Using Biomass Gasification Mineral Residue as Catalyst to Produce Light Olefins from CO, CO2 , and H2 Mixtures

Abstract

Gasification is a process to transform solids, such as agricultural and municipal waste, into gaseous feedstock for making transportation fuels. The so-called coarse solid residue (CSR) that remains after this conversion process is currently discarded as a process solid residue. In the context of transitioning from a linear to a circular society, the feasibility of using the solid process residue from waste gasification as a solid catalyst for light olefin production from CO, CO₂ , and H₂ mixtures was investigated. This CSR-derived catalyst converted biomass-derived syngas, a H₂ -poor mixture of CO, CO₂ , H₂ , and N₂ , into methane (57 %) and C₂ -C₄ olefins (43 %) at 450 °C and 20 bar. The main active ingredient of CSR was Fe, and it was discovered with operando X-ray diffraction that metallic Fe, present after pre-reduction in H₂ , transformed into an Fe carbide phase under reaction conditions. The increased formation of Fe carbides correlated with an increase in CO conversion and olefin selectivity. The presence of alkali elements, such as Na and K, in CSR-derived catalyst increased olefin production as well.

Keywords: CO2 hydrogenation; Fischer-Tropsch; biomass residue; iron; olefins.

Figure 1

HAADF‐STEM images of the fresh CSR sample (left) and EDX chemical mapping (right). Fe is shown in red, and Si is shown in green.

Figure 2

Characterization of the crystalline phases in the CSR sample and a reference Fe/SiO₂ catalyst material with XRD. (a) XRD patterns of the fresh and spent (T=450 °C, P=5 bar, and CO/CO₂/H₂/N₂=4.5 : 2.5 : 3 : 1) CSR sample. XRD pattern of the mineral gehlenite from the PDF‐4+ XRD database is added as a reference. (b) Fe/SiO₂ (7.7 wt %) fresh and spent (T=450 °C, P=5 bar, and CO/CO₂/H₂/N₂=4.5 : 2.5 : 3 : 1).

Figure 3

Catalytic performance of the CSR sample and the Fe/SiO₂ reference catalyst material. Catalytic testing of (a) CSR and (c) Fe/SiO₂ in CO/CO₂/H₂/N₂=4.5 : 2.5 : 3 : 1 at P=5 bar, T=250, 350, 450 °C, and gas hourly space velocity (GHSV)=3400 h⁻¹ (12 h per temperature). Stability testing of (b) CSR and (d) Fe/SiO₂ at 450 °C for 24 and 20 h, respectively, under the same gases and pressure as (a).

Figure 4

Catalytic performance in (consecutively) CO₂ hydrogenation reaction, FTO reaction, and subsequent CO₂ hydrogenation reaction of (a,b) CSR, (c,d) Fe/SiO₂, and (e,f) K−Fe/SiO₂. The CO₂ hydrogenation steps were carried out at T=250 °C, P=5 bar, H₂/CO₂=3, GHSV=3070 h⁻¹, while the FTO step was carried out at T=350 °C, P=5 bar, H₂/CO=0.7, GHSV=2425 h⁻¹. The hydrocarbon (CH₄ and C₂₊) selectivities displayed are CO and/or CO₂‐free. Prior to the first CO₂ hydrogenation step, the samples were pre‐reduced at 450 °C in N₂/H₂=2 for 1 h.

Figure 5

Operando XRD of the CSR sample. (a) CSR fresh, after reduction at 450 °C in H₂, and during CO hydrogenation (H₂/CO=0.7) for 70 h at 450 °C and 5 bar. (b) Zoom in of 16–25 ° 2θ, showing CSR contained a mixture of Fe₃O₄ and metallic Fe after reduction. Under reaction conditions (Fe₅C₂), Hägg carbide, evolved as active phase in the CSR sample. (c) CO conversion [%] and (d) product selectivities over time.

Figure 6

Catalytic performance of the CSR sample in CO/CO₂/H₂/N₂=4.5 : 2.5 : 3 : 1 at P=20 bar, T=250–450 °C, GHSV=3400 h⁻¹ (6 h per temperature).

Figure 7

Operando Raman micro‐spectroscopy on the CSR sample (a) during the reduction procedure in H₂/Ar=1 and during CO/CO₂ hydrogenation (CO/CO₂/H₂=2.2 : 1.2 : 1.5) at P=20 bar and T=450 °C. The Fe₂O₃ peaks are indicated with a gray dashed lines and the carbon D and G bands with black dotted lines. (b) Photograph of the high‐pressure Raman cell and microscopy image of a CSR particle. (c) MS signals for CH₄ (m/z=15) and C₂–C₄ olefins (m/z=26).