Calcium silicate-based catalytic filters for partial oxidation of methane and biogas mixtures: Preliminary results

Highlights

• Catalytic filters were developed and tested for biogas partial oxidation reaction.

• The catalytic experiments were performed at varying temperatures.

• La addition affects the reaction over 850 °C.

• Calcium silicate filters have a high potential for catalytic application.

Abstract

Development and testing of catalytic filters for partial oxidation of methane to increase hydrogen production in a biomass gasification process constitute the subject of the present study. Nickel, iron and lanthanum were coated on calcium silicate filters via co-impregnation technique, and catalytic filters were characterized by ICP-MS, XPS, XRD, TEM, TGA, TPR and BET techniques. The influences of varying reaction temperature and addition of Fe or La to Ni-based catalytic filters on methane conversion, and hydrogen selectivity have been investigated in view of preliminary results obtained from reactions with 6% methane-nitrogen mixture, and catalytic filters were tested with model biogas mixtures at optimum reaction temperature of each filter which were 750 °C or 850 °C. Approximately 93% methane conversion was observed with nearly 6% methane-nitrogen mixture, and 97.5% methane conversion was obtained with model biogas containing CH₄ which is 6%, CO₂, CO, and N₂ at 750 °C. These results indicate that calcium silicate provides a suitable base material for catalytic filters for partial oxidation of methane and biogas containing methane.

Introduction

In the past few years, although the emission of greenhouse gases (GHG) had slowed down briefly after rising for decades owing to the technology-driven transition to low carbon energy, and this situation increased hope for bending down the GHG emission curve [1]; decarbonizing the world economy will be a slow process. The decrease in GHG emission is vital to achieving another goal which is limiting global warming to 1.5 °C [2]. Intergovernmental Panel on Climate Change (IPCC) and the Conference of the Parties (COP) are jointly working on this subject because it has significant benefits to people and natural ecosystems. On the other hand, most recent developments are not very encouraging. For example, energy-related CO₂ emissions reached another peak since 2013 rising by 1.9% in 2018 [3].

Alternative energy sources are employed to overcome the problem of global warming, environmental pollution, and energy shortage [4]. Biomass is a renewable source containing agricultural products & residues, animal waste, woody biomass, and municipal solid waste [5], and recycles carbon dioxide thus decreasing GHG emission. The biomass gasification process is a mature technology to convert biomass to hydrogen and other products without combustion [6].

One of the main problems in the biomass gasification process is tar formation which decreases hydrogen yield and causes blockage of catalyst units, filters or pipes [7]. Moreover, separation and purification steps are necessary for hydrogen production as a final step in the process. Additional hydrogen can be produced from undesirable tar compounds and the CH₄ in the biogas. Catalytic filters are suitable for combining tar or methane cracking and hot gas filtration steps in the biomass gasification process to eliminate particles and increase total hydrogen production [[8], [9], [10], [11], [12], [13], [14], [15]].

The thermal stability of hot gas filter elements has significant effects on the catalytic filtration process, and alumina and silicon carbide-based catalytic candle filters have been investigated [10,12]. Rapagna et al. [15] coated SiC-based candle filters with a fine MgO–Al₂O₃ suspension, preferred nickel for catalytic activation, and obtained 93.5% average overall tar conversion. Nacken et al. [10] prepared SiC filter elements by catalytically activating with MgO–NiO catalysts, and achieved a complete tar conversion with naphthalene as a model compound at 800 °C. Zhao et al. [9] catalytically activated alpha Al₂O₃-based filter with nickel using a precipitation-deposition method with urea and investigated the effect of nickel loading, gas velocity, and the reaction temperature on methane and naphthalene conversion. On the other hand, Turan et al. [16] prepared nickel coated calcium silicate based catalytic candle filters and performed the activity tests under simulated H₂S-free and H₂S-containing biomass gasification atmosphere; methane and benzene conversion values, between 77% and 88% were obtained in this study.

Although steam methane reforming (SMR) is a mature technology for hydrogen production, it causes a large amount of CO₂ emission and needs high energy consumption [17]. On the other hand, partial oxidation of methane is an alternative route for synthesis gas production [18], and an economically promising process because it is mildly exothermic and does not require high operating pressures [19]. The main reaction for partial oxidation is given in Equation (1) [20]:

$${\text{CH}}_4+\left(1/2\right)+{\text{O}}_2\to \text{CO}+2{\text{H}}_2\Delta \text{H}=-35.59\text{KJ}/\text{mol}$$

Various catalysts containing Ni, Co, Ru, Pt, and others, have been studied for catalytic partial oxidation of methane [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. Even though researchers reported that nickel-based catalysts are commercially promising for methane partial oxidation reaction because of their low cost, and high catalytic activity and selectivity to hydrogen in the reaction [31], these catalysts have some drawbacks including carbon deposition on the surface resulting in blocking the active sites of the catalyst [32] and sintering [33]. To overcome these problems, several studies have been reported including the addition of noble or non-noble metals [25,[34], [35], [36], [37]]. Choudhary et al. [34] investigated the effects of the addition of Pt, Pd, and Ru to Ni/Al₂O₃ on methane conversion to H₂ and CO and selectivity and reported that the addition of noble metals decreased reaction starting temperature, and slightly increased the methane conversion except the catalysts with Pd. Besides, the highest methane conversion, CO and H₂ selectivity with the catalysts containing 2.5% Ru were 90.8, 97.7 and 98.8% at 800 °C. On the other hand, the high cost of noble metals such as Pt, Pd, Ru is another challenge for large scale production [38]. Therefore, the addition of non-noble metals with low cost and high resistance against coke is another economically feasible way to inhibit coke formation [39].

Researchers have investigated the effect of the addition of Co, Fe, Ce, etc. as a non-noble second metal to Ni-based catalysts [25,37,40]. Koh et al. [25] synthesized Ni–Co bimetallic catalysts supported on CaAl₂O₄/Al₂O₃ and reported that 1 wt% Co and 2 wt% Ni bimetallic catalyst is highly selective and active for the reaction. Wang et al. [41] synthesized Ni–Fe/La₂O₃ catalysts by impregnation technique and reported positive effects of plasma treatment on the selectivity and activity of catalysts for partial oxidation of methane and the conversion of methane has been found around 60% at 950 °C. Also, researchers synthesized Ni–Fe/Al₂O₃ catalysts and investigated the selectivity and activity of catalysts. Even though methane conversion and hydrogen selectivity with plasma treated Ni–Fe/Al₂O₃ catalyst was around 97% at 875 °C, a decrease in temperature had significant effects on the conversion which was 60% at 700 °C [40]. Various supports such as Al₂O₃, CaO, La₂O₃, MgO and SiC have been used as well with Ni catalysts [24,[42], [43], [44]].

The aim of this study is designing calcium silicate based catalytic filters for partial oxidation of methane and biogas mixtures to increase hydrogen production in a biomass gasification process. Powdered calcium silicate supported catalysts were used for catalytic thermal decompositions of methane in one of our previous studies [45]. In the present study, calcium silicate filter material was used as a support for Ni catalysts coupled with La for stability and prevention of coke formation [46,47] and Fe for extended lifetime and for stability at higher temperatures [[47], [48], [49]]. The effects of varying reaction temperature and calcium silicate based material on activity and stability of catalytic filters have been investigated for partial oxidation of methane and biogas mixtures containing methane. Also, it is expected that the catalysts will be applicable for additional hydrogen production by tar conversion in actual biogas mixtures resulting from biomass gasification.