Measurement of Fixed Carbon, Volatile Matter, and Ash of Biocoke

Significance of the topic

Thermogravimetric analysis (TG) combined with differential thermal analysis (DTA) provides a compact and information-rich approach to characterize solid fuels such as biocoke. Determination of moisture, volatile matter, fixed carbon and ash content is essential for assessing combustion performance, storage stability, transportability and suitability as a fossil-fuel substitute. Simultaneous TG-DTA further adds thermal effect information (endotherms/exotherms) that helps to distinguish pyrolysis from combustion events and to evaluate reactivity and ignition behavior.

Objectives and overview of the study

This application study evaluated combustion characteristics and proximate composition of two biocoke types produced from waste wood and buckwheat husks. The aims were to: (1) use a DTG-60 simultaneous TG/DTA to resolve thermal events during heating in air, (2) determine volatile matter, fixed carbon and ash by a nitrogen-to-air switching protocol consistent with proximate analysis practice, and (3) compare combustion behaviour and reactivity between the two feedstocks.

Methodology

• Sample preparation: biocoke samples were ground and sieved to collect particles smaller than 250 µm.
• Combustion-characteristics measurement (air atmosphere): samples (~8 mg) were heated at 10 °C/min from 30 °C to 700 °C under air to record TG, derivative TG (DrTGA/DrTG) and DTA traces. This resolved moisture loss, volatile combustion, and fixed-carbon surface combustion as discrete mass-loss steps.
• Proximate analysis / fixed carbon determination (N2 → air): samples (waste wood 8.37 mg; buckwheat husk 8.26 mg) were heated at 10 °C/min from 30 °C to 1000 °C with an initial nitrogen atmosphere up to 900 °C, then switched to air and heated to 1000 °C. Mass losses were apportioned to moisture, volatile matter (pyrolysis under N2), fixed carbon (combustion after switching to air), and residual ash. Results were evaluated on a dry basis following JIS M 8812 principles.

Instrumentation used

• Shimadzu DTG-60 simultaneous thermogravimetric analyzer / DTA.
• Software and workstation typically used with the system (e.g., LabSolutions TA) for data acquisition and derivative curve calculation.

Main results and discussion

• TG-DTA behaviour in air: both biocoke types showed a characteristic three-step mass loss: (a) moisture evaporation up to ~120 °C; (b) mass loss between ~250 °C and ~400 °C attributed to combustion of volatile matter (pyrolytic volatiles oxidizing under air); (c) mass loss above ~400 °C corresponding to combustion of fixed carbon (surface carbon burnout).
• Differences between feedstocks: the relative magnitudes of the second and third steps differed by feedstock, reflecting different volatile content and carbon reactivity.
• Derivative and DTA interpretation: DrTG peak positions and heights differed for the two samples. Peak height correlates with reactivity (higher peak = higher instantaneous reaction rate), while higher peak temperature indicates lower reactivity. DTA traces provide combustion start and end temperatures and distinguish endothermic pyrolysis from exothermic combustion events.
• Proximate composition (dry basis):
  • Waste wood biocoke: volatile matter 62.8 wt%, fixed carbon 17.8 wt%, ash 19.4 wt%.
  • Buckwheat husk biocoke: volatile matter 78.4 wt%, fixed carbon 19.6 wt%, ash 2.0 wt%.
  These values indicate that buckwheat-husk biocoke releases a larger fraction of volatiles (faster ignition/combustion onset) and contains much lower ash, whereas waste-wood biocoke retains substantial inorganic residue (higher ash) and a slightly lower volatile fraction.
• Nitrogen-to-air switching: performing pyrolysis under inert gas followed by combustion in air permits clean separation of volatile release and subsequent carbon oxidation, enabling straightforward calculation of fixed carbon and ash residues representative of proximate analysis.

Benefits and practical applications

• Quality control: TG-DTA gives a reproducible method to compare biocoke batches and to screen feedstocks or production conditions for desirable combustion traits.
• Fuel selection and blending: knowing volatile/fixed-carbon/ash ratios helps predict ignition behavior, calorific delivery profile, slagging potential and ash handling requirements in industrial furnaces and boilers.
• Process optimization: TG-DTA traces help identify optimal pyrolysis/pressing conditions for producing biocoke with targeted reactivity and ash content.
• Comprehensive thermal insight: simultaneous measurement of mass change and thermal effects (endotherms/exotherms) allows differentiation of physical dehydration, pyrolysis and combustion steps in a single run.

Future trends and potential uses

• Integrating evolved-gas analysis (e.g., TG-FTIR or TG-MS) to identify volatile species and better link composition to emissions and combustion chemistry.
• Kinetic modelling of TG/DrTG data to extract activation energies and reaction mechanisms for different biomass-derived chars and biocokes; useful for reactor design and scale-up.
• High-throughput screening and statistical analysis across larger sample sets to map how raw-material blends and production parameters affect final fuel properties.
• Standardization: establishing validated TG-DTA protocols that complement existing proximate analysis standards to improve cross-laboratory comparability for alternative fuels.
• Application to blended fuels and co-combustion studies to design mixes that balance ignition, burn rate and ash behavior.

Conclusion

Simultaneous TG-DTA using the DTG-60 effectively characterizes biocoke combustion behavior and provides proximate-analysis data (volatile matter, fixed carbon, ash) when using a nitrogen-to-air switching protocol. The method resolves moisture, pyrolysis/volatile release and carbon combustion steps while delivering thermal-effect information that aids interpretation of reactivity and combustion temperatures. Differences observed between waste-wood and buckwheat-husk biocokes illustrate how feedstock selection impacts volatile fraction, fixed carbon content and ash, with direct implications for fuel performance and handling.

Reference

1. Tamio Ida, Kunihiko Namba, and Hiroshi Sano: A Study of Thermal Analyses and Fundamental Combustion Characteristics for Thermal Utility with Biomass Volatile Matter, Journal of High Temperature Society, 2007, Vol. 33, No. 1.
2. Atsushi Yamada: Fry-drying of wood biomass fuel, Journal of Forest Products Research Institute, No. 548, 2021.
3. Paolo Ghetti, Leandro Ricca, and Luciana Angelini: Thermal analysis of biomass and corresponding pyrolysis products, Fuel, Vol. 75, No. 5, 1996.