Chemical looping reforming of toluene as bio-oil model compound via NiFe₂O₄@SBA-15 for hydrogen-rich syngas production

Highlights

• A decoupling strategy for biomass chemical looping reforming was proposed.

• Fe₂O₃@SBA-15, NiO@SBA-15, and NiFe₂O₄@SBA-15 were prepared by embedding strategy.

• NiFe₂O₄@SBA-15 achieved toluene conversion of 95.34% and average H/C ratio of 1.97.

• The possible reaction mechanism of oxygen carrier and toluene was revealed.

Abstract

Hydrogen-rich syngas was a clean energy and an important industrial material. Based on the decoupling strategy of biomass chemical looping gasification process, this paper proposed a strategy of metal oxides embedded into molecular sieves to prepare highly dispersed and nanosized oxygen carriers for producing hydrogen-rich syngas. NiO@SBA-15, Fe₂O₃@SBA-15, and NiFe₂O₄@SBA-15 were prepared by the impregnation method, and the reaction conditions on the chemical looping reforming of toluene were investigated. The results showed that NiFe₂O₄@SBA-15 had the highest toluene conversion rate of 93.4% and a relatively high CO selectivity rate of 80.7%. It was confirmed that the embedding strategy can effectively enhance the nanocrystallization and dispersion of metal oxides in oxygen carriers, which could effectively reduce sintering. The inverse spinel structure of NiFe₂O₄ made the oxygen carrier have more metal adsorption sites and a closer reaction distance, which were beneficial to the adsorption and reaction of the fuel. After testing, the optimum reaction temperature was 750 °C, and the optimum weight hourly space velocity was 1.168 h⁻¹. In the 10 cycles of testing of 20 NiFe₂O₄@SBA-15, the average conversion rate of toluene was 95.34%, the moderate selectivity of CO in the gaseous product was 94.83%, the average H/C ratio was 1.97, which indicated that the cycle stability is good. It provided a reference for developing and designing future oxygen carriers of biomass chemical looping reforming.

Introduction

Biomass chemical looping conversion is an important way to efficiently utilize biomass energy and reduce carbon emissions, which is an efficient fuel conversion process for direct hydrogen-rich syngas by using solid metal oxides as oxygen carriers [1,2]. In the traditional biomass chemical looping gasification process, the complex parallel or series of reactions are usually carried out. To solve the problems existing in the traditional biomass chemical looping gasification process, a biomass chemical looping reforming process based on a decoupling strategy was proposed, as shown in Fig. 1(b). The biomass chemical looping reforming process based on a decoupling strategy was divided into three parts: pyrolysis reactor (PR), fuel reactor (FR) and air reactor (AR). Firstly, the biomass was pyrolyzed into volatiles and biochar in the PR, then the volatiles entered the FR, and the char entered the AR. The volatiles produced by biomass pyrolysis can be divided into macromolecular and micromolecular volatiles according to whether they can be condensed at normal temperatures. Macromolecular volatiles can be condensed, and micromolecular volatiles cannot be condensed. The liquid phase products formed by condensation are macromolecular volatiles, also known as bio-oil, accounting for about 70 wt% of the pyrolysis volatile components. In the FR, the macromolecular volatiles generated hydrogen-rich syngas under the catalysis of the active component and the partial oxidation of lattice oxygen in the oxygen carrier. The micromolecular volatiles developed hydrogen-rich syngas under the partial oxidation of lattice oxygen in the oxygen carrier [3,4]. In the AR, char and air occurred during the combustion reaction, and the reduced oxygen carrier reacted with air to supplement lattice oxygen. The regenerated oxygen carrier carried sufficient lattice oxygen into the FR for the next reduction reaction. Studies have shown that hydrothermal liquefaction of biomass (rapid pyrolysis) before steam reforming can effectively increase the yield of renewable hydrogen and cost-effectiveness [5,6].

In the biomass chemical looping reforming process based on the decoupling strategy, the preparation of hydrogen-rich syngas by pyrolysis volatile chemical looping reforming was a key step [4,[7], [8], [9], [10], [11]]. Due to the high content and ease of condensing at low temperatures of macromolecular volatiles, which could easily cause problems such as blockage and corrosion of equipment. Therefore, the reforming of macromolecular volatiles is very important. To achieve efficient and clean biomass gasification, and reduce the cost and operation difficulty of replacing equipment caused by equipment corrosion, it is very important to study the reforming of macromolecular volatiles in biomass pyrolysis. The chemical looping reforming reaction of macromolecular volatiles has many requirements in the case of oxygen carriers. Firstly, the oxygen carriers needed to play a catalytic role in the FR to break the C-C and C-H bond in macromolecular volatiles, so the macromolecular volatiles were broken into micromolecular volatiles. Secondly, the oxygen carrier needed to provide lattice oxygen to partially oxidize micromolecular volatiles formed by the cleavage of macromolecular compounds to generate H₂ and CO. The oxygen carrier entered the AR should react with oxygen to eliminate carbon deposition and supplement lattice oxygen. Therefore, the excellent oxygen carriers should have strong catalytic activity, good lattice oxygen transport rate and storage capacity, moderate partial oxidation performance, and superior heat carrying capacity [12].

In recent years, the study of chemical looping reforming of macromolecular volatiles has been carried out using single metal oxides, composite metal oxides, and perovskite as oxygen carriers. Due to the poor thermal stability and mechanical strength of single metal oxides, it is easy to sinter at high temperatures. The structure was easy to change after the reduction reaction reduced the active sites. Studies have shown that composite metal oxides and perovskite oxygen carriers perform better than single metal oxides [13]. In addition, due to the complex composition of macromolecular volatiles, researchers often used macromolecular volatiles with high component content (such as acetic acid, glycerol) or difficult to decompose at high temperatures (such as benzene, toluene, naphthalene) model compounds for oxygen carrier performance test [8,9]. Huang [[14], [15], [16]] prepared MFe₂O₄ (M = Cu, Ba, Ni, Co) oxygen carriers and tested the chemical looping reforming characteristics of toluene as a model in a fixed-bed reactor. They found that NiFe₂O₄ has the best reactivity. In further research of NiFe₂O₄, the effects of adding steam and oxygen carrier reduction state on the product composition and toluene conversion rate were tested. And the reaction mechanism and carbon deposition process in the chemical looping reforming of toluene were also studied. The carbon deposition formation mechanism and the possible reaction path of chemical looping reforming of macromolecular volatiles were proposed. However, after multiple redox cycles, some metal ions may be lost from the main structure of the oxygen carrier, thereby reducing the reactivity of the oxygen carrier was not solved.

Embedding Fe₂O₃ into molecular sieves can effectively reduce the sintering and wear of Fe₂O₃, and nanocrystallization of Fe₂O₃ effectively overcame the relatively weak lattice oxygen transport activity and low catalytic activity of Fe-based oxygen carriers [17]. Using the confinement effect of a molecular sieve, embedding Fe₂O₃ in a molecular sieve was one of the effective ways to realize Fe₂O₃ nanocrystallization. SBA-15 molecular sieve has good mechanical strength, large specific surface area, regular pores, and large specific heat capacity, which could be used as a suitable carrier for the nanocrystallization of Fe₂O₃ [[18], [19], [20]]. The composite ferrite formed by adding other catalytic active components (such as transition metal elements) to the Fe-based oxygen carrier had an inverse spinel structure. In the octahedral site of the inverse spinel structure, the surface with the M_oct atom (transition metal) as the terminal has more metal adsorption sites. In the tetrahedral site of the inverse spinel structure, the surface with the Fe_{tet 1} atom as the terminal has a closed space between the oxygen atom layer and the fuel molecule. The space possibility of reacting with Fe_{tet 1} was higher, which could realize the preferential adsorption and accelerated transformation of macromolecular compounds and effectively improve the reactivity and selectivity of metal-oxygen carriers [14,21].

In this paper, Fe₂O₃@SBA-15, NiFe₂O₄@SBA-15 were prepared by adding the transition metal Ni to further improve the oxygen carrier's catalytic performance. In addition, the effects of active metal loading in NiFe₂O₄@SBA-15, reaction temperature, weight hourly space velocity and steam addition on the chemical looping reforming of toluene were investigated.