Numerical study of the combustion behaviors and NOx formation in NH3/coal co-combustion under MILD mode: Effect of NH3 injection modes

Co-combustion of pulverized coal with NH3 is a crucial strategy for achieving a low-carbon transition in the coal-fired power plant, while unstable combustion and high NOx emissions are the key technical challenges for its widespread application. The Moderate or Intense Low-oxygen Dilution (MILD) combustion technology attracts increasing attention due to its advantages of stable flame and low NOx emissions, thus, MILD combustion technology is applied to coal/NH3 co-combustion that can overcome the challenges. This work systematically studies the effect of five different NH3 injection modes on combustion behaviors and NOx emissions in coal/NH3 co-combustion under MILD mode. Compared with other injection modes, the center-in mode has a higher share of the MILD regime in the furnace (similar to 90 %) and a lower temperature fluctuation (4.18 %), which further improves the temperature uniformity and enhances the combustion stability. The char average burnout rate of center-in mode increases by 2.56 times compared to the regular premixed injection mode. The order of overall NO emission concentration under the different NH3 injection modes is Inner injection >> Center-out injection > Mixed injection approximate to Outer injection > Center-in injection. Compared to the original Mixed injection mode, the NO-sum concentration under the Center-in injection mode is decreased by 33.8 %. The Center-in injection mode balances well the high char burnout and low NOx emissions, which provides theoretical references for the design of new low-NOx burners for the co-combustion of NH3 and coal.