An Introduction to Coal – How is it analyzed as a fuel?

In my previous post, I shared about how coals are classified by rank and this time around I’d like to share on how geologists or engineers analyzed coals as a fuel. So if you’ve noticed on the table found in the last part of my previous post especially on the data that was provided, well, it is the result of the method of analysis being conducted per rank of coal. And with that, let us now prepare ourselves in getting to know the two known methods of analyzing coal as a fuel and these are as follows: proximate analysis and ultimate analysis.

Why it is so important to analyze coals?

Since the focus of this article is solely for power generation; analyses being done to coals helps the operators of coal mining site to identify if the coal they are mining is suitable and profitable for use in power generation or to other specific uses such as in the production of coke which is best used in the production of steel. Most importantly, it helps engineers and power plant operators on what coal to use in order to produce greater amounts of energy and at the same time, have the capability to release reduced amounts of the so-called products of combustion such as sulfur dioxide.

What is Proximate Analysis?

The tests that are being used in ranking by classification of coals is called as proximate analysis. It is a method in analyzing coal wherein the goal is to obtain the amount of moisture, volatile matter, fixed carbon and ash; for which these are all expressed in percentage.

How are the percentage in moisture, volatile matter, ash and fixed carbon obtained?

• Moisture content
  The moisture content of a coal are classified into four, namely the: surface moisture, inherent moisture, decomposition moisture and lastly, mineral moisture.

  • Surface moisture also known as free moisture is the moisture that is found in the surface of the coal’s particles.
  • Inherent moisture also known as hygroscopic moisture is the moisture that was held by the capillary action that has occurred in the microfractures of the coal.
  • Decomposition moisture is the moisture that is produced by the organic matter that is found in coal.
  • Mineral moisture is the moisture that accounts for the crystal structure of hydrous silicates and the inorganic minerals found in coal. When talking about minerals found in coal, there are 33 that occur in most coal samples and is further reduced to 8 when talking about substantial quantities to be considered as a major constituent of a coal.

  The proximate analysis for moisture is conducted either to determine the total or residual moisture content of the coal. The total moisture content of coal consists of surface and inherent moisture. Among the two, total moisture content of coal is the heavily favored by the power plant engineers and operators since this analysis takes on the coal on an as received basis which is also known as the as fired basis. And most importantly, the proximate analysis for moisture is generally done by subjecting a 1-gram sample of the coal to a temperature of 220 to 230 degree Fahrenheit (105 to 110 degree Celsius) for a period of 1 hour. After one hour, the loss in mass of the 1-gram sample of coal is expressed as the amount of moisture that is found in the coal and is expressed in percentage.

• Volatile Matter content
  The volatile matter found in coal constitutes the hydrogen, hydrocarbons and incombustible gases like carbon dioxide (CO₂) that can be easily removed by way of heating. The proximate analysis for volatile matter is conducted by placing a 1-gram sample of coal in a covered platinum crucible for which it is subjected to heating with temperatures as high as 1740 degree Fahrenheit (~950 degree Celsius) and unlike the proximate analysis of moisture, the analysis only takes 7 minutes to complete. The loss in mass of the 1-gram sample of coal represents the total loss in moisture and volatile matter and since the moisture content of the other 1-gram sample has been obtained from the proximate analysis of moisture, the percentage for volatile matter would be obtain by just subtracting the moisture content from the total moisture and volatile matter content.

• Ash content
  In obtaining the ash content of a coal, the method in obtaining it is by using the samples that was used in conducting the analysis for moisture content. Those sample are then subjected to a heat for which the temperature ranges from 1290 to 1380 degree Fahrenheit wherein the sample is placed in an uncovered crucible and exposed with good circulation of air until all the samples are burned completely or in layman's terms, the coal is subjected to a complete combustion wherein the end result is the ash. The resulting ashes from each sample are then subjected to weighing and the constant weight readings are taken as the ash content of the coal.

• Fixed Carbon content
  In conducting proximate analysis of coals, determination of the carbon content is the last thing to do, wherein it is just equal to 100% subtracted by the respective percentages for moisture, volatile matter and ash contents.

What is Ultimate Analysis?

The ultimate analysis of coal determines the percentage by weight of the major elements that are found in coal which are the so-called CHONS which stands for Carbon, Hydrogen, Oxygen, Nitrogen and Sulfur. In this test, moisture content of the coal is taken as directly proportional to the hydrogen and oxygen content. Additionally, the name ultimate suggests that it is a thorough examination of the coal sample for which it is advisable that the analysis must be done by a chemist.

• Carbon and Hydrogen Content

Carbon (C) and hydrogen (H) are the main combustible elements found in coal. Usually coals comprises 60 – 95% carbon on a weight basis and for hydrogen, if the coal has less than 90% carbon content it has utmost of 5% hydrogen content and for coals that have 95% carbon content, close to 2% hydrogen content is possible. Well, for the determination of the carbon and hydrogen content in an ultimate analysis, it has to undergo methods such as Liebig method, wherein a known mass of coal is being burned to a stream of dry oxygen and at a high temperature in a closed system for which other gases can’t enter. In this method, all hydrogen is converted into water (H₂O) and all the carbon are converted to carbon dioxide (CO₂). These products of combustion (say for carbon dioxide and water) are readily absorbed by suitable reagents and are determined gravimetrically. And since an incomplete combustion takes place in the early part of this method, copper oxide (CuO) is used in achieving complete combustion. And lastly, since coal has traces of elements such as sulfur (S), chlorine (Cl) and nitrogen (N); the oxides of the latter elements are also readily absorbed by reagents, wherein the oxides of sulfur (SO_x) are absorbed by lead chromate (PbCrO₄), oxides of nitrogen are absorbed by grains of manganese dioxide (MnO₂) and for chlorine, it is absorbed by a silver gauze roll.

• Nitrogen Content
  The average content of nitrogen in almost all coals falls between 1-2% and the most commonly used method in obtaining the content for nitrogen is the so-called Kjeldahl method. This method was developed by Johan Kjeldahl in 1883 and was developed to analyze the nitrogen content of both organic and inorganic substances. In this method, pulverized coal is exposed to heating with strong acid which is sulphuric acid (H₂SO₄) that contains potassium sulphate (K₂SO₄) and degrades the coal for which ammonium sulphate ((NH₄)₂SO₄). After releasing the ammonium sulphate, distillation takes place for which ammonium sulphate is added with a small amount of sodium hydroxide (NaOH) for which the products of the reaction includes ammonia (NH₃). The next process of this method is to capture ammonia and is being done by dipping the ammonia to a solution of boric acid (BH₃O₃) and after that, the products of chemical reaction are then undergoes back titration with the aid of sodium carbonate (NaHCO₃). This method has gained criticism since one of the catalysts that were used to accelerate the decomposition of organic substances (coal) is mercuric oxide (HgO). Additionally, selenium can be used also as a catalyst.

• Sulfur Content

Sulfur content of the coal is very critical in analyzing the coal and despite being proven to take up 0.5-2.0% of coal, this element is the main culprit of air pollution. When coal is being burned in the furnace, the resulting products of combustion includes the sulfur oxides or famously known amongst scientists as SO_x emissions. In the quantitative analysis for sulfur content in coal, three methods were developed namely: Eschka method, bomb washing method, high-temperature combustion test method. All of those methods are developed to ensure high precision in obtaining the sulfur content of coal. For the Eschka method, a sample of coal is mixed with a mixture of two parts of magnesium oxide (MgO) and one part of anhydrous sodium carbonate (Na₂CO₃) which is also known as Eschka mixture; wherein the mixtures are then heated to a temperature of 800°C for which the goal is to achieve oxidation. In this method, the sulfur in coal is converted to magnesium sulfate (MgSO₄) and sodium sulfate (Na₂SO₄). The sulfates are then extracted with the aid of hydrochloric acid solution (HCl) and with the addition of barium chloride (BaCl₂); precipitation takes place for which the products of chemical reaction include barium sulfate (BaSO₄). The resulting barium sulfate is then filtered and weighed. For the bomb washing method, this method is conducted after the usage of oxygen bomb calorimeter (I will discuss in my next article the usage of calorimeters). The term washing was derived from the procedure after using bomb calorimeters wherein it is washed with the use of water and the barium sulfate is then gravimetrically determined in the bomb washings. And lastly, the high-temperature combustion method is used for rapid determination of the total sulfur content of a coal wherein a sample of coal is heated in tube furnace for which there are streams of oxygen for 30 minutes at a temperature of 1350°C. In this method, sulfur dioxide is amongst the products of combustion and with that sulfur content of coal is obtained by the usage of titration techniques such as acid-base titration and iodimetric titration. For acid-base titration method, the products of combustion such as chlorine gas (Cl₂) and sulfur dioxide (SO₂) are absorbed by a solution of hydrogen peroxide (H₂) which produces hydrochloric (HCl) and sulfuric acids (H₂SO₄). The resulting acids are then titrated with the usage of sodium hydroxide (NaOH) which is a base. For iodimetric titration method, the resulting sulfur dioxide is absorbed by an aqueous solution that contains iodine for which iodine is converted to iodide. Special to this method is the usage of iodine titrant that is being added proportionately for which the excess iodine is being replenished and the volume of iodine titrant that has been used in replenishing the iodine is used to compute the sulfur content of the coal.

What comes next for the results of these analyses?

The data that is being obtained from the aforementioned analyses are then used by engineers in expressing the coal are as follows: for proximate analysis, it helps in classifying the coal by rank and for ultimate analysis the coal is expressed in the basis such as "as received or as fired", "dry or moisture free", "moisture and ash free or combustible" and "moisture, ash, and sulfur free"; and most importantly, in the computation of the higher heating value or gross calorific value and the theoretical and actual air-fuel ratio, which I also intend to share in my next posts.

References

1. Potter, P. (1959). Power Plant Theory and Design (2^nd ed.). New York: The Ronald Press Company.
2. https://www.usea.org/sites/default/files/042014_Coal%20sampling%20and%20analysis%20standards_ccc235.pdf. Retrieved on July 30, 2018.
3. http://nptel.ac.in/courses/113104058/mme_pdf/Lecture2.pdf. Retrieved on July 30, 2018.
4. https://en.wikipedia.org/wiki/Coal_assay. Retrieved on July 30, 2018.
5. https://pubs.usgs.gov/circ/c1143/html/text.html. Retrieved on July 30, 2018.
6. https://www.ems.psu.edu/~radovic/Chapter7.pdf. Retrieved on July 30, 2018.
7. https://www.sis.se/api/document/preview/18665/. Retrieved on July 30, 2018.
8. https://www.911metallurgist.com/blog/coal-coke-analysis. Retrieved on July 30, 2018.
9. https://en.wikipedia.org/wiki/Kjeldahl_method. Retrieved on July 30, 2018.
10. https://encyclopedia2.thefreedictionary.com/Eschka+mixture. Retrieved on July 30, 2018.
11. https://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/20_2_PHILADELPHIA_04-75_0099.pdf. Retrieved on July 30, 2018.

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