The present work describes the steps to design a pyrolizer oven that uses microwave to produce a low-cost activated carbon starting from waste tyres. A viability test on using microwave to transform waste rubber into carbon black was done using a small chamber that could be filled with 100 grams of rubber, treated with 500 W of microwave power: the result gave the start to the design of a lab scale oven that could be easily scaled up in case of success of the entire process. The cavity should be able to accept a minimum of 10 kg of waste rubber: as the process time was estimated near 10 minutes, the hour capacity was 60 kg (that roughly corresponds to twelve tyres processed per hour). The oven shape was a steel cylinder with diameter and length of 400 and 1400 mm respectively, with a total microwave power of 12 kW. Using an EM simulation tool (Concerto) it has been possible to evaluate the best input configuration of the 4 microwave generators, to optimize the port dimensions, to design a rectangular to cylindrical transitions, as well as the double quartz and Teflon window necessary to withstand the high temperature and pressure inside the oven-vessel.