Wisconsin-based WEC Energy Group (WEC) is working to reduce emissions across its energy subsidiaries in the Midwest. The company has a goal to become carbon-neutral by 2050 and cut emissions 80% by 2030, from 2005 levels.

To this end, WEC is exploring the use of low-carbon fuels. One of its subsidiaries, Upper Michigan Energy Resource Corporation (UMERC), hosted a hydrogen-natural gas blending demonstration at the A.J. Mihm Generating Station. The demo, conducted in the fall of 2022, involved blending hydrogen in one of the three grid-connected 18.8 MW Wärtsilä reciprocating engines at the plant.

The partners, which included the Electric Power Research Institute (EPRI), demonstrated 25% hydrogen by volume fuel blending in the engine that was tested. Other project team members included Blue Engineering, Burns & McDonnell, Certarus, Lectrodryer, and Mostardi Platt. EPRI released the results of the blending demonstration in March, and we had the opportunity to discuss them with Dr. Andrew Maxson, a senior program manager with the nonprofit.

Link to executive summary for the blending project

Why engines?

Calling it a “seminal study,” Maxson said this was the first hydrogen blending test on a commercial-scale, grid-connected and operating reciprocating engine.

Reciprocating engines have superior fuel and operational flexibility compared with gas turbines, he said. They can start quickly and ramp to balance the grid in areas with high renewable penetration. Engines are good at burning virtually any fuel and can accordingly be sited in places where the fuel quality isn’t stellar. Maxson said that was a factor in choosing the plant and location for the blending test.

“In this particular region, there is lower pressure natural gas and varying natural gas quality, which engines can handle better than turbines,” said Maxson. “So that was one of the primary reasons why they went with engines.”

Site safety and preparation

Because hydrogen is so flammable and can leak easily, a detailed plan was followed to ensure safety on site.

Safe operating conditions of the engine were identified ahead of time, as well as corrective actions to be implemented if key performance indicators exceeded established thresholds.

“We took it very seriously and we had an experienced team involved in this,” said Maxson.

An aerial photograph of the A.J. Mihm generating station during the testing. Photo by Electric Power Research Institute (EPRI).

All contractors visiting the A.J. Mihm plant were required to complete an online environment, health and safety orientation to confirm all personnel on site were familiar with the plant’s existing safety policies and procedures.

Both written and verbal forms of communication were employed, with signs placed throughout the plant to indicate hazardous areas or where additional personal protection equipment was required.

A restricted access zone was placed around the perimeter of the hydrogen blending system equipment. The location of the blending equipment, brought in from out of state, was determined based on access within the facility, roads, proximity to fuel gas tie-ins and preferred engine in addition to meeting requirements of the National Fire Protection Association 2 – Hydrogen Technologies Code (NFPA 2).

All the equipment and piping that touched the pure hydrogen was American Society of Mechanical Engineers (ASME) code certified. Hydrogen leak detection tape – yellow tape that turns black if hydrogen contacts it – was applied on all flanged connections for piping that contained either pure hydrogen or the hydrogen/natural gas blend. All flanged connections were inspected before each engine startup and after each engine shutdown or trip.

When the engine was operating, no one was allowed into the engine hall. Hydrogen sensing monitors were placed there to detect any possible hydrogen leaks.

Team members working directly with the hydrogen supply, pressure reduction and fuel blending equipment were required to wear personal hydrogen gas monitors as part of their normal operating procedures.

In the end, there was no evidence of any hydrogen leaks during testing, including from the engine itself.

“We were pretty careful, and we learned a lot,” said Maxson. “I think we learned a lot of lessons learned that we’re going to pass on the industry about handling hydrogen as a result.”

Engine loads, emissions and efficiencies

The grid-connected 18.8 MW Wärtsilä reciprocating engine was tested at different engine loads and to operate on various fuel blends, ranging from 10–25% hydrogen by volume.

As different hydrogen levels were tested, Maxson stressed that it was important for the teams to see how the engine would perform without any mechanical modifications. As it turns out, the engine, on a 25% hydrogen blend (the highest % tested), did not require mechanical modifications. Maxson said the only modifications made involved some manual tuning at increased engine loads.

For the 50% engine load runs, engine tuning was not performed as the engine was able to operate reliably on hydrogen blends up to 25% by volume.

For the 75% and 100% engine load runs, the charge-air pressure and ignition timing were adjusted to maintain stable operation of the engine.

For each of the 50%, 75%, and 100% engine load runs, the engine was able to achieve the full load setpoint at all hydrogen blends, with the exception of the 25% hydrogen blend.

At that blend, the engine was only able to make 95% capacity. As Maxson explained, engines have a closed volume and hydrogen is much less energy-dense than natural gas. Therefore, getting enough hydrogen into the [engine] cylinder to produce all the power the engine can provide is a challenge.

“We were thinking when we were at 25% blend ratio, we weren’t going to be able to provide the full capacity of the engine,” said Maxson. “And we were expecting as much as a 15% reduction. And we only saw 5%, which everyone was ecstatic about, that the engine was still able to put that much output out.”

The teams measured emissions, heat rate and efficiency at various engine loads and hydrogen blends, relative to a 100% natural gas baseline.

As expected, carbon dioxide levels decreased as increasing levels of hydrogen were introduced. CO2 was reduced by approximately 10% at 25% by volume hydrogen co-firing.

“These were all things that we expected but we were happy to see,” said Maxson. “It’s always good when measurements back up your expectations.”

Efficiency and emissions data as a percentage of the baseline for each load and fuel blend. Table by EPRI.

Carbon monoxide (CO) and nitrogen oxides (NOx) were also measured at both the engine outlet and the outlet of the selective catalytic reduction (SCR) system going to the stack. One environmental concern with using hydrogen is because it burns hotter than other fuels like natural gas, it can produce more thermal NOx.

When blending hydrogen without NOx controls, Maxson said the teams did see a NOx increase coming out of the engine in some cases. But after the NOx SCR controls, Maxson said there was little change in these emissions coming out of the stack compared to the baseline.

“It wasn’t dramatic,” said Maxson, speaking of the uncontrolled NOx emissions. “And we did change how the engine operated a little bit by increasing the air fuel ratio so that it burned a little bit leaner. That reduces some of this higher temperature coming out.”

 Uncontrolled Emissions

• At 50% Engine Loads: CO emissions decreased by 21–35% as a result of faster, more complete combustion with increased hydrogen blend ratios. By contrast, NOx increased by 21–74% at higher hydrogen content due to increased cylinder temperatures. No engine tuning was done in these test runs.

• At 75% Engine Loads: The engine was retuned after performing the baseline to lower NOx emissions, resulting in NOx emissions actually being lower at 10% and 15% by volume hydrogen blends and then increasing to 20% above the baseline at 25% by volume hydrogen. The teams noted that further engine tuning could have been done to maintain even lower NOx emissions levels. CO emissions decreased by 10–25% over the tests, with the reductions increasing with hydrogen content.

• At 95% engine load: CO and NOx emissions were substantially lower than the baseline as the engine was manually tuned to reduce NOx and the CO was reduced in part due to the lower carbon content in the fuel.

• At 100% engine load: CO emissions increased by 20% during the 12% by volume hydrogen full-load testing because the air-fuel ratio and ignition timing were changed to keep NOx low. NOx emissions were substantially lower than the baseline by 58%.

Controlled Emissions

Stack emissions of CO and NOx after emission controls were kept well below the regulatory permit limits of the plant in all cases and test runs.

• At 50% engine loads: CO emissions decreased by up to 15%, and NOx decreased by 13–17%.

• At 75% engine loads: CO emissions decreased by 12–18%, and NOx increased by 10–20%.

• At 95% engine load: CO emissions increased by 18%, while NOx decreased by 2.5%.

• At 100% engine load: CO emissions increased by 54% during the 12% hydrogen by volume full-load testing, while NOx was comparable to the baseline.


At the 25% hydrogen blend, the heat rate and efficiency were almost identical compared to the natural gas baseline. From a thermal performance point of view, Maxson said the engine basically operated the same.

“In some cases, they were a little higher – in some cases, a little lower – but nothing appreciable,” said Maxson. “So that was great news that hydrogen really doesn’t impact the efficiency of an engine.”


Maxson said the test helped provide data to engine manufacturer Wärtsilä, who can use these results to accelerate the development of its hydrogen-capable fleet. The manual tuning to accommodate increasing volumes of hydrogen during testing would ideally be implemented into the company’s future engine design.

Wärtsilä aims to have commercially available 100% fired hydrogen-fired engines by 2025.

Several mechanical changes will be needed for engines to be able to handle 100% hydrogen, Maxson said.

One of them is to change the compression ratio to reduce temperatures, thus avoiding NOx increases and engine knocking. Maxson said another tweak could be implementing pre-chamber combustion to better control the ignition.

Piping will also need to be code-certified for 100% hydrogen to prevent leaks.

“Over the long term, hydrogen also can destroy certain metals because of embrittlement,” said Maxson. “So you have to use the right materials to be able to handle 100% hydrogen coming in and out of the engine.”

NOTE: We are currently accepting speaker submissions for presentations at POWERGEN International on January 23-25, 2024 in New Orleans. Topics include hydrogen co-firing through our track Unlocking Hydrogen’s Power Potential. Submit an abstract for a chance to join our speaker lineup here.

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