OCR Text |
Show tip. Thermal NOx, on the other hand, is generated further down the flame where the NOx forming species are exposed to sufficiently high temperatures for a relatively longer of time. Pre-heated air-based flames generate mainly thermal NOx. An oxygen-based flame will generate more prompt NOx than a pre-heated air based flame at an equivalent firing rate. It is believed that by pulsating at least one of the gaseous feeds, the fraction of NOx promptly generated is significantly decrased. Prompt and thermal NOx formation is very sensitive to both flame temperature and local O2 concentration. The maximum flame temperature is obtained for the stoichiometric composition (R=l), since any excess of either oxygen (lean flame, R<l) or natural gas (rich flame, R)l) will dilute the flame. For small O2 enrichment levels, up to R~0.8, an increase in NOx formation is observed. Below R=0.8 the dilution effect lowers the flame temperature enough that a decrease of the NOx concentration in the flue gas, relative to the stoichiometric value, is observed. Consequently, both lean and rich flames generate less NOx than a stoichiometric flame. This is the basic principle behind the use of pulsating combustion for NOx control. An overall stoichiometric flame, which gives the best combustion efficiency, can be obtained by a succession of periodically alternating lean and rich flames. Such a sequence is obtained by establishing a gas flowrate which varies versus time in at least one of the feed lines. A minimum gas flowrate needs to be maintained in both lines, in order to avoid flame extinction and high levels of CO emissions due to incomplete combustion. An idealized example of pulsated flowrate follows the square signal shown on figure 1. Characteristics of this signal are: Q (Nm 3 / h) pulsated gas flovrate C (Nm 3 /h) Minimum flowrate and constant fraction of the gas flowrate P (Nm3 / h) Amplitude of the pulsated fraction of the gas flowrate 0 (s) Duration of the maximum flowrate (Q C + P) F (s) Duration of the minimum flowrate (Q = C) f (Hz) Pulsation frequency (1/(0 + F» t Dephasing between the two pulsations when both gases are pulsated Vhen pulsating only one gas, the other gas flowrate is kept constant and equals to A*(C + P/2) (A equals 2 or 0.5 if oxygen or natural gas a re respectively the pulsated gas). If the two gases are pul sated the opti mum dephasing is 1/2. Experiments has been r un in order to determine the feasibility limits of the technique and t he opti mum value of parameters C, P, 0, F and t. 3. Experimental set-up Two different furna ces we re used ~ h ich val ' ed in the inner refrac tory lining material and in f i ri ng ra te capabi li y. Furnac nO l, l ined with refractory wool, was us ed for the first exp riments at 20 kY f i ring rat e. and Furnace n02, lined wi th alumina brick wa s ed at 50 kY and relativel' 6 |