| OCR Text |
Show The thermal N 0 X mechanism is initiated with the high temperature disassociation of atmospheric N 2 molecules. Atmospheric nitrogen has a triple atomic bond, and is extremely stable. Atmospheric nitrogen will only decompose at the very high temperatures within the flame. Thermal N O x is therefore very temperature dependent. The resultant nitrogen atoms are unstable and will precede rapidly to form either N O or a new N 2 molecule. For fuel lean combustion, the Zeldovich mechanism is accepted as an approximation of N O x formation. N 2 + 0 - > N O + N (RATE LIMITING) (1) N + 02 -> NO + O (FAST) (2) In staged combustion, thermal N O x is minimized by: 1. Reducing Peak Flame Temperatures, 2. Reducing the Oxygen Levels at the Peak Flame Temperatures, 3. Reducing the Time of Exposure to the Peak Flame Temperatures. For fuels such as residual oil, the oxidation of nitrogen that is organically bound to the fuel molecule becomes the primary source of total N O x emissions. For this reason overall N O x reduction efforts are geared toward reducing the conversion of fuel bound nitrogen (FBN) to N O x . Recall that in thermal N O x formation, the nitrogen source is the extremely stable (triple atomic bond) atmospheric nitrogen. The source of nitrogen for fuel N O x is actually single bonded directly to the fuel oil molecule. As the fuel burns, all bound nitrogen disassociates to form highly unstable nitrogen radicals. These nitrogen radicals will either be oxidized to form N O x , or bond with another nitrogen radical to form N 2 (atmospheric nitrogen.) To minimize the oxidation of FBN, w e try to delay mixing of stoichiometric air. The less oxygen available to the nitrogen radicals, the more the reaction will favor the formation of N 2. The desired result are long, narrow flames with fuel rich zones to inhibit the conversion of F B N to NOx. Ideally, the combustion reaction will not be complete until just before the gasses turn into the convection tube bank. Thermal NOx and Fuel NOx are both being formed in the combustion reaction. Although the formation mechanisms are widely different, minimization techniques are similar. The long fuel rich flame established to niinimize F B N oxidation also lowers the flame temperature and reduces thermal N O x . The delayed mixing of stoichiometric air also reduces the amount of atmospheric nitrogen available at the peak temperature zones at the base of the flame. |
