SCR delivers on emissions and efficiency

The simplest technical solution to meet restrictions on NOx emissions for ships is also the one that minimizes total cost of ownership while maximizing uptime.

Selective Catalytic Reduction (SCR) is both proven and the most cost efficient option to meet latest emissions standards for ships, according to Caterpillar Marine. The company, whose engine reach extends into every industry and market segment, says SCR is the technology that will deliver both  lowest total cost of ownership and highest uptime to ship owners, in line with the latest maritime regulations.  

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.

January 2016 is also fast-approaching, when 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. The fact that, as a company, Caterpillar supplies both SCR and EGR solutions across different sectors, adds particular significance to its technical preference for the marine industry.

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. 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.”

Across its full product line-up for wider Tier 4 Final compliance, Caterpillar has delivered different combinations of emissions-reduction technologies based on various requirements, from mining trucks and wheel-loaders to oil high pressure pumping engines and stationary generator sets. According to Darnell: “The decision to select an SCR-based solution to meet EPA Tier 4 / IMO III marine requirements came after careful evaluation of the technical options available, and a comparison of each of these options against critical customer requirements within the marine industry.”

Simplicity and reliability mean that SCR has already earned a place in the marine industry, a sector well-known for its reluctance to commit to unproven technologies.  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.”

“The SCR option is not about redesigning the engine. Meeting Tier 4 Final using solely EGR has implications for engine efficiency and durability. Dramatic technological countermeasures can soften the impacts, but not eliminate them. The EGR option cannot claim validation in the harsh environments that engines face in marine applications. We’re very comfortable that we have not developed our Tier 4 Final marine solution around a technology that’s untried in its market which could compromise customer uptime.” 

In contrast, the Cat® SCR option minimizes design and installation costs for shipyards and is fully accessible for service engineers, Darnell emphasises, reducing downtime and replacement/overhaul parts costs for the customer. Nor does SCR add any engine-related service such as stricter fuel filtration requirements or EGR valve and cooler maintenance.

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’, and that has been our guiding principle. The Caterpillar marine solution is also the most efficient SCR system on the market due to achieved lower overall fluid and fuel consumption for improved TCO, minimal new engine content for greater reliability and establishment of common designs and processes across the platform which reduces variability..”

Meanwhile, Caterpillar points out that DEF infrastructure is already in place in the automotive, on-highway truck, off-highway machines, and other land-based industrial and petroleum sectors. DEF suppliers have been gearing up energetically for marine and are eager to continue working with customers to understand the various delivery and storage needs across the vast industry.

“EPA Tier 4 Final-compliant C32, 3500E, and C280 series marine engines are being offered with SCR because this technology offers not only compliance, but ease of installation, increased performance, and the lowest TCO. As a company, Caterpillar has advanced competencies in both SCR and EGR technologies; our position is based on selecting the right technology for the applications.”