You will first need to check with the government agency that regulates emissions in your area. They will provide you with very specific criteria you need to meet with respect to particulate matter levels, residence/dwell times, operating temperatures, etc. The afterburner (referred to as a thermal oxidizer in the industry) for a 10 kilo roaster is going to be considerably different than one for a 15 kilo roaster. There will be a considerable price difference between the two. To spec the proper afterburner, you need to provide the manufacturer with information regarding roasting chamber temperatures, exhaust flow in cubic feet per minute, and BTU output of the roaster's burner. There will be a considerable variation between 10 and 15 kilo roasters. Also, you need to determine whether you have adequate natural gas supply and pressure to run the afterburner. Keep in mind the afterburner uses a considerably higher amount of fuel than the roaster. Most burners on afterburners that meet my local air quality mandates require natural gas pressure of 1/2 psi or 14 inches above water column. This would require most commercial sites to upgrade to a medium pressure gas system.
Just a few thoughts to consider when sizing an afterburner. It is not as simple and straightforward as most people think. Hope this helps...