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.
Ultra-low loads may be necessary for large coal plant survival, but they represent uncharted territory for operations and maintenance staff. Stay informed on the latest trends for success and expect the unexpected.
“This is … coffee?” Marnie Surfaceblow, vice president of Surfaceblow & Associates International, stared balefully at the weak concoction in her paper cup. “My dear friend, Mary, this is simply atrocious! No wonder you have so many problems here. With coffee like this, your staff likely just … loses their will to live.” Shaking her head, Marnie continued, “We were roommates in the engineering dorms, and yet, you disrespect me like this?”
Mary Short, day manager of the Cottonwood Power Plant and a friendly rival of Marnie’s since university, smiled wanly. “Remember that funny sign at the coffee shop we worked at? ‘Don’t complain about our coffee—someday you may be old and weak yourself!’ ”
Marnie could not suppress her shudder. “It was by Upton Sinclair, and like most things he wrote, it was highly misunderstood. Like how every single high school teacher thinks The Jungle was about the meatpacking industry. It was about socialism! Not hot dogs, but socialism!”
Maya Sharma, Marnie’s lead field engineer, tapped notes into her computer while adding, “Ma’am, let us focus upon this problem. Or, perhaps, we can continue with topics not of importance. I could relate to Ms. Short your attempt to create an ultrasupercritical coffee maker, and what happened to our laboratory …”
“Another time,” interrupted Marnie, her fair Scottish face blushing slightly. “It looks like the meeting is about to start.”
As the clock showed 7:00, the plant lead staff filed into the conference room. There was the traditional confusion of handshakes, business cards, and finding seats, and then with a clearing of her throat, Mary kicked the meeting off and addressed the Surfaceblow duo.
“I told you most of the situation in our last call. The plant was commissioned in 1998, and like all of us, it’s starting to show its age.” Only Maya heard Marnie’s muttering of “speak for yourself,” and gently kicked her boss’s leg under the table. “We’re the last operating coal plant in our system, mainly because we’re still paying off the loan.”
“But the world has changed, and those who do not adapt shall perish,” Marnie noted. “Somehow you managed to keep mostly baseload operation going until five years past, and I see that when you moved to deep cycling operation, you did your best to accommodate the new regime.”
Walt, the maintenance lead, spoke up. “We started seeing really bad sulfur corrosion as the flue gas temperatures dropped, so we started blending in low-sulfur Central Appalachian coal and then eventually switched away from our design high-sulfur Illinois Basin coal. Our ash content went up, but at the lower loads, fly ash erosion wasn’t so much a problem anymore,” he said.
Mary continued, “When the Cowskin Creek solar project came online last year, we shifted to parking at the lowest load possible—about 45% of MCR (maximum continuous rating)—to supply constant low baseload generation. But even that sometimes put us out of the money because it was too much power at the wrong time. We’d already changed the low-pressure turbine section to improve final steam quality, and after we upgraded our control systems, and swapped the main plant fans and the boiler feed pumps to VFDs (variable frequency drives), we could reduce our minimum stable load all the way down to 20% of MCR.”
“Ma’am, with your boiler being natural circulation, was that not a problem?” inquired Maya.
Mary nodded toward Keith, their water-side lead, who answered, “We added an electric booster pump to help out at really low loads, and after some trial by fire, it seems to work.” Pausing and frowning deeply, he continued, “I doubt we could go too much lower though, even without all the other problems we’ve had. Did you tell them everything?”
Mary walked to the whiteboard and started listing bullet-items of pain for the plant operators. “Number one, we started noticing about six months ago that we were clogging the underflow lines in the wet limestone scrubber from high fly ash carryover. We fully oxidize our scrubber sludge to make gypsum. We don’t make much money from it, but it’s a lot better than paying $30 a ton for landfilling. But the problem was a moving target. Once every couple of months we would see serious scrubber fly ash carryover, so bad we had to blow the system down for days, AND we were paying $100,000 a month to landfill contaminated gypsum.”
Maya raised her hand, and asked, “Ma’am, when this event occurs, do you have high unburned carbon in the ash? And do you have opacity monitors downstream of the precipitator?”
Mary shook her head. “No problem with fly ash LOI (loss on ignition), and we aren’t regulated on opacity. We pass every particulate emissions test pretty easily though. Our limit is 0.1 lbm/MMBtu, and we’ve never tested above half that.”
An ‘Unhealthy’ System
Kelly, the scrubber lead, added, “Don’t forget what else happens with that fly ash in the scrubber. It starts to line the inside of the pipes, like arteries clogging, and last fall a big piece of fly ash concrete came loose and destroyed a recycle pump.”
“Like a blood clot suddenly breaking free, to carry on the cardiac analogy,” added Marnie, with a somber look. After a minute’s thought, she added, “I really should eat healthier, but they lock the yogurt fridge before I even get down to breakfast at the hotel …”
“That is not an acceptable excuse ma’am,” interjected Maya. “You are merely attempting to justify your traditional morning sausage and biscuits.” As Marnie looked open-mouthed at her assistant, unable to develop a defense, Maya carried on. “I see there is an unplanned outage this morning. What is the root cause?”
“We had another scrubber recycle line clog and pump damage, but also we didn’t see much of a fly ash upset this time. That’s another strange thing that’s been happening.” Mary paused and spoke into her plant radio, “Jessica, have you got that pump apart now? … You do? … OK, great! Later this morning I’m bringing some overpriced consultants down to pester you.” Mary smiled at Marnie. Marnie grinned back and said, “It’s alright Mary. I recognize with your advanced age you likely need to hire people like me to give you a hand. Are we all hitching a ride on your mobility scooter?”
Mary cursed, and muttered, “I’m only two months older than you!”
1. Marnie and Maya study plant drawings to get a better understanding of system designs. Source: POWER
For the next hour Marnie asked for various design drawings of the air quality control systems, tracing through the flow path with Mary. Meanwhile, Maya input the scrubber process flow diagrams (PFDs) and design data into one of her custom-built simulators, occasionally viewing the drawings with Marnie (Figure 1) to check her data, then ran some simulations of the system.
When it was time to walk to the maintenance bay, Marnie walked over to Maya and glanced at her spreadsheet of simulation results. “Do you see what I see, my faithful apprentice?”
Maya began to answer, then suddenly her eyes widened and she said in a nervous whisper, “Ma’am! One thing I DO see is you are drinking diet soda!”
Marnie took a sip and shrugged, “It’s stronger than the coffee.”
Somewhat disbelieving of her senses, Maya gave her boss the side-eye while she reported on what she had found via her simulations. Marnie nodded approvingly and said, “Well done. Let’s go take this can of Diet … whatever this is … on a tour of the plant.”
‘Clotted’ Pumps and Pipes
As Marnie and Maya waited for an elevator that moved at glacial speed to take them to the maintenance bay, Maya asked, “Ma’am, I am certain I know what one of the problems is, but there are actually two problems, yes?”
“Indeed,” replied Marnie. “You could call them twin problems—not identical, but more fraternal.”
Maya sighed, “Each year I understand more of your metaphors, but today I have failed.”
Marnie laughed and patted her assistant on the shoulder. “Don’t worry, Maya. Sometimes I don’t understand them either.”
A scrubber recycle pump on the half-shell lay exposed on a sturdy workbench. A frustrated-looking maintenance technician was chiseling what appeared to be semi-friable concrete from the pump inlet and outlet manifolds. Taking a short break, the maintenance tech introduced herself as Jessica and explained the situation, adding, “It’s not so bad this time. The impeller seems good, just a little grinding and balancing and it should be ready by tomorrow. I’d like to change the bearing just in case, and we don’t know how much of this rock is in the other pipes, so if a piece breaks off and hits the pump …”
“It would be a metaphor for a venous blood clot, just like I said!” Marnie chimed, while Maya shook her head. “You should also purge the entire scrubber vessel tank, then send new slurry in from the limestone mills at maximum pump flow. In fact, I recommend strongly you operate the limestone feed system in a surge mode at low unit loads. It’ll throw your scrubber chemistry off a little bit, but then most OEMs (original equipment manufacturers) don’t really have recommended chemical balances at very low loads. Few provide a PFD at operation less than 50% of MCR, and for this plant 60% is the lowest operating PFD I could find,” Marnie said.
“I have conducted many chemical equilibrium simulations, and I believe I can optimize your limestone use to reduce the required solids flow. Particle drop-out will still vex you, it will simply be less common,” added Maya. “Your piping is sized for greater volume flows than you utilize at low loads. The reduced sulfur content from your current coal reduces the volume flow further. The limestone drops out at your very low flow rate. You can witness this in your DCS (distributed control system) historian, where there are occasional pump pressure surges as the obstruction grows, then a release of pressure when it breaks free. It’s not practical to replace all your piping with smaller pipe, as that would limit your full-load capability, so I suggest you change the pump flow and pressure profile.”
Upgrades Could Pay Off
Jessica and Mary looked down at the pump parts. “That makes a lot of sense to me. What if we installed a VFD on the pump, so we could cycle the speed regularly during operation?” Jessica asked.
Maya and Marnie nodded in approval. “Indeed, and at the very low flow rates you wish to operate at, the use of a VFD may save you some energy as well.”
Jessica rubbed the back of her neck. “Sure, but that costs money, and I don’t know if the energy saved would pay for the upgrade.”
“Oh, almost certainly not,” Marnie replied matter-of-factly. “But then add in the maintenance parts and labor, and most of all, the lost market opportunity cost. This is a 750-MW net unit, and even at 20% load and market power prices of $20/MWh, you’re losing probably a couple thousand dollars of energy sales per hour. I’ll wager you end up with a payback period of one year or less.”
“But part of the problem is all this fly ash contaminating the scrubber, causing, well, this,” Mary said, waving her hand at the concrete-laden pump under repair. “So, what’s behind the fly ash contamination? Some sort of dropout in the electrostatic precipitator (ESP)?”
Marnie laid her gloved hand gently on Mary’s forearm, and gave her a very serious look, green eyes hard. “Oh no, my friend, I fear that it is much more dangerous. Maya and I are going to check one thing, while I’d like you to get everyone together for a meeting later this afternoon. I hope my fears aren’t justified about what I think your fly ash problem is. Whatever the case, never fear, as Surfaceblow & Associates is on the job, just as we’ve been for more than a century!”
“So, you must be one of the founders?” Mary quipped, and Maya smirked, as Marnie muttered, “Walked into that one … not enough coffee …”
A Heavy Problem
The danger was subtle; few would notice without prompting, but once pointed out, Maya realized she just couldn’t un-see the problem. Between the air heater gas outlet manifold and the ESP inlet header, the flue gas ductwork bowed threateningly downward (Figure 2). Walking nearby but avoiding the underside of the ductwork, Marnie fished in her handbag and retrieved her customized green laser pointer, highlighting areas where the casing was buckling and critical fasteners had already sprung free.
2. At very low loads, the velocity of the flue gas was too low to transport the ash, causing it to accumulate in the ductwork, resulting in a noticeable sag in the structure. Source: POWER
Moving to the ESP side, the two women climbed the staircase until they were even with the bottom of the duct. Referring to a spirit level attached to her laser, Marnie traced the dangerously downward dust droop.
Maya gasped, as she noted the ductwork had sagged almost half a meter out of true at its midpoint. “Record this please,” said Marnie, and Maya immediately started taking video.
After a minute of taking measurements and photographs, Maya asked her mentor, “So, is this what you mean by ‘twins’? Here the velocity of the flue gas is too low to transport the ash at very low loads, and this is not helped by the higher coal ash content. So, ash accumulates in the ductwork, and then during ramp-up to higher loads, sometimes it breaks free and overwhelms the ESP?”
“Pretty much,” Marnie responded. “And there is not much that can be easily done to solve this problem, although note I didn’t say it couldn’t be solved. There are options. They could pull more ash from the economizer and air heater hoppers, trying to reduce the ash loading at the air heater outlet. They could install a damper system, that could restrict the gas flow under control by the operators, or even automatically. They could install shaker solenoids onto the ductwork, to occasionally stir up the ash. Alternately, they could install a new ash hopper in the middle of the ductwork, but that would require a bit of additional support structure, as well as tying it all into the rest of the fly ash system.”
A Creative Solution
“Think on this possibility, ma’am. What if a grid of fluidizing air nozzles was installed in the bottom of the ductwork, once a day during load sending plant air at five to 10 atmospheres through the nozzles to liberate the ash from the duct floor back up into the flue gas stream?” Maya suggested.
Marnie’s response was a slow nod. “I think, my friend, you have found what may be the best solution,” she said. “Large dampers are awkward and maintenance problems, and shakers could damage that long run of ductwork over time. Although they will need to repair or replace the duct anyhow, there’s no need to break it again. Great idea! Why don’t you draw up a sketch and I’ll develop a cost estimate, and we can help keep this annoyance from becoming a deadly disaster. My cousin Maxine saw one of these collapse before, and it was terrifying.”
“This is … very, very bad, ma’am. We must inform your friend and her employees at once!” Maya said.
Marnie nodded her head, then reached again into the seemingly bottomless Prada purse, producing two rolls of yellow plastic tape with the words “DANGER! HAZARDOUS AREA!” printed repeatedly on it. “You know the drill, my friend. Let’s cordon off the area, being careful not to walk anywhere remotely under it. First, we protect the staff, then we propose the solution, and then we fix the problem,” Marnie said.
“As you have always taught me, ma’am. And then?” asked Maya, one eyebrow raised.
Marnie finished tying off the first piece of warning tape, and sighed, “First, we stop for decent coffee. Then, we return home, and I finally finish my ultrasupercritical coffee maker!”
“Very good, ma’am. Thank you for the notice. This time I shall inform the fire department well in advance,” remarked Maya.
—Una Nowling, PE is an adjunct professor of mechanical engineering at the University of Missouri-Kansas City.