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- Englisch (7)
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- Limiting oxygen concentration (3)
- Calculated adiabatic flame temperature (2)
- Lower explosion limit (2)
- Abschätzmethode (1)
- Adiabatic flame temperature (1)
- Brand- und Explosionsschutz (1)
- Chemische Sicherheitstechnik (1)
- Closed vessel combustion (1)
- Explosion (1)
- Explosion of gases (1)
- Flammability (1)
- Flammability limit (1)
- Flammable (1)
- Fuel (1)
- Hydrogen (1)
- Laminar burning velocity (LBV) (1)
- Methane (1)
- Minimum inert concentration (1)
- Natural gas (1)
- Sauerstoffgrenzkonzentration (1)
Organisationseinheit der BAM
The limiting values of fuel concentration in a flammable fuelair mixture are the LEL (lower explosion limit) and UEL (upper explosion limit). The addition of an inert component to fuel/air mixtures determines the increase of LEL and decrease of UEL, until these values finally merge at the inerting point. The maximum oxygen amount of a non-flammable fuelairinert mixture is the LOC (limiting oxygen concentration), an important safety characteristics.
The investigation of a comprehensive set of flammability data at elevated temperatures and ambient pressure taken from literature sources was made for systems containing nitrogen, carbon dioxide and water(vapour) as inert components, at 100°, 200° and 250 °C. The adiabatic flame temperatures at LEL (CAFTLEL) and LOC (CAFTLOC) were calculated by taking into account the dissociation of gases within the flame. A linear correlation of CAFTLOC versus CAFTLEL was empirically derived for the examined systems. The slope and intercept of the correlation are dependent on temperature and on nature of the inert gas. The correlation allows the development of a simple procedure for estimating LOC, when the LEL of fuelair and the equivalence ratio of the fuelairinert mixture at the inerting point are known.
Knowing the scarce information concerning the flammability of fuelairinert mixtures at temperatures higher than ambient, the proposed procedure brings about an useful tool for estimation of LOC.
The limiting oxygen concentration (LOC) of fuel / air / inert premixed gaseous systems are usually determined from measurements of explosion limits at progressive dilution with inert gas of fuel / air mixtures, which is a long and cumbersome procedure. An alternative procedure to evaluate LOC would be of great interest for all fields of activity involving the use of flammable mixtures, especially when less characterized fuels are used. The paper describes a new procedure (algorithm) meant to estimate the LOC of fuel / air / inert premixed systems, using the values of lower explosion limit (LEL) of the fuel / air mixture and the calculated adiabatic flame temperature (CAFT) both at LOC and LEL when nitrogen is used as an inert gas. It is based on an empirical correlation established between the CAFT computed for fuel / air / nitrogen mixtures at LOC and CAFT at LEL, for a large number of flammable gases and vapors. This requires only the measurement of LEL. The correlation was derived from flammability data taken from literature sources (German and American recommended values). The method is based upon the assumption that mixtures at LOC have an equivalence ratio 8 / 1.250, which is close to the equivalence ratio of the most reactive fuel / air systems.
Inverse calculations made with this new algorithm for nine fuel / air / nitrogen mixtures allowed the determination of LOC with a relative deviation of 2/ 22%.
The flammable hydrogen-blended methane–air and natural gas–air mixtures raise specific safety and environmental issues in the industry and transportation; therefore, their explosion characteristics such as the explosion limits, explosion pressures, and rates of pressure rise have significant importance from a safety point of view. At the same time, the laminar burning velocities are the most useful parameters for practical applications and in basic studies for the validation of reaction mechanisms and modeling turbulent combustion. In the present study, an experimental and numerical study of the effect of hydrogen addition on the laminar burning velocity (LBV) of methane–air and natural gas–air mixtures was conducted, using mixtures with equivalence ratios within 0.90 and 1.30 and various hydrogen fractions rH within 0.0 and 0.5. The experiments were performed in a 14 L spherical vessel with central ignition at ambient initial conditions. The LBVs were calculated from p(t) data, determined in accordance with EN 15967, by using only the early stage of flame propagation. The results show that hydrogen addition determines an increase in LBV for all examined binary flammable mixtures. The LBV variation versus the fraction of added hydrogen, rH, follows a linear trend only at moderate hydrogen fractions. The further increase in rH results in a stronger variation in LBV, as shown by both experimental and computed LBVs. Hydrogen addition significantly changes the thermal diffusivity of flammable CH4–air or NG–air mixtures, the rate of heat release, and the concentration of active radical species in the flame front and contribute, thus, to LBV variation.
The present paper aims to re-examine the validity of the linear correlation found between AFTLOC, the adiabatic flame temperature at the apex of the flammability range of fuel-air-inert mixtures (where LOC, the Limiting Oxygen Concentration, is measured) and AFTLFL, the adiabatic flame temperature at the lower flammability limit of fuel-air mixtures (LFL). New sets of experimental measurements of LFL and LOC referring to fuel-air mixtures diluted with N2, CO2 and H2O(vap) from trusted literature sources form a comprehensive database for such evaluation. Both the slope and intercept of correlations AFTLOC = a + b*AFTLFL are dependent on the nature of inert gas and on initial temperature. Based on the linear correlation between AFTLOC and AFTLFL, a procedure for calculation of LOC and MIC (Minimum Inert Concentration) of fuel-air-inert mixtures is presented, using measured or calculated LFL of fuel-air mixtures and their corresponding AFT. The method predicts with reasonable accuracy LOC and MIC of fuel-air-inert mixtures (relative deviations ranging between -14 and +17% when calculated and measured LOC and MIC for fuelair-nitrogen and fuel-air-carbon dioxide at ambient initial conditions are examined).