Study On Catalytic Removal Of NO_x In Marine Exhaust Gas By Non-thermal Plasma Assisted Activated Carbon

Exhaust pollutants such as SOx,NOx and PM emitted by marine diesel engines are harmful to human health and environment,especially near port cities.With growing concerns on environmental protection,regulations become more and more stringent.And marine emissions also receive increasing attention.In this paper,existing ship exhaust control technologies were reviewed,including pre-treatment,in-cylinder cleaning and after-treatment technologies.The technologies to meet the SOx emission requirements of IMO regulations mainly include low sulfur fuel oil and wet scrubber.And the selective catalytic reduction technology(SCR)is the main technology to meet the NOx emission requirements.The desulfurization technologies for ships have been mature,and the denitration technologies are hot and difficult topics for current researches.Because the big price fluctuation of low sulfur fuel oil,the market still holds a wait-and-see attitude towards the investment of wet scrubber.SCR denitration technology which can be applied to both desulfurization tower and low sulfur fuel oil technology will have a broader application prospect.Vanadium-based catalysts are mainly used for marine diesel exhaust denitration.However,their poor catalytic ability at low temperature and sulfur tolerance,as well as high toxicity and cost,are big turnoffs.Therefore,it will be of great value to find a new route of denitration technology.In summary,a synergistic denitration technology based on activated carbon(AC)and non-thermal plasma(NTP)is proposed in this paper.The current research works are as follows:(1)Firstly,based on a set of diesel generator bench,the emission characteristics of six engine working points at rated speed equipped with DOC and vanadium-based SCR were tested.The effects of DOC on CO,NOx and PM were analyzed.The denitration efficiencies of SCR at different power points and urea injection were investigated.The results show that DOC could convert NO to NO2,but it has little effect on the total concentration of NOx.The denitration efficiency of SCR is closely related to temperature and NH3/NOx ratio.It is almost 100%when the temperature higher than 250? and NH3/NOx=1.The denitration efficiency of vanadium-based SCR decreases to about 70%when the temperature is about 200?.(2)Secondly,a simulated exhaust gas system of diesel engine was set up.The coal-based AC was used for simulated exhaust gas denitration in different conditions.The results show that the NO removal ability of AC is poor in the NO/N2 system.The NO2 removal ability is excellent in the NO2/N2 system,where NO is desorbed.The NOx removal efficiency is 95%when the temperature is higher than 200? in the NO2/NH3/N2 system.When the temperature is lower than 100?,AC can catalytically oxidize NO to NO2 in the NO2/O2/N2 system.The near-stable catalytic efficiencies of AC for a slow SCR reaction,a standard SCR reaction,and a fast SCR reaction at 300? are 12.1%,31.6%,and 70.8%,respectively.(3)Thirdly,the experimental study on exhaust denitration was carried out by using dielectric barrier discharge(DBD)reactor to generate non-thermal plasma(NTP).The power efficiency and the denitration efficiency of different gas components by NTP were discussed.The exhaust gas reaction mechanism was analysed.The experimental results show that the NO removal efficiency is close to 100%by NTP in the NO/N2 system.For the NO/O2/N2 system,the critical oxygen concentration increases with NO concentration.The O2 concentration plays a decisive role in the denitration performance of the NTP.H2O contributes to the oxidative removal of NO.And NH3 improves the removal efficiency at low ED while slightly reduces the denitration performance at high ED.CO2 has little effect on NTP denitration performance,but as the ED increases,the generated CO gradually increases.When simulating typical diesel engine exhaust gas condition,the removal efficiency increases first and then decreases with the increase of ED in the NO/O2/CO2/H2O/N2 system.After adding NH3,the removal efficiency of NOx reaches up to 40.6%.(4)Fourthly,the combination of NTP generated by dielectric barrier discharge(DBD)and AC was used to remove NOx from the simulated marine diesel engine exhaust gas.The results show that with the simulated exhaust gas in the range of 100?300?,for AC+NH3 system or NTP+AC system,the denitration efficiency decreases first and then increases with temperature.The denitration efficiency of the NTP+AC+NH3 system can reach 91.8%after 1 hour's NTP action at 0.5kJ/L energy density.The AC characterization results show that NTP increases the specific surface area of AC.Around 300?,AC is rather stable and the NTP+AC+NH3 system can maintain high denitration efficiency for at least 5 hours.(5)Last but not the least,the removal of PM,CO,and NOx from real exhaust gas by the combination of AC and NTP was studied.The results show that PM removal efficiency reaches up to 77%without NTP,while the pressure drop increases with running time.AC may degenerate to some extent with the increase of temperature,resulting in a negative removal efficiency of CO.When turn off the NTP,the denitration efficiency of AC is up to 34.5%without urea,and further increases to 44.8%after spraying urea,still a distance from industzial applications.When turn on the NTP,the denitration efficiency can reach 92.5%for real exhaust gas by NTP+AC+NH3 system,which is comparable to the vanadium-based SCR catalyst.This research shows that the denitration efficiency of the synergistic denitration technology for ship exhaust gas based on activated carbon and non-thennal plasma is comparable to that of vanadium-based SCR technology.From the denitration performance,the synergistic denitration technology of NTP+AC can be used as an alternative technology of vanadium-based SCR.And because of its non-toxic and low-cost,it has good application prospect and great economic and environmental value.This paper lays a foundation for the industrialization of this technology.