Storage Question 1
We are having a 1.5Mt six-stage Precalciner kiln operating with South African coal having a VM of 28 per cent. Now we thinking to switch over to Indonesian coal with high VM content (up to 42 per cent) and Chinese coal (VM up to 32 per cent). The coal mill is a VRM having hot gases from cooler. Up to what maximum VM we can go without having explosion problems. With high VM coal what are the other changes to be carried out to regarding coal residue so that we do not get problems in the flame shape.
There are well established guidelines for the safety operation of coal grinding equipment. These fall into two broad categories:
(i) explosion prevention, and
(ii) explosion protection.
For prevention the best solution is to ensure that there is less than 12 per cent oxygen in the atmosphere. You cannot do that when cooler exhaust air is used for coal drying. As a general guideline drying with cooler exhaust air is suitable with direct firing systems. Other prevention measures are to ensure there are no fine coal dust accumulations in the system and no possible sources of ignition. These are determined by the design of the coal milling and storage system.
Storage Question 2
How can the bulk density of cement change? Kindly describe the factors responsible? What are the consequences of bulk density of cement on concrete? How to increase the bulk density of cement? Do you suggest to increase iron ore content in raw meal to increase cement density? What is the range of bulk density and specific gravity for ordinary Portland cement (BS 1997)?
The bulk density of a particular cement can vary due to compaction or due to chemical reactions with the atmosphere or any water the cement comes into contact with. If you specifically want to change the bulk density of freshly ground cement then you would need to adjust the chemical and mineralogical composition. Increased iron oxide would do this but the effects would be very slight. I would also expect problems with excessive coating and ring formation in the kiln. There are certain concrete applications where high density concrete is specified (eg radiation shielding). This is achieved by using heavyweight aggregates such as barium carbonate or sulphate rather than by increasing the bulk density of the cement itself. The bulk density of cement is around 1.5 tonnes per cubic metre.