Stanford University Thermosciences

Flow Reactor Study of Controlled Combustion Kinetics

GCEP Global Climate and Energy Project

C. T. BowmanK. Walters

Mechanical EngineeringDepartment

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Outline

• Background

- Motivation

- Objectives

• Approach

• Experimental Setup

• Results

• Future work

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Why Controlled Combustion?

• In conventional combustion devices, the chemical conversion offuel and oxidizer to products occurs rapidly in an uncontrolledand highly irreversible process (flame) and prior to work extraction,leading to a loss in cycle efficiency.

10

20

30

40

50

0 5 10 15 20 25Cycle Pressure Ratio

y

Second Law Cycle Efficiency - %

Fuel Exergy Loss in Combustion - %

Simple Brayton Cycle

T04 = 1200K

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Why Controlled Combustion?

• Modification of work producing cycles to match work extractionand heat release rates can lead to increased cycle efficiencyand, hence, reduced greenhouse gas emissions.

Cycle OPR nth Gain

Simple 20 41.2%

Reheat 92.6 48.0%13.8%

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What is Controlled Combustion?

• In controlled combustion, the rateof the fuel conversion process isvaried by imposing prescribedconditions (temperature and massfractions of the oxidizer/diluents),leading to potential reductions inirreversibilities in energy conversion(improved efficiency) and reducedemissions of pollutants andgreenhouse gases.

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Controlled Combustion

The Challenge to Implementation:

• A new regime of combustion that is poorly understood at thefundamental level needed for design optimization, especiallyfor high-pressure combustion systems, such as gas turbinesand diesel engines (HCCI).

The Objective of this Study:

• To investigate the combustion mechanisms of fuels in theintermediate preheat temperature range (1000-1300K) forpressures up to 50 bar with dilution by inert and chemicallyactive species.

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Controlled Combustion – Expected Outcomes

• Understanding of the effects of inert and chemically activeadditives on combustion rates at intermediate temperaturesand high pressures.

• Detailed and reduced mechanisms for controlled flamelessoxidation of model fuels for use in modeling and designinglow-irreversibility combustion engines, including HCCI andmulti-stage turbine burners.

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Range of Conditions Investigated

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Approach

• Flow Reactor Experiments– Measure the spatial evolution of temperature and composition– Experimental parameters:

• Temperature• Pressure• Oxygen concentration• Fuel concentration• Fuel composition: C2H6, C2H6/CH4 mixtures, surrogates, and

oxygenated fuels• Bath gas composition

• Compare the experimental data to the model results using adetailed reaction mechanism

• Optimize and validate the reaction mechanism• Model reduction by principal component analysis

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High-Pressure Flow Reactor

Gas Analyzers

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High-Pressure Flow Reactor

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Modeling Approach

• Initial studies have been conducted using CH4, C2H6and CH4-C2H6 mixtures to simulate natural gas.

• The starting reaction mechanism is GRI-Mech 3.0.

• The Chemkin and Senkin computer codes are used tomodel the reaction progress and to conduct sensitivityand reaction path analysis.

• Future studies will focus on surrogate andoxygenated fuels.

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1500 ppm C2H6: 0.5% O2

P = 1 bar

Experimental and Model Results

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1500 ppm C2H6: 3.0% O2

T0 = 1170K P = 1 bar

Experimental and Model Results

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Sensitivity Analysis Results

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1050 ppm C2H6: 0.5% O2

T0 = 1250K P = 2 bar

Experimental and Model Results

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Sensitivity Analysis Results

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Sensitivity Analysis Results

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Experimental and Model Results

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Conclusions

• Combustion times can be varied from 10 ms – 100 msec by varying initial O2concentration and diluents.

• For the fuels considered, combustion times increase with increasing pressure.

• Existing detailed reaction mechanisms capture some of the featuresof combustion of methane and ethane in the controlled combustionregime.

• The model consistently under predicts the initial rate of CO formation.

• In the controlled combustion regime, RO2 chemistry becomes increasingly important as the initial O2 concentration decreases and pressure increases.

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Future Work

• Additional experiments– Vary fuel composition and concentration– Vary O2 concentration– Vary pressure

• Measurements of stable intermediates– Aldehydes (CH2O, CH3CHO)– Alcohols (CH3OH, C2H5OH, C3H3OH )

• Update the mechanism• Mechanism reduction