"The more we value things outside our control, the less control we have." - Epictetus
The previous career autobiography post is here:
Many many years ago, I worked at a coal-fired power plant. I did a long-ish post about it 15 years ago. It was without a doubt the most dangerous job I've ever had. It was a daily routine to play with lava-hot ash.
At the time the ACE plant was built (1991), it was an exciting new combustion technology - one that delivered reduced sulfur and other emissions by using an inventive and fairly new combustion technique. The new technique used a boiler with a Circulating Fluidized Bed (CFB) combustor.
The project was built as a demonstration of this technology, and also provided (as-needed) steam to a chemical processing facility next door. It was far more lucrative financially to sell the electricity than to divert the steam to the chemical facility, but it was part of the contract, and when one of their boilers went offline, we shipped them steam.
The dominant coal-fired combustion technology for large power plants has always been the Pulverized Coal (PC) boiler. In this type of coal-fired boiler, coal is ground into fine dust by pulverizers. It is then carried into the furnace suspended in a stream of hot air, and ignited. It burns instantly, at very high temperature, almost like gasoline.
Below: A small Pulverized Coal burner on a test stand at minimum fire. A power plant will use a dozen or more of these burners, each throwing long column of flames into the center of the boiler.
Below: A top-down view inside a Tangential-fired coal furnace. The flame swirls to allow good mixing and to lengthen the time the coal remains in the boiler. There will be several stacks of burners arranged in each corner of the furnace.
Pulverized coal technology is still in widespread use. It is well understood, easy to implement, and - these dinosaurs still deliver a lot of power at a reasonable financial price. But Pulverized Coal technology has some significant drawbacks in the modern age - ignoring for the moment the huge environmental issues of CO2, Mercury, and solid waste.
- The temperature at which pulverized coal burns generate large amounts of Nitrous Oxides - Nitric Oxide (NO) and Nitrous Oxide (NO2). Together these two gases are called NOx, and they are responsible for the brown haze you sometimes see over large cities. These gases also react with other chemicals (like gasoline vapor) to form ground level Ozone, which is toxic. NOx requires post-combustion treatment to reduce the levels released. This process is known as SNCR, or Selective Non-Catalytic Reduction. Typically Ammonia will be introduced into the furnace where the combustion gas temperature is reduced to the point where Ammonia will best react with the NOx to create harmless water vapor and Nitrogen gas.
- Any sulfur in the coal will be converted to gaseous Sulfur Dioxide (SO2) as the coal is burned. This SO2 will go out the stack to later combine with water vapor in the air, and rain down later as dilute sulfuric acid - also known as "acid rain". Removing SO2 from the combustion gas requires expensive post-combustion treatment to reduce the amount released into the environment. This sulfur removal is known as "FGD" - Flue Gas De-sulfurization".
- Coal contains contaminants - it's been buried underground for millions of years, and even though the coal goes through a washing process, it still has dirt embedded in it. When coal burns this hot, it melts the embedded dirt - which is mostly silica. This melted silica will deposit as slag (a glassy substance) in cooler regions of the furnace, and has to be dealt with. Occasionally slag forces a shutdown for removal.
- Pulverized coal furnaces require coal with very high energy content, and very low ash content - to reduce slag formation.
The Fluidized Bed technique of burning coal offered a new combustion approach that reduced or eliminated nearly all of the disadvantages of pulverized coal boilers. The ACE Project was one of the earliest boilers in the US that utilized this combustion process. If you aren't familiar with the concept of fluidization, it is possible to make a mass of solid particles liquefy if you distribute enough air underneath. Below is a video demonstrating this process.
Side note: If you do this with water instead of air, you get quicksand!
When you add granulated coal and heat the fluidized bed, the granules of coal will burn - and they will burn at a much lower temperature than Pulverized Coal. This eliminates several disadvantages of using pulverized coal.
- The lower combustion temperature reduces NOx significantly. Downstream SNCR is used to reduce the NOx to even lower levels.
- The fluidized bed is mainly composed of limestone. During combustion of coal, this limestone reacts with oxygen to break down into quicklime, which then reacts to capture the sulfur dioxide (SO2) created during coal combustion. The product of quicklime and SO2 reaction is Calcium Sulfate - Gypsum. Gypsum is the same harmless stuff that drywall is made from, and this becomes part of the ash stream. Because sulfur is captured during combustion, there is no need to install an expensive Flue Gas De-Sulfurization system.
- The combustion temperature is too low to melt silica, so there is no slag build-up in the furnace.
- The combustion process isn't sensitive to the heating value (BTU content) of the fuel, nor to the ash content. A fluidized boiler can burn almost anything - agricultural waste, tire waste, petroleum coke, wood chips, or coal mining waste - piles of garbage that a PC boiler can't burn.
Fluidized bed combustion technology is pretty neat, really. The furnace at ACE was a circulating fluidized bed, which is a slightly more complicated beast. To burn a lot of coal quickly, so that you can make a lot of steam and electricity, you need *a lot* of air flow. When you introduce that much air underneath the bed, the greater air velocity will carry a portion of the fluidized bed upwards out of the combustion area of the furnace. Some of that material will be unburned coal that you are continuously feeding in, so that will reduce fuel efficiency.
To counter this loss of bed and fuel caused by the higher airflow, the exhaust gas is run through a cyclone separator (or two or three). The cyclone causes heavy stuff containing unburned coal to be flung to the outside. It then falls to the bottom of the cyclone, and is allowed to flow back into the furnace and burn. The lighter stuff containing fly ash and combustion gas flows through some heat exchangers to capture the remaining heat, then into a filter house to remove ash, and then it goes out the stack. The combustion process has a few quirks, but it's really cool.
The Quirks? For example, you might be wondering why there is a loop seal below the cyclone separator. Because so much air flow is required to fluidize tons of material in the bed, the pressure at the bottom of the furnace is quite a bit higher than it is at the top. The loop seal prevents gas from exiting in reverse (upwards) through the cyclone. Material separated by the cyclone will not begin to circulate back into the boiler until a lot of material has fallen into the bottom of the cyclone. Fluidizing air is blown into a header in the loop seal, to ensure the separated material flows back into the boiler.
If you watched the second video of the fluidized sand in the glass column, you may have thought of something else: Erosion. It's a huge problem in these boilers. The fluidzed bed contains silica, coal, limestone, and bits of rock and sand. There is an incredible amount of airflow, and there are eddies and swirls containing suspended abrasives - which makes conditions inside of these boilers a lot like sandblasters. For this reason, the bottom part of the boiler containing the bed is lined with refractory several inches thick. The cyclone inlet and outlet, and the cyclones themselves are lined with fire brick.
However, the water tubes above the bed must be in contact with the combustion gas for good heat transfer and high efficiency. At a result, it's always just a matter of time before the abrasive material etches a hole in a boiler tube, and another leak shows up.
You might think that the bottom of a CFB furnace would just be a floor full of holes to distribute the air. This would work - until you had to shut down. Then the bed would fall to the floor and drain out like an hourglass through those holes and into the air duct, plugging it up. Instead "arrowheads" are welded to the floor. These direct the air downwards, so that once the boiler is shut down, the ash falls to the floor. It cannot pass upwards into the arrowhead.
Below: A side view of an arrowhead, showing normal airflow. Up, then out each side. Solid material cannot flow in the reverse. Simple, robust, and less likely to fail in a harsh environment than a hinged plate.
Below is a video of some people working inside the combustion part of a CFB boiler. Note the interface between the refractory and the water wall tubes, as well as all the arrowheads set into the floor.
Before workers can enter the combustion section after the furnace has been in operation, there is a long process to make it safe. First, the boiler has to be cooled down and the bed removed. Coal feeders are shut off, and the fuel in the bed finishes burning. Then airflow is maximized for 20-24 hours and the bottom ash screws are run at maximum speed. This removes much of the heat, and removes the bed.
Below is a video of what it looks like while the furnace is in operation. It's a sloshing bucket of lava, kept in place by a thick wall of refractory.
Even after fanning down the boiler, there is still red-hot ash remaining in the loop seals, and there is still red-hot ash settled on the cyclone inlet. When there is a pile of ash, only the surface will have cooled off. The center will continue to be glowing hot. Ash in both locations must be removed before it is safe to enter the combustor - which is where the guys in the above video are working. Sadly there have been cases when ash sluffed down onto a worker in the bottom of the boiler.
Below are a couple of pictures inside of the ACE boiler where I once worked. It's a bit larger than the one in the video above. The loop seal return is just to the right of the ladder - it's a fairly large opening. The start up burners are inside the brick pipes right and left, and another inside the large hole on the far side. The other holes are for secondary air, which helps reduce emissions and improves combustion.
Below: It appears that a section of refractory has fallen off of the waterwall tubes. Just to the left of the top of the ladder, you can see exposed tubes, so this will need repair. This is the section where the fluidized bed sloshes around, so the abrasive bed likely eroded a boiler tube, which took the boiler offline.
Below: Arrowheads on the floor. There's always ash, and you always wear a filter mask!
Below is a video of a start-up burner heating the bed during start up. The start-up burners use natural gas to heat the bed. Once the bed is up to the firing temperature of coal, a coal feeder is started. The plant where I worked at had six start-up burners (SUBs) to get the fluidized bed hot enough to support coal combustion. It is difficult to warm the bed, because fluidizing air carries off so much heat. Another issue is that the bed would slosh around and snuff out the flame on the burners. For several hours during start-up, the burners would constantly trip. The ACE boiler needed at least five of the six to make any progress warming the bed.
You cannot visually tell when coal combustion begins, and switching over to coal can be perilous if there is not enough heat to burn it - you can create a bomb by adding combustible material in an oxygen-rich environment. The biggest clue was stack oxygen. Once the coal combustion begins, excess oxygen would immediately drop, and by quite a bit. Exhaust temperatures would also rise rapidly, compared to running the start up burners.
Operationally, the furnace was extremely unusual. The lag time between adding fuel and actually seeing the increased steam pressure was minutes. Let me say that again: If you made a small move on the fuel, it would be minutes before steam pressure increased! A big move on fuel would be seen more quickly, like in a minute or two - but still very laggy. It made for an interesting automated control system, and for manual moves during start-up. You wouldn't want to drive a car with huge time lags between input and response :)
Below: Looking down. This is inside the boiler during an outage, on top of scaffolding that has been assembled to inspect the water wall tubes. Ultrasonic testing is performed on each tube at various elevations to determine the thickness remaining. Remember, the abrasive environment wears away the tube metal!!! Once the tubes fall below the specified thickness, they are replaced. Usually entire sections of tubes were cut out and replaced with new panels. At the very bottom of the furnace are the arrowheads (center of the photo).
Edit: Found and scanned the picture. Looks like in the later photo (above) that they added a couple of safety rails and platforms.
The location of this facility was Trona, California.
Below: A couple of views from the top of the boiler. The coal barn. That's Telescope Peak, over in Death Valley, in the background.
Below: The switchyard and a few water tanks.
It was an interesting job, and there were some incredibly intelligent people working there. I learned a lot about the power industry and operating while I was there. Somehow I also and managed to not get maimed or killed too!
EDIT: I scanned a few more photos of this very cool power plant, that is currently being sold off and scrapped.
Below: Most of the power plant. It's bigger than it looks. At the bottom left is the limestone crusher. Big rigs would drive into the building and drop limestone into a pit, where it would be lifted by a conveyor into the crusher. After crushing, the powdered limestone would be blown up into the smallest silo on the boiler structure, which held 70 tons of limestone. The two larger silos each held 210 tons of coal, and had to be refilled twice daily from the coal conveyor
Below: View of the cooling tower from the top of the boiler. One of the ash landfills is above the right hand side of the cooling tower. You can see the white and gray streams where the ash has been blended with water to eliminate dust.
Below: Air flow meters and adjusting valves for one of the two loop seals. The Fluidizing Air blowers above provided the air just for this.
Below: The Primary Air Fan. I think the motor that drove this fan was 2500 horsepower. It was ungodly noisy to be near - like 'rattle your chest' loud.
Below: Bottom of a coal silo, and three of the four coal feeders. The feeders were variable-speed belts that weighed and metered the amount of coal fed into the boiler. They were sealed and pressurized with air, because the lower portion of the boiler was pressurized. To keep fire from coming out into the coal silos, air had to be forced in.
Below: View of the stack, the coal conveyor, and the boiler structure. The ash silos are at the left. The horizontal pipe in the foreground belonged to another company and their process.
Below: My own shot of the coal barn and Telescope Peak, about 35 miles away.
Edit: I found a cutaway drawing of the ACE boiler online. How cool is that???
Down at the bottom right of the drawing (next to the dude) is the Primary Air fan. The fan blows into pre-heaters that are in the yellow and light orange backpass section. Combustion gas still has some heat in it, and this is used to pre-heat combustion air, to improve efficiency and extract as much heat as possible out of the exhaust gas. The Primary air then goes to a couple of ring headers around the boiler and to the bottom, to fluidize the bed. The drawing even shows one of the start up burners.
The deep orange is the coal system. Fuel handling at the top, the big silo, then the feeder, and the chutes that allow the coal to slide down into the boiler.
Feedwater enters the backpass about halfway up from the left side, goes through the economizer to preheat the water, then up into a yellow pressure vessel - I no longer remember the purpose of this - then into the drum at the top left. Green is water and yellow is steam. Water circulates internally in the boiler down the green downcomers and up through the waterwall tubes that make up the sides of the boiler.
Steam leaving the drum goes into the backpass for primary superheating, then over into the boiler (two red tube panels stretching across the boiler), then back to the backpass for polishing superheat. From there to the steam turbine.
One thing about this boiler: If you didn't have enough bed material, the secondary superheaters would overheat the steam. I swear to god, it was possible to bring steam the temperature of a volcano into the steam turbine during start-up, when you didn't have enough bed material. High steam temp turbine trips are a thing.
Final image is of all the industry in the Searles Valley in California. The stack of the ACE Cogeneration plant is at the left side of the image, just to the left of the one putting out steam. I do miss the desert.
Update April 2023.
The ACE project has been decommissioned, and is being taken down. I watched a drone video here, and took a screen-cap of the ACE plant as the drone flew past. It appears that the current owners are selling off the major components, and will scrap whatever can't be sold off. The transformers are gone, and the hole in the side of the building is where the electrical switchgear once was. Sad to see.
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