These elements are only present in very small trace amounts in biofuels. Their effect and importance is mainly related to there relatively high toxicity, which due to the increased concentrations in formed ashes can be relevant.
Cd, Pb, Zn, Cr
Analysis of the trace elements Cd, Pb, Zn and Cr are typically made as these heavy metals are also toxic. Zn, Pb and Cd are partly volatile during combustion and do thus accumulate in small ash particles in the fly ash, which is either emitted with the flue gases or collected in the fine particulate precipitation units (e.g. ESP filter). Moreover, in case the ash is attempted to be reused as a fertiliser the concentrations of these elements can be a limiting factor on the amount that can be recycled to the soil.
are quite common in earth minerals, but in very low concentrations. They have little relevance for biomass as none are nutrient and not especially toxic. High concentration in the fuel is typically an indicator of a dirty fuel with significant amounts of contamination. Ti is also very common in white paint and could also indicate that waste wood has been mixed in the biomass fuel.