Program topic: Combustion technologies examination of changing char reactivity with isothermal oxy-fuel combustion by a simplified atomistic simulation

During oxy-fuel combustion, combustion, and gasification the char reactivity commonly changes during conversion. Through the use atomistic simulations this process can be explored with great fidelity. Here the change in char reactivity during oxy-fuel combustion was explored. Two simplistic yet large-scale (40,000 atoms) char structure were constructed that captured a distribution of chemical and physical features: namely stacking extent, local order, and an assumed pore size distribution. The chars differ in the extent of porosity (a low porosity microporous model and a higher porosity char with additional micro-and mesopores). An automated oxy-fuel combustion simulation was performed with a sequence of oxygen diffusion, close contact calculations to identify reacting reactive-edge atoms, and reactive atom deletion (but retention of the oxygen). Through scripting the size of the char, reactive carbons, carbons removed, number of atoms, relative rate, and char transformations were captured. Post analysis identified the changes in pore size distribution, surface area, and helium density. Graphite with a similar number of carbon atoms was also examined as a reference material. The higher porosity char had the fastest rate (72% more rapid than graphite based on the number of steps), similar to the lower porosity char (75%). Relative rate examined as a function of conversion followed an expected trend of increasing, peaking, and then slowly declining. Transitions were followed showing the inclusion of mesoporosity slightly increased the rate with the peaking occurring over a longer conversion range (30 - 60%) in comparison to the lower porosity char (∼30%). The peak reactivity (number of carbon atoms deleted in a step) followed the maximum surface area (with a delay of ∼5% conversion), than decreased - as there were fewer reactive carbons. Both chars had a changing density combustion mode initially transforming into a combined changing density and shrinking core after ∼35% conversion. Graphite was less reactive and followed a shrinking core as expected (reactions were limited to edge carbon atoms).