To analyze effects of aggregation in powder/gas reactions quantitatively, particle packing models were applied to aggregate structures, and theoretical equations of tile reaction rate, the reaction-equivalent panicle radius, the acid gas concentration exponent and the particle radius exponent were deduced. While confirming the validity of these equations by comparing experimental values (data on SOx removal in aerosol reactions and stationary reactions) to these equations, other experimental data were analyzed using these equations. The results show that : the dependency of lime conversion on the particle size decreases sharply as the aggregation of particles in the aerosol advances; injecting limestone powder into the furnaces, using a high rate dispersion nozzle, reduces the degree of aggregation and elevates the lime conversion; and in thermobalance analysis of SOx/limestone reactions, bulk limestone powder behaves like a single giant particle, and the diffusion process inside the aggregates is controlling. It is concluded that hexagonal closest packing is the optimal packing model for the particles constituting aggregates in aerosol.