Analysis of Alternative Fuels Using XRF

X-ray fluorescence (XRF) is a primary analytical tool for assuring the proper composition of materials, including alternative fuels, by detecting major, minor, and trace elements (especially heavy and transition metals).

Other important analytical tools include:

• Microscopy: Used to examine phase sizes, amount distribution, and other characteristics that reveal information about clinker history and kiln temperature profiles, aiding in improving clinker grindability.
• X-ray diffraction (XRD): Useful for determining mineralogical composition of clinker and identifying unknown materials that may cause problems like kiln buildups or cyclone chokes.
• Thermal analysis: Quantifies organic emissions from fuels and secondary materials, and investigates cement sulfate forms and hydration products, aiding in problem-solving for operational issues.

WDXRF systems like Rigaku's ZSX Primus series are designed for elemental analysis. For alternative fuels, which are often complicated, heterogeneous, and high in organic content (carbon, hydrogen, oxygen, nitrogen), traditional quantitative methods with calibration standards are difficult to set up due to the lack of suitable analytical standards. Therefore, WDXRF utilizes a "semi-quantitative analysis" method called SQX, which involves:

• Qualitative scan: Identifying detected elements and their characteristic X-ray lines.
• Fundamental Parameters (FP) method: Quantifying concentrations using a factory-set sensitivity library based on measured intensities from the unknown sample.
• Scattering FP method: Particularly useful for organic-rich alternative fuels, this method estimates the balance of organic components by measuring the Compton scattered X-ray line, which is influenced by hydrogen, carbon, oxygen, and nitrogen content. This allows for successful measurement of materials like plastics, tyres, and biomass.

Due to the heterogeneous nature of alternative fuels, proper sample preparation is crucial to ensure representative and accurate XRF results. The preferred method involves:

• Drying: Removing moisture from the material.
• Cutting/shredding: Passing the dried material through a cutting mill to reduce particle size.
• Splitting: Obtaining a representative sample from the larger, shredded material.
• Cryogenic milling: Treating the material with liquid nitrogen to make it hard and brittle, followed by pulverization to achieve a homogeneous fine powder.
• Pressing (hot pressing for organic-rich samples): Compressing the pulverized powder into a stable pellet with a defined density and smooth analytical surface. Hot pressing, which uses heating plates, is especially beneficial for high-organic content materials like plastics, as it melts and consolidates the sample, ensuring infinite thickness for accurate analysis of both light and heavy elements. Ashing is generally not preferred as it can lead to the loss of volatile lighter elements like chlorine.

Several challenges exist when analyzing alternative fuels with XRF:

• Heterogeneity: Alternative fuels are often physically diverse. This is addressed by meticulous sample preparation involving drying, shredding, cryogenic milling, and pressing to achieve a more homogeneous sample. However, it's often recommended to take multiple measurements from highly heterogeneous samples.
• Complex matrix: The varied composition, especially the high organic content, makes traditional quantitative analysis difficult due to the lack of suitable calibration standards. The "scattering FP method" in WDXRF software is specifically designed to handle these organic-rich matrices by estimating the balance of carbon, hydrogen, oxygen, and nitrogen.
• Volatile elements: Ashing samples can lead to the loss of volatile elements like chlorine. Cryogenic milling followed by pressing is preferred to retain the entire elemental composition.
• Analyzing depth: Heavier elements have a deeper X-ray penetration. It's critical to ensure the prepared pellet is thick enough to satisfy the required analyzing depth for accurate measurement of these elements; otherwise, results may not be trustworthy.

Rigaku's WDXRF spectrometers (ZSX Primus series, Simultix 15, Supemini200) are norm-compliant with international standards like ASTM, EN, and ISO (e.g., ISO 29581 for fusion and pressed powder methods). They offer:

• Pre-calibration packages: These provide wide-range calibration for raw materials, clinker, and cement (e.g., OXIDE-FB-Pak) and narrower ranges for specific materials like cement, raw meal, and clinker.
• Pressed powder calibration sets: Available for cement, slags, clays, limestones, dolomites, sand, gypsum, and bauxite.
• Matrix matching calibration: Rigaku assists customers in certifying their in-house reference materials (secondary standards) to achieve matrix-matched calibrations, which are crucial for high accuracy and precision, especially with the pressed powder method.
• Software capabilities: Include both empirical and fundamental parameters calibration, as well as features for calibration maintenance (check samples, drift corrections) and data integration with external systems like LIMS.
• Service and support: Rigaku provides help desk support, extensive technical and application support, and flexible service contracts, emphasizing their rugged, reliable, and user-friendly XRF equipment designed for industrial environments.