# Presentation: 500067

## 		Effects of Mixing	on	Ammonia Oxidation	in	Combustion Environments	at	Intermediate Temperatures

- Joseph Grcar1  Peter Glarborg2  John Bell1
- Marcus Day1  Antonio Loren2  Anker Jensen2

- 1 Center for Computational Science and Engineering
-   Lawrence Berkeley National Laboratory
-   Berkeley, CA 94720 USA

- 2 Department of Chemical Engineering
-   Technical University of Denmark
-   2800 Lyngby, Denmark

## Outline

- _Outline_

- 		Flame Background
- 		Experiment
- 	Plug Flow	Predictions
- 	DNS	Predictions
- 	 Zwieterung	Predictions
- 		Conclusions

## Biomass releases nitrogen compounds as:
Ammonia
Hydrogen cyanide
Fuel-bound nitrogen oxidizes to N2 or NO
NO selectivity varies with flame type:

Sarofim et al., AIChE Symp. Series (1975)
Sullivan et al., Combust. Flame (2002)

- Biomass releases nitrogen compounds as:
- Ammonia
- Hydrogen cyanide
- Fuel-bound nitrogen oxidizes to N2 or NO
- NO selectivity varies with flame type:
- Sarofim et al., *AIChE Symp. Series* (1975)
- Sullivan et al., *Combust. Flame* (2002)

- Ammonia in Flames

- Diffusion flames: 	Selectivity declines 	with more Ammonia
- Premixed flames:	Linear response

## Eg: Methane Diffusion Flame

- 	Flame:	Sullivan et al., *Combust. Flame* (2002)
- 	Mechanism:	Glarborg et al., *Combust. Flame* (1998)
- 	Fluid Dynamics:	Day and Bell, *Combust. Theory Modelling* (2000)

_**Fuel Ammonia**_
**1****st**** 500 ppm NH****3**** **
****
**≈ ****200**** ppm NO**
**2****nd**** 500 ppm NH****3**
****
**≈ ****75**** ppm NO**

## NH3NO selectivity depends on 2 effects:
Reactions between 2 nitrogen-bearing species,

	So more NH3 lessens NH3NO selectivity
Transport of NO to the rich side of the flame

- NH3NO selectivity depends on 2 effects:
- Reactions between 2 nitrogen-bearing species,
- 	So more NH3 lessens NH3NO selectivity
- Transport of NO to the rich side of the flame

- Selectivity in Diffusion Flames

- 		N + NO	 N2 + O
- 		NH + NO	 N2O + H
- 			 N2 + OH
- 		NH2 + NO	 N2 + H2O
- 			 NNH + OH

## Flame cycling of NO’s N atoms

- (left) Two-step process:
- NO forms on the lean side
- Some NO diffuses to the rich side and forms either N2 or HCN.  The HCN then moves back to the lean side and forms either N2 or NO ...
- Sullivan et al., *Combust. Flame* (2002)
- (right) Stochastic particles track N atoms that form NO
- Black point: create only
- Color point: first of 1 or more destruction cycles
**RED  = 1 cycle**
**PURPLE  > 10 cycles**
- Bell, et al., *JCP *(to appear)

- X(NO)

- (NO)

- particle events

## Outline

- _Outline_

- 		Flame Background
- 		Experiment
- 	Plug Flow	Predictions
- 	DNS	Predictions
- 	 Zwieterung	Predictions
- 		Conclusions

## Isothermal Reactor

- Characterized by:
- Temperature set point (900-1500 K)
- Fuel : Oxidizer flow ratio (1 : 3,  1 : 1,  3 : 1)

| L / m | Fuel | Oxidizer |
| --- | --- | --- |
| CH4 | 1.0e-3 |  |
| NH3 | 3.0e-4 |  |
| O2 |  | 4.0e-2 |
| H2O |  | 2.0e-2 |
| N2 | balance | balance |

- Always same total flow
- 	(2.0 L / m in standard T & P)
- Always same reactant dosage
- N2 apportioned to choose the flow ratio

## Observations

- Below 1300 K
- 	NO independent of flow ratio
- 	(50 ppm variation)
- 1300-1400 K
- 	Qualitative change in behavior
- Above 1400 K
- 	NO sensitive to flow ratio

## Outline

- 		Flame Background
- 		Experiment
- 	Plug Flow	Predictions
- 	DNS	Predictions
- 	 Zwieterung	Predictions
- 		Conclusions

- _Outline_

## Mechanism of Glarborg, Alzueta, Dam-Johansen, and Miller, Combust. Flame (1998)
Chemkin’s SENKIN by Lutz, et al. (1987)
Premixed inflow so no flow ratio dependence (residence time is 1.274 s per 1000 K)
“Good” agreement with experiment up to 1300 K
60 ppm variation at highest experimental flow ratio

- Chemkin’s SENKIN by Lutz, et al. (1987)
- Premixed inflow so no flow ratio dependence (residence time is 1.274 s per 1000 K)
- “Good” agreement with experiment up to 1300 K
- 60 ppm variation at highest experimental flow ratio

- Plug Flow Model

- Conclude: reactor has premixed reaction zone below 1300 K.

## Outline

- 		Flame Background
- 		Experiment
- 	Plug Flow	Predictions
- 	DNS	Predictions
- 	 Zwieterung	Predictions
- 		Conclusions

- _Outline_

## 2-D Calculations

- Models:
- 65 species, 447 reaction methane-nitrogen  	(Glarborg, et al., 1998)
- CHEMKIN kinetics and transport 	(Kee, et al. 1983, 1986)
- Low Mach number fluid dynamics	(Day, Bell, 2000)
- Setup:
- R-Z coordinates 	(1.25 cm radius, 25.0 cm length)
- 193  resolution in reaction zone 	(adaptive mesh)
- Evolve to steady state 	(1.5 to 2.0 model seconds)

| NO ppm | 1200 K | 1300 K | 1500 K |
| --- | --- | --- | --- |
| 1:3 F/O | 76 | 81 | 85 |
| 1:1 F/O |  |  | 122 |
| 3:1 F/O |  | 125 | 151 |

## 2-D Calculations

- Models:
- 65 species, 447 reaction methane-nitrogen  	(Glarborg, et al., 1998)
- CHEMKIN kinetics and transport 	(Kee, et al. 1983, 1986)
- Low Mach number fluid dynamics	(Day, Bell, 2000)
- Setup:
- R-Z coordinates 	(1.25 cm radius, 25.0 cm length)
- 193  resolution in reaction zone 	(adaptive mesh)
- Evolve to steady state 	(1.5 to 2.0 model seconds)

| NO ppm | 1200 K | 1300 K | 1500 K |
| --- | --- | --- | --- |
| 1:3 F/O | 76 | 81 | 85 |
| 1:1 F/O |  |  | 122 |
| 3:1 F/O |  | 125 | 151 |

## 2 Types of Reaction Zones

- 1200 K

- 1300 K

- 1:3 F/O Flow Ratio

## 2 Types of Reaction Zones, cont.

- 1200 K

- 1300 K

- Note these reaction zones are not self-sustaining flames.

- Low temperatures
- Thick, parabolic shape extends across reactor tube
- Occurs after fuel and oxidizer streams are well mixed
- 	(for the 1:3 fuel-oxidizer flow ratio, about 33% of centerline fluid is oxidizer at 3 cm)
**Premixed reaction zone**
- Higher temperatures
- Thin, cone shape is anchored at lip of inner, fuel tube
- Occurs at stoichiometric surface
**Non-premixed reaction zone**

## DNS Predictions

- At extremes of the temperature range, the predictions are “good”
- match at 1200 K
- within 20% of the experiment at 1500 K
- At bifurcation points, the reactor and the model can be out of synch
- the NO predictions are bad
- several possible reasons ...

- Conclude: reactor has nonpremixed reaction zone above 1400 K.

## Nonpremixed Reaction Zone

- Predictions are within 20% of measurements when both reactor and model have nonpremixed reaction zones
- At higher fuel : oxidizer flow ratios, the nitrogen chemistry moves toward the lean side of the reaction zone

- 1500 K

## Shift to Lean-Side Reactions

- Most of the change is caused by
**NH****2**** + O ******** HNO + H **
- which is enhanced in the forward direction when NH2 moves to the lean side of the reaction zone.

- Subtract Reaction Paths

## Outline

- 		Flame Background
- 		Experiment
- 	Plug Flow	Predictions
- 	DNS	Predictions
- 	 Zwieterung	Predictions
- 		Conclusions

- _Outline_

##  Zwieterung Model

- Used to described mixing effects in reactors
- Assume exponential entrainment of the oxidizer stream into the fuel stream over a mixing time 
- Here,  is determined from the 2-D simulation as the time to obtain 95% mixing of the two streams on the centerline of the reactor
- Implemented using Chemkin’s SENKIN
-  Zwieterung, *Chem. Eng. Sci.* (1959)
- Alzueta, et al., *Energy Fuels* (1998)
- Røjel, et al., *Ind. Eng. Chem. Res.* (2000)

##  Zwieterung Model

- Very good agreement with experiment for the 1:1 and 3:1 F/O flow ratios

- Conclude: the mixing model captures surprisingly well the experimental trends.

## Outline

- 		Flame Background
- 		Experiment
- 	Plug Flow	Predictions
- 	DNS	Predictions
- 	 Zwieterung	Predictions
- 		Conclusions

- _Outline_

## Conclusions

- Laminar flow reactor bifurcates between stable premixed and stable nonpremixed reaction zones
- Bifurcations determined by temperature
- Nonpremixed reaction zone moderates NO production from NH3
- As in flames, NO production at intermediate temperatures is sensitive to mixing

- Inexpensive 2-D DNS simulations can be used to visualize complex mixing in laminar flow reactors
- All calculations were done on desktop personal computers
- Entrainment models can be constructed to correctly predict reactor behavior

## Conclusions

- Laminar flow reactor bifurcates between stable premixed and stable nonpremixed reaction zones
- Bifurcations determined by temperature
- Nonpremixed reaction zone moderates NO production from NH3
- As in flames, NO production at intermediate temperatures is sensitive to mixing

- Inexpensive 2-D DNS simulations can be used to visualize complex mixing in laminar flow reactors
- All calculations were done on desktop personal computers
- Entrainment models can be constructed to correctly predict reactor behavior

- Contact author regarding collaborative use of the 2-D DNS software.