amount of ammonia injected:
72.3 kg/min x 12 min = 867.6 kg
ammonia concentration = 0.382%
[tex]\tt 0.382\%=\dfrac{mass~amonia}{mass~ammonia+mass~natural~gas}\times 100\%\\\\0.00382=\dfrac{867.6}{867.6+mass~natural~gas}\\\\mass~natural~gas=226,252.88~kg/12min=1131264.4~kg/h[/tex]