Using Ball Mills in the Energy Industry
Experimental studies on coals of different metamorphic grades and various fractional states were conducted in 2000 by the Institute of Thermal Physics of the Russian Academy of Sciences, at experimental thermal energy facilities. These showed that fine-ground coal, milled to a particle size of 15-30 microns, develops a highly reactive property that is analogous to fuel oil – to which it can become an alternative. The experimental facility was rated at up to 1000 kW, and equipped for use with ultra-fine ground coal (produced with an ultra-fine ball mill); burning (pre-furnace and furnace equipment); a plasma system and gas starter for ignition and supplementary firing; combustion control (an automated post for combustion control) and cleaning (a vortex scrubber). The results produced in these experiments can be used to establish the parameters needed in technological facilities for ignition systems and supplementary burning using coal-dust boilers – a replacement fuel for gas and fuel-oil boilers.
The conclusions from these theoretical and experimental studies pointed to the technical and economic viability of using ultra-fine ground coal as a new oil-free technology for the ignition and stabilisation of combustion in coil-fired boilers at power plants, in addition to the ability to replace liquid fuels in boilers.
The primary technological facilities for making use of this new technology are: equipment for ultra-fine milling (ball mills), and the supplementary equipment for supplying and combustion of coal. Technical designs for the supplementary equipment have been developed, which are essential for wheeling-out the new technology (muffle furnace apparatus, input nozzles for the coal dust, accelerating devices for igniting the primary fuel mixtures, feeders for fuel discharge, hoppers for storage, and so forth). Factories able to manufacture the new supplementary equipment already exist in Russia. The last-mentioned also produces milling equipment, and specifically ball mills for ultra-fine milling processes.
This new technology is low-cost, with a short return-on-investment cycle which will hit break-even in no more than 2 to 3 years. The additional financing costs are in producing the ultra-fine ground coal (the purchase of ball mill machinery) – the additional machinery also has a short investment payback cycle due to the economics of the fuel supply industry.
The new Plasma-fuel technology has now passed the final stages of certification – for pilot industrial use. This allows assessment of the risks of the new technology – and if required, it can be further honed to optimise its operation prior to finalising the business case which can be put to potential investors.
Converting oil-fired boilers to run on ultra-fine ground coal
The primary task is moving to rejecting the use of fuel oil by the facility in future. Of course, in places where it is available, it makes sense to change to using natural gas. However, where this is not an available option, then such facilities can be converted to run on ultra-fine ground coal. The economic result of making the change from fuel oil to ultra-fine ground coal will be in the greatly reduced cost of fuel. Over and above this, there is an environmental gain to be made – since there will be a marked reduction in the emission of sulphurous oxides into the atmosphere. This has a further economic benefit, in terms of decreased payments to be made for such emissions.
When making the changeover to using ultra-fine coal, the issue of disposal of the ash waste which it produces needs to be addressed. For facilities currently using fuel oil, this can be problematic. In the first instance, this issue could be resolved by making agreements to remove the ash and slag waste from the boiler room to nearby ash dumps or industrial sites. This process could lead to a loss of some of the cost benefits of making the changeover. But in a more positive light, the ash and slag waste can be recycled as a component in the manufacture of construction industry materials, mineral components, and similar by-products. Installing a production line for the recycling of slag and ash is not only a responsible way of negating environmental pollution – but can similarly cull in economic benefits.
This means that the issue of converting oil-fired power stations to run on ultra-fine ground coal can be easily resolved both technically and administratively. Each individual case for conversion should properly be put through a business plan, including a technical survey of the boiler equipment, and the prevailing economic situation.
Evaluating the efficiency
Energy efficiency can be determined by making a comparison with the costs of fuel oil operation (i.e. the current costs), against the projected costs of transferring the facility’s operation to ultra-fine coal (the current costs, plus the cost of additional equipment). To make these estimates for the current costs, it follows that the costs of the current in-purchasing of fuel oil should be compared against the costs of purchasing coal, plus the additional electricity costs incurred in the grinding process. Particularly concerned with this latter cost, it pays to consider the choice of grinding machinery in the light of its electrical consumption costs. It makes obvious sense to purchase machinery with the lowest energy operating costs. Furthermore, when weighing up the decision to switch from fuel oil to ultra-fine coal, the operation of installing the additional equipment needed for ignition of the ultra-fine coal must be carefully considered.
The ball mill for grinding ultra-fine coal is essential. This kind of coal-grinding apparatus to create combustible fuel is traditionally divided into several categories. Quiet-Operation Slow Ball Mills operate with a rotation speed of 16-23 revs per minute. Fast-Action Tangential Mallet Mills have an operational speed of 590 to 980 revs per minute; and there are also Medium Roller Mills which rotate at 40 to 78 revs per minute. The table of ultra-fine coal dust obtained is below, depending upon the type of machinery chosen.
- Ball drum mills are used for grinding anthracite and bituminous coal with a milling operational range of ≤ 1.1 and low volatility required fine grinding (6… 7 %). If the raw source material to be milled includes some presence of pyrite sulfur fuel ( up to SP > 6 % ) then only ball mills can be used.
- Hammer mills are used for brown and black coal of relatively high volatility (Vg > 30 % ).
- Medium Roller Mills are used for grinding coals with a milling operational range of at least 1.1 Wp and humidity of no more than 16 %, with an ash content of no more than Ar 30 %
Additional Equipment required for producing the ultra-fine coal fuel:
- crushed coal bunker
- coal feeder
- high-pressure fan
- coil-dust burner
- muffle furnace extension
- blowing fan
- milling shelf
- cyclone dust collector
The primary rationale for undertaking the technical changeover is to replace expensive and increasingly-scarce fuel oil – traditionally used for the ignition and stabilisation of combustion in coal-fired thermal plant boilers – and also to replace liquid fuel oil in boilers. Issues of economic efficiency in tandem with environmental responsibility should underlie decisions for making the changeover. Coal combustion can make up part or the whole of the combustion process in combination with other resources, and offers enhanced productivity for boiler equipment along with reductions in fuel consumption when producing energy through combustion processes, along with an improved level of cleanliness and purification in the resulting flue gases.