This paper addresses the theoretical development and experimental validation of optimized recirculating reverse-flow gas cyclones. The simulation of these systems is based on the predictive properties of a finite diffusivity model, modified to include partial recirculation of the cyclones' emissions. Experimental validation was obtained at laboratory and pilot scales at low temperatures (up to 350 K) and for cork waste biomass boilers at higher temperatures (up to 600 K). Under certain circumstances, with recirculation, the proposed system showed a better performance than an online pulse jet bag filter, and substantially better than with multicyclone systems. The generally observed unexpected high collection of submicron particles, which occurs with inlet concentrations as low as 100 mu g/m(3), is attributed to turbulent dispersion, either by promoting fine particle capture by larger ones, much like what occurs in recirculating fluidized beds, or by bringing fine particles near the cyclone wall. The extremely fine particle size distributions exiting from the recirculation system, as measured off-line at laboratory, pilot and industrial scales, were confirmed at pilot scale using online measurements through a laser monitor. (c) 2006 Elsevier B.V. All rights reserved.