Dongjing Liu

CeO2-La2O3/ZSM-5 sorbents for high-temperature H2S removal

Performance of CeO2-La2O3/ZSM-5 sorbents for sulfur removal was examined at temperature ranging from 500 oC to 700 oC. The sulfur capacity of 5Ce5La/ZSM-5 was much bigger than that of CeO2/ZSM-5. H2 had a negative impact on the sulfidation; however, CO had little influence on sulfur removal. The characterization results showed that CeO2 and La2O3 were well dispersed on ZSM-5 because of the intimate admixing of La2O3 and CeO2, the major sulfidation products were Ce2O2S and La2O2S, the XRD and SEM results revealed that ZSM-5 structure could remain intact during preparation and sulfidation process, the H2-TPR showed that the reducibility of CeO2 can be remarkably enhanced by addition of La.

Status of Coal Gas H 2 S Removal

Coal is found in huge amounts throughout the world and has lower cost as compared to other fossil fuels, it represents at present about 70% of the world’s proven fossil fuel resources; therefore, coal is probably to remain one of the most important sources of primary energy for a long time, playing a strategic role in the medium–long-term energy production systems. However, coal utilization has a few negative impacts on the environment and atmosphere; the critical issue in promoting coal utilization is environmental pollution control without reducing the energy efficiency. Coal is a complex chemical mixture composed of carbon, hydrogen, and dozens of trace elements. When coal is severed as a fuel source, some of these elements would convert to gaseous emissions, such as sulfur dioxide (SO2) or hydrogen sulfide (H2S), nitrogen oxides (NO x ), mercury, and other chemical by-products via the coal combustion or thermal decomposition. These emissions have been established to possess detrimental effects on the environment and human health, which contributes to acid rain, lung cancer, and cardiovascular disease.

KINETIC BEHAVIOR OF ELEMENTAL MERCURY SORPTION ON CERIUM- AND LANTHANUM-BASED COMPOSITE OXIDES

The kinetic behavior of gas phase mercury sorption on ZSM-5 supported cerium- and lanthanum-based composite oxides in the temperature range of 120–240∘C is investigated. The Hg0 sorption abilities all first increase and then decrease with the elevation of the temperatures, the cerium-based composite oxides show better Hg0 sorption abilities than that of the lanthanum-based composite oxides. CeO2–Mn2O3/ZSM-5 performs the best Hg0 sorption ability with Hg0 removal efficiency of nearly 100% at 180∘C. Doping Cu or Mn into La2O3 lattice causes the decrease in the Hg0 removal efficiency of the lanthanum-based composite oxides due to the formation of perovskite or perovskite-like compounds. The sorption of elemental mercury over metal oxides is governed both by diffusion processes as well as chemical reactions, and the sorption of elemental mercury on the cerium- and lanthanum-based composite oxides are chemisorption processes.

Desulfurization Kinetics and Thermodynamics

The high-temperature desulfurization is a complicated gas-solid non-catalytic redox reaction, the sulfidation activities of cerium-based, lanthanum-based desulfurizers and nano elemental metal (nano Cu and nano Fe) desulfurizers are governed by chemical factors (active components) as well as physical factors (such as surface areas, pore structures, and particle sizes).

Rare-Earth Oxide Desulfurizers

The IGCC coarse gas derived from coal gasification process usually contains large amounts of sulfur containing components, such as H2S, COS, and CS2, 90% of them are H2S (~0.1 to 1.5 vol.%) which could cause severe corrosion of the pipelines, facilities, and catalysts in the subsequent coal gas conversion process; thus, the sulfur compounds need to be removed prior to syngas utilization. The hot coal gases are often purified with solid sorbents (single metal oxides or bimetal oxides), namely, called as dry desulfurization. Recently, the most commonly used sorbents are transition-metal oxides and their composites, for instance, zinc oxide, copper oxide, ferric oxide, and manganese oxide as well as zinc ferrite, zinc titanate, which are called as the first-generation high-temperature desulfurizers (FHDs)

Nano Elemental Metal Desulfurizers

It is well known that the support of sorbents plays a crucial role in gas diffusion and sulfidation reaction, and the supports of big specific areas and large pore volumes are beneficial for H2S adsorption [1]. Mesoporous carbon aerogel, a 3-D network structure of interconnected nanosized primary particles, is regarded to be a preferable support. Carbon aerogels have a unique porous structure, including well-developed and controlled interparticle mesopores and intraparticle micropores, huge pore volume, and large specific surface area. Additionally, the carbon aerogels remain stable under high temperature, and they could be formed into various shapes and can be used without any further forming treatment, which makes these materials attractive as the support of the desulfurizer [2, 3, 4].

H 2 S and Its Effect on Devices and Environment

Hydrogen sulfide, the chemical compound with the formula H2S, is a colorless gas with the characteristic foul odor of rotten eggs, and it is very poisonous, corrosive, acidic, and flammable [1]. Swedish chemist Carl Wilhelm Scheele has firstly discovered hydrogen sulfide in 1777. Hydrogen sulfide usually results from the microbial breakdown of organic matters in the absence of oxygen gas, such as in swamps and sewers; this process is commonly known as anaerobic digestion. Hydrogen sulfide could also occur in volcanic gases, natural gas, and in some sources of well water. The human body produces small amounts of hydrogen sulfide and uses it as a signaling molecule.