The exhaustion of fossil fuels and increasing environmental concerns, have recently begun to motivate research into various alternative energy source, with biomass being the most popular. The products of biomass use, such as CO, water and ash, can be absorbed by living organisms, circulating in the environment and considerably reducing greenhouse gas emissions. Using biomass for energy production is generally accomplished either through the fermenting of glucose to ethanol or by a thermo-chemical process that utilizes supplied oxygen or air, such as pyrolysis, gasification, or incineration. Gasification is the most widely researched and applied as it emits fewer contaminants such as SOx and NOx, and produces combustible gas which is readily useful.
Gasifying lignocellulosic biomass produces gaseous materials, such as CO, CO2, H2 and CH4, as well as tar, char, ash and liquid materials. Tar production is particularly problematic, in that it causes fouling and blockage of process pipes, gas engines and gas turbines efficient tar reduction or removal is therefore necessary for increased commercialization. There are five different types of tar, including GC-undetectable, heterocyclic aromatic, light aromatic(consisting of 1ring), light polycyclic aromatic hydrocarbon(PAH, consisting of 2rings), and heavy PAH(consisting of 4rings). Benzene and toluene generally make up the largest proportion of tar.
The hydrocarbons present in tar can be treated by reforming, electrochemical, photochemical, and biological processes. Reforming processes, such as steam reforming, partial oxidation reforming and auto-thermal reforming, are attractive in that they operate at relatively low temperatures. Steam reforming is especially popular in hydrogen production, fuel cell generation, and DME processes.
Metal catalysts are generally used in steam reforming. Ni is the most commonly used metal catalyst because it provides high activity and is comparatively inexpensive; however activity decreases over time due to sintering, sulfur poisoning, and, especially, carbon production. To prevent this issue and enhance the function of active materials, promoters are added to increase catalytic activity.
Given the high costs of metals and rare earth metals, catalysts prepared with such materials involve high manufacturing costs. Compared to transition metals, however, they have greater activity and can be more carbon-resistant. They are also easier to purify and reuse. The burden of high costs can be reduced by using an adequate catalyst support to enhance long-term stability.
As such, this study focused on the steam reforming of toluene, using the Ru-based RUA and Ni-based FCR-4 catalysts. At high concentrations of model biomass tar, the effect of temperature, S/T ratio, and SV were analyzed. Both catalysts showed carbon conversion of over 90% at 800oC, with RUA showing slightly higher activity. Carbon conversion and H2 production increased with in creasing temperature in all experiments, while the conversion dropped as SV increased. The carbon conversion also increased with S/T, eliciting a rise in CO2 production due to the increased water content that drove the WGS. Mean while, SEM result reveals significant carbon deposition for FCR-4, and TGA showed minimal carbon production on RUA for all toluene concentration, but significant graphite accumulation for FCR-4, explaining the observed decrease in activity over time. In addition, XRD analysis showed increasing particle size for the spent catalyst as toluene concentration in creased, demonstrating that sintering on the surface of the catalyst caused the observed deactivation in that case.
In the light of that ,catalysts are manufactured for toluene steam reforming by the impregnation method and the multistage impregnation method
As a result, The carbon conversion and H2 production increased according to temperature with similar tendency. Catalytic activity was enhanced by adding rare earth element as promoter. Catalytic activity was maintained by using rare earth metals, especially the Dy promoter. The different results obtained with varying supports imply that supports affect the durability, life and activity of the catalysts. Because the type and amount of deposited carbon is closely related to deactivation by poisoning, the effect of carbon deposition on catalyst activation and stability was determined by analyzing the carbon structure and morphology.
The activation energy values were found to exist in the order of
-Catalysts prepared by the impregnation method
: Ce(65.58-66.88kJ/mol) > Nd(38.52-56.52kJ/mol) > Sm(47.00-51.83kJ/mol) > Gd(49.25-60.88kJ/mol) > Dy(37.06-37.49kJ/mol)
-Catalysts prepared by the multistage impregnation method
: Ce(47.94-59.29/mol) > Nd(44.90-51.07kJ/mol) > Sm(41.02-42.62kJ/mol)
> Gd(38.92-40.24kkJ/mol) > Dy(29.49-36.79kJ/mol).
SEM analysis showed that catalysts with a ZrO2 support and Ce promoter had high a mounts of carbon fiber.
Through this study result, it is expected to be useful for selecting the material and composition of the catalyst, when developing the catalyst that is possible to apply to reformation of tar in the future.