Coal Analysis Insights | Combustion Aids for Calorific Value Testing: More Than Just Lens Tissue
Professionals familiar with Chinese national standards will know that GB/T 213, Determination of Calorific Value of Coal, provides a clear solution for coal samples that are prone to spattering or difficult to burn: wrapping the sample in lens tissue before testing.
This method helps stabilize the combustion process, promotes complete combustion, and ensures accurate and reliable calorific value results.
However, two questions are still frequently raised:
- Why was lens tissue selected as a combustion aid in GB/T 213?
- Is lens tissue the only combustion aid that can be used in calorific value testing?
This article reviews the background behind the relevant standard provisions and examines the key properties of lens tissue and other materials, helping laboratories better understand the selection criteria for combustion aids used in coal calorific value determination.
01 Why Lens Tissue Became the Preferred Option in GB/T 213
Many people assume that lens tissue was the only material considered during the development of GB/T 213. In fact, this was not the case.
During the early drafting and validation stages of the standard, the drafting team evaluated a variety of combustion aids, including benzoic acid powder, anthracite, and other combustible materials.
Repeated comparative tests, however, revealed several practical limitations. Some materials caused non-uniform combustion, while the calorific value correction associated with the added material was not sufficiently reliable for low-calorific-value coal samples. These factors could distort the test results and prevent the method from meeting the required level of precision.
To address these problems, the drafting team referred to the established Japanese practice of wrapping samples in gampi paper. Following multiple rounds of comparative testing and validation, lens tissue was found to provide the best overall performance.
Lens tissue meets the principal requirements for a suitable combustion aid:
- Its calorific value is stable and controllable.
- Its combustion products are simple and cause minimal interference.
- It helps poorly combustible coal samples achieve complete combustion.
- It reduces the risk of sample loss caused by spattering.
Because of these advantages, lens tissue was formally incorporated into GB/T 213 and became a commonly used combustion aid in coal calorific value testing.
02 Key Clarification: Combustion Aids Are Not Limited to Lens Tissue
The key conclusion is straightforward: lens tissue is not the only possible combustion aid for coal calorific value testing. It is one recognized and widely used option.
In principle, a material may be used as a combustion aid when it satisfies the essential requirements for stable heat correction, interference-free combustion, and effective promotion of complete sample combustion, subject to the provisions of the applicable standard and method validation.
One alternative that has been explicitly included in a relevant draft standard is analytical-grade starch.
Analytical-Grade Starch: A Newly Recognized Option
In the group standard Fully Automated Testing Methods for Coal Used in Power Generation—currently at the submission-draft stage and pending publication—analytical-grade starch has been explicitly included as a permissible combustion aid.
This challenges the common assumption that lens tissue is the only acceptable material.
Like lens tissue, analytical-grade starch satisfies the three fundamental requirements for a combustion aid.
Stable and Controllable Calorific Value
Analytical-grade starch has a well-defined chemical composition and high purity, helping ensure a stable and reliable calorific value for the required correction.
Simple Combustion Products with Minimal Interference
Starch consists only of carbon, hydrogen, and oxygen. Under complete combustion, it produces simple combustion products without leaving solid residue that could interfere with the test.
Promotion of Complete Combustion
Extensive testing has been conducted using low-calorific-value coal samples that are difficult to burn.
The performance of starch was evaluated from several perspectives, including residual carbon in the combustion residue, repeatability of the calorific value results, and measurement accuracy. The results demonstrated that starch can effectively assist difficult-to-burn samples in achieving complete combustion.
03 Support from International Standards
The scientific basis for using starch-based combustion aids is also supported by international standards.
ISO 1928, Coal and coke — Determination of gross calorific value, refers to rice paper as a suitable combustion aid. Rice paper is primarily made from starch-based material.
This provides further support at the international-standard level for the use of starch and starch-based materials as combustion aids in calorific value testing.
04 Summary: The Fundamental Criteria for Selecting a Combustion Aid
A review of the development of calorific value testing provisions shows that different standards may specify different materials.
GB/T 213 includes lens tissue, whose principal component is cellulose. ISO 1928 refers to rice paper, whose principal component is starch. Both materials are composed primarily of carbon, hydrogen, and oxygen.
Lens tissue, analytical-grade starch, and rice paper are suitable not because a standard has arbitrarily designated one unique material, but because they satisfy the fundamental requirements for an effective combustion aid:
- Stable and reliably determined calorific value
- Simple combustion products with minimal analytical interference
- Effective promotion of complete sample combustion
- Prevention or reduction of sample loss caused by spattering
As coal testing technologies continue to develop and laboratory requirements become increasingly specialized, new materials with better combustion performance and greater stability may emerge.
After rigorous experimental validation, such materials may eventually be incorporated into relevant standards and used as alternative combustion aids for coal calorific value testing.