Char in-situ (char[is]) obtained from corn stalk pyrolysis was evaluated as a catalyst to upgrade corn stalk pyrolysis vapors. A catalyst evaluation device was introduced to conduct the experiments. The effects of reaction temperature and char[is] dose on catalytic performances in biomass pyrolysis were evaluated. The results showed that the char in-situ had a remarkable effect on the pyrolysis products. Under the action of char[is], the primary compounds of pyrolysis vapors were catalytically converted into phenolic products, such as phenol and 4-ethyl-phenol, while the acetic acid content was evidently reduced. The product selectivity was not dependent on the polar functional groups on the char[is]’s surface according to the Fourier transform infrared (FTIR) results, but might have been dependent on the mesoporous structure and the basicity sites of the charis as well as the metallic species in the char[is]. A possible reaction mechanism for phenols production and acetic acid inhibition was proposed.
Phenols Production from Online Catalytic Conversion of Corn Stalk Pyrolysis Vapors using Char in-situ
Yunchao Li,a Xianhua Wang,b,* Huawei Song,a Jingai Shao,bHongtao Ma,a and Hanping Chen b
Char in-situ (charis) obtained from corn stalk pyrolysis was evaluated as a catalyst to upgrade corn stalk pyrolysis vapors. A catalyst evaluation device was introduced to conduct the experiments. The effects of reaction temperature and charis dose on catalytic performances in biomass pyrolysis were evaluated. The results showed that the char in-situ had a remarkable effect on the pyrolysis products. Under the action of charis, the primary compounds of pyrolysis vapors were catalytically converted into phenolic products, such as phenol and 4-ethyl-phenol, while the acetic acid content was evidently reduced. The product selectivity was not dependent on the polar functional groups on the charis’s surface according to the Fourier transform infrared (FTIR) results, but might have been dependent on the mesoporous structure and the basicity sites of the charis as well as the metallic species in the charis. A possible reaction mechanism for phenols production and acetic acid inhibition was proposed.
Keywords: In-situ upgrading; Pyrolysis; Char; Bio-oil; Phenols
Contact information: a: Huadian Electric Power Research Institute Co., Ltd., Hangzhou 310030, China; b: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China; *Corresponding author: email@example.com
Due to the shortage of fossil fuels and increasing environmental concern, renewable energy has attracted great attention in the past few decades. Lignocellulosic biomass, due to its renewability, low cost, wide availability, and CO2 neutral features, is being studied as a promising alternative energy source. Pyrolysis is considered as an efficient technology to convert biomass into liquid fuel (bio-oil), solid char, and non-condensable gas (Papari et al. 2017). Among them, bio-oil has the potential to replace fossil fuels and can also be used as a feedstock for valuable chemical production (Elkasabi et al. 2015). Actually, many value-added chemicals exist in bio-oils, such as levoglucosan (LG), furfural, phenolic products, and more. However, currently, it is uneconomical to recover these valuable compounds from bio-oils due to their low concentrations (Ren et al. 2017). Through specific pre-treatment or catalytic pyrolysis of biomass, previous studies have been developed to upgrade bio-oil into fuels or chemicals. Although it may result in the formation of additional water or coke, or decrease the yield of bio-oil, the in-situ catalytic cracking of pyrolysis vapors is efficient to upgrade bio-oil (Lødeng et al.2013). By using appropriate catalysts, the production of target compounds will be promoted and the undesirable compounds will be suppressed.
In previous studies, the traditional zeolite catalysts have been extensively studied, for example HY, HZSM-5, etc. (Sharma and Bakhshi 1993; Qi et al. 2007; Lu et al. 2010a; Stefanidis et al. 2011). The zeolite catalysts were found to effectively transform the oxygenated compounds to the low oxygen-containing organic substances, such as hydrocarbons. However, the zeolite catalysts are easily inactivated because of the coke deposition. The yield of liquid would be decreased, and the polycyclic aromatic hydrocarbons could abundantly form (Lu et al. 2010a). In recent studies, mesoporous catalysts, such as SBA-15, Al-MCM-41, and others, have been used for upgrading the biomass pyrolysis vapors because of their specific porosity and acidic nature (Adam et al. 2006; Iliopoulou et al. 2007). However, compared to the zeolite catalysts, the mesoporous catalysts are hydrothermally unstable and have a high production cost, thus inhibiting their applications. In addition, most of them may suffer deactivation caused by coke deposition on their surfaces (Mortensen et al. 2011).
Pyrolysis char is a by-product from biomass pyrolysis. It has a low cost and a porous structure, containing some alkali metallic species. Due to its special physical and chemical properties, pyrolytic char is regarded as a potential catalyst (Bartocci et al. 2018). Sun et al.(2011) used pine wood char to crack the tar and found it active catalytically for the tar cracking at 500 °C to 600 °C. Keown et al.(2008) investigated the influence of char-volatile interactions on the alkali and alkaline earth metals (AAEM) species’ volatilization during the biomass gasification process. However, there has been a lack of studies that focus on the online catalytic conversion of biomass vapors using char in-situ to obtain high quality bio-oil.
In this paper, the char is used as a catalyst for in-situ upgrading of pyrolysis vapors of biomass. The corn stalk is selected as the experimental sample. The effects of reaction temperature and char dose on catalytic performances in biomass pyrolysis are evaluated. The ultimate products are analyzed and the possible reaction mechanism is proposed. It should be noted that although the characteristics of the char would be slightly changed in the reaction, the char is still defined as a catalyst in this paper.
The corn stalk (Caidian Farm, Wuhan, China), with a particle size from 40-mesh to 200-mesh, was used as the pyrolysis feedstock in a catalyst evaluation device (WFS-3070; Xianquan Co., Tianjin, China), as shown in Fig. 1. The properties of the corn stalk are given in Table 1. The char in-situ (charis) was also pre-prepared in the device by pyrolyzing the corn stalk at different temperatures (400 °C to 600 °C). The pyrolysis residence time for charis preparation was 90 min to ensure that the corn stalk was completely decomposed and that no secondary cracking would occur in the following catalytic process.
Table 1. Properties of Corn Stalk
For each pyrolysis experiment, approximately 4.0 g of corn stalk sample was loaded into the left tube (the pyrolysis reactor, with a length of 630 mm and an inside diameter of 24 mm) with loose quartz wool packing. The charis was kept in the right tube (the catalytic reactor, with a length of 450 mm and an inside diameter of 18 mm) which was pre-heated to the set temperature (the same temperature as the carbonization temperature). The corn stalk was then heated from room temperature to the pre-set temperature (the same as the temperature of the pyrolysis reactor) with a heating rate of 20 °C/min. All of the pipes were insulated, and the inner atmosphere was nitrogen gas with a flow rate of 200 mL/min. The pyrolysis vapors then passed through the charis. The mass ratio of the charis to corn stalk was 0.125 (0.5 g) and 0.25 (1 g), respectively. After leaving the reactor, the pyrolysis vapors were condensed in an ice-water condenser operating at approximately 0 °C. The liquid products (bio-oil) were quantitatively weighed and collected using acetone as the solvent. The char yield in this paper was defined as the solid residue (abbreviated as charsr) in the pyrolysis reactor, which was collected and weighed. The reacted char (charis) in the catalytic reactor was also weighed. The yield of the uncondensable gas was calculated by the difference. For comparison, tests without charis in the catalytic reactor were also performed. It should be noted that the residence time of the pyrolyzer was also kept as 90 min to make sure the reaction was complete. In contrast, the gas analyzer used in the experiment had a certain lag because of the gas residence time in the condensation system. All of the experiments were conducted twice to guarantee the results were reproducible.
Fig. 1. Schematic diagram of the catalyst evaluation device
Characterization of the products
The main components of bio-oil were determined by gas chromatography-mass spectroscopy (GC-MS, Agilent 7890A/5975C; Agilent Technologies Inc., Santa Clara, CA, USA). The capillary column was HP-5-MS (30 m × 0.25 mm × 0.25 µm). The temperature of GC oven was first 40 °C maintained for 0.5 min and then raised to 270 °C for 5 min at a rate of 5 °C/min. The temperature of the injector was 280 °C with a split ratio of 50:1. After 2 min of solvent delay, the sample was injected into the ion source of a Hewlett-Packard model 5975 series mass-selective detector (Agilent Technologies Inc., Santa Clara, CA, USA) and scanned over a range of 30 m/z to 500 m/z. The software MSD Chemstation (Agilent Technologies Inc., E.01.00.237, Santa Clara, CA, USA) was used to identify the compounds based on the retention time. Other instrument settings and operating parameters can be found in the authors’ previous study (Li et al. 2016). The organic functional groups of the charis before and after reactions were detected in a VERTEX 70 spectrophotometer (Bruker, Ettlingen, Germany) (Li et al. 2014). The surface area of the charis was measured by nitrogen adsorption (Micromeritics, Norcross, GA, USA), and the basicity of the chariswas determined by Boehm titration (Li et al. 2013, 2014). The gas components were determined by an on-line infrared gas analyzer with a thermal conductivity detector (TCD) for H2 (Huamin, Wuhan, China), and the relative content of the different components wascalculated from their peak areas.
RESULTS AND DISCUSSION
The yield distributions of bio-oil, charsr, and uncondensable gas from corn-stalk pyrolysis at the temperatures between 400 °C and 600 °C, are shown in Fig. 2. The horizontal axis of Fig. 2 represents the experimental conditions. For instance, “400” means the pyrolysis temperature was 400 °C without char present; “400-1.0” means that the pyrolysis temperature was 400 °C and the char dose was 1.0 g. As shown, the yield of bio-oil increased with increased temperature from 400 °C to 500 °C, and reached its maximum value (37.1 wt%) at 550 °C. The charsr yield was reduced as the temperatures increased. Higher temperatures increased the gaseous product yield.
Fig. 2. Product-distribution from corn stalk on-line catalytic pyrolysis
After interacting with the charis, the yield of the bio-oil declined, and as the charis dose increased, the bio-oil yield further decreased. For example, the yield of the bio-oil was 37.1 wt% at 550 °C, and it decreased from 33.05 wt% to 27.7 wt% as the charis dose was 0.5 g and 1.0 g, respectively. This decrease might have been due to the decomposition of bio-oil promoted by the charis, polymerization to form chars, or adsorption into the charis pores. The gas yield also increased with the charis present, and the increase might have been caused by the secondary cracking reaction.
Table 2. Composition of Bio-oil Determined by GC/MS at 550 °C
Table 2 lists the composition of bio-oil at 550 °C. As is known, the peak area percentage of the GC/MS chromatogram is considered linear with its relative compound content (Lu et al. 2010b). As shown, the chemical compounds of the original bio-oil without charismainly consisted of acetic acid (31.42%, the peak area percentage), 2,3-dihydrobenzofuran (17.71%), phenol (7.77%), and hydroxyacetone (6.07%), with a small amount of dehydrated sugar (1.35%). Acetic acid was the major component in the original bio-oil. After interacting with the charis, the composition of bio-oil was notably changed. The acetic acid content was remarkably reduced from 21.73% to 17.19% when the charis dose changed from 0.5 g to 1.0 g. The content of 2,3-dihydrobenzofuran was also remarkably reduced. Simultaneously, the phenol increased from 7.77%, without charis present, to 8.82% and 15.96% with charis doses of 0.5 g and 1.0 g, respectively. The catalytic mechanism was discussed in the following sections.
To clearly show the change of compositions, the bio-oil components detected by GC/MS were classified into twelve groups (acids, phenols, furan, ketones, cyclopentenone, sugars, hydrocarbons, alcohols, phenanthrene, esters, ethers, and other compounds), as shown in Fig. 3. The highest relative content of phenols was obtained at 550 °C with a charis dose of 1.0 g, and the peak area percentage could reach up to 55.6%, while it was only 25.5% without catalytic process. The peak area of the acids also declined from 34.4% to 23.6% (0.5 g) and 17.2% (1 g), respectively. The charis reduced the formation of acids and promoted the phenol production. The phenols’ selectivity increased as the charis-to-biomass ratio increased (Fig. 3f). Lu et al. used a NiO-catalyst and found that the content of phenol increased from 26.5% to 32.6% (peak area percentage) after catalytic reaction. The NiO-catalyst was thought to be promoting the depolymerization of lignin from the phenolic compounds (Le et al.2010b). Adam et al. (2006) used a transition metal modified catalyst and found that the phenols yield was improved. Lu et al. also used the Pd/CeTiO2 catalyst and found the peak area percentage of the phenols obviously increased, which indicated that the Pd incorporation would improve the cracking activity for lignin pyrolysis (Lu et al. 2010a). Sun et al. (2011) used pine wood char for tar decomposition and found that the carboxylic acids, furans, etc. substantially decreased with the phenols’ increase. These results were similar to what the authors had achieved. As is known, the acids present in the bio-oil have a negative effect on the property of bio-oil. Thus, the reduction of its content is beneficial to bio-oil quality. Phenols are valuable chemicals that are widely used in resin production. Therefore, the catalyzed bio-oil may be used as a promising raw material for phenol recovery. Other organic components were detected as well, but in very low concentrations, and thus were not discussed further. As the charis achieved its best catalytic activity at 550 °C, the temperature of 550 °C was chosen in the following discussion.
Properties of Gaseous Product
The gas compositions before and after charis catalysis at 550 °C are shown in Fig. 4. The major components in the gas phase were CO2(7857.1, peak area, relative content) and CO (6022.1), with small amounts of CH4 (2459.1) and H2 (435.5). As shown, the charisintensely facilitated the decomposition of pyrolysis vapors to gases in the forms of CO, CO2, etc., and it was beneficial for the production of hydrogen and methane. After catalysis, the relative contents of CO2, CO, CH4, and H2 were 12879.3, 8955.6, 4176.6, and 1372.2, respectively, with the charis dose of 1.0 g at 550 °C.
Fig. 3. Summary of classification of bio-oil components before and after charis catalysis (400 °C to 600 °C)
The production of CH4 and H2 in the catalytic pyrolysis might be attributed to the catalytic cracking of polysaccharide (Mante and Agblevor 2011). In the pore channel of charis, the vapors might have undergone a series of dehydrogenation, decarboxylation, decarbonylation, dehydration, and isomerization reactions that led to the formation of the phenolic substances and the small molecule gases. The gasification reaction or secondary cracking of charcoal might occur simultaneously, which resulted in the enhancement of the gas yield.
Fig. 4. The gas released under different pyrolysis conditions
Properties of Solid Chars
The charis in the catalytic reactor was weighed, and the result showed that the amount of the charis was not changed remarkably. This might have been due to the combined effect of the secondary cracking reaction (decreasing weight) and secondary coke forming or bio-oil adsorption (increasing weight). Figure 5 shows the infrared spectra, as well as the pore size and surface area of the chars before and after catalytic reaction. As shown, the polar functional groups on the surface of charis were not obviously changed. The surface area of original charis was small (1.96 m2/g), with an average pore size of 20.0 nm. The surface area of the charis increased after catalysis (4.78 m2/g to 7.93 m2/g), which might have been due to further evolution of the charis pore structures with prolonged residence time. Simultaneously, the vapors might also have been covered on the charis surface so that some pores were clogged. The increased surface area of the charis indicated that the pore development was the main process. The average pore size was not changed noticeably after catalysis, and was mainly consisted of mesopores. The presence of the mesopores might have provided a reaction channel during the catalytic reactions. The Boehm titration result showed that the basicity of charis decreased (from 0.175 mmol/g to 0.125 mmol/g and to 0.130 mmol/g) after interacting with the vapors. The ash composition revealed that the main element in charis was K, followed by Si, Cl, and Ca. Perhaps these metallic species played an important role in the catalytic reaction. Keown et al. (2008) confirmed that the presence of alkali and alkaline earth metals in the char affected the char’s activity and selectivity. Further experiments will be conducted to confirm the effects of these alkali metals and alkaline earth metals.
Fig. 5. Characteristics of the chars (a: FTIR of pore structure and Boehm titration results; b: ash composition)
Catalytic Mechanism of Chars
As is known, acetic acid is mainly derived from the deacetylation of hemicellulose, furans are mainly from the degradation of cellulose, and phenols are from the degradation of lignin (Melligan et al. 2012). In this study, due to the effect of the charis, the acids in the bio-oil decreased and the phenolic substances increased. The acetic acid decomposition path is shown in Eqs. 1 through 3. Because the basicity of charis decreased after catalysis according to the Boehm titration result, the decrease of acetic acid might also have been attributed to the neutralization reaction in Eq. 4. Figure 6a shows the possible ways for acetic acid reduction that occurred in the pore channel of the charis.
In the charis catalytic process, the lignin may be catalytically converted into monomeric phenols through the following path shown in Fig. 6b (Melligan et al. 2012). The mesoporous structure of the charis provided a channel for the reaction. Iliopoulou et al. (2007) believed that the surface area and porous structure are important for enhancing the formation of phenols. As the previous results showed, after the charis catalysis the content of 2,3-dihydrobenzofuran was remarkably reduced. Thus, a second possible reaction path for phenols production is proposed as shown in Fig. 6c. Directional chemical bond cut occurred, and the 1-2 and the 3-4 bonds were broken, which resulted in phenol and ethane production. With an increased dose of charis, the volatile organics were further catalytically converted into phenol products, and the acetic acid was evidently reduced.
Fig. 6. Possible mechanism of catalytic reaction under charis: (a) acids inhibition; (b) lignin to phenols in char’s action; (c) phenol promotion
- The present study demonstrated that it was feasible to use char as the catalyst for in-situ upgradation of biomass pyrolysis vapors. The quality of bio-oil was improved as the phenols were increased, and the acids were decreased in comparison to when there was no charis present.
- The highest relative content of phenols was 55.6% (peak area percentage), obtained with a charis dose of 1.0 g (the charis to feed ratio was 0.25) at 550 °C, while the acetic acid was reduced from 31.4% to 17.2% (peak area percentage).
- The product selectivity was not dependent upon the polar functional groups on the surface of charis according to the FTIR results, but it might have been dependent on the pore structures (mainly mesoporous) and the basicity sites of the charis, as well as the metallic species in the charis.
- Three possible reaction mechanisms for catalytic process were proposed. Further experiments will be done to collect the evidence for the catalytic mechanism in detail.
The authors express their great gratitude for the financial support from Huadian Electric Power Research Institute Co., Ltd., the National Natural Science Foundation of China (51506071, 51622604), and the Fundamental Research Funds for the Central Universities.
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Article submitted: February 13, 2018; Peer review completed: April 8, 2018; Revised version received: April 27, 2018; Accepted: April 28, 2018; Published: May 10, 2018.