Research paperDurability studies of limonite ore for catalytic decomposition of phenol as a model biomass tar in a fluidized bed
Graphical abstract
Introduction
Biomass has attracted much attention as an energy resource because it is a carbon-neutral resource that is readily available worldwide. Thermochemical conversion of biomass by gasification is an effective way to produce combustible gases (H2, CO, CH4, and other hydrocarbons) that can be used for the production of electricity via fuel cells, internal combustion gas engines, and turbines, or for the synthesis of liquid fuel by the Fisher-Tropsch reaction. This process is generally conducted at high temperatures (>1000 °C); however, low-temperature gasification is desired to increase the cold gas efficiency of the gasifier. The problem with low-temperature gasification is the formation of tarry compounds (tar), composed of a complex mixture of aromatic hydrocarbons, because the condensation of tar causes plugging of lines and filters, leading to major operation problems in the gasification process.
To overcome these problems regarding low-temperature gasification, metal catalysts, which possess catalytic activity for the decomposition of tar and increase the gas yield, have been widely used. Generally, transition metals, such as Ni, Fe, and Co loaded onto metal oxides show high catalytic activity for the decomposition of tar [1]. From a point of a practical view, catalysts should be satisfactory with regard to both activity and cost. Our group has developed highly active and inexpensive catalysts, such as Ni-loaded chicken droppings [2], Ni-loaded brown coal char [3], [4], and limonite ore [5], for decomposition of tars derived from biomass and coal.
Limonite ore is a low-grade, inexpensive iron ore that includes iron species having catalytic activity for tar decomposition [6]. Li et al. [5] reported that an Indonesian natural limonite ore showed catalytic activity for coal tar decomposition to produce light fuel gas. Moreover, Zhao et al. [7] studied the catalytic reformation of biomass volatiles derived from corncob pyrolysis using an Indonesian limonite and found that the reforming activity of the limonite at 650 °C was comparable to a commercial Ni/Al2O3. One of the critical problems using catalysts for tar decomposition is the formation of deposit carbon on the active metal, which leads to the deactivation of the catalyst.
We also developed an internally circulating fluidized-bed gasifier for biomass gasification [8], [9]. This system consists of two chambers (gasification and combustion chambers) and a bed material that circulates between the two chambers. This process can utilize the heat from the combustion of biomass char for the gasification of biomass as follows. In the former chamber, the biomass is gasified to produce combustible gases and biomass char. The char formed is moved to the combustion chamber by a circulating bed material and burned with air to generate heat in the combustion chamber. The generated heat and the hot bed material are then supplied to the gasification chamber for application as a thermal source for gasification. During this process, if we used the catalyst for tar decomposition with the bed material, the carbon deposited on the catalyst during the gasification could be burned away, and the heat generated from this process could be used as the thermal source for biomass gasification. Therefore, the durability of the catalyst for O2 regeneration in the fluidized bed is important for the success of this process.
In this study, we evaluated the durability of the catalyst for long-term decomposition of biomass tar with O2 regeneration treatment and introduction of steam. The catalytic decomposition was conducted at a low temperature (650 °C) in a fluidized bed including a calcinated limonite ore. As a model tar, we used phenol, one of the main components of the tar in woody biomass [10]. The effects of the O2 regeneration treatment for the catalyst and introduction of steam during the tar decomposition on durability were evaluated in terms of gas production, tar formation, and carbon deposition.
Section snippets
Materials
Limonite is a natural iron-containing ore; the limonite used in these experiments was produced in Indonesia. The elemental composition of the limonite ore analyzed using X-ray fluorescence spectroscopy (XRF, EDX-700; Shimadzu Corp., Japan) is shown in Table 1. The limonite ore was calcinated by the following procedure before being used as a catalyst. First, the limonite powder was kneaded with distilled water to form balls 2–3 cm diameter. The obtained balls were calcinated at 900 °C for 30 min
Phenol decomposition activity of limonite catalyst and sand for 3 h
The Cgas percentages in the carbon balance using limonite and sand were 45% and 0.2%, respectively, indicating that the use of limonite dramatically increased the gas yield from the decomposition of phenol. Fig. 2 shows the gas yield during the phenol decomposition in N2 for 3 h using the limonite and sand beds. The gas production rate using limonite (Fig. 2b) was much higher than that using sand (Fig. 2a) during the 3-h reaction. When limonite was used as the bed material (Fig. 2b), the gas
Conclusions
Limonite calcinated at 900 °C was used as a bed material in the fluidized bed and showed catalytic activity for phenol decomposition to produce H2 and CO at 650 °C. Long-term durability tests of the phenol decomposition were conducted using the limonite with O2 regeneration treatment and the introduction of steam. The H2 and CO were constantly produced during the 25-h phenol decomposition, whereas the carbon deposited during the phenol decomposition could be removed repeatedly by periodic O2
Acknowledgements
This study was financially supported by the “Advanced Low Carbon Technology Research and Development Program (ALCA)” of the Japan Science and Technology Agency (JST).
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