OCR Text |
Show The NH2 is then available for reaction with NO to eventually yield N2. The above results suggest that the key parameters for the enhancement of burnout zone chemistry in staged combustion or reburning are: • Reaction temperature (8S0·C), • CO level (0.5% or less), and • NH3 species. Advanced Reburning Apparently the conventional reburning proces does not provide the required environment. An advanced reburning process 7, which combine reburning with selective NO~ reduction (SNR) via ammonium sulfate injection, was designed and tested. ~lgure 2 shows two hybrid schemes with 20 percent and 10 percent gas reburning, respectively. With 20 percent reburning (SR2 = 0.9), the burnout air was divided into two streams to yield an SR3 of 0.99 and an SRt of 1.15. With 10 percent reburning, the reburnlng zone stoichiometry (SR2) was 0.99 and the burnout air stoichiometry (SRt ) was 1.15. In both ca~es, an aqueous solution of ammonium sulfate was atomized with the final burnout air and injected at 8S0·C at an N to NO molar ratio of 1.5. Figure 3 presents the results obtained with natural gas as the primary fuel. The natural gas was doped with NO to provide two levels of primary NOx' 600 and 240 ppm (dry, 0 percent O2), Twenty and ten percent reburning were applied separately in both cases. These data indicate that a hybrid process which utilizes 10 percent reburning fuel can achieve similar overall efficiency to a process using 20 percent reburning. It is apparent that there exists a tradeoff between natural gas premiums and the cost of ammonium sulfate. Pilot Scale verification A series of experiments were first carried out with natural gas as the primary fuel to define the optimum stoichiometry distribution for the advanced reburn i ng process at pi 1 ot scale. Measurements of CO and 02 concentrations were conducted at the ammonium sulfate injection location, approximately at 900·C. Results obtained indi.cated that in order to have an optimum CO level between 1500 and 2000 ppm (corresponding to approximately 0.7 percent 0.2) at the reagent inject ion 1 ocat ion, it was necessary to increase the rocal stoichiometry from 0.99, as defined in the bench scale studies, to 1.03 to account for the finite rate mixing and higher CO concentrat ions in the pi 1 ot scale furnace. Experi ments were subsequent 1 y carried out with an Indiana coal as the primary fuel. The Indiana coal produced an uncontrolled NOx emission of 800 ppm (dry, 0 percent O2) at 15 percent excess air. The prlmary NO at SRI = 1.13 was 680 ppm. Figure 4 presents the results and indicate t~at as seen in the bench scale studies, both advanced concepts were equally effect i ve in NOx reduct ions. Simi 1 ar results were also oetained with a Utah coal as the prlmary fuel (primary NOx = 850 ppm, Figure 5). Figure 5 summarizes all of the pilot scale results and compares with the bench scale data. The rat i os of (NO ) EX to (NO) are presented as a function of (NOx)o concentrations. It crnoe seen th~tpfor both conventional reburning and advanced reburning, the process efficiency depends on the (NOx) level, higher efficiency at higher (NOx)' Also, substantial scale effec~s were observed with conventional reburniRg. However, with advanced reburning the effect of scale was much less significant. Approximately ~ percent loss in ~ocess efficiency was observed when scaled up from 100 x 10 Btu/hr to 5 x 10 Btu/hr at a factor of 50 times. 2 |