Efficient mechanical separation of CO2 from combustion effluent affects the utilization potential of high CO2 producers such as coal. Novel mechanical separations of condensing CO2 from gas flows need to be able to capture the small condensed droplets below the cyclone cut-off limit of 20 µm. We describe the thermodynamics, the energy costs and droplet formation of CO2 phase separation from combustion effluent and natural gas. We report the first measurements of condensing CO2 droplet sizes from gas. This shows that application of homogeneous condensation of CO2 yields much smaller droplets in flue gas (N2/CO2) than from contaminated natural gas (CH4/CO2). These small droplets can only be efficiently removed at high throughputs using the novel centrifugal method we describe. Such mechanical separations are preferable to the current standard chemical methods because of the much lower environmental footprint.