GB/T 211—2007 Method for determination of total moisture in coal
GB/T 211-2007 replaces GB/T 211-1996
1 Scope
This standard specifies the reagents, equipment, operation steps, calculation of results, and precision for determining the total moisture in coal. The method of drying in a nitrogen stream (Method A1 and Method B1) is applicable to all types of coal. The method of drying in an air stream (Methods A2 and B2) is suitable for bituminous coal and anthracite. The microwave drying method (Method C) is used for bituminous coal and lignite.
Method A1 is designated as the arbitration method.
2 Normative references
The terms in the following documents become part of this standard through reference. For dated references, subsequent amendments (excluding errata) or revisions do not apply to this standard. However, parties entering into agreements based on this standard are encouraged to investigate whether the latest versions of these documents are available. For undated references, the latest edition applies.
Preparation method of GB 474 coal sample
GB/T 19494.2 Mechanized sampling of coal - Part 2: Preparation of coal samples (GB/T 19494.2-2004, ISO 13909-4:2001, NEQ)
Industrial analysis method for GB/T 212 coal (GB/T 212-2001, eqv ISO 11722:1999, eqv ISO 1171:1997, eqv ISO 562:1998)
3 Method summary
3.1 Method A (two-step method)
3.1.1 Method A1: Drying in a nitrogen stream
A certain amount of coal sample with a particle size less than 13 mm is dried to constant mass at a temperature not exceeding 40°C, then crushed to less than 3 mm and dried in a nitrogen stream at (105–110)°C until the mass is constant. Total moisture is calculated based on the mass loss after two-step drying.
3.1.2 Method A2: Drying in an air stream
Similar to A1, but using air instead of nitrogen for the second drying step.
3.2 Method B (one-step method)
3.2.1 Method B1: Drying in a nitrogen stream
A coal sample with a particle size less than 6 mm is weighed and dried in a nitrogen stream at (105–110)°C until constant mass is achieved. Total moisture is determined by mass loss.
3.2.2 Method B2: Drying in an air stream
A similar procedure is followed, but using air instead of nitrogen for drying.
3.3 Method C (microwave drying method)
A coal sample with a particle size less than 6 mm is placed in a microwave oven. Water molecules generate frictional heat due to the alternating electric field, causing rapid evaporation. Total moisture is calculated based on mass loss after drying.
4 Reagents
4.1 Nitrogen (GB/T 8979): purity 99.9%, oxygen content less than 0.01%.
4.2 Anhydrous calcium chloride (HGB3208): chemically pure, granular.
4.3 Color-changing silica gel (GB/T 7822): industrial grade.
5 Instruments (for Methods A and B)
5.1 Air drying box: automatic temperature control, air blowing device, capable of maintaining temperatures between (30–40)°C and (105–110)°C, with proper ventilation.
5.2 Nitrogen drying oven: automatic temperature control, maintains (105–110)°C, with nitrogen inlet and outlet, sufficient ventilation.
5.3 Shallow plate: made of heat-resistant materials, can hold up to 500g of coal sample, load per unit area not exceeding 1g/cm².
5.4 Glass weighing bottle: 70mm diameter, 35–40mm height, with tight grinding cap.
5.5 Analytical balance: accuracy of 0.001g.
5.6 Industrial balance: sensitivity of 0.1g.
5.7 Dryer: contains color-changing silica gel or granular anhydrous calcium chloride.
5.8 Flowmeter: range of 100–1000 mL/min.
5.9 Drying tower: 250mL capacity, filled with silica gel or anhydrous calcium chloride.
6 Samples
6.1 Coal sample
For total moisture, a coal sample with particle size less than 13 mm should be at least 3kg; for less than 6 mm, at least 1.25kg.
6.2 Sample preparation
6.2.1 Full-moisture coal samples with particle size less than 13 mm should be prepared according to GB474 or GB/T19494.2.
6.2.2 For full-moisture samples with particle size less than 6 mm, crush the sample using a crusher that minimizes moisture loss, then reduce to at least 1.25kg and store in a sealed container.
6.3 Before measuring total moisture, check the seal of the container. Wipe clean, weigh to 0.1% accuracy, and compare with the labeled weight. If there's a small discrepancy, it may indicate moisture loss during transport.
6.4 Mix the sample thoroughly for at least 1 minute before weighing.
7 Determination steps
7.1 Method A (two-step method)
7.1.1 External moisture (Methods A1 and A2, air dried)
Weigh approximately 500g of coal sample (500 ± 10g), spread evenly in a shallow pan, and dry in an air drying oven at ambient or not higher than 40°C until constant mass. Record the mass loss.
Calculate external moisture using formula (1).
7.1.2 Intrinsic moisture (Method A1, nitrogen drying)
After determining external moisture, break the sample to less than 3 mm, weigh 10 ± 1g into a pre-dried weighing bottle, and place it in a nitrogen-drying oven preheated to (105–110)°C. Dry for 1.5 hours for bituminous coal, and longer for lignite and anthracite.
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