Effect of Lignin Components on Gasification of Japanese Cedar

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Effect of Lignin Components on Gasification of Japanese Cedar (Cryptopmeria japonica) Wood and Bark Using an Entrained-FlowType Gasification Reactor Tomoko Ogi,* Masakazu Nakanishi, and Yoshio Fukuda National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ABSTRACT: Japanese cedar (Cryptomeria japonica) wood and bark were gasified using an entrained-flow-type gasification reactor at 900 °C in the presence of H2O or H2O + O2. With H2O alone, the cedar wood was gasified well: carbon conversion into gas [CC(g)] = 94.3%; however, cedar bark was not gasified so well: CC(g) = 73.4%, and relatively large amounts of solid residues were produced: carbon conversion into solid residues [CC(sr)] = 14.0%. Gas produced from both cedar wood and cedar bark were suitable compositions for catalytic liquid fuel synthesis: [H2]/[CO] ≅ 2−3. Tar yields were low: that from the cedar wood was wood > bark > o-lignin, in all of these atmospheric conditions. Effects of feedstock and atmosphere on the gasification and thermogravimetric analyses had the same tendencies. We, therefore, concluded that the lower CC(g) of the cedar bark than the cedar wood was attributable to difficulties of gasifying the lignin components.

1. INTRODUCTION The Japanese government revised their energy scenario for introducing renewable energy (including biomass energy) in 2006. In the revised scenario, the term “biofuel” was first nominated, and the government expected that biomass-toliquid fuel (BTL), which is catalytically synthesized liquid fuels from gasified biomass, would be widely introduced by 2020 in the earliest case or by 2030 in the latest case.1 Although almost 100% of Japanese transportation (vehicle) fuel is currently derived from oil, the government hopes to reduce this to 80% or less by adopting fuels derived from biomass and other renewable sources. The Basic Energy Plan announced that commercial use of the “next-generation fuel”, including BTL, was targeted by 2030.2 The development of a process for synthesizing liquid fuel via gasification of inedible biomass is, therefore, urgently required. Gasification is one of the most promising biomass-to-energy conversion processes. Because gasification technologies were first studied and developed for coal and because both compositions and characteristics of biomass greatly differ from those of coal, both gasifiers themselves and related equipment and gasification conditions must be modified suitable for biomass feedstock. Part of the biomass projects sponsored by the New Energy and Industrial Technology Development Organization (NEDO), Mitsubishi Heavy Industries, Ltd., (MHI), Chubu Electric Power Corporation (CEPCO), and our research group in the National Institute of Advanced Industrial Science and Technology (AIST) developed a test plant with a biomass scale of 2 t day−1 for studying © 2016 American Chemical Society

the integrated process of biomass preparation, gasification, gas purification, methanol synthesis, and off-gas treatment, for which an entrained-flow-type gasifier with a partial oxidation process was adopted.3−5 An entrained-flow-type gasification has many advantages: a simple structure, a high carbon conversion into gas [CC(g)] without a catalyst, a low tar yield, easy control of the gas components, and applicability to wide varieties of biomass as feedstock. Easy and strict control of the gas compositions and gas purification, both of which the entrained-flow-type gasifier easily satisfies, are keys for the BTL, such as methanol, Fischer−Tropsch oil, liquefied petroleum gas (LPG), and dimethyl ether (DME). Amounts of H2 and CO and their molecular ratio [H2]/[CO] were important parameters, and [H2]/[CO] should be controlled to be 2−3, depending upon catalysts and target products, because the liquid fuels were generally synthesized from H2 and CO. We have also been studying the gasification of various kinds of biomass using a laboratory-developed entrained-flow-type gasification reactor to clarify the most suitable conditions for synthesizing liquid fuels and found that gasification properties depended upon characteristics of feedstock biomass, including unused wastes, as reported before.6−8 Special Issue: In Honor of Michael J. Antal Received: April 11, 2016 Revised: June 2, 2016 Published: June 22, 2016 7867

DOI: 10.1021/acs.energyfuels.6b00848 Energy Fuels 2016, 30, 7867−7877

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Energy & Fuels Table 1. Elemental and Chemical Analyses of Feedstock (a) Elemental Analyses content (wt %, dry) cedar wood cedar bark delignified cedar bark o-lignin

C

H

48.6 49.1 44.9 66.3

6.1 6.0 5.9 9.3

O

a

46.1 43.2 48.4 27.5 (b) Ash Analyses

N

S

ash

0 0.3 0.2 0.2

ndb nd nd nd

0.2 1.4 0.6 0.04

content (wt %) cedar wood cedar bark

SiO2

Al2O3

Fe2O3

10.3 3.3

3.3 2.2

3.6 1.2

CaO

MgO

46.4 7.0 4.7 55.0 2.5 1.1 (c) Chemical Analyses (wt %)

P2O5

Na2O

K2O

ash

4.1 3.0

1.7 0.1

6.8 6.7

0.26 1.44

Klason lignin

holocellulose

35.2 42.3

64.3 51.4

cedar wood cedar bark a

SO3

Calculated by difference. bUnder detection: 95 wt %), and the primary tars (intermediate products generated by thermal decomposition of cellulose and hemicellulose and lignin to a lesser extent) reacted with H2O to quickly degrade further into smaller compounds. The cedar wood, cedar bark, and o-lignin had relative weights at 600 °C of 0.13, 0.24, and 0.34, respectively, all of which were higher than that of the delignified cedar bark (0.04), but also decomposed well at 900 °C, leaving only ash. The TG curves of the cedar wood and cedar bark between 400 and 800 °C decreased in parallel. Their difference was almost constant of 0.10−0.15, almost equal to the difference of their lignin contents, which were 35 and 47 wt %, respectively. The o-lignin, which had the highest lignin content, had the highest relative weights (i.e., the least decomposition) among the four feedstock samples until 750 °C. The relatively low weight of the o-lignin at 750 °C and above might be caused by the fragmented lignin that decomposed more easily at a high temperature. The addition of O2 to H2O further accelerated decomposition of all four samples. The decomposition began at a lower temperature and mostly completed by 600 °C, even for the o-lignin and cedar bark.

Figure 3. TG curves of four samples in different atmospheric conditions: (a) in He alone, (b) in He + H2O, and (c) in He + H2O + O2.

Table 2. Carbon Conversions into Gas [CC(g)] and Solid Residues [CC(sr)] in Four Feedstock (900 °C and [H2O]/ [C] = 2.5) (%) CC ([O2]/[C] = 0)

cedar wood cedar bark delignified cedar bark o-lignin

CC ([O2]/[C] = 0.35)

gas

solid residue

difference

gas

solid residue

94.3 73.4 96.5

0.8 14.0 0.1

4.9 12.6 3.4

95.4 87.5 103.6

0.7 2.6 0.07

3.9 9.9

59.9

18.1

2.2

81.4

7.1

11.5

difference

between 380 and 1050 °C, with its relative weight of 0.09 at 900 °C, actual gasification temperature, and was almost completely decomposed at around 1100 °C. The delignified 7870

DOI: 10.1021/acs.energyfuels.6b00848 Energy Fuels 2016, 30, 7867−7877

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Energy & Fuels

Figure 4. Results of gasification of four feedstock (left) without O2 and (right) with O2.

delignified cedar bark, and some of the tar adhered to the conduits and was lost. The elemental analysis of the tar from the cedar wood and cedar bark without O2 condition was C, 93.0 wt %; H, 6.7 wt % (for the cedar wood) and C, 94.0 wt %; H, 6 wt % (for the cedar bark), corresponded to 0.6 and 0.2 carbon conversion into tar, respectively. In our experiments, biomass feedstock was once pelletized and then fed into the reactor for the purpose of stable feed. The pelletized biomass feedstock fell through the conduit, then easily broken into pieces (about 1 mm), entrained in the gasification reactor on upward flow of the gasification agent, and gasified. In the early stage of our study, we experimentally certified that gasification properties were almost independent of physical properties of feedstock when their sizes were smaller than 2−3 mm. We used a transparent reactor made of quartz and directly observed feedstock biomass. When their sizes were smaller than 2−3 mm, they were entrained in the reactor and quickly shrank/gasified within a couple seconds. We also studied effects of retention (residence) time of the produced gas in the reactor and found that gaseous components were almost independent of time when longer than 10−20 s.20 The residence time in the reactor, which depended upon its volume and amount of produced gas, was roughly estimated to be from 10 to 20 s; accurate estimation was very difficult because the gas amount varied as the gasification reaction progressed. Compositions of the produced gas under any condition were far different from those estimated in their equilibrium states. When H2O alone was used, the cedar wood and delignified cedar bark were gasified well, with CC(g) of the cedar wood and delignified cedar bark of 94.3 and 96.5%, respectively, but the cedar bark and o-lignin were not gasified well, with CC(g) of the cedar bark and o-lignin of 73.4 and 59.9%, respectively. Large amounts of solid residues were produced, with CC(sr) of the cedar bark and o-lignin of 14 and 18%, respectively. These low CC(g) of the cedar bark and o-lignin agreed well with the results of the thermogravimetric analyses under He + H2O. Lignin, as described before, has a very complicated structure with various types of functional groups (−Ph−OH, R−OH, −OCH3, and −CHO) and bond linkages (R−O−, Ph−O−, C−C, Ph−, and CC−). Lignin thermally decomposed to smaller fragments (lower molecular weight products), and these intermediate products decomposed further, finally to gases. At the same time, these intermediate products were also

Table 3. Elemental Analysis of Solid Residues [Element and Ash, wt %; CC(sr), %] cedar wood cedar bark delignified cedar bark o-lignin a

C

H

Oa

N

ash

CC(sr)

64.3 68.8 3.2 81.3

1.9 1.3 1.0 3.4

26.0 13.4 11.9 15.0

0.1 0.3 0 0.3

7.7 16.2 83.9 0.01

0.8 14.0 0.1 18.1

Calculated by difference.

All of these results indicated that thermal decompositions of the biomass feedstock were strongly dependent upon their lignin contents, which strongly suggested that lignin much more hardly decomposed than cellulose and hemicellulose. The thermogravimetric analyses and gasification properties had the same atmospheric tendencies as discussed in the next section, although the heating rate of the thermobalance was 20 °C/min and, consequently, the heating time was much longer than the residence time in the gasification reactor of about 10−20 s. 3.2. Gasification. 3.2.1. Carbon Conversions into Gas and Solid Residues. The products obtained after gasification were gases, solid residues, tar, and water-soluble products. Table 2 shows carbon conversion into gas [CC(g)] and solid residues [CC(sr)]. In the difference (missing), tar and water-soluble products are classified. Tar yields were low in 0.1−0.3 wt % (not carbon based but weight based). Little amounts of watersoluble products with low molecular weights, such as methanol, formic acid, and acetone, were trapped in the drain and detected probably because they easily evaporated. Gases and solid residues were main products, accounting for 78−100% of the carbon in the feedstock. When gasifying the delignified cedar bark (with gasification conditions: [H2O]/[C] = 2.5 and [O2]/[C] = 0.35), the carbon conversion into gas [CC(g)] was 103%, which was within the range of uncertainties that resulted from variations in the moisture content and feedstock compositions. In the delignified cedar bark, gasification completely proceeded at 900 °C and a small amount of white ash was left. The cedar wood and delignified cedar bark were gasified well [95−100% CC(g)] at 900 °C. Tar yields were low, especially for the cedar wood and delignified cedar bark, with values as low as 0.1 wt %. More tar was produced from the cedar bark (0.3 wt %, without O2 condition) and o-lignin than from the cedar wood and 7871

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some of the fragments generated were further degraded into smaller compounds, changing from primary tars to secondary tars and finally to gases, whereas some fragments recombined with other parts of the lignin molecule, polymerized, and carbonized to produce solid residues that were rich in carbon. The same phenomena would occur for the cedar bark, with polymerization and carbonization leading to the evolution of H2, H2O, and other compounds with a small molecular weight and producing solid residues. The effect of adding O2 was less pronounced in cases of the cedar wood and delignified cedar bark because of their already high CC(g) with H2O (Table 2). On the other hand, because of the low CC(g) of the cedar bark and o-lignin, adding O2 greatly increased their CC(g) (by 14−21%; Table 2) It is well-known alkali and alkali earth metals, such as K, Ca, and Mg, function as catalysts on gasification [as both a primary catalyst, biomass decomposition, and a secondary catalyst, reforming hydrocarbons (tars)].22 In our experiments, K, Ca, and Mg contained in ash in biomass feedstock functioned as catalysts and accelerated gasification. We performed to gasify cedar bark in an addition of Ca (OH)2 in another experimental series and found that carbon conversion into gas [CC(g)] was increased (partially presented in ref 23). We also investigated the behavior of ash (alkali and alkaline earth metals) in wood after gasification.24 However, in our experiments in this paper, effects of the lignin component were larger than that of ash (alkali metals), because in both cedar wood and cedar bark, Ca (major component of ash, nearly half of ash), K, and Mg were contained and the total ash content in the cedar bark of 1.44% was about 5 times larger than that in the cedar wood of 0.26%. Nevertheless, carbon conversion into gas [CC(g)] from the cedar bark was lower than that from the cedar wood. The CC(g) from the delignified cedar bark, of which the ash content was 0.4%, was higher than that of the cedar wood. The effect of the ash content was minor compared to the effect of lignin in this experimental series. 3.2.2. Product Gas Composition. Figure 4 shows the molecular ratio of each evolved gas to carbon in the feedstock biomass. CO, CH4, CO2, and H2 were the primary gases evolved, and C2+ gases accounted for o-lignin. The thermogravimetric analyses and gasification results showed the same tendencies, which suggested that the lower gasification rate of the bark was attributable to difficulties of gasifying its lignin components. Japanese cedar bark is produced in large quantities in Japan but is mostly unused and wasted. Our results suggest that it can be gasified using H2O + O2 as gasification agents, because of its high enough CC(g), which was slightly lower compared to that of the wood. The lower CC(g) of the bark was attributable to its high lignin content.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-29-861-5567. Fax: +81-29-861-8181. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the NEDO biomass project. The authors thank Dr. Takeno (MHI) for a valuable discussion of gasification. The authors thank Dr. Kuboyama (FFPRI) for providing Japanese cedar feedstock. The authors thank Dr. Magara (FFPRI) for performing the chemical analysis of the Japanese cedar. 7876

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DOI: 10.1021/acs.energyfuels.6b00848 Energy Fuels 2016, 30, 7867−7877