Article pubs.acs.org/IECR
3D Matrix Burners: A Method for Small-Scale Syngas Production Vladimir S. Arutyunov,* Vladimir M. Shmelev, Ayan N. Rakhmetov, and Oksana V. Shapovalova Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygina, 4, Moscow 119991, Russia ABSTRACT: An urgent need in flexible low-scale gas-chemical technologies for processing and transporting unconventional and remote low-debit gas resources makes it necessary to develop methods for effective low-scale conversion of natural gas into syngas. The paper describes a principally new type of re-former based on the gas-phase conversion of hydrocarbons into syngas in 3D matrix burners, which can be used in many low-scale applications. The effective convective and radiant recuperation of heat of combustion products to the matrix and then to incoming fresh gas, along with the absence of radiation losses in the closed cavity of the matrix, lowers the limit of stable combustion of rich methane−air mixture to an oxygen excess coefficient of α < 0.4. This simple noncatalytic gas-phase process makes it possible to attain H2 and CO yields very close to thermodynamically equilibrium values.
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INTRODUCTION Recent developments in unconventional gas production have shown that natural gas will be one of the world’s main sources of energy and hydrocarbon feedstock, at any rate up to the end of this century. However, these new vast hydrocarbon resources are spread over large areas and usually have low debit and lifetime. Therefore, the gas industry needs new technologies for their processing and transportation. These technologies must be more flexible and low-scale than the existing gas chemical processes. The most complex, costly, and energy consuming stage of the modern methods of natural gas conversion into value-added products and chemicals is the production of syngas. This stage consumes up to 60% of the total capital and operating costs.1 Its complexity restrains many practical applications of chemical processing of gaseous hydrocarbons, especially for low-scale production. Of the possible processes of conversion of hydrocarbons into syngas, the partial oxidation is the most attractive, especially for low-scale applications, because it is an exothermic reaction, with no need for external heat sources. From the thermodynamic point of view, the optimal conditions for the reaction
local sources, including unconventional and low deposit resources, by means of efficient low-scale production of various liquid fuels and chemicals.
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BASIC PRINCIPALS So-called “flameless” combustion stabilized on the surface of a solid porous or perforated planar matrix, is widely used for many technological applications, first of all as a source of IR radiation.4,5 In this case, the flame front is stabilized at some distance above the surface, with its temperature being low enough, ∼1000−1200 °C, due to intense convective and radiation heat transfer from the flame front to the surface. The surface heated in this manner intensively radiates in the IR region and can be used as an effective source of IR radiation for many industrial applications. However, intense convective and radiation loss from the flame front significantly narrows the combustion limits. To widen the combustion limits and ensure the combustion of lean gas mixtures with low NOx emissions, it was suggested to use 3D permeable matrixes with a closed inner cavity.6,7 It was demonstrated that, in such matrix burners, the stable combustion of very lean mixtures at an oxygen excess coefficient of α > 2 can occur at a specific combustion power of up to 30−40 W/cm2. These conditions provide very low concentrations of nitrogen oxides and carbon monoxide. Theoretically estimated temperatures of the flame front and of the working and back surfaces of such 3D matrices were found to be in close agreement with the available experimental data.6,7 However, the same organization of the combustion process makes it possible to widen the combustion limits for rich mixtures as well, thus providing necessary conditions for an effective conversion of hydrocarbon fuels into syngas.7−9 The
CH4 + 1/2 O2 → CO + 2H 2
are attained at an oxygen excess coefficient of α = 0.25 (The coefficient α characterizes the divergence of the fuel−oxidant mixture composition from stoichiometric ratio necessary for its complete combustion, for which α = 1, by definition; for methane, α = [O2]/2[CH4]). However, it is very difficult to maintain the stable conversion of such rich hydrocarbon− oxygen mixtures in the noncatalytic process. Standard burners can rarely operate at α < 0.5. Although a number of technologies for the partial oxidation of natural gas into syngas have been proposed, 2 none of them proved to be technologically applicable. The present paper describes a principally new way for conversion of gaseous hydrocarbons to syngas through their partial combustion near the inner surface of 3D permeable matrixes with locked IR radiation.2,3 Such simple devices can provide highly efficient conversion of hydrocarbons into syngas, thereby making it possible to utilize natural gas from different © 2013 American Chemical Society
Special Issue: Recent Advances in Natural Gas Conversion Received: Revised: Accepted: Published: 1754
July 14, 2013 November 17, 2013 November 18, 2013 November 18, 2013 dx.doi.org/10.1021/ie4022489 | Ind. Eng. Chem. Res. 2014, 53, 1754−1759
Industrial & Engineering Chemistry Research
Article
(combustion) in a very narrow flame front, and the absence of interaction of the reaction products with the matrix surface, which males it possible to avoid soot formation and many problems it entails, especially challenging in catalytic processes.
principal scheme of a syngas re-former based on a 3D permeable matrix and its general view are shown in Figure 1.
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RESULTS AND DISCUSSION The limiting conditions for the combustion of a rich gas mixture on the surface of planar and 3D permeable matrixes were theoretically investigated in ref 10. The critical combustion conditions were evaluated on the basis of the energy balance equation, with the chemical energy of the gas mixture being converted into the thermal energy of the combustion products and thermal radiation energy. To test the calculation results, which predict that the transition from a planar to a 3D permeable matrix significantly widens combustion limits, we performed experiments with a wide variety of 3D matrixes of different shapes and designs, made of very different permeable materials, including perforated ceramics, metallic foams (nickel, chromal, and alumel alloys), with different thickness and porosity, metallic wool, metallic gausses, etc. As predicted theoretically, the effect of transition from a simple planar matrix to a closed 3D matrix was pronounced. Due to effective heat recuperation from the reaction products to the inner surface of closed matrix and then to the fresh gas mixture, the fuel mixture enters the combustion front already preheated to a temperature approximately equal to the surface temperature. Along with eliminating radiation loss, it significantly widens the combustion limits. It should be noted that at low temperatures of the matrix surface (
