(617c) Chemical Looping Combustion from Biomass Derived Syngas Using a Fluidizable Ni-Co-La/?-Al2O3 Oxygen Carrier: CLC Performance and CPFD Modelling

Biomass steam gasification yields a blend of H₂, CO, CH₄ and CO₂, designated as syngas. Syngas can be further combusted using fluidizable oxygen carriers (OC). This operation is known as Chemical Looping Combustion (CLC). With this end, a Ni-based oxygen carrier is implemented with a Co and La modified γ-Al₂O₃ support. This type of OC limits NiAl₂O₄ formation. This novel nickel on g-alumina OC was prepared using an Incipient Wetness Impregnation (IWI) method as described in previous research[1]. This OC also includes Co and La additives. Co and La additives have valuable effects on both NiAl₂O₄ and carbon formation reduction, respectively [1]. Syngas is a typical biomass gasification product[2].

In spite of the valuable progress made with OCs by our research team [2,3], the 750^ºC calcination temperature use was shown to lead to: a) strongly bounded Ni species on alumina and b) undesirable NiAl₂O₄ [1]. These species inherently limit the OC oxygen capacity in the 550-650^ºC operating range. To circumvent this, a new highly performing OC was prepared by reducing all the formed nickel species at 900^ºC for 2 hours and this prior to 750^ºC calcination. This methodology ensures the complete decomposition of strongly bounded Ni on alumina, as well as that of NiAl₂O₄. This yields a re-dispersing nickel phase which has little interaction with the g-alumina support. This new highly performing oxygen carrier (HPOC) can provide more than 90% of the nominal oxygen lattice at temperatures between 550-650^ºC. The promising HPOC was characterized using BET, XRF, XRD, H₂-TPR, NH₃-TPD and pulse chemisorption.

CLC runs of syngas and the new OC were performed using the CREC Riser Simulator. This is a mini-fluidized bed reactor with high gas mixing that operates under batch conditions[4]. This unit is designed for oxygen carrier evaluation at expected industrial process conditions. An automatic valve allows the transfer of products to an online gas chromatograph (GC) to detect all product chemical species (CO, CH₄, CO₂ and H₂) accurately. The evaluation of this highly performing OC (HPOC) was developed at 550-650^ºC, 5-40s and with a ratio of oxygen to be consumed over maximum available oxygen, ψ = 0.5 to 1. A syngas was used in the experimental runs using a 20v% CO, 10v% CH₄, 20v% CO₂ and 50v% H₂ mixture. CLC runs of syngas showed that 60-92% CO₂ yields can be obtained within these operating conditions. The CO₂ yields and the CH₄ conversion steadily increase with reaction time for ψ=1. It is also shown that CO and H₂ conversions stabilize and decrease respectively for ψ =1. These trends were attributed to the influence of reforming reactions under OC starving oxygen conditions. An adjustment of the OC oxygen supply to ψ=0.5, leads to all CO, H₂ and CH₄ species being 78-88% converted.

For the point of view of implementation as an industrial process, a set of twin fluidized bed (oxidizer and reducer) is considered in the present study. Regarding the selected twin fluidized bed configuration, both fluidized beds are interconnected to facilitate oxygen carrier particle circulation. This oxygen carrier particles have a 75 μm mean diameter and a 2500 kg/m³ particle density. Considering 150KW power generation capacity, and ψ =1, the diameter of oxidizer was designed as 780 mm diameter and with an overall length 4500 mm including transport disengaging height. Similarly, reducer was designed as 800 mm and 3750 overall length. Typical 30 seconds total retention time for reducer and 15 seconds for oxidizer were considered. Reduced particles are collected in a loop-seal pot and furthered transported to the top of oxidizer bed. In a similar manner oxidized particle are transported to reducer bed. Product gas (Flue gas) and exhausted air leave from the reducer and oxidizer top section via cyclone particle separators.

To analyze this twin fluidized bed configuration a multi-scale (Wen-Yu) drag model coupled with computational particle-fluid dynamics (CPFD) and CLC volume averaged kinetics are being considered[5]. This Hybrid Barracuda CPFD model uses the Eulerian-Lagrangian approach called MP-PIC (multi-phase particle-in-cell). The software’s numerical methodology considers a direct element method wherein solids are modeled as discrete particles with both size and density distributions, and the fluid is modeled as a continuum. The formation of particle cluster is also considered based on cluster size distribution[6].

On the basis of the data obtained this study allows the following: (a) to establish the twin fluidized bed system performance for CLC; (b) to describe the required oxygen carrier particle circulation; (c) to evaluate the extent of CO₂ gas leaking via the loop-seal.

References

[1] M.R. Quddus, M.M. Hossain, H.I. de Lasa, Catal. Today. 210 (2013) 124–134.

[2] J. Mazumder, H. de Lasa, Appl. Catal. B Environ. 160–161 (2014) 67–79.

[3] M.M. Hossain, K.E. Sedor, H.I. de Lasa, Chem. Eng. Sci. 62 (2007) 5464–5472.

[4] I. Ahmed, S. Rostom, A. Lanza, H. de Lasa, Powder Technol. 316 (2016) 641–649.

[5] M.A. Hamilton, K.J. Whitty, J.S. Lighty, J. Energy Resour. Technol. 138 (2016) 42213.

[6] A. Lanza, H. de Lasa, Comput. Chem. Eng. 101 (2017) 226–242.