Blend design tools for Medium Combustion Plants (MCP) firing biomass wastes

A feasible alternative for agricultural or forestry waste management is the operation of a distributed network of sustainable Medium Combustion Plants (MCPs). However, one of the main factors that hinder its development is the propensity to operational problems derived from corrosion, slagging and fouling characteristics of both bottom and fly ashes. Therefore, a cost-effective approach for these multi-product MCP could be based on predictive tools for an optimal formulation of a fuel blend. This work focuses on the assessment of the ability of these methods to provide guidance for preventing ash-related operational problems and to provide fuel-blending rules. The more widespread tools pertain to two types: compositional classification based on chemical analysis of laboratory ashes, and thermodynamic prediction of the most likely species and phases. Both criterion numbers and compositional maps are ranking methods based on the chemical analysis at a given ashing temperature. Thermodynamic equilibrium modeling is not constrained by any difference in the physical conditions of the MCP compared to those in the laboratory. Both kind of prediction tools have been validated in an MCP firing olive tree pruning residues as well as its typical blends in order to mimic a plausible pattern of fuels along a full year operating campaign. An intensive experimental campaign encompasses plant monitoring and off-line analysis of the ashes along the process line. Interpretation of compositional plots has revealed to be potentially sensitive to ashing temperature. Here are presented examples showing how this variable could lead to either insignificant differences or to a substantial disparity in the a priori fuel diagnosis. Some inconsistencies have been observed between the predictions based on criterion numbers, even for the same fuel and for ranking rules specifically formulated for biomasses. Moreover, it does not match consistently with the information obtained from phase diagrams. Therefore, their use should be limited to the case of a well-established selection of a fuel index for a well-defined fuel provided empirical evidence of an enough good description of the ash behavior, which is not the most frequent case. Thermodynamic equilibrium calculations allow a more precise prediction of the main species in the condensed phase, without the constraint of the ashing temperature. Elemental closure of main ash-forming elements with the chemical analysis of the process ashes presents small differences, and their proximity localization on the phase diagrams denote similar prediction between predicted and process ashes.