The cement industry is one of the largest pollutant emitters. One way to cope with high pollutant emissions is to co-combust biomass with pulverized coal. A mathematical model was developed, which is detailed enough to consider the complex physical and chemical behavior of the co-combustion process but simple enough to perform simulations with a real geometry of cement rotary kiln within reasonable time. Numerical simulation with a 20-% share of pulverized biomass of total fuel heating value was performed. An industrial rotary kiln geometry was simulated; temperature and velocity fields along with mass fractions of released volatiles and combustion products were analyzed. The model allows better insights in the co-firing process with the main goal to reduce CO2 emissions by optimizing the combustion process inside the rotary kiln.