1 71 st IEA-FBC, Seoul, Korea, Nov.4-6, 2015 N 2 O Decomposition by CaO under Conditions of Carbonator of Calcium Looping Cycle T. Shimizu,, S. Sizuno, K. Ito, R. Houshito, H.j. Kim, L.y. Li Niigata University, Japan
Principle of CaL process for CO 2 separation CaL process consists of a carbonator and a regenerator. In the cabonator, flue gas from air-blown combustor is introduced. In the regenerator, fuel (coal) is burned to supply heat to decompose CaCO 3 to CaO. CO2-free gas CO2, H2O CaO+CO2 CaCO3 CaO CaCO3 CaCO3 CaO+CO2 Flue gas (CO2 10-15%) Carbonator Fuel O2 CO2 Regenerator 2
Flue gas from air-blown FB combustor CaL process consists of a carbonator (absorber) and a regenerator. In the regenerator, fuel (coal) is burned to supply heat to decompose CaCO 3 to CaO. CO2-free gas CO2, H2O CaO CaO+CO2 CaCO3 Flue gas fr CaCO3 CaO+CO2 CaCO3 Flue gas (CO2 10-15%) Carbonator Fuel O2 CO2 Regenerator 3
transportation from regenerator to carbonator Coal combustion in regenerator. à Formation of char and transportation of char with CaO à Formation of CO and/or CO 2 CO2-free gas, CO CO2, H2O, NOx CO CaO CO2 CaCO3 CaO CaCO3 NOx Fuel-N Flue gas (CO2 10-15%) Carbonator Fuel O2 CO2 Regenerator 4
Possible roles of char in carbonator is expected to have different roles in carbonator: Oxidation of char-n : formation of N 2 O C+N 2 OàN 2 : N 2 O reduction { NOx, N2O Increase (?) Decrease CO2 Recirculated CO2 NOx, N2O N- O2 N2, N2O NOx, N2O CaO CaCO3 CO2 Volatile matter Flue gas (CO2, O2, NOx, N2O) Carbonator Coal O2 Regenerator 5
6 N 2 O formation from char oxidation/no reduction N 2 O emissions from char combustion / NO reduction by char were only slight (<10 ppm = 0.3% CO 2 -equivalent). N 2 O emissions from carb. [ppm] 20 15 10 5 0 MVB NO feed 196-200 ppm Without NO feed 0 5 10 15 20 25 Carb. fluidizing gas O 2 conc. [%] N 2 O emissions from carb. [ppm] 20 15 10 5 0 SA NO feed 193-205 ppm Without NO feed 0 5 10 15 20 25 Carb. fluidizing gas O 2 conc. [%] (Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015)
Reduction of fed N 2 O in carbonator For semi-anthracite, about 16% of N 2 O in fluidizing gas was reduced in carbonator. N 2 O in flue gas from carbonator [ppm] 300 250 200 150 100 50 Fuel: SA, O 2 in carb. =4% Slope = 0.84 0 0 100 200 300 N 2 O fed to carbontor [ppm] (Shimizu et al., 6 th HTSLC Meeting (Milan, Italy, 2015) 7
Expected N 2 O reduction by CaO in carbonator CaO is expected to catalyze N 2 O decomposition and reduction by CO which is formed by char oxidation. CO2 free gas NOx, N2O reduction CO2, H2O O2 N2 CO NOx, N2O CaO CaO CaCO3 VM CO2 CO2 gas recycle to dilute O2 Flue gas (CO2, O2,NOx, N2O) Fuel O2 CO2 Carbonator Regenerator 8
9 This work Basic study on N 2 O decomposition by CaO using quartz FB reactor N 2 O-containing gas was fed to a bubbling fluidized bed of CaO at 600 o C to evaluate N 2 O decomposition. Also batch feed of char to the CaO bed was conducted.
Experimental apparatus batch feed CaO bed N 2 O (480ppm), O 2 (8%), N 2 mixture Heater Filter GC Quartz FB ID 26mm Limestone (0.35-0.42 mm) calcined at 900 o C Static bed height H s =7 cm Temp. 600 o C U gas = 17cm/s (H s /U gas =0.4 s) 10
11 batch feed was prepared by heating at 900 o C in N 2. Batch feed was conducted. Analyses of char Ash (dry) Moist. (sample) C (daf) H(daf) N(daf) 23.3 2.0-3.5 95.8 0.6 1.2 VM content: 2-3% of combustible part. (TGA weight loss in N 2 between 200 600 o C)
12 Results and Discussion ØN 2 O decomposition by CaO without char Ø batch feed to CaO/inert bed
13 N 2 O decomposition without char feed Silica sand bed: nearly 0% conversion (no thermal decomposition at 600 o C) CaO bed: nearly 100% decomposition (480ppm at inlet à <1 ppm at outlet) kt = ln(in/out) > 6.2* k: first-order rate constant :[1/s] t: contact time [s] (*: This value includes bubble-emulsion mass transfer resistance)
14 Rate of N 2 O reduction by CaO In dry gas, rate expression is: k N2O (0) = 2.8x10 7 exp(-1.057x10 4 /T)=154 [1/s]. (Shimizu et al., Energy Fuels 2000, 14, 104-111) Gas-solid contact time: t = 0.2 s àk N2O t = 30 àcontribution to N 2 O reduction.
15 N 2 O decomposition by CaO in the presence of H 2 O In the presence of water, rate expression is: k1 k2 kn 2O = + 1 + K P 1+ K P H 2O k N2O (0)=k 1 +k 2, k 2 /k N2O (0) 2000, =0.0011T 14, 104-111) - 0.685 k N2O (0) = 2.8x10 7 exp(-1.057x10 4 /T) K 1 = 43exp(4.19x10 3 /T), K 2 =2.7 At 600 o C and and P H2O =0.1 atm, k N2O =34 s -1 à Contribution to N 2 O reduction is expected. 1 2 H 2O 0.1 0.2 Water vapor pressure [atm] (Shimizu et al., Energy Fuels T [K]= 1023 T [K]= 973 T [K]= 873
16 batch feed to CaO bed During the experiments, char batch feed (40 50 mg) was conducted. Formation of CO, CO 2 and change in N 2 O were observed. Under the present condition, however, the effect of char feed on N 2 O decomposition was not clear because sufficiently high N 2 O decomposition was observed without char feed.
Anticipated problem with char transportation Transportation of char to carbonator à Low temperature oxidation of char à Formation of CO CO2-free gas, CO CO2, H2O, NOx CO CaO CO2 CaCO3 CaO CaCO3 NOx Fuel-N Flue gas (CO2 10-15%) Carbonator Fuel O2 CO2 Regenerator 17
Formation of CO and CO 2 in carbonator (dual-fb) transportation to carbonator àco & CO 2 formation when sand was employed as bed material for dual-fb experiments. CO+CO 2 conc. in carbonator flue gas [%] 4 3 2 1 0 SA MVB 0 5 10 15 20 25 O 2 conc. in carbonator fluidizing gas [%] (Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015) 18
Previous results of CO emissions from carbonator When sand was employed as bed material, 20 40% of carbon oxidized in the carbonator was converted to CO. àco in gas = 1000 2000 ppm 1 1 CO/(CO+CO 2 ) In carbonator flue gas [-] 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 a)hvb O2 in carb: 21% O2 in carb: 10% O2 in carb: 4% 2 3 4 5 6 7 8 CO/(CO+CO 2 ) in carbonator flue gas [-] 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 c) SA O2 in carb: 21% O2 in carb: 15% O2 in carb: 10% O2 in carb: 4% 2 3 4 5 6 7 8 O 2 conc. in regenerator flue gas [%] O 2 conc. in regenerator flue gas [%] (Shimizu et al., J. Jpn. Inst. Energy, 94, 841-850, 2015) 19
20 Present results of CO formation from char Selectivity to CO was reduced from 0.45 for sand bed (similar to dual-fb results) to zero (<detection limit) for CaO bed. CaO bed can suppress CO emissions even when char is transported to carbonator. Selectivity to CO Sand bed CaO bed 0.45 0 CO conc. [ppm] 1600 1200 800 400 Sand bed, O 2 8%, 42.2 mg, Gas feed 1.7 NL/min CO CO2 Sand bed 0.2 0.15 0.1 0.05 CO 2 conc [%] 0 0 0 1000 2000 3000 Time [s]
21 Conclusions It is expected that carbonator of CaL process can work as N 2 O abatement reactor because CaO can work as N 2 O decomposition catalyst. particles are known to be transported from regenerator to carbonator and form CO there. The presence of CaO is expected to reduce CO emissions from carbonator.
22 Acknowledgements The authors express their thanks to the financial supports from The Iwatani Naoji Foundation.