The Clean Catalytic Combustion of Gasified Biomass

(after R.Burch and B.W.L.Southward, J.Catal. 195, 217 (2000))

Catalyst Characterisation
NH₃ Temperature Programmed Desorption

• Regenerable, low temperature, trapping / oxidation catalysts were investigated to facilitate the selective oxidation of NH₃.

• NH₃ TPD was examined to see if the acid / base characteristics of the fuel could provide specific adsorption / reaction of NH₃

(see Figure 1.)

Samples first purged in flowing Helium 150 °C for 1 hour, then dosed for 1 hour in a 1000ppm NH₃ in He flow at 150 °C

Sample re-purge at 150 °C for 30 min in He to remove physisorbed species

Ramped at 12 °C / min in He flow - evolved species monitored by HPR20

• PtCu has the highest NH₃ uptake during dosing

• PtCu had the largest desorption of NH₃ upon heating

• PtCu also exhibited the least oxidation of NH₃
i.e. minimal H₂O production.

• PtCu showed high NH3 uptake under ideal conditions, how was this affected by fuel components and temperature?

Samples purged as previously, then exposed to NH₃ under varying conditions:

a) Dosed at 100, 250, 300°C for 1 h / 1000 ppm NH₃ / He

b) Dosed at 100, 250, 300°C for 1 h / 1000 ppm NH₃ / 5.1%CO / 3.4%H₂ / He

Sample re-purge 30 min in He to remove physisorbed species

• Increasing Temperature favours NH₃ desorption

• High fuel concentrations increase competition for surface sites

• Hence both factors decrease total NH₃ 'uptake'

(see Figure 2.)

• Do the NH3 adsorption / desorption characteristics relate to its activity and selectivity in NH3 oxidation?

NH₃ TPD data generated as described previously.

NH₃ Oxidation activity and selectivity conditions:
60mg PtCu, 1000 ppm NH₃ / 6% O₂/ balance He, Total flow 300 ml/min, Gas Hourly Space Velocity (ml of gas /ml of catalyst /hour) 240,000 h^-1

• Excellent activity / selectivity (>97% N₂) over a wide T range

• 'Window' > previous eatalysts, corresponds to NH₃ desorption profile

• Indicates a link between presence of NH_xads and high N₂ yield

(see Figure 3.)

• How does the presence of the highly reactive fuel components in the reaction mixture affect the performance of the catalyst for NH3 oxidation?

PtCu, PtBa and Pt-Al₂O₃ catalysts
NH₃ Oxidation activity and selectivity
INITIAL conditions (0-180s): 60mg, 200 °C, / balance He, Total flow 300 ml/min, Gas Hourly Space Velocity 240,000 h^-1
Cycling conditions (after 180s): 30s / 30 s switch between:
1000 ppmNH3 / 5.1% CO/ 3.4% H₂/ 0.5% O₂ - FUEL RICH
1000 ppmNH3 / 5.1% CO/ 3.4% H₂/ 9.3% O₂ - FUEL LEAN

• Fuel components 'compete' for the surface - previous performance is lost

• Low N₂ due to combustion exotherm in the catalyst at steady state

• N₂ yield dramaticallv increased under cyclic operation (high / low O₂)

• Full Product Spectrum 60 mg PtCu: Conditions as previous.

At low O₂ / fuel rich phase NH3 is adsorbed on the CuO

At high O₂ / fuel lean phase Pt catalyses NH₃ -> NO

BUT

NO+NH₂ads -> N₂ + H₂O

Note the QIC provides real-time analyses of gases and WATER vapour

• Is it an exclusive reaction between NO and NH_xads?

• Examined by dosing catalyst, purging then expose to a 2nd stream

Treatment	PtCu Peak Integration.	PtBa Peak Integration.
CO/H2, purge, NO/He	m/z 28: 7.34E-O9	m/z 28: 6.44E-08 m/z 44: 2.85E-08
NH3, purge, NO/02	m/z 28: 3.84E-07 m/z 44: 1.68E-07	m/z 28: 1.88E-07 m/z 44: 9.50E-08
NO/02, purge, NH3	m/z 28: 2.25E-08 m/z 30: 5.89E-O9 m/z 44: 4.47E-O9	m/z 28: 1.37E-07 rnz 44: 2.07E-O9
NO/02, purge, H2	m/z 44: 3.12E-O9	m/z 28: 4.3 lE-08 m/z 30: 7.36E-10

Conditions:

Purge sample in He 30 min 200°C

Expose to first gas / mixture in He - 15 min

Purge 15 min in He

Expose to 2nd gas / mixture - monitor evolved gases

• High N₂ only seen for PtCu for NO / O₂ exposure post NH₃ adsorption

• Contrasts with PtBa - shows multiple N₂ production reactions.

Figure 1. NH₃ TPD from 1%Pt-AI₂O₃, 1%Pt 20%BaO-AI₂O₃ (PtBa), 1% Pt 20%CuO-AI₂O₃ (PtCu)

Figure 2. Ammonia uptake

Figure 3. Oxidation and TPD profiles

Figure 4. Fuel rich / fuel lean cycles

Figure 5. Full product spectrum

Figure 6. Portion of Fig. 5 in detail

Conclusions:

• The low temperature clean catalytic combustion of NH₃ - bearing biomass is possible using a Pt-based trapping / oxidation catalyst in cyclic operation.

• N₂ production occurs via an iSCR / internal Selective Catalytic Reduction mechanism in which NO_x formed on Pt is reduced by NHxads on the CuO.

• The HPR-20 was invaluable in this study enabling both the in-situ characterisation and catalyst activity evaluation.

• The HPR-20 facilitated with its internal cryopanel option which enabled the minimisation of background water, essential for quantitative NH3 determination (due to m/e 17 overlap), and the combination of QIC and rapid data acquisition / analysis of complex reaction product streams to give real time results comparable to chemiluminescence.