Re: aux. furnace coal consumption

xxxx, whoever you are,

The picture below shows what a typical answer to your question might be. (never trust any model !)
For this calculation, I made some assumptions that might not be suitable for your application (specially concerning safety):

- coal with Low Heat Value of 25.9 GJ/t (wet basis 1% moisture)
- coal analysis CHONS-Ash = { 66%, 5%, 5% , 1.5%, 1%, 20.5%}
- recirculation of 70% of the hot gases
- mill power consumption of 400 kW, fan power consumption of 40 kW

As you can see, 51 kg/h of coal are needed, in this case.
This is equivalent to an heat consumtion of 2.05GJ/tWaterVapor.
This is rather low and it is explained by the heat generated by the mill itself.
The sensible heat from the hot gases represent a power of 367 kW, which is slightly lower than the power of the mill.
The oxygen content in the exhaust gases is rather high: 16.5% .
This might be very undesirable depending on the volatile content and the fineness.
I maintained the requested gas speed at mill end (1.5 m/s).
Finally, the dew point of the exhaust gases is 54°C which is nearly 30°C between the gas temperature.

Increasing the recirculation will decrease the oxygen level.
Increasing the recirculation will also decrease the heat consumption.
For example, with a recirculation of 90%, the oxygen level drops to 9.7% and the heat consumption to 38 kg(coal)/h  or 1.53 GJ/tWaterVapor.
Note that increasing the recirculation also increases the dew point in the gas circuit. Eventually, the dew point will come too close to the gas temperature, and the risk of condensation on cold points will increase. Here, for 90% recirculation, the dew point rises to 77°C which is only 7°C below the gas temperature.

Overall, I like pictures!

Michel

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