Sampling Source and Coal Grade: Assessing Variability in Proximate Properties and Gross Calorific Value

Coal is a heterogeneous sedimentary fuel composed of organic matter together with variable proportions of mineral matter. Consequently, its chemical and energy-related properties can change over relatively short spatial scales and according to the material selected for sampling. This heterogeneity is particularly important for coal-quality measurement because sampling, sample preparation and analysis represent successive sources of variability. Sampling theory and coal-quality practice therefore emphasize that the collected sample should represent the population of coal for which the analytical result will be used (Moodley and Minnitt, 2009; Hand, 2014; Thomas, 2020).

The representativeness problem becomes more relevant when samples are collected at different stages of the mining system. Drill cores and seam or face samples are intended primarily to characterize coal in its geological occurrence, whereas samples taken from conveyors or other moving coal streams characterize material after extraction and handling. The two categories need not represent exactly the same material population. Differences may arise from inherent seam heterogeneity, out-of-seam dilution, mineral-matter distribution, particle segregation, selective extraction, handling and environmental exposure. Sampling standards therefore prescribe systematic procedures designed to reduce bias and obtain representative increments from heterogeneous coal streams (ASTM D2234/D2234M-20; ISO 18283:2022; BIS IS 436).

Proximate analysis remains one of the most widely used routine approaches for characterizing coal. Moisture, ash, volatile matter and fixed carbon provide complementary information on fuel composition and are closely associated with combustion and utilization behaviour (Lotfi and Jafari, 2018). Gross calorific value (GCV) provides the energy content of the fuel and is especially important in India because non-coking coal is classified commercially according to GCV. Empirical and experimental studies have repeatedly demonstrated relationships between proximate constituents and heating value, particularly the negative influence of ash and moisture and the contribution of the combustible organic fraction represented by volatile matter and fixed carbon (Channiwala and Parikh, 2002; Parikh et al., 2005; Majumder et al., 2008; Aich et al., 2020; Kumari et al., 2019).

Indian Gondwana coals are commonly characterized by relatively high mineral-matter and ash contents, although substantial variation exists among coalfields, seams and individual samples. Studies from Singrauli and other Indian coalfields have documented variation in moisture, ash, volatile matter, fixed carbon and GCV and have linked compositional differences to coal quality and utilization (Dwivedi and Kumar, 2022; Sajid et al., 2022). Work on Indian coal has also shown that ash and moisture can exert strong effects on calorific value, making their measurement important for quality assessment (Saravanan et al., 2023; Kumari et al., 2019). Such evidence establishes the importance of heterogeneity, but most studies have focused on geological/seam variability, inter-mine comparisons, or prediction of coal properties rather than direct comparison of multiple sampling points within the same underground mine.

This distinction provides the motivation for the present study. A sampling-source comparison can address a practical question relevant to mine-level quality control: does the measured quality of coal remain sufficiently similar when samples are collected from different points in the mining and handling system? If not, the choice of sampling point may influence not only individual proximate parameters but also the GCV-based grade assigned to the material. Although representative sampling has been extensively addressed in standards and sampling literature, comparatively limited published work directly compares drill, seam and conveyor-belt samples from the same underground Indian mining operation.

The present study therefore examines coal from Sharda Underground Mine in the Kanhan Area of Western Coalfields Limited, Madhya Pradesh. Moisture, ash, dry-basis volatile matter, fixed carbon and GCV are compared for drill, seam and belt samples. The objectives are to (i) quantify differences among the three sampling sources, (ii) examine how these differences are reflected in GCV and Indian coal-grade classification, and (iii) interpret the findings in relation to representative sampling and coal heterogeneity. The study is intentionally framed as a mine-level case study; the dataset is not intended to provide a mine-wide statistical estimate of coal quality.