Removal of sulfur dioxide and nitric oxide by lignite and various other lignite-derived adsorbents

Abstract

The purpose of this work was to develop an adsorbent from Saskatchewan lignite that is efficient for the removal of sulfur dioxide (SO2) and nitric oxide (NO) from flue gases and, also, to develop the models for their removal processes. The development of the adsorbents was achieved by optimizing a number of adsorbent preparation parameters using extensive physical characterization of the adsorbents as well as their SO2 and NO removal performance as the tool. The carbonization of lignite to produce char and subsequent activation of char to produce activated carbon (AC) as well as the SO2 or NO removal performance evaluation were performed in stainless steel fixed-bed reactors. The operating conditions for carbonization were: carbonization temperature, 350-550°C; time, 30-120 min. Those for activation were: activation temperature, 550-50°C; time, 15-45 min; activation agent, steam and carbon dioxide (CO2). SO2 and NO removal performance evaluation experiments used: particle size, 2-5.6 mm; adsorption temperature, 75-175°C; residence time, 4-10 sec; influent SO2 concentration, 1000-5000 ppm; influent NO concentration, 400-1100 ppm; O2 concentration, 0-6.5 vol%. SO2 removal performance depended strongly on the fraction of supermicropores in the adsorbents, types of metals impregnated, SO 2 residence time and influent concentration, and on the presence of O2. Maximum SO2 removal (86 mg/g) was achieved with the reduced Cu/AC adsorbent. On the other hand, NO removal efficiency depended on the surface area of the adsorbent, type and valence state of the metal impregnated, and presence of O2. High NO removal efficiency was achieved with reduced Cu/AC (46%) and Ni/AC (47.5%) adsorbents in the presence of O2. For the first time, a kinetic model has been developed for predicting NO removal from a mixture of NO-N2 over char and AC adsorbents. Also, an adsorption model has been developed for the removal of SO2 over lignite. The results from these studies have also shown, for the first time, that methanol can be used for complete removal of NO in the absence of O2 over a Mn-promoted co-precipitated Cu-Al catalyst at temperatures as low as 200°C.