Several papers addressed changes in particle size distribution both in fluidized bed and in pulverized combustion of coal. Recently, a single particle pyrolysis-combustion fragmentation model has been developed [1, 2] to predict the propensity of coal particles to fragment under a wide range of heating conditions. The model describes heat up and devolatilization of coal and then combustion of the residual char. Fragmentation occurs as a consequence of mechanical failure of the particle. Stress inside the particle arises from thermal shock, associated to particles' heat up, as well as from overpressure generated by volatiles release upon devolatilization. If fragmentation takes place during pyrolysis, it is referred to as primary fragmentation. The model was able to predict nicely the changes in particle size distribution of coal in a laminar flow reactor operated at 1573 K in CO2 rich atmosphere. Under such conditions, particles fragmentation was mainly due to the heat-up and occurred over timescales comparable with pyrolysis, but much shorter than char gasification with CO2, which in fact had no effect on fragmentation. The model has now been used to calculate the propensity of coal particles to undergo fragmentation in the early stages of oxy- combustion, with gaseous atmospheres of 5-30% O2 in CO2. In particular, the paper investigates the possibility that char conversion modifies the particle's porosity, affecting the solid's mechanical strength, volatiles diffusion out of the particle and, ultimately, the fragmentation probability. Reference has been made to both entrained flow and fluidized beds reactors, accordingly particles size of 0.1-10 mm have been assumed, reactor temperatures of 1123 and 2073 K, heating rates of 100 and 10000 K/s. Results showed that under entrained flow reactor conditions the particles break in the first 20-30 ms, producing relatively coarse fragments and a bimodal particle size distribution. Under fluidized bed conditions, the particles undergo explosive fragmentation after 1-2 s, well before pyrolysis is complete. This produces a multitude of fragments with broad particle size distribution. In both cases primary fragmentation occurs over very short timescales compared to char combustion and gasification therefore the pattern of primary fragmentation is not influenced by the changes in the gaseous atmospheres. Operative conditions where fragmentation occurs before or in parallel with char combustion or gasification can be inferred by comparing on an Arrhenius plot the timescale of fragmentation and heterogeneous reactions for a larger array of operating conditions (heating rate, particle size, reactor temperature). The figure reveals that gasification of char cannot be neglected for high reaction temperatures, more reactive coals, larger particles size. In these situations moreover gasification reactions can have an important role in secondary and percolative fragmentation, enhancing internal porosity.
Effect of O2/CO2 atmospheres on coal fragmentation
2019
Abstract
Several papers addressed changes in particle size distribution both in fluidized bed and in pulverized combustion of coal. Recently, a single particle pyrolysis-combustion fragmentation model has been developed [1, 2] to predict the propensity of coal particles to fragment under a wide range of heating conditions. The model describes heat up and devolatilization of coal and then combustion of the residual char. Fragmentation occurs as a consequence of mechanical failure of the particle. Stress inside the particle arises from thermal shock, associated to particles' heat up, as well as from overpressure generated by volatiles release upon devolatilization. If fragmentation takes place during pyrolysis, it is referred to as primary fragmentation. The model was able to predict nicely the changes in particle size distribution of coal in a laminar flow reactor operated at 1573 K in CO2 rich atmosphere. Under such conditions, particles fragmentation was mainly due to the heat-up and occurred over timescales comparable with pyrolysis, but much shorter than char gasification with CO2, which in fact had no effect on fragmentation. The model has now been used to calculate the propensity of coal particles to undergo fragmentation in the early stages of oxy- combustion, with gaseous atmospheres of 5-30% O2 in CO2. In particular, the paper investigates the possibility that char conversion modifies the particle's porosity, affecting the solid's mechanical strength, volatiles diffusion out of the particle and, ultimately, the fragmentation probability. Reference has been made to both entrained flow and fluidized beds reactors, accordingly particles size of 0.1-10 mm have been assumed, reactor temperatures of 1123 and 2073 K, heating rates of 100 and 10000 K/s. Results showed that under entrained flow reactor conditions the particles break in the first 20-30 ms, producing relatively coarse fragments and a bimodal particle size distribution. Under fluidized bed conditions, the particles undergo explosive fragmentation after 1-2 s, well before pyrolysis is complete. This produces a multitude of fragments with broad particle size distribution. In both cases primary fragmentation occurs over very short timescales compared to char combustion and gasification therefore the pattern of primary fragmentation is not influenced by the changes in the gaseous atmospheres. Operative conditions where fragmentation occurs before or in parallel with char combustion or gasification can be inferred by comparing on an Arrhenius plot the timescale of fragmentation and heterogeneous reactions for a larger array of operating conditions (heating rate, particle size, reactor temperature). The figure reveals that gasification of char cannot be neglected for high reaction temperatures, more reactive coals, larger particles size. In these situations moreover gasification reactions can have an important role in secondary and percolative fragmentation, enhancing internal porosity.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
