1156 posts
Fuels Question 5
Recently we have successfully commissioned our project of conversion to coal from furnace oil. We have imported the coal stacking, reclaiming and grinding system from Hefei Cement Research and Design Institute, China and dosing, firing system from FLS Denmark. We are writing to you to seek your opinion regarding effects on thermal efficiency of our kilns by use of coal in place of furnace oil. We have studied in Cement Data Book by Walter H. Duda which says that coal generates less combustion gases compared with furnace oil and as a result, the heat consumption while using coal is always less compared to the furnace oil. The heat consumption, while using 100 per cent furnace oil in our Kilns was found to be in the range of 770 to 800 kcal/kg of clinker. We will like to have an idea that what kcal should we expect by use of 100 per cent coal in place of furnace oil.
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1156 posts
Re: Fuels
I expect that you will see an increase in output and slight reduction unit thermal energy consumption when burning coal rather than furnace oil. This will arise for the reasons you have mentioned, ie a reduction in the volume of combustion product gases and the incorporation of the coal ash into the clinker. This coal ash does raise the possible problem of heterogeneity in the clinker mineralogy, however I would not expect it to be a major problem. I do not agree with the projected loss of 11 per cent of the calorific value of the coal for the following reasons:
1. The coal should be dried by excess hot air from the grate cooler therefore additional energy does need to be supplied to dry the coal.
2. I consider coal handling losses of two to three per cent to be excessive. The handling equipment must be designed and sealed to minimise these losses.
3. The coal transport air injected with the coal dosing supplements the primary air in the burner rather than being excess air. 4. The sensible heat input to the kiln in the fuel will be lower compared with furnace oil, however I expect the lower combustion product gases and coal ash incorporation to more than offset this.
1156 posts
1156 posts
Re: Fuels
The criteria for petcoke grinding should be the residue on a 90 micron sieve. You need to find the optimum by conducting trials however you can anticipate that you may need to grind the petcoke to less than 10 per cent residue on the 90 micron sieve. The problem with petcoke is the low level of volatiles. With a coal particle the particle "explodes" when it enter the kiln atmosphere due to rapid evaporation of the volatiles in the coal. This causes very rapid combustion. With the petcoke there are no volatiles so the particle can fall into the bed of material before fully combusting. This can cause locally reducing conditions and badly affect the clinker quality. This is why petcoke is usually ground to a high fineness.