Research On Some Important Problems During SCR Of NO Over V₂O₅/AC Catalyst

Coal is the richest fossil energy in China. The dominant role of coal inthe structure of primary energy in China will do not change for a long time inthe future. Combustion is the traditional mode of coal utilization in China,which takes up more than80%of the total consumption. Combustion of suchgreat amount of coal has already led to serious coal-burning air pollution inour country. NOx is one of the main pollutants in flue gases, which hasbrought tremendous threat on human health and ecological environment.Among various technologies developed, selective catalytic reduction(SCR) of NOXwith NH₃is recognized to be the most widespread and effectivetechnology for NOXabatement from flue gases of stationary sources. Aimingat the problems of V₂O₅/TiO₂based mid-temperature catalysts (usable ataround400oC), activated coke supported V₂O₅(V₂O₅/AC) catalyst wasinvented, which has a high low-temperature (～200oC) SCR catalytic activityand resistance to SO₂poisoning. Thus, this catalyst was studied extensively inthe past decade including preparation techniques, reaction behaviors, effects ofSO₂and H₂O, reaction mechanism and kinetics etc. So far, however, effect ofAC's properties on stability of ammonium sulfates, mechanism of SO₂ influence on SCR behavior, SCR selectivity to N₂, thermal stability of catalyst,and effect of Hg on SCR activity are not well understood, which are studiedsystematacially in this paper.The major conclusions obtained are as follows:(1) NH₄HSO₄tends to anchor at strong adsorption sites of AC initially atlower loadings and then progressively at weak adsorption sites with theincrease of NH₄HSO₄. Decomposition of NH₄HSO₄on AC follows two-stepreaction mechanism: formation of NH₃and H₂SO₄(leading to release of NH₃)starting at about170oC, and reduction of H₂SO₄(or SO₂-4) by AC to form SO₂at higher temperatures. The presence of NO and V₂O₅do not affect thistwo-step reaction mechanism.(2) The reduction of H₂SO₄to SO₂by AC support is responsible for theV₂O₅/AC catalyst's low-temperature resistance to SO₂poisoning since it freesthe pores of AC. In the experimental conditions, the BET surface area andpore volume of AC do not affect the reduction behavior of H₂SO₄, regardlessof the presence of NO. The oxygen and nitrogen containing functional groupsin AC benefit the reduction of H₂SO₄, while the minerals in AC areunfavorable to the reduction of H₂SO₄. V₂O₅loaded on AC and NO in the gasboth inhibit reduction of H₂SO₄to SO₂.(3) NH₄HSO₄inhibits the SCR activity of AC below175oC, and theinhibiting effect aggravates with the increase of NH₄HSO₄; NH₄HSO₄promotes the SCR activity above175oC, and the promoting effect increases with the increase of NH₄HSO₄. In the experimental conditions, the BETsurface area and pore volume of AC do not affect its SCR activity, and alsohave no relationship with the effect of NH₄HSO₄on SCR activity.(4) The effect of on-line gaseous SO₂on SCR activity of V₂O₅/ACcatalyst is different from that of pre-loaded NH₄HSO₄. SO₂has multi-role onSCR activity of V₂O₅/AC: on one hand, the sulfate species formed by SO₂andO₂+H₂O+NH₃can promote the catalytic activity; on the other hand, blockingthe pores of catalyst by sulfate species and competitive adsorption of SO₂withNH₃on V₂O₅surface both inhibit the catalytic activity. In the experimentalconditions, the inhibiting effect by competitive adsorption of SO₂with NH₃isobviously larger than the effect by sulfate species, thus SO₂inhibits SCRactivity of V₂O₅/AC.(5) The N₂O formation over V₂O₅/AC catalyst during SCR of NO by NH₃is generally low, less than10ppm in the temperature range of150-250oC.N₂O is formed from reduction of NO by both AC and NH₃. V₂O₅contributeslittle to the N₂O formation while SO₂+H₂O slightly accelerate N₂O formation.SCR selectivity to N₂is obviously promoted by V₂O₅and0.91wt%V₂O₅issufficient to yield a N₂selectivity of higher than95%.(6) Oxidation of the AC support to CO₂occurs in the SCR process and itsrate increases with an increase in V₂O₅loading and reaction temperature. SO₂in the gas promotes AC oxidation to CO₂due to formation of sulfuric acid onthe surface, which reacts with the carbon-and-oxygen containing functional groups on the AC. Some of the carbon-and-oxygen containing functionalgroups do not react with O₂under the conditions used, but can be oxidized bysulfuric acid.(7) SCR activity of V₂O₅/AC catalyst increases with the increasingtemperature, regardless of the adsorption of Hg on its surface. Hg adsorptioninhibits the catalytic activity, and the inhibiting effect aggravates with theincreasing Hg adsorption quantity and reaction temperature. The presence ofH₂O+SO₂strengthens the inhibiting effect of Hg on V₂O₅/AC. The adsorptionand oxidation behaviors of NH₃over V₂O₅/AC are not influenced by Hgadsorption.