Ash

Coal ash does not contain, but containing inorganic substances such as minerals. Ash is the inorganic residue of coal combustion products, composed of metal oxides such as Fe2O3, MgO, Na2O, K2O, etc., and non-metal oxides such as SiO2, P2O5, etc.. Determination of ash is part of the proximate analysis. Principle of this designation is a number of examples that have been refined coal burned at temperatures in the propagation of a particular heating up obtained residue (ash). Obtained residue was weighed and the amount calculated in percent. Burning example on a standard method BS and AS carried out at a temperature of 815 degrees Celsius with the propagation of a particular heating until constant weight gained, but in general, the combustion is carried out for 3 hours, and is considered constant, except when the residue is still visible example yet burning with perfect, so that the burning must be added for 15 minutes. In the standard method ISO, burning example is done in two stages. The first firing was started from room temperature to a temperature of 500 degrees Celsius for 60 minutes, then being held at that temperature for 30 minutes (to brown coal and lignite should be held for 60 min), then heating was continued until the temperature of 815 + / - 10 degrees Celsius. In the standard method ASTM burning example is generally carried out at a temperature of 750 degrees Celsius for 4 hours, but at certain old coal combustion can be reduced or increased depending on the type of its instances. For example certain coke and coal are non-reactive baking time may need to be added, but for some old coal combustion can be less than 4 hours before the time when it had reached a constant weight. Value of the coal ash is always smaller than the value of mineral content. This happens because during combustion has occurred in the coal chemical changes, such as the evaporation of crystal water, carbon dioxide and sulfur oxides. Mineral content can be calculated from its ash content of the formula, one of which is the Parr Formula: MM = 1.08*A + 0.55*TS. 

Reference:
ISO 1171 - Solid mineral fuels - Determination of ash
ASTM D 3174 - Test Method for Ash in the Analysis sample of Coal and Coke from Coal 

Manual Sampling

Manual sampling is a sampling method that making its increments using a manual/person.

Mechanical Sampling

Mechanical sampling is a sampling method that making its increments using a mechanical device.

Sampling

Sampling is the process of taking a portion of the population that will be quality tested. Part is called the sample taken or instance.

Bias

If the difference in the results of an analysis with an outcome that is considered true (reference) is always greater or smaller, then the event is called bias.

Bias can occur at any stage of sampling, either manual or mechanical sampling, and the time distribution of samples in the sample preparation stage, or other conditions. Although this bias occurs in both stages of sampling and sample preparation stage, but very difficult to be able to measure the level of bias.

Processes that are most likely to do the measurement bias in the sampling process is the conveyor belt.

The most common and many people do is measurement bias (bias test) at sampling mechanically (mechanical sampling).

Bias test is done by comparing the results of analysis (usually using ash as a comparison parameter) of the stopped belt samples (as a reference) with the results of analysis of samples taken with the mechanical sampling.

Analysis results obtained from the reference sample (taken with a stopped belt sampling) is considered as the result of the closest to the actual values.

The true value of a parameter no one ever knew. The value of a parameter that is considered closest to the true value is the average value of the results of analyzes performed by the laboratory as much as possible.

Reference :
AS 4264 : Part 4 - Determination of precision and bias
AS 2646 : Part 8 - Determination of precision and bias
ASTM D6518-03a - Practice for Bias Testing a Mechanical Coal Sampling System

Accuracy

What is meant by accuracy of a measurement is large or small deviation measurement results to the true value. If the difference is very small then it is said that the measurements are very accurate, and vice versa if the difference is large then it is said that the measurements are not accurate. The real value is never anyone knows, there is only approximate.

Value approach can be obtained by :

1. Averaging as much as possible the results of measurements

2. Determine how and where sampling test results are considered as the true value (eg stopped belt).

See Bias!

Coal

Coal is sedimentary rocks that can be burned, brown or black, consisting of crop residues are buried for millions of years.