Alkali metal bifunctional catalyst-sorbents enabled biomass pyrolysis for enhanced hydrogen production

Journal Pre-proof Alkali metal bifunctional catalyst-sorbents enabled biomass pyrolysis for enhanced hydrogen production

Ming Zhao, Muhammad Zaki Memon, Guozhao Ji, Xiaoxiao Yang, Arun K. Vuppaladadiyam, Yinqiang Song, Abdul Raheem, Jinhui Li, Wei Wang, Hui Zhou PII:

S0960-1481(19)31876-2

DOI:

https://doi.org/10.1016/j.renene.2019.12.006

Reference:

RENE 12715

To appear in:

Renewable Energy

Received Date:

21 April 2019

Accepted Date:

02 December 2019

Please cite this article as: Ming Zhao, Muhammad Zaki Memon, Guozhao Ji, Xiaoxiao Yang, Arun K. Vuppaladadiyam, Yinqiang Song, Abdul Raheem, Jinhui Li, Wei Wang, Hui Zhou, Alkali metal bifunctional catalyst-sorbents enabled biomass pyrolysis for enhanced hydrogen production, Renewable Energy (2019), https://doi.org/10.1016/j.renene.2019.12.006

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Alkali Metal Bifunctional Materials Carbonate

CO2

CO2 sorbent

CO Biomass

Tar

Catalyst

Char Catalyst

H2

Catalyst

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Alkali metal bifunctional catalyst-sorbents enabled biomass pyrolysis for enhanced hydrogen production Ming Zhao,a Muhammad Zaki Memonb, Guozhao Jic, Xiaoxiao Yang,d Arun K Vuppaladadiyam,a Yinqiang Song,e Abdul Raheem,a Jinhui Li,*, a Wei Wang,a and Hui Zhou*, e a

b

School of Environment, Tsinghua University, Beijing 100084, People’s Republic of China Energy and Environmental Engineering Department, Quaid-e-Awam University of

Engineering, Science and Technology, Nawabshah 67480, Pakistan c

School of Environmental Science and Technology, Dalian University of Technology, Dalian

116024, People’s Republic of China d

Tsinghua University-University of Waterloo Joint Research Center for Micro/Nano Energy &

Environment Technology, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, People’s Republic of China e Department

of Mechanical and Process Engineering, ETH Zürich, Leonhardstrasse 21, Zürich

CH-8092, Switzerland * Corresponding authors. Email: [email protected] (H. Zhou), [email protected] (J. Li)

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ABSTRACT: Alkali ceramics are well-known high temperature CO2 sorbents in forms of zirconates or orthosilicates with catalytic tar cracking ability prior to or after carbonation. In this study, Li2ZrO3, Li4SiO4, and Na2ZrO3 were selected as catalyst-sorbent bifunctional materials to enhance the pyrolysis of sawdust. This study investigated the synergy between the alkali metal bifunctional materials under pyrolytic conditions. The weight loss data and gas yield trends in the temperature between 200 and 800 °C demonstrated a combined catalytic process and gassolid reaction. A metal carbonate phase was formed after the reaction of capturing CO2. The CO2 capture promoted H2 production because of Le Chatelier principle and the formation of carbonate phase assisted tar cracking reactions. H2 production increased from 5.73 mmol g-1 to 8.87 mmol g-1, 15.85 mmol g-1, and 13.67 mmol g-1 in the presence of Li2ZrO3, Li4SiO4, and Na2ZrO3, respectively. At temperatures around 700 °C, CO was released due to secondary cracking reaction and the Boudouard reaction of CO2 released from the sorbents. Overall, the alkali ceramics present the catalyst-sorbent bifunctional activity for enhancing H2 production during biomass pyrolysis.

Keywords: alkali ceramics, bifunctional catalyst-sorbent, biomass pyrolysis, hydrogen production, char gasification

1. Introduction Current scenario of increasing energy demand, depleting fossil fuel reserves, and climate change has highly motivated energy experts and researchers around the globe to search for alternative energy resources. Biomass is one such ubiquitous, cheap, and carbon-neutral renewable source of hydrocarbons with which a wide range of fuels and chemical feedstock, depending upon conversion pathways, can be generated [1]. The utilization of biomass as a

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sustainable fuel helps ease the dependency on fossil fuels. In addition, biomass, such as agricultural waste and energy crops are considered to be eco-friendly compared to fossil fuels in view of global warming [2,3]. Lignocellulosic biomass, in particular, could be an ideal feedstock for thermochemical processes to produce energy-related products [4]. It is estimated that China produces 728-750 Mt agricultural residues and 200-220 Mt forest biomass (including firewood and forest residue) annually [5]. Amongst the available thermochemical conversion pathways, pyrolysis presents a set of advantages, such as atmospheric pressure and low temperatures, which make it affordable and easy to operate [6]. However, the products obtained from pyrolysis of biomass are not appropriate for the production of valuable products for reasons such as acidity, high moisture and viscosity, and low energy density [7,8]. These reasons triggered exploration of diverse routes, such as co-pyrolysis and/or catalytic pyrolysis, for refining the quality of pyrolysis products. In this regards, synergistic interaction between feedstocks and catalysts during pyrolysis is an important area of research. One of the developing research interests is to achieve process intensification by the integration of catalytic and chemical CO2 absorption functionalities over single particle [9]. Combining the catalyst and sorbent functionalities in a single particle is one way to overcome the inefficiency caused by conventional two-pellet systems. The advantages of the bifunctional catalyst-sorbent particle are not limited to circumventing mass transfer barriers, but also, they can reduce the material cost and have potential impact on the concentration profiles of gases at particle level [10]. Many studies suggest that bifunctional catalyst-sorbent materials are favored compared to conventional mixing of catalysts and sorbents [11–13]. In the literature, the most popular choice of the sorbent is CaO [9,14], owing to its fast CO2 absorption kinetics and high CO2 absorption

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capacity (17.8 mmol-CO2 g sorbent−1) [15]. The disadvantage of CaO is that it is prone to sintering at high temperatures so it requires stabilization which ultimately increases the material costs [16]. Even with the addition of support, CaO tends to lose its efficiency over a multi-cycle process [17]. Therefore, an alternate set of materials may be considered for biomass conversion with single-pellet material. Alkali metal sorbents, binary metal oxides having one metal from group I elements, have been widely investigated for post-combustion capture of CO2 [18]. The list of materials that can be grouped as alkali metal sorbents is long; however, not all of them are good choices as bifunctional material on the basis of their chemical CO2 absorption capabilities [18]. Among the alkali metal sorbents, Li2ZrO3, Li4SiO4, and Na2ZrO3 (Eqns. 1-3) are the most investigated under different conditions such as varying temperatures and varying CO2 partial pressures over multicycle CO2 capture and release [19]. Li2ZrO3(s) + CO2(g) ⇌ Li2CO3(s)+ ZrO2 (s),

∆H298 = -160 kJ mol-1

(1)

Li4SiO4(s) + CO2(g) ⇌ Li2CO3(s) + Li2SiO3(s),

∆H298 = -142 kJ mol-1

(2)

Na2ZrO3(s) + CO2(g) ⇌ Na2CO3(s) + ZrO2(s),

∆H298 = -149 kJ mol-1

(3)

In addition, alkali metal carbonates have been used in biomass or coal gasification as catalysts for water-gas shift (WGS) reaction [20,21]. Na2ZrO3 has exhibited catalytic ability to oxidize CO to CO2 that can be subsequently trapped by sorbents, allowing them to perform the roles of both catalyst and sorbent [22]. The performance of Na2ZrO3 as the bifunctional catalyst and sorbent during pyrolysis of cellulose was previously investigated by our group. The results illustrated that Na2ZrO3 was able to perform catalytic and chemical CO2 absorption activities, to enhance H2 yield from cellulose pyrolysis [23]. Chen and co-workers investigated the sorption enhanced catalytic steam gasification of wood sawdust with a Pd/Co–Ni catalyst and dolomite as a CO2

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acceptor [24]. In this study, the catalyst and dolomite are placed separately. To our best concerns, there are no studies on the utilization the sorbent-catalyst bifunctional materials in the thermochemical conversion of real biomass feedstock. Hence, it is worth investigating to establish that catalyst-sorbent bifunctional ability of alkali ceramics can be extended to pyrolysis of the real biomass containing cellulose, hemicellulose, lignin, and extractives. In this study, biomass (sawdust) was pyrolyzed with the presence of Li2ZrO3, Li4SiO4, or Na2ZrO3 in thermogravimetric analyzer coupled with mass spectrometer (TG-MS). An attempt has been made to examine the influence of alkali salts on the thermal behavior of sawdust during pyrolysis. The catalytic and sorption ability of three selected alkali bifunctional materials were tested.

2. Materials and Methods 2.1 Material synthesis and characterization Lithium nitrate (LiNO3) and zirconium nitrate (Zr(NO3)4) were used for the synthesis of Li2ZrO3; lithium nitrate (LiNO3) and fumed silica (SiO2) were used for the synthesis of Li4SiO4; and sodium oxalate (Na2C2O4) and zirconium nitrate (Zr(NO3)4) were used for the synthesis of Na2ZrO3. Soft-chemistry method was employed for the synthesis of the materials. The stoichiometric molar ratios of precursors were used. In case of Li2ZrO3 and Na2ZrO3, the appropriate amount of precursors was added in 100 mL of deionized (DI) water, and then, to obtain clear solution, oxalic acid (H2C2O4·2H2O, 0.4 mol L-1) was added dropwise [25]. To prepare Li4SiO4, lithium nitrate was dissolved in ethanol and dissolution of nitrates appropriate amount of fumed silica was mixed in the solution [26]. The clear solutions were placed at hotplate with continuous magnetic stirring for evaporation of water. The powders obtained after

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drying were calcined at a high temperature (850 °C for Na2ZrO3 and 750 °C for Li2ZrO3 and Li4SiO4) for 6 h in a muffle furnace. The phases of the as-prepared materials were characterized by XRD (Rigaku, Model D/Max2500 V+/PC). Brunauer-Emmett-Teller (BET) surface area, along with Barrett-JoynerHalenda (BJH) average pore size and pore volume were determined by N2 physisorption (Quantachrome autosorb iQ-C). 2.2 Biomass characterization Pine tree residue (mixture of stem, bark, and branches) obtained from a lumber mill in the vicinity of Beijing was selected as waste biomass for pyrolysis experiments. The biomass samples were shredded in grinder and sieved to less than 0.2 mm. Carbon, hydrogen, nitrogen, and oxygen content were determined by a EuroEA3000 Elemental Analyzer, and sulfur fraction was determined with 5E-AS3200B Automatic Coulomb Sulfur Analyzer. Higher heating value (HHV) of sawdust was determined with Isoperibol oxygen bomb calorimeter Parr 6400. The proximate analysis, ultimate analysis, and HHV of the waste biomass are shown in Table 1. Table 1. Proximate analysis, ultimate analysis, and higher heating value (HHV) of sawdust. Proximate Analysis (wt. %)

Ultimate analysis (dry, wt. %)

HHV (kJ kg-1)

Moisture content

9.85

Carbon

47.27

17095.45

Volatile matter

72.92

Hydrogen

6.44

Ash

0.27

Oxygen

45.84

Fixed carbon

16.97

Nitrogen

0.12

Sulfur

0.04

2.3 Determination of catalytic-sorption activity

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Pyrolysis tests were conducted in a TGA (SDT Q600) coupled with an MS (HPR20). The pyrolysis experiments were carried out under inert atmosphere, with a constant Ar purge rate of 500 mL min-1. TGA was connected to MS via a heated transfer line, maintained at 200 °C to ensure that evolved light gases i.e. H2, CO, CO2, CH4 do not condense in the line. A high Ar purge rate is important in two ways: firstly, it safeguards delivery of evolved gas species to MS with haste; secondly, the constant high purge rate is quite essential for semi-quantitative analysis as refined ionic signal recorded by MS can be represented as instantaneous volume concentrations of evolved species relative to Ar [27]. Florin and Harris noted that MS was biased towards low molecular weight species which made this method suitable to assess and analyze the trends of syngas yield from pyrolysis of selected samples [27]. The sawdust samples were labeled as SD and alkali salts were labeled as LZ (Li2ZrO3), LS (Li4SiO4), and NZ (Na2ZrO3), respectively. For TG-MS experiment regime, the temperature ramping rate was set at 40 °C min-1 and pyrolysis was carried out up to 850 °C. Approximately 3 mg of biomass was taken for experiments. The weight ratio of biomass to alkali materials was set at 1:1. The biomass and alkali materials were vigorously mixed with the help of vortex shaker. Experiments in TG-MS were done in triplicate and the results reported below are the refined averages of the data obtained. The method described by Zhao et al. was adopted to estimate generation rate and production of gas species [28]. The raw signals received obtained from MS were normalized relative to constant Ar purge and weight of biomass (Eqn. 4). Normalized signal for key molecule fragments ‘i’=

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IC  500 i

ICAr  wt.biomass

(4)

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where ICi stands for molecular m/z signals for molecular ion fragments ‘i’ (arbitrary unit); ICAr is the m/z signals for Ar; and wt.biomass is the weight of sample (g). The normalized ion current signals from MS were expressed as instant volume concentrations of gas products contained in carrier gas (Ar, 500 mL min-1, 25 ° C, and 1 atm). To obtain evidence of chemical CO2 absorption by alkali sorbents during pyrolysis, 1 g of sawdust was mixed with 1 g of freshly calcined bifunctional materials on a vortex shaker and placed in a sealed horizontal furnace. N2 was purged to remove air from furnace and after sufficient time pyrolysis was initiated up to desired temperature. The resulting residue was analyzed with XRD. If any CO2 was captured during the pyrolysis, XRD patterns would reveal the formation of carbonate and secondary metal phase.

3. Results and discussion 3.1 Characterization of bifunctional materials XRD analysis confirmed that all materials were successfully synthesized with no evident impurities (Fig. 1). Patterns from Li4SiO4 sample fit profile of monoclinic Li4SiO4 (JCPDS card # 37-1472) and no other phase was registered by XRD. The Li2ZrO3 XRD pattern is dominated by the tetragonal Li2ZrO3 (JCPDS card #41-0324), which is reported to be more active than its monoclinic counterpart (JCPDS card #33-0842) in CO2 absorption [29], with peaks at 35.8°, 39.8°, 42.6°, and 59.6°. However, traces of monoclinic Li2ZrO3 were also observed by XRD analysis at 22°. Na2ZrO3’s crystalline patterns showed existence of two phases; monoclinic (mNa2ZrO3, JCPDS card # 35-0770) and hexagonal (h-Na2ZrO3, JCPDS card # 21-1179), where mNa2ZrO3 appeared to be the dominant phase. By comparing the relative peak intensity ratios of prepared Na2ZrO3 with standard intensity ratios of model m-Na2ZrO3 and h-Na2ZrO3, we

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established the peak at 32.6° was likely from h-Na2ZrO3, while remaining peaks were closer to m- Na2ZrO3 [23]. Overall, m-Na2ZrO3 appeared to be the dominant phase, which was a desirable result as m-Na2ZrO3 is reported to be active in CO2 capture [30].

Fig. 1. XRD patterns of synthesized samples Surface area, pore size, and pore volume of prepared materials are shown in Table 2. Alkali ceramic materials are quite dense materials with low surface area and low pore volume. The parameters obtained from N2 physisorption are close to the parameters reported in similar studies [25,31].

Table 2. Surface area, pore size, and pore volume of prepared materials.

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Material

BET surface area Average pore size Pore Volume (m2 g-1) (nm) (cm3 g-1)

Li2ZrO3

1.1

3.1

0.010

Li4SiO4

0.7

4.2

0.003

Na2ZrO3

3.0

3.4

0.016

Thermogravimetric characteristics of biomass pyrolysis with bifunctional materials Pyrolysis is an anaerobic thermochemical conversion pathway that can yield solid (char), liquid (bio-oil), and gaseous fuels (syngas) depending upon operating parameters such as temperature ramping rate and residence time of evolved species (Eqn. 5). CnHmOp + Heat → ∑liquid CaHbOc + ∑gas CxHyOz + ∑solid C

(5)

To obtain higher syngas yields from pyrolysis of a biomass feedstock, the residence time of evolved species is extended so as to promote secondary reactions [32]. Initial pyrolysis occurs at temperature range of 200-500 C producing char and condensable gases through degradation of cellulose, hemicellulose, and lignin. Cellulose, hemicellulose, and lignin yield both condensable gases and non-condensable gases. Condensable gases are heavy hydrocarbons that turn into liquid upon cooling, while non-condensable gases include CO2, CO, CH4, C2H6, and C2H4. It should be noted that condensable gases can be converted into non-condensable gases through thermal or catalytic cracking. Tar is also a product of initial pyrolysis, and it is produced by the depolymerization of cellulose, hemicellulose, and lignin. Compared to tar generated from other thermochemical procedures, tar from pyrolysis processes is mostly oxygenated and has little commercial value [33,34]. At temperatures above 500 °C, tars can be converted through various cracking and reforming reactions to produce hydrogen (Eqns. 6-11, CnHm indicates tar, Cn-xHm-y indicates lighter species of tar). Steam and CO required to drive these equations are released during the pyrolysis of the biomass.

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Thermal tar cracking: CnHm → Cn-xHm-y + H2 + CH4 + C

(6)

Tar steam reforming: CnHm + nH2O → (n+m/2) H2 + nCO

(7)

Dry reforming: CnHm + nCO2 → (m/2) H2 + 2nCO

(8)

Water-gas shift: CO + H2O ⇌ CO2 + H2

(9)

Methane steam reforming: CH4 + H2O ⇌ CO + 3H2

(10)

Carbon steam gasification: C + H2O ⇌ CO + H2

(11)

The TG and DTG curves, obtained by pyrolysis of samples, are presented in Fig. 2. For all samples, there are four weight losses registered. An initial evaporation weight loss was observed at temperature < 200 °C, followed by a major pyrolysis weight loss at temperature range of 250400 °C. For samples with alkali salts, a peak following the pyrolysis peak was noticed at 400500 °C along with a very small weight loss around 650 °C. The first peak can be attributed to the release of moisture shown in all samples. For the pyrolysis weight loss, impact of the alkali metals as catalyst is significant. The main weight loss peak for SD was noted at 388 °C. The weight loss peak appeared at lower temperatures for the samples containing alkali salts, which suggested that presence of alkali salts may have initiated onset of pyrolysis at lower temperatures.

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Fig. 2 (a) DTG and (b) TG curves obtained from pyrolysis of biomass with different additives. LS was slightly more active in lowering the peak pyrolysis temperature (359 C) than NZ (361 C), while LZ was only able to shift maximum weight loss peak to 375 °C. This is an interesting observation because any catalytic activity performed by alkali salts here is not due to presence of carbonate phases, which are well-known for catalytic ability [35], but rather the active catalytic sites coming from Na2ZrO3, Li4SiO4, and Li2ZrO3. The results here matched our previous work, where sample bearing the same ratio of cellulose to Na2ZrO3, was able to lower the pyrolysis activation energy [23]. Following the pyrolysis, DTG curves for SD were stabilized but curves from SD-NZ, SD-LS, and SD-LZ exhibited a sustained weight loss period up to 550 °C before stabilization. This small plateau of weight loss indicated a continuous period of catalytic breakdown of lignin. The final weight loss peak occurred at temperatures above 650 °C could be related to char gasification, which is discussed in the later part. 3.2 Enhanced hydrogen production from catalytic-sorption activity The H2 evolution showed that the gas was mainly evolved from two stages of pyrolysis. For the SD sample, the hydrogen was released from the main pyrolysis stage and then later at

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temperature over 500 °C (Fig. 3 (a)). It should be noted that in the second phase of H2 production, which occurred at temperatures above 500 °C, no weight loss was recorded by TGA, signifying that H2 was produced from tar cracking (Eqns. 6 and 7), tar reforming (Eqn. 8), and other secondary reactions (Eqns. 9-11). A clear contrast was observed when alkali salts were added. The presence of alkali salts in the later stage of pyrolysis led to an increased hydrogen production. In the range of 450-650 °C, NZ mixed samples showed enhanced hydrogen production. Similarly, Li4SiO4 also enhanced production of H2, but the increased H2 production with Li4SiO4 was only observed above 500 °C. This difference is probably due to the different CO2 absorption abilities of respective materials (as shown in Fig. 4). According to Le Chatelier’s principle, both WGS (Eqn. 9) and carbon steam gasification (Eqn. 11) favor the production of H2 if the produced CO2 is removed from the system. CO2 absorption on the part of alkali sorbents explained sudden spike in H2 production; Na2ZrO3, the more active CO2 sorbent, was able to capture CO2 at a lower temperature hence it had earlier H2 production. Li4SiO4 captured CO2 at temperatures over 500 °C, while SD-LZ samples produced a slightly higher H2 yield but the H2 generation appeared in the same region as the SD sample without any additives. The slight enhancement in H2 generation of SD-LZ over SD samples is probably related to presence of Li as a catalyst for cracking reactions (Eqns. 6 and 7) and reforming reactions (Eqn. 7) [36]. Fig. 3 (b) showed that Li2ZrO3 could absorb some CO2. As shown in Fig. 3 (d), methane yield remained insignificant in both SD and alkali salts mixed samples. The gas generation trends of CO and CO2 will be discussed in detail in next section.

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Fig. 3. Gas formation rate from pyrolysis of biomass with alkali salts (a) H2, (b) CO, (c) CO2, and (d) CH4. Accumulative gas yields of H2, CO, CO2, and CH4, as well as H2 fraction are presented in Table 3. Li4SiO4 and Na2ZrO3 produced more than double H2 and also improved the H2 fraction considerably compared to that without addition. However, Li2ZrO3 did not increase the H2 yield significantly, but it did improve the H2 fraction. CO yield was increased in the presence of alkali salts while CO2 yield was decreased. This may be attributed to Boudouard reaction generating CO at the expense of CO2. The combined catalytic and sorption effect of alkali salts on the enhancement of H2 production was calculated as follows: Mass of detected H2

(12)

H conversion = Mass of H in biomass × 100%

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Pyrolysis of biomass without addition converted close to one-fifth of the available elemental hydrogen to H2. SD-LS and SD-NZ released a higher fraction of H2 than SD-LZ, and this displays the importance of sorption abilities of alkali sorbents for the enhancement of H2 production. Table 3. Accumulative gas yields, H2 fraction, and H conversion during biomass pyrolysis. Accumulative gas yield (mmol g-1) H2

SDa.

CO

SD.

CO2

SD.

CH4

SD.

H2 fraction (%)

SD

5.73

0.18

4.76

0.12

6.49

0.36

0.69

0.10

32.41

18.62

SD-NZ

13.67

0.52

5.73

0.24

5.91

0.36

0.69

0.04

52.56

44.41

SD-LS

15.85

0.30

6.01

0.11

4.02

0.02

0.81

0.06

59.36

50.14

0.25 8.87 SD-LZ a Standard deviation.

3.81

0.07

3.90

0.73

0.64

0.04

51.52

20.12

Sample

H conversion (%)

3.3 Evidence of CO2 absorption from solid phase identification To get insight on possible correlation between hydrogen production and CO2 absorption, sawdust samples mixed with alkali salts were pyrolyzed up to three different temperatures, i.e. 500, 550, and 600 °C. The results are elucidated in Fig. 4. The bottom patterns, labeled as NZblank [Fig. 4(a)], LS-blank [Fig. 4(b)], and LZ-blank [Fig. 4(c)], are obtained by mixing sawdust with Na2ZrO3, Li4SiO4, and Li2ZrO3. It can be observed that all peaks belong to pure alkali salts, but a slight raise can be noticed in the patterns that lie between 20° and 27° for all three materials. The similar raise did not appear when similarly mixed samples are pyrolyzed, which suggested that concerned patterns appeared due to presence of biomass. In Fig. 4 (a), the evidence of chemical CO2 absorption is observed in the patterns of all pyrolyzed samples. In the patterns of SD-NZ sample pyrolyzed at 500 °C peaks of ZrO2 manifest at 30° and 59°, and at 35° peak relating to Na2CO3 appears. These peaks gain more distinction in the SD-NZ sample

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pyrolyzed at 550 °C and in SD-NZ pyrolyzed at 600 °C, two new peaks of ZrO2 appear at 28° and 31°. In Fig. 4 (b), phase transformation for SD-LS samples can be observed; however, peaks were identified only for Li2SiO3 at 27° and 33°. XRD analysis did not register Li2CO3 phase. In Fig. 4 (c), the peaks of Li2ZrO3 remain the same throughout the three pyrolysis runs up to 500, 550, and 600 °C, which meant that LS-SD samples were unable to absorb CO2 significantly. There is clear evidence suggesting that Na2ZrO3 can capture CO2 produced from pyrolysis of biomass sample at the temperatures lower than 500 °C. The co-existence of Na2ZrO3, Na2CO3, and ZrO2 in all pyrolyzed samples confirms that Na2ZrO3 is not completely converted into Na2CO3 and ZrO2 phases. It was noticed that, the H2 peak from NZ mixed samples coincided with the temperature range, under which carbonate and secondary metal phase co-existed. The phase change of LS can be observed as the temperature of pyrolysis increased. Li2SiO3, the secondary phase of Li4SiO4 carbonation reaction (Eqn. 2), appeared in the temperature range where H2 production from LS samples increased.

Fig. 4. XRD patterns of (a) SD-NZ, (b) SD-LS, and (c) SD-LZ pyrolyzed at 500 °C, 550 °C, and 600 °C.

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Although the formation of Li2CO3 is likely to occur according to Eqn. 2, the quantity might be too small to be detected. It is important to note the role of PCO2. Although TG-MS setup cannot determine local partial pressure of evolved species, local PCO2 was high enough to initiate absorption reaction for Na2ZrO3 and Li4SiO4. According to previous studies, Na2ZrO3 and Li4SiO4 are active sorbents in PCO2 as low as 0.025 bar at 575 °C and 0.2 bar at 500 °C, respectively [30,37]. 3.4 Discussion Production of CO came from two peaks for the alkali salts addition samples (Fig. 3 (b)). In the SD sample, CO was mainly produced from the biomass pyrolysis. CO is released from oxygen containing functional groups during pyrolysis, and different groups breakdown at different temperatures [38]. C=O and C-O-C bonds breakage is primarily responsible for CO release during pyrolysis [39]. The presence of Na2ZrO3’s appeared to influence the release of CO at low temperatures. For SD-LS, Li4SiO4 led to CO release at lower temperature and a small peak of CO release can be seen from SD-LZ sample. A low quantity of CO was produced from the SD sample after the completion of main pyrolysis and this can be attributed to the secondary tar reactions [33]. Alkali salts mixed samples displayed similar trend, except that CO production had another peak at around 600 °C. This CO production was related to cracking and reforming reactions and Boudouard reaction, where char and CO2 can generate CO (Eqn. 13). C + CO2 ⇌ 2CO,

∆H298 = 172 kJ mol-1

(13)

CO2 evolution trend, as shown in Fig. 3 (c), displayed one peak before 500 °C in all samples, which appeared during primary pyrolysis. In the presence of alkali salts, CO2 peaks were suppressed in comparison to peaks in SD samples. As observed from XRD results in Fig. 4, Na2ZrO3 and Li4SiO4 showed evidence of CO2 absorption and a partial transformation to

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carbonate was observed, yet there was no CO2 desorption from the result of MS. The likely cause of the limited CO2 is that it could be consumed by Boudouard reaction (Eqn. 13). Alkali metal carbonates are known to catalyze Boudouard reaction by participating in a cyclic reduction, oxidation, and carbonation mechanism [40]. The effective role of alkaline catalyst (K, Na, and Li) has been demonstrated in promoting the conversion of CO2 and coke into CO at low temperatures (< 750 C) [41]. Differential weight loss curves of all samples in the temperature range 500 - 800 °C are circled in Fig. 5. Between 500 and 550 °C, the DTG curves of all samples were tending towards stabilization after major weight loss during pyrolysis. A minor weight loss peak was registered for all alkali salts mixed samples at the temperatures above 600 °C. The most likely cause for this weight loss were secondary reactions that produced H2 and CO. Sawdust had almost no ash content (Table 1), so at the temperature above 600 °C, the residue mass from pyrolysis was char which was catalyzed by alkaline catalysts to produce syngas. SD-LS and SD-NZ showed the most prominent weight loss above 600 °C (Fig. 5), and both SD-LS and SD-NZ yielded the highest quantity of CO and H2. It can be concluded that formation of alkali carbonates promoted multiple tar cracking and reforming reactions that enhanced the production of H2 and CO at later stages of pyrolysis in SD-LS and SD-NZ cases. SD-LZ samples did not transform into carbonates but SD-LZ displayed a smaller weight loss than other alkali salts mixed samples. This weight loss may be related to presence of Li which acted as a catalyst [36]. In contrast, there was no noticeable movement for the curves of SD sample and it can be concurred that there are no more reactions taking place.

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Fig. 5. DTG curves of pyrolyzed samples (weight loss peaks between 500 and 800 °C are highlighted). Another factor for CO production could be the release of trapped CO2 in alkali sorbents. The regeneration temperatures of Li2ZrO3 and Li4SiO4 are typically 700 °C and above, and regeneration temperature of Na2ZrO3 is 800 °C and above [19]. At temperatures lower than 850 °C, char gasification reaction rate is controlled by surface reaction of CO2 to carbon [42]. As char and alkali catalysts were well mixed, any CO2 released from sorbent was close to catalyst site and char. In mixed SD samples where there was no mass transfer barrier a rapid conversion of CO2 and char to CO was achieved.

4. Conclusions Alkali ceramics, Na2ZrO3, Li4SiO4, and Li2ZrO3, were investigated as bifunctional catalystsorbent materials under pyrolytic conditions. Results showed that presence of Na2ZrO3 and Li4SiO4 increased H2 yield significantly during pyrolysis. The presence of alkali metals increased H2 yield from 5.73 mmol g-1 (SD) to 8.87 mmol g-1 (SD-LZ), 15.85 mmol g-1 (SD-LS) and 13.67 mmol g-1 (SD-NZ). Na2ZrO3 and Li4SiO4 could capture CO2 released from pyrolysis and

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transform into carbonate phase. Removal of CO2 promotes WGS reaction and tar cracking reactions towards H2 production. Presence of Li2ZrO3 had limited improvement in H2 generation and further testing showed that Li2ZrO3 was unable to capture CO2 during pyrolysis. The catalytic activity of alkali salts was not just limited to their carbonate phase but Na2ZrO3, Li4SiO4, and Li2ZrO3 themselves could lower the primary pyrolysis temperatures of biomass samples. Carbonate catalysts participated in tar cracking and reforming reactions that enhanced the yield of CO, and when captured CO2 was released, alkali salts acted as catalyst for Boudouard reaction and converted CO2 and char into CO. Altogether, alkali metal participated in multiple important pyrolysis reactions over a wide temperature range, both as a catalyst and sorbent. The results obtained favor further investigation of alkali metal based bifunctional material in scaled up experimental conditions. Acknowledgement The work was supported by National Natural Science Foundation of China (grant number: 51506112). Declarations of interest None. References [1] V.S. Sikarwar, M. Zhao, P. Clough, J. Yao, X. Zhong, M.Z. Memon, N. Shah, E.J. Anthony, P.S. Fennell, An overview of advances in biomass gasification, Energy Environ. Sci. 9 (2016) 2939–2977. [2] Z. Li, C. Liu, Z. Chen, J. Qian, W. Zhao, Q. Zhu, Analysis of coals and biomass pyrolysis using the distributed activation energy model, Bioresour. Technol. 100 (2009) 948–952. [3] S. Al Arni, Comparison of slow and fast pyrolysis for converting biomass into fuel, Renew. Energy. 124 (2018) 197–201. https://doi.org/10.1016/j.renene.2017.04.060.

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Journal Pre-proof Author Contribution Statement Ming Zhao: Conceptualization, Methodology, Data curation, Writing- Original draft preparation Muhammad Zaki Memon: Investigation, Writing- Original draft preparation Guozhao Ji: Investigation Xiaoxiao Yang: Investigation Arun K Vuppaladadiyam: Investigation Yinqiang Song: Investigation Abdul Raheem: Investigation Jinhui Li: Supervision, resources Wei Wang: resources Hui Zhou: Conceptualization, Supervision, Writing- Reviewing and Editing

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights 

Bifunctional catalyst-sorbent activity of alkali ceramics was investigated.

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Alkali ceramics were active in catalytic and chemisorption activities of pyrolysis.

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Presence of alkali ceramics increased H2 yield by two-fold.

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At 700 °C alkali ceramics participated in gasification of char.