Simultaneous Tar Reforming and Syngas Methanation for Bio

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Kinetics, Catalysis, and Reaction Engineering

Simultaneous Tar Reforming and Syngas Methanation for Bio-SNG Junjie Zhang, Guangyong Wang, and Shaoping Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02085 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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Industrial & Engineering Chemistry Research

Simultaneous Tar Reforming and Syngas Methanation for Bio-SNG Junjie Zhangb, Guangyong Wanga,*, Shaoping Xub,** a

State Key Laboratory of Comprehensive Utilization of Industrial Exhaust Gas, National Center for C1 Chemical Engineering and Technology, Southwest Research & Design Institute of Chemical Industry, No.393 Jichang Road, Chengdu 610225, China

b

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, No.2 Linggong Road, Dalian 116024, China

*Corresponding address: *[email protected] (G. Wang); **[email protected] (S. Xu)

Abstract: Biomass steam gasification in a free-fall reactor followed by the as-produced biogenous syngas upgrading over Ni/olivine and Ni/olivine+CaO in a countercurrent moving bed reactor was performed in this study. In the newly developed moving bed configuration, the tar in the syngas was steam reformed and simultaneously the syngas was methanated. As a preliminary part of this research, the influences of tar on syngas methanation at different temperatures and H2O contents were studied as well in a fixed-bed reactor with toluene as a model tar compound. It was found that the carbon deposition behavior was greatly affected by the reaction conditions. During raw biogenous syngas upgrading over Ni/olivine in the countercurrent moving bed upgrading reactor, both CH4-rich gas production and tar elimination with good resistance towards carbon deposition were achieved. With the introduction of CO2 sorbent into the upgrading reactor further, CH4/H2 mixture was achieved. Keywords: Bio-SNG; Methanation; Tar reforming; Carbon deposition; Biomass 1. Introduction Biomass gasification is a well-developed process for synthesis gas production, which could be converted to electric power, heat or fuels.1, 2 Typically, substitute natural gas (SNG) production from the biogeneous syngas, as a carbon-neutral energy 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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and distributed by the existing natural gas grid, could be an important complement to the traditional fossil fuel.3-5 However, contaminants such as condensable hydrocarbons (i.e. tar) and ash are generally contained in the raw syngas6,

7

and

detrimental to the downstream methanation. The tar in the gas reaches to the magnitude of 100 g/Nm3, 10 g/Nm3 and 1 g/Nm3 for the updraft, the fluidized beds and the downdraft gasifiers, respectively.8 Low temperature scrubbing was commonly adopted before methanation for the removal of tar.9 Alternatively, an integrated gasification-methanation process with hot gas-cleaning could significantly improve the efficiency by avoiding cooling and reheating of the producer gas and lower the investment-costs as well, especially suitable for the decentralized biomass-fuelled SNG production.10, 11 As a promising measure for tar elimination from hot gas, catalytic steam reforming/cracking of tar has been developed to accelerate the kinetically limited tar elimination reactions,12 in which the tar was converted into permanent gases that were downstream usable, rather than removed from the syngas stream. The catalytic steam reforming of tar was usually performed at 800-1000 oC utilizing the steam and sensible heat of the hot producer gas.13 It might be in-bed over active bed materials (olivine, dolomite, Ni/olivine, Ni/MgO, etc.)14-16 or downstream the gasifier over a secondary catalyst bed (Ni/Al2O3, Ni/dolomite, char, etc.).17-19 The in-bed tar elimination is favorable because no secondary tar reforming unit is needed.12, 20 In this respect, a fluidized two-stage process has shown excellent performance even in commercial plants. The running data showed that the tar content in the gasified gas was as low as 0.4 g/Nm3 at temperatures of 700 oC for the fluidized bed pyrolyzer and 850 oC for the transport fluidized bed gasifier.21, 22 Such tar destruction even with in-bed catalysts, however, is still insufficient for the conventional tar-free syngas

2 ACS Paragon Plus Environment

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methanation process, due to the interactive gasification and tar reforming in the gasifier which makes it difficult to efficiently strengthen the tar destruction.13, 23 The downstream reformers were proved to be efficient and necessary.13, 24

C x H y Oz ( tar ) + H 2O → CO + H 2 + CO2 △HR > 0

(1)

(the △HR depends on tar composition, for example,

C7 H 8 ( toluene ) + 7 H 2O → 7CO + 11H 2 △HR = 876 kJ/mol) 25 CO + 3H 2 ↔ CH 4 + H 2O △HR = -206 kJ/mol

(2)

CO2 + 4 H 2 ↔ CH 4 + 2 H 2O △HR = -165 kJ/mol

(3)

CO + H 2O ↔ H 2 + CO2 △HR = -41 kJ/mol

(4)

As a development of the commonly used stratagem introducing the reformer as a guard bed before methanation reactor, tar reforming incorporated in methanation could utilize the heat of the exothermic methanation. Several works have demonstrated the feasibility of catalytic tar steam reforming at moderate temperature (500-700 oC).25-28 Such temperature range is also conducive to methanation in consideration of kinetics and heat exchange efficiency. Furthermore, both tar reforming and syngas methanation could be promoted by identical nickel catalysts. Methanation of raw biogenous syngas over Ni/MgO-SiO2 in a fixed-bed reactor led to temperature-rise up to 500 oC from 350 oC and as a result, in-situ tar conversion reached to more than 90%.10 In respect of the carbon deposition behavior, tar of the biogenous syngas, consisting mainly of mono- and polyaromatic hydrocarbons, is prone to polymerization and deposition over the catalysts.13, 29 The influence of the small amount of tar in the biogeous syngas or its residual after reforming/cracking on carbon deposition in methanation condition, however, has been paid little attention.10, 30

The surface properties of Ni/Al2O3 and its catalytic activity for methanation were

evaluated under a lab version of biosyngas, with the addition of CH4, C2H4, C2H2, 3 ACS Paragon Plus Environment


C3H6 or C2H6 at 400 oC.30 The presence of C2-C3 in the feed gas was found to promote carbon whiskers formation at the catalyst surface and slightly decrease its methanation activity. During the methanation of raw biogenous syngas over Ni/MgO-SiO2 in a fixed-bed reactor,10 as mentioned above, the inlet-zone of catalyst bed exposed to the tar-containing feeding gas at temperature as low as 350 oC was prone to deactivation and the deactivated zone expanded gradually. Increasing the inlet-zone temperature to 450 oC, instead, resulted in a rapid carbon deposition and reactor blockage. At the relatively low temperatures the coke formation rate would be higher than the coke removal one (by gasification with agents such as H2O and CO2), and a net buildup of coke would appear, generating a catalyst deactivation.13 It seems necessary to carry out the integrated tar reforming and syngas methanation above 500 oC to destruct tar and inhibit carbon deposition as well. In comparison, syngas methanation is thermodynamically preferred at much lower temperature, as a result, CH4 content in the product gas could be low in the compromised conditions. Additionally, the product gas has to be purified before feeding the natural gas grid, during which the CO2 separation process consumes up to 10% of its heating value.31, 32 To improve both the carbon deposition resistance and the CH4 content, reforming and methanation of biogenous syngas in a gas-solid countercurrent moving bed has been developed in present work. Over the catalytic bed material of the moving bed, both reforming of the trace amount of tar and syngas methanation occur. The bed material is heated by the exothermic methanation and delivered downward. As a result, an axial temperature gradient along the moving bed is built, i.e. the up-zone at relatively lower temperature and the down-zone at higher temperature. The introduced tar is mainly reformed over the hot bed material in the down-zone of the moving bed, meanwhile, the low-tar or even tar-free syngas flows upward for further methanation.


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Specifically, biomass steam gasification in a free-fall reactor followed by upgrading of the as-produced biogenous syngas over Ni/olivine in a countercurrent moving bed reactor for bio-SNG was performed. In the upgrading reactor, the tar in the syngas was steam reformed and simultaneously the syngas was methanated. By introducing CO2 sorbent into the catalytic bed of the upgrading reactor further, CH4/H2 mixture was achieved. As a preliminary study, tar reforming and syngas methanation over Ni/olivine in a fixed-bed reactor were conducted, in which toluene and CO2 + H2 (H2/CO2 = 3.2) were used as model for tar and syngas, respectively. The performances for both the tar reforming and the syngas methanation and the interaction between them were investigated in the fixed bed tests. 2. Experimental 2.1 Reforming and methanation of syngas containing toluene in a fixed-bed reactor The influence of toluene on syngas methanation at different temperatures and H2O contents was studied at atmospheric pressure in an electric heated fixed-bed quartz reactor with 8 mm inner diameter. The toluene was used as a model tar compound as it represents the stable aromatic structure of tar from biomass gasification.26, 33 An olivine supported 6 wt.% nickel, i.e. Ni/olivine, was used as the reforming and methanation catalyst. It was prepared by incipient wetness impregnation of the 600 oC calcined olivine with an aqueous solution of nickel nitrate followed by calcination at 350 oC for 4.5 h. More details about the catalyst have been discussed in our previous work.34 1 g Ni/olivine was placed in the middle of the reactor with a K-type thermocouple placed in the center of the catalyst bed to monitor the reaction temperature. The syngas was controlled by a mass flow controller and admitted down through the catalyst bed at 130 ml/min, an hourly space velocity (GHSV) of 11,000 h-1. Toluene was added to the feeding gas with liquid hourly space velocity (LHSV) of



0.6, 1.2 and 2.4 h-1, corresponding to toluene content of 45, 90 and 180 g/Nm3, respectively. The toluene and steam were transported by two syringe pumps and preheated to 250 oC before entering into the reactor. The heat and mass transfer limitations during the methanation reaction have been proved to be negligible.34 The concentrations of CO, CO2, CH4 and other gas products were monitored using a GC 7890Ⅱ gas chromatograph equipped with both TCD and FID detectors. The CO2 conversion ( Cco 2 =1-

CO 2 out CH 4 out ) and CH4 selectivity ( SCH 4 = ) were CO 2 in CO 2 in - CO 2 out

calculated based on the gas composition, where CO2 in was the mole feed rate of CO2, CO2 out and CH4 out the mole rate of the effluent CO2 and CH4, respectively. To illustrate the accompanied toluene reforming during the methanation of syngas containing toluene, catalytic steam reforming of toluene over Ni/olivine was performed at 600 oC under the mixture of H2O and toluene with H2O/C mole ratio of 2 and toluene LHSV of 1.0 h-1. Toluene conversion was calculated basing on the carbon-based conversion of toluene to gaseous products. 2.2 Simultaneous tar reforming and syngas methanation in a countercurrent moving bed upgrading reactor downstream a free-fall gasifier Reforming and methanation of biogenous syngas was investigated in a lab-scale gasification coupled with upgrading reaction system, consisting of a free-fall gasifier, a separator and a countercurrent moving bed upgrading reactor, as shown in Figure 1. The free-fall reactor with a length of 1900 mm and an i.d. of 26 mm, is heated by five electric heaters. Five K-type thermocouples of the reactor tube are inserted in the middle of the five heaters closing to the outer wall, respectively. The gas-solid separator connected to the bottom of the reactor tube is heated by an electric heater with a K-type thermocouple installed at the outside wall of the separator. The


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gas-solid countercurrent moving bed upgrading reactor with 26 mm i.d. and 370 mm height, is heated by two independent electric heaters to compensate the heat loss with a K-type thermocouple inserted in the middle of each heater closing to the outer wall of the reactor. Catalytic bed material is loaded in the upgrading reactor with the updating rate of the bed material controlled by a rotary valve. All the reactors are made of 316L stainless steel.

Figure 1. Lab-scale gasification coupled with upgrading reaction system. Silica sand of 20-40 mesh was used as the moving bed material in the upgrading reactor, incorporating methanation/reforming catalyst and CO2 sorbent of the same particle size for hot raw gas upgrading. The Ni/olivine and calcined limestone were used as the dual functional catalyst and CO2 sorbent, respectively. The limestone came from Changxin, Zhejiang and was calcined at 900 oC for 4.5 h. It contains 91.5 wt.% CaO. The biomass feedstock used in this work was pine sawdust from Dalian 7 ACS Paragon Plus Environment


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City, Liaoning Province and its proximate and ultimate analyses are presented in Table 1. The pine sawdust was crushed and sieved to 20-40 mesh and dried for 4 h at 105-110 °C before tests. Table 1. Proximate and ultimate analyses of the pine sawdust. Proximate analysis (wt.%, ad) Moisture

Ash

Volatiles

8.26

0.61

78.40

Fixed carbon 12.73

Ultimate analysis (wt.%, daf) Carbon

Hydrogen

Oxygen1)

Nitrogen

Sulfur

47.75

6.98

44.84

0.07

0.36

1) by difference

Prior to test, the bed material of the upgrading reactor was loaded and purging nitrogen with flow rate of 300 ml/min was turned on to insure an air-free atmosphere. Then the reactors were heated up to the desired temperatures. At a steady state of the gasifier temperature (800 oC), the separator temperature (800 oC) and the upgrading reactor temperature (up-zone: 350-500 oC/down-zone: 350-600 oC), biomass steam gasification began by feeding biomass at feeding rate of 60 g/h and preheated steam 0-24 g/h. Meanwhile, the bed material of the upgrading reactor started to be updated at updating rate of 600 g/h. The height of the catalyst bed was maintained to be 200 mm and equally distributed in the two independent-heated zone of the upgrading reactor with the top and bottom surfaces of the catalyst bed near the center of each heater. The raw biogenous syngas from biomass steam gasification in the free-fall gasifier was separated from the nascent char in the separator and then delivered to the countercurrent moving bed upgrading reactor, underwent upgrading over its catalytic bed matreials. The product gas was extracted from the upgrading reactor with the help of a vacuum pump and cooled in four sequential glycol-cooled (-12 oC) condensers, in which the condensable components and particulate matter were separated from the permanent gas. The effluent product gas underwent further aerosol removal and was 8 ACS Paragon Plus Environment

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measured by a wet type gas flowmeter and analyzed every 10 minutes by a gas chromatography GC 7890Ⅱ equipped with both TCD and FID detectors. The tar in the product gas was sampled via the sampling points before the condensers. The tar sampling was performed by condensing and dissolving the tar components out of the raw gas with six 250 ml impinger bottles basing on the protocol CEN/TS 15439.35-37 The impinger bottles were located in a cooling bath cooled down to -12 oC by a cryostat, with bottles 1-5 filled with toluene and bottle 1, bottle 5 and bottle 6 glass beads. The liquid phases in the impinger bottles were unified after sampling and the tar was obtained by evaporating off the toluene solvent of toluene phase at 50 oC under reduced pressure. GC/FID (Agilent 5975C) with a HP-5MS column (30 m × 0.25mm × 0.25 µm) was used to quantify the different compounds in the toluene phase. Helium was used as the carrier gas. The injector temperature was 320 oC and an injector split ratio of 50:1 was employed. The temperature program was as follows: 50 oC for 3 min, followed by ramping at 5 oC/min to 300 oC and maintained for 10 min. The relative proportions of the detected compounds were determined basing on the area (%) of the GC-FID peaks.38

3. Results and discussion 3.1 Reforming and methanation of syngas containing toluene in a fixed-bed reactor The influence of toluene on methanation of H2/CO2 over Ni/olivine was investigated at different reaction temperatures. As shown in Figure 2, CO2 conversion and CH4 selectivity decrease remarkably by adding 45 g/Nm3 toluene at 450 oC, while increase slightly at 550 oC. The reforming/cracking of toluene by CO2 and the produced H2O during methanation at 550 oC, providing syngas and consuming H2O, could contribute to the improved CH4 production.



Figure 2. Methanation of H2/CO2 over Ni/olivine at different reaction temperatures in the presence and absence of toluene. The methanation activity variation towards toluene contents in the feeding gas was further studied. CO2 conversion and CH4 selectivity decrease with the increase of toluene content, as indicated by Figure 3. A rapid coke formation occurred leading to reactor-blockage with 180 g/Nm3 toluene addition.

Figure 3. Methanation of H2/CO2 over Ni/olivine with different toluene additions at 550oC. Thermal gravimetric analysis (TGA) of the spent catalysts was performed under CO2 flow at a heating rate of 10 °C/min on a DTU-2B thermal balance. The carbon deposition on the catalyst could be determined by the weight loss. TG curves (Figure 4) of the spent Ni/olivine catalysts indicate that weight loss of 1.0 wt.% happens at 400-550 oC for the catalyst exposure to H2/CO2 in presence of 45 g/Nm3 toluene at 10 ACS Paragon Plus Environment

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450 oC. But remarkable weight loss of 49.0 and 1.5 wt.% appears at 550-700 oC for the samples from the surface and bottom of the catalyst bed experienced 550 oC reaction, respectively. These two peaks are attributed to the gasification of encapsulating carbon film and carbon filaments, respectively, in accordance with the published works on carbon species formation during steam reforming of hydrocarbons over nickel catalysts.26, 29 The rate of hydrocarbon adsorption exceeds the rate of cracking at relatively low reaction temperature and thus a polymer film is formed, which encapsulated the metal surfaces of the catalyst resulting in loss of catalytic activity,29 confirming the results displayed in Figure 2. The filamentous carbon formed at higher temperature, with loose structure and low adhesion to the catalyst surface, would not cause a loss of catalyst activity before plugging of the pores.29, 30 Still, the regions of filamentous carbon forming potential must be avoided, as the rates of filamentous carbon deposition are sufficiently high to cause pore plugging and reactor-blockage once initiated,10, 29 as the case under 180 g/Nm3 toluene in Figure 3.



Figure 4. TG curves of the spent Ni/olivine after reaction at different temperatures in H2/CO2 with 45 g/Nm3 toluene, with or without 15 vol.% H2O addition (b: Two samples from the surface and bottom of the catalyst bed reaction at 550 oC were analyzed due to their remarkable difference). The carbon deposition behavior was suggested to be determined by catalyst structure26 and its reaction conditions.29 As indicated in Figure 4b, serious carbon deposition happened in the surface layer of the catalyst bed at 550 oC, but much less carbon deposition in the bottom layer. Such distinct difference could be related to the increase of H2O content in the feed gas during its penetration in the catalyst bed as a result of methanation. It has been reported that coke removal by gasifying agents (e.g. H2, H2O and O2 that remove coke as CH4, CO and COx, respectively) could contribute to reduce carbon deposition39. Typically, carbon species formed in steam reforming of hydrocarbons on nickel catalysts was greatly affacted by the H2O content29. The increasing H2O content alleviated the carbon deposition over the catalyst downstream the surface layer, but could not totally prevent the potential carbon deposition at 550 o

C. Higher temperature in addition to the increasing H2O content may work, to

promote gasification of the deposited carbon or its precursor and inhibit carbon deposition. Herein, the carbon deposition behavior over Ni/olivine under H2/CO2 mixture with 45 g/Nm3 toluene and 15 vol.% H2O addition at 600 oC, was further evaluated. The carbon deposition was not detectable in this condition as expected (Figure 4c). Meanwhile, Ni/olivine showed evident catalytic activity for steam reforming of toluene at 600 oC, indicated by Figure 5. The H2-rich product gas from tar steam reforming could contribute to CH4 production during methanation of syngas containing tar.


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Figure 5. Toluene conversion and product gas composition on dry basis during toluene steam reforming with H2O/C mole ratio of 2 and toluene LHSV of 1.0 h-1, over Ni/olivine at 600 oC. Methanation of H2/CO2 over Ni/olivine at 600 oC with the addition of 15 vol.% H2O and 45 g/Nm3 toluene sequentially is displayed in Figure 6. CO2 conversion and CH4 selectivity decrease a lot by steam addition compared with the dry feeding gas as a result of thermodynamic equilibrium shifting. The catalytic performance remained stable when toluene was added to the H2O-containing feeding gas, in accordance with the negligible carbon deposition as indicated above.



Figure 6. Methanation of H2/CO2 over Ni/olivine at 600 oC, with the addition of 15 vol.% H2O and 45 g/Nm3 toluene sequentially. In optimized condition, typically, with 15 vol.% H2O at 600 oC here, steam reforming of toluene could be integrated within the methanation without deteriorating its performance. In such condition, however, the compatibility between tar reforming and syngas methanation was achieved at the expense of unfavourable syngas to CH4 due to the thermodynamic equilibrium restriction. Typically, CO2 conversion and CH4 selectivity were as low as 44% and 36%, respectively (Figure 6), due to the relatively high temperature and steam content. To improve CH4 yield during methanation of syngas containing tar, an additional catalyst bed was introduced into the fixed-bed reactor, with the up-layer catalyst (0.5 g) at 600 oC as guard bed for tar elimination and 500 oC the down-layer (0.5 g) as guaranteeing bed promoting methanation. An improved result with 53% CO2 conversion and 84% CH4 selectivity was achieved, as shown in Figure 7a. CH4 concentration increases to 19% from 5% over the single catalyst bed at 600 oC (Figure 7b). CO content decreases a lot by the intensified methanation and water gas shift reaction (WGS). 14 ACS Paragon Plus Environment

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Figure 7. Methanation of H2/CO2 with 15 vol.% H2O and 45 g/Nm3 toluene addition: (a) CO2 conversion and CH4 selectivity over two-layer Ni/olivine; (b) product gas composition over the two-layer and single Ni/olivine. Alternatively, a countercurrent moving bed methanation process could be preferred for efficient H2O-rich biogenous syngas upgrading. The hot biogenous syngas flows upward in the moving bed during which catalytic methanation proceeds. The fed low temperature catalytic bed material is heated by the exothermic methanation reaction and goes downward promoting steam reforming of tar. In this way, steam reforming of tar happens over the moderate temperature down-zone catalyst (guard bed) and sufficient methanation of the well upgraded gas occurs over the low temperature up-zone catalyst (guaranteeing bed). The hot spent catalyst is then cooled and may also experience periodic regeneration before circulating back. 3.2 Simultaneous tar reforming and syngas methanation in a countercurrent moving bed upgrading reactor downstream a free-fall gasifier Biomass steam gasification occurred in the free-fall reactor and the as-produced hot raw syngas underwent simultaneous tar reforming and syngas methanation over the countercurrent moving bed upgrading reactor. To determine the raw producer gas as well as the biomass gasification performance, the upgrading reactor was heated to 350 o

C with SiO2 as bed material to avoid tar condensation and decomposition.10 The



outlet gas composition as measured against time at 800 oC is shown in Figure 8. The gas compositions of H2, CO, CH4, CO2, C2-C3 maintained a stable value during 1 h test. Raw syngas with gas yield of 0.71 Nm3/kg, tar content of 96.6 g/Nm3 and lower heating value (LHV) of 17.6 MJ/Nm3 was achieved at 800 oC and steam to biomass mass ratio (S/B) of 0.4. CO content was as high as 44.4% and CH4 content reached to 15.8% in the raw gas, due to the short residence time of volatiles40 and limited gas-solid contact in the gasifier. The results agree well with the reported ones from SilvaGas and MILENA developed by Battelle and Energy research Centre of the Netherlands (ECN), respectively.3, 41 Ni/olivine was then introduced into the upgrading reactor, with the up-zone of the moving bed at 350 oC and the down-zone 600 oC. The CH4 content increases to 22.0% and CO content decreases to 6.8%, confirming the improved methanation over Ni/olivine-SiO2 (25 wt.%-75 wt.%) during raw gas upgrading in the countercurrent moving bed. Meanwhile, the tar contained in the raw gas was effectively eliminated to 4.6 g/Nm3 and the gas yield increases to 1.00 Nm3/kg as well.

Figure 8. The outlet gas compositions as measured against time. Table 2. Key parameters and performances for catalytic biomass gasification with SiO2, Ni/olivine-SiO2 and Ni/olivine-CaO as the bed materials of the upgrading 16 ACS Paragon Plus Environment

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reactor. Feeding rate (g/h)

60

60

60

60

Gasifier temperature (oC)

800

800

800

800

350/350

350/600

500/600

500/600

SiO2

Ni/olivine-SiO2

Ni/olivine-CaO

Ni/olivine-CaO

S/B (kgH2O/kgfuel,db)

0.4

0.4

0.2

0

Carbon conversion (%)

65.1

70.9

10.0

15.9

0.71

1.00

0.30

0.53

LHV (MJ/Nm )

17.6

12.8

17.4

17.2

Tar content (g/Nm3)

96.9

4.6

3.3

5.7

H2

24.8

37.5

71.1

73.7

CO

44.4

6.8

0.3

0.2

CH4

15.8

22.0

26.9

25.5

Upgrading reactor temperature (oC) Bed material in upgrading reactor

3

Gas yield (Nm /kg) 3

Dry gas composition (vol.%)

CO2

9.5

33.7

1.6

0.5

C2H4

4.9

-

-

0.1

C2H6

0.5

-

-

-

C3H6

0.1

-

0.1

-

C3H8

-

-

-

-

The catalytic tar steam reforming over Ni/olivine during raw gas upgrading was further illustrated by tar compounds analysis, as shown in Table 3. Compared to the complex tar with abundant compounds of C8-C24 over SiO2 at 350 oC, a much simplified tar was achieved after the as-produced raw gas upgraded over Ni/olivine. Classification of the tar compounds was performed for a concise presentation of tar properties and the catalytic behavior of Ni/olivine. The tar compounds classification basing on the their behaviors (water solubility and condensation) in the downstream processes, proposed by ECN, was used in this study to characterize the detected tar compounds.38,

42

The ECN classification groups the tar compounds into five tar

classes. Class 1 comprises high molecular components which are prone to condensing at high temperatures and not detectable by the GC. Classes 2 to 5 are heterocyclic compounds and polyaromatic hydrocarbons distinguished by means of heterocyclic character and the number of aromatic rings. As shown in Figure 9, the fractions of 17 ACS Paragon Plus Environment


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class 4 and class 5 account for the dominant proportion, i.e. 67% and 24% of the total tar over SiO2. In comparison, class 5 tar was completely eliminated and class 4 tar was significantly decreased to 6.5% over Ni/olivine. The results indicate that Ni/olivine was efficient for polyaromatic hydrocarbons conversion at moderate temperature, in agreement with the published works on biogenous tar elimination over nickel catalysts.10,

43

Meanwhile, 79% class 3 tar was achieved, which are not

important in condensation and water solubility issues.42 Toluene as a typical compound in class 3 (its GC/FID peak overlapped by the toluene solvent and not included in Table 3 and Figure 9), was generally present in biomass tar, especially those from the downdraft and the fluidized bed gasifiers.33, 44 The limited toluene conversion over Ni/olivine at 600 oC is in accordance with the undesirable class 3 tar conversion during hot gas upgrading. Table 3. Tar components distribution from raw gas upgrading over SiO2 and Ni/olivine-SiO2. SiO2 Compound

Ni/olivine-SiO2 Formula

Content (%)

Ethylbenzene

C8H10

0.38

Compound 1-Methyl-2-methylenecyclohexane

Formula

Content (%)

C8H14

3.25

p-Xylene

C8H10

0.52

Ethylcyclohexane

C8H16

1.38

Styrene

C8H8

2.08

Ethylbenzene

C8H10

6.39

Phenol

C6H6O

0.85

p-Xylene

C8H10

18.17

3-Hydroxyphenylacetylene

C8H6O

0.40

Phenylethyne

C8H6

1.03

3-Methylphenylacetylene

C9H8

2.56

Styrene

C8H8

35.78

Naphthalene

C10H8

27.40

Benzaldehyde

C7H6O

4.89

1-methyl-Naphthalene

C11H10

2.60

Phenol

C6H6O

2.23

Biphenyl

C12H10

2.71

2-propenyl-benzene

C9H10

4.42

Biphenylene

C12H8

8.20

3-Hydroxyphenylacetylene

C8H6O

7.11

1-(2-propenyl)-Naphthalene

C13H12

0.30

3-Methylphenylacetylene

C9H8

8.86

Acenaphthene

C12H10

0.32

C10H8

6.50

Dibenzofuran

C12H8O

2.12

Fluorene

C13H10

3.07

Phenanthrene

C14H10

12.84

2-phenyl-Naphthalene

C16H12

0.30

Naphthalene


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2-methyl-Phenanthrene

C15H12

0.37

4H-Cyclopenta[def]phenanthrene

C15H10

1.58

2-phenyl-Naphthalene

C16H12

0.99

Fluoranthene

C16H10

6.52

Pyrene

C16H10

6.22

1-methyl-Pyrene

C17H12

0.27

11H-Benzo[b]fluorene

C17H12

1.69

2-methyl-Fluoranthene

C17H12

0.34

Benzo[c]phenanthrene

C18H12

1.43

Benz[a]anthracene

C18H12

2.38

Triphenylene

C18H12

1.47

7,8-dihydro-Benzo(a)pyrene

C20H14

0.33

9H-Cyclopenta[a]pyrene

C19H12

0.32

Benzo[e]pyrene

C20H12

3.36

Benz[e]acephenanthrylene

C20H12

3.04

Perylene

C20H12

0.31

1-phenyl-Pyrene

C22H14

0.31

Dibenzo[def,mno]chrysene

C22H12

1.30

Benzo[ghi]perylene

C22H12

0.80

1,2:4,5-Dibenzopyrene

C24H14

0.31

Figure 9. Comparison of relative content and total amount of different tar groups over SiO2 and Ni/olivine-SiO2. TG curves of the spent Ni/olivine from the upgrading reactor show almost no weight loss (Figure 10), indicating that the formation of encapsulating carbon film or carbon filaments was inhibited in the reaction condition. The pronounced carbon deposition resistance here supports the analysis basing on Figure 4.



Figure 10. TG curves of the spent Ni/olivine from the upgrading reactor. Furthermore, by introducing CO2 sorbent into the catalytic bed material, sorption-enhanced simultaneous tar reforming and syngas methanation was evaluated, as shown in Table 2. The proportion of CH4 and H2 in the product gas reached 99.2% over Ni/olivine-calcined limestone (25 wt.%-75 wt.%). Meanwhile, the gas yield decreased a lot upon the introduction of CaO. It has been reported that the increase of H2O feeding could improve the activity of CaO by preventing its sintering and resulting in smaller CaO particles.32, 45 As a result, CO2 removal and CO conversion to CO2 via WGS were promoted during sorption-enhanced SNG production from syngas combining CO methanation, water-gas shift, and CO2 capture.32 Herein, the effect of H2O content on the sorption-enhanced simultaneous tar reforming and syngas methanation process was studied. It was found that increasing H2O feeding deteriorated biomass to CH4, as carbon conversion decrease to 10.0% from 15.9% with S/B increase from 0 to 2. Consequently, more carbon in biomass was bound in CaCO3 rather than converted to CH4 in the product gas. The favorable results above verify the feasibility of simultaneous tar reforming and 20 ACS Paragon Plus Environment

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syngas methanation over Ni/olivine and Ni/olivine+CaO in a countercurrent moving bed for CH4-rich product gas. Similarly, two-stage dual fluidized bed gasification (T-DFBG) has been developed to enhance the in-bed hot gas upgrading.46 In the upper stage of the two-stage dual fluidized bed, not only the catalytic material has its highest activity on catalyzing reforming and WGS reactions (because the material being just activated in the combustor) but the high reaction temperature in the upper stage also facilitates the catalytic reactions. The gas upgrading would increase the conversion of tars into gas and enhance WGS, thus hopefully increasing gasification efficiency and causing the produced gas to have less tar and hydrocarbon but more H2. Such attempts, basing on hot gas upgrading, provide new choices for biomass-fuelled energy/fuel production.

Conclusions The fixed-bed tests showed that the influence of tar on syngas methanation was affected by the reaction temperature, H2O and tar contents. The encapsulating carbon film and whisker-like carbon were formed at 450 oC and 550 oC, respectively, during the methanation of H2/CO2 containing toluene. The encapsulating carbon film resulted in Ni/olivine deactivation. The whisker-like carbon would not cause a distinct loss of catalyst activity, but accumulated rapidly at higher toluene content to cause reactor-blockage. The introduction of H2O at higher temperature, i.e. 15 vol.% H2O addition at 600 oC, contributed to toluene steam reforming and inhibited both the encapsulating carbon film and filamentous carbon deposition. The relatively high temperature and steam content, however, were detrimental to CH4 selectivity due to the thermodynamic equilibrium shifting. Biomass steam gasification in a free-fall reactor with the as-produced biogenous syngas upgraded in a countercurrent moving bed upgrading reactor was performed.



Both CH4-rich gas production and tar elimination with good resistance towards carbon deposition were achieved, verifying the feasibility of simultaneous tar reforming and syngas methanation over Ni/olivine in the countercurrent moving bed. With the introduction of CO2 sorbent into the bed materials of the upgrading reactor further, CH4/H2 mixture was achieved.

Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51306029 and No. 50776013) and the National High Technology Research and Development Program of China (No. 2008AA05Z407).

ORCID Guangyong Wang: https://orcid.org/0000-0002-4202-8955

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Graphic abstract


Simultaneous Tar Reforming and Syngas Methanation for Bio

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