Shizuo Kawano

Removal of SOx and NOx over activated carbon fibers

Abstract The recent development of de-SOx and de-NOx using activated carbon fibers (ACF) in Japan is introduced in this review comparing it with the conventional commercialized system. Pitch-based ACFs showed higher de-SOx activity than other ACFs from different precursors. De-SOx activity can be further modified by the heat-treatment, continuous and complete removal of SOx. ACF is also effective for removing NOx in the presence of NH4. The mechanisms of de-SOx and de-NOx on the surface of ACFs are proposed and discussed. Based on fundamental research results de-SOx, de-NOx and their combined processes are designed and proposed.

High catalytic activity of pitch-based activated carbon fibres of moderate surface area for oxidation of NO to NO2 at room temperature

Catalytic oxidation of NO (380 ppmv) to NO2 over activated carbon fibres of moderate surface area (∼800 m2 g−1) at room temperature was carried out, to develop oxidative removal of NO from flue gas. The heat treatment of pitch-based activated carbon fibres of moderate surface area markedly increased the conversion of NO to 87% in dry air, 62% in air of 80% relative humidity and 24% in wet air (100% r.h.) at 25°C and a ratio of fibre mass to gas flow rate of 1.0 × 10−2g min mL−1. The strong inhibiting effect of humidity on the activity of the as-received fibres was moderated by heat treatment of the fibres at 850°C. A lower concentration of 10 ppmv NO markedly reduced the oxidation, the conversions being 24 and 5% in dry and wet (100% r.h.) air respectively. The catalytic activities of other pitch-based fibres of different surface area were also much inferior in moist air above 60% r.h.

NO oxidation over activated carbon fiber (ACF). Part 1. Extended kinetics over a pitch based ACF of very large surface area

Abstract Kinetics on the oxidation of NO was studied over a pitch based activated carbon fiber of very large surface area (pitch ACF) by changing NO and O2 concentrations in a flow reactor. NO was initially removed through adsorption until saturation, increasing gradually its outlet concentration. In the mean time, NO2 was found in the gas phase after an induction period to increase its concentration very nearly to the stationary one. It is noted that a maximum NO concentration was observed just before reaching the stationary concentration, where the adsorption of NO was almost saturated. High concentrations of both O2 and NO increased the amount of saturated adsorption and the stationary conversion of NO, Freundlich equation being well fitted to the observed adsorption and conversion. The reaction order larger than unity in NO suggests multi-molecular intermediate over the ACF surface for oxidation as the dimer has been proposed in the gas phase oxidation of NO. The major adsorbed species found was NO2 over the ACF surface in the stationary state, although some NO was also found in TPD profile of adsorbed species even in the stationary state. Some reduction of surface NO2 at its desorption is suggested. A peculiar peak of the maximum NO concentration observed before the stationary conversion may reflect participation of NO to the multi-molecular intermediate which requires a large coverage of adsorbed NO2 before rapidly releasing NO2 from the surface to the gas phase.

The modification of pore size in activated carbon fibers by chemical vapor deposition and its effects on molecular sieve selectivity

Abstract Pore size control of a series of activated carbon fibers was attempted by chemical vapor deposition (CVD) of benzene to clarify the influence of the pore distribution on the development of molecular sieving ability. Weight increase by CVD was found to saturate at a certain level respective to the fiber, reflecting their surface areas. However, when saturation was obtained, the molecular sieving selectivity between CO2 and CH4 was induced only in the smaller surface area fibers, whereas the fibers with larger surface areas lost the adsorption activity for both gases. Straight micropores developed from the surface can be controlled in their slit size, by carbon deposition selectively onto their wall, if benzene molecules can get into the pore. In contrast, the size of micropores developed on the walls of mesopores in the fibers with large surface area are difficult to control since the carbon deposition continues until the whole wall of the mesopore is covered by the deposited carbon, which plugs the micropores.

Molecular sieving selectivity of active carbons and active carbon fibers improved by chemical vapour deposition of benzene

Abstract Modification of the pore size of several types of carbon adsorbents by chemical vapour deposition of carbon from benzene at 1000 K was examined. The carbons included commercial molecular sieve carbon (MSC), pitch based active carbon fiber (ACF), super and commercial active carbon (S-AC, AC). Greatly improved selectivity for the separation of CO2 and CH4 was achieved by this method for MSC and ACF. ACF is expected to exhibit rapid adsorption and desorption rates during molecular sieving separations. Carbon adsorbents which contain micropores of uniform size appear to have the highest potential for improvement in selectivity by CVD. Saturation of benzene CVD was observed in such carbons, indicating the deposition in the pore. In contrast, carbons which contain micropores in combination with large concentrations of mesopores lose adsorption capacity by filling of the mesopores with deposited carbon. The concentration of benzene and deposition temperature are key factors in achieving homogeneous carbon deposition on the pore wall but not on the outer surface which leads to improved selectivity.

Removal of SO2 over ethylene tar pitch and cellulose based activated carbon fibers

The activities of activated carbon fibers (ACFs) from ethylene tar pitch and cellulose with various surface areas were examined for the oxidative conversion of SO2 into aq.H2SO4 in the presence of H2O and O2, and compared to those of coal tar pitch and polyacrylonitrile based ACFs. Cellulose ACF with surface area of ∼1000 m2/g after calcination at 1000°C in N2 showed the highest activity among the ACFs of similar surface area. The higher activity of cellulose ACF appeared to be obtained through the larger amount of CO evolution during the calcination. The SO2 adsorption was enhanced while H2O adsorption was inhibited over the ACFs by the calcination regardless of the sources of ACFs. Activities of fibers decreased significantly with increasing adsorption temperature.

Kinetic study of the continuous removal of SOx on polyacrylonitrile-based activated carbon fibres: 1. Catalytic activity of PAN-ACF heat-treated at 800°C

Continuous removal of SO2 as aqueous H2SO4 over polyacrylonitrile-based activated carbon fibres (PAN-ACF) was studied kinetically at room temperature to determine the effects of SO2 (20–1000 ppmv), O2 (0–10 vol.%) and H2O (0–10 vol.%) concentrations and WQ ratio (mass of ACF/volumetric flow rate of gas). An oxygen level > 3 vol.% was sufficient to provide steady-state removal of SO2. Higher inlet SO2 concentrations gave higher SO2 outlet concentrations, while more H2O and a higher WQ ratio increased the SO2 removal. On the basis of this kinetic study, the rate-determining step is postulated to be aqueous H2SO4 desorption from the ACF bed, which makes sites available for further SO2 adsorption. Hence a lower SO2 concentration, more H2O, and a higher WQ ratio are compensating factors in achieving the complete removal of SO2 by continuous recovery of aqueous H2SO4 at the outlet.

Kinetic study of the continuous removal of SOx using polyacrylonitrile-based activated carbon fibres: 2. Kinetic model

Abstract Sulfur dioxide was continuously converted to aqueous sulfuric acid using a polyacrylonitrile-based activated carbon fibre catalyst (PAN-ACF) in a packed-bed reactor system. The qualitative effects of residence time and oxygen, nitrous oxide, sulfur dioxide, and water concentrations were reported in Part 1. The conversion conditions were chosen to be typical of those expected in flue gas streams: sulfur dioxide 20–1000 ppmv, oxygen 0–10 vol.% and water 0–20 vol.%. A power-law model was developed to describe the steady-state concentration of sulfur dioxide in the exit gas. Sulfur dioxide reaction rate was proportional to catalyst weight, oxygen concentration to the 0.25 power, sulfur dioxide concentration to the 0.123 power and water concentration to the 1.01 power. The model was used to compare the effects of reactor operating conditions on the outlet concentration of sulfur dioxide.