Quantification of the release of inorganic elements from biofuels

The release of inorganic elements, mainly K, Na, Zn, Pb, S, and Cl, from a number of well-characterized biofuels (wood chips, bark, waste wood, and straw) was quantified as a function of temperature in a lab-scale fixed-bed reactor, and as a function of residence time in a lab-scale entrained flow reactor. The fuels were characterized by use of wet chemical analysis, and also by advanced techniques: chemical fractionation analysis (of the raw fuels) and simultaneous thermal analysis (of ash samples derived from the fuels at 550 °C). In parallel to the experimental release investigation and fuel characterization, global equilibrium analysis, simulating the experimental combustion conditions, was performed.

The objective of this work was to compare the lab-scale release data with the results from the fuel characterization techniques and from the equilibrium calculations. Furthermore, the results from the lab-scale fixed-bed release tests were compared to pilot-scale mass balance tests.

While large differences were seen between the lab-scale release data and the release information obtained by the fuel characterization techniques, a good correlation was found between the lab-scale and pilot-scale fixed-bed release data.

In conclusion, it is recommended to perform the described lab-scale tests in order to obtain reliable quantitative data on the release of inorganic elements under grate-firing or suspension-firing conditions. Advanced fuel characterization by use of chemical fractionation and simultaneous thermal analysis, and global equilibrium analysis of thermal fuel conversion systems, provide valuable information on the association of inorganic elements in the fuel, the transformations and release of inorganic species upon heating, and the possible forms in which the inorganic elements are thermodynamically stable as a function of temperature. This information is needed for the interpretation of the lab-scale release data. Thus, for the purpose of modeling ash or aerosol formation, fuel characterization methods should be combined with lab-scale release measurements. Pilot-scale mass balance test runs provide useful information on the distribution of the inorganic elements among the different ash fractions during grate-firing, from which the release of inorganic elements can be calculated.

This paper provides an outline of each of the experimental and analytical methods applied, a comparison of the release data obtained, as well as a discussion of the limitations and the assumptions made.