Steam reforming of naphthalene as model biomass tar over iron-aluminum and iron-zirconium oxide catalyst catalysts

In this study the catalytic properties of iron-based mixed metal oxides such as iron-alumina (Fe-Al) and iron-zirconia (Fe-Zr) were investigated at 850 °C in a fixed bed reactor for the steam reforming of naphthalene as a model biomass tar compound. The effects of addition of copper species (CuO) to the iron-based mixed metal oxide catalysts were also examined. For Fe-Al catalysts, the catalytic activities for naphthalene conversion increased with increasing Fe content except for 100Fe-0Al. The catalytic activities of Fe-Al and Fe-Zr were comparable at steady state conditions. Compound oxides were formed in the cases of Fe-Al, but not in Fe-Zr. A strong peak in the vicinity of 2θ = 45° for metallic iron was observed after catalytic experiments in the XRD patterns of all catalysts, which could be related to the active sites of the catalysts. The addition of CuO increased the activities and stability of the Fe-Al catalysts. The reasons for catalytic activity enhancement due to CuO addition can be explained as follows: copper dispersed evenly in the compound oxides facilitate the reduction of iron oxides to metallic iron and prevent the catalytic deactivation due to decrease in surface area of the catalysts during the reaction.     

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.     

Zirconia: Selective oxidation catalyst for removal of tar and ammonia from biomass gasification gas

Catalysts containing zirconia and alumina were tested for their activity in the selective oxidation of tar and ammonia in biomass gasification gas. Their performance was compared with that of nickel and dolomite catalysts. Synthetic gasification gas with toluene as tar model compound was used as feed. In the presence of oxygen, zirconia and alumina-doped zirconia gave high toluene and ammonia conversions even below 600 °C. They were the most active catalysts for toluene oxidation below 700 °C and for ammonia oxidation below 650 °C. At higher temperatures than these, the impregnated ZrO₂/Al₂O₃ catalysts performed better: oxidation selectivity was improved and toluene and ammonia conversions were higher. The presence of both zirconia and alumina in the catalyst promoted toluene and ammonia conversions at low temperatures: zirconia enhanced the oxidation activity, while alumina improved the oxidation selectivity. The presence of H₂S had little effect on the activity of alumina-doped zirconia.     

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.     

Development of cobalt catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification

The catalytic performances of Co/MgO catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, CO adsorption, and CO-TPD) showed that large-sized Co metal particles were formed over the catalysts pre-calcined at 873 K with high Co loading via reduction of Co₃O₄ and MgCo₂O₄ phases. A few Co metal particles were obtained over the catalysts pre-calcined at 1173 K with all Co loading values after reduction.The catalytic performances data showed that 12 wt.% Co/MgO catalyst pre-calcined at 873 K exhibited the best catalytic performance (conv., 23%, 3 h) for the steam reforming of naphthalene among the catalysts tested in this study, due to the existence of Co metal and the low amounts of coke deposition. On the other hand, the data also revealed that the reaction of steam reforming of naphthalene proceeds over all Co-loaded catalyst pre-calcined at 1173 K initially; however, the deposition of the polymer of C_nH_m radicals and the oxidation of catalysts by H₂O led to the decrease of activity.It should be noted that 12 wt.% Co/MgO catalyst pre-calcined at 873 K showed high and stable activity under the low steam/carbon mole ratio (0.6), with H₂ and CO₂ as main products. These two excellent advantages serve to increase the overall biomass gasification system energy efficiency and allow using the product gas for fuel cell system. Thus, Co catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed.     

Partial oxidation reforming of biomass fuel gas over nickel-based monolithic catalyst with naphthalene as model compound

With naphthalene as biomass tar model compound, partial oxidation reforming (with addition of O₂) and dry reforming of biomass fuel gas were investigated over nickel-based monoliths at the same conditions. The results showed that both processes had excellent performance in upgrading biomass raw fuel gas. Above 99% of naphthalene was converted into synthesis gases (H₂+CO). About 2.8 wt% of coke deposition was detected on the catalyst surface for dry reforming process at 750 °C during 108 h lifetime test. However, no coke deposition was detected for partial oxidation reforming process, which indicated that addition of O₂ can effectively prohibit the coke formation. O₂ can also increase the CH₄ conversion and H₂/CO ratio of the producer gas. The average conversion of CH₄ in dry and partial oxidation reforming process was 92% and 95%, respectively. The average H₂/CO ratio increased from 0.95 to 1.1 with the addition of O₂, which was suitable to be used as synthesis gas for dimethyl ether (DME) synthesis.     

生物质气化焦油裂解用颗粒催化剂的再生性能

为开发适用于生物质气化焦油裂解的多孔白云石颗粒催化剂,以乙酸为生物质气化焦油的模型化合物,讨论了积炭对多孔白云石颗粒催化能力以及再生性能的影响。结果表明,在积炭率0～4.4%时,多孔白云石颗粒的催化能力随积炭率呈线性下降。当积炭率从0增加到4.4%时,颗粒的孔隙率从0.74 cm³·g^-1减小到0.48 cm³·g^-1,乙酸裂解率从99.3%降至32.3%。采用900 ℃和2 h焙烧法对积炭多孔白云石颗粒进行再生处理后,多孔白云石颗粒的强度基本不变,孔隙率增至0.55 cm³·g^-1。在再生多孔白云石颗粒上,乙酸裂解率达到99.5%。多孔白云石颗粒催化剂具有良好的再生性。     

Ni基生物质焦油重整催化剂的研究进展

Ni基催化剂对生物质气化过程中生成焦油的催化裂解重整具有较好的催化活性,但在催化反应中存在容易积炭进而失活的现象。如何提高Ni基催化剂的催化活性和抗积炭能力是Ni基催化剂研究中的难点。本文从催化剂的活性组分、载体和助催化剂3个方面入手,详细综述了近几年关于提高Ni基生物质焦油重整催化剂催化活性和抗积炭能力的研究进展,讨论了催化剂的活性组分、载体和助催化剂对催化剂催化活性的影响。指出Ni-Fe、Ni-Co、Ni-Cu催化剂均表现出比Ni基催化剂更好的催化活性,助剂的加入有利于提高吸附剂的抗积炭能力和抗烧结能力;催化剂载体方面,钙钛矿型混合金属氧化物载体、煤焦和生物质焦载体具有较好的研究前景。     

Experimental and kinetic study of the NO-reduction by tar formed from biomass gasification, using benzene as a tar model component

The present work studies the combustion of biomass syngas to characterize the NO-reduction by tar, benzene being selected as the representative model tar component. Experiments were carried out in a tubular flow reactor at atmospheric pressure and at different operating conditions i.e. an equivalence ratio of 0.5 to 2.5, temperatures between 1173 K and 1673 K and a reaction residence time of 50 to 100 ms. Kinetic parameters were determined from the experimental results. A simplified scheme of NO-reduction by benzene was presented and the effects of operating variables were concluded. The NO conversion is favored by an oxygen-rich condition, and the reduction efficiency falls with the rise of equivalence ratio, especially in the range of 0.5-1.0. Although high temperature enhances the reduction reactions, soot formation at high temperatures (1573 K, 1673 K) will seriously hinder the reduction of NO, which should be prevented. The ideal temperature is about 1473 K. 50 ms is found to be an insufficient residence time for the reduction. An increased contact time is of higher benefit at 1673 K where the heterogeneous reactions between NO and graphitized heavy hydrocarbons have a lower reaction rate. High pre-exponential factors and low activation energies are achieved under oxygen-rich conditions.     

Modeling circulating fluidized bed biomass gasifiers. A pseudo-rigorous model for stationary state

A 1-dimensional model for an atmospheric circulating fluidized bed biomass gasifier (CFBBG) under stationary state is presented in this paper. The model is based on the kinetic equations for the reaction network solved together with mass and heat balances and with several hydrodynamic considerations. Kinetics used include both our own kinetic data and published equations with some corrective factors. The reaction network used involves twelve different reactions. A sub-model for the tar generation-elimination in the CFBBG is included in the whole model. The model has an academic structure, but several assumptions were made because of lack of accurate data in some areas. The overall model has some empirical aspects and can therefore be considered as semi-rigorous. Hydrodynamics in the model were checked with a survey carried out worldwide among the existing pilot and commercial CFBBGs. The axial profiles of concentration of ten different species (H₂, CO, CO₂, tar, char, …) and temperature can be calculated with this model which was conceived to optimize both design and operation of CFBBGs.     

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.     

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.     

Comparison of Co/MgO and Ni/MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification

The catalytic performances of 12 wt.% Co/MgO catalyst pre-calcined at 873 K and of Ni catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, and CO adsorption) for Ni catalysts showed that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases. A few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K with all Ni loading values.The catalytic performance data showed that Co/MgO catalyst had higher activity (conv., 23%, 3 h) than any kinds of Ni/MgO catalysts tested in this study, under lower steam/carbon mole ratio (0.6) and higher concentration of fed naphthalene (3.5 mol%) than those used in the other works. The steam reforming of naphthalene proceeded when there was a stoichiometric ratio between the carbon atoms of naphthalene and H₂O over Co catalyst; however, the activation of excess H₂O happened over the Ni catalyst and this phenomenon can lead to having lower activity than Co catalyst. We concluded that these observations should be attributed to different catalytic performances between Co/MgO and Ni/MgO catalysts.     

固体超强酸Cl-/Fe2O3的制备及催化液化生物质

通过水热合法制备了固体超强酸催化剂Cl—/Fe2O3,并将其应用于稻壳,毛竹和玉米秸秆等生物质的催化液化。考察了催化剂用量、反应温度、反应时间等对催化剂性能的影响,试验表明在300℃、反应时间3 h、催化剂的用量为原料质量分数的4%时,可以获得较好的液化效果。以玉米秸秆为原料的催化剂寿命试验结果还表明,催化剂可以多次重复使用。     

Metallic iron as a tar breakdown catalyst related to atmospheric,fluidised bed gasification of biomass

Tar formation is a major drawback when biomass is converted in a gasifier to obtain gas aimed for utilisation in power production plants or for production of chemicals. Catalytic cracking is an efficient method to diminish the tar content in the gas mixture. In this study, the capability of metallic iron and iron oxides to catalytically crack tars has been experimentally examined. To obtain metallic iron, small grains of hematite (Fe₂O₃) were placed in a secondary reactor downstream the gasifier and reduced in situ prior to catalytic operation. The fuel used in the atmospheric fluidised bed gasifier was Swedish birch with a moisture content of approximately 7 wt%.The influence of temperature in the range 700–900 °C and λ values (i.e. equivalence ratio, ER) between 0 and 0.20 have been investigated. In essence, the results show that raising the temperature in the catalytic bed to approximately 900 °C yields almost 100% tar breakdown. Moreover, increasing the λ value also improves the overall tar cracking activity. The iron oxides did not demonstrate any catalytic activity.     

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.     

Modeling circulating fluidized bed biomass gasifiers. Results from a pseudo-rigorous 1-dimensional model for stationary state

Results from a 1-dimensional and semirigorous model for atmospheric and circulating fluidized bed biomass gasifiers (CFBBGs), presented in the (previous) paper by Corella and Sanz [J. Corella, A. Sanz, Modeling circulating fluidized bed biomass gasifiers. A pseudo-rigorous model for stationary state. Fuel Process. Technol. 86 (2005) 1021–1053], are shown here. Process variables predicted by the model are gas composition (H₂, CO, CO₂, CH₄, C₂H_n, H₂O and O₂ contents), gas yield, tar content in the flue gas and char concentration in the solids. Both axial profiles in the riser and values at the gasifier exit are calculated from the model and are shown here for some selected sets of process variables. Variables analyzed in depth are: total air flow (used as equivalence ratio, ER), percentage of secondary air flow, height (location) of the secondary air flow, biomass moisture and biomass flow rate, expressed as the biomass weight hourly space velocity in the gasifier. All the results from the model agree both with known published data and with some tests made to check the model.     

旋转滑动弧降解垃圾气化焦油组分中的萘

采用新型磁场和气流协同驱动旋转滑动弧等离子体,选取萘作为垃圾气化焦油模拟组分,在氮气气氛下开展了焦油裂解初步实验研究。重点考察了进样浓度、进气流量和预热温度对萘裂解效果及气体产物的影响;此外,采用气相色谱质谱联用仪（GC/MS）对液体副产物进行了表征。结果表明,当进气流量恒定时,随着进样浓度的提高,萘的降解率先升后降,在萘浓度为6mg/L时降解率达到最大值88.3%;随着进气流量从2L/min增加到12L/min,萘的降解率持续下降,由92.1%降至82.5%;提高预热温度可促进萘的降解。实验产生的主要气体产物为H₂和C₂H₂,其选择性与萘的降解率变化趋势基本一致,最高分别为44.0%和13.7%;液体副产物主要有苯乙炔、茚、苊烯等,其GC/MS峰面积比萘低2~3个数量级;在此基础上,对滑动弧反应区域中萘的降解路径及机理进行了初步探讨与分析。     

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.     

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.