03.08.2015   Technologies for Converting Biomass to Useful Energy

Fuel nitrogen conversion efficiency

(c/n) * Xno XCO2 + XCO

Подпись: NCONV Подпись: (3.37)

During combustion, only a fraction of fuel N is converted into NOX (about 20% to 30% in sub­bituminous coal (Bowman, 1991, 2001)), while the remaining fuel N is released as N2 with the flue gases. The reaction of N with oxygen is inhibited by carbon radicals bonding with available oxygen to form CO and CO2. The nitrogen conversion efficiency is defined as the amount of fuel nitrogen that gets converted to NOX. Annamalai and Puri (2007) showed that overall fuel nitrogen conversion efficiency can be approximated by:

where c/n is the atom ratio of the empirical carbon and nitrogen respectively, XNO is the mole fraction of NOX, XCO2 is the mole fraction of CO2, and XCO is the mole fraction of CO. All gases were measured in the exhaust stream. Note that the equation assumes that all NOX originates from fuel nitrogen and hence it presents an upper bound on fuel nitrogen conversion efficiency. In addition, when fuel nitrogen conversion efficiency is very low, it means that most fuel-bound nitrogen is converted to something other than NOX.

Note that as the equivalence ratio increased, less nitrogen was converted to NOX. In the extremely rich region, the conversion efficiency was nearly 0%. The largest decrease in con­version occurred when the flame went from stoichiometric to rich. Figure 3.35 presents the fuel nitrogen conversion efficiency forTXL andTXL:DB blended fuels. Also note that in general, the

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Figure 3.32. Effect of fuel onNOj forWYO and WYO:DB blended fuels. Note howNOx decreases in the near lean region for blended fuels (adopted from Lawrence, 2007).

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Figure 3.33. Effect of fuel on NOX for WYO and WYO:DB blended fuels corrected to 3% O2 (adopted from Lawrence, 2007).

♦WYO ■ 95-5 WYO lA-PC-DB-S«pS *90-10 WYO:LA-PC-Oe-S«(>S • 80-20 WYO LA-PC-Oe-S*pS

Effect of Fuel on NO, for WYO and WYO:OB Blended Fuels

 
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Figure 3.35. Effect of fuel on nitrogen conversion efficiency for TXL and TXL:DB blended fuels. Note that the conversion efficiency is less than coal for almost all TXL:DB blended fuels (adopted from Lawrence, 2007).

 

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Figure 3.36. Effect of reactor length on NO emissions (adopted from Arumugam et al., 2005). Modified reactor: length is longer.

DB blended fuels converted less nitrogen to NOX. The same trend was observed with WYO and WYO:DB blended fuels.

As with NOX reduction for AgB and coal blends, (section, 3.4.1), it is hypothesized that the increased volatile matter of AnB under co-firing conditions resulted in the increase of fuel N loading, greater depletion of O2 and sometimes even in reduction in NOX (Arumugam et al.,

2005) . However, the increased fuel N inAnB produces more NH3 due to urea type N in AnB. Thus, more NH3 type species are produced in blend coal:AnB fuels. Under appropriate temperature and oxygen concentration, the NOX reduction due to NH3 + NOX reactions may dominate the reduction process of NOX.