by Vylen AZATYAN, RAS Corresponding Member, Head of the Laboratory of Chain and Heterophasic Reactions, Institute for Structural Macrokinetics and Materials Science Problems, Russian Academy of Sciences (RAS)
Technical progress spurs the use of hydrogen (H2) in many industrial fields - in the production of ammonia and alcohols, reduction of metals, to name but a few. Hydrogen is regarded as a prospective fuel, too. The practical use of this gas, however, is always a hazard, as it is both readily combustible and explosive. But this risk can be minimized. The RAS Institute for Structural Macrokinetics and Materials Science Problems (ISMAN) has developed effective methods for controlling the processing of H2 combustion, explosion and detonation.
Hydrogen offers indisputable advantages as a fuel. Namely the heat of combustion specific calorific (heat) value compared with organic fuels as well as the better combustion rate generating more energy per time unit.* We should also add the wide concentration range (limits) of hydrogen combustion, i.e. the range from minimal to maximal concentrations (percentagewise) enabling steady combustion of an air/hydrogen mixture. For higher efficiency of combustion its composition can be varied much more than that of organic fuels like methane, propane, butane and gasoline having the flame propagation velocity of 0.35 to 0.57 m/s. This value is several-fold as high for hydrogen-2.6 m/s; that is it bums much more vigorously and thus boosts the performance of a power unit-say, a jet engine - it fuels. This high-efficiency fuel, H2, is ever more frequently used in electrochemical generators. It is an eco-friendly fuel, with water being the only product of combustion. Small wonder that some countries have adopted projects for making automobiles operating on hydrogen. Such work is in progress here in Russia, too.** Hydrogen holds out yet another advantage due to its low viscosity, a factor facilitating its pipeline delivery. This mode of gas transportation is already employed in Germany and the United States.
Hydrogen is obtained through steam conversion by and large. There are also other options. Say, H2 is released as
* See: B. Sokolov. S. Khudyakov, "Fuel of the Future", Science in Russia, No. 5, 2004; V. Rusanov. "Hydrogen and Hydrogen Power Engineering", Science in Russia, No. 6, 2004. - Ed.
** See: Ye. Multvkh, "Ecotransportation in the Spotlight", Science in Russia, No. 4, 2004. - Ed.
a by-product in the electrochemical production of chlorine in equal volume amounts as CI. Good prospects are also offered by water electrolysis with the use of energy from big electric power stations (nuclear and thermal power plants run by the state at the regional level) at night- and daytime hours when peak loads are off.
Since hydrogen is highly explosive, its use is on a wide scale is problematic. Hydrogen-air mixtures are more prone to uncontrolled combustion, explosion and detonation, and are sensitive to weak igniters (static electricity including). While an electric discharge of 0.26 mJ can ignite a stoichiometric* mixture of propane and as much as 0.3 mJ is needed to set off methane, the critical value is only a tenth for H2-namely 0.019 mJ.
Up until recently it was common practice to use largely nonchemical methods for controlling the combustion and explosion of hydrogen-air mixtures, namely protective barriers, flame arresters as well great additions of inert gases as diluents, and the like. Among chemical remedies were refrigerants (freons) used restrictedly-these are fluorine- chlorine- and bromine substituted hydrocarbons. But the effect of such agents is not high and, in addition, they are highly corrosive, toxic, have short storage life and are costly to boot. These substances are effective only if supplemented in very large amounts. Since freons destroy the ozone layer, their output has been discontinued.
The absence of effective chemical methods for controlling the combustion and explosion of gases may be attributed in many ways to the holdover view of the basic factors and regularities implicated in such processes.
Let us recall that ignition (inflammation) and follow-up combustion are reactions proceeding at increasing self-acceleration as long as the initial reactants are consumed; heat and light are released by the accelerating reaction. Two different factors are at work to cause ignition and combustion.
The first one comes from the heating of the mixture by energy liberated in the course of reaction which speeds up with temperature increase to intensify the reaction and thereby heat buildup. So that a positive feedback between the reaction rate and temperature rise is realized. This is thermal combustion.
The other factor causing ignition and combustion consists in the multiplication of chemically active intermediate particles, free atoms and radicals, which are in fact fragments of molecules. The latter are regenerated in vigorous (fast) reactions with starting (initial) molecules, their number soars and, as a consequence, the consumption of the starting substances is accelerated. The reactions of atoms and radicals alternate periodically to form reaction chains. The breeding active particles give rise to new chain branches. Apart from the reactions of regeneration and multiplication atoms and radicals enter into other reactions, e.g. they may combine amongst themselves (recombination) to give rise to valence-saturated molecules. The active particles perish in the end, and the chains break up. Under certain conditions multiplication of active species is faster than their destruction. As a result the multiplication of atoms and radicals proceeds in the geometric progression, the growth of concentration of these particles becomes avalanche-like. This is a fast self-stimulating process, one which is likewise accompanied by heat and light release; it also involves such phases as ignition and combustion. But in its very nature and characteristics this
* With reference to stoichiometry, or the determination of the atomic weights of elements, the proportions in which they combine, and the weight relations in any chemical reaction. Also, the branch of chemistry dealing with the relationships of elements in combination, especially with quantitative relationships. - Ed.
Dependence of heat release (q+) and heat removal (q-) rates on temperature.
process is quite different than thermal combustion - it is the chain combustion.
It was commonly believed up until now that the chain-branching avalanche in the combustion of gases is much implicated only at pressures dozens and hundreds of times as low as the atmospheric one. But if the pressure is normal or high, the temperature increase was thought to be the only cause of combustion. In his capital work Acad. Nikolai Semenov (Nobel Prize, 1956)-an eminent physicist and chemist, and the author of the theory of chain processes and combustion-has this to say on the third limiting pressure of inflammation, i.e. related to the atmospheric and high pressures: "The nice agreement between calculation and experience leaves no doubt in that the third limit of hydrogen and oxygen mixtures is of thermal nature." In their work two American scientists, Bernard Lewis and Giinter von Elbe, say that in the hot flames so-called, with which one has to deal in practice, self-stimulation proceeds along the thermal, not the chain path. The same viewpoint is upheld by the authors of the articles on combustion in the latest editions of the chemical and physical encyclopedias. Even the publications of the international symposia and conferences on combustion problems (2005) ignored the role of the chain factor.
And yet we at the 1SMAN Institute have found the following: that the chain proliferation (breeding) of free atoms and radicals evolves as the basic factor underlying the combustion of hydrogen and vapor of organic compounds at any pressures and at any self-heating. The branching and breaking competition of reaction chains is a determining factor in all forms of combustion, flame propagation and detonation. Self-heating, though not necessary in this combustion, accompanies intensive combustion and enhances the chain avalanche. Under certain conditions a thermal reaction may occur, too. Combustion implicating these two avalanche-like processes simultaneously is remarkable for superhigh intensity. This is a heat explosion with a chain-branching mechanism.
Our innovative approach has allowed to get down to chemical methods for gas combustion control. Yet, this has become possible after all owing to the further development of the theory of chain processes advanced by Acad. Semenov.
Our proposals boil down to the purposeful variation of the rates of reaction chains branching and breaking with the aid of selected chemically active molecular additives-inhibitors, which react readily with atoms and radicals, and produce low-activity products, thus preventing the proliferation of dangerous reagents. This averts their inflammation and combustion (acting in the same mode are graphite rods intercepting neutrons in a nuclear reactor)*. A molecular additive acts upon chain combustion only, but has no effect on thermal combustion occurring in the absence of reaction chains.
Inhibitors slow down the reaction rate and, as a consequence, also diminish the self-heating of the mixture and that, in turn, arrests the process furthermore. As a result, this hinders combustion from developing into detonation. And should an inhibitor speed up chain breaking so much that chain branching proceeds more slowly than breaking, combustion becomes outright impossible.
We at ISMAN have developed and tested efficacious, corrosion-safe and low-cost compounds which, if added in small amounts to hydrogen-air mixtures (Just a few percent would be enough), make it possible to control
* See: V. Subbotin, "Nuclear Power: From the Past into the Future", Science in Russia, No. 6, 1996 and No. 1, 1997. - Ed.
The effect ofAKM-3 inhibitor and halon 114B2 on the explosive concentration limits of hydrogen-air mixtures. The yellow-coded part corresponds to a heat explosion by the chain-branching mechanism.
Detonation prevented during bench tests of a hydrogen-operating ramjet engine.
inflammation, spread of flames and detonation, and thus render such mixtures explosion-proof. We have opted for light olefin hydrocarbons-their molecules contain double carbon bonds. These are ethylene, propylene and butylenes. Colliding with hydrogen atoms, the molecules of such compounds cause a break (nick) in one of the double bonds, which captures H atoms readily, thus withdrawing them from the combustion reaction; that is, the reaction chain is disrupted, broken thereby. Such inhibiting compounds are far more effective than halons. Small amounts of olefin hydrocarbons supplementing the mixture at 1 - 2 percent of its volume make it explosion-proof even when combustion is continued, if need be. This idea was materialized by ISMAN and the Moscow Aviation Institute (MAI) in their jointly patented method of explosion prevention in an internal combustion engine working on a hydrogen fuel (in the event of "knocking combustion").
Next, our technique makes it possible to control the combustion of other gases as well, methane including, and of synthesis gas (a mixture of hydrogen and carbon oxide*), now in wide use in the chemical industry. Methane explosions still claim victims in coal mines.** However, they can be warded off, as seen in the results of our laboratory experiments in the inhibitor-initiated suppression of methane ignition and explosion.
Hydrogen offers fine prospects for flying vehicles and aircraft with light fillers - in balloons, airships and probes now finding ever wider application in many areas. Helium is still being used for the purpose. But hydrogen is much lighter and less expensive. It is an explosive gas, true. We at ISMAN have developed and tested inhibitors not condensable down to minus 50 degrees on the centigrade scale; if supplemented at 2 to 3 percent, they make hydrogen explosion-proof in aircraft. Their testing carried out jointly by ISMAN and MAI came off well, and we obtained a state certificate.
Thus and so, we have developed chemical methods enabling effective control of the combustion of hydrogen-air mixtures, and allowing to minimize the danger of their sudden inflammation and explosion in different producer industries, in gas storage, transportation and uses. So we have coped with a major problem of hydrogen energetics.
The patented inhibitors have been used with much success in our cooperative works with the M. Planck Institute of Germany, with the Moscow-based Central Institute of Aircraft Motor Engineering, and other research and industrial centers of Russia.
* See: "Breakthroughs in Power Engineering", Science in Russia. No. 6. 2005. - Ed.
** See: A. Ruban, B. Zaburdyayev, "Coal Methane", Science in Russia, No. 3, 2006. -Ed.
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