SCR duct loading

[athomas236]

Hello,

I have today been asked to give advice on the design conditions that should be used for the flue gas ducts from a boiler to the SCR back again 600MW units.

Each boiler has two ducts from the boiler economizer outlet to the SCR and two ducts from the SCR back to the airheater inlet. Each duct will be approximately 3.6 m deep by 9 m wide by 50 m long. This duct size is based a flue gas velocity of 18 m/s at boiler MCR.

My problem is to advise what ash loading should be used for he design of the supporting steel work

We have an ASCE document that suggests ash loading from 150mm thick layer to 25% of duct depth. In our case this suggests a duct loading of 85 tonne for 150mm thick ash layer to 520 tonnes for a 25% layer.

If any one has any suggestions about the thickness of the ash layer and density I would be pleased to hear.

Best Regards,

athomas236

 

[boilerone]

I designed ductworks with ash loads many times. Normally just need to consider under worst condition how much ash can pile up without being carried over to downstream and it is totally case-by-case. For a horizontal duct that ASCE recommendations sounds reasonable if not overkilling. For a sloped duct then ash load could be much less or even nothing but at the bottom downstream near the hopper we normally have to draw a imaginary shape of ash assuming the hopper is full to the height of center line of flue gas duct outlet. The shape can be simplified as a straight line which goes from midpoint of gas outlet upward 65 degrees till it hits the duct.

The density of accumulated fly ash is related to the fuel and method of firing but any duct that is below economizer must consider the eco leaking then ash gets wet therefore water density (which is normally higher than ash) should be used instead.

In addition to just get weight of ash for ductwork support design, the shape and dimensions of ash accumulation are also important information for the design of ductwork itself. Wall thickness, stiffener spacing and sizing are all affected by ash loading.

Hope these help,

Boilerone

 

[athomas236]

boilerone,

Thanks for your information.

I am struggling with how to "consider under worst condition how much ash can pile up without being carried over to downstream". The main problem is to determine the ash loading for the 50m horizontal section.

The gas flow is 330kg/s and the ash content of the gas is 3.5kg/s. we are assuming an ash density of 1.28tonne cubic metres.

Can you offer any further guidance.

Best regards,

athomas236

 

[boilerone]

This is very much related to your ash property and gas flow velocity. Low velocity plus large ash size and weight of course make the worst condition.

The most extreme situation that I have seen was very sticky (enen though very fine) ash from a chemical recovery boiler and at a place where duct turns (there is a dead zone) and the duct was blocked for almost 90%. For your case, if it is regular PC boiler fly ash and the duct is not over-sized so the velocity is not low, 6 inches (150mm) should be enough according to experience but I won't be responsible especially if someone wants to challenge. The most scientific way, of course, is to take the ash sample to a lab and test it in a duct of the similar flow conditions. I don't think there is a trustworthy simple theoretic formula to let you calculate the result because there are so many factors involved with.

Regards,

Boilerone

 

[athomas236]

boilerone,

Thanks again for your advice.

I am beginning to think that we have problems because we do not have firm values of the ash density and ash level which if wrong could result in duct failure. So I think we should remove the problems by changing the design philosophy.

By this I mean that we should select values of these parameters based on the available data and use the combined ash and duct weight to design the duct supports. We could then monitor the weight of the duct and ash using load cells or similar. When the combined weight exceeds the design value an alarm is initiated and the operator has to shut the plant down and clear out the ash.

In this way if we are wrong with the ash density and level wrong there is no risk of duct failure, the only consequence is that the duct could have to be cleaned out more frequently than expected which is a commercial issue not a technical issue.

Best Regards,

athomas236

 

[boilerone]

That sounds like a good solution. I would also recommend adding ash hoppers at the bottoms of the ducts if you can not avoid long horizontal runs therefore you don't have to shut down the plant when ash loads become high. Just open up the slide gates and discharge the ash to ash bins/dumpsters underneath. This is a very common practice. If need to continuously remove the ash or want to do a clean job then a automatic ash removal and transportation system can be considered.

Regards,

Boilerone

 

[athomas236]

boilerone,

I have been thinking some more about the problem and will be proposing that instead of selecting the ash levels based on available data that we ask the contractor to carry out a flow study using CFD software on the ductwork from the boiler to the SCR and back to the boiler.

In this way ash levels, location of hoppers and effects of ash burdens, gas velocities and part load operation can be investigation. On the basis of this study the basis for duct design will be agreed.

Regards,

athomas236

 

[boilerone]

CFD is used very oven on boiler air/gas flow-related designs but the contractor needs to know very well about the ash properties which many times even the boiler supplier cannot give or guarantee. Sorry about the late reply.

 

[athomas236]

Boilerone,

Thanks again.

The project is on an existing plant so we can get ash properties.

Currently, awaiting management response to proposals.

Regards,

athomas236