A zeolite based SCR catalyst for NOx reduction using a reducing agent for treating exhaust streams from industrial and commercial boilers is provided. The reactor system has a zeolite based catalyst arranged in catalyst cassettes in a modular fashion where the reactor containing the zeolite based SCR catalyst cassettes is placed in a perpendicular direction to the exhaust exiting the industrial and/or commercial boiler. The catalyst selectively reduces nitrogen oxides to nitrogen with a reducing agent at low to medium temperatures. The reactor results in high NOx conversions and very low ammonia slip and is active for a wide range of boiler firing conditions. Boilers with low NOx and/or ultra low NOx burners can be replaced with a standard conventional burner for overall emissions reduction performance, efficiency improvements and energy savings. Boilers with low NOx and ultra low NOx burners can also be fitted with this zeolite based SCR catalyst reactor for additional NOx reductions and energy savings.