Cracking behavior of asphaltene in the presence of iron catalysts supported on mesoporous molecular sieve with different pore diameters

Cracking of a mixture of petroleum asphaltene and 10 wt% Fe catalyst supported on mesoporous molecular sieve (SBA-15) possessing a hexagonal array of uniform mesopores has been studied with a fixed bed reactor at 573 K under atmospheric He. When average pore diameter of Fe/SBA-15 is varied between 4.5 and 15 nm, asphaltene conversion increases almost linearly with increasing pore diameter up to 12 nm and reaches 65%, though the increment is small beyond this value. On the other hand, yield of maltene formed is almost independent of the diameter and less than 15% but greatly improved by using pressurized H₂ in place of He. Although pore volumes of all Fe/SBA-15 catalysts decrease by mixing with feed asphaltene, the extent of the decrease is larger for the catalyst with a larger pore diameter, which shows that higher asphaltene conversion may arise from the presence of larger amounts of asphaltene molecules held inside the larger mesopores. The N₂ adsorption measurements reveal that pore structures of Fe/SBA-15 catalysts are almost unchanged after cracking and subsequent re-calcination to remove deposited coke. The X-ray diffraction analysis and temperature programmed oxidation after cracking suggest that Fe species are highly dispersed inside the mesopores and present as the sulfided phases at the outermost layer.     

Design simplification of a small nuclear reactor for large-diameter neutron transmutation doping silicon using control rods

A design concept for a small nuclear reactor for neutron transmutation doping silicon (NTD-Si) using a Pressurized Water Reactor (PWR) full-length fuel assembly was proposed in our previous work. The excess reactivity was suppressed by a combination of Gd₂O₃ and soluble boron, which results in a flatter flux profile over the core than with control rod insertion; however, the soluble boron system for reactivity control is quite complex and expensive. The removal of this system would make the design much simpler. In the current work, the removal of soluble boron is considered. Criticality, neutron transportation and core burn-up calculations were performed using the MVP/GMVP II code and MVP-BURN code. The calculation results show that the insertion of control rods in five of the nine assemblies is enough to suppress reactivity. The thermal hydraulic analysis showed that heat removal from the core was possible under 1 atm operating pressure. Silicon ingots up to 30 cm in diameter could be irradiated with sufficient uniformity in the irradiation channels.     

Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification

Biomass gasification produces a low to medium-BTU product gas (or syngas) containing primarily CO₂, H₂, CO, CH₄ and (C₂ + C₃), as well as some contaminants such as tars, NH₃, H₂S and SO₂. In order to achieve better efficiencies of the syngas applications, these contaminants must be removed before the syngas is used for internal combustion, gas engines, and in particular for fuel cells and methanol synthesis. Compared with the wet scrubbing technology, hot-gas cleanup technology to remove tar, ammonia and other contaminants at the “hot” state is more advantageous with respect to energy efficiencies. This paper provides an overview on recent advances in catalysts for hot-gas removal of tar and ammonia from biomass gasification. The review focuses on the recent development and applications of dolomite catalysts, iron-based catalysts, nickel and other metal supported catalysts, and the novel carbon-supported catalysts for hot-gas tar removal and ammonia decomposition. The barriers in applications of hot-gas cleanup processes and catalysts for full-scale biomass gasification, and areas for future research, are also discussed.     

Design concept of a small nuclear reactor for large-diameter NTD-Si using a conventional PWR full-length fuel assembly

A design concept for a small nuclear reactor dedicated to large-diameter neutron transmutation doping silicon (NTD-Si) is proposed. Conventional PWR (Pressurized Water Reactor) full-length fuel assembly is used to assure stable and reliable supply of fuel. Criticality, neutron transportation, and core burn-up calculations are performed using the MVP/GMVP II code and MVP-BURN code. The calculation results show that the proposed reactor can be critical over 18 years, and excess reactivity can be suppressed by a combination of Gd₂O₃ burnable poison and soluble boron. Preliminary steady-state single-channel thermal hydraulic analysis showed that heat removal from core is possible under 1 atm operating pressure. Si ingots up to 30 cm in diameter can be irradiated in the reactor irradiation channels, and the uniform irradiation condition can be achieved for a large-diameter Si ingot.     

Conceptual design of a small nuclear reactor for large-diameter NTD-Si using short PWR fuel assemblies

A design of a small nuclear reactor for a large-diameter NTD-Si using a conventional Pressurized Water Reactors (PWR) full-length assembly was proposed in previous works. The height of the full-length assembly was 400 cm, and the overall size of the reactor and reflector around the core became large. In addition, the irradiation channel became very long, making handling of the Si ingots in the channel more difficult. The use of a short PWR fuel assembly, with a height of 100 cm, was considered in the current work. With the shorter assembly, the design of the reactor became compact and more practical. Gd₂O₃ and control rods were used to suppress excess reactivity. Criticality, neutron transport, and core burn-up calculations were performed using the MVP/GMVP II code and MVP-BURN code. Steady-state single-channel thermal hydraulic analyses were also performed. The calculation results showed that the reactor could be critical over 1200 days, and that heat removal from core was possible under 1 atm operating pressure. Large-diameter ingot up to 20 cm in height could be doped with sufficient uniformity. The reactor semiconductor production rate was estimated, and varied between 48 tons/year and 70 tons/year for the 50 Ω cm target resistivity depending on the position of the control rod.