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Energies 2020, 13, 420 45 of 96 The metal catalyst is subjected to deactivation processes during methanation reactions, including chemical deactivation due to poisoning and vapor–solid reactions, thermal deactivation due to sintering, and physical deactivation due to fouling and attrition [575]. Poisoning of the catalytic material is due to contaminants in the inlet gas, such as chlorides, tars, ammonia, alkali metals, sulfuric acid and coke [554]. Contaminants are chemisorbed on the catalyst surface, leading to the formation of new compounds and blocking the active sites. The electronic and geometric structure could be modified, altering the catalyst activity and selectivity irreversibly [576]. The poisoning by sulfur compounds (i.e., H2S) is a critical issue in numerous catalytic process, including methanation. Bartholomew et al. [577] explore the effect of operating conditions on H2S poisoning. They found that the increasing of H2/COx ratio and temperature promote sulfur poisoning. Moreover, only the MoO3 promoter increases the sulfur tolerance of nickel catalyst. Catalysts can be regenerated under controlled oxygen atmospheres or with inorganic oxidizing agents (i.e., KMnO4) that oxidize liquid phase and the adsorbed sulfur [578]. Vapor–solid reactions decrease catalyst activity. Catalysts react with inlet gas, support or promoters resulting in the formation of volatile compounds. Nickel reacts with carbon monoxide, generating nickel tetracarbonyls (Ni(CO)4) that increase nickel crystals size [579]. The vapor–solid reaction occurs at temperatures above 300 ◦C. Normally, the operating temperature of industrial plants are higher, but the problem exists at the start-up and shut-down phase of the methanation plant [580]. Sintering is a thermal degradation enhanced by high temperature and pressure that reduces the activity of catalysts. The total active area is reduced by the support collapse, the nickel encapsulation, and the agglomeration of the catalyst nanoparticles [581]. Steam promotes particles agglomeration, leading to the formation of Ni2+ doped alumina phase that promotes the loss of activity [582]. The particle growth could occur via two mechanisms: the particle migration, collision and coalescence, and the atom migration or vapor transport [583]. The former mechanism occurs at low temperatures, while the atom migration is the dominant sintering mechanism at high temperatures (550–600 ◦C) [584]. The deposition of dopants (i.e., ZrO2) onto the catalyst surface [585] and the encapsulation of colloidal catalyst nanoparticles prevent the degradation process [586]. Further, the regeneration of the deactivated catalysts due to sintering and carbon deposition is practicable. The calcination of the catalyst removes the carbon deposition, and the reduction of the NiAl2O4 lattice regenerates the metallic Ni particles with small size [587]. Fouling is the mechanical deposition of chemical compounds on the catalyst surface blocking the active sites and pores. Fouling also provokes the detachment of the metal catalyst from the support material [66]. The main deposited species are carbon, produced by CO cracking, and coke, generated by the decomposition or condensation of hydrocarbons. Carbon and coke deposition is restrained by the control of operating conditions [588]. Attrition is the breakage or abrasion of particles induced by the mechanical motion between particles in the feed fluid and the catalytic material. Many variables, including the direction of the collision between the catalyst and moving particles, the velocity and the contact stress, govern the attrition effects [589]. Indeed, either abrasion or fragmentation could occur. Abrasion removes fine particles of the catalyst surface, fragmentation shutter the surface in several parts [590]. The product yield, the gas composition and the carbon deposition depend on operating conditions including temperature, pressure (see Figure 20) and concentration ratio between inlet reactants (H2/COx) (see Figure 21).PDF Image | Green Synthetic Fuels
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