Yingyue Teng,Xiaoting Bian,Xiaojuan Fu,Yinmin Song,Xue Bai
Effect of Iron Component on the Structural Evolution of Carbon Bonds in Hydrochloric Acid-Demineralized Lignite During Pyrolysis
The pyrolysis characteristics of hydrochloric acid-demineralized Shengli lignite (SL+) and iron-added lignite (SL+-Fe) were investigated using a fixed-bed reactor. The primary gaseous products (CO2, CO, H2, and CH4) were detected via gas chromatography. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy techniques were used to study the carbon bonding structures of the lignite and char samples. In situ diffuse reflectance infrared Fourier transform spectroscopy was used to better understand the effect of the iron component on the transformation of the carbon bonding structure of lignite. The results showed that CO2 was released first during pyrolysis, followed by CO, H2, and CH4, and this order was unaffected by the addition of the iron component. However, the iron component promoted the generation of CO2, CO (<340 °C), and H2 (<580 °C) at lower temperatures and inhibited the formation of CO and H2 at higher temperatures while also inhibiting the release of CH4 throughout the pyrolysis process. The iron component may form an active complex with C=O and a stable complex with C–O, which can promote the fracture of carboxyl functional groups and inhibit the decomposition of ether bonds, phenolic hydroxyl groups, methoxy groups, and other functional groups, thus promoting the decomposition of aromatic structures. At low temperatures, it promotes the decomposition of aliphatic functional groups and finally the bonding and fracture of functional groups in coal, leading to the change of the carbon skeleton, resulting in the change of gas products. However, it did not significantly affect the evolution of −OH, C=O, C=C, and C–H functional groups. According to the above results, a developing reaction mechanism model of Fe-catalyzed lignite pyrolysis was established. Therefore, it is worth doing this work.
Effect of Iron Component on the Structural Evolution of Carbon Bonds in Hydrochloric Acid-Demineralized Lignite During Pyrolysis

The pyrolysis characteristics of hydrochloric acid-demineralized Shengli lignite (SL+) and iron-added lignite (SL+-Fe) were investigated using a fixed-bed reactor. The primary gaseous products (CO2, CO, H2, and CH4) were detected via gas chromatography. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy techniques were used to study the carbon bonding structures of the lignite and char samples. In situ diffuse reflectance infrared Fourier transform spectroscopy was used to better understand the effect of the iron component on the transformation of the carbon bonding structure of lignite. The results showed that CO2 was released first during pyrolysis, followed by CO, H2, and CH4, and this order was unaffected by the addition of the iron component. However, the iron component promoted the generation of CO2, CO (<340 °C), and H2 (<580 °C) at lower temperatures and inhibited the formation of CO and H2 at higher temperatures while also inhibiting the release of CH4 throughout the pyrolysis process. The iron component may form an active complex with C=O and a stable complex with C–O, which can promote the fracture of carboxyl functional groups and inhibit the decomposition of ether bonds, phenolic hydroxyl groups, methoxy groups, and other functional groups, thus promoting the decomposition of aromatic structures. At low temperatures, it promotes the decomposition of aliphatic functional groups and finally the bonding and fracture of functional groups in coal, leading to the change of the carbon skeleton, resulting in the change of gas products. However, it did not significantly affect the evolution of −OH, C=O, C=C, and C–H functional groups. According to the above results, a developing reaction mechanism model of Fe-catalyzed lignite pyrolysis was established. Therefore, it is worth doing this work.

Efficient utilization of low-rank coals such as lignite has attracted significant research attention. In addition to its high water content, large amount of volatile matter, and low combustion efficiency, lignite also contains significant amounts of active organic structures and minerals. Through pyrolysis, the water content in lignite can be removed to obtain high-added value char with improved structural properties, which improves its subsequent combustion or gasification performance. Therefore, pyrolysis is an important pathway toward efficient utilization of lignite resources. For this reason, it is of key importance to study the pyrolysis characteristics of lignite. The organic structure in lignite plays an important role in determining the products generated during pyrolysis. It is believed that the distribution of pyrolysis products is related to the fracturing of bond structures, such as Cal–O, Cal–Cal, Car–O, and Car–Cal (al: aliphatic and ar: aromatic), as well as to the polycondensation of aromatics. However, the yield and composition of pyrolysis gases are affected by the bond dissociation of carbon in carbonyl and aliphatic groups. It has been shown that the mineral components in lignite can increase the degree of disorder of aromatic structures, promote the condensation of aromatic ring clusters, and inhibit the development of side chains, which improve the pyrolytic properties of lignite, promote the generation of pyrolysis gases, and change the yield of tar. The catalytic effect of the iron component has also been found to be significant and is associated with the evolution of carbon bonds, which helps to improve the pyrolysis conversion rate of lignite and effectively regulates the distribution and composition of pyrolysis products. However, owing to the complex nature of different structural components of lignite, the mechanism by which the iron component affects the conversion properties of lignite during pyrolysis is not well understood. Jiang found that the iron component inhibited the polymerization of char, reduced the heating value of the pyrolysis gases, and increased the yield of tar. Gong et al. put forward that the catalytic pyrolysis of the iron component improves the combustion reaction performance of lignite, changes the structure of the produced char, and promotes the formation of free radicals. Fu et al. suggested that the iron component noticeably increases the yield of pyrolysis gases and char and reduces the bond energy of oxygen-containing functional groups. Their results also indicated that the iron component promotes the decomposition and hydrogenation of long chains and the dissociation of hydrogen-containing compounds, thereby further improving the reaction performance. Wang et al. found that the iron component facilitated the transfer of aromatic hydrogen and that the catalytic effect of FeCl3 was stronger than that of Fe(NO3)3. The largest lignite field in China is the Shengli coal field in Inner Mongolia, which possesses more than 22.4 billion tons of lignite reserves. The efficient utilization of Shengli lignite (SL+) is therefore of great significance to the economic development of Inner Mongolia. Therefore, in order to study the influence of iron components on the evolution characteristics of the carbon bonding structure during lignite pyrolysis, this paper selects hydrochloric acid-demineralized SL+ and iron-added lignite (SL+-Fe) as experimental samples. The formation of pyrolysis gas of lignite was investigated by a fixed-bed device and online gas chromatography (GC). The change of functional groups during pyrolysis was studied by in situ infrared analysis. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the bonding of carbon elements during lignite pyrolysis.

Lignite from no. 4 mine in the Shengli coal field of Inner Mongolia was selected as the experimental sample. Raw lignite with particle sizes in the range of 0.18–0.42 mm was dried at 105 °C for 4 h. The resulting sample, labeled as SL, was pretreated via a solution method to avoid interference from the active minerals in lignite, as follows: first, SL was mixed with hydrochloric acid (18%) at room temperature in a ratio of 1 g:10 mL. After stirring for 24 h at 100 rpm, the mixture was filtered, and the filter cake was washed with water until the filtrate contained no Cl– ions (AgNO3 test) and dried at 105 °C for 4 h to obtain the hydrochloric acid-demineralized lignite sample, labeled as SL+. Proximate and ultimate analyses were carried out on SL and SL+ according to GB/T212-2008 using an industrial coal analyzer (China 5E-MF 6200, China) and an elemental analyzer (Vario EL Cube, Elementar, Germany), with the results shown elsewhere. Full contact between the catalyst and the coal sample was achieved via blending and impregnation methods using soluble transition-metal salts. It is generally believed that the impregnation method can infiltrate transition-metal ions into the voids of pulverized coal to maximize the effectiveness of the catalyst. Therefore, iron component-added SL+ was prepared using an impregnation method from the literature, as illustrated in Figure 1. Based on a 5.0% iron content accounting for the mass percent of the coal sample, FeCl3·6H2O was dissolved in water and mixed with SL+ while stirring for 12 h. The mixture was then dried at 105 °C for 24 h and labeled as SL+-Fe. As shown in Table 1, X-ray fluorescence spectroscopy (ZSXPrimusII, Rigaku, Japan) showed that the SL+ contained 3.5% iron component. Therefore, compared with the amount blended, the iron content obtained via impregnation was lower than that expected. However, this did not significantly impact the study of the pyrolysis process. Proximate and ultimate analyses of the SL+-Fe samples were also carried out according to GB/T212-2008. The results of the samples described elsewhere indicate that 35.23% of ash in SL was removed by hydrochloric acid, and the ash content increased by 9.56% after adding the iron component, confirming its successful loading via impregnation.

The pyrolysis of SL+ and SL+-Fe was carried out in a fixed-bed reactor under an argon atmosphere, as shown in Figure 2. The mass of the coal samples used in the pyrolysis experiments was 1.5–2 g. Other parameters were as follows: a final pyrolysis temperature of 900 °C, a heating rate of 4 °C/min, a holding time of 90 min, and an argon flow rate of 200 mL/min. The resulting pyrolysis char products were labeled as SL+-900 and SL+-Fe-900, respectively. The char yield (Ychar), along with proximate and ultimate analyses of the char, is given in Table 2. The tar byproduct was condensed in cold water, while the pyrolysis gas was condensed and analyzed via GC. The working conditions of GC were set according to the retention time of the pyrolysis gas, as follows: TCD detector, a cylinder temperature of 160 °C, a sampler temperature of 180 °C, a hotwire temperature of 200 °C, and a gas collection interval of 7 min.

The organic functional groups of the lignite and char samples were analyzed using a NEXUS 6700 (Nicolet, USA) FTIR spectrometer, with a wavenumber range of 4000–400 cm–1 and a resolution of 4 cm–1. The sample and KBr were dried at 105 °C for 4 h. The sample and KBr were mixed and ground in a ratio of 1:200, pressed, and then scanned in the IR spectrometer.

In order to further explore the influence of the iron component on the structural evolution of carbon bonding during the pyrolysis of lignite, changes in the functional groups of lignite were investigated using an IRTracer-100 (Shimadzu, Japan) in situ diffuse reflectance infrared Fourier transform spectrometer. The lignite sample was placed in the sample holder, and 15 mL/min nitrogen gas was introduced into the reaction cell for pyrolysis experiments to determine the infrared spectrum of the lignite sample. The experimental temperature ranged from 20 to 750 °C (the maximum temperature of the reaction cell) at a heating rate of 4 °C/min, a scan number of 5, and a wavenumber range of 600–4000 cm–1. Starting from 150 °C, 11 temperature points were measured for every 50 °C temperature increment. Samples were directly ground and evenly put into the in situ infrared tank, in order to avoid the influence of KBr on sample pyrolysis at high temperatures.

The surface element compositions of the lignite and char samples were characterized using an ESCALAB-250Xi (Thermo Fisher Scientific, USA) X-ray photoelectron spectrometer. The XPS C 1s spectrum was peak-fitted using the Avantage software from Thermo Fisher.

Figure 3 shows the generation rates and cumulative amounts of the four main pyrolysis gases (CO2, CO, H2, and CH4). The results show that the CO2 gas is generated first, followed by CO, H2, and CH4, and that the addition of the Fe component does not change the pyrolysis gas generation sequence. The characteristic temperatures and amounts of gas accumulated during pyrolysis gas generation are listed in Table 3. From Figure 3 and Table 3, it is evident that in the initial stage of lignite pyrolysis, CO2 is the main component of the pyrolysis gas and is mainly generated through the breaking of the carboxyl functional group. After adding the Fe component, the CO2 temperature zone for generation is shortened and its rate of formation is increased significantly. As a result, the gas accumulation is increased by 7.1%, indicating that the complexation of the Fe component with the carboxyl group facilitates its rupture, thereby promoting CO2 generation. During lignite pyrolysis, CO is mainly generated through the breaking of functional groups such as aldehyde, carbonyl, phenolic hydroxyl, and ether. At low temperatures, CO is generated via aldehyde group decomposition, while at high temperatures, it is mainly derived from the decomposition of methoxy groups, secondary reactions of tar (involving the breakage of the ether bonds connecting the aromatic rings), and the breakage of ethers, hydroxyl groups, and oxygen-containing heterocyclic structures. After adding the Fe component, the CO generation temperature and the gas accumulation both decreases, the latter by 30.8%. At pyrolysis temperatures <340 °C, the CO generation rate increases, indicating that the Fe component promotes the decomposition of aldehyde groups at this stage. However, when the pyrolysis temperature is >340 °C, the CO generation rate decreases, indicating that the Fe component inhibits the decomposition of functional groups such as ether, phenolic hydroxyl, and methoxy, as well as the secondary reaction of tar. Hydrogen is generally considered to be a secondary pyrolysis product of lignite and is mainly derived from the polymerization of aromatic substances and the high-temperature dehydrogenation of aromatic rings. After adding the Fe component, the generation temperature of H2 and the gas accumulation both decreases, the latter by 1.7%. When the pyrolysis temperature is <580 °C, the generation rate of H2 increases significantly, indicating that the Fe component promotes the decomposition of aromatic structures, resulting in the breakage of the aliphatic side chains connected to it and the removal of large numbers of hydrogen radicals. These results are consistent with those of Fu. When the pyrolysis temperature is >580 °C, the rate of generation of H2 decreases, indicating that the Fe component inhibits further polymerization of free radicals and therefore the generation of H2, by inhibiting the breakage of oxygen-containing heterocyclic rings and the condensation of phenolic hydroxyl groups and agglomerates on the carbon surface. During the pyrolysis of lignite, CH4 mainly originates from the pyrolysis of methoxy groups and the breakage of aliphatic side chains in the lignite (at low temperatures) and the breakage of aromatic side chains (at high temperatures). After adding the Fe component, the formation rate of CH4 and the gas accumulation both decreased, the latter by 60.5%, indicating that the Fe component suppresses the pyrolysis of methoxy groups, aliphatic side chains, and aromatic side chains.

Figure 4 shows the FTIR spectra of the lignite and char samples. As the figure shows, the FTIR spectra of the lignite and char samples were classified into five sections as follows: 3675–3000 cm–1 (I), 2990–2800 cm–1 (II), 1780–1410 cm–1 (III), 1340–900 cm–1(IV), and 900–650 cm–1(V). Differences between the FTIR spectra of the SL+ and char samples can be clearly observed in Figure 4. In Section I (3675–3000 cm–1), the peak around 3426 cm–1 is the hydroxyl (−OH) bond absorption peak. The effect of iron composition on the −OH group cannot be accurately judged, as FTIR does not completely guarantee the absence of moisture. In Section II, 2935 and 2859 cm–1 are C–H characteristic peaks; the intensity of the aliphatic C–H characteristic peak of the chars is significantly weakened, with that of SL+-Fe-900 being slightly stronger, which indicates that the Fe component reduces the amount of aliphatic functional groups’ decomposition during pyrolysis to a certain extent. This is consistent with the CH4 generation data seen earlier. In Section III, C=O characteristic peaks at 1707 cm–1 and aromatic C=C characteristic peaks at 1610 cm–1 are mainly present; the intensity of the characteristic peaks corresponding to the C=O functional group decreases, indicating that C=O is easily decomposed during pyrolysis. The iron component showed no obvious effect on the changes in these functional groups. In Section IV, the characteristic peaks are at 1000–1300 cm–1 of oxygen-containing functional groups such as alcohols, phenols, ethers, and/or esters, and their intensity decreases. The intensity of the characteristic peaks corresponding to the functional groups of the chars is reduced. In particular, the intensity of the characteristic peaks of SL+-Fe-900 is relatively enhanced in the range of 1300–1000 cm–1, indicating that the iron component inhibits the decomposition of C–O to some extent. This is consistent with the amount of CO released in the high-temperature stage (>340 °C). In the fifth part (Section V), the iron component showed no obvious effect on the variation of aromatic C–H bonds.

Figure 5 shows the pyrolysis diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of the two lignite samples at different temperatures, and the organic functional groups are same as those observed in the FTIR spectra in Figure 4. According to the relevant literature, the measurement range can be divided into five distinct regions (labeled 1–5), same as the infrared partition method. Figure 5 also shows that there are three main types of organic functional groups present in the lignite and char samples, namely aliphatic structures, aromatic structures, and oxygen-containing functional groups. As the temperature increases, hydroxyl (−OH) absorption peaks appear in the spectrum, and the intensity of the characteristic peaks decreases in region 1 (3580–3720 cm–1). The aliphatic C–H structure, the characteristic peak of the C=O functional group, and the intensity of the C–O characteristic peaks in region 2 (2800–990 cm–1), region 3 (1410–1780 cm–1), and region 4 (980–1340 cm–1) were significantly decreased, which were the same as the FTIR results. The intensity of the characteristic peaks of C–H functional groups in region 5 (680–900 cm–1) is not obvious. Compared with FTIR, DRIFTS can better show the changes of functional groups in the pyrolysis process. To further semi-quantitatively study the changes in functional groups, curve fitting analysis was performed on the DRIFTS spectra of lignite samples at different temperatures using a peak fitting software, with the results shown in Table 4. According to a method from the literature, the infrared spectra of the samples were divided into five wavenumber segments. As an example, Figure 6 shows the SL+ spectrum at 150 °C. Figure 7 shows the functional group content of lignite samples at different temperatures, calculated as followswhere A is the peak area and the subscript χ can refer to either aromatic C–H, C–O/C–OH, C=C, C=O, aliphatic C–H, or free −OH structures. Figure 7 is the fitting result of in situ DRIFTS spectra at different temperatures. As lignite pyrolysis gas completely escaped before 900 °C, the maximum upper limit temperature of DRIFTS is 750 °C. Figure 7 shows that as the pyrolysis temperature increased, the aromatic C–H bond content in SL+ increased, indicating that the related functional groups underwent pyrolysis or condensation reactions. This is consistent with the removal of the aromatic side chains to a certain extent, resulting in an increase in the number of aromatic C–H bonds. On the other hand, the contents of free C–O/C–OH, C=O, and −OH structures decreased, indicating that the oxygen-containing functional groups in lignite are easily decomposed. By contrast, the C=C bonds in aromatic hydrocarbons were relatively stable, indicating that the carbon skeleton of the lignite barely changed. The iron component also had no obvious effect on the aliphatic C–H, −OH, and C=O conversions in the aromatic structures. After adding the iron component, however, the amount of aromatic C–H bonds decreased, indicating that the Fe component inhibits the polymerization of aromatic structures, which is consistent with the H2 generation results seen earlier. The contents of C–O/C–OH and −OH bonds were also reduced in the presence of the Fe component, indicating that it has an inhibitory effect on the decomposition of functional groups such as ether bonds, phenolic hydroxyl groups, and methoxy groups. This result is consistent with the generation of H2, CO, and CH4. The Fe component also increases the C=O and COO contents of the lignite sample, while the rate of decomposition of COO increases. This demonstrates that the complexation of the Fe component with carboxyl groups facilitates the rupture of these groups, which is consistent with the CO2 generation results. Additionally, the Fe component increases the content and the rate of decomposition of aliphatic C–H bonds, indicating that it promotes the formation of free aliphatic groups while promoting the decomposition of aliphatic functional groups. The aromatic C=C bonds in SL+-Fe decreased significantly, and the C=C bonds increased with increasing temperature, which could be due to changes in the carbon skeleton caused by the formation and rupture of functional groups.

Figure 8 shows the XPS C 1s fitting results of the lignite and char samples; hence, it can be seen that the carbon element bonding structures can be classified into four types: aromatic units and their substituents (C–C/C–H, 284.8 eV), ether or phenolic hydroxyl groups (C–O/C–OH, 286.1 eV), carbonyl groups (C=O, 287.6 eV), and carboxyl bonds (COO–, 289.0 eV), where C–C and C–H are the main bonding forms of the carbon element. The results show that the four functional groups still exist after pyrolysis of SL+-Fe. The XPS C 1s fitting parameters of the samples are listed in Table 5. Table 5 shows that within the experimental temperature range, the content of C–C/C–H structures in the lignite increased and that of C=O structures decreased, consistent with the results of FTIR. After adding the Fe component, the C–C/C–H content of the char samples decreased, the C–O/C–OH content increased significantly, the C=O content increased only slightly, and the content of the COO– structure decreased. In conjunction with the DRIFTS results, it is evident that the iron component promotes the fracture of the C–C/C–H structures during pyrolysis, which may be due to the formation and dissociation of bonds between the iron component and the organic structure, leading to changes in the carbon skeleton. This is consistent with the generation of H2 in the low-temperature stage (<580 °C). The Fe component inhibits the decomposition of phenolic hydroxyl groups, ether bonds, and methoxy groups and may form stable organic salts containing the C–O structure, to increase its content, which is consistent with the generation of CO and CH4. Simultaneously, the presence of iron component may result in the formation of active C=O–Fe, COO–Fe, or Fe–COO, Fe–C=O organic salts with C=O and COO– structures, which promotes the dissociation of aldehyde groups and carboxyl groups, thus increasing the content of C=O and decreasing the content of COO. This result is consistent with the FTIR and CO2 generation results seen earlier.

During the pyrolysis of lignite, the formation of iron-containing organic salts can promote the cleavage of aliphatic side chains and aromatic rings containing aldehydes or esters and inhibit the dissociation of oxygen-containing heterocycles, ethers, and phenolic hydroxyl aromatic rings. As discussed in Section 2.1, the Fe component promotes the formation of CO2, CO (<340 °C), and H2 (<580 °C) and inhibits the formation of CH4. A combination of FTIR and XPS structural characterization showed that the aromatic macromolecular system of carbon was destroyed, resulting in changes in the produced gases. The formation and dissociation of bonds between the Fe component and the organic structures led to changes in the carbon skeleton. Due to the changes in oxygen-containing structures such as aldehyde and ester groups in the aromatic ring side chains, oxygen-containing active free radicals and aliphatic side chains were obtained, facilitating the easy escape of CO2, CO, and H2 and hindering that of CH4. The oxygen-containing radicals reform carbon-containing organic substances when they encounter stable carbon structures, forming char. The reaction mechanism of Fe-catalyzed lignite pyrolysis is illustrated in Figure 9.

In this paper, the effect of FeCl3 on the evolutionary characteristics and carbon structure of lignite pyrolysis gas was investigated. The results show that the Fe component in lignite changes the reaction pathway of oxygen-containing functional groups in lignite, thus changing the distribution of pyrolysis products and significantly affecting the structure of aromatic rings in semi-coke samples. More carbon–oxygen bonds (single and double bonds) were retained in the final char. In contrast, the contents of C–C/C–H, carboxyl, and ester structures were reduced.