OCR Text |
Show IFRFDocNoK70/y/113 IJmuiden, April 1997 -11- GB (annulus) / Sewage (annulus) 3500 3000 O 2500 CO <^ 2000 E \ 1500 O z 1000 500 0 0 5 10 15 20 25 30 Primary rebum fuel fraction [ % ] 16 33 41 48 64 80 Gas velocity [m/s] Figure 3.8 The Effect of Internal Fuel Staging on N O x Introducing 25 % natural gas (based on fuel input) as rebum fuel had an immediate effect of reducing N O x from the baseline level of 2678 and 3096 mg/m3 0 to ~ 1150 mg/m3 0 (6 % 02) with both the 20 % and 5 0 % sewage flame. With the 5 0 % sewage flame N O x decreased linearly with increase in natural gas but with the 20 % sewage flame N O x was dramatically reduced at fuel fractions >10 % (gas velocity >33 m/s). It is possible that this sudden drop in N O x emissions was caused by the effect of reburn gas injection momentum. For the Type U flame the rebum gas was injected at a low velocity. Ignition occured from the inside to the outside of the coal jet, within the LRZ, where the combustion front reached the sewage layer. At this point sufficient combustion air was entrained to create high N O x levels. At rebum gas fractions greater than 12.5 %, a Type I penetration flame was created and the ignition pattern was reversed. This resulted in the sewage layer igniting whilst insufficient combustion air had been entrained. In this situation the natural gas is preferentially burnt, resulting in the flame front propagating towards the fuel rich flame centre and less N O x is formed. 3.3 Fuel Staging with Coal Blend 3.3.1 Fuel staging with bluff bodies The concept of fuel staging, by injecting fuel through the centre of the burner, has shown to be an effective way of reducing N O x . This study, on the firing of blends, has investigated further this concept, and in particular the effect on N O x , when bluff bodies are introduced onto the central coal gun. Figure 3.9, shows a 60/40 % blend (by mass of total fuel input) of Polish coal with Douglas Preumium 2. For all conditions tested, the central coal gun was positioned flush with the primary coal gun. The tgun position' in the legend refers to the coal gun position relative to the throat of the burner, and the bluff body is the type used on the central coal I - - l^_^ - - - - - --p. rX ~~^o i 'x y O - o 1 - o - 20% Sewage o 50% Sewage Q. " -Q 1 . 1 . pun |