Preston and Sanders RD Calculation
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ESTIMATING THE IN SITU RELATIVE DENSITY OF COAL K.B. PRESTON (Pacific Coal Pty. Limited) & R.H. SANDERS (Quality Coal Consulting Pty Ltd) Determination of the in situ density of coal from borecores is essential for the accurate estimation of reserves, especially for low rank (high bed moisture) coals. There is a confusion of terminology describing different types of density, and a wide range of opinion on the relevance of each type to the subject of reserves. In many cases known to the authors, coal reserves have been incorrectly calculated due to incorrect estimation of density. A study carried out to identify a suitable method for estimating the in situ relative density of coal concluded that, providing the bed (in situ) moisture and the moisture of the analysis sample are accurately known, the density of the coal in situ can be determined with acceptable accuracy from the relative density determined using the density bottle method, providing that the basis is changed as follows: Relative density (in situ) RDadX (100- Mad) 100+ RDadX (ISM- Mad) - ISM
INTRODUCTION The relative density of coal is a fundamental physical parameter which should be well understood by geologists, who need to know the in situ relative density of coal for use in reserve calculations. It is recognised that for resource evaluation, particularly where resources are large, accurate in situ coal relative density estimates are not critical. However, where mineable reserves are being determined for mine planning purposes, accuracy is important. A 5% error in relative density, over a typical reserve for a 20 year mine, is equivalent to a full year's production. In spite of this, the relative density of coal appears to be a subject which is poorly understood. The literature is of little assistance, with virtually no relevant work on the subject having been identified, apart from that of Svenson (1987) and Smith (1991). Hence the application of relative density in reserve calculations is at best uncertain, at worst incorrect. After numerous debates on the subject within Pacific Coal, Quality Coal Consulting Pty Ltd (QCC) was commissioned to assist in identifying a method for estimating the in situ relative density of coal. Paramount in the study was consideration of the relationships between coal density, coal porosity and moisture. The impact of coal rank, type and mineral matter content were not specifically considered, although it is acknowledged that these are important in the overall study of coal relative density. Ancillary study aims were to obtain a better understanding of the subject, the terms used, tests applied, and the causes of variability in results. This paper discusses some of the major findings of the study. REQUIREMENTS OF AN IN SITU DENSITY ESTIMATION METHOD It was recognised at the outset of the study that there are four requirements to be met by any estimation method being proposed. These are as follows: (i)The method must be appropriate - it must recognise the conditions under which coal exists in situ and be able to simulate these conditions. (ii)The method must be of acceptable accuracy at least as accurate as the estimates for other reserve evaluation parameters. (iii)The method must be applicable to sets of spatially distributed data. Accurate density estimation at only one or two points, will not allow lateral and vertical variations to be accommodated in the overall reserve estimate. (iv)The method must be available from micro scale tests - preferably laboratory testing of borecores. Macro scale tests are difficult and expensive to perform. OBSERVATIONS Porosity
It is well established that coal is a porous substance, and that both the pore size distribution and total pore volume vary~ depending on a number of factors. The pore sizes have been conveniently grouped into three types, by a number of researchers. The groups are: Large pores, with diameters >30 nm (30-3000 nm), Intermediate size pores, with diameters of 1.2 to 30 nm, Micropores, < 1.2 nm in size. Sub-bituminous and lower rank high volatile bituminous coals tend to be relatively high in total porosity ai-3 have a high proportion of intermediate size pores. High rank bituminous coals have no intermediate size pores and appreciable microporosity (50%). Lignites have little pore volume in the, intermediate sizes, and have high levels of macroporosity. Terminology
General Density The density of a material is its mass per unit volume, usually quoted in kg/m~, or g/cm . Prior to metrication this property was known as specific gravity.
Relative Density (RD) The density of water is 1.0 g/cm 3 at a temperature of 5C. So the relative density of a material is its density, relative to the density of water at 5C (taken as 1.0). This property is a ratio and is therefore, dimensionless. It is numerically equivalent to the value of the density of the material. Clearly all determinations of relative density require the mass and volume of a sample to be measured. The mass determination is always simple. However the volume determination is the one which causes problems, both of measurement and of understanding. This relates to the porous nature of the coal and the variable degree to which the different methods cope with this aspect of the determination.
"Types" of Relative Density "True" or "Absolute" Relative Density These terms should only be used to describe the relative density of a unit volume of pore free dry coal. In practice coal may exist dry but it is never pore free. Pore free in this sense means that in determining the volume of the sample, the medium used must occupy all pores. In practice this is difficult. The helium density method is the recognised method for determination of this parameter. Helium, being the smallest atom, has the best chance of penetrating the greatest number of pores, although it is recognised that there are some pores that even helium does not penetrate. Because coke has very large pores, the results obtained in water by the density bottle method are said to give the "true" relative density. Unfortunately, many results determined in the past on coal by this method have also been (incorrectly) referred to as "true" relative density. The availability of absolute relative density values for coal samples would make the estimation of in situ relative density a relatively straightforward process. In practice these values are rarely available and we have to struggle with inferior data.
"Apparent" Relative Density or "Coal Particle" Relative Density This term describes the relative density of lump coal which may contain pores, fissures and moisture. The degree of preservation of these features in the actual sample can be variable. Because of this fact and the relative imprecision of the method (displacement of lump coal in a water bath) the results are regarded to be inconsistent and unreliable. The method is described in AS1038.21 Item 5. A more precise method of determination exists, namely the mercury density method. However, like the helium method it is not available on a routine basis. If the method was generally available and the sample state known to be similar to in situ,
(particularly regarding moisture) then this method could be used to directly estimate in situ relative density. Unfortunately this is not the case.
In Situ Relative Density This refers to the relative density of coal in the ground. The coal will contain pores and fissures which will be filled with water and dissolved gases. The relative density of the coal in situ is the value required for estimating coal reserves.
Relative Density "AS1038.21 Item 4" The most common method of determining coal density is via this Australian Standard method or its predecessor (pre 1983), the ACIRL method. Most coal geologists have vast sets of relative density measurements of this type. Whilst the method is cheap and easy to apply, the state of the sample when tested, does not simulate a state which is useful for coal reserve estimation. That is to say, the sample is: •
ground to -212 micron size, thereby removing fissures and some pores,
air dried i.e. retaining some but not all of its in situ moisture.
The method involves measuring the amount of liquid displaced by the coal, either in a density bottle or in a volumetric flask, and hence determining its volume. This is then related to the original weighed mass. The major problem with the method is the inability of the liquid (either water and wetting agent or methylated spirits) to occupy all pores within the coal i.e. displace all air and water. The result approximates neither absolute relative density nor in situ relative density. However it probably approaches the former more closely than the latter. Prior to the publication of the Australian Standard method of determining relative density using the density bottle, the method commonly used in Australia was the NCB Analyst's Handbook Method, on which the current Standard is substantially based. (NCB was the British "National Coal Board", now British Coal). Note that the top size of the test sample is 212 micron, while the diameter of a typical macropore is only 1.0 micron. Clearly, whilst much of the test sample is finer than 212 micron, many of the pores are much finer than the particles. Some of the pores are removed in grinding; some are not. For the purposes of further discussion the relative density value determined according to the AS1038.21 Part 4 procedure will be referred to as the standard relative density. Relative Density Values NOT to Use in Determining Coal Reserves Standard Relative Density As noted these relative density values are cheap and plentiful. It could be claimed that this abundance gives a wide lateral and vertical picture of any coal seam and hence the values are statistically significant i.e. a lot of incorrect values are better than a few correct ones. The standard relative density does not replicate the conditions which need to be met in a determination of in situ relative density. Consequently it should not be used for coal reserve calculations. Use of the standard relative density in reserve calculations is estimated to result in an overstatement of reserves of 2.5-4.5%. Standard Relative Density - Adjusted UP for Moisture Some reserve estimators have recognised the fact that the values determined by this method are reported at air dried basis rather than in situ moisture basis (which is invariably higher). To compensate for this a moisture adjustment has been made as follows: RD in situ =RDairdried x 100- Moisture(airdried) 100-Moisture(insitu)
This process increases the relative density but incorrectly so. It is based on the premise that adding water to the coal must make it heavier i.e. more dense. AUSTRALIAN COAL GEOLOGY This would be true if water could be added without any increase in volume i.e. that the number being adjusted (RD air dried) represents a state where pores, fissures etc. are preserved but are only partly water filled. This is not the case. The coal in the Standard test has much of its pore space and contained moisture removed. The remaining moisture is contained in closed pores or bound to the margins of pores. The moisture lost on air drying cannot be added without first "adding" the pores back in, thereby increasing volume. Adjustments of this kind can easily add an error of 4-6% to an already overstated relative density figure. The Change of Basis Equation The correct equation to use for the conversion of coal relative density from one moisture basis to another is: RD2 =
RD1 x (10 0- M1) 100+ RD1 x (M2 - M1) - M2
where RD1 = old RD RD2 = new RD M1 = old moisture M2 = new moisture Or specifically for the in situ density calculation from standard relative density values: RD1 = standard RD (RDad) RD2 = in situ RD M1 = air dried moisture (Mad) M2 = in situ moisture (ISM) When considering how to convert coal relative density from the Standard basis to an in situ basis, it is instructive to consider what happens to the coal when being processed in the opposite direction, i.e. when is it converted from in situ to Standard basis. The following processes are believed to occur: Fissures will be removed by grinding. Any contained water will be drained or driven off in drying. With the exception of micropores all other open pores (i.e. pores intersected by coal particle faces) will either by removed or opened to penetration by the displacement fluid. Contained water will be driven off during air drying. Closed pores will be retained together with their contained water. Some open micropores may be removed and their water driven off. Many may lose some of their water in air drying and possibly some of their volume. These pores may not all be filled by displacement fluid therefore their volume is not entirely lost.
IN SITU CONDITION Solid coal; under pressure; saturated; all voids filled with water and dissolved gases.
------- effective coal volume 1 2 3
open fissure open micropore closed micropore open intermediate pore closed intermediate pore open macropore
4 5 6
solid grey hatching dry coal Mountain type hatching water & dissolved gas
AIR-DRY CONDITION - 'GROUND' COAL IN DISPLACEMENT FLUID Coal ground to –212 micron; closed pores still filled with water and dissolved gases, large open fissures, open macropores and open intermediate pores filled with displacement fluid; open micropores contain their portion of the air-dry moisture Only (that is, they are not penetrated by the fluid in the density bottle, even under vacuum).
....... ----------------1 2 3 4 5 6
effective coal volume
open fissure open micropore closed micropore open intermediate pore closed Intermediate pore open macropore
Solid Grey Hatching
Mountain type hatching
water & dissolved gas
Broken stick type hatching water Grey Dots
The figure shows diagrammatically the state of the coal, in situ and in the Standard (air dry) condition. Clearly while the same schematic model is used for both of these diagrams (both solid and ground coal) and the shape is retained, the coal in the lower diagram will of course be broken up on grinding. Fissures and large macropores will disappear. However, this makes no difference to the analytical process. The relationship between mass and volume, determined under the conditions of the test, will be constant irrespective of the physical state of subdivision of the model. An alternative way of stating this is that, for a given level of grinding, the mass of sample determined by weighing and the volume of the sample "seen" by the density bottle, will be the same as that "shown" in the schematic model. In the process of converting a coal sample to a ground, air-dry state, the volume is reduced by removal of pores and contained water. This reduction in volume has a greater effect on the relative density than has the loss of mass held in that volume. Thus the relative density of the sample is effectively increased by this process. More simply put, as voids and contained water are removed, the density of the sample trends towards the absolute density of the coal. The estimation of in situ relative density, from values determined by AS1038.21 Part 4, involves reversal of the process described above. The voids must be restored to their original state and refilled with water. In doing this both mass and volume of the sample will increase but volume will increase at a greater rate than mass. Hence, sample density will decrease, trending towards 1 (the relative density of water). The process of restoring the voids and refilling them with water is simulated by the change of basis equation. In conclusion, it is considered that the change of basis equation, given reliable input, will enable relative density of coal to be converted from one basis to another. In particular it is the appropriate formula for estimating in situ relative density from standard relative density. PRACTICAL APPLICATION The AS1038.21 Item 4 method of relative density determination should continue to be carried out routinely. As previously noted it is cheap and readily available and therefore does allow a large spread of results to be obtained. It is also important to acknowledge that: • • •
the method is relatively precise, it is an good indicator of relative density variations resulting from mineral matter content, it is the reference point from which the in situ relative density can be estimated.
Clearly estimation of in situ relative density from the above requires accurate determination of in situ moisture. This is not an easy property to measure. However, it should not be omitted because of this fact. Knowledge of in situ moisture is important both to density calculations (hence reserves, coal handling mass calculations, etc.) and to estimation of product coal total moisture. It is recommended that for all deposit evaluations, particularly where it appears that a significant resource may exist, a systematic effort to determine in situ moisture should be made. Sufficient samples should be taken to represent the normal range of ash values. If there are significant rank and/or type variations these should also be represented in the sampling. In general, it is thought that sampling should be done by coring; chip samples are too finely divided and will contain free moisture. Coring by clean water circulation will probably be necessary to avoid air blast drying of the coal. Prompt bagging and sealing of the core, and total reclamation of the moisture at the laboratory will be necessary. Once the appropriate moistures and Standard relative density are known, estimation of sample in situ relative density is a matter of simple calculation. REFERENCES CAN, H., NANDI, S.P. & WALKER, P.L. Jr., 1972: Nature of the porosity in American Coals. Fuel, 51, 272-277. PARKASH, S., CHAKRABARTTY, S.K. & DU PLESSIS, M.P., 1984: Porosity in Alberta subbituminous coals. Coal Research Department, Alberta Research Council, Edmonton, Alberta, Canada. SHARKEY, A.C. & McCARTNEY, J.T., 1981: Physical Properties of Coal and Its Products. In Elliott, M.A., (Editor):Chemistry of Coal Utilization, Second Supplementary Volume. Wiley and Sons, New York, 159-284.
STANDARDS ASSOCIATION OF AUSTRALIA: Determination of the Relative Density and Apparent Relative Density of Hard Coal, AS 1038.21 - 1983. SMITH, C.C., 1991: Theoretical Estimation of InSitu Bulk Density of Coal. CIM Bulletin, 84 (949), 49-52. SVENSON, D., 1987: The technical evaluation of hard rock (black) coal deposits - a review of AS2519. Australian Coal Geology, 7, 47-71. VAN KREVELEN, D. W., 1961: Coal as a solid colloid. In Coal. : Elsevier, Amsterdam, 127-149.