Pyrolysis Behavior of the Black Liquor Derived from Soda

Article pubs.acs.org/EF

Pyrolysis Behavior of the Black Liquor Derived from Soda− Anthraquinone and Soda−Oxygen Pulping of Rice Straw at Different Reaction End Points Lilong Zhang and Keli Chen* Faculty of Chemical Engineering, Kunming University of Science and Technology, Yunnan 650500, China ABSTRACT: Black liquor (BL) with inorganic cooking chemicals and combustible material is the alkali and power resources for pulping. For rice straw pulping, BL obtained from soda−anthraquinone (AQ) pulp (SABL) could hardly be combusted in a traditional recovery furnace, because of the involvement of a great deal of silica in it. On the contrary, soda−oxygen pulping may have the majority of the silica depositing in pulp owing to the oxidation, instead of its dissolution in the BL (SOBL). The pyrolysis behavior of SOBL was investigated in this paper, and composition and thermal degradation were comparatively studied by elemental analysis and TG-MS/FTIR. The reaction mechanism of pyrolysis of SOBL at different pulping end points is first revealed. The results suggested that SOBL had reasonable silicon content about one-eighth of that of the SABL, and the volatiles can be produced under relatively low temperature. For different reaction end points, with deeper oxidization, more volatiles were released at lower temperature.

1. INTRODUCTION As a post-harvest seed crop, rice is a valuable, renewable, and abundant bioresource with global yields of up to two billion tons per year. China, one of the largest agricultural countries in the world, produces 700 million tons of rice straw every year, about 30% of the world’s production according to Food and Agriculture Organization of the United Nation.1 In traditional agricultural industry, this resource is mainly used for feeding and composting. By contrast, modern biomass conversion methods,2 such as biogas, bioethanol, and biosolid fuel processing, have been developed to reuse this bioresource. However, as Table 1 shows, these methods are, generally, not economically viable and inefficient in the conversion. Therefore, only less than 40% of the collected straw residues8 are being used. The rest of the straw is burned on-site, which results in considerable environmental pollution and a significant waste of biomass resources. Pulping is an alternative method to deal with large amounts of straw residue, and straws have been used as the main pulp materials in China for decades. However, growing environmental policy pressure has led to a decrease of the proportion of the straw used for clean pulping in recent years so that only about 20 million tons of agricultural residuals are being used to produce paper products, and rice and wheat pulp account for only 4% currently. The inherent disadvantages, high content of ash and silicon accompanied with parenchyma cells,9 not only make the black liquor (BL) hard to extract and concentrate but also bring about the silicon disturbance in the recovery system.10 As a result, the BL obtained from straw pulping is hard to apply in the traditional recovery system, which becomes the bottleneck for development of straw pulping. Many methods have been developed to make up for the inherent deficiency. Dry and wet stock preparation could decrease the ash and silicon content of wheat straw. Vertical digester for producing straw pulp could significantly reduce the cost of heat while increasing the efficiency. Recycling of the © 2016 American Chemical Society

black liquor during the cooking process could increase solid contents in the resultant black liquor.11 With the help of them, ash and silicon in the BL obtained from wheat pulp can be largely cut down, which also could overcome obstacles of industrialization. However, for BL obtained from pulping of rice straw with an extremely large amount of silicon, the abovementioned technologies still cannot meet the need for its application in a traditional alkali recovery furnace. Consequently, wheat straws account for 70−75% of the total pulp production from straw while rice straws are only sparingly used.8 Silicon in the BL can also be largely reduced by soda−oxygen pulping for rice straw. Through oxidation, silicon largely could be kept in fibers, and BL appeared to have an acceptable silica content and a lower viscosity, which is closer to the wood.11 It is the straw BL we like best, as soda−oxygen pulping relieves traditional straw BL of additional treatment of desilication and viscosity reduction. Unfortunately, such a pulping method superior to the traditional alkali pulping in both pulp quality (brightness and yield all exceeded 50%) and BL feature (reasonable low silicon content and low viscosity) has not yet been attained in an industrial application.12 Lack of the particular knowledge about the characteristics of the BL during the recovery furnace is the main obstacle to its industrialization. Besides, BL of straw pulping is generally regarded as having a low heating value (HV), let alone the BL from alkali-oxygen pulping of rice straw, which is accompanied by heat loss ascribable to the oxidation. This BL with HV additional to some heat loss might be easily bracketed with low heat recovery efficiency, so that there is no more information about application of its BL in the alkali recovery system. The prospect of commercialization of the very promising alkali− Received: July 16, 2016 Revised: December 15, 2016 Published: December 15, 2016 514

DOI: 10.1021/acs.energyfuels.6b01735 Energy Fuels 2017, 31, 514−522

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Energy & Fuels Table 1. Comparison of Different Biomass Convention Methods3−7 typical method biogas bio-oil bioethanol biosolid BL gas a

high heating value (HVV)

fixed-bed gasifer entrained flow reactor enzymatic treatment carbonization fixed-bed gasifer

energy yielda (c.a.)

convention (c.a.)

3

3

13−14 MJ/(N m ) 22.5−25.7 MJ/kg 24 MJ/L 13.8−14.3 MJ/kg 20.9 MJ/kg

0.6 (N m )/kg 32.1−49.3 g/g 0.26−0.29 L/kg 36−45 g/g 70 g/g

8.1 9.8 6.6 5.7 14.6

Energy yield (MJ/kg) = HVV (median) × convention (median).

Table 2. Chemical Composition of Rice Straw (wt %) ash

SiO2

1% NaOH extractives

pentosan

benzene−ethanol extractives

holocellulose

cellulose

klason lignin

acid-soluble lignin

8.47

13.06

45.46

18.92

1.16

69.05

38.32

14.86

2.83

Yunnan province. Cooking trials conditions are displayed in Table 3. After cooking, each BL was separated from the pulp by centrifugation

oxygen pulping technology remains undecided with decades elapsed. Hence, it is very important to investigate the pyrolysis mechanism and the release raw of pyrolysis products of the straw pulping BL. The physical properties of BL are directly dependent on its composition. BL contains almost all the cooking chemicals along with lignin and other organic matter separated from the raw material. In the most traditional mill, the solid content of the extracted black liquor varies between 15% and 40% by weight, and organic compounds account for ca. 50−70% of the total solid composition.13 The sodium salts, existing as two main forms, organic bound Na salts and soluble salts, could catalyze the pyrolysis and gasification of BL. Wigmans14 explored the influence of pretreatment conditions on the activity and stability of sodium and potassium catalysts in carbon−steam reactions for Kraft wood BL. Kumar 15 researched about the pyrolysis and gasification of lignin obtained from Kraft wood pulping and the effect of alkali addition. Guo16 investigated the catalytic gasification mechanism of organic bound sodium groups and inorganic sodium salts in the wheat straw soda−anthraquinone (AQ) pulping BL. But, little attention has been paid to the soda−oxygen pulping black liquor for rice straw. Therefore, the catalysis of alkali on the pyrolysis of BL obtained from soda−oxygen pulp is the unsolved question to be addressed. Now that the thermal performance of straw SOBL becomes an essential foundation for the advancement of its efficient treatment in an alkali recovery system, this paper will focus not only on the catalytic gasification of the sodium salts for SOBL for rice straw but also on the effect of different reaction end points on the gasification characteristic of the BL by thermogravimetric analyzer (TGA) coupled with Fourier transform infrared (FTIR) spectrometry and mass spectra (MS). For comparison, the BL from soda−AQ pulping of rice straw was also used in this study.

Table 3. Cooking Trials

Tmax (°C) soda−AQ soda− oxygen1 soda− oxygen2 soda− oxygen3

EA [% (g/g raw tmax (min) t (h) material)]

AQ loading (g/g raw material)

solidtoliquor ratio (g/v)

final pH of the BL

140 110

30 75

2.5 2.5

15 18

0.5% no

1:5 1:5

12.0 9.3

110

60

2.5

18

no

1:5

11.0

110

45

2.5

18

no

1:5

12.0

and washed with hot water until more than 95% of the total solids in it was obtained. Part of the BL was freeze-dried to yield the dry BLS. 2.3. Component Analysis of the BLS. The organic/inorganic of the BLS was determined by combustion followed by ash analysis according to the standard the “Analysis of Soda and Sulfate Black Liquor” method (TAPPI T625 cm-14). Silicon content was determined by TAPPI T632-11 “Analysis of sodium silicate”, for which inorganic was calcined at 1000 °C after treatment with concentrated sulfuric acid and nitric acid. The analysis of the BL elements was implemented in a Vario-I elemental analyzer and an ICP inductively coupled plasma emission spectrometer. The measurement parameters were oxidation furnace temperature: 1150 °C, reduction furnace temperature: 850 °C, carrier gas flow rate of the measuring cell: 90 mL/min, carrier gas flow rate of the reference cell: 20 mL/min, and oxygen flow rate: 30−80 mL/min. 2.4. TG-FTIR/MS Method. The thermogravimetric analyzer (STA 449 F3 Jupiter, NETESCH) was coupled with an FTIR spectrometer (TENSOR 27, OMNIC) to determine the mass loss of the BL and online evolution of gaseous products. At the same time, mass spectra of the gases evolved from TGA were recorded by quadruple mass spectrometer (QS422, Pfeiffer Vacuum). The transfer lines connecting the TGA, FTIR, and MS were heated to 200 °C in order to prevent the condensation of the gaseous products. The experiments were done on the TGA at a heating rate of 20 °C/ min within the temperature range from 50 to 800 °C, and high purity nitrogen was used as carrier gas with a flow rate of 20 mL/min. About 30 mg of BLS was put in the ceramic crucible each time. The FTIR spectrum was obtained in the range 4000 to 650 cm−1. The mass spectrometer was set at 70 eV. The m/z was carried out from 1 to 100 amu to determine which m/z has to be specified during the TG experiments. The ion curves that were close to the noise level were omitted. Finally, only the intensities of nine selected ions (m/z = 16, 28, 39, 44, 45, 96, 109, 123, 138, and 152) were monitored with the thermogravimetric parameters.

2. EXPERIMENTAL SECTION 2.1. Chemical Composition of Rice Straw. Standard Methods of the Technical Association of the Pulp and Paper Industry (TAPPI, Atlanta, GA) were used to determine the chemical composition of the material, including 1% NaOH extractives (T 212 om-07), acidinsoluble lignin (T 222 om-06), pentosan (T 223 cm-01), ash (T 244 cm-99), SiO2 (T 245 cm-98), and cellulose content based on the nitric acid−ethanol method (T 203 om-93). 2.2. Preparation of Black Liquor Solids (BLS). Rice straw was sampled from Wuding, Yunnan Province. The results of the chemical composition are listed in Table 2. All of the black liquor samples were prepared from pulping of rice straw with soda−oxygen and also traditional soda−AQ in Pulping and Papermaking Research Center of 515


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3. RESULTS AND DISCUSSION 3.1. Composition and Element Analysis. The inorganic and organic of BL are shown in Table 4. Two of them are

more lignin dissolved in the BL results in the increase of C composition. As the addition in the pulp process, oxygen cooperating with alkali has a more powerful oxidation than soda−AQ which leads to the O elemental taking apart more constituent. With prolonging the reaction time, degree of oxidization for the BL increased as follows. O takes apart 41.95% of SOBL1’s organic elements which is the biggest proportion of all BLs while SABL just has 36.63%. The other apparent difference occurs in S elemental: SOBLs contain about 0.4% which is just one-third of SABL. During the process of the alkali recovery system, where BL burned in the recovery furnace, sulfur could transform into sulfur compounds like SO2 which are harmful to the air environment. So, the desulfurization process of the stack gas treatment is needed for the traditional Kraft pulp factory. However, the extremely low content of sulfur in the SOBL could reduce pressure of the process and even remove this part of the treatment. 3.2. TG and DTG Analysis. Through TG-DTG analysis of the BLS, their pyrolysis behaviors could be obtained as shown in Figure 1. Based on the curves, it can be divided into three parts which represent specific volatiles released from the BL. Among them, the weight loss of ca. 5% in the initial pyrolysis stage before 200 °C is mainly attributed to the release of water and to the pyrolysis of the carbohydrate and lignin in the second and third stages, from 200 to 300 °C and 300 to 650 °C.17 The basic characteristics of this BL pyrolysis may be narrated in detail below. In the second stage, as the main weight loss of ca. 20% of the whole process, the peak of the mass loss rate increased rapidly. As showed in the DTG figure, SOBL curves were all under the SABL’s. It means the former had higher decomposition rate than the latter. Besides, the maximum decomposition peak of SABL occurred at 280.2 °C being higher than that of the SOBLs. So, it could be concluded that SOBL had better thermolabile characteristic in consideration of the lower pyrolysis temperature with higher decomposition rate. For SOBLs, the maximum degradation rate (−4.84%/min) of SOBL1 occurred at 264.9 °C. Similar to the SOBL1, the maximum degradation rate of the SOBL2 happened at 264.2 °C but with the slowest decomposition rate of −3.87%/min. As an exception the date of SOBL3 appeared to have a lower temperature with 235.4 °C. Such an unusual temperature falling near ca. 30 °C may result from the catalytic effect of sodium compounds in black liquor during pyrolysis.18 Another unexpected result is that SOBL1 had the highest rate with relatively low temperatures. Liu19 reported that hemicelluloses started the decomposition at 220−315 °C. The minimum amount of silica in SOBL1 (see also Table 4) additional to the continuing organic dissolution during a prolonging oxidation cooking would bring the BL with more carbohydrates than SOBL3 as well as SOBL2 did. The result is that its decomposition rate was accelerated during pyrolysis in this stage. In the third stage, the pyrolysis rate slowed down in the range from 400 to 500 °C and the loss weight take up about 20%. This weight loss should correspond to the decomposition of the lignin.20 It should be, from the DTG curves in Figure 1, noted that SOBL1 had two prominent peaks that happened at 388 and 443.8 °C with decomposition rates 2.91%/min and 2.7%/min, respectively, but the peak around 388 °C even faded away with the final SOBL pH rising. More specially, SABL has notobvious peak in this stage. The above-mentioned result may be explained by the different pulping processes. With oxidation

Table 4. Composition of the BLS inorganic content [% (g/g of BLS)]

organic content [% (g/g of BLS)]

organic/ inorganic

SiO2 [% (g/g inorganic]

22.38 22.59 24.45 25.41

77.62 77.41 75.55 74.59

3.46 3.43 3.09 2.94

32.50 4.38 12.43 23.60

SABL SOBL1 SOBL2 SOBL3

expressed as the mass percentage of this content to total dry solids, and SiO2 is expressed as the mass percentage of this content to inorganic. These values have been obtained from a series of three tests, with a standard deviation lower than 5%. It can, from this table, be found that the inorganic content in SOBL declined with the increase of its alkalinity at the reaction end point. Although a longer cooking would help the dissolution of the more organic from the straw, the contribution of obvious shifting silica from the BL onto the pulp to the inorganic declination seemed much superior to that from the subsequent dissolution of the organic. The most convincing proof was the lowest silica proportion 4.38% in the inorganic of SOBL1, being about one-fifth of SOBL3’s. In other words, 22.59% of the inorganic decrement for SOBL1 resulted from the removal of silica from its BL, if the data of SOBL3 was taken as a reference. The reason for the reduction of silica in SOBL was that the acidic groups produced from oxidation of the straw caused pH decreasing of the BL during cooking, leading to the deposition of dissolved silicate onto the pulp, and the lower the final pH, the less the silica in SOBL. By contrast, the silica in SABL without oxidation accounted for almost a third of the inorganic. This is why the soda−oxygen method can alleviate significantly and even overcome silicon disturbance in recovery of the BL from straw pulping. However, SABL has the highest organic content. It may result from, although the charge of sodium hydroxide in the soda−AQ pulp is lower than that in the soda−oxygen, reaction temperature in the former is higher than in the latter. So, even though the reaction time is shorter than that of SOBL, the soda−AQ pulping process has more powerful destruction for lignocellulose. More organic matter could be released into the BL, and without the soda− oxygen synergistic effect, silicon cannot be absorbed in the fiber. As the result, by contrast with the SOBL, SABL has the higher organic amount (60.25%) and SiO2 accounts for 32.5% of the ash. As Table 5 shows the elements of the BLS, carbon (C) and oxygen (O) are the dominating elements in the BL, with small amounts of nitrogen (N) and hydrogen (H) and little sulfur (S) elements. SABL has the highest C content of 52.51% while SOBLs contain less, which means with deeper delignification, Table 5. Composition of the BLS Organic Elements

a

samples

C (wt %)

H (wt %)

N (wt %)

S (wt %)

Oa (wt %)

SABL SOBL1 SOBL2 SOBL3

52.51 48.68 48.62 49.96

7.92 7.30 7.67 7.66

1.78 1.65 1.57 1.68

1.16 0.42 0.39 0.49

36.63 41.95 41.75 40.21

By difference. 516


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Figure 1. TG and DTG curves of SABL and SOBL at different pulping final points.

Figure 2. Typical 3D infrared spectra from SABL (a) and SOBL (b) pyrolysis and gasification.

properties especially for residual sodium hydroxide, and the silica content had a particularly negative impact on the pyrolysis of the BL. On one hand, a higher inorganic proportion in the solid could decrease the HV of the BL. Otherwise, sodium hydroxide existing in the BL could catalyze the pyrolyzation. SOBL3 has lower pyrolysis peak temperature and higher pyrolysis rate than SOBL2 even after weaker reaction process. On the other hand, lowering the silica content could greatly reduce interference with the thermal expansion of the BLS. The maximum devolatilization rate (4.84%/min) was obtained at the minimum silica content 4.32% of SOBL1, which is just oneeighth of the SABL. Although the TGA curve could reflect the pyrolysis characteristic of the BLS obtained from different pulping conditions, the precision of constitution produced during the gasification could not be gained. More information on them could be obtained from MS/FTIR analysis of the evolved gases. 3.3. TG-MS/FTIR Analysis. Thermal degradation of polymers is a complex process that produces a wide range of low-molecular-weight (LMW) compounds. Although the MS analysis of the volatile products evolved during TG measurements offered detailed information on the thermal behavior of the BL, drawbacks still exist, such as one m/z signal from MS analysis possibly corresponding to various fragmentation ions like the m/z 18 representing H2O+ and NH4+ and m/z 44 for CO2+and C2H4OH+.22 However, temperature evolution during TG analysis might differ from some compounds, and FTIR also offers its characteristic functional group. So, comprehensive analysis of this information could help us discriminate

and delignification deeper during soda−oxygen pulping, the lignin content in the BL increased, and the lignin would be destructed to smaller fragments,21 which are easier to decompose thermally. Another important favor which cannot be neglected should be the catalysis from sodium hydroxide. Even SOBL3 resulted from the shortest-time oxidization; compared with the other two trials, the pyrolysis peak occurred at 433.8 °C with 2.92%/min pyrolysis rate which appears to be a better pyrolysis property than that of SOBL2. As for the appearance of a slightly slower increase of decomposition rate from SABL without an obvious peak, a reasonable explanation should be that whether such an evident fluctuation in thermolysis rate occurred or not was mainly dependent on the amount of the activity structures in the organic matter. And obviously, the existence of such structures from SABL was far inferior to that from SOBL subjected to oxidation during pulping. Overall, the relationship between the pyrolysis property and the composition of the BL is very complicated. It is hard to correlate the amount of silica or organic content etc. in the BL with its pyrolysis quantitatively. Both our previous research22 and and that of Cardoso23 have examined the effect of silicon on the physicochemical properties of BL, which reflected that silicon hampered the pyrolysis process. Zhang24 found that silica particles and lignin molecules are connected to form hydrogen bonds through silanol groups (Si−O−H), which limited the combustion of BL seriously. In this paper, in combination with the results in Figure 1 and Table 2, it is clear that inorganic materials could affect the thermodynamic 517


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Figure 3. FTIR spectra of volatile products at the maximum weight loss rate from BLS (a) SOBL1, (b) SOBL2, (c) SOBL3, and (d) SABL.

compounds volatilized from the pyrolysis. Similar problems also appeared in TG-FTIR analysis where complex mixtures of gas and the rotational bands complicate the interpretation. To overcome these difficulties, in this paper more than one single m/z signal or IR band was analyzed and represented the considered compounds. 3.2.1. FTIR Analysis. During the TG determining process, the volatiles evolved were swept into the gas cell in real time. At the same time, the FTIR spectrum and mass spectrum could be obtained by 3D infrared spectrum and quadruple mass spectrometer. Figure 2 provides the information on the BL samples as a function of wavenumber and time. According to the heating curves of the TG in Figure 1, temperature of variation release from the pyrolysis could be calculated. When one of the two factors was fixed, the other could be obtained to analyze the composition of the gas and its occurring temperature. The intensity of the peaks of the spectra represent the concentration of the variation in the whole process, which can reflect the tendency of product yields of the gas species. In order to analyze the pyrolysis of the whole process, three typical temperatures were chosen, based on the maximum TG peaks above-mentioned. Figure 3 showed the FTIR spectra of the volatiles generated during pyrolysis of the four BLs. In turn, Table 6 listed the typical functional groups and the FTIR signals from the possible pyrolyzed gas components. Combined with this information, the volatiles could be determined as certain compounds occurred at particular temperatures. It is helpful for discriminating the MS spectrum assignment. First of all, the small molecule gas volatiles like water, carbon dioxide, and methane were released in the first stage (200−300 °C), where FTIR peaks happened at 3964−3000, 2217−2391, and 3200−2850 cm−1, respectively. Among them, CO2 had strong peaks while the characteristic peak of CO at 2112 and 2180 cm−1was weak. SABL displayed the similar volatile patterns with those of SOBL. However, SOBL1 appeared to have its special characteristic, where absorption peaks at about 1700 and 3500 cm−1 were stronger than those of the others. This means more water and carboxylic acids were released.

Table 6. Main Functional Groups of Volatiles during Pyrolysis25−29 wavenumbers (cm−1) 3964−3000 3200−2850 2860−2770 2217−2391 1760−1580 1507 1040−1180 965−1435 860

functional group

compounds

OH CH stretch CO stretch CO stretch COOH aromatic skeleton vibration CH stretch OH or CO bending stretch

water alkanes aldehydes carbon dioxide carboxylic acids aromatic alcohols furaldehyde (R) hydroxyacetaldehyde

Second, in the second stage (300−500 °C), the pyrolyzed volatiles mainly generated from the lignin, with peaks at 1760− 1580, 1040−1180, and 965−1435 cm−1, correspond to carboxylic acids, alcohols, and furaldehyde, respectively. An unexpected result occurs at the SABL curve, and other than SOBL’s curves, the peak of 3000 cm−1 appeared weaker. It may be result from the lower density of methane released from pyrolysis of SABL. According to the previous report about the pyrolysis of the ligin, CH4 is mainly production for the pyrolysis decomposition of lignin. One conclusion can be drown: that SOBL could release a greater amount of CH4 than SABL in the same pyrolysis process. Finally, in the third stage of the gasification (500−800 °C), SABL had unique peaks around 1000 cm−1, indicating furaldehyde continuously was released until the end of the pyrolysis as well as the CO characteristic peak. However, for the absorption band of CO2 at 2217−2391 cm−1, SOBLs have a stronger signal, and with deeper oxidation, the trend becomes more obvious. CO2 was still released even at high temperature, and it may be result from the crack of the oxidized group. According to the discussion above, it could be concluded that the patterns of pyrolysis for SABL and SOBL are almost the same, while some differences still existed in several respects: 518


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Figure 4. Evolution of H2O (m/z 18), CH4 (m/z 16), and CO2 (m/z 44) for (a) SOBL1, (b) SOBL2, (c) SOBL3, and (d) SABL. Part 1: The release behavior of CH4, H2O, and CO2.

Figure 5. Evolution of acetic acid (m/z = 45) and furfural (m/z = 39/96) for (a) SOBL1, (b) SOBL2, (c) SOBL3, and (d) SABL. Part 2: The release behavior of acetic acid and furfural.

and carbohydrates with soda−oxygen pulping,30−34 which the oxidation groups like carboxyl and carbonyl group added after synergistic effect of soda and oxygen. As a result, LMW acid could be released by pyrolysis in the first stage, with more CH4 and CO2 produced in the second and third stages.

that SOBL1 released more aromatic small groups than the other three BLs at the first stage, the amount of CO produced from SOBL was larger than that from SABL, and furfural production from SABL still lasted during pyrolysis in the entire stage. Those results are inseparable with the change of lignin 519


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45 curve from SAB becomes narrowed, the main peak occurred at 400 °C, and two shoulder peaks sequentially appeared at 500 and 560 °C. Peak height reaches the maximum at ca. 400 °C, and shoulder peaks decreased rapidly to just half and one-third of the maximum. These three peaks should be correlated with several kinds of particular structures that were prone to releasing acetic acid via the cracking gas-generating stage at these corresponding temperatures. For SOBLs, expect SOBL1 with almost identical peak shape to that from SABL, neither SOBL2 nor SOBL3 had conspicuous peaks at 500 and 560 °C especially for SOBL2 subjected to the moderate oxidation. To clarify, the falling rate of SOBL2’s curve after the maximum peak was slower than that of the other BLs; that is, the comparatively higher mass-to-charge ratio intensity from 400 °C to near 600 °C implied the release of more acetic acid, so that the rear peaks at 500 and 560 °C could have been overlapped to some extent, as the two shoulders can still be observed at 500 and 560 °C. And of course, good understanding of the origin of SOBL2’s peak curve change distinct from other BLs may need to be studied further. It is also worth noting that the temperature of releasing acetic acid to the maximum peaks from SOBLs was ca. 25 °C lower than that from SABL, owing to the former organic matter with more oxidative groups. Last but not least, the mass curves of m/z 39 and m/z 96 representing furfural reach to the maximum peak at 520 °C. Compared with acetic acid, the pyrolytic temperature of furfural was ca. 100 °C higher than that of acetic acid, and furfural was not produced any more from the BLs over ca. 620 °C. There still existed some difference in the behavior of furfural release from BLs between both pulping methods. As shown in Figure 5d, the furfural curve from SABL had two peaks; one was weak, and the other was strong occurring at 530 and 585 °C, respectively. Unlike SABL, the curves of SOBLs owned a big peak at ca. 570 °C, seeming first to slow, and then quickly down with the temperature rising; then the abundance value fell fast. What is interesting is that the peak temperatures of furfural nearly coincided with with the small peak ones of acetic acid for SABL. It cannot be inferred that there existed a close relation between the two above-mentioned volatiles; however, furfural was released from cracking carbohydrates.34 It is assumed that acetic acid could be also produced at the same time, and one cannot rule out the possibility of the decomposition of the composition with the same structure, for example, alduronic acid etc., into furfural and/or acetic acid, correlated with the two peaks at 530 and 585 °C. By contrast, furfural curves of SOBLs all just had one big peak, and the SOBL1 peak temperature remained consistent with that of the small peak (550 °C) of its acetic acid curve, while both SOBL2’s and SOBL3’s kept pace with their last small peak ones (580 °C). This implies that although this part just represents a small part of the acetic acid, those two volatiles could be more or less released from correlated structures in SOBL via pyrolysis. Additionally, the peak temperature of the furfural curve from SOBL1 was 30 °C lower than that of the rest of the two SOBLs. The advancement of the temperature of starting thermal degradation may be attributed to its more activated structures in SOBL1 which suffered from the deepest oxidation in pulping. Volatile furfural discussed previously can be attributed to the pyrolysis of carbohydrates in the BLs, and volatile aromatic compounds mostly result from the thermal cleavage of lignin in BLs. As shown in Figure 6, guaiacol (G), methylguaiacol

By means of the current analysis technology, only the evolution profiles of the volatiles with distinct absorption bands could be achieved. However, the rule of each volatile evolved during the gasification should be determined by MS. 3.2.2. MS Analysis. Water and carbon dioxide are the main degradation products of biomass, and about 30% of the mass balance for the pyrolysis of BL also accounts for the majority of O elimination from BL. As Figure 4 shows, water was released in two stages (ca. 200−300 °C and 300−500 °C) while the carbon dioxide just has one main peak (ca. 400 °C). For water curves, SABL had two placid DTG peaks while SOBLs have one prominent peak range from 300−500 °C. With the pH of the end points increasing, temperatures of peaks of SOBLs became lower (from 400 to 370 °C), which verified the theory of Guo32 that sodium salts have catalysis for BL gasification. However, the SOBL1 curve appeared to have an unexpected apparent peak at 280 °C which may result from the degradation of carbohydrate during prolonged oxidation in the soda− oxygen process. Another cruve at the m/z 16 signal standing for CH4 has two peaks, and the maximum degradation rate occurred at 400 and 580 °C. Similar to the law of the m/z 18, the first peak of SOBL3 lightly moved left from 400 to 380 °C. SOBL1 has one unexpected peak at ca. 300 °C. This might be due to the prolonging oxidation for the carbohydrate, and this kind of the strong oxidation degraded the carbohydrate group to the LMW group like some kind of organic acid. Even under low temperature heating, such a structure would decarboxylate and lead to the peak obtained at low temperature. Through analysis of the dehydration and the decarboxylation of the BL in the low molecule weight range, effect of the different pulping conditions on the characteristic of gasification is obviously shown: 1, the carbohydrate decomposed badly after soda−oxygen pulp and the fragments dissolved in BL is easier to gasification than SABL, and 2, with the oxidation becoming more serious, the gasification is easier to release. The m/z 45 and m/z 39/96 represent acetic acid and furfural, respectively. The former should be derived from carbohydrates as well as lignin, whereas the latter probably only from carbohydrates in the BLs during prolysis.33 Both volatiles were all released from the four BLs in the temperature range from 300 to 600 °C, as shown in Figure 5. The typical bands at 1760−1580 cm−1 belonging to carboxyl groups and 2680−2720 cm−1 to aldehydes each from the two volatile components also can be identified in Figure 3 (also see Table 2). However, the production of each compound varying from thermal temperature was greatly related to the pulping methods but were similar to one another in the same way regardless of oxidation extent or the final BL alkalinity. First, the different pyrolysis property of BLs can be observed by half-peak width (HPW) which represents the temperature span of the pyrolysis process. In the curve of m/z 45 the biggest difference between SOBLs and SABL is that the latter releases acetic acid until the temperature reaches 600 °C while this process of SOBL could be prolonged to ca. 630 °C. As a result, SOBL had greater HPW than SABL. Peak height is the other difference; SABL forms a main peak when pyrolysis temperature reach to 300 °C, meanwhile SOBL1 has an apparent peak at 250 °C. Both the greater HPW and the lower peak temperature are a reflection that SOBL has a more complex structure than SABL. Second, different peak intensities in Figure 5 were compared for the concentration ratios of the volatiles. As the HPW of m/z 520


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Figure 6. Evolution of guaiacol (m/z 109), methylguaiacol (m/z 123, 138), and ethylguaiacol (m/z 152) for (a) SOBL1, (b) SOBL2, (c) SOBL3, and (d) SABL. Part 3: The release behavior of aromatic compounds during the pyrolysis process.

(MetG), and ethylguaiacol (EtG) represent the crack of the lignin during the tarring process and steam gasification35 and the corresponding mass to charge ratios are m/z 109 for G, 123 and 138 for MetG, and 152 for EtG, respectively. It is worth noting that there existed two relatively simple curves of mass to charge ratio in four lignin fragments, being from m/z 109 and 123. The maximum intensities of both fragments from SOBL1 occurred earliest at ca. 500 °C, compared with a lag at ca. 30 °C from the rest of the two SOBLs, while the peaks from SABL appeared near 540 °C. The lignin structure subjected to deeper oxidation easily became thermally cracked into fragments like G and also MetG groups under lower temperature. Besides, the possibility of silica interferrence with the pyrolysis cannot be rule out, since the latter three BLs with delaying peak at higher temperatures contained 3−8 times the silica content of SOBL1. Nevertheless, the silica amount was found not to be closely related to the temperature and led to maximum release of G and MetG. As for fragment EtG in Figure 6, except for a major peak and similar peak temperature to G and MetG from SOBL1 in the thermal temperature range, the other three BLs gave the fluctuation curves, which were more complicated than imagined. With the molecular weight of volatile organic compound (VOC) increasing, pyrolytic reaction became complex, and the release of VOC at high temperature became irregular.

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decreased to less than 10, and silicon content of SOBL is just one-eighth of the SABL. 2. SOBL of gasification provides many advantages: it is a more rapid gasification with lower reactor temperature. Acetic acid produced from the SOBL pyrolysis has a release temperature 25 °C lower than that of SABL as well as furural release required a 30 °C lower temperature, and the syn-gas of the gasification contains a larger amount of volatile organic compound. 3. Different pulping conditions have different effects on the BLS pyrolysis property, with deeper oxidation and more volatiles released in lower temperature during pyrolysis. SOBL1, the deepest oxidized condition for pulping, could release H2O at 200 °C and acetic acid at 300 °C while the other two SOBLs have main peaks at 400 °C.

AUTHOR INFORMATION

Corresponding Author

*(K.C.) E-mail: [email protected]. ORCID

Keli Chen: 0000-0001-9287-7632 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21276119).

4. CONCLUSIONS Through analysis of the results, the reaction mechanism of pyrolysis and steam gasification of BLs derived from soda−AQ and soda−oxygen pulping of rice straw at different reaction end points can be shown. 1. Soda−oxygen pulping could reduce the content of the silicon in the BL, when the end point of the reaction

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Pyrolysis Behavior of the Black Liquor Derived from Soda

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