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Show sign heat input rate of 6-million Btu/hr. This facility simulates furnace/convective pass gas temperature profiles and residence times, NOx levels, cyclone slagging potential, ash retention within the resulting slag, unburned carbon, and fly ash particle size of typical full-scale cyclone units. A comparison of baseline conditions of these units is shown in Table 1. STACK 'UHf AnA DEPOSITION - PROlE SECONDAAY AlA " ,. , • ~ } , , _ SLAG COLLECTOR ; / AND FURNACE WATER SEAL Rgure 2. Small Boller Simulator (SBS) Facility Tabl81 COMPARISON OF BASELINE CONDrTlONS FOR THE SBS FACILITY AND COMMERCIAL UNITS Typic.' CyclOM • SBS Boll.,. Cyclone Temperature >3000·F >3000·F Residence Time 1.4 seconds" 0.7 - 2 seconds Furnace Exit Gas Temperature 2265·F 2200· - 23SO·F NO. level 900 - 1200 ppm 600 - 1400 ppm Ash Retention 80-85% 60- 80% Unburned Carbon <1% in ash 1-20% Ash Particle Size (MMD; Bahco) 6 - 8 microns 6 - 11 microns • At full load Two reburning burners were installed on the SBS furnace rear wall above the cyclone furnace. Each burner consists of two zones with the outer zone housing a set of spin vanes while the inner zone contains the reburning fuel injector. Air and flue gas recirculation (FOR) can be introduced through the outer wne. Overfue air (OFA) ports are located on both the front and rear walls of the SBS at three elevations, with each elevation containing two ports. Two air-cooled deposition probes and simulated commercial sootblowers are available in the convective section (simulating secondary superheater and reheater tubes) in order to allow fouling (deposition) studies to be performed. J Pilot-Scale Results B&W's 6-million Btulhr small boiler simulator (SBS) was used to duplicate the operating practices of Nelson Dewey Unit No.2 (such as excess air, combustion air temperature, and boiler residence time). Baseline and coal reburning tests were performed using the Nelson Dewey demonstration coal (Lamar - a medium bituminous, 1.8% sulfur coal from Indiana). Rebum coal fmeness was varied from 63 to 90% through 200 mesh. During the reburning tests, the cyclone was firing at 10% excess air and at a coal flow rate equivalent to 66 to 80% of the total heat input The remaining 20 to 34% of the heat input was introduced through reburning burners under sub stoichiometric conditions to obtain reburn zone air/fuel stoichiometries of 0.86 to 0.95. The balance of air was introduced through OFA ports to achieve an overall stoichiometry of 1.15 to 1.2. NOx emissions and potential side effects were evaluated and are explained below. NO][ Emissions. Figure 3 shows NOx emissions as a function of the reburn zone stoichiometry and reburn coal fineness at 6-million Btu/hr. During all reburning tests, the cyclone stoichiometry was held constant at 10% excess air in order to minimize impact on cyclone slagging and corrosion. The baseline NOx level was 1025 ppm at 3% excess oxygen (furnace stoichiometry of l.16). As expected, the NOx concentrations decrease with decreasing reburn zone stoichiometry. A 49 to 73% NOx reduction was achieved when varying the reburn zone stoichiometry from 0.95 to 0.86. NOx levels were insensitive to reburn coal fineness, despite its wide variation (63, 78 , and 90% through 200 mesh). Similar NOx reductions were also achieved at 75% boiler load. When FOR was added into the reburning burner secondary air stream, the NOx reduction slightly improved. However, FGR can be utilized more effectively in larger utility boiler retrofits to enhance mixing between reburn fuel and combustion flue gases. Combustible Loss. Unburned carbon and CO emissions were measured during the baseline and rebuming phases. An inherent characteristic of cyclone furnaces is that combustion occurs mainly inside the cyclone furnace. Since cyclones will continue to be operated in an excess air mode, their combustion characteristics will not be altered. However, the amount of unburned char that does not burn within the cyclone will now enter a reducing environment in the reburning zone, with introduction of the remaining combustion air delayed until the OFA ports. During coal reburning, unburned carbon may increase since the reburning fuel is introduced into an oxygen deficient environment Although the reburn coal devolatilizes and partially burns, complete burnout will be delayed until the burnout zone. If FOR is introduced, unburned combustible levels may also increase since the residence time in the burnout zone decreases due to increased mass loading through the furnace and the associated lower gas temperature profile within the reburn wne region. Efficient |