Catalytic biomass gasification: Simultaneous hydrocarbons steam reforming and CO2 capture in a fluidised bed reactor

https://doi.org/10.1016/j.cej.2009.04.054Get rights and content

Abstract

Tars and CH4 generated from biomass gasification processes contribute significantly to the energy content of the producer gas: catalytic tar and CH4 steam reforming allows to clean the gaseous fuel and improve the H2 yield; in addition, the use of a CO2 sorbent minimises carbon oxides. As a result of the whole process, a H2 rich fuel gas may be obtained. This experimental work is addressed to study the practical feasibility of such concepts, choosing CH4, toluene and 1-methyl naphthalene (1-MN) as biomass gasification key primary products. Ni is used as a catalyst for steam reforming, and dolomite as a sorbent for CO2 capture. Two kinds of catalytic systems are tested as bed material: a mixture of dolomite and commercial nickel catalyst, and a new Ni/dolomite combined catalyst and sorbent. The experimental investigations have been carried out in a fixed bed microreactor and a bench scale fluidised bed reactor rig. Both combinations of catalyst and sorbent are found to be very effective in tar removing, with conversion values near to 100% for the compounds tested; simultaneous CO2 sorption reveals itself as the key process step, improving significantly the performance of the catalytic system that may then decrease considerably after sorbent saturation.

Introduction

It is well known that biomass is one of the most important primary and renewable energy sources, with a neutral balance in the carbon dioxide cycle. In order to obtain a readily usable energy vector from biomass, gasification is the conversion process closest to industrial exploitation [1]. Biomass gasification is a thermo-chemical conversion process utilizing air, oxygen and/or steam as gasification agents, which produces a fuel gas rich in hydrogen and carbon monoxide, with a significant content of methane and carbon dioxide. Steam and nitrogen are also present in the producer gas, in addition to organic (tar) and inorganic (H2S, HCl, NH3, alkali metals) impurities, and particulate. High molecular weight hydrocarbons (tar) are an undesirable and noxious by-product, with concentration ranging from 5 to 100 g/N m3 of the producer gas in fluidised bed gasifiers. Tar is a complex mixture of cyclic and polycyclic aromatic hydrocarbons [2] very harmful for toxicity and cancerous properties. Moreover, corrosive and pollutant characteristics of tar compounds prohibit direct utilization of the gas product stream. Catalytic steam reforming seems to be the best way to eliminate tar compounds, converting them into syngas, and thus recovering their energy content. Steam reforming of model tarry compounds as well as toluene [3], [4], [5], [6], phenol [7], [8], naphthalene [7], [9], [5] and 1-methyl naphthalene [10] has been yet studied in literature, but further investigations are necessary for a successful application of biomass-derived producer gas [11]. It is also of practical interest to maximise the H2 content in the fuel gas by simultaneous steam reforming of its CH4 fraction.

Steam gasification processes are able to convert the chemical energy of biomass into a hydrogen-rich syngas containing up to about 50% by volume of hydrogen on dry basis [12]. Nowadays considerable interest is focussed on a pure-hydrogen energy vector, and biomass, is the only feedstock that could assure H2 production in a sustainable way, without consumption of fossil fuels. In this respect, the steam gasification process appears as an optimum candidate, provided that a reliable and economically convenient process is developed to extract hydrogen from the producer gas. This can be obtained either by the utilization of selective membranes permeable to hydrogen small molecules, or by capture of carbon-containing gas components by means of an appropriate sorbent [13]. Both topics, steam reforming of hydrocarbons (methane and tar) and carbon dioxide capture, are therefore relevant to enhance the applicability of biomass gasification processes and to render them suitable for renewable hydrogen production. They are addressed in an innovative, unified perspective in this paper.

Recently, in the literature new developments have been reported to run a gasification process including CO2 capture [14]. It has been proposed to add a CO2 sorbent (a natural mineral substance, such as limestone or dolomite) to the FICFB (fast internally circulating fluidised bed [15]) reactor bed inventory. The sorbent circulates between a gasifier – CO2 capture bubbling bed, and a combustor – calciner riser, in order to run the whole process continuously.

In the reactor chamber devoted to biomass gasification and CO2 capture, the endothermic gasification and the exothermic solid carbonation processes combine well together and their coupling reduces the amount of the solid circulation rate required to sustain thermally the devolatilization and gasification reactions. On the other hand, the riser provides the calcined solid sorbent and the thermal loading, by combustion of residual char (and/or additional fuel). When this is performed utilizing pure oxygen, a CO2 stream is easily obtained (by steam condensation), available for storage and sequestration.

The thermodynamic constraints of the reaction between CO2 and CaO impose, at ambient pressure, a temperature level for gasification somewhat lower (650–700 °C) than the usual one (800–850 °C), as it is clear from the equilibrium equation [16], [17] (Eq. (1)).PCO2eq(atm)=4.137×107×exp20,474T(K)where PCO2eq(atm) is the equilibrium CO2 pressure expressed in atmospheres, and T (K) the system temperature expressed in K.

However, the experimental evidence [14] does not show any negative effect on tar production in these conditions of reduced temperature.

The final goal of our research project is to optimise the granular, mineral solid material for a dual bed system with a CO2 sorbent performing a calcination–carbonation loop. This is done in different steps, starting from the study of simultaneous hydrocarbon reforming and CO2 capture by means of commercial, readily available materials (a nickel catalyst mixed with calcined dolomite), and progressively moving to improve the catalytic activity of dolomite for reforming reactions in order it to perform the double function of CO2 sorbent as well as reforming catalyst.

Monocyclic aromatic hydrocarbons are considered the most representative tar compounds, reaching 46% of overall biomass tar [2]. Two rings aromatic hydrocarbons represent about 28% of biomass tar [2]. So, the reactivity of about 75% of biomass tar can be studied by choosing the appropriate model tar compounds: in this work, toluene and 1-methyl naphthalene (1-MN). The results of methane, toluene and 1-methyl naphthalene steam reforming and water gas shift tests are reported. The experiments have been carried out in a bench scale rig, with a fluidised bed inventory allowing for simultaneous hydrocarbon reforming and CO2 sorption by means of a commercial nickel catalyst mixed with calcined dolomite. In addition, toluene has been steam reformed in a fixed bed microreactor in presence of a 4 wt% Ni/dolomite supported catalyst.

It is worth mentioning here that dolomite is known to have a catalytic activity in tar reforming [18], [19], [20], [21], and at the same time is a very attracting material to absorb CO2 from syngas [13], [14], [15]. Nickel, on the other hand, is a well known, low-cost catalyst for steam reforming [4], [22], [23]. In this work, the activity of these materials is evaluated with different hydrocarbons (methane, toluene, 1-MN). The last two are representative of most tars formed during gasification, and the activity of methane is well known in steam reforming with Ni catalysts. The present contribution is only the first part of a more complex study including hydrocarbons reactivity in presence of all gas compounds produced during steam reforming of biomass.

Section snippets

Materials

Two different catalytic systems have been used. The former consists of a commercial nickel catalyst (Johnson Matthey Plc) mixed with dolomite kindly provided by Pilkington (see Table 1 for elemental analysis (a) and specific surface area (b) of dolomite).

The latter consists of the same dolomite as above, impregnated by nickel (Ni/dolomite); this catalyst contains 4% of nickel by weight. The preparation method is the following: a stirred suspension of calcined dolomite (pre-calcined at 900 °C for

Catalyst characterization

The characterization of Ni/dolomite catalyst, prepared according to the methodology explained in Section 2.1, highlights the interactions between NiO and CaO–MgO dolomite substrate.

When calcined dolomite and Ni/dolomite samples are compared, the X-ray diffraction analysis (Fig. 4) shows that there are not new phases in the Ni/dolomite catalyst with respect to the raw calcined dolomite.

This means that a solid solution between calcined dolomite and NiO is produced, and in particular, due to the

Conclusions

The results obtained for methane reforming and simultaneous CO2 capture confirm those reported in the literature [27] and indicate a promising sorption performance of dolomite in cyclic operation, while those related to water-gas shift, toluene and 1-MN reforming allow to extend to a biomass syngas the experimental evidence on simultaneous CO2 capture and heavy hydrocarbons (tar) reforming, showing the potential of generating a hydrogen energy vector by using a particulate solid acting as CO2

Acknowledgements

The authors would like to acknowledge the financial support received from the Italian Ministry of Research under the special program FISR-TEPSI. In addition, L. Di Felice would like to acknowledge the Italian-French University for his co-tutelle PhD grant.

References (28)

  • D. Swierczynski et al.

    Appl. Catal. B

    (2007)
  • D. Swierczynski et al.

    Chem. Eng. Process.

    (2008)
  • Z. Abu El-Rub et al.

    Fuel

    (2008)
  • K. Polychronopoulou et al.

    J. Catal.

    (2006)
  • L. Devi et al.

    Fuel Process. Technol.

    (2005)
  • B. Dou et al.

    Appl. Therm. Eng.

    (2003)
  • S. Rapagnà et al.

    Int. J. Hydrogen Energy

    (1998)
  • F. Garcia-Labiano et al.

    Chem. Eng. Sci.

    (2002)
  • D. Sutton et al.

    Fuel Process. Technol.

    (2001)
  • C. Courson et al.

    Catal. Today

    (2002)
  • R. Coll et al.

    Fuel Process. Technol.

    (2001)
  • K. Johnsen et al.

    Chem. Eng. Sci.

    (2006)
  • K. Gallucci et al.

    Int. J. Hydrogen Energy

    (2008)
  • A.V. Bridgewater

    Chem. Eng. J.

    (2003)
  • Cited by (60)

    • A novel hybrid iron-calcium catalyst/absorbent for enhanced hydrogen production via catalytic tar reforming with in-situ CO<inf>2</inf> capture

      2020, International Journal of Hydrogen Energy
      Citation Excerpt :

      A gravity crushing method [45] was used to measure mechanical strength of different absorbents. 1-methyl naphthalene (1-MN) was selected as biomass tar model compound in this work because two-ring aromatic hydrocarbons represent about 28% of biomass tars [46] and have significant effect on condensation behavior of tars [33]. A self-design tar reforming reaction system, manufactured by Tianjin Xianquan Industry and Trade Development Co., Ltd, was used for 1- MN reforming tests (Fig. 1).

    View all citing articles on Scopus
    View full text