In order to formulate a blend to satisfy a coal specification it is necessary to understand whether the coal properties are additive. Additive properties are those that can be calculated as a weighted average of the properties of the component coals. For practical purposes these properties include:
Moisture
Ash
Volatile Matter
Carbon
Hydrogen
Nitrogen
Sulphur
Forms of sulphur
Chlorine
Phosphorus
Ash analysis2
Calorific value
Trace elements
Properties that are not additive include:
Ash Fusion Temperature:
It is well established from phase equilibrium theory that the melting behaviour of mixtures of ash oxides are not additive. Occasionally blends can have AFTs that are higher than or lower than those of either of the component coals.
Hardgrove Grindability Index:
The HGI of a blend is normally lower than the weighted average. Figure 1 shows HGI measured for blends between many pairs of coals as well as of the component coals. The connecting lines are not straight but generally curved downward.The measured HGI therefore provides an unfavourable indication of the pulverising characteristics of the blend which is often not justified in practice. It is not recommended that the HGI of a blend be reported if it can be avoided.
2) This may not apply to sulphur (as SO3) because not all of the coal sulphur remains in the ash. In such a case the other elements would need to be normalised to a constant sulphur level.
Figure 1: Measured HGI for Blends of Coal Pairs
Abrasion Index: Experience suggests that the Abrasion Index may not be additive, though this may be a result of poor precision of the test.
Calculating the Coal Analysis for Additive Properties
In principal it is necessary to convert all analytical results to the as-received basis before determining the weighted average. Once this has been done the results can be converted back to any other basis if required.
The Ash Analysis of a blend is an important example of the above. In this case it is necessary to firstly calculate the percentages in the coal rather in the ash. The required steps for calculating the CaO in a 50/50 blend of two coals A and B are:
(1) CaOA (arb) = CaOA (% in Ash) x AshA (arb) / 100
(2) CaOB (arb) = CaOB (% in Ash) x AshB (arb) / 100
(3) CaOblend (arb) = 0.5 x CaOA (arb) + 0.5 x CaOB (arb)
(4) Ashblend (arb) = 0.5 x AshA (arb) + 0.5 x AshB (arb)
(5) CaOblend (% in Ash) = CaOblend (arb) / Ashblend (arb) x 100
A spreadsheet can easily be made to perform these steps without risk of errors. Alternatively a more compact spreadsheet can be made to do the calculations more directly and for blends of several coals 3).
Optimising the Composition of a Blend
Blends may be designed to satisfy a number of criteria such as top or bottom-limits on coal composition parameters using an optimising program4. At the same time it is possible to optimise the solution, for example:
Two coals are to be blended to achieve less than 15% total moisture, less than 0.9%
sulphur, a Fuel Ratio less than 2, and Na2O in ash between 0.5 and 2%. While
achieving this it is desired to minimise the cost of the coal, that is to maximise the proportion of the cheaper component coal. The required solution is the proportions of the two coals that satisfy all of these criteria at minimum cost.
The above example includes the Fuel Ratio which is derived from two parameters (Fixed Carbon/Volatile Matter). It is possible to include more complex parameters such as a Slagging Index. Occasionally there is no possible solution to the problem as defined, that is all of the requirements cannot be satisfied simultaneously. In such a case it is necessary to either relax one of the requirements or look at different component coal(s).
3) An Excel spreadsheet will do this using the SUMPRODUCT function. It is wise to check the calculations once using the long-hand method described above.
4) For example the Add-in Solver operation in Excel.
A Blending Scenario
Table 1 gives the analysis of two coals of very different properties, and the analysis of a 70/30 blend calculated as described above.
Table 1: Calculated Composition of a 70/30 Blend
In this case each of the coals has properties that may make them unattractive to coal customers. Because they are not the same properties for the two coals, the blend has properties that are generally more attractive, demonstrating that there may be mutual benefits with blending. Some of the features that are improved by blending are listed in Table 2.
Table 2: Properties that may be improved by blending Coals A and B
Worth noting is that in the case of sulphur, Fe2O3 and Na2O, intermediate values may be preferable to low or high values, so that blending improves these values for both coals. Table 1 includes estimates of the HGI and Abrasion Index of the blends event though these may not be strictly additive. As mentioned above, a measured HGI on the blend would probably be lower (perhaps around 48) than the value in Table 1, however experience suggests that the calculated value is a better indicator of mill performance than a measured value.
Ash Fusion Temperature. As indicated above, the AFT of a blend cannot be predicted by weighting the numbers. Nevertheless is reasonable to calculate a hypothetical value by this means (taking into account the different ash contents of the component coals), then confirming by direct measurement on a laboratory sample of the blend.