Basic Principles of CHN (Ultimate) Analyses

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Published Date Written by Sharon Banks

The TruSpec CHN is an analytical instrument that determines the C (carbon), H (hydrogen)and N (nitrogen) content of a variety of materials. The sample weight range capability of the Truspec is 50 milligrams to 1.5 grams. The instrument is connected to an external PC and uses a Windows - based software program to control the system operation and data management. There are three phases during an analysis cycle: purge, combustion and analyse.
In the purge phase, the encapsulated sample is placed in the loading head, sealed, and purged of any atmospheric gases that may have entered during sample loading. The ballast volume (zero volume at this point) and gas lines are also purged.
During the combustion phase, oxygen flows into the furnace to aid ignition of the sample. The sample is dropped into a hot furnace (950oC) and flushed with oxygen resulting in a very rapid and complete combustion. The chemistry of the combustion process is as follows:
Coal Sample + O2 + Heat = CO2 + NOX + SO2 + H2O + Ash
The gaseous products of combustion are passed through a secondary furnace called the Afterburner, which is set at 850oC, for further oxidation and particulate removal.The Afterburner contains 3 different reagents that remove potential corrosive compounds and assist in keeping conditions in an oxidising environment.
Alumina pellets present a surface area for combustion gases to stabilise in the dioxide state whilst maintaining an oxidising condition.
The furnace reagent removes sulphur oxide from the flow stream. The life of the furnace reagent depends upon the levels of sulphur contained in the materials analysed.
Magnesium oxide removes halogens from the flow stream. Fluorine and chlorine contained in the sample are removed to prevent corrosion in the system.
Particulate oxides are removed by the quartz wool and honey comb filter. The combustion gases exiting from the Afterburner are then collected in the ballast.
Ultimately the analyses phase; the homogeneous combustion gases in the ballast is now passed via two infrared detectors and a 3cc aliquot loop. Once the gases have equilibrated, carbon is measured as carbon dioxide by the CO2 detector and hydrogen is measured as water vapour in the H2O detector. The gases in the aliquot loop are transferred by a helium carrier flow, swept through copper at 750oC to remove excess oxygen and convert NOx to N2;
O2 + 2Cu = 2CuO
2NOX + 2Cu = 2CuO + N2
The combustion gases now flow through Lecosorb and Anhydrone to remove carbon dioxide and water, respectively. A thermal conductivity detector is used to determine the nitrogen content.
The final results are displayed as weight percentage (%w) or in parts per million (ppm) as selected by the operator.

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