Isaac A. Zlochower

Flammability Of Methane, Propane, And Hydrogen Gases

This paper reports the results of flammability studies for methane, propane, hydrogen, and deuterium gases in air conducted by the Pittsburgh Research Laboratory. Knowledge of the explosion hazards of these gases is important to the coal mining industry and to other industries that produce or use flammable gases. The experimental research was conducted in 20 L and 120 L closed explosion chambers under both quiescent and turbulent conditions, using both electric spark and pyrotechnic ignition sources. The data reported here generally confirm the data of previous investigators, but they are more comprehensive than those reported previously. The results illustrate the complications associated with buoyancy, turbulence, selective diffusion, and ignitor strength versus chamber size. Although the lower flammable limits (LFLs) are well defined for methane (CH4) and propane (C3H8), the LFLs for hydrogen (H2) and its heavier isotope deuterium (D2) are much more dependent on the limit criterion chosen. A similar behavior is observed for the upper flammable limit of propane. The data presented include lower and upper flammable limits, maximum pressures, and maximum rates of pressure rise. The rates of pressure rise, even when normalized by the cube root of the chamber volume (V1/3), are shown to be sensitive to chamber size.

Flammability Of Gas Mixtures Containing Volatile Organic Compounds And Hydrogen

Abstract An experimental program was conducted to evaluate the accuracy of some current methods for predicting the flammability of gas mixtures containing hydrogen and flammable or nonflammable volatile organic compounds (VOCs) in air. The specific VOCs tested were toluene, 1,2-dichloroethane, 2-butanone, and carbon tetrachloride. The lower flammability limits (LFLs) of gas mixtures containing equal molar quantities of the components were determined in a 19.4-l laboratory flammability chamber using a strong spark ignition source and a pressure criterion for flammability. All but one of the LFL values for the individual components were in agreement with earlier literature values. However, the LFL of 1,2-dichloroethane was found to be significantly lower than the range of values reported for previous determinations in smaller chambers. Two methods for calculating the LFL of mixtures were considered. The Group Factor (atomic) Contribution Method was determined to be generally more accurate than the LeChatelier Method for estimating the LFL of the gas mixtures reported here, although the LeChatelier Method was usually more conservative. The Group Factor Method predicted higher values (nonconservative) for the LFLs of several mixtures than were experimentally measured. For the case of a mixture of hydrogen and carbon tetrachloride, the Group Method estimation of the LFL was seriously in error.

Experimental Mine And Laboratory Dust Explosion Research At NIOSH

Abstract This paper describes dust explosion research conducted in an experimental mine and in a 20-L laboratory chamber at the Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational Safety and Health (NIOSH). The primary purpose of this research is to improve safety in mining, but the data are also useful to other industries that manufacture, process, or use combustible dusts. Explosion characteristics such as the minimum explosible concentration and the rock dust inerting requirements were measured for various combustible dusts from the mining industries. These dusts included bituminous coals, gilsonite, oil shales, and sulfide ores. The full-scale tests were conducted in the Lake Lynn experimental mine of NIOSH. The mine tests were initiated by a methane–air explosion at the face (closed end) that both entrained and ignited the dust. The laboratory-scale tests were conducted in the 20-L chamber using ignitors of various energies. One purpose of the laboratory and mine comparison is to determine the conditions under which the laboratory tests best simulate the full-scale tests. The results of this research showed relatively good agreement between the laboratory and the large-scale tests in determining explosion limits. Full-scale experiments in the experimental mine were also conducted to evaluate the explosion resistance characteristics of seals that are used to separate non-ventilated, inactive workings from active workings of a mine. Results of these explosion tests show significant increases in explosion overpressure due to added coal dust and indications of pressure piling.

Devolatilization wave structures and temperatures for the pyrolysis of polymethylmethacrylate, ammonium perchlorate, and coal at combustion level heat fluxes

Abstract Data are presented for the surface pyrolysis temperatures, T s , and for the develatilization mass loss rate per unit area, ṁ, for polymethylmethacrylate (PMMA) exposed to a laser beam at incident flux levels of 15–500 W/cm 2 . After a brief induction time during which the surface is being heated to T s , the measured ṁ values are thereafter steady state in time, and are linearly related to the net absorbed flux, I abs . The measured values for ṁ are in good agreement with that derived from the first law of thermodynamics: m = I abs ∫ T s T o C(T) dT+δ H v (T s ) −1 Kinetically, this equation represents a devolatilization wave front whose propagation rate and maximum temperature are both heat transport controlled, and determined by the magnitude of the driving flux, I abs . The available data for ammonium perchlorate and for coal are also shown to obey a similar relationship, and the same heat transport-controlled process appears to govern their pyrolysis and devolatilization kinetics. At high fluxes, the measured ṁ and T s values reported here for PMMA, and those reported by other investigators, can be interrelated to one another through an Arrhenius plot whose slope gives an “activation energy” that is essentially equal to the heat of vaporization, ΔH v ( T s ). That measured relationship between ṁ and T s is thus a reflection of the Clausius-Clapeyron equation for the equilibrium vapor flux emanating from the polymer melt at its surface decomposition temperature, T s . However, at low fluxes, the measured ṁ values are significantly lower than those predicted by the Clausius-Clapeyron line. The departure is attributed to the additional presence of mass transport limitations. Photographic data are presented for bubble structure and bubble location within the quenched PMMA samples that independently confirm the increasing significance of mass transport limitations at the lower fluxes.

Flammability limit measurements for dusts and gases: Ignition energy requirements and pressure dependences

Data are presented for the flammability limits of Pittsburgh Seam bituminous coal dust, polyethylene powder, and methane in air at pressures in the range 0.5 to 3 bar. Explosibility test chambers of 20 and 120 L were used, and ignitability limitations were overcome with efficient pyrotechnic ignitors with nominal energies of 500 to 5,000 J. The propagation criterion used was based on the maximum explosion pressure and the size-normalized maximum rate of pressure rise. The latter is a dynamic criterion that tends to minimize “overdriving” effects as the 20-L data are taken to their asymptotic limits at high ignition energies. The measured lean limits in air at atmospheric pressure are 90 g/m 3 for the coal dust, 35 g/m 3 for polyethylene, and 4.9 vol pct for methane. The rich limit for methane is 18–19 vol pct, whereas the dusts have no rich limits out to concentrations as high as 4,000 g/m 3 . A linear, lean-limit pressure dependence was measured for the dusts which was essentially the same as the pressure dependence measured for methane when all are expressed in comparable units: namely, mass concentration of fuel per unit volume of air. This observation further confirms a lean limit dust flame propagation mechanism that is controlled by the gas-phase reaction rate. The limit concentrations are then determined by the dust loading required to generate a lean limit concentration of pyrolysis products in the volatiles-air mixture, and the dust behaves as an equivalent “homogeneous,” premixed gas, regardless of the initial pressure.

Metal dust combustion: Explosion limits, pressures, and temperatures

Measurements are reported for the explosibility behavior in air of a variety of metallic and other elemental dusts. The data are useful for evaluating the hazards involved in their manufacture, transport, storage, and use. This study was also designed to help resolve, a fundamental issue of whether the combustion reactions in those dust flames proceed homogeneously in the gas phase or heterogeneously at particle surfaces. Data are reported for 14 dusts: from the more volatile metals such as magnesium and zinc, to the refractory metals such as tantalum, tungsten and niobium. Also included are some intermediate metals: hafnium, titanium, aluminum, iron, lead and copper; as well as the nonmetallic elements: boron, silicon, and carbon. Explosion pressures, rates of pressure rise, and continuum radiation temperatures were measured as a function of the dispersed dust concentration, using the Bureau of Mines 20-L explosibility test chamber and strong pyrotechnic ignitors. The lean limit fuel equivalence ratios varied from as low as 0.13 for the more reactive and volatile dusts to well in excess of 1.0 for the less reactive and less volatile ones. The copper and lead dusts were nonexplosible. For some dusts, preliminary data on particle size dependencies were obtained. The data are analyzed in terms of calculated adiabatic flame temperatures and the equilibrium vapor pressures or nonequilibrium vapor fluxes at those temperatures. An inverse correlation is observed between the measured lean limit equivalence ratio and the calculated equilibrium flame zone volatility for the dust. The correlation is analogous to one previously established for the carbonaceous dusts. Estimated absolute vaporization fluxes for the more flammable dusts are adequate to support a homogenous mechanism. Some refractory dusts, however, exhibit marginal (but finite) explosibility for the finer particle sizes even though their estimated vaporization fluxes appear to be too low to support a homogeneous mechanism.

Effect of volatility on dust flammability limits for coals, gilsonite, and polyethylene

Lean limit concentrations and limestone rock dust inerting requirements were measured in a 20-L chamber for a range of carbonaceous dusts including various ranks of coal, gilsonite, and artificial mixtures consisting of polyethylene and graphite. Although there is some uncertainty in the true yield of volatiles for some of the fossil mineral dusts, especially at flame flux levels, the data show that the combustible volatile content of the carbonaceous dusts is the dominant factor governing their combustion behavior in the dust explosions. The char residues and graphite are essentially inert on the rapid time scale required for flame propagation processes in explosions. The lean flammability limits of the various dusts correspond to invariant combustible volatile concentrations of about 30 to 40 g/m 3 . Total interting of the dusts occurs when the inert content of the mixture exceeds about 90 to 95 wt pct or, equivalently, when the combustible volatile content is less than 5 to 10 wt pct. The chars role is equivalent to that of the other inert constituents, such as ash and rock dust. In addition to the 20-L explosibility experiments, volatile yields of the various coals were measured using a high power, carbon dioxide laser at flux levels of 110 to 115 W/cm 2 . These measurements provide new data on the volatilities of 110 μm particles of the various coals, but some uncertainties remain in the absolute volatilities of the various sized coals used for the 20-L tests. A relatively simple volatility model for the various fossil mineral dusts can be used to explain their explosion propagation behavior and their extinction limits. When the uncertainties in volatility were removed by using the artificial mixtures of a completely volatilizable fuel (polyethylene) and the nonvolatilizable graphite, the model was confirmed.

Volatility model for coal dust flame propagation and extinguishment

A theoretical analysis is presented for the propagation and extinguishment of coal dust flames and of dust and gas flames containing inhibitor powders. The analysis is based on the established mechanisms for homogeneous flame propagation and the well known concept of a constant limit flame temperature for a given class of homogeneous fuels. The analysis is expanded to phase-heterogeneous systems such as coal dust by means of a volatility model. The analysis includes the singly heterogeneous system of a solid fuel dust in air; the singly heterogeneous solid inhibitor dust in a homogeneous fuel-air flame: and the doubly heterogeneous system consisting of a solid fuel and inhibitor dust mixture in air. The data for measured explosion pressures, flammability limits, and extinguishant requirements for heterogeneous systems are shown to be consistent with the established mechanisms and processes for homogeneous flame propagation provided that one adds an additional process: the heating and devolatilization of the solid fuel or inhibitor. The limitations on the rates of devolatilization of the solid particles become rate controlling at high burning velocities, at high dust loadings, and for large particle sizes. Devolatization rates are controlled by the intrinsic devolatilization rate constant for the solid fuel or inhibitor and the effective heating flux of the approaching flame front. The effective vield of volatiles is a function of those factors, the decomposition chemistry, and the time available for devolatilization. The fraction of the total volatiles that can be generated in the time available is the β-factor, and it determines the effective yield of fuel or inhibitor that participates in the flame propagation process. The data for explosion pressure, Flammability limits, and extinguishant requirements are readily understood in terms of those β-factors.

Inhibition and extinction of explosions in heterogeneous mixtures

The mass concentrations of various inhibitors required to extinguish explosions in stoichiometric methane-air and coal dust-air mixtures were measured in laboratory-scale systems of 8- and 20-L volumes. The additives studied were N 2 , CF 3 Br, CaCO 3 , KHCO 3 , and NH 4 H 2 PO 4 . The experimental method involved spherically-developing, adiabatic explosions at constant volume in which the test mixtures were taken to conditions of complete extinction. Strong, pyrotechnic ignition energies in the range of hundreds to thousands of joules were used in order to insure that inerting limits were truly independent of ignition energy. The order of effectiveness for the inhibitors was phosphate>nitrogen>CF 3 Br>carbonates. Essentially the same relative order of effectiveness was observed against methane as against coal dust; however, much higher absolute concentrations of the solid additives were required against methane than against coal dust. The phosphate, which was the most effective against either coal dust or methane, required 100 g/m 3 of the 7 μm powder to inert the most reactive coal dust concentration, and 280 g/m 3 of the same size powder was needed to extinguish stoichiometric methane. A preliminary theoretical analysis is presented based on a comparison of the measured inerting levels with those predicted from adiabatic, equilibrium thermodynamics using the established concept of limit flame temperatures. Three categories of thermodynamic systems are involved: fully-homogeneous ones, singly-heterogeneous ones, and doubly-heterogeneous ones. There are two types of singly-heterogeneous systems: those involving a solid fuel only and those involving a solid inhibitor only. The flammability and/or extinction behavior in the heterogeneous cases is controlled by limitations on the rates of devolatilization of the solid phase reactants. Preliminary data are presented for the rates of devolatilization of particles of coal dust, NH 4 H 2 PO 4 , and CaCO 3 pyrolyzed by a laser flux which simulates the flame heating flux. The data show that their measured rates of devolatilization are consistent with the inhibitors measured effectiveness in extinguishing explosions.

Particle size and surface area effects on explosibility using a 20-L chamber.

The Mine Safety and Health Administration (MSHA) specification for rock dust used in underground coal mines, as defined by 30 CFR 75.2, requires 70% of the material to pass through a 200 mesh sieve (<75 µm). However, in a collection of rock dusts, 47% were found to not meet the criteria. Upon further investigation, it was determined that some of the samples did meet the specification, but were inadequate to render pulverized Pittsburgh coal inert in the National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety and Health Research (OMSHR) 20-L chamber. This paper will examine the particle size distributions, specific surface areas (SSA), and the explosion suppression effectiveness of these rock dusts. It will also discuss related findings from other studies, including full-scale results from work performed at the Lake Lynn Experimental Mine. Further, a minimum SSA for effective rock dust will be suggested.