IMO Tier III requirements will start to be implemented for certain areas with date keel laying 1st January 2016. Besides other measures SCR applications are a well known measure to comply. For their 2-stroke engines WinGD promotes applications of high pressure and low pressure SCR.
The IMO Marpol 73/78 Annex VI, the International Convention for the Prevention of Pollution from Ships, regulation 13 and[ds_preview] 14 have become reality.
Regulation 13 is ruling the Nitrogen Oxide (NOx) emissions. The North American Emission Control Area (NECA) will implement the NOx cap as planned and as specified by the United States Environment Protection Agency (EPA) directive (Regulatory Announcement EPA-420-F-10-015).
Substitute to primary preventing the formation of hazardous emissions, one approach is to clean exhaust gases in a secondary approach by removing or converting existing SOx or NOx emission amongst others mainly by means of exhaust gas scrubbers or selective catalytic reduction (SCR). In this article we address the SCR installation on WinGD low-speed 2-stroke engines.
Technology
Selective Catalytic Reduction (SCR) installations have been around for a while and the technology was applied to 2-stroke low-speed engines since the 1980s. In general, it is agreed to be a mature, well understood Nitrogen Oxide (NOx) abatement which causes few issues when operated properly. Typically, the systems consist of:
Urea storage and transfer system
Engine load dependant urea dosing
Mixing arrangement with Urea injection
Reactor with catalytic elements
Sooth blowing system
Controls
The Urea dosing system delivers normally an aqueous 40% urea solution as reducing agent into the mixing duct where it vapours and in the process forms ammonia. This ammonia is then reacting on the active catalytic element substrate, converting nitrogen oxides and ammonia into nitrogen and water (vapour), see figure.
Integrated SCR application offer ship operators also fuel reduction benefit as the engine can be tuned fuel optimised with the SCR system compensating for to increasing raw NOx after cylinders. In this tuning flexibility regard, WinGD’s electronically-controlled common-rail engines offer full flexibility and are superior to other technical approaches presented to the market.
Compared with other NOx abatement technologies, the SCR solution is rather simple with few equipment causing low installation and operation costs. Though, the urea consumption has to be considered for Opex calculations and should be balanced with potential fuel saving scenarios. On the low-speed 2-stroke engines, SCR applications are on one hand applied at the low pressure (LP) side downstream from the turbocharger turbine side or at high pressure side before the turbine (HP).
LP-SCR
The LP-SCR is quite a simple system that in case of issues can be by-passed without affecting engine performance or causing downtime of the prime mover. The reactors are flexible arranged in a horizontal or vertical layout inside or outside the engine room. Particularly vessels that have the accommodation and funnels sitting at the stern provide the option to place a LP-SCR above deck in a funnel housing that is slightly wider or longer.
Compared to the later described HP-SCR application, the LP-SCR offers an advantage with multi turbocharger main engine installations. As the system is placed downstream from the turbochargers, it will not influence the balancing and control of them.
Measures to increase the reactor inlet temperature are partly done at the main engine by artificially increasing the temperature after the engine’s cylinder. This can be done by reducing turbine efficiency by a proportional by-pass valve that allows hot exhaust gases to by-pass the turbine. This hot gases will marginally increase the gas temperature after turbine as no expansion over the turbine takes place.
With by-passed turbine however, the more contributing factor is the reduced air flow through the cylinder caused by the lower turbine energy. The amount of residual gas with the cyclic scavenging of the cylinder will increase and therefore the combustion process temperature level will increase. Obviously, this approach is simple but bears the drawback of increased fuel consumption, slightly increased raw NOx and under certain circumstances an increased risk of liner/ piston ring cold corrosion due to the residual gas.
Consequently the catalyst can work in an ideal chemical reaction temperature range without adding heat from an auxiliary burner. Auxilary burners however are still needed for LP-SCR applications. They have the function of increasing the heat of the exhaust gas to a level the Urea can vaporize efficiently in a short distance before forming a homogenous gas mixture at the static mixers. Besides, burners can pre-heat the catalyst to operation temperatures while the vessel is still at the pier preparing to leave. Another useful function is the regeneration of the catalyst’s substrate.
When considering required engine modifications and project specific piping, and control interface adaption resulting in additional engineering work and manufacturing costs to the HP-SCR arrangement, an LP-SCR system has cost advantages.
HP-SCR
The HP-SCR reactor is an integrated part of the engine’s system. As the reactor is placed before turbine, it influences turbocharger performance and as a consequence engine performance. Additional flaps and by-pass lines must be installed on the engine and require design changes.
Provided that all boundaries like fuel and catalyst used are the same, the HP-SCR is requiring a reduced volume of approximately factor 3 compared to an LP-SCR application. This is due to the higher air density under pressure and the higher temperature of the exhaust gases. The HP-SCR system can be prepared for MGO and HFO. If sized for HFO, it will require a bigger volume of the reactor. When limiting the HP-SCR application to 0.1% sulphur fuels as applied in ECA areas, the reactor can be optimised for distillate fuel operation with low sooth and ash formation during combustion. The catalysts density, declared by »cells per square inch« (CPSI), is selected higher, therefore the reactor becomes compact. This compact sizing with sufficient exhaust gas temperature for an ideal chemical reaction are the prime advantages of the HP-SCR application.
The WinGD designed low-speed 2-stroke engines are accommodating the HP-SCR application by having generally high exhaust gas temperatures. This is only possible by the application of efficiently, bore cooled combustion chamber components in combination with an optimised scavenge concept of the cylinder. WinGD’s standard scavenge concept includes high compressor pressure ratios at the turbochargers and high auxiliary blower capacities.
With the modern generation common-rail engines, an automated detection algorithm for fuel used is implemented. With this algorithm, the minimum required operation temperature of the SCR can be calculated. At low engine loads, the available exhaust gas heat after the cylinder is with a Tier II tuned engine insufficient to reach the ideal reaction temperature in the SCR reactor, therefore turbine by-pass valves are applied to raise the overall level as described in chapter »LP-SCR«. When now an MGO instead of HFO is used, the target temperature can be lower, hence less or no turbine by-pass rate is needed. This gives a fuel consumption benefit while operating in Tier III mode. The post injection is a viable measure to increase temperatures at the reactor inlet and is now possible with the new X-engine generation. Quick acting valves enables an injection of fuel after the normal injection which provides more energy to the turbine. This methodology is applicable for the X-35 until the X-72 bore engines. As with the LP-SCR, also the HP-SCR provides for SCR by-pass in Tier II mode or when the SCR system fails. Further investigations include fully integrated control strategies allowing close loop reactor temperature control, urea dosing and acceleration boosts in case of high thermal inertia caused by the reactor. Such strategy could foresee isolating turbochargers at low load to have more boost from the turbochargers. The effect is the same as already applied to turbocharger cut out solutions on vessels in service. These optimising concepts will make the SCR application on a big bore engine an attractive alternative with low fuel consumption, in Tier II and III mode.
Dominik Schneiter
