 400 °C, while CO2 was produced at all of the temperature investigated. CO2 was always the dominant gas in all cases. At a relatively low temperature (300 °C), the adsorption of an oxygen molecule on the reactive center and the subsequent decarbonylation reaction lead to the production of CO2. Little CO and CH4 generated when the temperature was higher than 400 °C. Gravimetric results of pyrolysis tar indicated that the tar yield decreased from 0.3321 g/g of biomass (700 °C and 0% O2) to 0.1901 g/g of biomass (700 °C and 21% O2). Gas chromatography/mass spectrometry results showed that, under an oxidative atmosphere, primary tar components tended to be converted to secondary tar. The phenols would also be converted by the partial oxidation reaction under high oxygen concentrations. Oxygen promoted the development of the pore structure when the oxygen concentration was no more than 15%. However, oxygen would restrict the further development of the char pore under ultimate conditions, resulting from the high char combustion rate at high oxygen concentrations.

1. INTRODUCTION Biomass is one of the most potential renewable energy resources, and thermal−chemical conversion is an efficient method for biomass use, especially in China, where the biomass resource is dispersive and diverse. Pyrolysis of biomass receives great attention because it is a key and complex step in the thermal−chemical conversion process.1−3 Traditional pyrolysis is under an inert atmosphere without oxygen. However, in the biomass combustion process and some novel gasification technologies, biomass particles undergo pyrolysis under a certain oxygen concentration.4,5 In addition, oxidative pyrolysis can be used as a way to achieve an autothermal regime in conventional pyrolysis, by introducing small amounts of oxygen.6,7 Li et al.6 conducted oxidative pyrolysis of birch bark on a fluidized-bed reactor and found that autothermal operation was possible with oxygen addition in the pyrolysis reactor. Su et al.8 studied the degradation behavior, carbon production, and heat properties of the pinewood oxidative process on thermogravimetric analysis (TGA) coupled with mass spectrometry and differential scanning calorimetry (DSC) methods. However, the oxidative pyrolysis tar is not given attention, and the product gases were not investigated quantitatively. Senneca et al.9,10 studied the competition process between purely pyrolytic processes and heterogeneous oxidation during oxidative pyrolysis of non-fossil solid fuels in a thermogravimetric apparatus. They found that heterogeneous oxidation and pyrolytic processes played different roles depending upon the nature of the fuel, and a general feature of all fuels tested that burnoff could not be simply described as sequential reaction paths, namely, purely thermal degradation, followed by heterogeneous oxidation, while the weight loss curve had two distinct peaks. They suggested that the © 2014 American Chemical Society

synergistic effects between thermal degradation and heterogeneous oxidation needed to be included. Azik et al.11,12 investigated the change of structural characteristics of Turkish Beypazari lignite in air oxidation of 150 °C by Fourier transform infrared spectroscopy (FTIR). It was shown that, in the early stages of oxidation, −CO (carbonyl groups) and −COOH were formed. Sánchez and Rincón13 studied the oxidation paths of coking coal at 125 °C. The two steps of the oxidation reaction were found, in which phenolic hydroxyl and carbonyl were formed separately. The mechanism of coal coke oxidation was put forward. Maskos and Dellinger14 studied the radicals from the oxidative pyrolysis of tobacco under different oxygen concentrations and temperatures in a flow reactor system and explained the source and formation of radicals. Milhé et al.15 investigated the wood autothermal and allothermal pyrolyses in a continuous fixed-bed reactor. The temperature along the bed and the product yield were measured. They found that, under oxidative conditions, fixedbed pyrolysis led to smoldering and the formation of a stable ignition front below the bed surface. Oxidative conditions also decrease the yields of organic condensates and promote the yields of permanent gases, such as CO and CO2. However, the effect of the oxygen concentration and temperature on biomass oxidative pyrolysis, especially the tar and char characteristics, is not fully investigated. Most oxidative degradation investigations focus on coal or synthetic polymer,16−18 while seldom do Received: December 4, 2013 Revised: July 1, 2014 Published: July 7, 2014 5049

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Figure 1. Oxidative pyrolysis of solid fuel particles.

Table 1. Proximate and Ultimate Analyses of Pinewood ultimate analysis (wt %)

a

proximate analysis (wt %)

C

H

Oa

N

S

moisture

volatiles

fixed carbon

ash

Qb (MJ/kg)

46.35

5.56

47.95

0.14

0

3.05

69.08

24.32

3.55

18.20

Calculated by balance. bHigh heating value.

oxidative degradation investigations focus on biomass at a controlled forced oxygen flow condition. The typical heterogeneous oxidation of solid fuels is shown in Figure 1. Oxygen diffuses through the pore of particles and is adsorbed on the functional groups of solid particles. The formation of reactive functional groups will promote the degradation of solid fuels to release permanent gases and organic condensates. To reveal the oxidative pyrolysis mechanism of biomass and support the application in engineering of biomass thermal conversion, the oxidative pyrolysis mechanism of biomass was investigated on a fixedbed reactor called “hot-rod”. The effect of the temperature and forced convection oxygen concentration on oxidative pyrolysis was taken into consideration. The release of tar, biochar, water, and permanent gases was investigated qualitatively and quantitatively. This work focused on the two most important parameters: temperature and oxygen concentration. The results will serve the oxidative pyrolysis and gasification technology development.

2. MATERIALS AND METHODS Figure 2. “Hot-rod” fixed-bed reactor.

2.1. Materials. Pinewood received from the Nanhui area, Shanghai, China, was chosen as the raw material. The pinewood sample was sieved into small particles with a diameter ranging from 100 to 150 μm. The proximate analysis was conducted on STA 409PC (NETZSCH, Germany), and the element analysis was conducted on Vario EL IIIElementar (manufactured in Germany). The proximate and ultimate analysis results are shown in Table 1. 2.2. Experimental Apparatus and Methods. A bench-scaled fixed-bed gasifier called hot-rod was used to investigate the pinewood particle oxidative pyrolysis process, as shown in Figure 2. The detailed description of the hot-rod reactor was illustrated by Wu et al.19 and Pindoria et al.20 The hot-rod was made of 316-grade stainless steel with inner and external diameters of 12 and 16 mm, respectively, and length of 200 mm. In each experiment, 1 ± 0.01 g of pinewood sawdust with a diameter between 100 and 150 μm was used. The carrier gas was helium, which had a boiling point low enough to avoid condensation in the U-style tar trap part. The carrier gas flow rate was controlled at 200 mL/min by a mass flow controller (MFC, made by Sevenstar). A

K-type thermocouple was used to measure and control the pyrolysis temperature. The U-style tar trap was immersed in the liquid nitrogen to ensure that the tar was collected completely. The tar was condensed and collected in the liquid nitrogen bath. Most of the tar conducted in the reactor was condensed in the U-style tube, as shown in Figure 2. 2.3. Analysis Methods. The pyrolysis tar was collected and dissolved in the mixture solvent (chloromethane and methanol) with a volume ratio of CHCl3/CH3OH equal to 4:1. Each tar solution was analyzed by gas chromatography/mass spectrometry (GC/MS) using an Agilent 6890N gas chromatograph and an Agilent 5975C mass spectrometer. The GC split ratio was 10:1, and the oven temperature was set at 280 °C. A HP-5MS column (30 m × 0.25 mm, 0.25 μm) was used at a flow rate of 2.4 mL/min. The column temperature was increased from 45 to 180 °C at 5 °C/min and then to 300 °C at 45 °C/min, where it was held for 10 min. The sample injection volume was 1 μL. 5050


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Halenda (BJH)24 theories were used to calculate the specific surface, pore diameter distribution, pore volume, and some other parameters.

To quantitatively investigate the tar production and conversion pattern, the calibration of phenol on GC/MS was conducted, as shown in Figure 3.

3. RESULTS AND DISCUSSION 3.1. Effect of the Oxygen Concentration on the Pyrolysis Product Distribution. The mass yield ratio is defined as Myield = mi/mbiomass, in which Myield (g/g of biomass) is the product yield ratio, mi is the product yield, and mbiomass is the initial mass of the biomass particle used. mi is the product yield of gas, solid product, tar, and water. The gas yield was calculated by the sum of CO, CO2, and methane, which were analyzed and quantified in GC. Although there more gas components were produced, in our work, the yields of H2 and other hydrocarbon gases were quite small and not included. Therefore, the gas yield used here was a little lower than the true value. The solid product was mainly pyrolysis char and was obtained by the weight of pyrolysis char. Tar was first collected in CH3OH/CHCl3 solution, and then the solution underwent evaporation on a RE3000Arotary evaporator. The residue was put in a drying oven under an inert atmosphere at 35 °C for 1 h. After that, the tar was measure by weight on a electronic balance (Mtar = Mtar + beaker − Mbeaker). The oxygen consumption was calculated by the difference value of the supply amount and outlet content. The oxygen supply amount was calculated by multiplication of the MFC flow rate and sampling time. The oxygen content in the product gas was measured by GC−TCD. The water yield was calculated by balance. Figure 4a shows the pyrolysis products (water, gas, tar, and char) distribution under different oxygen concentrations at 500 °C. With the increase of the oxygen concentration, the char and tar yields decreased, while water and gas increased. The total product mass increased because more oxygen reacted in the oxidative pyrolysis under higher oxygen concentrations, as seen in Figure 4b. These results agree well with those obtained by Milhé et al.21 in a continuous fixed-bed reactor. The sum of measured and calculated product yields exceeded 100%. This was because of the oxygen reacted with biomass and converted into pyrolysis products. Mesa-Pérez et al.7 also observed a similar phenomenon with the increase of the equivalence ratio. However, Kim et al.22 observed that the yield of bio-oil showed little difference under different oxygen concentrations. This may be on account of the style of the reactor. The liquid

Figure 3. Calibration of phenol on GC/MS.

The permanent gases were collected in air bags and analyzed in Shimadzu GC-14B qualitatively and quantitatively. Four gas components (CO, CO2, CH4, and O2) were measured and analyzed. To reduce the effect of secondary reactions, the carrier gas of the reactor was chosen as 200 mL/min. Therefore, in the produced gases, the concentrations of CO, CO2, and CH4 were quite lower than oxygen. A TDX-01 column coupled with a flame ionization detector (FID) was used to analyze CO, CO2, and CH4. Each sample injection volume was 1 mL. The temperatures of the column, detector, and injection port were set as 100, 250, and 100 °C, respectively. The 5A molecular sieve column coupled with a thermal conductivity detector (TCD) was used to analyze oxygen. The temperatures of the column and detector were set as 60 and 120 °C, respectively. Each sample injection volume was 0.1 mL. In our previous work, we found that the amount of hydrocarbons, of which the molecule weight was larger than C2, was quite small, especially at the fixed-bed reactor, which was also proven by Milhé et al.21 and Kim et al.22 Therefore, the analysis of larger hydrocarbon gas components was not conducted here. The pore structure of char was analyzed by the TristarII 3020 automatic adsorption instrument. The analysis temperature was −196 °C (liquid nitrogen conditions), and high-purity nitrogen was chosen as the adsorbing medium, with a relative pressure ranging from 0.01 to 0.995. The Brunauer−Emmett−Teller (BET)23 and Barrett−Joyner−

Figure 4. Pinewood 500 °C pyrolysis (a) product distribution and (b) oxygen consumption under different oxygen concentrations. 5051


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product passed through a fixed char bed and underwent secondary reaction adequately. 3.2. Effect of the Oxygen and Temperature on the Gas Component Yields. Figure 5 shows three permanent gas (CO, CO2, and CH4) yields with the temperature and oxygen concentration. At 300 °C, the yields of CO and CH4 almost

show no difference under different oxygen concentrations, while CO2 shows a distinct difference, which means O2 promoted pinewood pyrolysis to form more CO2 at a relatively low temperature. When the pyrolysis temperature increased from 300 to 400 °C, the yields of CO and CH4 at oxidative atmosphere increased sharply, as shown in panels a and c of Figure 5. When CH4 is taken as an example, the yield increases from 0.00125 g/g of biomass (300 °C and 21% O2) to 0.03321 g/g of biomass (400 °C and 21% O2). This phenomenon indicates that the oxygen promoted the CO and CH4 yields between 300 and 400 °C remarkably. With a further increase of the pyrolysis temperature, the CO and CH4 yields were almost kept constant, which meant that, when the temperature was higher than 400 °C, only a little of CO and CH4 was released from pinewood particles. Unlike CO and CH4, the CO2 yield increased with the temperature at an oxidative atmosphere, even when the temperature was higher than 700 °C. However, in an inert pyrolysis process, no more CO2 was released at 500 °C or higher temperatures. At high temperatures, the pyrolysis char might react with oxygen to form CO2. In all cases, CO2 was always the dominant component and the content of CO2 was far more than CO and CH4. Milhé et al.21 also found similar results by comparison of autothermal and allothermal pyrolyses of wood. CO, CO2, H2, and CH4 were the four main permanent gas components in their products. All of these components increased under autothermal conditions. Kim et al.22 observed that the non-condensable gas yield was 13.2 g/ 100 g of biomass and CO and CO2 were the two major gases. Small amounts of CH4, C2H4, and H2 were produced. The sum of the yields of these three gases was less than 1 g/100 g of biomass. They attributed the accelerated formation of CO and CO2 with the increasing oxygen concentration to the partial oxidation reaction and stronger cracking reaction. 3.3. Water Yield Analysis. Water is a kind of important pyrolysis product. As shown in Figure 6, the water yield reached

Figure 6. Effect of the oxygen and temperature on the water yield.

a maximum of 0.0742 g/g of biomass at 500 °C under an inert atmosphere. However, with the addition of oxygen, the yield of water increased, and the higher the oxygen concentration, the larger the yield of water. Even when the temperature was higher than 700 °C, there was still quite an amount of water produced, which was similar to CO2.

Figure 5. Effect of the oxygen and temperature on gas release behavior.

RH → R•+H• 5052

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R•+O2 → ROO•

(2)

ROO• + RH → ROOH + R•

(3)

ROOH → RO• + HO•

(4)

•

•

2ROOH → RO +ROO +H 2O •

•

(5)

RO → CO + R′

(6)

ROO• → CO2 + R′•

(7)

R−CH3 → R•+CH3•

(8)

R−O−CH3 → R−O• +CH3•

(9)

CH3•+H• → CH4

secondary tar or permanent gases. Similar results and tendency were also obtained by Milhé et al.21 and Mesa-Pérez et al.7 To reveal the effect of the oxidative pyrolysis mechanism on the tar yield, the tar components under different oxygen concentrations was investigated and shown in Table 2. The relative concentration is defined as the ratio of the GC peak area of each component to the sum peak area of all components that were detected. Because of the sampling and analysis method that we used, components with small molecular weight (lower than 84, such as formic acid, acetic acid, and ketones) in the tar were not analyzed here. The yields of these components are quite abundant but not defined as gasification tar components. This work focused on the tar conversion, and such components were not analyzed. This will be improved in our future work. As shown in Table 2, 46 tar species are listed. The relative concentration of levoglucosenone, 1,4:3,6-dianhydro-α-D-glucopyranose, and 1,6-anhydro-β-D-glucopyranose (levoglucosan), which are homologous compounds similar to levoglucosan, decreased with the increase of the oxygen concentration. Levoglucosan is the most important product of cellulose pyrolysis. However, with the addition of oxygen, on one hand, the reaction path leading to the production of levoglucosan may be weakened, but on the other hand, the oxygen-increased in situ temperature thus promoted the conversion of primary tar components, including levoglucosan. Similar to levoglucosan, acids are also regarded as primary tar, which have weak thermostability. It can be seen from Table 2 that the relative concentrations of several acid components, such as benzoic acid, 4-hydroxy-3-methoxy-, methyl ester, benzoic acid, 4hydroxy-3-methoxy-, benzeneacetic acid, 4-hydroxy-3-methoxy-, and tridecanoic acid, decreased with the oxygen concentration. Furfural showed the same tendency. Furfural would be converted into furan under an oxygen atmosphere at certain temperatures. Furans, alcohol, and ketone tar increased with oxygen addition, which might result from the formation of the hydroperoxyl radical at an oxidative atmosphere. Oxygen markedly promoted the secondary reaction of primary tar. Phenol components were mostly derived from lignin in moderate-temperature pyrolysis. The weakly bonded ether linkage in lignin was easy to break down to fragments,29 such as phenols and phenol derivatives. These phenol derivatives were not chemically stable and would be converted into simple phenols. From Table 2, we can find that the relative concentration of substituted phenols, such as phenol, 2methoxy-4-propyl-, and phenol, 2-methoxy-4-(1-propenyl)-, (Z)-, decreased with the oxygen addition, while simple phenols, such phenol, increased. This phenomenon indicated that oxygen promoted the large lignin pyrolysis fragments to be converted into simple phenols by thermal and radical paths. As shown in Figure 8, the yield of phenol under inert pyrolysis was analyzed. The phenol yield increased with the temperature, from 0.16 mg/g of biomass (300 °C) to 8.44 mg/ g of biomass (700 °C), which was maybe mainly derived from the conversion of substituted phenols under higher temperatures. The phenol yields of oxidative pyrolysis at 500 °C are shown in Figure 9. It can be inferred that the phenol yield decreased and then increased with the oxygen concentration, which was also observed by Kim et al.22 In their study, the phenol yield increased again when the oxygen concentration reached 8.40%. A moderate amount of oxygen may promote the formation of substituted phenol and inhibit phenol via the formation of a

(10)

Equations 1−10 show the radical reaction to form CO, CO2, CH4, and water16,25,26 under an oxygen atmosphere. The weak bond of the biomass polymer is first broken to form an active center at a high pyrolysis temperature. Then, the oxygen molecule is adsorbed on these active centers to form peroxide groups, which undergo radical chain reactions accompanied by hemolytic and heterolytic reactions,16,27 as shown in eqs 2, 3, and 4. Carboxyl tends to form CO2, while carbonyl tends to form CO,28 as shown in eqs 6 and 7. As mentioned before, at a relatively low temperature (300 °C), O2 affects CO and CH4 yields little, while it promotes CO2 yields distinctly. This is because of the adsorption of O2 taking place quickly at a low temperature to form peroxide groups and further carboxyl. The amount of carboxyl depends upon the oxygen concentration, and carboxyl tends to form CO2 at a low temperature. Decarbonylation and demethylation reactions have higher activation energy and tend to take place at higher temperatures. 3.4. Effect of the Oxygen and Temperature on the Pyrolysis Tar Characteristics. Figure 7 shows the tar yields

Figure 7. Pinewood tar yield under different temperatures and oxygen concentrations.

under different temperatures and oxygen concentrations. It can be concluded that, the less tar produced under an oxidative atmosphere and the higher the oxygen concentration, the lower the yield of pyrolysis tar. The tar yield decreased from 0.3321 g/g of biomass (700 °C and 0% O2) to 0.1901 g/g of biomass (700 °C and 21% O2). This may be as a result of the more drastic radical reaction, which will convert primary tar to 5053


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Table 2. Pinewood 500 °C Pyrolysis Tar GC/MS Results under Different Oxygen Concentrations relative concentration (%) RT (min)

MW

formula

4.63 84 C5H8O 6.506 96 C5H4O2 7.382 98 C5H6O2 9.344 110 C6H6O2 9.453 84 C4H4O2 9.579 98 C6H10O 9.865 98 C5H6O2 11.284 110 C6H6O2 12.022 94 C6H6O 13.550 112 C6H8O2 14.511 116 C7H8O 15.209 108 C7H8O 15.393 126 C6H6O3 15.621 108 C7H8O2 16.337 126 C6H6O3 16.577 126 C7H10O2 16.669 126 C6H6O3 17.191 122 C8H10O 17.967 138 C7H6O3 18.414 138 C8H10O2 18.866 138 C8H10O2 19.066 110 C6H6O2 19.375 144 C6H8O4 19.959 126 C6H6O3 20.216 152 C9H12O2 20.828 140 C7H8O3 21.349 138 C8H10O2 21.641 124 C7H8O2 22.333 150 C9H10O2 23.329 154 C8H10O3 23.501 164 C10H12O2 23.752 166 C10H14O2 24.593 152 C8H8O3 24.822 164 C10H12O2 26.133 166 C10H14O2 26.802 166 C9H10O3 27.266 162 C6H10O5 27.552 182 C9H10O4 27.907 168 C9H12O3 28.627 168 C8H8O4 29.577 194 C11H14O4 30.590 182 C9H10O4 30.865 182 C9H10O4 32.438 178 C10H10O3 35.007 214 C13H26O2 35.248 208 C11H12O4 percentage of phenols percentage of listed tar components

compound

inert

2-butenal, 3-methylfurfural 2-furanmethanol ethanone, 1-(2-furanyl)2(5H)-furanone 2-cyclohexen-1-ol 1,2-cyclopentanedione 2-furancarboxaldehyde, 5-methylphenol 1,2-cyclopentanedione, 3-methylphenol, 2-methylphenol, 4-methylfurylhydroxymethyl ketone phenol, 2-methoxymaltol 2-cyclopenten-1-one, 3-ethyl-2-hydroxylevoglucosenone phenol, 2,4-dimethyl2,3-dihydroxybenzaldehyde 2-methoxy-5-methylphenol phenol, 2-methoxy-4-methyl1,2-benzenediol 1,4:3,6-dianhydro-α-D-glucopyranose 2-furancarboxaldehyde, 5-(hydroxymethyl)3,4-dimethoxytoluene 1,2-benzenediol, 3-methylphenol, 4-ethyl-2-methoxy1,2-benzenediol, 4-methyl2-methoxy-4-vinylphenol phenol, 2,6-dimethoxyeugenol phenol, 2-methoxy-4-propylvanillin phenol, 2-methoxy-4-(1-propenyl)-, (Z)phenol, 2-methoxy-4-propylethanone, 1-(4-hydroxy-3-methoxyphenyl)1,6-anhydro-β-D-glucopyranose (levoglucosan) benzoic acid, 4-hydroxy-3-methoxy-, methyl ester homovanillyl alcohol benzoic acid, 4-hydroxy-3-methoxyphenol, 2,6-dimethoxy-4-(2-propenyl)benzeneacetic acid, 4-hydroxy-3-methoxybenzaldehyde, 4-hydroxy-3,5-dimethoxy4-hydroxy-2-methoxycinnamaldehyde tridecanoic acid 3,5-dimethoxy-4-hydroxycinnamaldehyde

hydroperoxyl radical under an oxidative atmosphere, as mentioned before, while too much oxygen would lead to the partial oxidation of substituted phenols by the radical reaction triggered by oxygen.30,31 3.5. Oxidative Pyrolysis Char Analysis. Biochar is another important product of biomass pyrolysis. The physical and chemical structure analyses may reveal the mechanism characteristics of the heterogeneous reaction between oxygen and biomass. In this part, the pore structure analysis of pinewood char derived under different conditions was

2.36 2.73 0.35 1.72 0.65 3.42 0.52 0.52 1.35 1.12 1.01 0.33 3.58 0.87 0.29 0.62 0.42 0.39 0.60 5.19 2.98 0.98 1.52 0.43 1.83 1.67 1.42 5.83 1.38 1.58 1.10 0.75 6.07 0.32 1.80 1.52 1.56 0.45 0.32 1.67 0.37 4.46 0.44 0.46 37.21 68.95

5% O2

10% O2

15% O2

21% O2

0.13 1.19 2.90 0.40 1.83 1.00 1.90 0.87 1.54 1.03 1.51 1.52 0.33 4.37 1.52 0.20 0.12 0.47 0.68 0.54 5.59 2.86 0.98 0.67 0.48 1.93 1.72 1.55 5.94 1.22 1.33 0.56 0.50 5.83 0.18 2.00 1.29 0.84 1.54 0.24 0.18 1.31 0.26 4.90

0.19 1.69 3.30 0.34 1.82 0.90 2.02 0.71 1.86 0.71 1.59 1.54 0.28 4.71 1.50

0.63 1.25 3.20 0.40 2.57 0.97 1.51 0.80 1.95 0.59 1.64 1.15 0.22 4.43 1.62

0.72 1.11 3.33 0.41 2.66 1.01 1.32 0.82 5.21 0.32 1.66 0.98 0.19 4.32 1.68

0.48 0.66 0.53 5.69 2.67 0.15 0.50 0.43 1.83 1.71 1.52 5.77 1.20 1.00 0.36 0.48 5.50 0.24 2.85 1.63 1.30 1.56 0.13 0.24 1.19 0.19 4.43

0.46 0.94 0.52 5.87 2.43 0.10 0.48 0.43 1.86 1.70 1.97 5.99 1.13 0.96 0.35 0.65 5.32 0.22 3.34 0.33 0.88 1.78

0.44 1.05 0.51 5.92 2.22 0.09 0.33 0.42 1.85 1.68 2.02 6.05 1.09 0.88 0.32 0.66 5.22 0.21 3.45 0.21 0.76 1.84

0.22 1.02 0.14 5.17

0.20 0.97 0.11 5.62

0.54 39.82 68.36

0.56 39.41 67.77

0.61 39.27 67.17

0.71 38.11 69.85

conducted. The specific surface area of char samples is shown in Figure 10. The specific surface area of pinewood char samples shows great difference when the temperature is higher than 400 °C under different oxygen concentrations, as shown in Figure 10. The heterogeneous oxidation promoted the development of the pore structure, especially the micropore. However, the specific surface area decreased at high temperatures (>600 °C) under relatively high concentrations (>10%). Kim et al.22 investigated the oxidative pyrolysis of red oak on a fluidized bed and 5054


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oxygen concentrations. The accurate control of the oxygen concentration is the key for char structure regulation and control. 3.6. Oxidative Pyrolysis Mechanism of Biomass. At high temperatures, the oxidation reaction between O2 and char can lead to the production of CO2 and CO. According to the results by Su et al.,8 the oxidative pyrolysis of pinewood is a exothermic process and inert pyrolysis is a endothermic process. The generated heat may improve the in situ temperature and promote the generation of a free radical, which is chemically reactive to convert primary tar to a secondary tar component. Oxygen promoted the development of the pore structure but would restrict the further development of the char pore under ultimate conditions. The integrated process of biomass oxidative pyrolysis is shown in Figure 11. Figure 8. Quantitative analysis of pinewood inert pyrolysis tar.

4. CONCLUSION Oxidative pyrolysis showed a distinct difference compared to inert pyrolysis of biomass. The quantitative description of oxidative pyrolysis product distribution was essential for the reveal of the oxidative pyrolysis mechanism and the use of biomass thermochemical conversion. Oxygen improved the yields of permanent gas and water but reduced the yields of tar and char. The higher the oxygen concentration, the more distinct the results would be. In comparison to char and water, oxidative pyrolysis had a greater effect on permanent gas and tar yields. The total mass yield of the pyrolysis product exceeded the initial biomass mass because of the consumption of oxygen. About 0.2401 g/g of biomass of O2 was consumed at 500 °C and under 21% oxygen concentration. Three kinds of permanent gases (CO, CO2, and CH4) and water were analyzed quantificationally. CO2 was the dominant gas component in both inert pyrolysis and oxidative pyrolysis. The majority of CO and CH4 yields between 300 and 400 °C in oxidative pyrolysis, and when the temperature was higher than 400 °C, very little of CO and CH4 was released. The CO and CH4 yields depend upon the content of carbonyl and methyl/methoxy, respectively. CO2 was produced at all temperatures investigated. The adsorption of O2 on the reactive center to form carboxyl tended to form CO2, and this process took place at a relatively low temperature (300 °C). At high temperatures, the oxidation reaction between O2 and char would lead to the production of CO2. The oxidative pyrolysis of pinewood was an exothermic process, and inert pyrolysis was an endothermic process. The generated heat might improve the in situ temperature and promote the

Figure 9. Quantitative analysis of pinewood 500 °C oxidative pyrolysis tar.

analyzed the pore structure of biochar. They also found that the BET surface area of biochar increased significantly from 2.4 m2/ g under an inert atmosphere to 93.1 m2/g when 4.2% (v/v) oxygen was added and then decreased to 77.6 m2/g when the oxygen concentration further increased. They regarded this as the competitive relationship between the rate of oxygen molecule diffusion into the pore and the rate of the oxidation reaction. Amutio et al.32 also observed a similar phenomenon. They also attributed it to the high char combustion rate at high

Figure 10. Pinewood char surface area variation with the temperature and oxygen concentrations. 5055


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generation of a free radical, which was chemically reactive to convert primary tar to a secondary tar component. Oxygen promoted the development of the pore structure but would also restrict the further development of the char pore under ultimate conditions, resulting from the high char combustion rate at high oxygen concentrations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and/or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support of the National Natural Science Foundation of China (NSFC) (Grant 51176120) and the Science and Technology Commission of Shanghai Municipality (Grant 09dz12018) .

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Experimental Investigation of the Oxidative Pyrolysis Mechanism of

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