| OCR Text |
Show choice of a given operating regime depends on the desired benefits and the overall economics which are considered later. HIGH N 2 E N V I R O N M E N TS There are a number of potential sources of N2 in combustion systems utilizing oxygen-enhanced combustion. One source of high N 2 is from the oxidizer. Figure 1 shows a common method of using low levels of oxygen-enrichment in a combustion system, which is to blend higher purity 0 2 into the combustion air. This technique is often referred to as premix enrichment. In industrial applications, this is typically done for low level O E C , up to an oxygen concentration in the oxidizer of up to about 30%. Because only modest oxygen enrichment is used, large amounts of N 2 remain in the oxidizer stream. Beyond 3 0 % there are safety considerations which may require the existing piping system to be modified or replaced to handle higher levels of 0 2 enrichment. Another problem with higher levels of 0 2 enrichment is that the burner itself which was originally designed for air/fuel combustion may not be able to tolerate the added heat resulting from the faster rate of combustion and resulting higher flame temperatures within the burner and burner quarl. Premix enrichment often will result in a shorter visible flame than the air/fuel base situation. The original air/fuel burner may also need to be modified for higher levels of enrichment [2]. AIR OXYGEN • FUEL * Figure 1. Schematic of premixing 0 2 with air. A variation of premixing the 02 into the combustion air stream is to inject the 02 into the flame, typically from the bottom (see Figure 2). This technique has some advantages over premixing which include: no modifications to the existing air/fuel burner are typically required, higher levels of 0 2 enrichment are possible as all of the 0 2 delivery equipment is already designed for pure 0 2 so there are no safety considerations of putting high levels of 0 2 into piping originally designed for air, and the flame can be staged which typically produces a longer, more luminous flame with minimal effects on N O x [3]. This technique is used in low and medium level OEC, but most commonly for overall O E C below 40%. Again, because only modest oxygen enrichment is used, a large amount of N 2 remains in the oxidizer stream. 2 |
