Key Takeaways & Executive Findings
- •• • CO2 torrefaction at 300 °C raised the softening temperature (ST) of walnut shell ash by 251 °C and vinegar residue ash by 117 °C compared to N2 torrefaction, directly reducing slagging propensity in boilers. • • CO2 atmosphere increased the HCl-soluble and insoluble AAEMs fractions, as quantified by chemical fractionation, indicating a shift toward more stable, high-melting-point mineral forms. • • XRD analysis confirmed a higher relative content of alkaline earth aluminosilicates in ash after CO2 torrefaction, which are known to elevate ash fusion temperatures. • • The weight loss and ash yield of biomass were higher under CO2 than N2 torrefaction, suggesting that CO2 actively participates in devolatilization and mineral transformation, enhancing fuel upgrading.
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Abstract
This study investigates the migration and transformation behavior of alkali and alkaline earth metals (AAEMs) during CO2 and N2 torrefaction of walnut shell (WS) and vinegar residue (VR), and their subsequent impact on ash fusion characteristics. Chemical fractionation and ICP-MS quantified AAEMs speciation and content. CO2 torrefaction increased biomass weight loss and ash yield compared to N2, and significantly elevated ash fusion temperatures: deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). At 300 °C under CO2, WS ash ST increased by 251 °C and VR ash ST by 117 °C. Chemical fractionation revealed that CO2 atmosphere promoted increases in HCl-soluble and insoluble AAEMs fractions. XRD confirmed increased relative content of high-melting-point alkaline earth aluminosilicates. CO2 torrefaction effectively mitigates ash deposition and slagging issues in biomass thermal conversion. The findings provide a mechanistic basis for tuning torrefaction atmosphere to control AAEMs speciation and enhance ash fusibility, offering a viable strategy for improving biomass fuel quality and boiler operational stability.
1. Introduction
Biomass thermal conversion faces persistent operational challenges due to the inherent richness of alkali and alkaline earth metals (AAEMs), which induce ash deposition, slagging, and fouling on heat transfer surfaces, compromising boiler stability and efficiency. Conventional torrefaction under inert atmospheres (e.g., N2) improves fuel properties but does not sufficiently mitigate AAEM-related ash issues. The use of CO2 as a torrefaction atmosphere has emerged as a promising alternative, potentially altering AAEM speciation and ash fusion behavior through chemical interactions, while also offering economic and environmental benefits such as CO2 utilization and reduced N2 purification costs.
Despite these advantages, the mechanistic role of CO2 in modulating AAEM migration and ash fusion characteristics remains poorly understood. This study addresses this gap by systematically comparing CO2 and N2 torrefaction of two typical Shanxi industrial biomasses—walnut shell (WS) and vinegar residue (VR)—at temperatures from 150 to 350 °C. Through chemical fractionation, ICP-MS, and XRD analyses, we elucidate how CO2 atmosphere promotes the formation of high-melting-point alkaline earth aluminosilicates and increases HCl-soluble and insoluble AAEM fractions, thereby elevating ash fusion temperatures. The findings provide a foundation for optimizing torrefaction atmosphere to enhance biomass fuel quality and mitigate ash-related operational risks.
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WANG Weishu, QIN Yuhong, WANG Yuefeng, GAO Songping, GUO Shugang (2026). Regulatory Mechanism of AAEMs Migration and Transformation on Biomass Ash Fusion Characteristics under CO2 Torrefaction Atmosphere. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9655
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Frequently Asked Questions
What is the quantitative impact of CO2 torrefaction on ash fusion temperatures compared to N2 torrefaction?
At 300 °C, CO2 torrefaction increased the softening temperature (ST) of walnut shell ash by 251 °C and vinegar residue ash by 117 °C relative to N2 torrefaction. This significant elevation directly reduces slagging and fouling tendencies in thermal conversion systems.
How does CO2 atmosphere alter the speciation of AAEMs in torrefied biomass?
Chemical fractionation revealed that CO2 torrefaction promotes an increase in HCl-soluble and insoluble AAEM fractions. This shift indicates transformation of AAEMs into more stable, high-melting-point forms, such as alkaline earth aluminosilicates, as confirmed by XRD analysis.
What are the practical implications for boiler operation and maintenance?
The elevated ash fusion temperatures (DT, ST, HT, FT) achieved under CO2 torrefaction mitigate ash deposition and slagging on heat transfer surfaces, reducing downtime and maintenance costs. This enhances boiler reliability and operational efficiency when firing torrefied biomass.
Can CO2 torrefaction be economically viable at industrial scale?
CO2 torrefaction offers dual benefits: utilization of CO2 (reducing emissions) and elimination of N2 purification costs. The improved fuel quality (higher energy density, better grindability) and reduced ash-related issues can offset operational expenses, though detailed techno-economic assessments are required for specific industrial contexts.
What are the optimal torrefaction conditions for maximizing ash fusion temperature improvement?
The study identified 300 °C under CO2 as particularly effective, with ST increases of 251 °C for WS and 117 °C for VR. However, optimal conditions may vary with biomass type; systematic testing across temperatures (150–350 °C) is recommended to tailor torrefaction for specific feedstocks.
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