Emission controls for NOx compliance Challenges & Applications Presentation @ Greener Shipping Summit November 2017 Dr. John Emmanuel Kokarakis Bureau Veritas
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High pressure SCR (NH 2 ) 2 CO NO x N 2 -- H 2 O CO 2 3
Exhaust Gas Recirculation Sludge Water Treatment Cooler Blower 4
EGR vs. SCR Where does the arrow point? EGR FEATURE SCR Capital Cost - CAPEX Operational Cost - OPEX Fuel Flexibility Sludge Production Compactness Impact on efficiency Impact on durability Experience Commissioning/Certification Scrubber versatility 5
SCR Catalysts Plate-type catalysts have lower pressure drop. Less susceptible to plugging. But larger more expensive Honeycomb configurations are smaller with higher pressure drops and plug much more easily. SCR Catalyst must be sized to allow for adequate residence time and not increase engine back-pressure. 6
SCR Operational Challenges Sensitive to contamination and plugging. Pores can be plugged by fine particulate, ammonia sulfur compounds, ammonium bisulfate (ABS) and silicon compounds. Plugging & back-pressure Poison compounds alter the chemistry of the catalyst affecting NOx reduction and cause unwanted oxidation of ammonia to NOx. Must ensure uniform distribution of ammonia. Inefficient NOx reduction means excessive ammonia slip. Problems with control system and NOx monitoring. Corrosion in exhaust pipe with urea leaking onto engine. Higher sulphur reduces lifetime of catalyst and requires bulkier SCR to increase residence time. 7
Exhaust Gas Temperature is important For the catalytic reactions to occur in the SCR system, a certain exhaust temperature is needed. This temperature is higher if the content of sulphur oxides in the exhaust gas is high (i.e. when high sulphur fuels are used). 2% to 3% oxygen in the flue gas is required. Oxygen acts as a catalyst for the NOx conversion. At temperatures exceeding 350 C, oxidation of ammonia producing NOx begins to become significant Sulphur content in fuel influences the minimum exhaust gas temperature required. 8
Impact of SCR Urea increases CO 2 emissions by 1% as typical urea consumption is about 6.5 g/kwh resulting in about 4.8 kg CO2/MWh Used SCR elements are treated as hazardous material and have to be handled in line with the local disposal laws SFOC increase by 2-3 g/kwh at low loads and by 0 g/kwh when the engine load is sufficient to ensure proper exhaust gas temperature SCR Catalyst must be sized correctly to allow for adequate residence time for catalytic activity to occur and not adversely affect engine backpressure. 9
SCR at Low Load or how to increase exhaust temperature via intake air bypass 10
The pure ammonia question Urea is safer but it does not have the same potential as gaseous ammonia. Gaseous ammonia will allow for a higher NOx reduction rate as well as reduced weight and size of the system, injection unit complexity, costs. Gaseous ammonia allows for expanding the SCR operating temperature range, Anhydrous ammonia is highly toxic, hazardous, and requires thick-shell, pressurized storage tanks and piping due to its high vapor pressure. It is stored under pressure (5-10 bar) or stored at atmospheric pressure if kept at -33 C Aqueous ammonia, e.g. ammonia with added water or diluted in water, NH3 H2O, is less hazardous and easier to handle. Must be vaporized to be used. Solid AdAmmine releases ammonia when heated. 11
Dual Engine Rating 12
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