Discussion on the additivity of coal blending volatiles

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Discussion on the Additivity of Volatiles in Coal Blending. Deng Yanping, Coking Plant, Chongqing Jianghe Coal Chemical Co., Ltd. Abstract: Understanding the properties of volatiles is essential for accurately predicting the volatility of coal blends made from different types of coal, thereby reducing randomness in production and providing better guidance for coal blending. Keywords: coal blending, volatiles, additivity I. Introduction: 1. Principle and significance of measuring coal volatiles: Coal is heated to around 900°C in an air-free environment; the organic substances and a portion of the minerals in the coal decompose into gases or liquids and escape. By subtracting the moisture content in the coal from this value, what remains is the volatiles content. The residue remaining after the removal of volatile substances is called cinder; since volatile substances are not inherent to coal but rather products of thermal decomposition at specific temperatures, the content of volatile substances in coal should be referred to as the volatility yield. Since the volatile matter yield and the characteristics of cinders can be used to preliminarily determine the processing and utilization properties of coal as well as its calorific value, the volatile matter yield is the most important indicator for coal classification in countries around the world as well as in China. 2. Prediction of coal volatile matter: Since it is not an inherent property of the coal, like ash content, moisture, and sulfur content, it also exhibits strong additivity. Depending on the requirements, volatile matter can be expressed in various ways, such as ash-free and moisture-free basis volatile matter, or dry basis volatile matter. In the criteria for classifying coal, classification is carried out strictly based on the ash-free and moisture-free basis values of the coal; for example, our plant’s traditional method of blending coals involves calculating a weighted average using the ash-free and moisture-free basis volatile matter of individual coal types. Through numerous experiments, the author believes that calculating based on the anhydrous and ash-free volatile matter content yields significant differences from the actual values; therefore, using dry-base volatile matter to predict the volatile matter content of blended coal is a more scientific and reasonable approach. II. Experimental conditions: 1. Two types of coal were mixed in different proportions; the parameters for 1/3 coking coal are as follows: Ad%: 11.87 ； Mad%:1.04 ； Vdaf: 27.43%, Vd%: 24.17; the parameters for primary coking coal are Ad%: 12.23 ； Mad%:1.21 ； Vdaf: 21.46%, Vd%: 18.84%. The experimental results are as follows:
Table 1: Mixing of two types of coal samples in different proportions
Experiment No. | Proportion | Experimental result Vdaf% | Calculated value Vdaf% | Difference D1 | Experimental result Vd% | Calculated value Vd% | Difference D2
A | 1 | 5:5 | 23.19 | 24.45 | 1.26 | 20.41 | 21.51 | 1.10
| 2 | 5:5 | 23.42 | 24.45 | 1.03 | 20.62 | 21.51 | 0.89
| 3 | 5:5 | 23.24 | 24.45 | 1.21 | 20.46 | 21.51 | 1.05
B | 1 | 4:6 | 23.73 | 25.04 | 1.31 | 20.92 | 22.04 | 1.12
| 2 | 4:6 | 24.39 | 25.04 | 0.65 | 21.50 | 22.04 | 0.55
| 3 | 4:6 | 24.83 | 25.04 | 0.21 | 21.89 | 22.04 | 0.15
C | 1 | 3:7 | 24.72 | 25.64 | 0.92 | 21.83 | 22.57 | 0.74
| 2 | 3:7 | 24.37 | 25.64 | 1.27 | 21.52 | 22.57 | 1.05
| 3 | 3:7 | 24.43 | 25.64 | 1.21 | 21.57 | 22.57 | 1.00
D | 1 | 2:8 | 24.53 | 26.24 | 1.71 | 21.69 | 23.10 | 1.41
| 2 | 2:8 | 24.99 | 26.24 | 1.25 | 22.10 | 23.10 | 1.00
| 3 | 2:8 | 24.30 | 26.24 | 1.94 | 21.48 | 23.10 | 1.62
Average difference: 1.16, 0.97

2. When mixing coals with different ash contents, there are cases where the properties of the coals are similar, and there are also cases where the qualities of the coals differ significantly. The results are shown in Table 2 below:
Table 2: Comparison of mixtures with different ash contents
Experiment No. | Ash content of mixed coal | Experimental result Vdaf% | Calculated value Vdaf% | Difference D3 | Experimental result Vd% | Calculated value Vd% | Difference D4
1 | 16.66 | 16.59 | 17.18 | 0.59 | 13.68 | 13.54 | -0.14
2 | 20.48 | 21.69 | 23.32 | 1.63 | 17.04 | 17.8 | 0.76
3 | 41.06 | 23.35 | 33.20 | 9.85 | 13.66 | 14.91 | 1.25
4 | 40.21 | 23.85 | 32.51 | 8.66 | 14.00 | 14.95 | 0.95
5 | 43.03 | 31.15 | 38.65 | 7.50 | 17.77 | 19.21 | 1.44
6 | 19.07 | 19.65 | 19.01 | -0.64 | 15.74 | 16.98 | 1.24
7 | 20.19 | 22.17 | 23.86 | 1.69 | 17.54 | 18.69 | 1.15
8 | 20.76 | 23.04 | 25.09 | 2.05 | 18.62 | 19.54 | 0.92
9 | 18.50 | 20.79 | 20.18 | -0.61 | 15.44 | 16.13 | 0.69
10 | 17.94 | 18.38 | 18.95 | 0.57 | 15.01 | 15.28 | 0.27
11 | 17.37 | 17.64 | 17.72 | 0.08 | 14.45 | 14.42 | -0.03
Standard deviation: 3.87, 0.53

From the above data, it can be seen that using the weighted average based on the ash-free and moisture-free basis to calculate the volatile matter content of blended coal leads to significant errors. Moreover, when the ash contents of the mixed coals differ greatly—as seen in Experiments 3, 4, and 5 in Table 2—the difference between the calculated values and measured results can reach as much as 9.85. This indicates that such calculations lack additivity. III. Conclusion ； 1. Our plant previously used an anhydrous, ash-free basis calculation method for the volatiles content of blended coals; the results obtained using this method contained larger errors compared to those calculated on a dry-basis basis. When the ash contents of different coal blends varied significantly, the calculated values differed even more from the actual values. We have now switched to using dry-basis values for calculating the volatiles content of blended coals. 2. In existing thermal coal blends, it is a common practice to mix high-quality coal with low-quality coal. For thermal coal used in applications where strict requirements are placed on volatile matter, it is more reasonable to use dry-base volatile matter as a benchmark for predicting the composition of the blend. This approach helps to reduce losses resulting from inaccurate calculations and also provides better guidance for coal blending processes. This post was last edited by ryn on 2009-4-3 22:21]