Feature 3 | ENGINE TECHNOLOGY MAN outlines Tier III solutions


MAN Diesel & Turbo offers an emissions guide for Marpol Annex VI regulations which have been enforced since the beginning of this year


T


he international NOx emission limits on marine diesel engines as determined by MARPOL


Annex VI are shown in Figure 1. Ships constructed aſter 1 January 2016 must not exceed Tier III limits when sailing inside NOx ECA, except in cases where the area is designated later than the construction date of the ship. When sailing outside NOx ECA, the Tier II limits must be met. MAN Diesel & Turbo offers two


alternative methods to meet the Tier III NOx requirement on two-stroke engines. Te first method, exhaust gas recirculation (EGR), is an internal engine process to prevent the formation of NOx by controlling the combustion process. Te second method, selective catalytic reduction (SCR), is an aſter-treatment method using a catalyst and an additive to reduce the NOx generated in the combustion process. Figure 2 shows the layout of an EGR and SCR configured engine. Te present EGR and SCR solutions are


designed for low sulphur fuels of 0.1% or less, due to the SOx requirements in the emission control areas. In case a Tier III solution for high sulphur fuel is needed, a more complex solution including a SOx scrubber is also available. A Tier III engine has two emission cycle


operating modes: Tier II for operation outside NOx Emission Control Areas and Tier III for operation inside NOx Emission Areas.


The EGR principle Exhaust gas recirculation is a method to significantly reduce the formation of NOx in marine diesel engines. By using this method, the Tier III requirements in NOx ECA can be met. In the EGR system, aſter a cooling and


cleaning process, part of the exhaust gas is recirculated to the scavenge air receiver. In this way, part of the oxygen in the scavenge air is replaced by CO2


from the combustion


process. Tis replacement decreases the O2 content and increases the heat capacity of the


scavenge air, thus reducing the temperature peak of the combustion and the formation of


32


Figure 2: Two-way approach for Tier III engines – EGR on the left and SCR solutions


Figure 1: NOx emission limits according to MARPOL Annex VI


• EGR with bypass, configured with only one turbocharger and used for engines


• EGR with Turbo Charger cut-out matching, configured with two or more


of bore 70 or less


turbochargers and used for engines of bore 80 or greater.


SCR – Selective Catalytic Reduction The SCR solutions here assume low-sulphur fuels (≤ 0.1%) for Tier III running modes. SCR is an exhaust gas treatment method by which the NOx generated in a marine diesel engine can be reduced to a level in compliance with the NOx Tier III requirements. The NOx reduction is obtained by


a catalytic process in an SCR reactor installed in the exhaust gas line after the combustion process. In the SCR reactor, the NOx is


reduced catalytically to nitrogen and water by adding ammonia Figure 3: The principal of how EGR systems work The Naval Architect March 2016


NOx. Te NOx reduction is almost linear to the ratio of recirculated exhaust gas. Two different matching methods are used for the EGR systems:


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48