Reactions between char and gasifying agents are usually the controlling step and of a core role for the overall biomass gasification process due to the relatively low reaction rate. Char reactivity will be greatly affected via interacting with volatiles, such as steam, hydrocarbons, tarry compounds, and other light gas species. By taking the updraft/downdraft moving bed and fluidized bed gasifier as examples, this review discussed the effects of char generation and evolution in various reactor environments on its following reaction behaviors. The characteristics of biomass and subsequent char gasification are further examined in terms of feedstock types and their inherent inorganics. Then, the effects of operation conditions and gasifying reagents on biomass gasification are outlined mainly from the point of char production and the subsequent volatiles−char interaction. Finally, some directions and suggestions considering char conversion and utilization are addressed for the design and betterment of the biomass gasification process.
Ash
fusion behavior is closely associated with ash-related problems
including fouling, sintering, and slagging, which results in a negative
effect on the utilization of petroleum coke (petcoke). Petcoke ash
contains high levels of vanadium (V), nickel (Ni), iron (Fe), and
calcium (Ca). The chemical composition of ash plays an intrinsic role
in determining ash fusibility. To better understand the modification
mechanism of the ash fusion temperatures (AFTs), this study investigates
the influences of ash composition (CaO, Fe2O3, and NiO) on the synthetic petcoke ash fusibility from the perspectives
of ash composition change and temperature rising. The AFTs of synthetic
ash samples were identified by the ash fusibility tester. X-ray diffraction
(XRD) and scanning electronic microscopy (SEM) were applied to explore
the relationships between the experimental AFTs and the variation
of mineral composition and microstructure of high-temperature ash
slag. Moreover, the ash melting process was predicted by the SiO2–Al2O3–V2O5–CaO–Fe2O3–NiO
system based on the FactSage modeling. The results show that the AFTs
of petcoke are closely related to the ash chemical composition. As
the CaO and Fe2O3 content increases, AFTs exhibit
continuous decline, while first decreasing slightly and then increasing
with the increasing NiO content, which is ascribed to the different
mineral transformation behaviors of high-temperature ash slag. The
dominant crystalline minerals formed in high-temperature ash slag
with different CaO, Fe2O3, and NiO content are
anorthite (CaAl2Si2O8), nickel orthosilicate
(Ni2SiO4), calcium pyrovanadate (Ca2V2O7), and quartz (SiO2). Fe may
form Fe-bearing amorphous matter with other minerals. The synergistic
effect between high-melting Ni2SiO4 and low-melting
Ca2V2O7 may contribute to the variation
of AFTs, which was well validated through thermodynamic equilibrium
calculations.
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