Low-Temperature H₂S Removal from Steam-Containing Gas Mixtures with ZnO for Fuel Cell Application. 1. ZnO Particles and Extrudates

Abstract

Sulfur removal is important for a fuel cell that uses a hydrocarbon fuel, such as natural gas, liquefied petroleum gas, and gasoline, to prevent the downstream sulfur poisoning of catalysts in the fuel processor and in the fuel cell anode. Although most sulfur species are removed prior to reforming, the reducing environment of the reforming stage (such as autothermal reforming) converts residual sulfur to hydrogen sulfide (H2S). H2S in the reformate must be removed to ensure longevity of the catalysts in downstream processing and in the anode chamber of fuel cell systems. A unique modified ZnO sample with a different morphology has been prepared and comparatively studied together with a commercially available ZnO sample under various conditions. Extremely low H2S outlet concentrationsas low as 20 parts per billion by volume (ppbv)have been observed over the modified ZnO sample for extended periods of times. The sulfur-trap capacity (the amount of H2S trapped before breakthrough) also is dependent on space velocity, temperature, steam concentration, CO2 concentration, and particle size. Higher capacity is observed at higher H2S inlet concentration of 8 ppmv, compared to lower inlet concentrations of 1−4 ppmv. The trap capacity decreases monotonically as the temperature increases. Steam in the reformate inhibits the capture of H2S by ZnO; it seems to shift the equilibrium of the reaction ZnO(s) + H2S(g) ⇔ ZnS(s) + H2O(g) to the left, toward ZnO and H2S. The effect of steam seems to be reversible. Increasing the CO2 concentration in the feed up to 12 vol % decreases the capacity of ZnO for the capture of H2S.

Low-temperature H2S removal for solid oxide fuel cell application with metal oxide adsorbents

“…As the influence of steam content was investigated by Kim et al (2007), only dry gas was used for the test runs. Novochinskii et al (2004) reported that an increase of steam concentration had a negative influence on the H 2 S capture capacity of the ZnO sorbent. The accuracy of the certified mixtures was 3 ppmv H 2 S for the 200 ppmv gas mixture and 30 ppmv H 2 S for the 1000 ppmv gas mixture.…”

“…For those experiments, 1 g of sorbent sample has been flowed through at 293 K and a space velocity of 4000 h À1 for 24 hours. The sulfur capture capacity of the sorbent samples was calculated using the following formula, which has been used by Novochinskii et al (2004) and Kim et al (2007). In addition, the results were cross checked by analytical chemistry.…”

Synthesis Gas Purification

“…Theoretical maximum sulfur loading on pure ZnO is 39.5 mass% corresponding to the complete conversion of ZnO into ZnS. In general, the achieved capacity is near 25 mass% [76]. The sulfur removal rate is also depending on the characteristics of the sorbent: its textural properties, porosity and specific surface area [68,70,77,78].…”

“…The sulfur removal rate is also depending on the characteristics of the sorbent: its textural properties, porosity and specific surface area [68,70,77,78]. Moreover, the presence of H 2 O, CO and CO 2 in the gas may influence the absorption of H 2 S on ZnO: -given the equilibrium reaction, presence of H 2 O in the gas may affect the desulfurization performance of the trap [61,76]. However, for temperature lower than 300°C, this is expected to show little impact on S removal; -competitive adsorption between CO 2 and H 2 S is reported to impact S trapping.…”

Hydrogen sulfide and ammonia removal from biogas using water hyacinth-derived carbon nanomaterials

“…The use of nanoZnO has achieved up to 100% catalysis (Sayyadnejad et al, 2008) but the reaction is water sensitive therefore the presence of water vapour in the biogas favour backward reaction (Novochinskii et al, 2004).…”