Determining Calorific Values
One of the main criteria to determine the monetary value of coal is its net calorific value (NCV). Coal is a natural product, and its quality can vary widely. In order to obtain a representative result of such a heterogeneous material, quite some effort is required. The biggest errors are often made during the sampling process and sample preparation - not with the analytical equipment itself.
After proper sampling, the coal needs to go through a number of drying and grinding stages before it is finally ground down to 212 µm and is ready for analysis. In order to allow everybody to achieve a comparable result, each of the required steps from sampling to the final NCV is described in standards of organisations, such as ISO, DIN, ASTM, GB or GOST.
To determine the gross calorific value (GCV) of coal, so-called oxygen bomb calorimeters are used. To put it simply, a calorimeter determines the heat that is released by 1 gm of sample burned in a closed pressure vessel (decomposition vessel) at 30 bar of oxygen under controlled conditions. The heat released during the burning process needs to be measured under such controlled conditions so that no energy can get into the measuring system or escape from the measuring system. One of the influencing parameters disturbing the measurements with a calorimeter is the room condition. A calorimeter needs to operate ideally in an air-conditioned (controlled) environment.
Any direct sunlight, other sources of heat close to the unit or air draft should be avoided. The calorimeter's measurement cell should also be operated close to the temperature of the room itself. The pressure vessel (decomposition vessel or "the bomb") in which the sample is burned, is usually surrounded by water. This is the so-called inner vessel of a calorimeter. A PT 1000 temperature sensor allows temperature changes of up to 0.0001 K to be measured within the water. The inner vessel is surrounded by an outer vessel (jacket) that contains water as well, which can be controlled in different ways. Depending on how the jacket's water temperature is controlled during a measurement, the measurement method is called either adiabatic or isoperibol.
In an adiabatic calorimeter, the temperature in the outer vessel (Tov) is equal to the temperature of the inner vessel (Tiv) throughout the experiment. This is as close to a "perfect isolation" as possible. The temperatures are stable before the sampling is ignited and after the burning process. No correction calculations need to be done when compared with the isoperibol calorimeter.
In an isoperibol calorimeter, the temperature in the outer vessel (Tov) is kept constant throughout the experiment. This does not allow a "perfect isolation"; there are still small temperature fluctua-tions. A correction factor (Regnault-Pfaundler = ?) will be calculated after the experiment that takes these temperature fluctuations into account.
A third method used is the so-called static-jacket calorimeter that has no controlled jacket that may or may not contain water. In this example, the outer vessel is a combination of the pressure chamber, insulating air and the housing of the unit itself.
The jacket is not controlled nor filled with water. It is static. Looking at the temperature profile of the inner vessel of such a calorimeter, it behaves just like an isoperibol calorimeter. The same Regnault-Pfaundler correction calculations used in an isoperibol calorimeter can be applied (see temperature / time graph for isoperibol calorimeter below). The temperature increase ?T is the value an oxygen bomb calorimeter actually measures. In order to know the energy behind the increase of temperature, a calorimeter needs to be calibrated using a suitable substance with a known calorific value. The calibration material of choice worldwide is benzoic acid and is available in pure form and produced by standard institutes, such as the National Institute of Standards (US).
It is important to pay attention to the possibly required corrections of the reference calorific value based on the calorimetric measurement conditions, such as:
- Volume of the pressure vessel;
- Amount of water placed inside the pressure vessel;
- Temperature at the end of the experiment;
- Weight of the sample.
After the combustion in a calorimeter, attention should be paid to a number of other reactions taking place during the combustion process. The main influences are caused by nitrogen (N) and sulphur (S). Since the decomposition vessel represents a closed (isochoric) system, the gases produced during the combustion cannot escape. They form acids and heat is released during the same formation, when dissolving in the water inside the decomposition vessel. Nitrogen is either present in the coal sample or can derive from the air that was inside the decomposition vessel when closed, unless the air was purged out of the pressure vessel before ignition. The amount of produced acid also depends on the achieved temperature increase respective of the calorific value of the sample.
Sulphur is preferably determined with sulphur analysers, since these can reach a higher temperature for a longer period of time. This also allows the cracking of inorganic bonds between sulphur and, for example, iron (FeS2). Some standards do allow determining the sulphur after combustion in a calorimeter in the washings by titration. Also the nitric acid formed during the combustion process is usually titrated manually.
These energies need to be deducted from the preliminary gross calorific value to obtain the final gross calorific value at constant volume:
Parallel to the determination of the calorific value, the coal sample is often dried in a nitrogen-purged oven at 105 °C until the weight is constant. Nitrogen purging avoids further oxidation of the coal during the drying process. Many coals show such behaviour and can appear even heavier after the drying process than before, if dried under normal atmosphere conditions. The moisture determined that way is called analytical moisture.
If the sample was not completely dried before analysis, the weighed-in quantity is not correct since a part of the sample weight was just water. This correction can show quite a strong effect on the final GCV result. At the same time, the GCV corrected for the analytical moisture of the coal is the only basis to be able to compare results with each other properly. The repeatability limit defined in DIN 51900, for example on a coal sample, is 120 J / g if determined consecutively at the same laboratory, by the same operator and under the same conditions.
The GCV is still not yet the final value that is usually reported by a coal laboratory or used to determine the monetary value of the coal. Further analysis and effort is required to get to the so called NCV at constant pressure Hu, p (an). The NCV represents a much more realistic value of energy that will be delivered when burned in a power plant. Therefore further analysis is required with elemental analysis equipment to determine the carbon, hydrogen and sulphur content. Again, the ash content needs to be determined in a furnace that complies to proper standards.
No matter which above mentioned measuring principles (adiabatic, isoperibol, static-jacket) is used by a laboratory, all must lead to the same result within the repeatability and reproducibility limits. Coal analysis laboratories have to verify and be able to prove the proper performance of all their instruments used in the laboratory anytime.
Especially when accredited, according to ISO 17025, it is required to ensure the results produced by the laboratory can be trusted. For this purpose, it is necessary to test the calorimeter's performance continuously and also record the results in control charts. Besides the standard control methods by using pure calibration standards or other so-called control standards with a known calorific value, it is required to regularly check the procedure with real coal or coke samples. Therefore the laboratories also have to participate successfully in proficiency testing programmes on a regular basis each year. It is demanded to participate in such a programme for each matrix (for example, coal, coke, wood) and each single parameter measured by either device needs to pass the test. The successful parti-cipation is expressed in the so call z-score, which is shown on the certificates of the proficiency test providing company. The z-score has to be = ± 2.