Elsevier

Catalysis Today

Volume 343, 1 March 2020, Pages 63-71
Catalysis Today

Syngas chemical looping combustion using a highly performing fluidizable oxygen carrier

https://doi.org/10.1016/j.cattod.2019.01.011Get rights and content

Highlights

  • A high performing 20 wt%Ni-1 wt%Co-5 wt%La/γ-Al2O3 oxygen carrier is reported.

  • The preparation method secures a stable fluidizable OC with enhanced reactivity.

  • This OC carrier is suitable for biomass gasification derived syngas combustion.

  • This OC yields up to 92% CO2 yields in the 550-650ºC range.

  • This OC is free from NiAl2O4 formation in the targeted temperature operating range.

Abstract

Biomass steam gasification produces a blend of H2, CO, CH4 and CO2, designated as syngas. Syngas can be further combusted using fluidizable oxygen carriers (OC). This operation which is known as Chemical Looping Combustion (CLC), is valuable for syngas combustion as it involves nickel oxidation and nickel reduction, which are both exothermic reactions. To improve syngas CLC and establish its application, a new Ni-based oxygen carrier was studied using a Co and La modified γ-Al2O3 support. This type of OC considerably limits the NiAl2O4 formation. A Highly Performing Oxygen Carrier (HPOC) was engineered using a special preparation methodology to exclude NiAl2O4 species formation. The evaluation of this HPOC was developed using the fluidized CREC Riser Simulator reactor at mild CLC 550–650 °C temperatures. Reaction conditions employed ranged from 2 to 40 s and used a 0.5–1 fuel to oxygen stoichiometric supply ratio (ψ). It is shown that under these conditions, the HPOC demonstrates a stable performance, yielding 75–92% CO2, displaying a 1.84–2.75 wt% (gO2/gOC) oxygen transport capacity with 40–60% solid phase oxygen conversion.

Introduction

Fossil fuelled power plants are one of the largest CO2 emissions contributors (˜33-40%) [1]. These power plant emissions typically contain CO2, NOx, CH4, with CO2 being the dominant species. One possible solution to reducing CO2 emissions is to capture CO2 followed by sequestration (CCS). However, for an effective sequestration, highly concentrated CO2 is required, which adds additional costs to CO2 capture.

In this respect, Chemical Looping Combustion (CLC) is a promising technology which can maximize combustion efficiency, separate CO2 inherently and reduce CO2 capture costs significantly [2]. In addition to this, CLC can provide: a) a high thermal efficiency b) flameless combustion and c) minute amounts of nitrogen oxide (NOx) formed.

CLC was first suggested by Lewis et al. [3]. These researchers discovered that CLC enables fossil fuel based power plants to operate with high thermal efficiency by minimizing energy losses [4]. As shown in Fig. 1, CLC can be configured with a set of dual interconnected fluidized bed reactors: a Fuel Reactor and an Air Reactor. In CLC, the solid oxygen carrier is circulated between the fuel and air reactors, to be reduced by fuel and oxidized by air, respectively. This yields a flue gas composed primarily of almost pure carbon dioxide and steam. The steam can easily be condensed and removed from the flue gas stream. Essentially, the CLC system eliminates the additional energy intensive CO2 separation process. Hence, the exiting water-free CO2 can be sequestrated and used for further applications.

However, as large amounts of fossil fuel derived CO2 emissions continue to steadily increase, it is of primary importance to improve CO2 capture processes. One valuable alternative is to use renewable feedstock such as biomass, in power stations. Biomass is the result of CO2 capture from the atmosphere via photosynthesis. Biomass can be converted into syngas, which can in turn, be used as a fuel in power stations. On balance, this leads to net zero CO2 fossil fuel based emissions.

Biomass derived syngas contains mostly CO and H2 with some CH4 and CO2. An overall description of reaction stoichiometry in the air and fuel reactors can be considered as shown in Eqs. (1) and (2), respectively:

Air Reactor: oxidationMyOx-1+12O2(air)MyOx+(air:N2+unreacted:O2);ΔHo=-264kJ/molsyngasinpresentstudy;(MyOx=NiO)

Fuel Reactor: reduction(n+m+4k)MyOx+nCO+mH2+kCH4(n+m+4k)MyOx1+(m+2k)H2O+(n+k)CO2;ΔHo=5.9kJ/molsyngasinpresentstudywith MyOx-1 being the reduced oxygen carrier; MyOx being the oxidized oxygen carrier; and m, n and k being the stoichiometric coefficients.

Regarding reaction enthalpies in the syngas CLC process, one can note that NiO reduction with syngas is mildly exothermic (-5.9 kJ/mole). This is due to the relatively low amounts of methane in the syngas CLC, with this contrasting with the large endothermic heat of reduction (156.5 kJ/mole) for methane CLC. These favourable conditions demonstrate that syngas CLC is of great value for power generation. Furthermore, the application of the CLC process side-by-side to the gasification unit may facilitate tar biomass derived conversion, which is a frequent issue encountered in biomass gasification plants. In spite of these several described benefits, the application of syngas CLC, has not yet been developed for large scale applications. One of the pending issues is the availability of stable OCs with the following properties available: a) the OC must be suitable for the 550–650 °C range, b) the OC must have a high oxygen transport capacity, c) The OC must be stable through many redox cycles, d) The OC must be resistant to agglomeration and attrition, e) The OC must display good fluidization properties.

The status of CLC has been reviewed in the technical literature in recent reviews by Adanez et al. and Hossain et al. [5,6], establishing its promising and potential benefits, with a very few studies reporting syngas CLC. Regarding Ni-based oxygen carriers, it has been shown that OCs can provide 1.84–6 wt% oxygen transport capacity and 40–50 wt% solid conversion at 750 °C–1000 °C under excess oxygen conditions [7,8]. However, these OCs displayed low stability. In contrast, carefully designed Ni-based OCs modified with La and Co can display high thermal stability and less metal support interaction as shown by our research team [9,10]. It was demonstrated that Methane CLC with these OCs, yields 1.84–3.45% oxygen transport capacity and 40–72% solid conversion at 600–680 °C. However, the observed NiAl2O4 formed still represents a significant drawback [[9], [10], [11]].

Given all of the above and the incentives for syngas CLC, the aim of the present study is to develop a novel fluidizable HPOC having the following properties: a) High performance at 550–650 °C within a 40 s reaction time range, b) Enhanced oxygen transport capacity and solid phase oxygen conversion, c) Excellent CO2 yields for efficient CO2 sequestration, d) OC free of NiAl2O4 formation.

Section snippets

Oxygen carrier (OC) and feedstock

An OC (20Ni1Co5La/γ-Al2O3) was developed which consisted of 20 wt% Ni over γ-alumina. It also included 1 wt% Co and 5 wt% La additives. Co and La additives have valuable effects on both NiAl2O4 and carbon formation reduction, respectively. This OC shares some common properties with the OC originally developed by Quddus et al. [9] and can further be referred in the present study, as OC-Ref.

The syngas used was selected with a composition comparable the one from biomass catalytic gasification

Characterization of OC-Ref and HPOC

Fig. 3 reports a typical H2-TPR analysis. One can observe that the OC-Ref yields 45% of the available lattice oxygen only, in the 300–650 °C range, requiring higher thermal levels for complete OC oxygen utilization. Exposing the OCs to thermal levels higher than of 750 °C, promotes NiO transformation into NiAl2O4 [16,17], making the OC-Ref inherently less reliable. On the other hand, one can also observe as per the TPR for HPOC, that a H2-uptake of 50 cm3/g was recorded in the 300-650 °C range.

Conclusions

  • a)

    A 20 wt%Ni 1 wt%Co 5 wt%La /γ-Al2O3 Highly Performing Oxygen Carrier (HPOC) was synthesized using an enhanced preparation method. This led to an OC essentially free of nickel aluminate. This was confirmed using XRD, BET and TPR/TPO.

  • b)

    The new HPOC was successfully tested at 550–650 °C for synthesis gas (H2/CO = 2.5). CLC used both stoichiometric (ψ = 1) and under stoichiometric (ψ = 0.5) oxygen surface available amounts. The observed oxygen carrying capacity was in the 1.84–2.75 wt% (gO2/gOC)

Acknowledgments

We would like to gratefully acknowledge the financial support provided by the NSERC Strategic grant program (2015-H. de Lasa grant), Canada and Recat Technologies Inc. to Imtiaz Ahmed at the University of Western Ontario. We would also like to thank Florencia de Lasa who provided valuable assistance in the editing of this manuscript.

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