SCR vs. EGR: Operators edging into Tier 4 weigh technology options


The market for Tier 4 tugboats is off to a slow start, and the president of Honolulu towing company Young Brothers has a good idea why.

“It’s partly because of a slowdown in the oil patch, but a lot of people were anticipating Tier 4 and wanted to (have new boats built) before Tier 4 became effective,” Glenn Hong said in a recent interview.

The theory is widely held in the industry and is a good explanation for the surge in tug construction over the past few years following a sharp drop in new orders. Not all operators are sitting on the sidelines, however, as McAllister, Harley Marine, Baydelta Navigation and others have announced Tier 4 tug projects.

Young Brothers also has taken a plunge into Tier 4, announcing a four-boat order with Conrad Shipyard of Morgan City, La. The 123-foot Kapena-class tugs will be powered by twin 3,000-hp General Electric 8L250MDC engines. The first delivery is expected in early 2018.

Young Brothers, a Foss subsidiary, is a quick-turn line-haul operation with seven towing tugs in its fleet ranging from 3,000 to 5,500 hp. Its tugs haul cargo seven days a week between the Hawaiian Islands, and Hong described the GE engines as a “perfect fit” for its needs.

“If this were a different application, such as a harbor application, we wouldn’t necessarily go with the choice we made, but in our situation we felt this was the best selection for us,” he said in a recent interview.

That decision did not come easy. Young Brothers has not taken delivery of a new tug since 1991 and its fleet does not have any Tier 3 engines. The company spent months reviewing different options before selecting an engine.

“We thought, ‘This is going to be our future for the next 35 years,’ but along with that was the fact that we had to choose a technology that didn’t have a long track record” in marine operations, Hong recalled.

The Environmental Protection Agency’s Tier 4 standards have introduced the maritime world to a host of new acronyms and the concept of exhaust aftertreatment needed to meet the tougher federal rules concerning nitrogen oxides (NOx) and particulate matter (PM). The rules call for a 76 percent reduction in NOx emissions and a 70 percent cut in PM over Tier 3.

MTU spent five years revamping its Series 4000 engines with selective catalytic reduction to meet EPA Tier 4 emissions standards. The latest engines in the series also are more fuel efficient than their predecessors.

Courtesy MTU

Engine manufacturers took two approaches to meet these standards. Caterpillar, EMD, MTU and Cummins are using selective catalytic reduction (SCR) as an engine aftertreatment to “scrub” emissions. This solution introduces the chemical urea into the exhaust system to remove NOx and PM.

GE, on the other hand, is using an exhaust gas recirculation (EGR) system in its Tier 4 engines. This system reduces NOx by targeting its formation within the engine’s combustion chamber, said Sander Jacobs, GE Marine Solutions international sales director.

“A portion of the exhaust gas is cooled and rerouted back to the engine-charged air. Since the specific heat capacities of the exhaust gas components are higher than that of air, this results in a reduced combustion temperature and consequently less NOx formation,” he said.

Both systems require additional engine components compared to Tier 3 mains, but SCR takes up more engine room space. Dosing tanks are required to store the urea, which must stay within a certain temperature range to maintain effectiveness. Meanwhile, there are new pipes, valves and an exhaust gas recirculation cooler needed with EGR engines.

Angst about these new engine systems has been widely discussed in the industry. Concerns about space, performance, cost, maintenance schedules and urea handling are just some of the questions operators have raised. Manufacturers have done their best to address these concerns but admit there is work to do.

“I think it’s probably still at the wary (stage) because there isn’t a lot of it out there in North America,” said Andrew Packer, senior manager of marine application engineering at MTU America. “Daily or weekly we have discussions with local designers and customers trying to get their heads around what is entailed with installing an engine with aftertreatment. They’re still in the learning stage.”

MTU, a Rolls-Royce subsidiary, spent five years adapting its Series 4000 engine to meet Tier 4 standards. The new engine maintains the same envelope and roughly the same weight as its Tier 3 counterparts but packs more power per cylinder. The company will introduce a 20-cylinder model to go along with its 12- and 16-cylinder mains for workboat applications.

As it approached this revamp, boosting fuel efficiency was a key goal, Packer said. The latest Series 4000 models will be about 5 percent more efficient, largely due to improvements to the turbocharger and changes in engine calibration.

“It’s more or less the same base engine,” he said. “Where the changes are on the engine side is more or less to optimize combustion knowing that we have the SCR system to reduce the emission output of the engine.”

The SCR unit is roughly half the size of the engine, and MTU has developed options for operators depending on the vessel design and engine constraints. One option is a cubic SCR box and the other is a flat box that can be mounted above the marine gear to save space. The goal, Packer said, was simplicity for the customers.

“We’re trying to make it as close to plug-and-play as we can,” he said.

GE’s Tier 4 marine engines were 10 years in the making and are based on its locomotive engines, which have logged more than 2 million operating hours.

The company’s Tier 4 engines maintain roughly the same footprint as Tier 3 mains while boosting total output by 15 percent. Fuel efficiency also has increased, although the company did not say by how much. The addition of high-pressure common rail fuel injection helps reduce the fuel burn, Jacobs said.

Caterpillar did not respond to requests about its Tier 4 engines. Cummins is still developing its new mains and a spokesman declined to discuss specifics.

For its series of new tugs, Young Brothers wanted engines that were reliable, allowed for relatively easy maintenance and would last up to four decades. It also wanted simplicity, power and fuel efficiency.

The company’s engineers and executives had concerns with both systems. They spent months familiarizing themselves with each and learning their potential benefits. They met with manufacturers, sent engineers to the engine makers’ facilities and hired a consultant to provide an outside review of performance claims.

“Ultimately, fuel efficiency was a big driver, along with power, density and size,” Hong said. “We had … a number of thought processes (including) the need to make on-time arrivals within our operational system.”

The question of availability of urea at some remote Hawaiian ports also gave the company pause. “At the end of the day, relative to the two technologies, the urea became a pretty big factor,” he said.

Young Brothers’ new Kapena-class tugs will be powered by General Electric 8L250MDC engines, which use gas recirculation to reduce nitrogen oxide emissions.

Courtesy GE

Improved fuel efficiency was something of a bonus. Achieving a 5 percent improvement would result in hundreds of thousands of dollars in savings each year per tugboat, Hong said. Fuel prices, operating conditions and other variables will affect the final number, but he said it could be a “very significant” boost to the bottom line.

The Tier 4 engine choice was a little simpler for McAllister Towing, a New York-based company with a 75-tug fleet. Its engineers ran a series of analyses when considering engines for Capt. Brian A. McAllister and Rosemary McAllister, z-drive tractor tugs under construction at Horizon Shipbuilding in Bayou La Batre, Ala. The company ultimately chose Caterpillar 3516E engines producing 6,770 total hp on each boat.

McAllister considered GE’s engines with the EGR technology. But Martin Costa, McAllister’s engineering manager, said the engines weren’t an ideal match for the company’s new harbor and assist tugs. For one thing, they’re heavier, meaning the tugs would sit lower in the water, reducing their escort rating.

“The cost analyses showed … definite savings with SCR and the Caterpillar engines,” Costa said. He added that fuel savings from the new engines are expected to pay for the urea aftertreatment.

“The GE engines, for a line-haul boat where you’re towing long distances, might be a good choice. But for us, for harbor service, we needed a quicker response, which Cat gave us,” Costa said. “On top of that, the majority of the high-speed engines in our fleet are Cat, so we wanted to stick with the same engine manufacturer.”

There were some design hurdles around fitting SCR units in the engine room. McAllister also had to figure out a way to keep the urea tank in each tug in the proper temperature range. The company solved the problem by enclosing the tank in a separate air-conditioned space.

“We looked at maintenance schedules and all that, but primarily it wasn’t as complicated as one might think,” Costa said. “A lot of the design is already there with Tier 3, so more or less you’re just putting an SCR on the unit.”

In other words, the process has been relatively painless. As for engine performance?

“Once the (boats are) in operation, I’ll let you know,” Costa said.

By Professional Mariner Staff