Marcos Chaos

Syngas Combustion Kinetics and Applications

Strong interest in the use of coal-derived syngas in gas turbines has led to recent experimental studies that highlight the important features of H2/CO combustion at high pressures and relatively low temperatures. In the present study these investigations are reviewed, evaluated, and chemical kinetic updates based on these new results are discussed. Disparities observed between experimental measurements and kinetic model predictions of high pressure ignition delay and burning velocity are noted and the effect that surfaces, trace impurities, and contaminants may have on the H2/CO kinetic system are elucidated. In particular, the impurity coupling with NOx is discussed in relation to energy conversion processes involving hydrogen as a fuel component. An example of its importance to pre-ignition in reciprocating engine applications is also demonstrated.

SPONTANEOUS IGNITION OF PRESSURIZED RELEASES OF HYDROGEN AND NATURAL GAS INTO AIR

Abstract This paper demonstrates the “spontaneous ignition” (autoignition/inflammation and sustained diffusive combustion) from sudden compressed hydrogen releases that is not well documented in the present literature, for which little fundamental explanation, discussion or research foundation exists, and which is apparently not encompassed in recent formulations of safety codes and standards for piping, storage, and use of high pressure compressed gas systems handling hydrogen. Accidental or intended, rapid failure of a pressure boundary separating sufficiently compressed hydrogen from air can result in multi-dimensional transient flows involving shock formation, reflection, and interactions such that reactant mixtures are rapidly formed and achieve chemical ignition, inflammation, and transition to turbulent jet diffusive combustion, fed by the continuing discharge of hydrogen. Both experiments and simple transient shock theory along with chemical kinetic ignition calculations are used to support interpretation of observations and qualitatively identify controlling gas properties and geometrical parameters. Although the phenomenon is demonstrated for pressurized hydrogen burst disk failures with different internal flow geometries, similar phenomena apparently do not necessarily occur for sudden boundary failures of storage vessel or transmission piping into open air that have no downstream obstruction. However, subsequent reflection of the resulting transient shock from surrounding surfaces through mixing layers of hydrogen and air may have the potential for producing ignition and continuing combustion. Much more experimental and computational work is required to quantitatively determine the envelope of parameter combinations that mitigate or enhance spontaneous ignition characteristics of compressed hydrogen as a result of sudden release, particularly if hydrogen is to become a major energy carrier interfaced with consumer use. Similar considerations for compressed methane, for mixtures of light hydrocarbons and methane (simulating natural gas), and for larger carbon number hydrocarbons show similar autoignition phenomena may occur with highly compressed methane or natural gas, but are unlikely with higher carbon number cases, unless the compressed source and/or surrounding air is sufficiently pre-heated above ambient temperature. Spontaneous ignition of compressed hydrocarbon gases is also generally less likely, given the much lower turbulent blow-off velocity of hydrocarbons in comparison to that for hydrogen.

Combustion Characteristics of Materials and Generation of Fire Products

Hazards associated with fire are characterized by the generation of calorific energy and products, per unit of time, as a result of the chemical reactions of surfaces and material vapors with oxygen from air. Thermal hazards constitute those scenarios where the release of heat is of major concern. On the other hand, nonthermal hazards are characterized by fire products (smoke, toxic, corrosive, and odorous compounds.) Generation rates of heat and fire products (and their nature) are governed by (1) fire initiation (ignition); (2) fire propagation rate beyond the ignition zone; (3) fire ventilation; (4) external heat sources; (5) presence or absence of fire suppression/extinguishing agents; and (6) materials: (a) their shapes, sizes, and arrangements; (b) their chemical natures; (c) types of additives mixed in; and (d) presence of other materials. In this handbook most of these areas have been discussed from fundamental as well as applied views. For example, the mechanisms of thermal decomposition of polymers, which govern the generation rates of material vapors, are discussed in Chap. 7, generation rate of heat (or heat release rate) from the viewpoint of thermochemistry is discussed in Chap. 5, Flaming ignition of the mixture of material vapors and air is discussed in Chap. 21, and surface flame spread in Chap. 23.

Temperature measurements on solid surfaces in rack-storage fires using IR thermography

The development of fire modeling tools capable of predicting large-scale fire phenomena is of great value to the fire science community. To this end, FM Global has developed an open-source CFD fire simulation code, FireFOAM. The accuracy of this code relies fundamentally on high-quality experimental validation data. However, at larger scales, detailed measurements of local quantities (e.g., surface temperatures) needed for model validation are difficult to obtain. Often, the information obtained from large-scale fire tests is limited to the global heat release rates (HRR) or point temperature or heat flux measurements from embedded thermocouples or heat flux gauges, respectively. The present study addresses this limitation by introducing IR thermographic measurements in a three- and a five-tier-high rack storage scenario. IR temperatures are compared against modeled results. The tested and modeled cases represent realistic industrial warehouse fire scenarios. The rack-stored commodity consisted of corrugated paperboard boxes wrapped around a steel cubic liners, placed on top of a hardwood pallet. The global heat release rate was measured using a 20- MW fire products collector located inside FM Global’s Fire Technology Laboratory. An in-house calibrated microbolometer IR camera was used to obtain two-dimensional temperature measurements on the fuel surfaces and on the surfaces inside the flue spaces. Maximum temperatures up to 1200 K were observed on the external surfaces of the test array. Inside the flue spaces between pallet loads, temperatures up to 1400 K were measured. The modeled fire spread results match well fire spread shown in the IR thermographic images. The peak modeled surface temperatures obtained inside some of the horizontal flue spaces were ~1400K, which agreed well with the peak temperatures seen by the IR camera. The effect of the flames present between the surfaces of interest and the IR camera only contribute to about 50 K increase in measured temperature due to the limited flame emissive power with low soot concentration in the long-wave IR regime. This study shows the capability of IR cameras to obtain high resolution temperature measurements in large-scale fire scenarios, which enhances existing large-scale model validation data set.

Kinetic Modeling of the H2/O2 Reaction in High-Pressure Flames

An updated H2/O2 chemical-kinetic model based on that of Li et al. [Int. J. Chem. Kinet. 36 (2004) 566-575] is tested against a wide range of combustion targets that include the previous validation set from Li et al. as well as new measurements that have become available for speciation during H2 oxidation, H2O decomposition, and H2O2 decomposition; ignition delay times in shock tubes and rapid compression machines; and high-pressure and/or low-flame-temperature flame speeds. During the construction of the present model, we have attempted to identify major sources of uncertainties in the model that result in uncertainties in predictions of relevant combustion behavior in order to facilitate further model improvements in the future. Here, we present analyses that suggest that improved characterization of bath-gas mixture behavior in unimolecular decomposition reactions is likely to be necessary to predict ignition delay times and flame speeds accurately. A number of rate constant expressions from recent elementary reaction studies have been incorporated. Predictions using the present model adequately reproduce all the targets used for validation of Li et al. and yield significantly improved agreement with more recent targets that include high-pressure, dilute flames.

An Experimental Study of Complex Fuel Burning Behavior Using Characteristic Fuel Unit Approach

This experimental work studies the burning behavior of a representative complex fuel used for industrial fire testing. The selected fuel is the cartoned unexpended plastic (CUP) commodity, which is complex due to multiple combustible materials and intricate geometry that are beyond detailed treatment by current numerical modeling capabilities. The objectives of this work are to design an experimental method for characterizing the burning behavior of a complex fuel at scales represented by a characteristic fuel unit and to explore this behavior for the CUP commodity. A series of freeburn experiments were conducted to measure global quantities such as heat release rate and local thermal impact variables such as surface heat flux and gas temperature. A long-wave infrared camera was also used to reveal burning phenomena of the complex fuel, including corrugated cardboard delamination and plastic melting. The measurements are currently being used to develop a simple model describing the thermal degradation of the complex fuel with satisfactory results. The present work helps understand the unique burning behavior of complex fuels and provides data for numerical model development and validation.