Ultimate analysis provides a convenient method for reporting the major organic elemental composition of coal. For this analysis, a coal sample is combusted in an ultimate analyzer, which measures the weight percent of carbon, hydrogen, nitrogen, sulfur, and ash from a coal sample. The total carbon, hydrogen, and nitrogen are determined at the same time from the same sample in the analyzer. Total oxygen is calculated from the other values. Standard practices for ultimate analysis are shown in ASTM D3176-09 (American Society for Testing and Materials, 2013, p. 510-513). Testing methods are described in ASTM D5373-08 (American Society for Testing and Materials, 2013, p. 628-636).

Total carbon: Total carbon in the ultimate analysis is the measured weight percent of carbon in a coal, including the carbon in volatile matter. It is determined using ASTM method D5373-08 (American Society for Testing and Materials, 2013, p. 628–636). Fixed carbon from the proximate analysis is a different value than total carbon from the ultimate analysis. Total carbon includes some organic carbon that escapes as gaseous volatile matter during combustion.

Total carbon can be measured in an ultimate analyzer as part of the ultimate analysis, or in a carbon-sulfur analyzer. Both machines are calibrated using standard reference samples of known carbon, hydrogen, nitrogen, sulfur and ash values. In the carbon-sulfur analyzer, 0.25 grams of sample are placed in a specialized sample vial made from refractory-grade clay. The vial is then combusted at a temperature of 1350 °C.

When coal is combusted the carbon is liberated as carbon dioxide (CO2) and as hydrocarbons (CH4). CO2 comes from the organic compounds in coal, but can also be liberated from carbonate (e.g., calcite–CaCO3) minerals in the inorganic fraction of a coal, if they are present. This means that the total carbon measurement may include carbon from the mineral fraction as well as the organic fraction of the coal.

To determine the inorganic (mineral) carbon and organic carbon components of a total carbon sample, a split of the sample should be analyzed in a coulometer. The carbon measured in a coulometer is total organic carbon, sometimes denoted as TOC. With a coulometer measure of total organic carbon and ultimate analysis or carbon-sulfur analysis measurement of total carbon, the various components of carbon can be calculated:

  1. Total carbon (from ultimate or c-s analyzer) = Total inorganic carbon + total organic carbon (from coulometer)
  2. Total organic carbon (from coulometer) = Total carbon (from ultimate or c-s analyzer) – total inorganic carbon (from coulometer)
  3. Total inorganic carbon = Total carbon (from ultimate or c-s analyzer) – total inorganic carbon (from coulometer)
Automated analyzer for total carbon and total sulfur measurement.

Total hydrogen: Total hydrogen in the ultimate analysis is the measured weight percent of hydrogen in the coal. Hydrogen is liberated from coal during combustion as water vapor (H2O), which can come from the organic compounds in coal, inherent moisture in coal, and the breakdown of clay minerals in the inorganic fraction of the coal (Thomas, 1992). Hydrogen values from an ultimate analyzer then need to be corrected to factor out hydrogen derived from moisture. This is especially important in low rank coals which have higher moisture contents. To correct the measured (as determined) hydrogen from the analyzer to fuel-based hydrogen (as received) value:

Hydrogen (fuel-based) = Hydrogen (as received, analyzer measurement) – ((0.1119 * moisture (as determined, measured from the ultimate analyzer))

Other corrections between as determined, dry, and as received bases can be found in ASTM D3176-09; American Society for Testing and Materials, 2013, p. 510–513).

Values of total hydrogen are used to calculate the amount of oxygen needed for efficient combustion in a boiler, and for material balances in coal conversion processes such as gasification and liquefaction (ASTM D5373-08; American Society for Testing and Materials, 2013, p. 628–636). Hydrogen is also an important component of emissions. Water combines with sulfur dioxide (SO2) to form sulfuric acid (H2SO4), which is a regulated pollutant that contributes to acid rain. Hydrogen can also combine with nitrogen (NOx) to form nitric acid (HNO3), which also contributes to acid rain (U.S. Government Accounting Office, 2000; Energy Information Administration, 2001; U.S. Environmental Protection Agency, 2004a; Greb and others, 2006).

Total oxygen: The total oxygen content of a coal cannot be measured analytically, so it is determined through a calculation (ASTM method D3176-09; American Society for Testing and Materials, 2013, p. 510–513). Essentially, total oxygen content is the remaining major element of the five major elements in coal–carbon, hydrogen, oxygen, nitrogen, sulfur–in addition to the noncombustible ash in coal. The total oxygen content is calculated from the measured (as-determined) values for the major elements and ash:

Total oxygen weight % = 100 – (total carbon weight % + total hydrogen weight % + total nitrogen weight % + total sulfur weight % + total ash weight%)

Similar to hydrogen, the measured oxygen (as determined) value from the ultimate analyzer may also contain oxygen derived from moisture in the sample. To correct for a fuel-based oxygen (as received) value:

Total oxygen (fuel-based, weight %) = Total oxygen (analyzer measured, weight %) – ((0.8881 * moisture (analyzer measured, as determined, weight %)

Other corrections between as determined, dry, and as received bases can be found in ASTM D3176-09; American Society for Testing and Materials, 2013, p. 510–513).

Oxygen occurs in both the organic and inorganic fraction of coal. It is present in oxidized organic compounds, as well as oxide, hydroxide, and sulfate minerals. Some of the oxygen in coal is liberated during combustion in water vapor, carbon dioxide, nitrogen oxides, and sulfur oxides. Some of the oxygen also remains in solid minerals in the ash after combustion. The oxygen content is one of the most important characteristics for determining the chemical properties of coal (van Krevelen, 1992).

Total nitrogen: Total nitrogen in the ultimate analysis is expressed as the measured weight percent of nitrogen in the coal. It is determined by ASTM method D5373-08 (American Society for Testing and Materials, 2013, p. 628–636).

Nitrogen is mostly found in the organic fraction of coal. Upon combustion, it is liberated as nitrogen oxides in the flue gas (emissions). NOX emissions from coal combustion are regulated in the United States. They are an environmental concern because they combine with water vapor to produce nitric acid (HNO3), which contributes to acid rain (Energy Information Administration, 2001). They also contribute to the formation of ground-level ozone, an ingredient of smog, another human health concern (Energy Information Administration, 2003; U.S. Environmental Protection Agency, 2004b; Greb and others, 2006).

Total sulfur: Total sulfur in the ultimate analysis is the measured weight percent of sulfur in the coal. It is determined by ASTM methods D5373-08, or D4239-02 (American Society for Testing and Materials, 2013, p. 628–636, 556–561, respectively). In the test, a coal sample is ground to a set size and weighed, then placed in a sulfur analyzer. In the analyzer, the sample is combusted at a temperature of 1,370°C in an oxygen atmosphere. The oxygen reacts with sulfur to form sulfur dioxide gas. The gas passes through an infrared absorption detector in the analyzer, which measures the concentration of sulfur.

Sulfur in coal occurs in three forms: (1) organic, (2) inorganic, and (3) elemental. Inorganic sulfur occurs mostly in minerals such as pyrite (FeS2). An additional test of sulfur forms can be run to determine the relative amount of the three forms of sulfur in a coal that contribute to the total sulfur value determined by ultimate analysis.

Total sulfur is an important criteria in steam coal because sulfur combines with oxygen in boilers to form sulfur dioxide (SO2), which is emitted in the gas stream. SO2 can react with water vapor to form sulfurous acid (H2SO3), which oxidizes to sulfuric acid (H2SO4), a major component of acid rain. Sulfate deposition and acid rain can contribute to crop and forest degradation, increased acidity in lakes and streams, and human health risks. For these reasons, sulfur emissions are regulated in the United States and many industrialized countries (U.S. Government Accounting Office, 2000; U.S. Environmental Protection Agency, 2004a). In order to meet emissions standards, many utilities have strict guidelines on the amount of total sulfur in the coals they use.

For power plants with flue-gas desulfurization (scrubbers), the total-sulfur value helps to estimate the amount of sulfurous gases that will pass through the scrubbers, the amount of reagent (lime solids or wet solutions) needed for the volume of sulfurous gas, and the amount of solid gypsum that will be produced from the reaction with the reagants. Knowledge of the total sulfur content in a combusted coal is also important because sulfur can lead to fouling and corrosion in boilers. In the steel industry, low-sulfur coals are used for coking. In general, total sulfur contents less than 0.8 percent (air-dried basis) are used to make coke for steel (Stach and others, 1982; Thomas, 1992; Suárez-Ruiz and Crelling, 2008).

Compare to Proximate analysis

  • If you have specific questions about this analysis, please contact Jason Backus.

 

Reference

 

 


Last Modified on 2017-03-30
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