(698e) Reactive Absorption and Kinetics of Nitric Oxide (NO) Removal by Aqueous Persulfate Activated by Temperature and Iron(II) Ions

The combustion of fossil fuel (e.g., in coal-fired plants) releases a large amount of NO_x (mainly NO and NO₂) and SO₂ into the atmosphere.  Both NO_x and SO₂ contribute to the formation of ground-level ozone, urban photochemical smog, acid rain, eutrophication, respiratory and cardiovascular diseases. The efficient, cost-effective and environmentally friendlier removal of NO_x from flue gases poses a considerable industrial problem. Available control technologies such as Selective Catalytic Reduction (SCR), used for high NO_X removal has very high capital cost and suffer from various problems including disposal of spent toxic catalysts.  Selective Noncatalytic Reduction (SNCR, urea injection) is highly temperature dependent and SNCR, ammonia injection (Thermal DeNO_X) has moderately high capital cost and ammonia handling problems.  Low-NO_X burners (LNB) also has moderately high capital cost. Aqueous scrubbing with chemical oxidants provides alternative treatment techniques. While a number of chemicals such hydrogen peroxide, peroxymonosulfate (oxone) and sodium chlorite have been used, there are continuing need for reagents that are inexpensive, efficient, and environmentally benign. Sodium persulfate (or peroxydisulfate) is one such compound.  The peroxydisulfate anion (S₂O₈^2-) is a strong oxidizing agent and because it is stable prior to activation, it is becoming increasingly popular in the in situ chemical oxidation. In this work, the absorption and kinetics of nitric oxide (NO) oxidation by aqueous solutions of sodium persulfate (Na₂S₂O₈) activated by temperature with and without Fe²⁺ were investigated via experimental and modeling studies in a bubble column reactor operated in the semibatch mode. The effects of persulfate (0.01−0.2M), Fe²⁺ (0−0.1M), and gas-phase NO (500−1000ppm) concentrations, and temperatures (23−90°C) on fractional conversion of NO were investigated. Initial runs conducted at 50^oC showed 0.01M Fe²⁺ as the optimum for NO absorption for the different persulfate concentrations studied. At 0.1M Na₂S₂O₈ and 0.01M Fe²⁺ concentrations, fractional conversions of NO (percent of inlet NO removed) were 52.1% and 63.4% respectively, for temperature-alone and combined temperature-Fe²⁺ activation. In general, the NO conversion at all temperatures in or without the presence of Fe²⁺ increased sharply with persulfate concentration up to about 0.1M before leveling off. Increased temperature led to increased conversions of NO at all persulfate levels. At 0.1 M persulfate concentration, conversions up to 69% and 92% were observed at 70^oC and 90^oC, respectively. However, in the presence of 0.01M Fe²⁺ concentrations, conversions of up to 79% and approximately 100% were observed at 70°C and 90°C, respectively. The results indicate that Fe²⁺ activation further improved NO removal by about 10% at all temperatures. Solution pH and chloride concentration both were found to affect NO absorption at the higher temperatures but the effect of initial NO concentration was found to be marginal. The results also demonstrate the synergistic and antagonistic effects of Fe²⁺, which is dependent on the persulfate-Fe²⁺ ratio. In addition to mechanistic reaction pathways proposed, models were developed to correlate the experimental data and estimate kinetic rate constants, reaction activation energies, and mass-transfer parameters for the NO_x-persulfate-iron (II) reactions. The results of this work and current evaluation of the effects of SO₂ and other flue gas components will be presented.

References:

1. Adewuyi, Y.G.; Sakyi, N.Y. Kinetic and Mechanistic Investigations of Nitric Oxide (NO) Removal by Aqueous Persulfate Activated by Temperature and Iron (II) Ions. Environmental Science and Technology. 2012. In review.

2. Khan, N.E.; Adewuyi, Y.G. Absorption and Oxidation of Nitric Oxide (NO) by Aqueous Solutions of Sodium Persulfate in a Bubble Column Reactor. Industrial & Engineering Chemistry Research. 2010, 49 8749-8760.

3. Adewuyi, Y.G.; Owusu, S.O. The Aqueous Absorption and Oxidation of Nitric Oxide with Oxone for the Treatment of Tail Gases: Process Feasibility, Stoichiometry, Reaction Pathways and Absorption Rate. Industrial & Engineering Chemistry Research, 2003,42, 4084-4100.

4. Adewuyi, Y.G., Khan, N.E. Modeling the Ultrasonic Cavitation-Enhanced Removal of Nitrogen Oxide (NO) in a Bubble Column Reactor. AICHE J. 2011 http://onlinelibrary.wiley.com/doi/10.1002/aic.12751/pdf