Removal of dibenzothiophenes in kerosene by adsorption on rice husk activated carbon

Abstract

The capacity of rice husk activated carbon (RHAC) to adsorb refractory sulfur compounds of dibenzothiophenes (DBTs) from commercial kerosene was evaluated in terms of their textural and chemical characteristics. Rice husk activated at 850 °C for 1 h showed an acceptable adsorption capacity for DBTs, despite a much lower specific surface area (473 m²/g) and total pore volume (0.267 cm³/g), when compared to micro-porous activated carbon fiber with a large specific surface area (2336 m²/g) and total pore volume (1.052 cm³/g). The volumes of ultramicropores acting as DBTs adsorption sites, and of mesopores leading DBTs into the ultramicropores were closely related to the DBTs adsorption capacity of the RHACs.

Introduction

An extremely low sulfur content of the hydrogen produced from the reforming of hydrocarbon fuels is a key factor for fuel cell use. To suppress poisoning of the reforming and generating catalysts, the sulfur content of the hydrocarbon fuel should be as low as possible prior to fuel injection [1]. It is generally agreed that a sulfur content of <100 mass ppb is required for fuel cell applications [2]. Conventional hydrodesulfurization (HDS) is widely applied to reduce the sulfur content, but this does not remove all types of sulfur compound. Refractory sulfur compounds need to be removed by other cost-effective means, to promote the use of fuel cells. Song and Ma have comprehensively reviewed the approaches for deep desulfurization of hydrocarbon fuel [3].

Dibenzothiophene and its derivatives (hereafter called DBTs) contained in fuel oils such as kerosene and diesel oil are refractory poly-aromatic sulfur compounds. In particular, alkyl-substituted DBTs, i.e., 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), are highly refractory [3], [4]. Modified HDS methods using catalysts such as CoMo and NiMo [5], [6], [7], [8], [9], [10], [11], [12], [13], γ-Irradiation using additives and catalysis [14], biodesulfurization [15], oxidative desulfurization [16], [17], and adsorptive desulfurization using porous materials [2], [18], [19], [20] have been proposed for the efficient removal of refractory DBTs from fuel oils. Desulfurization using activated carbons [21], [22], [23] or catalyst-loaded activated carbons [24], which can be performed at ambient temperature and pressure, has been intensively studied. This seems to be more realistic than other methods in economic terms and for operational safety reasons. Toida demonstrated that activated carbon with a specific pore structure could be used for the efficient and selective removal of DBTs [25]. An activated carbon with a composite micro-mesoporous structure and a large pore volume is required.

A huge quantity of rice husk (ca. 2 million metric tons) is produced every year as agricultural waste in Japan. A micro-meso composite structure was shown to be obtainable from rice husk subjected to chemical (alkaline) activation [26]. However, this activation process requires extensive alkali treatment and rinsing, which hinders its industrial realization and mass production. However, rice husk that is now considered as agricultural waste in Japan [27] may be converted into a cost-effective adsorbent to remove DBTs without the use of chemical activation.

In the present study, rice husk was converted into an activated carbon intended for DBT removal from kerosene using a CO₂ gas activation method. Kerosene is widely used in Japan as a home heating fuel for portable and installed kerosene heaters, and can be readily purchased at filling stations or delivered to homes. Kerosene is a promising hydrogen source for fuel cells for domestic use in Japan. The DBTs adsorption capacity of rice husk activated carbon (RHAC) for commercial kerosene was evaluated in terms of the textural and chemical characteristics. The usefulness of activated carbon fiber (ACF) for ultra deep desulfurization of diesel oil has been examined by Sano et al. [28]. So, commercial micro-porous ACF was also tested as a reference.