An experimental study on the effect of operating parameters and sodium additive on the NOxOUT Process

An experimental study on the nitric oxide (NO) reduction through the NOxOUT Process has been conducted on a drop tube furnace experimental system. At 1223 K, when stoichiometric ratio of urea to NO (β) is fixed at 1.5 and 2, the maximum NO reduction efficiency of 70.19% and 78.89% could be achieved, respectively. The efficiency curve is presented as a reversed “V” as a function of temperature and the corresponding “temperature window” is 108 K and 154 K at β = 1.5 and 2. Also, these two β values are preferable to determine injected urea quantity. As oxygen concentration is increased from 1.5% to 4.5%, efficiency is continuously depressed from 68.35% to 55.37% at β = 1.5 and from 77.87% to 62.76% at β = 2. At the same time, residence time (τ) should be guaranteed at least 0.61 s for thorough NO reduction. When 50 ppm of NaOH, 25 ppm of Na2CO3 or 50 ppm of CH3COONa is injected as promoted additive, the maximum NO reduction efficiency is, respectively, heightened to 82.07%, 81.36% and 81.81% at β = 1.5 and these values are even higher than the ones which are achieved at β = 2 if no additive is injected. For both β = 1.5 and 2, efficiency improvement becomes slow when NaOH is increased to 20 ppm. Meanwhile, when NaOH is injected at β = 2, efficiency of above 90% could be achieved and this value is comparable to the one achieved in advanced reburning.

Research highlights▶ Characteristic of NO reduction during the NOxOUT Process is investigated on a drop tube furnace experimental system. Influencing factors of the reaction temperature, the normalized stoichiometric ratio of nitrogen reducing agent to NO (β, β = 2 × molar ratio of NH2CONH2/NO for urea), the oxygen concentration (O2) and the residence time (τ) are investigated. ▶ The maximum NO reduction efficiency of 70.19% and 78.89% could be achieved at 1223 K when β is set at 1.5 and 2, respectively. The corresponding “temperature window” is 108 K and 154 K. Efficiency is continuously depressed from 68.35% to 55.37% at β = 1.5 and 77.87% to 62.76% at β = 2, when O2 is increased from 1.5% to 4.5%. τ should be guaranteed at least 0.61 s. ▶ Sodium hydroxide (NaOH), sodium carbonate (NaCO3) and sodium acetate (CH3COONa) are selected as the additives. Their promoted effect on the NOxOUT Process is emphasized. ▶ When 50 ppm of NaOH, 25 ppm of Na2CO3 or 50 ppm of CH3COONa is injected, the maximum efficiency is heightened to 82.07%, 81.36% and 81.81% at β = 1.5. These values are higher than the ones under the β = 2 condition without additive injection. For both β = 1.5 and 2, improvement of efficiency becomes slow when NaOH is increased to 20 ppm. When NaOH is injected at β = 2, efficiency of above 90% could be gained. This is comparable to the one achieved in advanced reburning.