Regeneration of initial activity of a pitch-based ACF for NO–NH3 reaction at ambient temperature
Introduction
NO released from the combustion of any fuels has been recognized to contaminate the atmospheric environment, especially in places of poor ventilation[1]. Although a low concentration of NO is required to be removed at room temperature, no technology is available for use in limited space, especially in humid air[2].
The present authors have reported that PAN and pitch-based active carbon fibers of moderate surface area (∼800 m2/g) can reduce NO with NH3 at room temperature3, 4, 5, 6, 7, 8, 9, 10by a moderate rate in dry air. Inhibition by humidity of above 60% is a problem at room temperature[6]. Calcination at 850°C in inert atmosphere is very effective in enhancing the activity under humid conditions[7], nevertheless, its stationary activity is not sufficient[8].
The authors have found in a previous paper[11]that the ACF exhibited a complete removal of NO in humid air for the first few hours at W/F value of 3×10−2 g min/ml. The high activity is found to be due to adsorption as well as reduction of NO before the adsorption is saturated. The ACF after the initial period was confirmed to carry both NO and NH3 by their desorption. Hence, the reduction of adsorbed NO (including NO2) with adsorbed NH3 regenerates the initial activity of ACF. Such a reduction among adsorbed species has been observed before[12].
In the present study, the activity at an initial period and regeneration of a pitch-based ACF of moderate surface area after calcination at 850°C, was studied in order to design the effective removal of NO with NH3 in wet air at ambient temperatures by switching the removal of NO and regeneration.
The regeneration was attempted by flowing air or NH3 in air to reduce the adsorbed NO. In the latter case, adsorbed NH3 was expected to reduce adsorbed NO on the same ACF by a prolonged reaction time. The necessity of O2 at regeneration is a point of interest.
Section snippets
Active carbon fibers and their characterization
The pitch-based active carbon fiber (OG-8A) was supplied by Osaka Gas Co. As-received active carbon fiber was calcined at 850°C in the inert atmosphere in this study.
The calcined ACF carried C:93%, H:0.6%, O:5.0%, N:0.3%. Its surface area and pore volume measured using the BET method were 730 m2/g and 0.38 ml/g, respectively.
Reaction of NO–NH3
The reaction of NO (200 ppm) with NH3 (200–260 ppm) was carried out using wet air (relative humidity 80%) as a carrier gas, a U-shaped fixed-bed flow glass reactor (10 mm in
Activity of the ACF
Fig. 1 illustrates the activity of the ACF for the NO–NH3 reaction in 80% humidity at W/F=1.0–3.0×10−2 g min/ml. NO was completely removed for the first few hours, the length of time depending upon the amount of ACF; 2 h by 1 g of ACF, 4 h by 2 g, and 6 h by 3 g, respectively. The activity decreased gradually after the complete removal for several hours at the stationary conversion of 40–60% depending on the amount of the ACF.
Regeneration by NH3
Fig. 2 illustrates the activity and regeneration of the ACF (3 g) for the NO–NH
Discussion
The present study proposed a regeneration of initial activity of the ACF by wet air at 30°C for the complete removal of NO with least leakage of NH3, which includes both the steps of reduction of NO with NH3 and adsorption of NO during the reaction in wet air. Some of the adsorbed NO was oxidized into NO2 during the reaction as revealed by TPD[11].
The alternative operations of reductive and adsorptive removal and air regeneration may allow the complete removal of NO at room temperature in wet
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