No abstract

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.

Biomass gasification is a primary process in the thermochemical conversion of biomass into biofuels, chemicals, and electricity. The produced raw gas consists of permanent gas species, such as hydrogen (H 2 ) and carbon monoxide (CO), with variable amounts of heavier/larger species, depending on the gasification technique and process conditions employed. These heavier species are often referred to as tar, which is herein defined as all species with boiling points that lie between the boiling points of benzene and coronene. In this work, experiments were conducted in the Chalmers 2-4-MW dual fluidized bed gasifier utilizing equipment that allows for simultaneous quantification of the cold gas and the tar species, together with the total raw gas yields of C, H, O, and N. The obtained results are used to describe the effects of temperature, steam-to-fuel ratio, residence time, and active materials on both the gas composition and the carbon balance of the system. Furthermore, as the carbon balance is fulfilled, the char conversion, oxygen transport, and amounts of carbon in unidentified condensable species can be determined. The unidentified condensable species comprise a group of compounds that are not measured as part of the other groups [cold gas and SPA tar, measured using the solid phase adsorption (SPA) method]. In addition, this group is shown to be readily converted into SPA tar, and cold gas as the severity of the gasifier, in terms of temperature and residence time, was increased.

Low-temperature gasification of biomass is a primary process step for production of biofuels or electricity using gas-fired engines or turbines. In addition to the desired product gas, the raw gas produced through gasification inevitably contains condensable hydrocarbons, known collectively as tar. The amount and composition of the tar have relevance for the efficiencies of the downstream processes. Tar can be measured using both online and off-line methods. However, many of these methods currently lack information regarding their implementation and accuracy levels for large-scale systems. In this work, the SolidPhase Adsorption (SPA) method for measuring tar in industrial applications is evaluated. The individual steps of the method were examined for their effects on the overall performance of the analysis. Sample collection was found to be the most prominent source of error, and this was mainly due to human factors. Omitting to determine the temperature and pressure of the sampled gas contributed to this error, as the sampled volume of gas under normal conditions could not be correctly calculated. Inconsistencies in the treatment and storage of the collected samples were shown to affect the more volatile species with boiling points similar to that of benzene. The gas chromatography (GC) analysis was performed with satisfying accuracy. However, the reliability of the estimations of the average composition and dew-point of the tar mixture were dependent upon the amount of the identified species. The current implementation of the SPA method yields values with a relative standard deviation within 10% for the majority of the compounds in a given sample. However, in line with the result of previous studies, the tar species with boiling points between those of benzene and xylene (i.e., the BTX compounds) were measured with a lower accuracy than those of heavier tars.

Biomass gasification produces a wide range of species, from permanent gases to condensable hydrocarbons, with different composition and boiling points. This complicates the mass balance of the system, as multiple techniques are needed to quantify the various components of the produced raw gas. In this study, a high-temperature reactor for thermal conversion of raw gas at 1700 °C was developed to generate a gas stream that consisted primarily of CO, CO2, H2, and H2O. The reactor was experimentally evaluated and subsequently used for measurements of the raw gas from the Chalmers 2-4–MW dual fluidized bed gasifier. The gas stream that exits the reactor is analyzed to obtain the total elemental flows of C, H, O, and N, which facilitate determinations of the fuel conversion and oxygen transport in a dual fluidized bed reactor. The proposed system was operated in parallel with a gas-cleaning system, to determine the yield of condensable species, including tar and GC-undetectable species. A simplified approach is proposed for quantifying the average energy content of the condensable species, thereby allowing the wet raw gas efficiency and lower heating value (LHV) to be calculated.