DIN 51722-1 Testing of Solid Fuels - Determination of Nitrogen Content - Semi-Micro Kjeldahl Method

The standard “DIN 51722-1 Testing of solid fuels - Determination of nitrogen content - Semi-micro Kjeldahl method”, published by the German Institute for Standardization (DIN), describes a standardized procedure for determining the nitrogen content in solid fuels.

This standard applies to solid fuels such as the following:

• Coal and lignite
• Kok
• Biomass fuels (e.g., wood pellets, briquettes)
• Other solid combustible materials where nitrogen content is important.

Nitrogen content is an important parameter in fuel analysis because:

• High nitrogen levels can lead to increased NOx (nitrogen oxides) emissions during combustion (environmental impact).
• It forms part of the final analysis of fuels (elemental composition: carbon, hydrogen, nitrogen, sulfur, oxygen).
• It helps to evaluate fuel quality, combustion behavior, and compliance with emission regulations.

The method is particularly suitable for routine laboratory testing in the quality control, research, and certification of solid biofuels or fossil fuels.

This method follows the classic Kjeldahl method (semi-micro scale), which is one of the most widely used techniques worldwide for the determination of total nitrogen in organic and many inorganic samples.

This method generally follows these steps:

• Digestion: A small sample (typically 0,1–0,5 g, therefore semi-micro) is digested at high temperature (approximately 350–420 degrees) in concentrated sulfuric acid with a catalyst (usually potassium sulfate + copper sulfate or, in older variants, selenium/mercury compounds). This converts the organic nitrogen into ammonium sulfate.
• Alkalization and distillation: The digested substance is alkalized with sodium hydroxide, releasing ammonia gas, which is then distilled and collected in a collecting solution (usually boric acid or standard acid).
• Titration: Trapped ammonia is measured by titration (e.g., using an indicator or potentiometrically with sulfuric acid or hydrochloric acid).

Nitrogen content is calculated from the amount of acid (or base equivalent) consumed and is expressed as a percentage by mass.

The advantages of the semi-micro Kjeldahl variant are as follows:

• Compared to the classic macro Kjeldahl method, it requires smaller sample sizes, is faster, consumes less reagent, and is suitable for routine analysis.
• Good accuracy and precision for most solid fuels.
• It is well-established and robust for samples in the nitrogen range of 0,1-5% (typical for coal and biomass).

This method has the following limitations:

• It does not distinguish between different forms of nitrogen (only total nitrogen).
• Compared to modern instrumental methods (e.g., element analyzers using combustion and thermal conductivity detection), it is time-consuming.
• Some compounds (nitrates, nitro groups, azo compounds) may require modifications (e.g., salicylic acid pretreatment) for full recovery. This addresses that for standard fuel matrices.

Although the DIN 51722-1 standard remains a significant reference for laboratories and fuel testing (particularly coal and early biomass standards), many applications now refer to the following:

• ISO 29541 Solid mineral fuels - Determination of total carbon, hydrogen and nitrogen content - Instrumental method
• ISO 16948 Solid biofuels - Determination of total carbon, hydrogen and nitrogen content.
• EN/ISO equivalents for biomass and solid recovered fuels.

Today, laboratories analyzing solid fuels often use automated CNS analyzers for faster results, but the Kjeldahl method remains a reference wet chemical technique and is still used when instrumental methods need to be validated or for specific regulatory purposes.

In conclusion, the DIN 51722-1 standard represents a reliable and classic approach to determining nitrogen in solid fuels, using semi-micro Kjeldahl digestion and distillation. This method has been a fundamental approach in fuel chemistry for many years.

In short, the DIN 51722-1 standard describes a semi-micro version of the classic Kjeldahl method used for nitrogen determination in solid and liquid samples. This standard is widely applied, particularly in the analysis of coal, coke, and similar fuels.

Semi-micro, according to the classical Kjeldahl method, refers to the following:

• Smaller sample sizes (typically between 0,1 and 1 g)
• Less chemical consumption
• Lower waste levels
• Faster analysis time.

These features provide efficiency, especially in routine laboratory analyses.

The areas of application of this standard are:

DIN 51722-1 is used particularly in the following areas:

• Coal and lignite analysis
• Coke and carbon-based fuels
• Solid fuel quality control
• Combustion characterization in the energy sector.

Nitrogen content is a critical parameter in fuels for the following reasons:

• Burning behavior
• NOx emission potential
• Environmental impacts

The DIN 51722-1 standard defines the following topics in detail:

• Sample preparation conditions
• Purity of the reagents to be used
• Digestion temperature and time
• Distillation parameters
• Titration method
• Calculation formulas
• Repeatability and accuracy criteria

Nitrogen percentage is usually calculated according to the formula given in the standard.

The DIN 51722-1 standard is considered important for the following reasons:

• It provides reliable nitrogen determination.
• Used in environmental emission calculations.
• It plays a critical role in fuel quality assessment.
• It contributes to process control in energy production facilities.

In conclusion, this standard has the advantages of high accuracy, internationally accepted principles, and a wide range of applications. However, it does have some limitations. For example, it does not directly measure nitrate and nitrite nitrogen, the analysis time is relatively long, and acidic and corrosive chemicals are used.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the DIN 51722-1 standard.