A system combining gas-phase oxidation and liquid-phase collision absorption for removing NO from marine diesel engine exhaust was proposed. This method was the first to utilize different physical states of the same mixed solution to achieve both pre-oxidation and impingement reduction absorption of exhaust gases. During the pre-oxidation stage, a mixture of (NH4)2S2O8and urea solution was atomized into a spray using an ultrasonic nebulizer to increase the contact area between the oxidant and the exhaust gas, thereby efficiently pre-oxidizing the exhaust gas in the gas phase. In the liquid-phase absorption stage, the (NH4)2S2O8and urea solution was used in an impingement absorption process, which not only enhanced gas–liquid mass transfer efficiency but also effectively inhibited the formation of nitrates. Experimental results showed that, without increasing the amount of absorbent used, the maximum NO removal efficiency of this method reached 97% (temperature, 343 K; (NH4)2S2O8concentration, 0.1 mol/L; urea concentration, 1.5 mol/L; NO concentration, 1000 ppm; pH, 7; impinging stream velocity, 15 m/s), compared to 72% using the conventional liquid-phase oxidation absorption method. Additionally, this method required only the addition of a nebulizer and two opposing nozzles to the existing desulfurization tower to achieve simultaneous removal of sulfur and nitrogen oxides from the exhaust gas, with low retrofitting costs making it favorable for practical engineering applications.
The impact of ship emissions on the environment cannot be ignored and should be controlled. The possibility of applying seawater electrolysis technology and a novel amide absorbent (BAD, C 12 H 25 NO) to the simultaneous desulfurization and denitrification of ship exhaust gas is entirely confirmed by using various seawater resources. Concentrated seawater (CSW) with high salinity can effectively reduce the heat generated during electrolysis and the escape of chlorine. The initial pH of the absorbent can greatly affect the NO removal capacity of the system, and the BAD could keep the pH range suitable for NO oxidation in the system for a long time. The use of fresh seawater (FSW) to dilute the electrolysis of concentrated seawater (ECSW) to make an aqueous oxidant is a more reasonable scheme; the average removal efficiencies of SO 2 , NO, and NOx were 97.10%, 75.41%, and 74.28%, respectively. The synergistic effect of HCO 3 − /CO 3 2− and BAD was shown to further restrict NO 2 escape.
With the aim to develop wet denitration process for marine application, it is still challenging to simultaneously achieve high-efficiency NO x removal from flue gas and low nitrate concentration in scrubbed solution. Herein a novel two-step method was proposed to solve this issue: phosphate buffer solution (PBS) with organic additive was used to absorb NO x efficiently from flue gas, and in next step, Na 2 SO 3 as a reductant was added in the scrubbed solution to reduce NO x off-line. Five typical organic additives were chosen to compose with PBS. The tetrabutylammonium bromide and phosphate buffer solution (TBAB/PBS) composite solution exhibited superior NO x absorption performance. Furthermore, the effects of TBAB concentration, NO x oxidation degree, solution pH, temperature and O 2 concentration on NO x absorption performance of TBAB/PBS composite solution were investigated systematically, and a high NO x absorption efficiency of >90% could be achieved. It was mainly attributed to a physical complex between PBS and NO 2 , N O 3 − $$ N{O}_3^{-} $$ , N O 2 − $$ N{O}_2^{-} $$ , together with a mass transfer enhancement by TBAB. A long-run test showed that nitrate concentration was much lower than the limit value of IMO. Cyclic absorption and regeneration tests confirmed that Na 2 SO 3 could effectively regenerate the scrubbed solution in the absence of O 2 .
A high nitrate concentration in scrubber water is a major factor that restricts ship applications of NOx wet absorption technology. In this paper, an absorption system (Na2SO3/NaHSO3) that can effectively improve NO2 absorption and reduce the concentrations of nitrate and nitrite was studied. Several factors that affect the NOx absorption efficiency and the concentrations of nitrate and nitrite were investigated in a bubble reactor by using Na2SO3 and Na2SO3/NaHSO3. In addition, the reaction mechanism for the process of NO2 absorption was deduced. The results illustrate that increases in S(IV) concentration and initial NO2 concentration could enhance the NOx absorption efficiency while increases in the NO/NO2 ratio and SO2 concentration decreased the NOx absorption efficiency. Moreover, the maximum NOx absorption efficiency was 97.8% at 0.15 mol/L S(IV) and 1400 ppm of NO2 when using the Na2SO3/NaHSO3 absorption system. The S(IV) concentration was the main factor affecting the NOx absorption efficiency and the concentrations of nitrate and nitrite. Increasing the SO2 concentration slightly affected the NOx absorption efficiency but was beneficial for decreasing the concentrations of nitrate and nitrite. The performance of the Na2SO3/NaHSO3 absorption system was better than that of Na2SO3 in the presence of oxygen. Furthermore, the minimum concentrations of nitrate and nitrite were 13.5 and 0 mg/L, respectively, in the Na2SO3/NaHSO3 absorption system for 1100 ppm of NO2 and 0.3 mol/L S(IV) and were much lower than current regulations for scrubber water. Finally, compared with other experimental results, the Na2SO3/NaHSO3 absorption system exhibits great potential for ship NOx wet absorption, and the addition of NaHSO3 plays a key role in decreasing the concentrations of nitrate and nitrite.
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