Catalytic performance of Ni/CeO2/Al2O3 modified with noble metals in steam gasification of biomass

In the steam gasification of biomass, the additive effect of noble metals such as Pt, Pd, Rh and Ru to the Ni/CeO₂/Al₂O₃ catalyst was investigated. Among these noble metals, the addition of Pt was most effective even when the loading amount of added Pt was as small as 0.01 wt.%. In addition, the catalyst characterization suggests the formation of the Pt–Ni alloy over the Pt/Ni/CeO₂/Al₂O₃.     

Catalytic performance of supported Ni catalysts in partial oxidation and steam reforming of tar derived from the pyrolysis of wood biomass

Activity test of Ni/Al₂O₃, Ni/ZrO₂, Ni/TiO₂, Ni/CeO₂ and Ni/MgO catalysts in the partial oxidation (POT) and steam reforming of tar (SRT) derived from the pyrolysis of cedar wood was performed. In these activity tests, the order of the performance in both reactions was similar. Catalyst characterization was also carried out by means of H₂ adsorption, TPR and XRD. From the combination of catalyst characterization with the results of the activity tests, it is suggested that the conversion of tar in POT and SRT is mainly controlled by the number of surface Ni metal. In addition, Ni/CeO₂ showed smaller amount of coke than other catalysts in the POT and SRT. From the TGA profiles of active carbon mixed with catalysts, it is found that Ni/CeO₂ promoted the reaction of active carbon with O₂ and steam. The function of the fluidized bed reactor in the POT with respect to coke and tar amount was discussed.     

Ni nanoparticles supported on carbon as efficient catalysts for steam reforming of toluene (model tar)

This paper investigated the influences of surface properties of carbon support and nickel precursors (nickel nitrate,nickel chloride and nickel acetate) on Ni nanoparticle sizes and catalytic performances for steam reforming of toluene.Treatment with nitric acid helped to increase the amount of functional groups on the surface and hydrophilic nature of carbon support,leading to a homogeneous distribution of Ni nanoparticles.The thermal decomposition products of nickel precursor also played an important role,Ni nanoparticles supported on carbon treated with acid using nickel nitrate as the precursor exhibited the smallest mean diameter of 4.5 nm.With the loading amount increased from 6 wt％ to 18 wt％,the mean particle size of Ni nanoparticles varied from 4.5 nm to 9.1 nm.The as-prepared catalyst showed a high catalytic activity and a good stability for toluene steam reforming:98.1％ conversion of toluene was obtained with the Ni content of 12 wt％ and the S/C ratio of 3,and the conversion only decreased to 92.0％ after 700 min.Because of the high activity,good stability,and low cost,the as-prepared catalyst opens up new opportunities for tar removing.     

Development of Ni catalysts for tar removal by steam gasification of biomass

Catalytic performance of Ni/CeO₂/Al₂O₃ catalysts prepared by a co-impregnation and a sequential impregnation method in steam gasification of real biomass (cedar wood) was investigated. Especially, Ni/CeO₂/Al₂O₃ catalysts prepared by the co-impregnation method exhibited higher performance than Ni/Al₂O₃ and Ni/CeO₂/Al₂O₃ prepared by the sequential impregnation method, and the catalysts gave lower yields of coke and tar, and higher yields of gaseous products. The Ni/CeO₂/Al₂O₃ catalysts were characterized by thermogravimetric analysis, temperature-programmed reduction with H₂, transmission electron microscopy and extended X-ray absorption fine structure, and the results suggested that the interaction between Ni and CeO₂ became stronger by the co-impregnation method than that by sequential method. Judging from both results of catalytic performance and catalyst characterization, it is found that the intimate interaction between Ni and CeO₂ can play very important role on the steam gasification of biomass.     

Performance of a catalytically activated ceramic hot gas filter for catalytic tar removal from biomass gasification gas

Silicon carbide-based filter elements were catalytically activated to provide filter elements for catalytic tar removal from biomass-derived syngas. The filter element support was coated with CeO₂, CaO–Al₂O₃ and MgO with a specific surface of 7.4, 15.9 and 21.9 m²/g synthesized by exo-templating with activated carbon. Doping of a MgO coated filter element with 60 wt% NiO has led to an increase of the specific surface from 0.15 to 0.21 m²/g, whereas in case of a MgO–Al₂O₃ coated filter element a decrease from 1.18 to 0.91 m²/g was found. An increase of the NiO loading from 6 to 60 wt% on a MgO coated filter element resulted in an increase of the naphthalene conversion from 91 to 100% at 800 °C and a face velocity of 2.5 cm/s at a naphthalene concentration of 5 g/Nm³ in model biomass gasification gas. In case of a MgO–Al₂O₃ coated filter element with 60 wt% NiO in addition to complete naphthalene conversion in the absence of H₂S, a higher conversion of 66% was found in the presence of 100 ppmv H₂S compared to 49% of the MgO–NiO coated filter element. After scaling up of the catalytic activation procedure to a 1520 mm long filter candle, which shows an acceptable differential pressure of 54.9 mbar, 58 and 97% naphthalene conversion was achieved in the presence and absence of H₂S, respectively. The calculated WHSV value of 209.6 Nm³ h⁻¹ kg⁻¹ indicates the technical feasibility of a further increase of the catalytic performance by an increase of the NiO loading.     

Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification

Biomass gasification produces a low to medium-BTU product gas (or syngas) containing primarily CO₂, H₂, CO, CH₄ and (C₂ + C₃), as well as some contaminants such as tars, NH₃, H₂S and SO₂. In order to achieve better efficiencies of the syngas applications, these contaminants must be removed before the syngas is used for internal combustion, gas engines, and in particular for fuel cells and methanol synthesis. Compared with the wet scrubbing technology, hot-gas cleanup technology to remove tar, ammonia and other contaminants at the “hot” state is more advantageous with respect to energy efficiencies. This paper provides an overview on recent advances in catalysts for hot-gas removal of tar and ammonia from biomass gasification. The review focuses on the recent development and applications of dolomite catalysts, iron-based catalysts, nickel and other metal supported catalysts, and the novel carbon-supported catalysts for hot-gas tar removal and ammonia decomposition. The barriers in applications of hot-gas cleanup processes and catalysts for full-scale biomass gasification, and areas for future research, are also discussed.     

Hydrogen production by the partial oxidation and steam reforming of tar from hot coke oven gas

Hot coke oven gas (COG) with a temperature of about 1050 K was produced from a test unit for coke production, the capacity of which was 80 kg of coal. The COG was introduced into an experimental unit with a tar converter where oxygen and steam were injected. Over 98% of the total carbon in the hot COG was partially oxidized, reformed with steam and converted to hydrogen and CO. About 1 Nm³/h of hydrogen was continuously produced for 5 h in this experiment. The experimental results suggest that three to five times the amount of hydrogen and CO that were present in the original COG could be recovered by this technology, utilizing the heat of the hot COG for the reaction. The feasibility study showed that hydrogen can be produced by this technology at a lower cost and higher efficiency than by the separation of cold COG.     

Effects of controlling parameters on production of hydrogen by catalytic steam gasification of biomass at low temperatures

Low temperature catalytic steam gasification of biomass is being investigated around the world as an environmentally friendly alternative to the existing techniques for hydrogen production. The aim of the present investigation was to gain a fundamental understanding about the catalytic steam gasification of some species under low temperature conditions. The research, in particular, focused on the role and relative importance of controlling parameters, such as reaction temperature and the heating rate on the composition of the products. It was found that higher temperatures and steam flow rates increased the gas yield. A relatively low reaction temperature of 600 °C and a high steam content of about 90% showed the strongest tendency for maximising the hydrogen production.     

Development of new nickel based catalyst for biomass tar steam reforming producing H2-rich syngas

Mayenite (Ca₁₂Al₁₄O₃₃ or 12CaO.7Al₂O₃) was previously developed and applied as Ni support for biomass tar steam reforming in the absence and presence of H₂S by our group because of its high oxygen restoring property in the structure [C. Li et al., Appl. Catal., B. 2008]. In this study, catalyst Ni/mayenite (mayenite as support) was prepared by impregnation method with nickel nitrate hexahydrate. Experiments were tested in a fixed-bed reactor, toluene as a tar model compound. The influence of the catalyst preparation and operating parameters (reaction temperature, steam to carbon ratio and space time) on catalyst activity and products selectivity were studied, and a long-time evaluation (more than 76 h) also exhibited excellent resistance to coking. These results were compared to these obtained by commercial-like catalysts: Ni/CaO_x/MgO_1−x and our previous NiO/mayenite, showing that Ni/mayenite exhibited excellent property for biomass tar reforming, with higher H₂ yield than that of Ni/CaO_x/MgO_1−x, and higher CO selectivity than that of NiO/mayenite. For kinetic model, the first order reaction used for toluene with activation energy of 80.24 kJ.mol^− 1 was coincident with literature data.     

Catalytic reforming of tar during gasification. Part I. Steam reforming of biomass tar using ilmenite as a catalyst

Ilmenite, a natural iron-containing mineral, has been investigated as an inexpensive catalyst for the steam reforming of volatiles (tar) from the pyrolysis of mallee woody biomass. The results indicate that ilmenite has good activity for the steam reforming of tar into gases due to its highly dispersed iron-containing species. The supply of external steam, in addition to the H₂O and CO₂ produced during the pyrolysis of biomass, plays an important role in minimising the formation of coke on the catalyst surface and thus the catalyst activity. The catalyst deactivation due to coke formation has more adverse effects on the reforming of larger aromatic ring system with steam than that of smaller ones. In addition, the supply of additional oxygen at low concentration changed the outcomes of tar reforming mainly because oxygen activated the smaller aromatic ring systems and polymerised them into larger aromatic ring systems in the gas phase.     

Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming

Char, char-supported catalysts and ilmenite were investigated for the steam reforming of biomass tar derived from the pyrolysis of mallee wood in situ. Special attention was given to the reforming of aromatic ring systems in tar. The results indicated that the char-supported iron/nickel catalysts exhibited much higher activity for the reforming of tar than the char itself. Ilmenite and the char-supported iron catalyst contained similar active phase but showed different tar reforming activities. Kinetic compensation effects demonstrated that the reaction pathways on the char-supported catalysts were similar but were different from those on ilmenite. The proprieties of support could play important roles for the activities of the catalysts and the reaction pathways on the catalysts. Char would not only disperse the catalysts but also interact with the catalysts to enhance their activity for the steam reforming of tar.     

Catalytic reforming of model tar compounds from hot coke oven gas with low steam/carbon ratio over Ni/MgO-Al2O3 catalysts

The catalytic reforming of toluene and naphthalene was performed to investigate the possibility for directly converting tar components from hot coke oven gas (COG) with lower steam/carbon (S/C) molar ratios to light fuel gases. The NiO/MgO-Al₂O₃ catalysts reduced exhibited excellent catalytic activity, stability and sulphur tolerance. The effects of various reaction conditions and S/C ratios on the catalytic performance were investigated in detail. Toluene and naphthalene were completely converted into small gas molecules at 700-800 °C and S/C = 0.28. An appropriate amount of steam benefited the methanation reaction of CO and H₂. The effects of N₂, CH₄ or CO in COG were also discussed. Relative to N₂, CO contributed to the conversion of toluene and the formation of CH₄, but the opposite was true for CH₄. The sulphur tolerance was tested by adding H₂S in the feed gas. The reaction results were explained by a water cycle mechanism.     

The effect of steam on pyrolysis and char reactions behavior during rice straw gasification

Steam gasification of biomass can generate hydrogen-rich, medium heating value gas. We investigated pyrolysis and char reaction behavior during biomass gasification in detail to clarify the effect of steam presence. Rice straw was gasified in a laboratory scale, batch-type gasification reactor. Time-series data for the yields and compositions of gas, tar and char were examined under inert and steam atmosphere at the temperature range of 873-1173 K. Obtained experimental results were categorized into those of pyrolysis stage and char reaction stage. At the pyrolysis stage, low H₂, CO and aromatic tar yields were observed under steam atmosphere while total tar yield increased by steam. This result can be interpreted as the dominant, but incomplete steam reforming reactions of primary tar under steam atmosphere. During the char reaction stage, only H₂ and CO₂ were detected, which were originated from carbonization of char and char gasification with steam (C + H₂O→CO + H₂). It implies the catalytic effect of char on the water-gas shift reaction. Acceleration of char carbonization by steam was implied by faster hydrogen loss from solid residue.     

H2 rich syngas by selective CO2 removal from biomass gasification in a dual fluidized bed system — Process modelling approach

A process model of dual fluidized bed gasification is presented based on mass- and energy balances. The model further covers the evaluation of thermodynamic equilibrium states. The gasification is investigated for the special case that CaO/CaCO₃ is used as bed material allowing selective transport of CO₂ from the gasification reactor to the combustion reactor by repeated carbonation and calcination. Experimental data are used to determine the model parameters. An empirical approach towards the kinetics of fuel conversion allows prediction of process behaviour at varied fuel water content. The selective transport of CO₂ results in high H₂ contents in the produced syngas. The lower operating temperatures in the gasification reactor increase the efficiency of energy conversion. The results are in agreement with experimental data and show the thermodynamic limitations of the technology.     

Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound

A Ni/olivine catalyst, previously developed for biomass gasification and tar removal during fluidized bed steam gasification of biomass, was tested in a fixed bed reactor in toluene steam reforming as a tar destruction model reaction. The influence of the catalyst preparation parameters (nickel precursor, calcination temperature and nickel content) and operating parameters (reaction temperature, steam to carbon S/C ratio and space-time) on activity and selectivity was examined showing a high toluene conversion and a low carbon formation compared to olivine alone. The steam reforming of toluene was found to be of zero order for water and first order for toluene. Activation energy required for Ni/olivine was determined to be about 196 kJ mol⁻¹ in accordance with literature. Catalyst activity and stability and its resistance against carbon formation were discussed on the basis of X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed oxidation (TPO) results. Characterization before test (XRD, temperature programmed reduction (TPR), Mössbauer spectroscopy) have shown the presence of NiO–MgO solid solution, formed on the surface of olivine support, which explains the efficiency of the catalyst calcined at 1100 °C. After test, Ni–Fe alloys were observed (TEM, Mössbauer spectroscopy). It was suggested that magnesium oxide enhanced steam adsorption, facilitating the gasification of surface carbon and that Ni–Fe alloys prevented carbon deposition by dilution effect.     

Simulated biomass tar removal mechanism by a Quench Coupled with ADsorption Technology (QCADT)

Tar removal is a bottleneck in the smooth commercialization of biomass gasification technology. Based on introducing adsorption process into Quench Coupled with ABsorption Technology (QCABT) previously proposed by the author's group, Quench Coupled with ADsorption Technology (QCADT) has been developed to narrow this gap. Additionally, benzene and naphthalene, which are more similar to the real tar for containing aromatic ring structures, were adopted as light and heavy simulated tar, respectively. Also their removal behavior by QCADT was investigated. The results show that the removal mechanism of QCADT is similar to that of QCABT, except for the higher overall tar removal rate due to adsorption effect. Adsorbents with both micro- and narrow mesopores exhibit a better benzene removal performance, while narrow mesopores play dominant roles in naphthalene removal. Penetration adsorption loading of benzene and naphthalene on AC-1 can reach 0.38 g·g^-1 and 0.34 g·g^-1, respectively. The sawdust hardly has any tar removal effect. Combined micro- and meso-pores, will benefit both deep tar removal and large adsorption rate, providing a high tar removal efficiency.     

Hydrogen from biomass-production by steam reforming of biomass pyrolysis oil

Thermo-conversion of biomass is one of the leading near-term options for renewable production of hydrogen and has the potential to provide a significant fraction of transportation fuel required in the future. We propose a two-step process that starts with fast pyrolysis of biomass, which generates high yields of a liquid product, bio-oil, followed by catalytic steam reforming of bio-oil to produce hydrogen. A major advantage of such a concept results from the fact that bio-oil is much easier and less expensive to transport than either biomass or hydrogen. Therefore, the processing of biomass and the production of hydrogen can be performed at separate locations, optimized with respect to feedstock supply and to hydrogen distribution infrastructure. This approach makes the process very well suited for both centralized and distributed hydrogen production. This work demonstrates reforming of bio-oil in a bench-scale fluidized bed system and provides hydrogen yields obtained using several commercial and custom-made catalysts.     

Solid oxide fuel cells in combination with biomass gasification for electric power generation

Biomass, a source of renewable energy, represents an effective substitute to fossil fuels. Gasification is a process that organics are thermochemically converted into valuable gaseous products(e.g. biogas). In this work, the catalytic test demonstrated that the biogas produced from biomass gasification mainly consists of H_2,CH_4, CO,and CO_2, which were then be used as the fuel for solid oxide fuel cell(SOFC). Planar SOFCs were fabricated and adopted. The steam reforming of biogas was carried out at the anode of a SOFC to obtain a hydrogen-rich fuel.The performance of the SOFCs operating on generated biogas was investigated by I–V polarization and electrochemical impedance spectra characterizations. An excellent cell performance was obtained, for example,the peak power density of the SOFC reached 1391 mW·cm~(-2) at 750℃ when the generated biogas was used as the fuel. Furthermore, the SOFC fuelled by simulated biogas delivered a very stable operation.     

Experimental comparison of biomass chars with other catalysts for tar reduction

In this paper the potential of using biomass char as a catalyst for tar reduction is discussed. Biomass char is compared with other known catalysts used for tar conversion. Model tar compounds, phenol and naphthalene, were used to test char and other catalysts. Tests were carried out in a fixed bed tubular reactor at a temperature range of 700–900 °C under atmospheric pressure and a gas residence time in the empty catalyst bed of 0.3 s. Biomass chars are compared with calcined dolomite, olivine, used fluid catalytic cracking (FCC) catalyst, biomass ash and commercial nickel catalyst. The conversion of naphthalene and phenol over these catalysts was carried out in the atmosphere of CO₂ and steam. At 900 °C, the conversion of phenol was dominated by thermal cracking whereas naphthalene conversion was dominated by catalytic conversion. Biomass chars gave the highest naphthalene conversion among the low cost catalysts used for tar removal. Further, biomass char is produced continuously during the gasification process, while the other catalysts undergo deactivation. A simple first order kinetic model is used to describe the naphthalene conversion with biomass char.     

Ultrahigh temperature water gas shift catalysts to increase hydrogen yield from biomass gasification

Noble metal (Rh, Pt, Pd, Ir, Ru, and Ag) and Ni catalysts supported on CeO₂–Al₂O₃ were investigated for water gas shift reaction at ultrahigh temperatures. Pt/CeO₂–Al₂O₃ and Ru/CeO₂–Al₂O₃ demonstrated as the best catalysts in terms of activity, hydrogen yield and hydrogen selectivity. At 700 °C and steam to CO ratio of 5.2:1, Pt/CeO₂–Al₂O₃ converted 76.3% of CO with 94.7% of hydrogen selectivity. At the same conditions, the activity and hydrogen selectivity for Ru/CeO₂–Al₂O₃ were 63.9% and 85.6%, respectively. Both catalysts showed a good stability over 9 h of continuous operation. However, both catalysts showed slight deactivation during the test period. The study revealed that Pt/CeO₂–Al₂O₃ and Ru/CeO₂–Al₂O₃ were excellent ultrahigh temperature water gas shift catalysts, which can be coupled with biomass gasification in a downstream reactor.