Article pubs.acs.org/EF
Evolution of Inherent Oxygen in Solid Fuels during Pyrolysis Pengwei Dong,† Gan Chen,‡ Xi Zeng,† Mo Chu,‡ Shiqiu Gao,*,† and Guangwen Xu*,† †
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China ‡ School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, People’s Republic of China ABSTRACT: Inherent oxygen in solid fuels is various in content and occurrence mode related to other elements. Three samples with different inherent oxygen contents were pyrolyzed in an externally heated fixed-bed reactor to characterize their oxygen transformation among solid, water, and gas products in addition to understanding their pyrolysis performances. It was found that the oxygen retained in the solid char decreased with increasing temperature. About 30−50% of the inherent oxygen has transferred into water after pyrolysis at 800 °C. The major oxygen-containing gaseous product was CO for the low-oxygen fuel, such as Fugu bituminous coal (FG), whereas this was mainly CO2 for the oxygen-rich fuels, such as Shengli lignite (SL) and herb residue (HR). Fourier transform infrared (FTIR) spectra were taken to analyze the evolution of five oxygen-containing functional groups. The CO bond existing in ester and carboxyl/carbonyl groups is easy to decompose and is nearly eliminated at 600 °C. The hydroxyl group was formed at a high temperature after decomposition at a low temperature, while the C−O single bond in the ether group increased because of the cleavage of CO bonds.
1. INTRODUCTION Various different solid fuels have been used not only to produce energy, such as electricity and steam, but also to be the raw materials for diversified conversion processes via, for example, pyrolysis and gasification.1 Of the major thermal conversion technologies, pyrolysis refers to a kind of mild decomposition process of solid fuels in inert atmospheres under heated conditions, and pyrolysis as a kind of reaction becomes involved in every thermal conversion process of solid fuels.2 The volatile from pyrolysis can be converted into fuel oil and valuable chemicals,3,4 while the residual solid can still be feedstock for gasification and combustion.5,7 Many novel pyrolysis technologies in different scales have been tested throughout the literature.6,7 To improve efficiency of the whole system, regulation on distribution of pyrolysis products has aroused great interest. Pyrolysis occurs via primary and secondary reactions, which makes it difficult to determine its final products. Many studies have been conducted to describe the pyrolysis process; however, the mechanism is still unclear. Variations in elemental composition and exhibition state in solid are big challenges for pyrolysis process control. Carbon and hydrogen consist of the major skeleton of solid fuels and their organic structures, which are easy to decompose at heated conditions. Nitrogen and sulfur occupy minor composition in solid, but their oxides are defined as environmental poisons. Transformation of nitrogen and sulfur during thermal conversions has been investigated in the literature.8−12 Trace elements, such as alkali and alkaline earth metals, which usually form in salt, can affect the quantity and quality of pyrolysis products.13−16 Many researchers focused on the environmental impact of heavy metal in solid.16,17 Oxygen is well-dispersed in solid, and its thermal transition is usually neglected. Oxygen (O2), as a reaction agent, has been widely used in gasification and combustion. The input of O2 requires rigorous calculation to obtain optimized and economic © 2015 American Chemical Society
operation, in which the inherent oxygen in solid is considered to have the same reactivity with external O2. However, variations on the content and exhibition states may affect its reactivity. The oxygen content in solid fuel increases with decreasing fuel rank according to literature data.18,19 For high-rank coal, such as anthracite, its oxygen content is below 5%, while in volatile-rich fuel, such as biomass, its oxygen content can be over 40%. Oxygen exhibits as organic and inorganic states in solid. Organic becomes decomposed easily and forms volatile products at a high temperature. The inorganic part exhibits as salt/oxide and mainly remains in residue after reaction. Mrazikova et al.20 investigated the distribution and variations of four oxygen-containing functional groups in five Czech coals by chemical analysis methods. All of the samples used were coal without considering oxygen-rich biomass. Liu et al.21 investigated the transformation paths of oxygen during pyrolysis for three Chinese coals. Most oxygen was found to release as gas in pyrolysis. However, their work did not take water into consideration. Luik et al.19 investigated oxygen transformation of pine bark in three pyrolysis conditions. Most oxygen in pine bark had been transferred into water finally. This work indicated that the redistribution of oxygen in pyrolysis was affected by experimental conditions. Shiju et al.22,23 studied the effects of external O2 on the decomposition of catechol. Their studies showed that pyrolysis was affected by two competing factors: radical enhancement and oxidative destruction, which was induced by additive oxygen. Many researchers tried to expound the complex pyrolysis mechanisms by component analyses. Demirbas24 discussed the pyrolysis of cellulose, hemicelluloses, and lignin in wood. He Received: December 24, 2014 Revised: March 10, 2015 Published: March 12, 2015 2268
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Energy & Fuels Table 1. Proximate and Ultimate Analyses of Raw Materials proximate analysis (wt %ada) b
a
ultimate analysis (wt %ad)
sample
volatile
moisture
fixed carbon
ash
C
H
N
S
O
FG SL HR
30.23 31.01 70.15
0.55 2.32 3.98
66.93 53.90 11.92
2.28 12.77 13.95
75.12 58.29 42.93
6.50 5.66 5.95
1.31 1.07 1.91
0.40 1.30 1.36
15.99 25.85 43.54
ad, air-dried basis. bFG, Fugu bituminite; SL, Shengli lignite; and HR, herb residue.
found that cleavage of the aromatic C−O bond would form a product with one oxygen atom, while cleavage of the methyl C−O bond would form a two-oxygen-atom product. Sharma et al.25 traced the change of element composition in pectin char. They found that it was a linear relationship between the H/C and O/C ratios and pyrolysis reactions, including dehydration, decarboxylation, and decarbonylation. In their series work,26 they employed instrument analyses on char from lignin. The results showed that oxygen was detectable even at a high temperature and the residual oxygen was mainly phenolic through nuclear magnetic resonance (NMR). The objective of this paper is to investigate how inherent oxygen in solid fuels with evidently different oxygen contents behave and transform during the pyrolysis process. Two coals and one biomass feedstock are pyrolyzed at the same conditions. The transformation paths of oxygen are traced and analyzed to obtain more information about the process. A comparison of three fuels is exhibited to show the effect of the fuel oxygen content on the pyrolysis performance. The fate of five functional groups are analyzed to state the evolution of organic oxygen.
Figure 1. Schematic diagram of the pyrolysis reaction system.
jar by the overwater method, and its total volume was determined with a measuring cylinder. Gas composition was analyzed using a gas chromatograph (Agilent Micro 3000, TCD). All liquid product including that from washing the pipeline was gathered into acetone solution. The tar−acetone solution was separated by evaporating the solvent in a vacuum rotary evaporator (below 0.1 MPa at 25 °C) to leave the residual tar for measurement. A parallel experiment with the same procedure was conducted to determine the pyrolytic water. Methanol rather than acetone was used in this parallel experiment to collect all water. Then, the water content was titrated by the Karl Fischer method (Metrohm Karl Fischer Titrator 870 Plus). The content of oxygen in solid was taken by a Vario EL elemental analyzer using the O model. Oxygen in water was deduced from pyrolytic water, and oxygen in gas was calculated from CO2 and CO contents. Oxygen in tar sample was calculated by difference. Chemical titration methods have been used to determine organic functional groups in coals by many studies18,27 and proven to be accurate for the carboxyl group. Fourier transform infrared (FTIR) technology has its advantage of easy application and is convenient to analyze the infrared (IR)-sensitive functional groups. It has been used in characterization of coal,1,28−35 biomass,36,37 and char residues.37,38 Mathematic methods have been introduced to process the spectrum. The FTIR spectrum zone of 1800−1500 cm−1 was processed to obtain the peak area of the carboxyl group, which had good correlation with the carboxyl group content determined by chemical titration methods.18 In this paper, the chemical analysis (ion-exchange) method was used to determine the carboxyl content following the protocols in the literature.27,39 Meanwhile, all solid were analyzed using FTIR (BRUKER TENSOR 27, 4000−400 cm−1). The ratio of the solid sample to KBr was strictly controlled to 1/100 mg for each pallet. The obtained FTIR spectra were processed following the literaturereported methods. Spectra at a wavenumber of 1800−1500 cm−1 were assigned to CO groups (carbonyl, carboxyl, ester, etc.).18,34 The baseline of the selected zone from a FTIR spectrum (1800−1500 cm−1) was created first before further processing. Position and number of peaks were established initially from the second derivative of the spectrum.35 The Gauss amplitude method was taken to obtain good fitness, and the R2 value was over 0.995.31−35,40,41 The peak area obtained was exported for further calculation. The content of oxygen-
2. EXPERIMENTAL SECTION 2.1. Fuels and Apparatus. Three solid fuels with different oxygen contents were chosen in this paper. Among them, Fugu bituminous coal (FG) and Shengli lignite (SL) represented typical Chinese coal with great reserve amount and herb residue (HR) was an industrial biomass solid waste in the Chinese medicine production process, leaving thousands of tons to dispose of every year. The proximate and ultimate analyses of three feedstock are listed in Table 1. From Table 1, one can see that the oxygen content of three samples are 15.99, 25.85, and 43.54%, respectively. All samples were crashed and sieved to 0.5−1.0 mm in diameter and dried at 105 °C for 3 h before experiments. About 20.00 ± 0.01 g of coal samples and 10.00 ± 0.01 g of biomass were used for each experiment to maintain the same height in the fixed-bed reactor shown below. A schematic diagram of the experimental apparatus is shown in Figure 1. The reaction system includes the sections of gas supply, reaction, and downstream separation. Highly pure N2 (99.999%) was used as a carrier gas with pressure and flow rate regulated by a pressure stabilizer and a mass flow controller. A vertical quartz-made fixed-bed reactor (inner diameter, 30 mm; length, 350 mm) coupled with an external electric furnace (Golden Furnace, 1.43 kW, 950 °C at maximum) made up the reaction section. The temperature was detected and controlled using a K-type thermocouple located in the middle of the reactor. The downstream separation consisted of a tube condenser (cold medium, air), a tar collector, a condenser (1:1 H2O/ glycol, −15 °C, provided by a chiller), a tar trapper (capture medium, acetone, in icy water bath), H2S remover (NaHCO3-saturated solution, in icy water bath), a gas collector, and a measuring cylinder. 2.2. Experimental Approaches. Nitrogen was regulated to 100 mL/min at the start until the end of a test. The reactor was heated to reach the preset temperature as 400, 500, 600, 700, and 800 °C at 100 °C/min. It took 45 min for each experiment, which included 30 min for heating and reaction and another 15 min for cooling and purging. The char product was measured by an electronic balance after it was cooled in N2 flow. Incondensable pyrolysis gas was collected in a 10 L 2269
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Energy & Fuels containing functional groups, except for carboxyl, was determined using the integral area ratio Rj/carboxyl in FTIR spectra. 2.3. Data Analysis Approaches. The yield of pyrolysis products was calculated as follows:
Yi =
mi × 100% mfeedstock
(1)
∑i ρi φiV
Ygas =
mfeedstock × 1000
× 100%
(2)
where Yi is the yield of pyrolysis product i, including char, tar, H2O, etc., mfeedstock is the mass of feedstock in each run, ρi is the density of gas composition i at 273.15 K and 100 kPa, ϕi is the volumetric concentration of gas composition i, and V is the total volume of pyrolysis gas. The distribution coefficient of inherent oxygen was calculated as follows: mchar ωchar × 100% mfeedstock ωfeedstock
Ochar =
OH2O =
Ogas =
mH2OωH2O mfeedstock ωfeedstock
× 100%
mCO2ωCO2 + mCOωCO mfeedstock ωfeedstock
× 100%
Otar = 100 − Ochar − Ogas − OH2O
(3)
(4)
(5) (6)
where Oi is the distribution coefficient of inherent oxygen in product i, ωi is the weight fraction of oxygen in sample i, and Otar is calculated by difference. The content of oxygen-containing functional groups in solid samples was calculated as Xcarboxyl =
Xj =
ωcarboxyl Ychar
× 100%
R j/carboxylωcarboxyl Ychar
× 100%
(7)
(8)
where Xj is the content of functional group j in raw feedstock, ωcarboxyl is the content of the carboxyl group determined from titration, and Rj/carboxyl is the ratio of FTIR spectra peak area of functional group j and the carboxyl group. Figure 2. Product yields varying with the pyrolysis temperature.
3. RESULTS AND DISCUSSION 3.1. Oxygen Transformation Paths. Figure 2 shows the yields of pyrolysis products for three feedstocks. As oxygen-rich fuel, biomass has higher reactivity than the two coals. When it comes to 400 °C, more than 40% weight loss of biomass (other than char) indicates more volatile released in comparison to less than 10 wt % of the two coals. Chars from FG and SL coals have similar decreasing trends in yield as the temperature increases from 400 to 800 °C. The char from HR does not change much from 700 to 800 °C. Little tar is observed below 400 °C for FG and SL coals. It reaches their respective maximums at 600 °C and then decreases because of the secondary reactions. In contrast, the HR sample has nearly 20% tar production at 400 °C, and this yield keeps increasing to more than 30% at 800 °C. The water generated in pyrolysis of FG and SL coals shows similar increasing trends, although their yields are different from each other. However, the yield of water from the HR sample does not change much with the temperature. Pyrolysis gas increases by about 10% in its yield for the three feedstocks at 400−800 °C.
In comparison of the preceding product distributions, the ratio of oxygen in each product to the initial oxygen in feedstock is defined in this paper as the distribution coefficient of inherent oxygen to characterize the transformation paths of inherent oxygen during pyrolysis. Figure 3 shows the variations of this oxygen ratio in all products as a function of the pyrolysis temperature for three feedstocks. As the temperature increases, oxygen in the solid residue decreases and transfers to other pyrolysis products for all feedstock, which means that the inherent oxygen participates in pyrolysis reactions. Although the HR sample possesses the highest initial oxygen content, its oxygen in solid residue drops more quickly than the other two coal samples with raising temperature. When the temperature becomes 400 °C, only 30% of inherent oxygen is left in the HR char residue compared to over 75 wt % for the other two coal samples. Meanwhile, water has become the major oxygen carrier for the HR sample at 400 °C, and a similar phenomenon happened to SL and FG coals at 700 and 800 °C, respectively. 2270
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redistribution of inherent oxygen in pyrolysis was investigated below 800 °C. For the two tested coal samples, the consequence for the final oxygen carrier out of solid is water, gas, and tar, while the HR sample has its released oxygen distribution in water, tar, and gas as an order. The HR sample has the largest oxygen conversion ratio, with less than 5% of initial oxygen left in the char residue. 3.2. Oxygen in Volatile Phases. Volatile products become released with different oxygen components in pyrolysis. In gas, oxygen mainly exhibits as CO and CO2; In pyrogenic water, only H2O existed. Tar has complicated composition with thousands of oxygen-containing compounds inside, and its detailed composition analysis is beyond the scope of this work. Figure 4 shows the oxygen distribution in volatile products
Figure 3. Allocation of oxygen in pyrolysis products.
Thus, the loss of inherent oxygen from char substantially occurs at 400 °C for HR but persists in 400−800 °C for two coals. It shows also that, after thermal pyrolysis at 800 °C, the contents of oxygen in the char residue for three feedstocks are comparative. While the SL char has a similar oxygen content with HR char as 10 wt %, the FG char contains 6.5 wt % oxygen. However, the difference lies in the respective transformation paths. In the pyrolysis conditions of this work, the major proportion (about 30−50%) of inherent oxygen has transferred to water for all feedstocks when the temperature reaches 800 °C, in accordance with a previous report.19 At temperatures above 600 °C and even above 400 °C for HR, oxygen in water does not change much because of competing consumption and generation reactions of H2O. On the occasion, the decrease of the oxygen content in char and the barely change of the oxygen content in tar and water indicate that oxygen prefers releasing as gas products at a high temperature. As Figure 3 indicates, the reactivity of internal oxygen has a certain correlation with fuel rank. For FG coal, the highest rank fuel with the least oxygen content, its internal oxygen behaves more stably than other fuels, so that oxygen release is slowest among the tested fuels. For the SL and HR samples, the char residue becomes the least carrier of inherent oxygen when the temperature reaches 800 °C. Of course, in this paper, the
Figure 4. Oxygen ratio in volatile products varying with the pyrolysis temperature.
according to its distinctive exhibition states, such as oxygen in CO (O−CO), oxygen in CO2 (O−CO2), oxygen in tar (O− tar), oxygen in water (O−H2O), and oxygen in total volatile phases (O−volatile). The ratio of O−volatile (against fuel inherent O) increases as the pyrolysis temperature elevated for all of the feedstocks, in accordance with the decreasing trend of O−char shown in Figure 3. O−CO keeps increasing with the temperature, and this O−CO ratio of FG coal exceeds the O−CO2 ratio at 2271
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Energy & Fuels temperatures above 600 °C, in contrast with the variation for the other two samples. For the O−CO2 ratio, it decreases at a low temperature while increases as the temperature is elevated. This phenomenon is observed for both FG and SL coals. For the HR sample, its O−CO2 ratio conserves only the increasing trend. The difference between generations of O−CO and O− CO2 is due to their different formation paths in pyrolysis. Generally, CO and CO2 are from decomposition of oxygencontaining functional groups, such as carboxyl, carbonyl, ether, etc. The final contents of CO and CO2 in pyrolysis gas are determined by competing reactions. At a low temperature, CO2 comes out early because of the easy breakage of carboxyl groups. As the temperature increases, its competing reactions for CO generation are superior. This results in the competition of CO and CO2 proportions when the total oxygen in gas does not change much at this temperature range. At a high temperature, CO and CO 2 generate mainly from the decomposition of volatile fractions. As a result, more oxygen enters into the gas phase, and both of the compositions become increased. As for the HR sample, only increasing trends of the oxygen ratio in CO and CO2 can be observed, even at 400 °C. The easy release of volatile material at a low temperature makes it difficult to distinguish the primary and secondary reactions for the HR sample. The O−H2O ratio keeps the same trend with water, as mentioned in section 3.1. The proportion of O−tar keeps a similar trend with tar production shown in Figure 2. From Figure 4, one can see that less than 10% of oxygen enters into tar for FG coal, while this ratio comes to 25% for SL coal and 30% for the HR sample. This agrees with our knowledge that the pyrolysis tar or oil from low-rank coal, such as lignite and biomass, has a high content of oxygen. 3.3. Oxygen in Solid Residue. It is assumed that the fuel solid only becomes involved in primary decomposition reactions, and the apparent evolution during pyrolysis of inherent oxygen from feedstock was studied. The oxygencontaining functional groups, such as carboxyl, hydroxyl, ether, carbonyl, etc., in solid fuel have been characterized by many methods.18,20,28−30,32−35,39−41 The chemical titration (ionexchange) method has been reported as effective to determine carboxyl group content with high accuracy. All of the titration experiments in this work have been conducted 3 times, and the error was within 5%. However, it is still difficult to determine other groups, such as ether, carbonyl, etc. In this work, FTIR spectra are applied for qualitative and quantitative analyses of oxygen-containing functional groups. Figure 5a shows the content of the carboxyl group of all samples measured by the ion-exchange method. The SL coal possesses the highest carboxyl group content among the feedstocks. The carboxyl group in char from two coals decreases as the temperature is elevated. In contrast, the content of the carboxyl group in HR char shows an opposite increasing trend, which is due to the great solid weight loss. When calculated on a dry basis of feedstock, the variation of carboxyl in solid is shown in Figure 5b. More than 80% of the carboxyl group in SL coal decomposed during pyrolysis when this ratio comes to about 55% for FG coal and HR sample at 700 °C. With a further temperature rise to 800 °C, the decomposition of the carboxyl group varied little for coal samples but increased to 75% for HR. Figure 6 presents the intensity of IR absorbance, showing that the signal decreases as the temperature increases, indicating an inactivation of functional groups because of
Figure 5. Carboxyl groups in char from different pyrolysis temperatures: (a) carboxyl group and (b) oxygen in the carboxyl group.
thermal decomposition. Some FTIR absorbance peak areas are narrowed with increasing the temperature to indicate the sensitivity of those functional groups to reaction temperature. For peaks at wavenumbers of 3100−2800 cm−1, which were assigned to aromatic and aliphatic C−H bonds, they decrease gradually and vanish at 600 °C for three samples. Thus, the aromatic and aliphatic C−H bonds decomposed or changed and no longer gave their IR adsorption at a high temperature. For the peak at 3600−3100 cm−1, an interval assigned to the hydroxyl group, it declined at a temperature below 600 °C but appeared again for high-temperature chars. This phenomenon is in accordance with the previous literature.20 The declining trend indicated that the initial hydroxyl group in samples decomposed at heated conditions. At a high temperature, the volatile fraction and solid residue would produce new forms of the hydroxyl structure, which require more energy for cleavage. Besides peak areas diminishing, some FTIR absorbance peaks shifted because of structure changes. For peaks at 1800 and 1500 cm−1, an interval usually assigned for a series of CO groups, such as carboxyl, carbonyl, ester, ketone, etc., there was an apparent red shift of absorbance peaks with increasing the temperature. It indicated that functional groups changed gradually. Because the characterization regions for ester, carboxyl, and carbonyl varies from 1800 to 1500 cm−1, the red shift of this peak indicated that the structure of CO groups changed with the temperature. The carbonyl group had the better thermal resistance than the ester and carboxyl groups, as detailed in the following comparison. The carboxyl group has also been determined by the chemical titration (ion-exchanging) method, as in Figure 5. For FTIR spectra, the peak area of the carboxyl group is overlapped with the other CO groups in 1800−1500 cm−1. A mathematical processing has been taken to deconvolute this 2272
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Figure 6. FTIR spectra of raw and pyrolyzed solid samples.
area into nine peaks following the work of predecessors.18,32,33,40,41 An example of curve fitting for FG coal is shown in Figure 7, in which the carboxyl group is assigned to
Table 2. Assignment of Oxygen-Containing Functional Groups in FTIR Spectra assignment carboxyl ester carbonyl ether hydroxyl
parameter center fwhm center fwhm center fwhm center fwhm center fwhm
(cm−1) (cm−1) (cm−1) (cm−1) (cm−1)
FG
SL
HR
1703 33 1725 33 1681 33 1180 189 3433 179
1721 38 1746 38 1697 38 1169 305 3398 172
1720 33 1743 33 1692 33 1155 205 3423 368
Figure 7. Deconvolution on FTIR spectrum in 1800−1500 cm−1 for FG raw coal.
area B.18 Table 2 lists the parameters defining the major oxygen-containing functional groups in FTIR spectra. Figure 8 shows the correlation between the integral peak area of the carboxyl group in FTIR spectra and the content of the group determined from titration. The good correlation makes it possible to use the peak area of the carboxyl group to calculate the amount of other functional groups. Figure 9a shows the content of the carboxyl group from chemical titration methods (dry basis of feedstock). Although SL coal has the highest content of the carboxyl group in feedstock, it loses nearly 75% at 600 °C, while FG coal loses 31% and HR sample loses 45%. The major decomposition of
Figure 8. Calibration curve for the carboxyl group in FTIR.
the carboxyl group happens at temperatures below 400 °C, and then the decomposition rate slows, as shown in the figure. Figure 9b shows the evolution progress of the ester group. The ester group had the lower content than the other oxygen2273
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Figure 9. Contents of oxygen functional groups varying with the temperature.
keeps increasing as the temperature is elevated. The breakage of the CO double bond at a high temperature contributes to this phenomenon. The HR sample has the highest content of ether initially, and its ether content decreases and loses most at a low temperature. The increasing trend at a temperature above 400 °C has the same trend as the coal samples. The hydroxyl group is a O−H single bond with a high content, especially in the HR sample. It becomes decomposed easily under heated conditions. As shown in Figure 9e, the hydroxyl group almost vanishes at temperatures above 400 °C. Because both generation and decomposition reactions happened to ether and hydroxyl groups in pyrolysis, it is complicated to distinguish the relationship between final products and the decomposition of these two groups.
containing functional groups. This diatomic oxygen group decreased quickly at heated conditions and vanished at a temperature higher than 500 °C. The evolution of carboxyl and ester groups had similar trends, which could be correlated with the trend of oxygen in volatile products, as shown in Figure 4. When the temperature increased from 400 to 600 °C, the increase of oxygen in volatile products, as shown in Figure 4, might come from decreasing carboxyl and ester groups in panels a and b of Figure 9, respectively. These diatomic oxygen groups would decompose into products with various oxygencontaining bonds in gas, tar, and water to improve the oxygen distribution in total volatile phases. In Figure 9c, the decreasing trends of the carbonyl group are similar for three samples. The carbonyl group decomposes fast at the temperature below 400 °C for SL coal and HR sample but around 500 °C for FG coal. In comparison to Figure 4, the increased oxygen in CO may come from the decomposition of the carbonyl group, which has agreed with the previous studies.42,43 The ether group has the highest content of the five analyzed functional groups. For FG and SL coals, the C−O single bond
4. CONCLUSION Three fuel samples with obviously different inherent oxygen contents were pyrolyzed under the same conditions. The pyrolysis performance was contrasted, and the transformation of inherent oxygen into different pyrolysis phases/products was investigated. Through mathematical processing on FTIR 2274
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spectra, more information on major oxygen-containing functional groups was obtained to learn their fates in thermal conversion. It was found that oxygen in solid residue decreased as the reaction progress. While oxygen-rich biomass lost more oxygen at a low temperature, low-oxygen fuel, such as FG coal, had its gaseous products mainly as CO compared to CO2 for the oxygen-rich solid, such as SL coal and HR sample. About 30−50% of inherent oxygen in the tested fuels transferred into water after reaction at 800 °C, and the following carrier of transformed oxygen was gas for coal and tar for biomass. The CO double bonds, such as carboxyl, ester, and carbonyl, decomposed with increasing the temperature. In contrast, the C−O single bond, such as the ether group, increased because of the cleavage of the CO double bond. The hydroxyl group became decomposed at heated conditions and almost vanished around 600 °C. However, some hydroxyl groups were detectable at a high temperature via FTIR analysis.
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AUTHOR INFORMATION
Corresponding Authors
*Telephone/Fax: +86-10-8254-4886. E-mail:
[email protected]. *Telephone/Fax: +86-10-8254-4886. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2014CB744303), the Natural Science Foundation of China (21306209), and the Strategic Priority Research Program of CAS on clean and high-efficiency utilization of low-rank coal (XDA07050400).
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