Soot inception limits in laminar diffusion flames with application to oxy-fuel combustion

Journal Article · Fri Aug 15 00:00:00 EDT 2008 · Combustion and Flame

DOI:https://doi.org/10.1016/J.COMBUSTFLAME.2008.03.008· OSTI ID:21081132

Kumfer, B M; Skeen, S A; Axelbaum, R L ^[1]

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1. Department of Energy, Environment, and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130 (United States)

For diffusion flames, the combination of oxygen enrichment and fuel dilution results in an increase in the stoichiometric mixture fraction, Z{sub st}, and alters the flame structure, i.e., the relationship between the local temperature and the local gas composition. Increasing Z{sub st} has been shown to result in the reduction or even elimination of soot. In the present work, the effects of variable Z{sub st} on soot inception are investigated in normal and inverse coflow flames, using ethylene as the fuel. Use of the inverse coflow flame underscores the validity of these concepts, since the convective field in the inverse flame directs particles into the fuel-rich region. Sooting limits based on particle luminosity are measured as a function of Z{sub st}. The sooting limit is obtained by varying the amount of inert gas until soot appears above a predefined height. For each limit flame, the adiabatic flame temperature is calculated and the flame temperature at the half-height is measured. The flame temperature at the sooting limit is found to increase with Z{sub st} for both normal and inverse flames. The effects of residence time are also investigated, and the sooting limit inception temperature is found to be dependent on fuel stream velocity for both the normal and inverse configurations. A simple model applicable to oxy-fuel combustion is presented which describes how increasing Z{sub st} results in the reduction and ultimately elimination of soot. This model assumes that soot inception can only occur in a region where critical values for species, temperature, and residence time are met. The soot inception region is shown to be bounded by two isotherms: a low-temperature boundary that is a function of residence time, and a high-temperature boundary that corresponds to the location of a critical local carbon-to-oxygen ratio. The effect of increasing Z{sub st} is to move the boundaries of the soot inception zone towards each other, until the zone is infinitely thin and thus the sooting limit is reached. By comparing the model to experimental data, a critical local C/O ratio of 0.53 and a sooting limit inception temperature of 1640 K (for a characteristic residence time of 22 ms) were determined for ethylene. (author)

• OSTI ID: 21081132
• Journal Information: Combustion and Flame, Journal Name: Combustion and Flame Journal Issue: 3 Vol. 154; ISSN CBFMAO; ISSN 0010-2180
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
CARBON
COMBUSTION
COMBUSTION PROPERTIES
Coflow flames
DIFFUSION
DILUTION
ETHYLENE
EXPERIMENTAL DATA
FUELS
HEIGHT
ISOTHERMS
Inverse flames
LAMINAR FLAMES
LIMITING VALUES
LUMINOSITY
MIXTURES
OXYFUEL COMBUSTION PROCESS
OXYGEN
OXYGEN ENRICHMENT
SIMULATION
SOOT
STOICHIOMETRY
SYNTHESIS
Sooting limits
TEMPERATURE RANGE 1000-4000 K
VELOCITY
ZONES