Long-time POWER readers may remember Marmaduke Surfaceblow, a fictional character whose engineering escapades were brilliantly portrayed in hundreds of stories published within POWER magazine’s pages over more than 30 years beginning in 1948. Today, the fictional series continues through Marmy’s granddaughter, Marnie, who is an engineering wiz in her own right.
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As science and technology advance, so too does plant engineering. Ensure you’re ready for the challenge by staying informed.
It was only a half-hour call, but Marnie Surfaceblow, vice president of Surfaceblow & Associates International, was worn out. She rested her head on her arms at the conference table, muttering, “I’m getting too old for this. I feel … exhausted.”
She sat next to her lead field engineer, Maya Sharma, who was enjoying a hot cup of chai while reviewing process flow diagrams of a carbon capture system (CCS). She’d only heard half the conversation between Marnie and her peer vice presidents, so she asked, “Ma’am? What was their objection to this project?”
Raising her head, Marnie seized the opportunity to soliloquy. “Nobody denies troubleshooting a first-of-a-kind CCS is challenging. Engineering is supposed to be challenging! Bringing new technologies into practical manifestation, balancing the environment, ethics, efficiency, effectiveness, and economy! Grandpa Marmaduke wouldn’t have quailed from this challenge—he’d have rolled up his sleeves and told the others ‘Bilgewater! If you nervous Nellies are so focused on profits, why didn’t you go to business school!’ ”
In This Issue
Full issueLaughing at Marnie’s impersonation of the legendary Marmaduke, Maya asked, “Did you win, ma’am? We can work on this project?”
Marnie threw back her head and laughed. “Yes! We’ll sail their stranded plant and CCS from the Sargasso Sea of constant forced outages out into the high seas of safe, reliable, profitable, clean generation! We won’t even need the full week on-site—we’re solving this problem in three days!”
A minute of silence passed, then as they heard the sound of the plant staff approaching their conference room for the kickoff meeting, Maya said, “They gave us three days?”
“They gave us three days,” Marnie confirmed.
Understanding Plant Operations and Emissions
The e-mail requesting their help had given the basic facts. Wolverine Power Unit 1, a 500-MW coal power plant, was located on the shores of Lake Michigan in northeastern Wisconsin. With easy port access and connected to two large rail lines, since 1986 the plant had burned coals from high-sulfur bituminous to low-sulfur subbituminous. Unlike many aging power plants, this one was built right. Major forced outages were rare, with an average availability of more than 88% over most of its life. Then, in 2020, future CO 2 regulations led to a hard decision for Wolverine Power: switching to natural gas, adding a CCS with 90% CO 2 removal efficiency, or shutting down.
“The owners gave us three options, but we sold them a fourth,” said Plant Manager Heinrich Altergott. “Wisconsin’s 45% forest, and we’re surrounded by lumber mills, kraft paper plants, and companies all around the Great Lakes shipping biomass pellets. But given the great condition of the plant, we looked for the most flexible future option. We modified the boiler systems to let us burn up to 50% natural gas or 50% black pellet biomass with coal, and we designed and worked with a reputable OEM [original equipment manufacturer] to make a CCS that was as reliable as possible. However, it’s possible that having too much flexibility is part of our problem. Dick, can you explain the specifics?”
“When we designed our system with the OEM, we took every lesson learned we could to create a customized CCS that would let us run for another 40 years if they let us,” began Richard “Dick” Anderson, the lead environmental engineer. “Our system takes lessons learned from every amine CCS ever built, so we could try avoiding as many problems as possible. We start by reducing the CO 2 production by leveraging natural gas and black pellet biomass as much as the economics allow. The CCS has tight requirements for pre-scrubbing the flue gas, so we use low-NO x burners, overfire air, and an SCR [selective catalytic reduction system] to reduce NO x by 99%. Lime sorbent is injected downstream of the SCR to reduce sulfur trioxides, and SO 2 is removed by a wet scrubber downstream, giving us a 99.5% SO 2 removal efficiency. The lime we inject, along with fly ash, is captured in an electrostatic precipitator [ESP] at 99% efficiency.”
As Dick paused, Heinrich added, “That 99% efficiency is less than the specification for our CCS, but it also has to work with our activated carbon injection [ACI] system that helps us capture about 95% of mercury emissions. Amelia, can you talk about our CO2 regulations?”
“Our permit limit is 0.101 ton/MWh, which was based on 90% CO2 removal when burning subbituminous coal. If we burn 50% natural gas, our required CCS removal efficiency is 87%. I know, you’d think it would be lower, but the math works,” explained Amelia Palmer, the plant’s operations manager.
Raising her hand, then speaking, Maya asked, “Ma’am, one assumes your CO2 limit is on a gross generation basis?” Receiving affirmation of her assumption, Maya then asked, “What is your required removal efficiency when burning black pellet biomass?”
Amelia nodded. “Thankfully the black pellets are considered net CO2 neutral, and …” noting that Marnie was about to forcefully interject, Amelia added, “We know black pellet production isn’t really carbon-free, but our permit says it is, so that is our reality. Given the reduced performance of our CCS, we burn as much of the black pellets as possible, depending on the market price.”
“And, of course, that is the problem—our reduced CCS performance,” Dick interjected. “Since we first started the system, our CCS hasn’t been able to scrub greater than 84% of our CO2. That means we’re typically burning anywhere from 16% to 50% black pellets. We suffer a substantial mill and primary air fan derate with the black pellets, so we’re usually limited to about 75% of our coal-based maximum generation.”
“And given your CCS requires 18% of your electrical generation and about 11% of your steam heat, your full output is already reduced roughly 25%,” Marnie calculated. She thought briefly, then continued, “What I’m hearing is you only meet your CO2 limits by operating the plant at about half your coal-based net generation. That’s not a great situation, but,” Marnie looked around the conference room quickly, “that’s not why you called us here.”
As the plant manager, detailing the task was Heinrich’s job. “Our CCS has never come close to 90% removal efficiency, but when we first brought it online at least it was stable. Barely six months into its operation, however, the CCS began a cycle of gradually losing efficiency until we’re forced into an outage. We clean out the system, then drain the system and recharge it with fresh reagent. Things work well for the first few weeks after each outage, then slowly but steadily the efficiency loss comes back to stay.”
“As a result,” continued Amelia, “we’ve never run longer than about three months between CCS outages. We can slow the decline by burning as much natural gas or biomass as possible, but the market price of each fuel can be prohibitive. And we’re not sure it even does any good. Even when burning the maximum possible biomass, it almost seems to make problems worse!”
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1. The plant manager passed out charts showing how carbon capture system (CCS) performance regularly declined over roughly three months of operation. Source: POWER |
Dick passed around printouts of the phenomenon (Figure 1), showing arrays of trendlines mapping every major performance output as the CCS CO2 removal efficiency dropped from nearly 84% post-outage, until the next shutdown between 60% and 70%.
After reviewing the charts for several minutes, Maya asked, “Sir, ma’am, could the underperformance of the ESP be the root cause of your CCS problem—say, from accumulation of ash in the amine? And what is impacting the ESP performance most?”
“Adding to Maya’s question, I have one too,” Marnie said. “Why didn’t you use a fabric filter baghouse, either to replace your ESP or as a gas polisher?”
Maintenance Supervisor Willie Hoppe spoke up for the first time. “When the plan was changed to include biomass fuel, we started to worry about fires from unburned fuel carryover. But even without the biomass, our ACI system was already dumping lots of unburned carbon into the flue gas, and that alone nixed planning for a baghouse. The carbon needs residence time to oxidize the elemental mercury in the flue gas, but we only have a very short ductwork run between the air heater outlet and the ESP inlet. This led to that strange drain trap-shaped ductwork. And just like a drain trap, we get a lot of carbon fallout in the bend, meaning we inject two to three times the carbon necessary, and a lot of that ends up in the ESP.”
“But sir, from my understanding you do achieve the mercury limits required by regulation as well as the CCS design, yes?” asked Maya. Upon receiving affirmation of both, she then asked, “Are you certain the poor ash removal performance is not the cause of your CCS amine deactivation?”
“Industry experience told us ash in the CCS primarily caused problems by plugging amine piping, especially heat exchangers. Our OEM proposed using larger heat transfer pipes with extra fin area, and it worked. Even with the ash levels in our circulating amine system, we never miss our target operating temperatures.”
A few more questions took them to the start of lunch. While the plant staff talked amongst themselves, Marnie quickly scrolled through some research papers on her laptop, closed it with an authoritative snap, and stood to address the room. “Thank you all ever so much. Let us know anything else that comes to mind. Heinrich, can you get your fuel buyer to come talk with us for just a wee minute?”
Fueling Speculation
As she drove the rental car into the woods surrounding the power plant (Figure 2), Maya observed, “Ma’am, I have learned much of your Scottish heritage via observation. Thus, I am not surprised that ‘a wee minute’ is actually very many minutes.”
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2. A road trip to Hodag Mills, the plant’s black pellet supplier, provided valuable information to help solve the CCS performance problems. Source: POWER |
“I thought you said we were going into town for lunch, not driving two hours to our biomass supplier!” exclaimed Emily Altergott, Heinrich’s niece and the plant’s fuel manager, who was tightly packed with miscellaneous test equipment in the back of the rental car.
Putting on a serious and conspiratorial attitude, Marnie turned to face their reluctant passenger. “Ms. Altergott, here’s the mission: We need the straight dope on the feedstock for your black pellets. Normally, biomass suppliers won’t give me any information because I’m not their customer. As the fuel buyer, Emily, you are the linchpin to our success. You are the decider. And right now, that makes you the most important person in the world. So, first, the mission, then, the feast.”
Eyeing Marnie warily, Emily kept her composure through the second hour of travel to Hodag Mills, the black pellet supplier for the plant. At first Marnie’s strategy proved sound, as within five minutes of entering Hodag Mills headquarters, all three women were seated with company president, Freddie Koch, in a luxurious office. “Well, shoot, you didn’t have to bring Emily all the way out here. I knew your father and your famous grandfather, Ms. Surfaceblow. He did us a good turn, so we always have time for the Surfaceblow family!”
Crossing her arms, Marnie gave Freddie “The Look,” and said, “If you know my family, Freddie—may I call you Freddie? You may call me Marnie—then you need to be straight with me, because you know I’ll discover the facts, by hook or by crook. My question is: Do you use post-industrial wood in the black pellets you sell to Emily?”
Freddie looked at Marnie in disbelief, then laughed, “Of course we do. There are huge supplies of construction waste, pallet scrap, and even landfill recovery hereabouts. Our contract with Wolverine Power specifies we can use up to 20% waste wood by weight in our pellets. Didn’t you look at the contract before driving two hours out here?”
Trying to maintain her composure, Marnie sternly added, “That may be so, but I’d like to review all lab analyses of your black pellet deliveries to Wolverine Power.”
Freddie again displayed disbelief. “I’ve emailed every analysis since day one to Emily. Didn’t you ask her for them? Well, it’s OK,” he said. Freddie picked up his phone, tapped on it for a few seconds, then said, “If you’ll give me your email address, Marnie, I’ll send a link to the cloud drive with every analysis.”
As Maya pulled out of the parking lot, Marnie sighed, turned to face the stony gaze of Emily in the back seat, and handed her a gift card while smiling sheepishly. “This is worth ten ‘Empress Treatments’ at Xanadu Day Spas,” she noted. “I hope this will help reduce the post-mission stress.”
Emily took the card, tucked it into her purse, and pronounced, “Fine. But we’re also stopping at that Brazilian steakhouse on the way back.”
Marnie nodded agreement, pretending not to notice Maya’s smirk.
Finding the ‘Best’ Solution
Early the next morning, Marnie and Maya met with the plant project team in the conference room again. When all the players had arrived Marnie began her big reveal.
“Some of this is not proven, as we say in Scottish courtrooms, so we’ll need to collect ash and reagent samples for lab analyses and we won’t know the results for a few weeks, but here’s how Maya and I see the situation,” she said.
Maya took a pen and in her elegant script drew a flow diagram on the whiteboard. “As happens in life, several small effects have joined to create a greater effect. We agree the problem lies in the amine reagent, but from what cause? With amine reagent systems, there are many contaminants that may reduce efficiency over time. Most of these are not present or implied, but two we believe are likely, which are chromium and zinc,” Maya suggested.
“Chromium and zinc?” asked Heinrich over the murmuring of the crowd. “Where is that coming from? We knew these metals were risks, so we specified no galvanized or chrome steel was used when the CCS was built.”
“It’s in the biomass itself,” Marnie replied. “Many of your supplier’s waste wood lab analyses show high chromium and zinc content. Zinc comes from bits of galvanized wire, nails, and screws from post-construction waste, whereas chromium is a common wood preservative, along with copper and arsenic.”
“As you burn greater quantities of biomass, the concentration of chromium and zinc steadily increases in your reagent,” continued Maya. “These elements chemically degrade the amine, and since you have no process in place to remove or neutralize them, their concentration increases until you must cease operations and add fresh reagent.”
Dick asked, “So, because our supplier mixes waste wood with virgin at random times from random sources, does that explain our variable CCS uptime?”
“Oh … so close, but I’ll give you a cigar anyway … if only I had one,” Marnie said while pantomiming patting her pockets. “The zinc and chromium are mainly carried in with the ash particles when burning biomass, and there’s several reasons for that. Maya, why don’t you walk everyone through it?”
Maya continued drawing her process flow diagram as she spoke. “Problem first: your ESP is old and was never designed to capture biomass ash. Wood biomass ash often has high electrical resistivity from its calcium oxide content. I have not checked, but I do guess you increased your ESP voltage to improve the collection efficiency, yes?” Seeing a nod from Willie, she continued. “Problem second: the surplus carbon in your flue gas stream from your poorly designed ACI system greatly reduces the ash electrical resistivity. When the carbon loading is too great, when you rap the ESP plates to drop the ash into the hoppers, the resistivity can be so small that the dropped ash can re-attach to the ESP collecting plates. In short, this creates highly variable ESP performance.”
Marnie jumped back in, “Your lime additive system could be described as a passive-aggressive co-culprit. By removing sulfur compounds upstream of the ESP, they don’t attach to the ash particles. The adsorbed and absorbed sulfur compounds not only can make the ash stickier, but they also improve its electrical resistivity.” Pausing, Marnie noted, “Since you tend to choose the black pellet biomass also at times when the price is lower—presumably due to having higher waste wood content—you have yet another variable to contend with.”
The conference room was filled with a cacophony of conversation, but quieted as Heinrich asked, “What’s the best solution?”
“Excellent!” exclaimed Marnie. “Most people ask if there is a solution, but you know there is always a solution, and want only the best. Maya is drawing up proposals for further studies to help us answer that question. Some of the solutions could include swapping out your ESP for a fabric filter—along with redesign of the ACI system to reduce your carbon spillover. Or possibly using a smaller fabric filter as a flue gas polisher downstream of the ESP. You could also change to a different biomass fuel source. What else, Maya?”
“Sir, other options include regular or continuous monitoring of chromium and zinc levels in your amine reagent.” Maya paused, suddenly self-conscious, then said, “I have one idea, sir, but I caution it is first-of-a-kind: employing a bespoke catalyst downstream of your SCR to oxidize the chromium and zinc, along with an additive mixed with your activated carbon to help remove the metals.”
As the three plant leaders and other staff thanked Marnie and Maya, Heinrich warmly shook Maya’s hand, and said, “A custom catalyst and additive for removing chromium and zinc? That is definitely new, but very interesting.”
Leaning over to pat Maya’s shoulder, Marnie smiled broadly and quipped, “Hey, first-of-a-kind plants require first-of-a-kind solutions? Patent pending …”
—Una Nowling, PE is an adjunct professor of mechanical engineering at the University of Missouri-Kansas City.
