If you just provide a lot of words to describe instead of a detailed equation and formula to find the final answer I will give a negative thumb.

The process needs to include assumption, Fick's equation, the law of conservation of mass, then integrate the things between the limits to get the final time answer. That kind of thing, I am not sure about it too.

A fluidized coal reactor has been proposed for a new power plant. If operated at 1145K, the process will be limited by the diffusion of oxygen countercurrent to the carbon monoxide (CO) formed at the coal particle surface. Assume that the coal is pure solid carbon with a density (c) of 12800kg/m3 and that the particle is spherical with an initial diameter of 150m. Air (21%O2 and 79%N2) exists several diameters away from the sphere. Under the conditions of the combustion process, the mass diffusivity of oxygen in the gas mixture (DAB) at 1145K is 1.3cm2/s. The rate of reaction between carbon and oxygen at this temperature is so high that the concentration of oxygen at the particle surface is essentially zero. Use a quasi-steady state analysis to determine the particle radius as a function of time. Determine the time necessary to reduce the diameter of the coal particle to 50m