Dino fueled power plant Q/A

[dr.diesel]

Inspired by f_e's recent post.  What would you like to know about a coal fired power plant, from the Sr. Engineer responsible for the control system?

Cliffs:

 - (2) 550MW generators
 - 30,000 physical IO points
 - Old, huge, big and bigger
 - 10,000 HP 13.8kv VFD motors
 - Redundant everything

Q/A only, no videos as I dislike Turkish prisons, maybe a pict or two if I get brave.

[marshallh]

I'm not familar with those systems, but you could always tell stories.. 

[ConKbot]

What was the biggest "Oh %#&(@&#%" or butt pucker moment that you can safely tell us about?

[free_electron]

Do they shovel lumps of coal in a big burner ? I don't think so.
Explain how it works. I read they grind the coal to a fine powder and basically use injectors , much like a diesel engine.

How to trap fine particles i the exhaust. Do they use co scrubbers .

[nctnico]

You could visit a power plant. A reasonably modern one also makes plaster out of the exhaust gasses.

[Stonent]

So what are they using on a modern plant? Anthracite, lignite, bituminous?

What about the technology behind it all? PLCs, SCADA?

[dr.diesel]

Do they shovel lumps of coal in a big burner ? I don't think so.
Explain how it works. I read they grind the coal to a fine powder and basically use injectors , much like a diesel engine.

How to trap fine particles i the exhaust. Do they use co scrubbers .

This is a good place to start, let's do:

Coal in > Control System > Massive amounts of hot air/exhaust > Future > ?

Coal comes from mines via train, already cleaned and washed, in chunks about the size of #5-4 gravel, 100 Tons per car.  From here it is usually feed directly into the inside coal bunkers or off loaded onto the pile outside, which is just like it sounds, a great big huge pile of coal.  Outside the pile gets rained/snowed on etc, the moisture causes many problems, most plants carry enough of a pile to last 45+ days worth.  The supply is not as regular as you'd think.  Coal on the pile is groomed/mixed/worked as each train car doesn't have the exact same type of coal, via a Cat 854G and D12 Dozer at my plant.  (These are HUGE)  Coal fired power plants range in size from 90MW on the low side to about 800MW at the top, per single unit.  My plant can burn 10,000 tons per day with both units at full load.

Coal from the bunkers is fed directly to the coal feeders, which control the amount of coal that is fed to the pulverizer, these comes in pairs.  A typical 550MW single unit will have 6 feeder/pulverizers, with each pair separately controlled, providing coal to different elevations of the boiler.  Once metered through the feeder gravity drops the chunked coal into the pulverizer, of which there are two main types:

 - Bowl Mill type.  These mills have large hardened steel wheels that ride on the floor of the pulverizer, crushing/pinching the coal as it is trapped between the wheel and the floor.  Coal will continue to be crushed in this manor until is is small enough to be blown out passing through the classifier veins.

 - Ball Mill type.  These are far less common, but use hardened steel balls about the size of tennis balls.  Imagine a 55 gallon drum on it's side, about the size of semi trailer, filled 1/2 full of these balls.  Coal is fed in while this drum it's spinning, being crushed by the balls, again trapped unit small enough to be blown out through the classifiers.

In either case above the coal exits the pulverizer in a fine powder, much smaller than fine salt or sand.  The back side of the pulverizer has an exhauster fan that simply blows the fine powder up and into the boiler via a coal conduit, which is simply a 30" diameter tube routed up and in.  The entrance into the boiler does have burner nozzle, which is only used to direct the coal flow up or down. 

Coal is blown in from each of the 4 boiler corners, at different elevations, all boilers are a rectangle standing up.  While in operation, that is a physical fireball in the upper center of the boiler, with dark glasses on you can see it burning through observation ports in the boiler!

A few specs:

 - Each Pulverizer/Exhauster is run via a 900HP, 4160v electric motor.
 - Forced Draft furnace has (2) 5000HP, 4160v fans blowing air in, (2) 10,000HP, 13,800v internal fans sucking air out, holding a 0.50" negative pressure inside the boiler.

There is no single point of failure in these systems, any one or multiple of the above can trip off without tripping the entire unit.  The control system is exceptionally robust, specifically tuned to deal with most circumstances.

p.s.  You could actually open doors at the upper elevations and shovel coil in if you wanted to!   

Questions, please ask.  Up next, the control system..

[dr.diesel]

You could visit a power plant. A reasonably modern one also makes plaster out of the exhaust gasses.

Yes, visiting a plant is possible, I've given many tours.  I'll get to this towards the end, plants with scrubbers do make a waste that ends up in dry wall, but with the housing collapse most of it is just land filled.   

[orion242]

I got lucky a long time ago and got a tour through a coal plant. http://www.consumersenergy.com/content.aspx?id=1332  I was amazed that the “boiler” was hung from ceiling because of expansion and the stresses evolved.  This “boiler” was several stories tall at the plant I was in.

What’s the normal steam pressure there?  Have you seen how they check for leaks on fresh steam piping?

I don't think getting tours in the US these days happens all that often with the ridiculous security.

[dr.diesel]

Yes, the entire boiler, steam drum are hanging from the ceiling of the power house.  From cold to hot the boiler will "grow" almost 2 full feet.  In my case the steam drum weighs in at 490,000lb, who knows how much the boiler adds to that.  Power house construction is like nothing most have ever seen, an absolute tank with monstrous I-Beams EVERYWHERE!  If you took a large steel bridge, stood it on end vertically, grouped about 100 of them together, that's what the inside looks like.

Drum boiler pressure runs about 2700psi in the drum, 2400psi of throttle pressure to the turbine.  Once through supercritical boilers have no drum and run pressures over 3500psi.

Boiler leaks are usually pretty easy to find @2400psi, most of the time the tube just bursts taking the unit offline.  Small leaks can usually be heard, but don't stay small for very long.

As for the tours   

[orion242]

2,700psi!!  holy crap. I guess small leaks would be short lived at that pressure.  I would have to guess closing a gate valve and finding a leaking valve packing makes for a bad day.

The steam heating systems I occasionally work with typically run 5-15psi, “high pressure” systems maybe 100-300psi.  On the last high pressure system, I saw these guys with paper wands waving them down a new run of piping.  When I asked what the heck they were doing, it was to check for leaks.  When the paper is cut in half, they know there is a leak…

[dr.diesel]

Control system

There are couple major players in this industry, each are specifically tailored to the power industry, the specifics are intentionally left out.

The control system has a unique layout:

 - Organized into what's called drops.  Each drop has two completely separate, but redundant controllers/processors, each individual controller has redundant networking, with redundant power supplies.  The redundant controllers have redundant networking between the pair (one of which is separate from the above), this is how they stay in sync, you can turn controllers off back and forth without losing control.

 - All networking switches are redundant, you can unplug entire switches without losing control.

 - Each drops redundant power supplies are fed from separate 30kva UPSs, with a bank of batteries bigger than most garages.  In the event the UPS faults, the load is automagically switched to AC, fast enough nothing is dropped.  The UPS batteries are separate from the station DC bus, which is the last line of defense when all else fails.  (which I've seen)

 - Each drop has cages of I/O cards.  Each cage can hold just about any type of I/O and can be mixed.  Common I/O types are digital in/out, 4-20ma in/out, serial, Ethernet, proprietary communication etc.  Critical plant I/O is triple redundant, going to different drops in-case an entire drop is lost.

 - I/O is polled at specific intervals and varies depending on purpose.  Normal non critical stuff usually once per second with turbine I/O at 100ms or faster.

 - Most I/O can be hot swapped, depending on what it's controlling. 

 - Each drop is a standalone entity and will continue the best it can, when neighboring drops fail, even if portions of it's logic depend on I/O points from other drops.  Some are programmed with heartbeats to detect certain conditions so the entire plant can be automatically taken down in the event of uncontrollable loss.

 - Controllers/Processors run a tailored version of VxWorks on Intel x86 hardware.  These are programmed via Solaris Sparc workstations in a logical (think AND/OR gate) type symbols on "logic sheets".  Imagine functional drawings of 74 series logic and you'll have a pretty good idea.

 - Logic can be modified on-the-fly, but is generally only done online in not critical areas to reduce the risk.  Just in-case one of those once in a million, stars are aligned type of events happen.

 - The entire control system network is physically separated from the outside world, there is no hacking in taking control by some unknown person 5000 miles away.  You'd have to drive to the plant, physically plug into a free network port, which would do you no good since free ports are disabled.  To even get to this point you'd have to cut one of my fingers off (not telling which one) to open the biometic lock on the door.

Obviously a brief overview of an incredibly complex beast.  The control system drives 30,000+ physical I/O points, from small solenoids in 1/4" tubing, to 10,000HP electric motors, to 500kv switch yard breakers, including some 150,000+ internal data points.  Note modern day coal/combine cycle plants will have 5x the physical I/O.

Even with all of this redundancy and complexity there are hard wired sensors with direct control (outside of the control system) of specific breakers to instantly stop all fuel to the boiler, or stop the turbine in the event of a complete loss of the control system.  The boiler can melt within just a few seconds of cooling water loss and the generator is filled with hydrogen gas, something we'd all prefer say together!  I don't recall which plant it was, but this has happened here in the US many years ago, it blew the entire plant to pieces on a hydrogen generator explosion.

Questions, please ask.   

[dr.diesel]

On the last high pressure system, I saw these guys with paper wands waving them down a new run of piping.  When I asked what the heck they were doing, it was to check for leaks.  When the paper is cut in half, they know there is a leak…

Thankfully the vast majority of tube leaks are towards the inside of the boiler, obviously because this is the side that is facing the heat.  I'm sure there are different sizes but ours are about 1.5" OD with a wall thickness of 3/16" maybe 1/4" (sorry, not a boiler mechanical guy).  When the tubes burst they typically "fish eye", sometimes flailing around like a wet noodle bending and banging all around.

External bursts are quite rare, but do happen and are very dangerous.

Fish eye burst example:

[Stonent]

They should have a gift shop with bumper stickers. "I <3 Carbon"

[dr.diesel]

Ha,

Most of the fellow employees truly believe in a clean safe environment.  I'll get to it in the next part but scrubbed plants are fairly clean (not accounting for the CO2), I don't know the percentage off the top of my head but most of the smoke looking exhaust is actually just steam off the Flue-gas desulfurization process.

EDIT:

Note not all plants in the US have scrubbers, most of these are being closed down in the next 2-3 years due to recent EPA regs.  Almost all of this generating capacity is not being replaced, at this time anyhow.

[Stonent]

Caterpillar got out of the truck engine business a few years back due to increased EPA regs. I guess despite being a very popular choice it wasn't worth it for them considering the EPA regs are much less stringent for tractors and backup generators. They started licensing out and selling engine blocks to Navistar  aka (International).   Nav was sure they could get the big engines EPA certified without a Urea injection system. But after several years of selling noncompliant engines Navistar couldn't get them clean and had to pay a 15k fine IIRC on each installed engine until they finally swallowed their pride (and shut down their plant in Dallas) and went to Cummins with their hat in their hand asking for help.

In a way that I'm sure sounded like Mater from Cars (Larry the cable guy) Cummins said "Well there's your problem! You ain't got no selective catalytic reduction urea injection system!  We've got a few of those for sale if you're interested! We'll even make you an adapter to fit your stinky old engine!'

[Stonent]

Regarding grades of coal back in the early 1900s the coal industry had an advertising  campaign for Anthracite. It had a young lady wearing a white dress putting coal into a stove and the tagline said "Anthracite keeps her dress white!"

I'm sure her father was very relieved.

[rollatorwieltje]

How does a big plant like that deals with a sudden blackout / disconnect from grid?
I know plants can start venting when the turbine speed gets too high, but does that still work if you suddenly have to get rid of 500MW+ worth of pressurized steam?
I live near a chemical plant that uses high pressure steam, when they had to vent due to some process error is was extremely loud and could be heard up to 10KM away. That was nowhere near 500MW worth of energy though.

[Stonent]

I'm guessing some kind of clutch or governor if the turbines get out of control but that's just what I would design it may not be right.

[rexxar]

What happens when one of the boilers goes down? Is there not so much demand that the other can keep up, or does it get overloaded? For that matter, what happens when the whole plant gets shut down, just black out the whole city, or does power come in from another station somewhere?

[dr.diesel]

A full load unit trip is a bit exciting and usually causes some minor damage.  The main turbine has two sets of hydraulically controlled valves, when something bad happens these valves SLAM closed, they have huge springs constantly pushing them closed at all times, you have to "hold" them open during normal operation.  When this happens a MFT (Main Fuel Trip) is initiated cutting all fuel to the boiler, almost instantly the boiler/drum over pressurizes (cause the normal path through the turbine is now closed) and vents all of the excess steam straight up into the air from the roof of the power house.  The cooling water pumps say active replacing the escaping water until the boiler cools down, which takes hours.  The entire building shakes around when this happens and is very loud, can be heard for miles.

When the generator breakers are closed it is pretty much impossible (there are ways, ie losing generator excitation) for it to over-speed, doing so would mean over powering the grid, which is not possible due to the massive load.  There is no clutch between the turbine and generator.  The generator it'self is the most protected piece of equipment at the plant, there are multiple redundant protective systems, to my knowledge a catastrophic over-speed has not happened in the US.  At least not in the past 50 years.

A bunch more could be said here, perhaps I'll add it to the list of write-ups.

[Alana]

Why you use hydrogen as coolant in generators?

[notsob]

from wiki
/snip

Generator cooling

While small generators may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing, is used because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. This system requires special handling during start-up, with air in the generator enclosure first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the air.

The hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress. The hydrogen must be sealed against outward leakage where the shaft emerges from the casing. Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing between the shaft and the seals. Seal oil is used to prevent the hydrogen gas leakage to atmosphere.

The generator also uses water cooling. Since the generator coils are at a potential of about 22 kV, an insulating barrier such as Teflon is used to interconnect the water line and the generator high-voltage windings. Demineralized water of low conductivity is used.
Generator high-voltage system

The generator voltage for modern utility-connected generators ranges from 11 kV in smaller units to 22 kV in larger units. The generator high-voltage leads are normally large aluminium channels because of their high current as compared to the cables used in smaller machines. They are enclosed in well-grounded aluminium bus ducts and are supported on suitable insulators. The generator high-voltage leads are connected to step-up transformers for connecting to a high-voltage electrical substation (usually in the range of 115 kV to 765 kV) for further transmission by the local power grid.

The necessary protection and metering devices are included for the high-voltage leads. Thus, the steam turbine generator and the transformer form one unit. Smaller units may share a common generator step-up transformer with individual circuit breakers to connect the generators to a common bus.

/endsnip

[SeanB]

There are a few power plants here in South Africa which get the coal from the mine by conveyor belt, about 30km of belting in sections linking the mine to the plant, and this grows with the mine moving along the seam.

A storey from my dad was of a routine belt replacement ( about 5km of rubber belt needing replacement) that went wrong when a work lamp on a support tower fell into the belt and the leg started to cut a rip in the belt during start up. They had to truck in and fly in about 3km of rubber from all over to quickly replace the torn section, and this took another day of lost production to get the belting, splice it in at the multiple spots where the individual rolls were joined and realign it again. There were a few mine storemen called at 3AM to go to stores, get all they had in stock out and place it on a lowbed hired at 3AM and ship it to arrive "as soon as possible or even faster" either by truck for the closer ones or on a cargo flight hired just for the few tons of belt they could fit in the plane. They drew straws at the site as to who had to go to the mine manager and tell him they would need an extra day to change the belt.

[TheWelly888]

As a 15 year old schoolboy decades ago, I went on a school outing to visit a 3000MW coal fired power station. The turbine hall was the highlight of the trip, the sheer power of the 6x 500MW turbo alternators roaring away.

Sadly that coal plant closed earlier this year as it was nearly life expired and too expensive to fit de-sulpherisation gear to the exhaust.