No abstract

A novel concept for secondary catalytic tar cleaning with simultaneous regeneration of the catalysts is presented in this work. The process is demonstrated by an initial experiment with producer gas from the Chalmers biomass gasifier using ilmenite (FeTiO3) as catalytic bed material. The tar cleaning system was operated at 800 °C and fed with raw gas from the Chalmers biomass gasifier, in which silica sand was used as bed material and the gasification was performed with steam. The tar content of the gas emerging from the gasifier was roughly 30 gtar/Nm3 gas. The experiment showed that the catalyst was continuously regenerated from carbon deposits, and the ilmenite reduced the total amount of tar by 35% at a gas residence time in the bed of 0.4–0.5 s. Branched hydrocarbons and phenols were more or less completely reformed, while there was an increase of stable aromatic rings (benzene, naphthalene). The catalyst showed high activity in water-gas shift reactions, and the H2/CO ratio was shifted from around 0.7 prior to the reactor to almost 3 after the reactor.

Biomass gasification plays an important role in the emerging production of second-generation biofuels. One of the major challenges facing biomass gasification is to find simple and efficient ways to reform tar components. While the tar causes operational problems, it can be reformed to increase the chemical efficiency of the gasification process. With respect to tar reforming, catalytic materials are of special interest. Many of the materials that have been proposed as promising catalysts are metal oxide-based materials. However, metal oxides also have the ability to transport oxygen when subjected to alternating oxidizing and reducing atmospheres, similar to that which occurs in a dual fluidized bed gasification system. In this work, ilmenite was used as the catalytic material in the Chalmers 2–4 MWth dual fluidized bed gasifier to decrease the yield of tar. The ilmenite was mixed with the silica sand, which was used as the bed material, to investigate how the level of ilmenite affected chemical efficiency and tar yield. Furthermore, energy balance calculations were established to elucidate the general aspects of oxygen transport in dual fluidized bed gasification systems. The results presented in this paper reveal that adding low levels of ilmenite reduces the tar yield by ∼50%mass. However, the oxygen transport induced by ilmenite caused a reduction in the chemical efficiency of the gasifier and the heating value of the gas, compared to using 100% silica sand as the bed material. The impact of adding ilmenite was found to be dependent upon the operational conditions of the gasifier; a low fluidization velocity gave the highest reduction of the tar yield, whereas higher fluidization velocities led to increased levels of heavy components. Overall, the use of ilmenite as a catalyst for reduction of the yield of tar appears promising, provided that the level of oxygen transport can be restricted.

Both agglomeration of bed material and corrosion of heat transfer equipment are issues related to combustion of biomass in a fluidized bed boiler. The biomass-ash component potassium is considered a major contributor for both phenomena. In this study, the conventionally used bed material, silica sand, was replaced with up to 40 wt % by the natural ore ilmenite in Chalmers 12 MW th circulating fluidized bed (CFB) boiler. In this study the purpose was to evaluate the physical and chemical changes ilmenite undergoes during this process. Close observations revealed that ilmenite underwent segregation of iron to the surfaces and an enrichment of titanium in the particle core. The ash formed a calcium-rich double layer on the particle, surrounding the iron layer. A diffusion of potassium into the particle core was also seen which led to the formation of KTi 8 O 16 . In addition to evaluating how ash components interact with the material, the ilmenite was leached and investigated as a possible potassium capturer. Leaching experiments on the used ilmenite showed that calcium and potassium were leachable to a very limited degree, namely, to less than 0.2 and 1 wt %, respectively, of the total content. The diffusion of potassium into the core of the particle could reduce both agglomeration and corrosion issues and could thereby be of great value for the improvement of the resistance of the bed material agglomeration in the fluidized bed boiler.

Biomass is recognized as a CO2-neutral energy resource. However, biomass is a challenging fuel to combust because of its heterogeneity with regard to the content of inorganic constituents, volatiles, and moisture. Oxygen carrier aided combustion (OCAC) is a process advancement that provides enhanced combustion in existing circulating fluidized bed (CFB) units. The oxygen carrier has a central role in the OCAC concept through the oxygen transport it provides. The natural mineral ilmenite (FeTiO3) has been identified as a promising potential oxygen carrier. In order to ensure the feasibility even for long-term operation in industrial-scale processes, it is imperative to understand the evolution of the material during an OCAC process. In the present study, ilmenite was used as the bed material in the Chalmers 12 MWth CFB boiler during OCAC with woody biomass as fuel. Bed material samples were extracted from the bed inventory at different time intervals ranging from 5 to over 300 h. This paper proposes a mechanism for migration and layer growth of biomass ash on the ilmenite used as the oxygen carrier in a CFB combustor. It was found that with increased time of exposure, potassium migrated into the particle core. Longer process times led to the formation of a calcium layer around the particle, and simultaneously, migration of calcium inward on the particle was observed. Thermodynamic calculations were used along with analysis techniques in order to build a hypothesis for the possible mechanism of ash–bed material interaction.