Determination of Nitrogen in Coal
Nitrogen is an important element in coal and has significant influence on utilization of coal. It is common that nitrogen content in coal varies greatly according to coal type, of which exceeds several thousand mg/kg. For instance, the nitrogen content in coal ranges from 0.3-3% in bituminous coal and up to 3-20% in lignite. Therefore, it is essential to analyze and determine nitrogen content in coal in order to determine its quality and utilize the coal.
At present, the most commonly used analysis method for determination of nitrogen content in coal is the Kjeldahl method. This method is based on the fact that nitrogen-containing compounds would be converted into ammonium sulfate under certain conditions and then ammonia gas is released and absorbed by sulfuric acid. Consequently, the amount of ammonium sulfate produced is proportional to amount of nitrogen in the original sample.
The first step of Kjeldahl method is sample size reduction by grinding. During this process, the sample should be broken down into fine particles to the maximum extent and powdered to a homogenous mass with particle size of about 1 mm. This ensures a uniform surface for the reaction, thereby improving the accuracy of the determination.
The second step involves digestion of sample. During this process, a sample is placed in Kjeldahl flask and mixed with a catalyst and concentrated acid. The acid helps to break down nitrogen-containing compounds into ammonium sulfate and ammonia gas, which would then absorb sulfuric acid. The amount of ammonium sulfate produced is proportional to the amount of nitrogen-containing compounds.
Detection and calculation of the produced ammonium sulfate is then conducted in the subsequent steps. First, the acid solution containing ammonium sulfate is titrated with a standard solution of sodium hydroxide to determine the amount of ammonia gas produced. Then, the nitrogen content is determined based on the amount of ammonium sulfate produced.
The accuracy of Kjeldahl method for determination of nitrogen in coal is generally accepted. However, certain limitations are present as well. The most apparent limitation is the need for sample digestion which is labor-intensive and time consuming. Furthermore, the digestion is prone to error due to incomplete reaction between sample and acid.
Due to such limitations, few modern methods have recently been proposed for determination of nitrogen content in coal. One of them is the UV spectrophotometric method which is based on the fact that the UV absorbance of nitrogen in coal corresponds to the maximum absorbance at a wavelength of 216 nm. This method has the advantages of fast and labor-saving compared to the Kjeldahl method and it is more accurate as well.
In conclusion, determination of nitrogen content in coal has significant influence on its usage and quality. Kjeldahl method is the most commonly used analysis method for this purpose. However, it is labor-intensive and prone to error due to incomplete reaction between sample and acid. Therefore, modern methods, such as the UV spectrophotometric method, have been proposed for more accurate determination of nitrogen content in coal.
