Combustion in microchannel reactors in context

Combustion in microchannel reactors in context Junjie Chen Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com Combustion is a chemical reaction between substances, usually including oxygen and usually accompanied by the generation of heat and light in the form of flame. The rate or speed at which the reactants combine is high, in part because of the nature of the chemical reaction itself and in part because more energy is generated than can escape into the surrounding medium, with the result that the temperature of the reactants is raised to accelerate the reaction even more. A familiar example of a combustion reaction is a lighted match. When a match is struck, friction heats the head to a temperature at which the chemicals react and generate more heat than can escape into the air, and they burn with a flame. If a wind blows away the heat or the chemicals are moist and friction does not raise the temperature sufficiently, the match goes out. Properly ignited, the heat from the flame raises the temperature of a nearby layer of the matchstick and of oxygen in the air adjacent to it, and the wood and oxygen react in a combustion reaction. When equilibrium between the total heat energies of the reactants and the total heat energies of the products (including the actual heat and light emitted) is reached, combustion stops [1, 2]. Flames have a definable composition and a complex structure; they are said to be multiform and are capable of existing at quite low temperatures, as well as at extremely high temperatures. The emission of light in the flame results from the presence of excited particles and, usually, of charged atoms and molecules and of electrons. Combustion encompasses a great variety of phenomena with wide application in industry, the sciences, professions, and the home, and the application is based on knowledge of physics, chemistry, and mechanics [3, 4]. Their interrelationship becomes particularly evident in treating flame propagation. In general terms, combustion is one of the most important of chemical reactions and may be considered a culminating step in the oxidation of certain kinds of substances [5, 6]. Though oxidation was once considered to be simply the combination of oxygen with any compound or element, the meaning of the word has been expanded to include any reaction in which atoms lose electrons, thereby becoming oxidized. As has been pointed out, in any oxidation process the oxidizer takes electrons from the oxidizable substance, thereby itself becoming reduced (gaining electrons). Any substance at all can be an oxidizing agent. But these definitions, clear enough when applied to atomic structure to explain chemical reactions, are not as clearly applicable to combustion, which remains, generally speaking, a type of chemical reaction involving oxygen as the oxidizing agent but complicated by the fact that the process includes other kinds of reactions as well and by the fact that it proceeds at an unusually fast pace. Furthermore, most flames have a section in their structure in which, instead of oxidations, reduction reactions occur [7, 8]. Nevertheless, the main event in combustion is often the combining of combustible material with oxygen. Combustion, with rare exceptions, is a complex chemical process involving many steps that depend on the properties of the combustible substance. It is initiated by external factors such as heat, light, and sparks. The reaction sets in as the mixture of combustibles attains the ignition temperature. The combustion spreads from the ignition source to the adjacent layer of gas mixture; in turn, each point of the burning layer serves as an ignition source for the next adjacent layer, and so on. Combustion terminates when equilibrium is achieved between the total heat energies of the reactants and the total heat energies of the products [9, 10]. Most reactions terminate when what is called thermal equilibrium has been attained, i.e., when the energy of the reactants equals the energy of the products. A relationship exists between the ignition temperature and the pressure of the mixture under specific conditions. The relationship for a mixture of hydrogen and oxygen has been determined. Only one temperature corresponds to a given pressure, whereas one or three pressures, called the explosion limits, may correspond to one temperature. The mechanism of the reaction determines the explosion limits: the reaction can proceed only when the steps in the sequence of reactions occur faster than the terminal steps [11, 12]. Thus, for combustion to be initiated with light or with a spark, the light intensity or the spark energy must exceed certain minimal values. The complexity of the combustion reaction mechanism and the rapidly varying temperatures and concentrations in the mixture make it difficult and often impossible to derive an equation that would be useful for predicting combustion phenomena over wide temperature and concentration ranges. Instead, use is made of empirical expressions derived for specific reaction conditions. Reactions of oxygen with hydrogen, with carbon monoxide, and with hydrocarbons are most important from the standpoint of theory and, at the same time, are of great practical value [13, 14]. Hydrogen combustion proceeds by complicated branched-chain reactions involving the interaction of hydrogen and oxygen atoms with oxygen and hydrogen molecules, respectively, to produce hydroxyl radicals. The final reaction product is water, formed by the combination of hydroxyl with hydrogen molecules. Reactions that terminate the chain, such as recombination of atoms and fragments of molecules, and the adsorption of active particles on solid surfaces, also play an important part in the mechanism of hydrogen combustion. The knowledge of rate constants for these processes derived empirically makes possible a quantitative description of all combustion characteristics, such as explosion limits, delay of ignition, and burning velocity [15, 16]. Combustion of carbon monoxide is mainly restricted to mixtures containing hydrogen or hydrogen compounds. In this case the reaction mechanism differs from that of hydrogen combustion chiefly in that it involves a step of fast interaction between hydroxyl and carbon monoxide. Pure mixtures of carbon monoxide and oxygen (or air) can be ignited only with sparks of high energy or under high pressures and temperatures. The chemical mechanism of their combustion is not yet clear, probably because oxidation of carbon monoxide, a reaction that is part of the combustion of practically all natural fuels, usually occurs in the presence of hydrogen or hydrogen compounds during burning produces carbon monoxide, hydrogen, and compounds of carbon and hydrogen. The mechanisms of combustion of hydrocarbons and of other organic compounds are known in general outline only. Many elementary steps of hydrocarbon combustion involving oxygen and hydrogen atoms, hydroxyl, and organic radicals are similar to those for hydrogen; however, the overall mechanism of hydrocarbon combustion is complicated by the diversity of molecules and radicals involved [17, 18]. Moreover, with oxidation, thermal decomposition, a breakdown of complex organic compounds without oxidation, also takes place [19, 20]. Two types of hydrocarbon combustion have been defined: slow combustion at temperatures below 500 °C, including cool flames observed at certain pressures, and combustion at higher temperatures, accompanied by hot flames. Ignition in two stages is characteristic of higher hydrocarbons, in which a cool-flame stage yielding readily oxidizable products precedes that of the hot flame. In addition to chemical reactions, physical processes that transfer mass and energy by diffusion or convection occur in gaseous combustion [21, 22]. In the absence of external forces, the rate of component diffusion depends upon the concentration of the constituents, pressure, and temperature changes, and on diffusion coefficients (a measure of the speed of diffusion). The latter are either measured or calculated in terms of the kinetic theory of gases. The process of diffusion is of great importance in combustion reactions, in flames, that is, in gaseous mixtures, and in solids or liquids [23, 24]. Diffusion heat transfer (by molecular means) follows a law (Fourier's law) stating that the heat flux (a measure of the quantity of heat) is proportional to the temperature gradient (the difference in temperature between two limiting temperatures). The coefficient of proportionality, called the thermal conductivity coefficient, is also measured or calculated in terms of the kinetic theory of gases, like the diffusion coefficient. Convective transport of mass and energy may be accounted for by buoyant forces, external forces, and turbulent and eddy motions. Convection is in the main responsible for the mixing of gases (e.g., in furnaces). Sublimation (the direct evaporation of a solid without intermediate liquid stage), evaporation of liquids, and the mechanical destruction of burning samples, all methods of transforming solids and liquids into gases, greatly contribute to their ease of combustion. In general, the combustion of condensed systems, i.e., liquids and solids, is more complex than that of gases, which accounts for the greater development of the gas combustion theory. Energy transport in combustion may also occur by the emission of light, mostly in the infrared. Reactors, in chemical engineering, are devices or vessels within which chemical processes are carried out for experimental or manufacturing purposes. Reactors range in size and complexity from small, open kettles fitted with simple stirrers and heaters to large, elaborate vessels equipped with jackets or internal coils for heating or cooling, nozzles or ports for adding and removing materials, sources of ultraviolet radiation or electrical energy, specially designed agitators or scrapers, and strong walls and tight seals to permit operation at high or low pressures [25, 26]. In many cases, these reactors are provided with instruments that measure temperature, pressure, pH, or other properties of the contents [27, 28]. They are constructed of materials selected for strength and resistance to attack by the substances being processed. For certain processes that are particularly adaptable to continuous operation, reactors are devised so that the chemical reaction may continue for weeks or months while starting materials are introduced continuously or intermittently and the finished product is removed in a steady stream or in periodic discharges. A microchannel reactor, or microreactor or micro-structured reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below one millimeter; the most typical form of such confinement are microchannels. Microchannel reactors are designed in the field of micro process engineering, together with other devices (such as micro heat exchangers) in which physical processes occur [29, 30]. The microchannel reactor is usually a continuous flow reactor (contrast with a batch reactor). Microchannel reactors offer many advantages over conventional scale reactors, including vast improvements in energy efficiency, reaction speed and yield, safety, reliability, scalability, on-site or on-demand production, and a much finer degree of process control. Using microchannel reactors is somewhat different from using a glass vessel. These reactors may be a valuable tool in the hands of an experienced chemist or reaction engineer [31, 32]. Microchannel reactors can remove heat much more efficiently than vessels and even critical reactions such as nitration can be performed safely at high temperatures. Hot spot temperatures as well as the duration of high temperature exposition due to exothermicity decreases remarkably. Thus, microchannel reactors may allow better kinetic investigations, because local temperature gradients affecting reaction rates are much smaller than in any batch vessel. Heating and cooling a microchannel reactor are also much quicker. As a result of the superior heat transfer, reaction temperatures may be much higher than in conventional batch-reactors. Many low temperature reactions as organo-metal chemistry can be performed in microchannel reactors. Microchannel reactors are normally operated continuously [33, 34]. This allows the subsequent processing of unstable intermediates and avoids typical batch workup delays. Especially low temperature chemistry with reaction times in the millisecond to second range are no longer stored for hours until dosing of reagents is finished and the next reaction step may be performed. This rapid work up avoids decay of precious intermediates and often allows better selectivity. Continuous operation and mixing cause a very different concentration profile when compared with a batch process. In a batch, reagent A is filled in and reagent B is slowly added. Thus, B encounters initially a high excess of A. In a microchannel reactor, A and B are mixed nearly instantly and B won't be exposed to a large excess of A. This may be an advantage or disadvantage depending on the reaction mechanism - it is important to be aware of such different concentration profiles. Although a bench-top microchannel reactor can synthesize chemicals only in small quantities, scale-up to industrial volumes is simply a process of multiplying the number of microchannels [35, 36]. In contrast, batch processes too often perform well on research and development bench-top level but fail at batch pilot plant level. Pressurization of materials within microchannel reactors (and associated components) is generally easier than with traditional batch reactors. This allows reactions to be increased in rate by raising the temperature beyond the boiling point of the solvent. This, although typical Arrhenius behavior, is more easily facilitated in microchannel reactors and should be considered a key advantage. Pressurization may also allow dissolution of reactant gasses within the flow stream. Although there have been reactors made for handling particles, microchannel reactors generally do not tolerate particulates well, often clogging. Clogging has been identified by a number of researchers as the biggest hurdle for microchannel reactors being widely accepted as a beneficial alternative to batch reactors [37, 38]. So far, the so-called microjet reactor is free of clogging by precipitating products. Gas evolved may also shorten the residence time of reagents as volume is not constant during the reaction. This may be prevented by application of pressure. Mechanical pumping may generate a pulsating flow which can be disadvantageous. Much work has been devoted to development of pumps with low pulsation [39, 40]. A continuous flow solution is electroosmotic flow. Typically, reactions performing very well in a microchannel reactor encounter many problems in vessels, especially when scaling up. Often, the high area to volume ratio and the uniform residence time cannot easily be scaled. Corrosion imposes a bigger issue in microchannel reactors because area to volume ratio is high. Degradation of few µm may go unnoticed in conventional vessels. As typical inner dimensions of channels are in the same order of magnitude, characteristics may be altered significantly. Combustion processes which happen in microchannel reactors are considered micro-combustion. The high surface-to-volume ratio increases specific heat loss. Quenching distance plays a vital role in stabilizing the flame in such combustion chambers [41, 42]. Combustion in microchannel reactors is the sequence of exothermic chemical reaction between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species at micro level. The release of heat can result in the production of light in the form of either glowing or a flame. Fuels of interest often include organic compounds (especially hydrocarbons) in the gas, liquid or solid phase. The major problem of combustion in microchannel reactors is the high surface to volume ratio. As the surface to volume ratio increases, heat loss to the walls of the microchannel reactor increases, which leads to flame quenching [43, 44]. The development of miniaturized products such as microrobots, notebook computers, micro-aerial vehicles and other small-scale devices is becoming increasingly important in ordinary daily life. There is a growing interest in developing small scale combustors to power these micro-devices due to their inherent advantages of higher energy density, higher heat and mass transfer coefficients and shorter recharge times compared to electrochemical batteries [45, 46]. The energy density of hydrocarbon fuels is 20-50 times higher than the most advanced lithium-ion concept based electrochemical batteries. Progress has been made towards the development and application of such small-scale devices to generate power through the combustion of hydrocarbon fuels [47, 48]. Micro-combustors are an attractive alternate to batteries as they have large surface area to volume ratio, due to which, significant amount of heat is transferred through the walls which leads to flame quenching. However, the increased rate of heat transfer through solid walls is advantageous in the case of steam reformers used for hydrogen production. Declaration of competing interest The author declares that there is no conflict of interest. References H. Wang and D.A. Sheen. Combustion kinetic model uncertainty quantification, propagation and minimization. Progress in Energy and Combustion Science, Volume 47, 2015, Pages 1-31. Y. Huang and V. Yang. Dynamics and stability of lean-premixed swirl-stabilized combustion. Progress in Energy and Combustion Science, Volume 35, Issue 4, 2009, Pages 293-364. B.T. Zinn. 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