Emissions and efficiency are nowadays the two most important aspects
for the shipping industry. Engine manufacturer Caterpillar Marine relies on Selective Catalytic Reduction (SCR)
According to the company, the simplest technical solution to meet restrictions on NOx emissions for ships is also the one[ds_preview] that minimizes total cost of ownership while maximizing uptime, in line with the latest maritime regulations. »SCR is both proven and the most cost efficient option to meet latest emissions standards for ships«, Caterpillar Marine says. Volatile fuel prices have given owners reason to pause on their future ship emissions compliance strategies, but Caterpillar Marine notes that air pollution restrictions are marching on regardless. US EPA Marine Tier 4 Final emissions requirements started phasing in January 2014. Engines with a power range of 2,682–4,962 hp (2,000–3,700 kW) were affected first but all new engines with maximum power 600 kW (804 hp) and greater must meet EPA Tier 4 standards by October 2017. In January 2016 separate but similar International Maritime Organization rules covering NOx emissions (IMO Tier III) will apply to new ships operating within the North American/US Caribbean NOx Emissions Control Area. For the offshore, workboat, ferry and small cargo ship sectors driven by medium speed and high speed engines, both Selective Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR) technologies enable compliance. However, Caterpillar Marine says SCR provides measurable fuel cost savings while using proven engine platforms. Cat Marine is implementing the SCR option across its medium and high speed Tier 4 Final certified C32, 3500E, and C280 series marine engines.
SCR technology allows for advances in combustion and fuel efficiency through NOx reduction downstream of the engine itself, says Ryan Darnell, Marine Product Value engineer within Caterpillar’s Large Power Systems Division. He goes on to say: »Our engineers were able to place their focus on driving down fuel consumption instead of adapting a technology unproven in the marine marketplace.« To meet Tier 4 Final, and by extension IMO Tier III requirements, it has been critical that the right technology was selected for the right application. According to Caterpillar, the SCR solution is both straightforward and simple to install; there are no additional components required specific to the engine using SCR. »It’s also durable, reliable and technically robust. Vessel owners considering their options know that their total costs of ownership (TCO) depend on a number of factors.« Customers expect that if emissions decrease, fuel consumption is going to go up, says Darnell. »SCR is not only easy to install for yards and naval architects; it is the solution that meets NOx rule requirements and delivers higher uptime, while saving operators money on fuel; the largest TCO attribute.« »What’s not so widely acknowledged is that SCR delivers the lowest total cost of ownership in the marine sector. One of the main reasons why is because the relationship between engine-out NOx and fuel consumption is inversely proportional; this means SCR maximizes engine efficiency. The amount of diesel fuel required to run a Tier 4 Final engine from Caterpillar with an SCR can be up to 9% lower than that required for a Tier 2 or Tier 3.« Caterpillar’s SCR system injects a solution of Diesel Exhaust Fluid (DEF) into the exhaust system where it is decomposed into NH3, which selectively reduces NOx over a catalyst. The saving in diesel fuel more than offsets the cost, Darnell says. »In general, SCR service periods have been designed to align with the engine overhaul intervals.« Around 1,250 SCR systems have been installed on marine vessels in the past decade, with over 500 applications in 2013 alone. Installations have been made on-board ferries, tankers, container ships, icebreakers, cargo ships, workboats, cruise ships, and naval vessels. By comparison, EGR is unproven in the real world of marine operations. Darnell suggests that, while the approach may promise compliance, it depends on »complex interactions of mechanical, electrical, and fluid processes working together in perfect unison.« EGR recirculates a portion of the exhaust gas to the engine cylinders, reducing the oxygen content intake. This lowers combustion temperatures and generates less NOx, but is also less efficient from a combustion point of view. The shortcoming is being addressed by marine EGR proponents through using common rail direct fuel injection systems operating at higher pressures. However, Darnell suggests that the consequent increases in preventative maintenance and service activities, such as the need to exchange fuel filters more frequently, can become maintenance issues.« He emphasizes, the Cat SCR option minimizes design and installation costs for shipyards and is fully accessible for service engineers, reducing downtime and replacement/overhaul parts costs for the customer.
Darnell also states that any technical challenges arising from the SCR option have been addressed through straightforward and practical solutions. Expectations of variable sulphur content in fuel, for example, have led Caterpillar to design a more sulphur-tolerant SCR solution, allowing for operation in regions with higher sulphur content fuels. He further highlights that Caterpillar has developed a variety of different orientations when it comes to housing the additional equipment, depending on the ship installation, and whether the kit is destined for the stack or the engine room. »The feedback we got from naval architects on this one was ›make it as flexible to install as possible‹«
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