Pulverizer oulet temperature 1

[yulinios]

Hi there

We have MPS coal pulverizers, we are operating controlling the mix air-temperature between 65 to 75 °C in oulet temperature in a relation of 2.5 air/coal.

Do you have some recomendation to this parameter to operate the pulverizer?, our problems are erosión of the parts, unburnt carbon as high as 11% in fly ash, and high CO.

Bye There

 

[davefitz]

sounds like the classifier is not properly rejecting the coarse back to the table. Alternatively, to much coarse is being elutriated from the table, perhaps by a high velocity air jet. If you can redcue the magnitude of the peak velocities in the air space between the tableand the classifier inlet , perhaps less coarse will carryout to the classifier.

Expensive solution is to add a dynamic classifier.

 

[yulinios]

Hi There

Davefitz
We have a rotary classifier which operates in 55 rpm, we have a vibration in pulverizer too, therefore we need to decrease the speed of classifier as low as 40 rpm, our fineness of coal after classifier is more than 70% passing mesh 200, I would like to know parameters like oulet temperature and air coal ratio in other coal plants.
Bye there

 

[JEB66]

Most of the MPS89 mills I have delt with operate with an air /fuel ratio of 1.8 to 2.2 at full load. Being too lean will cause excessive partiles on the 50 & 100 mesh area. These particles are the ones which will cause the slag and LOI. Most people like to see 99% passing through 50 mesh.

If the 50 , 100 and 200 mesh screens do not make a straight line on a log plot, then you have classifier bypass.

If you change the air to coal ratio, make sure you do not violate the minimum velocities. 3000 feet per minute for theburner line transport velocity and 7000 feet per minute minimum throat velocity. Lowering the air will reduce erosion. The erosion is proportional to the velocity raised to the 3.5 power.

Most likly the cause of your LOI and CO is improper tuning of your burners. Take a flue gas sample at the economizer and see if the O2 and CO are balanced. You can also check the control room excess air instrumentation as well. Many times an O2 probe can calibrate perfectaly fine, but read low sometimes as much as 25% excess air.

B&W, Storm engineering, ABB, GE, SAVVY Engineering can due these furnace diagnostics if you do not have the expierence in house.

 

[davefitz]

A different method of preventing vibration was demonstrated by MHI. See <

http://www.mhi.co.jp/tech/pdf/e383/p106.pdf>.

Apparently , the vibration would occur when the dynamic classifier ( at 55 rpm) achieves its best fineness,as excess coarse particles would build up on top of the bed of coal on the table; the vibration can be eliminated by mixing the recycled coarse with the raw coal at the center of the table as the coal is fed in. This required the add'l use of a fixed classifier upstream of the dynamic classifier. This arrangement allows full pseed, high fineness operation of the dynamic classifier without the vibration.

As JEB66 stated, you need to get the fineness down to less than 1% retained on 50 mesh in order to avoid 2-phase flow behavior of the coal-air mixture; this will simplify the balancing of the coal -air conduits and reduce the burner to burner coal flow unbalance to less than 15%. To limit CO and Lu wehnoperating with lo Nox burners( low stochiomentry in burner zone) , the burner to burner coal flow unbalance must be less than 15%.

 

[yulinios]

Hi There, thank you for your answers.

We have tested our pulverizers decreasing oulet temperature as low as 60°C (normally operates in 75°C), we didn't find differences to improve our coal combustion, therefore we think that is better work in 75°C because we are gaining heat from the wheel (air heater)improving our efficieny, manufacters manual set like max. 80°C.

On the other hand, we tested air/coal ratio at full load as low as 2.2 (normally operates at 2.5), we didn't find any disturbs in the pulverizer parameters in this value, decreasing to 2.1 wasn't possible because we had high vibration of pulverizer. Following the way setted by JEB66 we can save money decreasing our erosion parts.

About the fineness we are a little bit outside of limits in mesh 50, we have 1.5 %.

How is the way to test unbalance of ducts?, we measured the air flow duct by duct adjusting only in commissioning time, now in normal operation when the sampling of coal pulverized is taken we had a big diferences as high as 50% in weight of samples, on the other hand we took the samples in horizontal lines where gravity could influence, thus we are thinking took the samples in vertical spaces.

Bye there

 

[JEB66]

You should take samples in a verticle run. In a Horz, run, you will have a "coal rope".

Sampling method will have an effect as well. The Rotoprobe isw a very good device that is gaining acceptance in the US.

Pipe to pipe Clean air flows, should be +- 5% and pipe to pipe coal clows should be +-10%. That way tuning the secondary air will be easy.

 

[TGQuinn]

I echo everything that JEB66 is saying. especially the
air/fuel ratio of 1.8 the more air, the more fluidized the mill becomes, which will cause metal/metal contact instead of metal/coal contact, thus increasing vibration. The more the mill is fluidized, the more internal erosion. The more airflow, the higher the velocity in the coal pipes, the more erosion there as well.


With regard to LOI, larger coal particles are harder to burn. Especially coals with low/medium %volitile matter. Larger particle size WILL affect LOI. Furnace residence times are very short, and with staged combustion (OFA and low nox burners) larger particles spend even less time exposed to condtions that facilitate carbon burnout.

I would put great effort into lowering air/fuel ratio to the 1.8 range, then monitor vibration and erosion. Take great pains to break any operator habits to run at the leaner ratio.

Pulverizer temperature is primarilly a funcion of %volatiles. Use the temperature recommended with the mill for the type of coal you are using.