Biomass Combustion With Emphasis on Interactions Between Metals and Inorganic Particulate

Biomass is clean, stored solar energy. Not only is it a plentiful fuel, but its use also reestablishes the natural carbon cycle helping mitigate greenhouse gas emissions. This renewable energy source is nearly CO₂ neutral. Overall, it is possible to achieve a 93% reduction in net CO₂ emissions per unit heating value by switching from coal to biomass and a 84% reduction by switching from natural gas-fired cogeneration to biomass. Due to inherent advantages of the biomass in substituting fossil fuels, and increasing legislative pressures against CO₂ emissions (Kyoto Protocol), biomass-based power is genuinely considered. It seems practically impossible to meet Kyoto requirements by replacing fossil fuels combustion with nuclear energy, hydropower or fuel cells. Simply, there is not enough time. In this context, there exists a niche for the biomass-based power generation. This paper compares interactions between metals and solid particles evolving from biomass during the classical Fluidized Bed Combustion (FBC) and a new Low-High-Low temperature (LHL) combustion. Experiments, conducted at a pilot-scale, reveal a clear pattern of surface predominance of light metals (Ca, K) and core predominance of heavy metals (Cd, Cr) within the LHL-generated particles. No such behavior was induced by the classical FBC approach. Metals migration is linked to the evolution of inorganic particles. A composite picture of the metals rearrangements in the particles was obtained by the combination of independent analytical techniques including electron probe microanalysis, field emission scanning electron microscopy, inductively-coupled plasma spectrometry and X-ray diffractometry. It is suggested that the combination of (i) the high-temperature region in the LHL and (ii) changes in the surface free energy of the particles is the driving force for the metal-particle behavior. Important practical implications of the observed phenomena are proposed including removal of hazardous submicron particulate and reduction in fouling/slagging during biomass combustion. These findings may contribute to redesigning currently operating FBC units in order to generate non-hazardous, non-leachable, re-usable particles where heavy metals are immobilized while environmental and technological problems reduced.