Catalytic Steam Gasification of Biomass Surrogates: Thermodynamics and Effect of Operating Conditions

Accepted Manuscript Catalytic Steam Gasification of Biomass Surrogates: Thermodynamics and Effect of Operating Conditions Jahirul Mazumder, Hugo I de Lasa PII: DOI: Reference: S1385-8947(16)30135-8 http://dx.doi.org/10.1016/j.cej.2016.02.034 CEJ 14767 To appear in: Chemical Engineering Journal Received Date: Revised Date: Accepted Date: 21 November 2015 February 2016 10 February 2016 Please cite this article as: J Mazumder, H.I de Lasa, Catalytic Steam Gasification of Biomass Surrogates: Thermodynamics and Effect of Operating Conditions, Chemical Engineering Journal (2016), doi: http://dx.doi.org/ 10.1016/j.cej.2016.02.034 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Catalytic Steam Gasification of Biomass Surrogates: Thermodynamics and Effect of Operating Conditions Jahirul Mazumder and Hugo I de Lasa* Chemical Reactor Engineering Centre, Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario N6A 5B9 ABSTRACT Thermodynamic chemical equilibrium of biomass steam gasification is considered using both stoichiometric and non-stoichiometric analyses These thermodynamic analyses include gaseous products, tars and coke, as well as consider a wide range of operating conditions It is shown that both stoichiometric and non-stoichiometric approaches provide close results Catalytic steam gasification of biomass surrogate species (glucose and 2-methoxy-4-methylphenol) is developed in a CREC Riser Simulator under the expected conditions of a circulating fluidized bed gasifier A highly active and stable fluidizable Ni/La2O3-γ-Al2O3 catalyst is employed in this study, to investigate the effects of gasifier operating conditions This catalyst yields 98% carbon conversion of glucose to permanent gases with no tar formation and negligible coke deposition at 700 °C Catalytic gasification results with the variation of temperature and steam/biomass ratio show limited deviation from equilibrium predictions The deviation between experimental results and equilibrium predictions can be attributed to the relatively shorter reaction time (20s) As the reaction time is increased, experimental results approach chemical equilibrium For 2-methoxy-4-methylphenol gasification, 90% carbon conversion is obtained with only 5.7 wt% tar formation and 3.3 wt% coke deposition Again chemical equilibrium predictions are close The only exceptions are: a) Coke and methane being overestimated and b) Tars being underestimated Regarding synthesis gas from glucose and 2-methoxy-4-methylphenol gasification, the Ni/La2O3-γ-Al2O3 catalyst yields a H2/CO ratio greater than 2.0 Keywords: steam gasification of biomass, La2O3 promoted Ni/γ-Al2O3 catalyst, thermodynamic equilibrium, gasifier operating conditions * Corresponding author at: Chemical Reactor Engineering Center, Faculty of Engineering, Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, ON, Canada N6A 5B9 Tel.: +1 519 661 2144;fax: +1 519 850 2931; e-mail: hdelasa@eng.uwo.ca 1.0 Introduction Biomass refers to organic materials derived from all living matter Biomass from plants was the first fuel used by humans to meet their energy demands In the 19th century, the discovery of fossil fuels helped to industrialize the world and improve the standard of living Nowadays, the gradual depletion of accessible fossil fuels and the increased concerns about climate change are stimulating a renewed interest in the efficient utilization of biomass as an energy source [1–4] Biomass is considered as the primary renewable resource given its abundance, its CO2 neutral emissions and its lower sulfur content, in order to gradually replace depleting fossil fuels Using biomass as an energy source, zero net emissions of carbon dioxide can be achieved with the released carbon dioxide being recycled back into plants via photosynthesis [4–9] Different biomass conversion processes are utilized to produce heat and electricity, as well as chemical feedstocks In particular, biomass steam gasification has great potential to produce synthesis gas with a high heating value and a high hydrogen content [1,2,4– 6,9–14] Steam gasification of biomass can be implemented in fluidized beds This is a promising technology given the favorable effect of either inert sand or solid catalysts particles to: (i) Enhance biomass heating, (ii) Favor very effective heat and mass transfer between reacting species, (iii) Provide a close to uniform temperature established throughout the gasifier [5,6,15,16] One can view biomass gasification as a combination of primary and secondary reactions [2,15,17,18] Primary reactions transform vaporized biomass chemical species forming permanent gases, higher hydrocarbons, coke and tars: heat Cx H y Oz + H2O →  H2 + CO+ CO2 + H2O + Cn H2m + C(s) + tars (1) Secondary reactions crack the higher hydrocarbons formed into gases These gases can either be combust or be reduced: C n H m + nH O  → nCO + ( n + m ) H (2) Furthermore, permanent gases react to alter the gas composition depending on gasifier conditions as indicated below: Chemical equation Water-Gas Shift ௢ ∆‫ܪ‬ଶଽ଼ (݇‫ܬ‬/݉‫ )݈݋‬1 CO + H O ←→ H + CO -41.16 (3) CH + H O ← → CO + 3H 205.81 (4) Dry Reforming of Methane CH + CO2 ← → 2CO + H 246.98 (5) Char Gasification C + H O ←→ H + CO 131.29 (6) Boudouard Reaction C + CO ←→ 2CO 172.46 (7) Hydrogenating Gasification C + H ←→ CH -74.52 (8) Steam Reforming of Methane standard heat of reaction at 298K were calculated from the standard heat of formation The development of an efficient steam gasification process requires insights into biomass gasification mechanisms in order to predict the end-reaction products To date, different types of models have been developed for biomass gasification Thermodynamic equilibrium models [4,19–25] provide valuable tools to predict the maximum achievable yield of hydrogen or syngas Most of the reported thermodynamic studies are on air gasification of biomass There is scarcity in comprehensive equilibrium models for biomass steam gasification Salaices et al [17] developed a thermodynamic model for biomass steam gasification at atmospheric pressure without accounting for coke and tar formation On the other hand, the use of an active and stable catalyst enables biomass gasification at a low temperature operating close to thermodynamic equilibrium Catalytic steam gasification of biomass facilitates tar reforming eliminating costly downstream processing [9,26,27] Furthermore, the conversion of tar adds value to the syngas by increasing the yields of H2 and/or CO [26,28,29] As a result, catalytic biomass steam gasification yields a higher energy efficiency process producing high quality and tar-free synthesis gas [1,9,18,26] Ni catalysts are a promising choice for biomass gasification due to their high reforming activity and affordability [30–34] Fluidizable γ-Al2O3 is one of the most promising supports for a Ni-based catalyst due to its surface area and mechanical strength Moreover, La2O3 has been used as a promoter of Ni/Al2O3 catalysts [33–39] Preparation conditions and formulations of Ni/La2O3-γ-Al2O3 catalysts for biomass steam gasification have been extensively investigated in our previous studies [40–42] In the present study, a thermodynamic analysis of biomass steam gasification is developed considering all gasification species, including both coke and tars The thermodynamic equilibrium predictions are compared with the catalytic gasification of two biomass surrogate species (glucose and 2-methoxy-4-methylphenol) using a Ni/La2O3-γ-Al2O3 catalyst [41] It is proven that chemical thermodynamic equilibrium displays close trends to the ones observed experimentally These results highlight the potential value of thermodynamics for performance evaluations of catalytic biomass gasifiers 2.0 Experimental Methods 2.1 Catalyst Preparation and Characterization γ-Al2O3 (Sasol Catalox® SSCa5/200) was used as a catalyst support La2O3 promoted Ni/γ-Al2O3 catalyst was prepared using a specially designed multi-step incipient wetness technique with the direct reduction of metal precursors after each impregnation in a fluidized bed [41] Three main steps were involved in the catalyst preparation: a) support impregnation, b) drying, and c) metal precursor reduction After the impregnation of the precursor solution, the resulting paste was dried overnight at 140 °C The dried powder was then reduced in a specially designed fluidized bed chamber at 700 °C (3 °C/min) for h under the flow of 10% H2-He It has been found in our earlier study [41] that a local bed temperature rise inside the catalyst reduction chamber can occur due to the exothermicity of the metal nitrate reduction process This increase in bed temperature can cause severe catalyst sintering via γ-Al2O3 dehydroxylation Therefore, the catalyst precursor reduction step has to be carefully considered to prevent temperature runaway Gas flow during catalyst reduction can control the rise of catalyst bed temperature and hence, it plays a major role in the resulting catalyst properties such as surface structure, acidity-basicity, metal dispersion and crystal size The effect of reducing gas flow rate on the catalyst properties and their performance for steam gasification of biomass surrogate species were discussed in details in our previous work [41] On this basis, a 20 wt% Ni/5wt% La2O3-γ-Al2O3 catalyst was prepared under bubbling fluidization in the reduction chamber by flowing 12 L/min of 10% H2-He gas for every mole of impregnated metal nitrate This catalyst, designated as HF, is considered in the present study Characterization results of the HF catalyst are summarized in Table Details of catalyst characterization techniques and results can be found in our previous publication [41] Table 1: Characterization results of HF catalyst SBET Pore volume 166 m2/g 0.32 cm3/g Avg proe diameter 76 Å Ni reducibility 95% 2.2 Ni dispersion 4.52% Total acidity 550 µmol NH3/g γ-Al2O3 Total basicity 188 µmol CO2/g γ-Al2O3 Gasification in a Minifluidized Bed CREC Riser Simulator Cellulose (23-50 wt %) and lignin (10-29 wt %) are the two major components of biomass Cellulose is a glucose polymer, consisting of linear chains of glucopyranose units On the other hand, the building blocks of lignin are believed to be a three carbon chain attached to rings of six carbon atoms, called phenyl-propanes [1] Based on these, glucose and 2-methoxy-4-methylphenol were selected as the surrogate species presenting the cellulose and lignin content of biomass, respectively Steam gasification of these biomass surrogates were performed under the expected reaction conditions of a twin circulating fluidized bed gasifier using a CREC Riser Simulator [43] The CREC Riser Simulator is a bench-scale mini fluidized bed reactor with a volume of 50 cm3 A schematic diagram of the experimental setup is given in Figure A detailed description of the experimental setup and gasification procedure can be found in our previous work [42] In case of catalytic gasification experiments, 0.25 g of catalyst was loaded into the catalyst basket After the reactor reached the specified gasification temperature, the solution of surrogate species was injected through the injection port using a syringe Once the reaction time was reached, gasification products were evacuated from the reactor to the vacuum box Reactor and vacuum box pressure data against time were recorded by the Personal Daq Acquisition Card Gasification products were analyzed using an online GC-MS system The permanent gases were analyzed by passing through a packed-bed column (HaysSep® D) and a thermal conductivity detector (TCD) A capillary column (BPX5 and a mass spectrometer (MS) were used to analyze the heavier compounds Figure 1: Schematic diagram of CREC Riser Simulator Gasification setup Gasification experiments were performed by varying the gasification temperatures (600, 650 and 700 °C), steam/biomass (S/B) ratios (0.4, 0.6, 0.8 and 1.0 g/g) and reaction times (10, 15, 20 and 30 s) As the H/C and O/C of 2-methoxy-4-methyphenol are much lower than those of glucose, a higher S/B ratio (1.5 g/g) was used in case of 2-methoxy-4- the produced gas decreased from 2.49 to 2.13 as the gasifier operation temperature was raised from 600 to 700 °C at a S/B =1.0 g/g Figure also reports that the HF catalyst can deliver a H2/CO ratio greater than 2.0 from glucose gasification in the range of 600 700 °C temperatures having a S/B ratio of 1.0 g/g 3.0 ο 600 C 650 οC 700 οC H2/CO ratio 2.5 2.0 1.5 1.0 0.5 0.2 0.4 0.6 0.8 1.0 1.2 S/B (g/g) Figure 8: H2/CO ratio of product gas from steam gasification of glucose using catalyst HF at different S/B ratios (at 650 °C) and different temperatures (at S/B=1.0 g/g) [Cat/B = 8.75, 10, 11.25 and 12.5 g/g for S/B = 0.4, 0.6, 0.8 and 1.0 g/g, respectively; 20 s reaction time] 26 4.2 Steam Gasification of 2-Methoxy-4-Methylphenol Figure reports the dry gas yield, the carbon conversion, the tar yield and the coke deposition resulting from the steam gasification of 2-methoxy-4-methylphenol Figure shows that experimental results for only 20 s of 2-methoxy-4-methylphenol gasification using HF catalyst are comparable with the gasification thermodynamics It can also be observed in Figure 9a and b, that both the dry gas yield and the carbon conversion to permanent gases are improved with the increase in temperature When the gasification temperature was augmented from 600 to 700 °C, the carbon conversion was increased by 10.25% yielding a 34% higher amount of dry gas Figure 9c and d show that a higher gasification temperature also results in reduced tar and coke or char formation In agreement with the technical literature [19,22,23,45], the equilibrium model developed in the present study also predicted negligible amounts of tar from the steam gasification of 2-methoxy-4-methylphenol (Figure 9c) It can also be observed in Figure 9b and d that, thermodynamically, a complete gasification of 2-methoxy-4-methylphenol can be achieved at 684 °C using a S/B ratio of 1.5 g/g This temperature is known as the carbon boundary point (CBP) Beyond this point, the coke or char yield becomes zero 27 C-Conversion (%) Dry gas yield (mol/mol biomass) 100 (b) 18 15 12 90 80 70 60 (a) 50 600 650 600 700 650 700 ο Temperature (oC) Temperature ( C) 15 15 10 Coke/C inj (%) Tars/biomass (wt %) (d) 10 (c) 600 650 Temperature (οC) 700 600 650 700 Temperature (oC) Figure 9: Profiles of (a) dry gas yield, (b) carbon conversion, (c) tar yield (wt% of biomass) and (d) coke yield (% of carbon injected); with the temperature from steam gasification of 2-methoxy-4-methylphenol using catalyst HF Lines represent the corresponding equilibrium data [S/B = 1.5 g/g; Cat/B = 12.5; 20 s reaction time] Figure 9a and b also report that the thermodynamic equilibrium model overpredicts the dry gas yield and carbon conversion in comparison to the experimental data On the other hand, one should note that a coke deposition less than the equilibrium prediction was obtained at 600 °C This can be attributed to the significant difference between experimental tar yields and equilibrium predictions (Figure 9c) As a result, the 28 thermodynamic model overestimates coke below the CBP to compensate for underestimated tar In any event, these modest deviations of experimental yields versus the best possible achievable yields as dictated by thermodynamics, are excellent indicators of superior performance of the prepared catalyst 0.6 (a) CO Mole Fraction H2 Mole Fraction 35 0.5 0.4 0.3 (b) 25 15 05 600 650 700 600 ο 700 Temperature ( C) 0.35 0.25 (d) CH4 Mole Fraction (c) CO Mole Fraction 650 ο Temperature ( C) 0.25 0.15 0.20 0.15 0.10 0.05 0.05 0.00 600 650 700 ο Temperature ( C) 600 650 700 ο Temperature ( C) Figure 10: Profiles of product gas compositions (dry basis) with temperature: (a) H2, (b) CO, (c) CO2 and (d) CH4; from steam gasification of 2-methoxy-4-methylphenol using catalyst HF Lines represent the corresponding equilibrium data [S/B = 1.5 g/g; Cat/B = 12.5; 20 s reaction time] 29 Figure 10 compares the experimental and equilibrium compositions of the dry gas produced at different temperatures It shows that the fractions of H2 and CO in the produced gas increased with the temperature due to the greater extent of hydrocarbon and tar reforming Figure 10d also reports that the thermodynamic model overpredicts CH4 composition as in the case of coke deposition This shows that at chemical equilibrium CH4 is more favourable than tar formation Figure 11 describes the H2/CO ratio of the product gas from the steam gasification of 2methoxy-4-methylphenol at different temperatures As in the case of glucose gasification, the H2/CO ratio was reduced at higher temperatures due to the decrease in the extent of exothermic water-gas shift reaction H2/CO ratio 600 650 700 ο Temperature ( C) 30 Figure 11: H2/CO ratio of the product gas with the variation of temperature from steam gasification of 2-methoxy-4-methylphenol using catalyst HF [S/B = 1.0 g/g; Cat/B = 12.5; 20 s reaction time] Thus, one can conclude that the HF catalyst of the present study considerably enhances the cellulose and lignin species surrogate gasification Its performance as shown in the experimental runs in the CREC Riser Simulator becomes very close to thermodynamic equilibrium 5.0 Conclusions The following are the main conclusions of this paper: a) Thermodynamic equilibrium for steam gasification of biomass is analyzed on the basis of stoichiometric and non-stoichiometric approaches These two approaches provide essentially the same results for a wide range of conditions Coke and tar compounds are included in the equilibrium calculations together with the permanent gases b) The thermodynamic models considered are very adequate for the constant volume CREC Riser Simulator of the present study This allows a rigorous comparison with experimental results c) The fluidizable La2O3 promoted Ni/γ-Al2O3 catalyst performs very close to the thermodynamic chemical equilibrium for gasification of biomass surrogate 31 species The experimentally observed gasification product compositions are in general agreement with thermodynamic model predictions Moreover, product composition approaches chemical equilibrium as the reaction time is increased d) This fluidizable La2O3 promoted Ni/γ-Al2O3 catalyst yields 100% glucose gasification without detectable tars formed In the case of 2-methoxy-4methylphenol gasification at 700 °C, the 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90% C-conversion with small tar and char Catalytic gasification displays close trends with the chemical equilibrium 40