The coal samples used in the coal quality analysis process are mostly on air dry basis. Even in the weighing process with a balance, the sample is also required to be kept in a drying status. That is to say, before the coal sample weighing, user needs to remove the desiccant (calcium chloride or color changing silica gel) from the balance first; and then open the side doors of the balance to let the air in, making the balance realize a status of air dry basis; In this case, when the sample is weighed, the inherent moisture in the coal sample will not evaporate very quickly. After the sample weighing, put the desiccant back into the balance immediately. When the moisture baking is completed(or the ash of coal sample has been burned out, or the volatile matter has been determined), since the sample has reached a complete drying status, the desiccant in balance should not be taken out from the balance in the sample weighing process. Meanwhile, please try to open the balance door as little as possible. If the balance keeps a complete drying status in the sample weighing process, the sample weight will be found to be decreased gradually. For the lignite, long flame coal or non-stick coal with high concentration of moisture, the weight loss phenomenon is more obvious. In particular, when the coal sample has not reached a compete drying status (has lots of free moisture on the surface), the weight loss phenomenon is more serious. This will inevitably lead to an increase of the measurement error: such as the low measured external moisture value and high measured calorific value, etc.
In general, the coal sample weighing process should be fast and accurate. After the sample weighing, the sample is required to be in a complete drying status for the carbon and hydrogen determination. For other determinations, take the moisture loss as an example: a little moisture loss in coal sample will not affect the testing results.
In addition, before weighing coal samples, in order to achieve uniformity as far as possible, user must use a small spoon to stir the coal sample, that is why the bottle should not be too full of samples, or it will be difficult to stir. Especially for the high ash or high sulfur raw coal samples, since the density difference between minerals and pure coal is too large, they are prone to segregation. Therefore, the coal sample stirring is an effective measure to reduce the measurement error. After the selection, the cleaned coal sample will be much more uniform, so it will be okay to stir a bit properly in the sample weighing process.
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