Study on combustion and reduction of iron particles as an energy carrier

To address the global energy and greenhouse gas crisis, it is important to develop low-carbon, clean and renewable energy sources. Among the green energy candidates, metal powders have emerged as promising options due to their high energy densities and abundant availability. In this context, iron in particular is considered as one of the promising candidates among metal fuels. In the practical applications, the utilization efficiency and recyclability of the iron particles are critical.

During oxidation and reduction processes, iron may experience deactivation due to structural changes, which can shorten the cycle lifetime. Therefore, deactivation of iron particles in terms of changes in reactivity and morphology during the oxidation and reduction process was experimentally investigated, under both fixed-bed and suspension conditions. The effect of heating rate, reactor temperature, and particle-particle contact pattern (fixed bed and suspension) on particle characteristics, morphology, and reactivity was evaluated. It was found that simple thermal treatment in an inert atmosphere at 1200 °C had no influence on the reactivity of particles towards oxidation, even at long exposure resulting in mild agglomeration. In fixed-bed conditions, agglomeration and sintering of particles took place, limiting the efficiency of the subsequent cycles. In pulverized-fuel combustion in a drop tube reactor (DTR) at high heating rates (~10⁴ K/s), the intensive combustion and heat release led to melting of particles and morphology change from irregular to spherical. As the reactor temperature was increased from 800 to 1200 °C, the particle reactivity in terms of reduction degree (fractional conversion from Fe₂O₃ to Fe) slightly decreased from 78% to 72%, associated with a minor loss of surface area. Successive redox cycles in the Thermogravimetric analyzer (TGA) showed a steady decline in reduction reactivity (750 °C, 2.5% H₂), while the oxidation degree (950 °C or 1200 °C, 21% O₂) remained constant. Overall, both deactivation and loss of iron due to evaporation may serve to limit the energy release in iron combustion over extended cycles.

With a better understanding of the deactivation of iron particles during processes, the combustion of micron-sized iron particles was further investigated. Despite of extensive studies on combustion dynamics, studies combustion efficiency and utilization of iron particles are scarce. In this work, combustion of micron-sized particles of iron was investigated in a DTR under overall lean conditions (excess air ratio in the range of 1.1-1.4), and oxide products after combustion were analyzed, varying reactor temperature and particle size. A bimodal particle size distribution was observed after combustion: black micron-sized particles consisting of a mixture of iron oxides from the cyclone and reddish ultra-fine particles of Fe₂O₃ from the filter of the DTR. The combusted micron-sized particles had oxidation degrees ranging from 60% to 90% with no obvious dependency on the reactor set temperature, and displayed sizes comparable to those of the raw particles.

Modelling of the combustion using a simple 1D reactor model based on the Particle Equilibrium Composition (PEC) indicated that with an overall excess air ratio of 1.4, the residence time in the isothermal zone should be sufficient for conversion to Fe₃O₄. The model predicted larger oxidation degrees than observed experimentally. The difference was presumably due partly to short-comings in the modeling assumptions and partly to stratification in the drop tube reactor, resulting in local fuel-rich conditions. The low impact of reactor temperature on the final oxidation stage indicates that diffusion limitations, rather than kinetic barriers, are rate controlling.

To address the global energy and greenhouse gas crisis, it is important to develop low-carbon, clean and renewable energy sources. Among the green energy candidates, metal powders have emerged as promising options due to their high energy densities and abundant availability. In this context, iron in particular is considered as one of the promising candidates among metal fuels. In the practical applications, the utilization efficiency and recyclability of the iron particles are critical.
During oxidation and reduction processes, iron may experience deactivation due to structural changes, which can shorten the cycle lifetime. Therefore, deactivation of iron particles in terms of changes in reactivity and morphology during the oxidation and reduction process was experimentally investigated, under both fixed-bed and suspension conditions. The effect of heating rate, reactor temperature, and particle-particle contact pattern (fixed bed and suspension) on particle characteristics, morphology, and reactivity was evaluated. It was found that simple thermal treatment in an inert atmosphere at 1200 °C had no influence on the reactivity of particles towards oxidation, even at long exposure resulting in mild agglomeration. In fixed-bed conditions, agglomeration and sintering of particles took place, limiting the efficiency of the subsequent cycles. In pulverized-fuel combustion in a drop tube reactor (DTR) at high heating rates (~10⁴ K/s), the intensive combustion and heat release led to melting of particles and morphology change from irregular to spherical. As the reactor temperature was increased from 800 to 1200 °C, the particle reactivity in terms of reduction degree (fractional conversion from Fe2O3 to Fe) slightly decreased from 78% to 72%, associated with a minor loss of surface area. Successive redox cycles in the Thermogravimetric analyzer (TGA) showed a steady decline in reduction reactivity (750 °C, 2.5% H₂), while the oxidation degree (950 °C or 1200 °C, 21% O₂) remained constant. Overall, both deactivation and loss of iron due to evaporation may serve to limit the energy release in iron combustion over extended cycles.

With a better understanding of the deactivation of iron particles during processes, the combustion of micron-sized iron particles was further investigated. Despite of extensive studies on combustion dynamics, studies combustion efficiency and utilization of iron particles are scarce. In this work, combustion of micron-sized particles of iron was investigated in a DTR under overall lean conditions (excess air ratio in the range of 1.1-1.4), and oxide products after combustion were analyzed, varying reactor temperature and particle size. A bimodal particle size distribution was observed after combustion: black micron-sized particles consisting of a mixture of iron oxides from the cyclone and reddish ultra-fine particles of Fe₂O₃ from the filter of the DTR. The combusted micron-sized particles had oxidation degrees ranging from 60% to 90% with no obvious dependency on the reactor set temperature, and displayed sizes comparable to those of the raw particles.

Modelling of the combustion using a simple 1D reactor model based on the Particle Equilibrium Composition (PEC) indicated that with an overall excess air ratio of 1.4, the residence time in the isothermal zone should be sufficient for conversion to Fe₃O₄. The model predicted larger oxidation degrees than observed experimentally. The difference was presumably due partly to short-comings in the modeling assumptions and partly to stratification in the drop tube reactor, resulting in local fuel-rich conditions. The low impact of reactor temperature on the final oxidation stage indicates that diffusion limitations, rather than kinetic barriers, are rate controlling.