Use of Pd-Ag Membrane Reactors for Low-Temperature Dry Reforming of Biogas-A Simulation Study

Abstract

Biogas is a valuable renewable energy source that can help mitigate greenhouse emissions. The dry reforming of methane (DRM) offers an alternative hydrogen production route with the advantage of using two main greenhouse gases, CO₂ and CH₄. However, its real application is limited mainly due to catalyst deactivation by coke formation and the reverse water gas shift (RWGS) reaction that can occur in parallel. Additionally, the typical dry reforming temperature range is 700-950 °C, often leading to catalyst sintering. A low-temperature DRM process could be in principle achieved using a membrane reactor (MR) to shift the dry reforming equilibrium forward and inhibit the RWGS reaction. In this work, biogas reforming was investigated through the simulation of MRs with thin (3.4 µm) and thick (50 µm) Pd-Ag membranes. The effects of the feed temperature (from 450 to 550 °C), pressure (in the range of 2-20 bar), and biogas composition (CH₄/CO₂ molar ratios from 1/1 to 7/3) were studied for the thin membrane through the calculation and comparison of several process indicators, namely CH₄ and CO₂ conversions, H₂ yield, H₂/CO ratio and H₂ recovery. Estimation of the CO-inhibiting effect on the H₂ molar flux through the membrane was assessed for a thick membrane. Simulations for a thin Pd-Ag MR show that (i) CO₂ and CH₄ conversions and H₂ yield increase with the feed temperature; (ii) H₂ yield and average rate of coke formation increase for higher pressures; and (iii) increasing CH₄/CO₂ feed molar ratio leads to higher H₂/CO ratios, but lower H₂ yields. Moreover, simulations for a thick Pd-Ag MR showed that the average H₂ molar flux decreases due to the CO inhibiting effect (ca. 15%) in the temperature range considered. In conclusion, this work showed that for the considered simulation conditions, the use of an MR leads to the inhibition of the RWGS reaction and improves H₂ yield, but coke formation and CO inhibition on H₂ permeation may pose limitations on its practical feasibility, for which proper strategies must be explored.

Keywords: biogas; dry reforming; hydrogen production; membrane reactor; syngas.

Figure 1

Illustration of the MR configuration.

Figure 2

Parity plot comparing the experimental CH₄ and CO₂ conversions obtained by [22] with simulated values.

Figure 3

Temperature profiles for the retentate (T_r) and permeate (T_p) zones of the reactor (a) and permeation fluxes of hydrogen (b) when operating the MR with feed temperatures of 450, 500 and 550 °C.

Figure 4

(a) CH₄ and CO₂ conversions, H₂ yield and H₂/CO mole ratio profiles, and (b) H₂ recovery and H₂ mole fraction profile on the permeate zone along the reactor, operating with a feed temperature of 450 °C.

Figure 5

Mole fraction profiles in the retentate zone along the normalized reactor length for different biogas compositions.

Figure 6

H₂ molar flux along the normalized reactor length for different biogas compositions.

Figure 7

H₂ molar flux profiles for the MR with thin and dense membranes, with and without considering the CO inhibiting effect and operating with feed temperatures of 450 °C and 550 °C.