Non-Reacting Flow Characteristics and Emissions Reduction on Blends of Coal and Dairy Biomass in 30 kW_(t) Low NO_(x) Down-Fired Furnace
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Recently, coal-fired power plants have considered either to retire themselves or to use natural gas as the main energy source instead of coal due to more stringent air pollution regulations for nitrogen oxides (NO_(x)), mercury (Hg) and more recently the required CO_(2) reduction of 30% by 2030. Clean coal technology must be continuously developed in order to prevent people from losing their jobs and to decrease the negative impacts of firing coal on environment. The present research focuses on NO_(x) emissions which arise mainly due to oxidation of fuel-bound nitrogen using low NO_(x) burner (LNB) when fired with Wyoming Powder River Basin coal (PRB) and blends of coal and dairy biomass (DB). The DB was selected as co-fired fuel for possible elimination of DB from dairy feedlots which result in land, air and water pollution if not properly disposed of. LNB adopts staged air introduction in order to limit the availability of oxygen when nitrogen from fuel is released. To achieve the objective, the mixing patterns between fuel particle and air were predicted using non-reacting flow (NRF) simulation inside the cylindrical combustion chamber. The effects of varying burner parameters, fuel particle sizes, main burner equivalence ratios (ER_(mb)) and overall equivalence ratios (ER_(oa)) on mixing characteristics were investigated. Then, the LNB components were modified based on the results from NRF simulation. The modified main burner is a partially premixed swirl burner (fuel mixes with the primary air inside the fuel/primary air nozzle, and the secondary air is swirled by the straight-vane swirler) whose swirl angle and secondary air swirl number are 59? and 1.42 respectively. The circular over-fire air (OFA) nozzles are located 484 mm below the main burner exit, and the OFA is injected into the combustion chamber in the radial direction. The fuels used in the research were: 1) pure PRB and 2) the fuel blend of PRB and DB with the PRB-to-DB ratio of 90 to 10 on mass basis (90-10 PRB-DB blend). Fuel characteristics were first obtained, and empirical chemical formulae were deduced. The CO_(2), O_(2) and NO were measured as a function of ER_(oa) and ER_(mb) (ER_(mb) based on air flow without inclusion of OFA). The gas analyses were used to obtain the burnt fraction, respiratory quotient (RQ, = CO_(2) moles produced/O_(2) moles consumed) and equivalence ratio which is then checked against measured values. Uncertainty analyses were also performed. The optimum conditions for minimum NO_(x) emission that pass the EPA limit (210 g/GJ) were obtained as follows. With ER_(oa) = 0.95, firing pure PRB produced NO_(x) 220 g/GJ without OFA, and 179 g/GJ with OFA (ER_(mb) = 1.10) which is about 18.6 % reduction. Under same conditions, the co-firing of 90-10 PRB-DB blend decreased NO_(x) by 3.6% without OFA, and 22.2% with OFA (ER_(mb) = 1.10) compared to firing pure PRB at ER_(oa) = 0.95 without OFA. Furthermore, co-firing 90-10 PRB-DB blend with OFA at ER_(mb) = 1.10 and ER_(oa) = 0.95 (excess air 5.26%) emitted NO_(x) approximately 171 g/GJ whilst firing pure PRB without OFA at ER_(oa) = 0.85 (excess air 17.65%) emitted NO_(x) approximately 330 g/GJ which is 48% reduction and less than 210 g/GJ (the current EPA limit). This reduction could benefit 500-MWt power plants approximately $113,500 per year in case the efficiency of power plants is 35% and NO_(x) are traded at $15.89 per short ton.