Page 44: of Marine News Magazine (November 2013)

rine Diesel Engine to meet EPA Tier 4 with an all on-engine approach. COMPLIANCE APPROACH NO. 3 ? DIE-SEL WITH SCR AFTER-TREATMENT Among multiple after-treatment technologies, the urea-based Selective Catalytic Reduction (SCR) system as shown in Figure 2, represents the most mature and available solution at present day. An SCR works by using a catalyst to chemically reduce NOx to clean nitrogen (N2) using the urea as a reagent in the presence of high-tem- perature exhaust gases. SCR is com- patible with higher sulfur fuels and if necessary can be equipped with a soot blower to remove particulate matter. In October 2013, Caterpillar Ma- rine announced that their C280 and 3516C models will meet EPA Tier 4 using SCR after-treatment systems. Cummins Marine already uses SCR and indicates their planned use for higher horsepower marine engines to achieve Tier 4. Other engine manufac- turers have also indicated SCR as their planned approach to compliance. SCR AFTER-TREATMENT ? V ESSEL DESIGN IMPLICATIONS With indications that many, if not most, of the marine diesel engine makers are taking the SCR after-treat- ment route to EPA Tier 4 compliance, the industry is hungry for data. Over the last few years, both commercial and government stakeholders have re- quested vessel designs to meet Tier 4. Naval architects need to understand the space, weight, and technical im- plications of supporting such systems. Though increasingly more available, data is still limited as to what and when engine options will be available in a Tier 4 con guration.Gibbs & Cox partnered with Ten- neco, an emissions technology leader, to understand the vessel design impli- cations of incorporating a urea-based SCR system. Point design studies were performed to quantify the impacts of SCR systems on small and medium size commercial vessels. Two of these designs are highlighted in Figures 3 and 4. The SCR system comprises multiple subsys-tems including the mixing tube, catalyst module, and urea dosing system. First, the pump on the urea dosing system delivers the solution from the tank to the exhaust stream through injectors on the mixing tube. Then, in the presence of the hot exhaust gas, the urea decom- poses to form ammonia. The ammo- nia reduces the NOx to clean nitrogen gas (N2) and water within the catalyst module. The urea dose and timing is coordinated by the dosing control unit using sensors for temperature, pressure, and NOx at the SCR. The point design for an ocean tug showed that the added weight of the SCR system and urea equaled 1.7% of the vessel?s full load displacement. The added weight on a 350 ft multi- purpose vessel was 0.8% of its dis- placement. For both vessels, the SCR system occupied approximately 3% of EMISSIONS REGULATIONS mmÜÜWW==EEPPPPTTFFOOPPTTJJRRMMNNNN ccññWW==EEPPPPTTFF==UUPPTTJJTTTTUURRbbWW==ÜÜÉÉ~~ÇÇÑÑääììëëÜÜÉÉêê]]~~ÜÜÉÉ~~ÇÇíí~~ååââKKÅÅççããttÉÉÄÄWW==ïïïïïïKK~~ÜÜÉÉ~~ÇÇíí~~ååââKKÅÅççãã++==``ÜÜÉÉããááÅÅ~~ää==ooÉÉëëááëëíí~~ååíí==++==ooììëëíí==CC==``ççêêêêççëëááççåå==mmêêççççÑÑ====++==llÇÇççêê==ccêêÉÉÉÉ==++==aaììêê~~ÄÄääÉÉ==++==``ççããéé~~ÅÅíí====++==iiááÖÖÜÜíí==ttÉÉááÖÖÜÜííqqóóééÉÉ==ffff==ppÉÉïï~~ÖÖÉÉ==qqêêÉÉ~~ííããÉÉååíí==mmää~~ååíí==EEppqqmmFFrp=`ç~ëí=dì~êÇ=`ÉêíáÑáÉÇ ~åÇ=fjl=OMNM=^ééêçîÉÇ qqÜÜÉÉááëë==ÅÅççååëëííêêììÅÅííÉÉÇÇ==ççÑÑ==eeÉÉ~~îîóódd~~ììÖÖÉÉ==iiii==mmççääóóÉÉííÜÜÉÉääóóååÉÉ==ììëëááååÖÖ==ííÜÜÉÉ==ããççëëíí~~ÇÇîî~~ååÅÅÉÉÇÇ==ëëíí~~ííÉÉJJççÑÑJJííÜÜÉÉJJ~~êêíí==êêççíí~~ííááççåå~~ääããççääÇÇááååÖÖ==ééêêççÅÅÉÉëëëë==ÅÅêêÉÉ~~ííááååÖÖ==~~==ììååááíí==ííÜÜ~~íí==ááëëWW++==iiççïï==jj~~ááååííÉÉåå~~ååÅÅÉÉThe volumetric energy density of LNG when considered with its storage and support systems can be up to three times more demanding than diesels; a space penalty that many vessels cannot afford. If technically feasible, a total ownership cost analysis needs to compute a low enough payback period to justify the higher investment cost. Mark Masor, Gibbs & Cox Gulf Coast Operations Manager November 201344 MNMN November2013 Layout 32-49.indd 44MN November2013 Layout 32-49.indd 4410/28/2013 3:21:06 PM10/28/2013 3:21:06 PM