THE BCURA COAL SAMPLE BANK: A USERS HANDBOOK

The Coal Bank was originally established in 1982 as the CRE Coal Bank. The intention being to supply universities and other research organisations with small quantities of representative UK deep mined coals. In recent years the Coal Bank has expanded to include a number of internationally traded, primarily, power station coals. It now comprises 36 well characterised specimens, ranging from lignite to anthracite, and including both selected seam samples and commercial grades. In addition a small number of ex British Coal, coal liquefaction catalyst samples and a pitch sample are included in the Bank.

This document reviews the general requirements for the processing and storage of high quality reference coal samples. The policy of the now BCURA Coal Bank is discussed and the preparation and analysis of the samples described. Data sheets for the 36 coals are included together with instructions for requesting samples.

The BCURA Coal Bank is financed entirely by BCURA.

CONTENTS

Tables 1 and 2

Figures 1 to 5

Appendices 1 to 38

THE BCURA COAL SAMPLE BANK

Although collections of coal specimens for scientific studies have existed for many years, it is only comparatively recently that rigorous sampling programmes have been established to provide long-term supplies of reference specimens from the major coal basins of the world. These sample banks are a consequence of the oil crisis of the early 1970s and the re-emergence of coal as a principal world energy resource.

In earlier times, coal was mainly used near to the mine, and its properties probably varied little from year to year. However, with the rapid expansion of the international coal trade, coals of different geographical origin, and with every varying composition and properties, are being used. This diversity demands fuller characterisation procedures to ascertain their composition, and assess the influence of a coal’s characteristics on its utilisation properties.

At the same time, advanced combustion techniques, and new coal conversion technology for gasification and liquefaction, call for specific chemical and physical properties in the coal. To optimise such processes, it is necessary to define those parameters that correlate with performance, and examine their variation for the fullest range of commercially available coals.

As more sophisticated techniques for coal characterisation have evolved, it has become increasingly important that results from the various research activities should be comparable. It has been observed that coal science is often hampered by the fact that scientists and engineers base their work on samples that are “poorly selected, poorly collected, poorly prepared, poorly preserved and poorly characterised”.

To address this problem, a number of programmes have been established to collect representative single seam samples from the major coal basins, and to maintain their long-term availability for scientific and technological research. The principal requirements are that sub samples should be chemically and physically as identical as possible, have well characterised properties, and that they should be stored under conditions which ensure their stability over long periods of time.

It is extremely difficult to ensure that coal samples are maintained in a pristine condition. Great care is necessary during gathering, processing and storage. A channel sample will show all chemical and physical properties of the coal seam from which it is taken, but selective samples, rich in a particular maceral, may be desirable for some studies.

It is important that contact with oxygen should be minimised at all processing stages, but particularly so for finely ground samples. Thorough mixing before and after crushing is necessary to ensure uniform samples, but the use of mechanical blenders should be avoided since they may cause pulverisation of constituent macerals and even segregation. The use of a spinning riffler is preferable for obtaining multiple portions of identical composition. During processing, relative humidity and temperature should be controlled to maintain the equilibrium moisture of the original coal. Once processed, small specimens should be sealed in glass ampoules under an inert gas (nitrogen or argon). Larger samples may be similarly sealed in glass carboys; lumps of coal, as part of the original sample, may be stored under nitrogen or argon in closed drums.

Such a rigorous approach, necessary for high quality samples, is both labour-intensive and expensive. Nevertheless, it has been adopted in the Premium Coal Sample Programme of the Argonne National Laboratory (USA), and by the European Centre for Coal Specimens (SBN) at the Nederlands Meetinstituut (NMi) in Eygelshoven.

Both these programmes are concerned primarily with selected seam samples. The use of such pristine specimens is important in many aspects of coal science, such as organic geochemistry. However, in other areas, particularly coal utilisation studies, their relevance is more questionable. Here, the use of commercial grade samples, processed under more typical conditions, may be appropriate.

2. The BCURA Coal Sample Bank

It was the CRE Coal Bank that was originally established in 1982 to supply universities and other research organisations with small quantities of representative and well characterised UK coals. By so doing, it was hoped to achieve comparability of results through the use of common samples, and to co-ordinate to some extent the research effort on UK coals. Many institutions do not have ready access to coal supplies or possess comprehensive analytical facilities of their own, and the CRE Coal Bank became recognised as a source of reference coals.

In recent years financing of the Coal Bank has been through BCURA. The Coal Bank now comprises 36 coals, ranging from lignite to anthracite. The original specimens were selected seam samples, chosen to represent a rank series of bituminous coals. These specimens were distinguished by untypically low mineral contents. However, commercial grades have featured prominently in recent additions to the Bank. Such coals are more typical of those in industrial use, but they may represent blends from more than one seam in a particular colliery.

The current policy of the BCURA Coal Bank is to maintain a rank series of selected seam samples representative of the major UK coalfields. With the contraction of the UK coal industry, this now represents a unique archive of UK coal reserves. Commercial grade samples are added from time to time; some have represented opencast sites or new mines such as Selby and Asfordby, while others are coals with particular features that may have implications for their utilisation properties. Most recently, a small number of internationally power station grade coals have been added.

From the outset, it was recognised that preparation of Coal Bank specimens would be something of a compromise, since no facilities were available for processing large quantities of coal under an inert atmosphere. The current procedure involves the processing of an initial 100-200kg sample. The coal is thoroughly mixed prior to storage in sealed plastic drums, each with a capacity of ~20 kg. It is assumed that the composition of the material in each drum is identical; sampling from successive drums has failed to reveal significant between drum variations. Representative samples of the coal are taken for analysis, and crushed to the appropriate sizes for the BS tests. Priority is given to avoiding the unnecessary exposure of finely crushed coal to oxygen.

Depending on the nature of the requests, sample may be supplied directly from the bulk coal. Requests for material of a smaller particle size, eg <72 mesh (<212 µm), are met by crushing and screening representative samples of the lump coal.

On account of the diverse nature of the sub-samples, the analyses on the Coal Bank data sheets should be regarded only as “typical”. Although the information is given in good faith, and is believed to be accurate, final responsibility for ensuring that the analysis is fully representative of the actual sub sample provided must rest with the user. However, successive analyses have shown that variations between sub-samples are relatively minor. If required, full or partial analyses of individual samples by accredited methods can be provided at additional cost.

2.2 Analyses

The 36 coals are listed in Table 1 in decreasing rank order, as defined by their carbon content on a dry, mineral matter free (dmmf) basis. The nature of the sample (seam/grade) is indicated and the geographical origin (for the UK coals) is shown on the map in Figure 1.

The coals were analysed by standard methods (BS1016, or equivalent), as appropriate. The Coal Bank data sheets are reproduced as Appendices 1 to 36, in descending rank order. Mineral matter was calculated using the BCURA formula (BS1016, Part 16), and corrections for carbon present as carbonate and for hydrogen in the water of constitution of shale have been applied in converting elemental analyses to a dmmf basis. Organic sulphur was determined by difference in the forms of sulphur analysis (BS 1016, Part 11), and oxygen was obtained by difference making the assumption that 50% of the total chlorine is associated with the coal substance (BS1016, Part 16). Petrographic analyses were carried out by standard reflectance microscopy techniques using 400 or 500 observation points on mounted, polished samples.

The NCB/BCC (CRC) and ECE (ISO) codes for the Coal Bank coals are given in the data sheets (Appendices 1 to 36), and are summarised in Table 1. The 1956 ECE classification of hard coals has been retained, pending wider acceptance of the new (1988) codification system. The NCB (1964) and ECE (1956) schemes are shown in Appendices 37 and 38.

The location of the Coal Bank coals on a version of Seyler’s Coal Chart is shown in Figure 2. This chart conveniently summarises the interdependence of a number of rank related properties. The majority of the Coal Bank coals fall within, or very close to, the main coalification band. Individual parameters are plotted against carbon content (dmmf basis) in Figures 3 to 6. Figure 3 demonstrates the expected decrease in oxygen and hydrogen contents. By contrast, the nitrogen content appears to pass through an ill-defined maximum at approximately 85% carbon (Figure 5). The data for Hambach (Rheinish lignite) Appendix 36A, has been excluded from these correlations due to its significant difference from all the other samples.

2.3 Liquefaction Catalyst and Pitch Samples in the BCURA Coal Bank

Three samples of catalysts used in the hydrocracking step of the Liquid Solvent Extraction (LSE) coal liquefaction process have been placed in the BCURA Coal Bank, together with a sample of coal-derived pitch obtained by distillation of the hydrocracker feed stream. Brief descriptions of these materials are given below.

PBC90

This catalyst contains approximately 2.5% NiO and 12% MoO₃ on a rolled spherical alumina support. The support has a high macropore volume and is capable of functioning despite high levels of adventitious metals deposited from the feedstock. The nominal size range is 2.4 – 4.0 mm diameter and the bulk density is approximately 510 kgm^-3. Although not in commercial production, this catalyst is a direct analogue of a commercially available catalyst that has a smaller size distribution. This catalyst was used for the early part of the test programme at the 2.5 tpd Point of Ayr pilot plant.

EXP-AO-60

This is an experimental catalyst formulated by Amoco and produced in tonnage quantities for the former Wilsonville liquefaction programme. It contains approximately 3 % NiO and 17.5% MoO₃ on an extruded cylindrical alumina support. The support has a bimodal pore distribution, with significant pore volume in both the micropore and macropore range, the intention being to convey resistance to poisoning by metals and carbon laydown. The extrudate diameter is 1.5 mm, the nominal length being 5 – 6 mm and the bulk density approximately 510 kgm^-3. This catalyst was used for the majority of the test programme at Point of Ayr.

HDS 1443

This catalyst was commercially available from Criterion Catalysts. It contains approximately 4% NiO and 12.5% MoO₃ on an extruded alumina support. The support has relatively little pore volume in the macropore region. The extrudate diameter is 0.8 mm, the nominal length 5 – 6 mm and the bulk density approximately 570 kgm^-3. This catalyst was not used at Point of Ayr.

Pitch

This material (ex Point of Ayr) was produced by vacuum distillation of an LSE hydrocracker feedstock and represents the bottoms remaining at an equivalent atmospheric temperature of approximately 475 ^oC. It contains both freshly derived coal extract and material which has been recycled one or more times around the process as part of the solvent. It is a solid at room temperature.

2.4 Arrangements for Obtaining Samples

Requests for samples from the BCURA Coal Bank should be made (in writing only please) to Mr George Bradley, TES Bretby, P.O. Box 100, Burton-on-Trent, Staffordshire, DE15 0XD, Tel: 01283-554522, Fax: 01283-554431, E-mail: george.bradley@esgl.co.uk. Coal samples up to 0.5 kg will be prepared to the user’s specification. The quantities of liquefaction catalyst and pitch samples available will be dependent upon the reserves at the time of the request. No charge is made to researchers in UK academic institutions; this is made possible by the financial support from BCURA.

Requests for larger coal samples will be considered in the light of available stocks. However, it is emphasised that the principal objective of the BCURA Coal Bank is to maintain the long-term availability of small, representative samples for scientific research.

In order that the integrity of Coal Bank samples can be assessed all recipients are requested to advise BCURA of the results of any conventional coal analysis undertaken on the supplied samples. Comparison of this information with the Coal Bank data sheets will enable a record of sample consistency and integrity to be maintained. Any information supplied will be treated in confidence.

Table 1

R suffix to Ref. No. indicates that original coal has been replaced by fresh sample

N/A Not applicable; eg. ash > 10%w/w

Table 2:

Table of Data Used for the Preperation of Figures 3, 4 and 5

Link to appendices 1 to 5 (288 KB)
Link to appendices 6 to 10 (280 KB)
Link to appendices 11 to 15 (279 KB)
Link to appendices 16 to 20 (290 KB)
Link to appendices 21 to 25 (276 KB)
Link to appendices 26 to 30 (278 KB)
Link to appendices 31 to 35 (281 KB)
Link to appendices 36 to 38 (241 KB)
(Adobe Acrobat pdf documents)

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