Settling Velocity and Particle Size distribution- Coal Combustion. 1

[jjv2006]

I am in the process of designing a temporary wastewater sedimentation (settling tank) for a coal fired power plant. I am trying to get more information of particle size and settling velocity for coal combustion by-products. The data available based on literature review almost always suggests that it is dependant on the type of coal and the efficiency of the combustion process. Also, I was able to glean some data suggesting that particle size is less than 10 µ m with a varying range from 0 to 15 µm. Can anyone direct me on information regarding settling velocity information and their variation for various seasons (summer and winter) for Coal Combustion by-products, particularly coal ash and fly ash? Any particular column tests for the same processes will also be of help to me.

Thanks in advance.

 

[francesca]

Try the Waste disposal & treatment forum.

 

[Ron]

I would suggest that you get a sample of the material and run a particle size distribution using the hydrometer method. This will give the particle size and the settling velocity.

 

[stanier]

One of the problems is that the compoents of fly ash contain particles that react with water. At Bayswater power staion the aluminium salts would react and drive the pH to 6.0. Then the remainder would drive the pH to 11.

The particle size and componenets were ever changing. Iron salts proved to be the most problemmatic as they afect the apparent viscosity.

The slurry was pumped away at a solids density of 72% 11km to a mine site.

Settling was not an issue as the particle size s generally small. Bottom ash was handled separately.

Search the internet for papers on this project. BHRA may have papers on slurry pipelining of fly ash.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[jjv2006]

Thanks for all the information. Any column test standard that is applicable to design a settling tank for coal combustion by-products.


Thanks.
Mani

 

[bimr]

What is it that is temporary about your project?

As to a pilot test, you can probably send a sample to someone like Westech and have them predict the settling performance on a bench test.

If you are interested in something temporary, why don't you just rents some tanks from Baker Tanks. Those tanks are frequently rented for temporary use.

 

[jjv2006]

Thanks for your input. It is temporary because there is a lined wastewater basin that handles the peak flows throughout the year and also responsible for taking care of all Discharge to surface water requirements like oils,suspended solids and the like. But the basin liner needs to be replaced and hence a temporary arrangment is required. I will loook into the pricing of renting the tanks and also other requirements. Any other input will be welcome.

Thanks again.

 

[bimr]

jjv2006,

Use of rental containers such as those supplied by Baker Tank and others is common on construction sites for storm runoff, etc. Flow goes in one end of the tank and overflows the other end. Some users cascade from tank to tank to increase the efficiency of treatment. The tanks are basically semi tractor trailers and were originally used in the oil fields to separate drilling muds and other uses.

 

[SlideRuleEra]

jjv2006 - Maybe I don't understand what you are trying to do, but the volume of fly ash and/or bottom ash produced by a "typical" coal fired power plant in a few months (the time needed to replace the basin liner?) would likely be measured in acre-feet. That would require quite a large temporary tank.

http://www.SlideRuleEra.net

 

[jjv2006]

During the replacement of the basin liner, the influent needs to be treated to meet all NPDES guidelines. Since the flow is close to 6 MGD, a sedimentation tank to address oils and solids is the best solution and the Baker Tanks might not be big enough.

 

[DMcGrath]

Don't forget the floating portion of the ash, cenospheres I think they are called. You will have to skim them off periodically.

 

[bimr]

jjv2006 - Also don't understand what you are trying to do, since the flow of wastewater discharged from a coal fired power plant is substantially less than 6 MGD.

What is the purpose of relining the basin? And the type of liner.

Have you thought about just replacing a portion of the basin at a time. You can sheet pile half of the basin.

 

[jjv2006]

The average influent flow into the existing wastewater lagoon/stabilization basin is 6 MGD. The existing liner needs to be replaced because it is severely damaged. The existing lagoon is very deep (15 feet) and hence replacing the liner in portions and doing the vinyl sheet pile wall is not a good option. There needs to be a temporary sedimentation tank/ clarifier system that will handle all the influent and meet local NPDES effluent discharge requirements when the liner is being replaced. The design is geared towards the same. If anyone has any other thoughts, please do respond.I am relatively new to industrial wastewater treatment mechanisms and any input will be greatly appreciated.

 

[stanier]

If the fly ash is pumped as a dense fluid (72% solids, then you dont need a liner. At Bayswater power station in NSW you could walk on the fly ash after 24 hours.

At Stanmore power station in Queensland dense fly ash spews from a vertical pipe and forms a mound. The water drains and is collected and returned to the process. When the mound gets to a certian height they fix another length of pipe to it and keep pumping.

Consider dense phase pumping over lean phase. Saves power and water.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[SlideRuleEra]

jjv2006 - You probably will have to view this situation as more than a wastewater treatment problem. With a flow rate of 6 MGD and a lined permanent basin 15 feet deep, this sounds like a description of the primary sluice system to deliver the ash (fly ash, bottom ash, or both) to what we would call an "ash pond". How big is the pond (in acres) where the liner is being replaced? That will help clarify exactly what you are working on.

Your project may involve repairs to a much smaller pond to take care of "blowdown" (the electric utility term for discharge water), but it does not sound like it.

Also, what is the total nominal size (in electrical megawatts) of all units (there may be more than one) that your project will have to process? This will give a rough idea of how much ash will be coming in. For just about any commercial size coal fired generating station "dealing" with the ash is as much a material handling challenge as it is a waste treatment problem.

bimr - The 6 MGD flow rate makes sense if this is the primary ash system. The same sluice water is continuously recirculated to transport ash. As you have noted, the discharge flow rate to the environment is much lower.

Also, both fly ash and bottom ash contain heavy metals pollutants that tend to leach into groundwater. A pond liner is to keep this from happening.

http://www.SlideRuleEra.net

 

[bimr]

It sounds like you working with an old plant that uses large volumes of water.

Here is some general information on coal fired ash handling systems:

http://www.hnd.usace.army.mil/techinfo/UFC/UFC3-430-02FA/TM581015/chap6.pdf

http://www.hnd.usace.army.mil/techinfo/UFC/UFC3-430-02FA/TM581015/chap7.pdf

Do you have an ash dewatering bins? If not, you probably need one.

http://www.unitedconveyor.com/partsdata.html

Seems like you need more work to design an economical ash handling system rather than a sedimentation tank.

 

[jjv2006]

Thanks bimr for the links.

Slideruleera:
I think it is still a temporary wastewater influent treatment problem because the exisitng wastewater lagoon/basin (which is 13 acres in size) will be active after the liner is replaced and will be able to handle the normal and peak flows and meet discharge requirements.
There is a separate ash basin to handle all the ash that is settled in the existing wastewater basin.
The temporary wastewater system ( I am thinking more like a temporary aboveground sedimentation tank/solids contact clarifier that can be installed to take in the influent flows and handle all effluent discharge requirements). The important discharge requirements include suspended solids, oils and heavy metals).
I do not have exact information on the amount of ash generated. I am still looking at the logs and initially it looks like the particle size is 10 to 100 um and very fine with a suspended solids concentration of 2-10% in the influent...

Any modifiications or more efficient methodology will be of great help ..

Thanks....

 

[SlideRuleEra]

jjv2006 - I'll get off my soapbox
...but encourage you to find out how much ash is produced - I think you will be surprised. Got to "put" it somewhere. Bimr bring up a good point about checking with United Conveyor, they are a leader on this type problem. A competitor is Allen-Sherman-Hoff (ASH)

http://www.diamondpower.com/DP/ASH/ASH/Framesets/ASHProdFS.html

If you discuss your project with representative from either, or both of these companies - they will probably "jump thru hoops" to give you suggestions (on how to solve it with their products). Which may be what is needed.

http://www.SlideRuleEra.net

 

[TorchRed]

jjv2006 -
How are you coming along with this? I have a similar project for a permanent install. I'm looking for clarifier/thickener installations here in the U.S. for proof of principle. My client will want to visit an installation if they go forward with the design. You do have to take into account where you're going to pump the ash that you settle out of your flow. My initial flow is 8100 gpm (12 mgd) and @2% solids. You can determine your solids density if you know the higher heating value and ash content of the coal burned, the heat rate of the plant, as well as the load they are running the plant. For example, I have a 1350 MW plant with a heat rate of 10,000 btu/kwh; This gives me a heat input of 13500 MBH/hr at full capacity. My coal is assumed to have a HHV of 11,000 btu/lb. From this I calculate the tons of coal per hour that I'm burning. Now, the client told me that the coal has an ash content of 7.13% and that 90% of the ash is fly ash. My fly ash production rate is, therefore, 39 tons per hour at full load, or about 61 gpm (dry volume).

If you're dealing with hydroveyors like I am, then the water flow rate will remain constant (8000 gpm for my example), but the ash content will vary based on load. Toyo pumps has a picture of a treatment facility in New Mexico that uses clarifiers to thicken ash. I'm trying to track it down so I can track down the thickener manufacturer. My design to get 2% up to about 5-7% uses two 100 ft. diameter clarifiers. To go from 20% to 50% solids (another option we're looking at) it takes the same size clarifiers. The number I've been getting is something like 0.3 to 0.5 gallons per square foot of clarifier area for sizing. But as you've probably determined from your internet searches is that it's all a guess until you get the rheology report for your particular plant. The amount of carbon left in the ash as well as the condition of the ash (are the spheres broken and jagged) as well as the calcium content all have an effect on the performance of the fly ash. If you're burning PRB, you can run into cementitious problems. You might contact Dorr-Oliver Eimco for more assistance. They have an example of a facility they installed overseas (Gale Common, British Energy). You'll probably end up with a polymer additive so you get agglomeration and quick settling, which would not be that much more expensive or complicated for a temporary installation. I have to steer clear of complexity on my job. But you are talking a lot of expensive equipment to handle that flow rate to create a sludge that you can handle. The estimate I got for my two 100 ft. diameter clarifiers was between 1.5 to 1.7 million...just for the clarifiers; no pumps, nothing else. Just parts and installation of the clarifiers (metal aboveground tanks). I imagine you'll have to solidify to at least 60% so it can be stacked somewhere for drying prior to loading for landfilling? I'm just guessing. I hope you're still monitoring this thread b/c I think we may be able to help each other out.

 

[stanier]

Hi Torch Red,

I think you would be better off using dense phase pneumatic conveying to handle the fly ash into a silo. The fly ash from your precipitators or bag house canbe conveyed by airslides to the pots. Pneumatic conveyors can transport the ash 1km if necessary to the silo. From there you can blend it with water in an agitated tank to make a paste for dense phase pipelining (72% solids) or lean phase slurry (23% solids).

It seems a nonsense to mix with so much water and then try and separate the ash in a gravity thickener. The ash is so fine that you will use an enormous amount of coagulant & flocculant to get it to settle. On top of that the pH, temperature, chemical composition, solids distrubution etc will vary so much that you will never get your chemical dosing right.

The above dense phase process has been used in Australia on a 4 x 660MW power station and a 4 x 500 MW station.

Contact Kockums Bulk Systems for the pneumatic conveying.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au