A Generator of Synthesis Gas and Hydrogen Based on a Radiation Burner

ISSN 0040�5795, Theoretical Foundations of Chemical Engineering, 2010, Vol. 44, No. 1, pp. 20–29. © Pleiades Publishing, Ltd., 2010.Original Russian Text © V.S. Arutyunov, V.M. Shmelev, I.N. Lobanov, G.G. Politenkova, 2010, published in Teoreticheskie Osnovy Khimicheskoi Tekhnologii, 2010, Vol. 44, No. 1,pp. 21–30.

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INTRODUCTION

The widespread use of natural gas and hydrogen inenergetics, transport, and as raw materials for the chemi�cal industry is one of the primary contemporary techno�logical tendencies. Currently, the worldwide hydrogenproduction capacity is valued at 100 million tons per year.An overwhelming share of hydrogen is produced in theprocesses of natural gas and coal conversion into synthesisgas, and also in reforming and other oil refining and pet�rochemical processes [1]. However, as the importance ofreforming declines and the role of hydrogenation pro�cesses grows when switching to a new generation of pollu�tion�free high�octane motor fuels with lower level of pol�lution and a low content of aromatic compounds and sul�fur, petrochemistry itself is changing, to an increasingextent, from a supplier of hydrogen to its consumer. At thesame time, the quick and significant growth of hydrogenand synthesis gas consumption is predicted in other fields.For instance, the potential market for methanol only isestimated at 800 million tons per year, and this exceeds itspresent world production by 20 times [2]. Therefore, thegrowth of hydrogen production up to 400 million tons isexpected by 2050, while it would require almost 700 mil�lion tons of hydrogen to completely replace the hydrocar�bon motor fuels (approximately 2200 million tons) con�sumed in the world.

From an ecological point of view, it is attractive to usehydrogen in motor transport for the power supply ofonboard fuel cell–based engines, whose efficiency comesto 55%, twice exceeding that of internal combustionengines [3]. However, two serious problem stand in theway of its practical realization: it is necessary to increasethe capacity of onboard hydrogen storage systems up to5–7 kg and to create an efficient infrastructure for its pro�duction and distribution. At that, the production must belocated maximally close to the points of consumption in

order to avoid the transportation over significant distancesand the storage of great volumes of hydrogen, as its dailylosses come to approximately 5% even under good ther�mal insulation. As the distance between fuel stations mustnot exceed 3.2 km in cities and 40 km on highways,besides thousands of fuel stations, it will be required tohave hydrogen synthesis plants located near them ordirectly included in them.

The United States Department of Energy (DOE),actively advocating the use of hydrogen in transport, hasmade the reduction of its price to $2–3 per kilogram forthe ultimate consumer a top priority. Every day an individ�ual fuel station serves, on average, 150–170 automobiles;therefore, hundreds of effective autonomous hydrogensynthesis plants with a capacity at a level of 1500 kg/daywill be required [4].

Because of their technical complexity and high energyconsumption, the contemporary industrial hydrogen syn�thesis methods, which are based on steam or steam–oxy�gen natural gas conversion, will hardly be able to claim asignificant role in such a scattered production of hydro�gen. In hydrogen and synthesis gas–based technologicalprocesses, their share of the commercial product costexceeds 60% of the total expenses, and an acceptableprofitability level is reached only at huge productivecapacities from 1 million tons per year and higher [5]. Theother known methods, including electrolysis and the useof alternative energy sources, give a several times higherprime cost of produced hydrogen and have practically noindustrial use [6].

Steam and steam–oxygen natural gas conversion canhardly be imagined as a hydrogen source even for station�ary autonomous energy supply sources, which are pre�dicted to play an important part in energetics in the nearfuture. In addition, it is much harder to imagine the suc�cessive advancement of hydrogen transport energetics onthe basis of them. This situation has made engineerssearch intensively for new methods of hydrogen and syn�

A Generator of Synthesis Gas and HydrogenBased on a Radiation Burner

V. S. Arutyunov*, V. M. Shmelev, I. N. Lobanov†, and G. G. PolitenkovaSemenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia

* e�mail: [email protected] July 16, 2008; in final form, November 24, 2008

Abstract—The possibility of the conversion of methane into synthesis gas in rich methane–air mixturesunder conditions of stable surface combustion in a volumetric permeable matrix in a radiation field of lockedinfrared irradiation is shown. It is suggested to use the combustion of hydrocarbons in a volumetric permeablematrix as a simple and compact generator of synthesis gas and hydrogen.

DOI: 10.1134/S0040579510010033

† Deceased.

THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 44 No. 1 2010

A GENERATOR OF SYNTHESIS GAS AND HYDROGEN 21

thesis gas production. In recent years, several processesbased on natural gas conversion in different power sys�tems, such as internal combustion engines, gas turbines,and jet�propelled engines, were proposed [7]. In spite ofsome progress, the complexity of these technologies haspermitted none of them to attain an industrial level.

In the present work, the possibility of the production ofsynthesis gas and hydrogen on the basis of natural gas con�version during its surface combustion in a volumetric per�meable matrix under conditions of locked infrared irradi�ation is considered. Combustion on a standard plane per�meable matrix surface is realized by means of radiationand convective heat transfer from the flame front to theincoming mixture through the chain flame–matrix–ini�tial mixture. At that, the burning hot matrix surface is thesource of intensive infrared irradiation. Such a combus�tion scheme provides for a substantial reduction in theflame front temperature and, correspondingly, a decreasein the concentration of nitrogen oxides in the combustionproducts. However, at that, the combustion limits becomenarrower because of additional losses by radiation. Newpossibilities of surface combustion are opened up whendeep volumetric matrixes are used [8–10]. In this case,the combustion proceeds in the matrix cavity under theconditions of partially or completely locked infrared irra�diation, and this method permits, if not to exclude com�pletely but at least reduce them by many times, irradiationlosses and, thereby, the substantial expansion of the com�bustion limits. The experiment confirms the expansion ofthe limits towards the regions of both lean and rich mix�tures [11].

In a volumetric matrix made from a porous material(ceramics or metal), the flame front can be observed bothover the irradiating surface inside the matrix cavity andunder the surface in the matrix body. At that, the processparameters (mixture composition and combustion tem�perature) can be adjusted within very large intervals withthe retention of combustion stability, which is impossibleunder conventional conditions. Earlier, this approach wasused for the development of effective gas burners with a

low specific fuel consumption and carbon oxide emission[8–10].

The purpose of the present work is to demonstrate thepossibility of achieving high yields of Н2 and CO duringmethane conversion inside a volumetric honeycombmatrix cavity under conditions of locked infrared irradia�tion and to estimate the prospects of the application of asimilar device as a simple autonomous source of hydrogenand synthesis gas.

EXPERIMENTAL TECHNIQUE

The experimental test bench scheme is shown in Fig. 1.In this work, we used network gas as a fuel and air as an oxi�dizer. The fuel and oxidizer rates were adjusted accordingto the readings of rotameters and were more preciselymeasured by gas meters. After leaving the mixer, a homo�geneous air–gas mixture of a specified composition wasfed into the radiation burner with a deep volumetric matrix(Fig. 2).

The main part of the experiments was performed on aburner with a matrix whose internal cavity represented an80 × 40 ×115�mm right�angled parallelepiped confined bywalls, a bottom, and a top of perforated 15�mm�thickceramic permeated by cylindrical channels 1.2 mm indiameter. The ratio of the total channel cross�section areato the total surface area (porosity) was γ = Sc/S = 0.25. Weworked with a burner which was open or covered by a per�forated ceramic top 5. The air flow rate through the burnerwas 31–36 l/min in most experiments. All of the experi�ments were performed at atmospheric pressure. The tem�peratures of the matrix’s working surface and the combus�tion products inside the burner device cavity were mea�sured by chromel–alumel thermocouples.

For analysis, the conversion products were directlytaken from the burner cavity by syringes of 5 cm3 in vol�ume through a stainless steel capillary tube. The analysisof the products was conducted on an LKhM�8 chromato�graph. Hydrogen, CO, air components, and hydrocar�bons were determined on a 3�m�long column with NaX

Fig. 1. Experimental test bench scheme. GM is gas meters, R is rotameters.

Blower

Network gas

GMR

Fume�collecting hood

Mixer

BurnerGM

R

22

JOURNAL OF CTHEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 44 No. 1 2010

ARUTYUNOV et al.

molecular sieves, and СО2 and ethylene were defined on a2�m�long column with Poparack�Q. The temperature ofthe columns was 50°С; argon was used as a carrier gas.

EXPERIMENTAL RESULTS

The main value defining the temperature and thecharacter of the chemical conversion processes in aburner is the oxidizer excess coefficient α =[О2]0/2[СН4]0, showing the deviation of the mixturecomposition from the stoichiometric ratio. If in the caseof power plants, in order to use chemical energy morecompletely and reduce harmful emissions from theirincomplete combustion, as a rule, the burning of fuel atvalues of α ≥ 1 (stoichiometric and lean mixtures) issought, then for the production of chemical products,such as synthesis gas and hydrogen, it is necessary to con�duct the process at α < 1 (rich mixtures). The lowest valuesof α at which we succeeded in achieving stable burneroperation in air and obtained well�reproduced resultsunder conditions of the present work for the given burnerdevice were 0.35–0.37.

The temperature measurement experiments haveshowed that when the parameter α is varied from 0.35 to1, the surface temperature of the matrix changes from 350to 650°С with the maximum at α ≈ 1. At α = 0.4, it is closeto 400°С (Fig. 3). The temperature of the combustionproducts inside the cavity exceeds that for the matrixworking surface and grows with an increase in α in theregion of α ≤ 1. Note that the flame front temperatureexcess of at least 200–300°С over the irradiating surfacetemperature is also typical for the mixture combustionabove the plane matrix surface.

The adiabatic heating of the methane–air mixturecombustion products is equal to ΔТр ≈ 870°С at P =1 atm, α = 0.4, and a thermodynamic equilibrium com�position of the products. However, the analysis of theproduct compositions shows that the system does notattain a thermodynamic equilibrium over the time of res�idence in the flame front (Fig. 4). The calculation for α =0.4, taking into account the experimentally obtained finalproducts, gives a lower adiabatic heating ΔТ ≈ 500°С inthe flame front. The temperature measured experimen�tally behind the flame front is slightly higher (~500–600°С), and this is most likely caused by the recuperationof flue gas heat as a result of its radiative transfer into thematrix and further by the incoming fresh gas heated in thechannels of the burning hot matrix. Inside the burner cav�ity covered by a perforated ceramic top, the stabilizedtemperature of the matrix surface and the productsbehind the flame front is approximately 100°С higherthan that in the burner with an opened cavity due to thehigher degree of reaction heat recuperation.

In spite of the fact that chemical equilibrium is notattained in the flame front, the temperature of the com�bustion products in a locked radiation field is almost thesame over the whole burner device cavity.

Consequently, the key feature of the given process isthe possibility of the stable conversion of hydrocarbons at

Synthesis gas

Hydrocarbon–oxidizer mixture

1

2

3

4

5

Fig. 2. Scheme of a radiation burner with a deep volumet�ric matrix: 1 is the external shell, 2 is the volume with agas–air mixture, 3 is the gas outlet, 4 is the burner cavityside walls and bottom made from perforated ceramic, and5 is the burner cavity top made from perforated ceramic.

1200

1000

800

600

400

200

0 0.80.60.4

T, °C

α

1

2

Fig. 3. Dependence of (1) the temperature inside theopened burner cavity behind the flame front and (2) thetemperature of the internal matrix surface on the oxidizerexcess coefficient α.



low values of α < 0.5 and the flame temperature (~600–700°С). This opens up real prospects for its application asa generator of different chemical products and, first of all,of hydrogen and synthesis gas.

The experimental dependencies on the value α for theconcentration of methane conversion products in theburner with an opened cavity are shown in Fig. 4. It can beseen that the Н2 and CO concentrations grow sharplywith a decrease in α, and at α ≈ 0.4 they are close enoughto the values that are adiabatically equilibrium ones underthese conditions. Thus, the experimental Н2 concentra�tion is 18–22% (at α = 0.4, its equilibrium concentrationis ~25%) and the experimental CO concentration is 10–11% (at α = 0.4, its equilibrium concentration is ~14%).Accordingly, the СО2 concentration decreases monoton�ically and reaches 2% (its equilibrium concentration is~2%).

It is necessary to note that after the completion of oxy�gen conversion and the formation of the main products,their concentrations remain nearly constant throughoutthe burner’s internal cavity height up to its external edge(Fig. 5). At a temperature of ~600–700°С, the proceed�ing of any gas�phase processes with the participation ofСО2 and Н2О deep oxidation products, advancing thesystem towards thermodynamic equilibrium, during theperiod in which gases reside in the burner is unreal. Onlya monotonic drop in the concentration of methane,which seems to be subjected to thermal pyrolysis underthese anoxic conditions with the formation of heaviercondensation products that cannot be registered by us, isobserved. Undoubtedly, the kinetics of these processesmust be studied in more detail. The kinetic equilibriumestablished in the system makes the withdrawal of the syn�thesized target products with the retention of theirattained high concentrations easier. At low values of α, the

conversion of oxygen is nearly complete, and its small reg�istered concentration should be related to the disadvan�tages of the analytical method connected with the substi�tution of some hydrogen by air for the period between thewithdrawal of the sample and its injection into a chro�matograph.

The dependencies on α of the two main Н2/СО andСО/СО2 ratios characterizing the composition of the syn�thesized products are illustrated in Figs. 6 and 7. TheН2/СО ratio grows monotonically and comes to 1.8–2.2at α = 0.37 (the equilibrium H2/CO ratio is 1.68 at α = 0.4and 1.98 at α = 0.3). The СО/СО2 ratio also increasesmonotonically and reaches 5 at α = 0.37 (the equilibriumCO/CO2 ratio is 8.4 at α = 0.4). Hence, in spite of the factthat, on the whole, the thermodynamic equilibrium com�position of the products is not attained, the main param�eters characterizing the yield of CO and Н2 are closeenough to their thermodynamic equilibrium values.

It should be noted that the concentrations of methaneand ethylene, whose thermodynamic equilibrium con�centrations are very close to zero under these conditions,grow with a decrease in α (Fig. 4). At α = 0.37, the resid�ual СН4 concentration is 4.8–6.6%, corresponding to areduction in its conversion from 100% at α = 1.0 to ≈77%at α = 0.37.

The Н2 formation selectivity estimated by the balanceof hydrogen�containing products is approximately equalto 40%, and this gives a hydrogen yield of ≈30% duringmethane conversion in the area of 80%. The CO forma�tion selectivity evaluated by the balance of carbonic prod�ucts is approximately equal to 50%, and this gives a carbonoxide yield of ≈35% relative to the initial carbon. Thesedata obtained in the device, which is relatively simple incomparison with what is now applied in industrial tech�

20

14

12

10

8

6

4

2

00.90.80.70.60.50.40.3

16

18

1.0

C, vol. %

α

CO

H2

CH4

C2H4

CO2

Fig. 4. Dependence of the methane conversion product concentration С in the matrix cavity on the oxidizer excess coefficient α.

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ARUTYUNOV et al.

nologies, allow one to look forward to good practical pros�pects for the process.

The ethylene formation selectivity, which is high inrelation to the initial carbon and is equal to nearly 20%,gives an ethylene yield of approximately 15%. However, inthe burner whose cavity is covered by a metallic grid, theethylene concentration in the products drops nearly tozero. This is most likely caused by the fact that the internalburner cavity temperature exceeds that of the “ethylenegap” (>1100°С), connected with the change in the mech�anism of the interaction between methyl radicals and oxy�gen [12]. At that, traces of ethane, which was not regis�tered under other conditions, appear. However, in theburner cavity covered by a ceramic top, the ethylene yieldremains high and the concentrations of the remainingreaction products do not substantially differ from thoseobtained for the variant with the open cavity.

A series of experiments were performed in a cylindricalburner with an internal diameter of 64 mm, a cavity depthof 60 mm, and a matrix made from a corrugated metallicnonwoven material (metallic felt of Nichrome wire 40 μmin diameter with a porosity of 0.9–0.95). At that, noappreciable distinctions were revealed both in the flametemperature inside the burner cavity and in the concen�trations of the formed products in comparison with theirvalues for the above�described burner consisting of right�angled ceramic honeycombs.

RESULTS AND DISCUSSION

Historically, the combustion (oxidation) of hydrocar�bons was mainly considered as an energy source, andmost attention was concentrated on the kinetics of theprocesses in stoichiometric or lean mixtures, allowing forthe most effective transformation of chemical energy fuelinto thermal energy. Only a relatively short time ago did the

25

20

15

10

5

140100806040200 120

C, vol. %

H2

CO

CH4

C2H4 CO2O2

L, mm

Fig. 5. Dependence of the methane conversion product concentration С in the matrix cavity on the distance L to the burner bot�tom at α = 0.37.

H2/CO

2.5

2.0

1.5

1.0

0.5

00.3 0.5 0.7 0.9 1.1

α

Fig. 6. Dependence of the Н2/СО ratio on the oxidizerexcess coefficient α.

CO/CO2

00.3 0.5 0.7 0.9 1.1

α

8

6

4

2

Fig. 7. Dependence of the СО/СО2 ratio on the oxidizerexcess coefficient α.



hydrocarbon oxidation processes come to be regarded as asource of chemical products [13]. In any case, the possibil�ity of the synthesis of important products in the oxidationof hydrocarbons is connected with their incomplete (par�tial) oxidation; i.e., with the conversion of rich mixtures,mainly proceeding under severely nonequilibrium condi�tions, when the yield of the products is caused by the kinet�ics of the process. The prospects for the creation of newtechnologies for the synthesis of different products (hydro�gen, synthesis gas, oxygenates, olefins, and others) by theincomplete oxidation of hydrocarbons have stimulatedinterest in the technological aspects of rich hydrocarbonmixture oxidation (combustion) and in the kinetics ofthese processes (see, for instance, [6, 12–15]).

With respect to the synthesis of the most demandedconversion products (synthesis gas and hydrogen), themain technological problem is the organization of stablehydrocarbon conversion at a low coefficient α = 0.3–0.5,which is necessary for attaining a high yield of these prod�ucts. This is not a trivial task, as a converted mixture ispractically outside the ignition region under conventionalconditions at these values of α. A number of effectivemethods allow for the expansion of the region of stablecombustion. Among them are the stepwise combustion ofthe mixture, the preliminary heating of gases, the com�bustion of gases under increased pressures, the flue gasheat recuperation by a fresh mixture, the catalytic activa�tion or chemical promotion of the flammable mixture, theignition of the mixture by flame or by combustion prod�ucts from an independent source, and the activation of themixture by different physical methods. All of these meth�ods, to one extent or another, are applied for the organiza�tion of stable oxidation process, and in most cases severalmethods are used simultaneously. Some of the most typi�cal methods are briefly described below for comparisonwith the proposed one.

Autothermal reforming, which is a combination ofpartial methane oxidation and catalytic steam methanereforming, is a widely used industrial technology for theproduction of synthesis gas and hydrogen on the basis ofpartial methane oxidation. In this process, a low totalvalue of α is attained due to the stepwise partial com�bustion of methane under conditions close to stoichio�metric ones with the subsequent addition of methaneand steam to the hot oxidation products. The furthercatalytic conversion of heated gases allows for theobtainment of a nearly equilibrium composition ofproducts. The original variant of this process was devel�oped by the Halder Topsoe Company [16], whichremains one of its leading developers.

The conversion of methane–air mixtures preliminaryheated to 450–600°С into synthesis gas in a high�pressurebomb (P0 = 30 atm) at α ≥ 0.35 in a self�ignition regimeand at lower α < 0.25 in a forced ignition regime isdescribed in work [15]. The yields of Н2 and CO did notdepend on the ignition method and were close to the ther�modynamic equilibrium values. The deviation from the

equilibrium values did not exceed several percent, butgrew with a decrease in α. The authors drew the conclu�sion that it was not reasonable to work at very low values ofα ≤ 0.3, as an appreciable soot formation was observed forthis regime, and at α = 0.25 the residual methane concen�tration was already 11%.

In the case of natural gas conversion in an ener�gochemical internal combustion engine based on aggre�gates, in spite of the increased pressure (mixture compres�sion ratio of 37–40), the possibility of working at low val�ues of α ≥ 0.35 is provided either by the preliminaryheating of the mixture to 100°С or by its forced spark orprecombustion chamber ignition. The methane conver�sion is 82–86%. At α ≈ 0.42, the composition of the prod�ucts is as follows (vol %): 16.6 Н2, 11.2 CO, 2.5 CO2,3.2 CH4, and the H2/CO ratio is 1.5 [6, 15].

In the aggregates based on rocket technologies (liquid�propellant rocket engines), at α = 0.35–0.45 the possibil�ity of the methane–air mixture conversion is mainlyreached owing to an increased pressure inside the com�bustion chamber and the continuous work of an incendi�ary device (separate small�power liquid�propellant rocketengine). An additional contribution is made by the heat�ing of the preliminary oxidant owing to the partial recu�peration of flue gas heat transferred through the combus�tion chamber wall cooled from the outside by the oxidantflow and, evidently, by the high degree of reacting mixtureturbulization. The methane conversion comes to 81–85%. The outlet concentration of H2 is 52–58% and thatof CO is 35–38% for work with a methane–oxygen mix�ture at α = 0.35–0.45 [6, 15].

For the processes of partial hydrocarbon oxidationinto synthesis gas, the initialization by the “volley” injec�tion of radicals or a molecular promoter can hardly beeffective, as it requires very high concentrations for anintroduced promoter [17], evidently, due to the shortchains in these reactions. A wide range of woks is devotedto the investigation of the possibility of synthesizinghydrogen by the conversion of hydrocarbon gases in theplasma of an electrical discharge (see [18]). For example,in [19] hydrogen was obtained in the plasma of an arc dis�charge with the use of a granulated nickel catalyst locatedin the postplasma zone. The energy efficiency of hydro�gen emission was 1.21 MJ/kg at a methane conversion of90% and a H2 yield of 90%. However, as for the chemicalinitialization, it is necessary to continuously introduce alot of energy from an external source [18] to maintain theoxidation process because of the short reaction chains,and this fact makes these processes uninteresting forindustrial application at present.

The catalytic activation of combustion processes,including that for rich mixtures, with the purpose of theirconversion into synthesis gas is intensively studied. Fromthis point of view, the processes proceeding at short timesof contact with a monolithic catalyst (grids, sponge metal�lic blocks, blocks with transparent channels) and, conse�

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ARUTYUNOV et al.

quently, at high hourly space velocities of a flow, are ofespecial interest. At millisecond contact times on similarcatalysts, the oxidation processes pass nearly adiabaticallyin an autothermal regime with a high volumetric capacity.The process begins as a catalytic reaction on the surfaceand further continues in the gas phase at the expense ofradicals and the active products formed on the catalyst[20]. On the sponge Rh/α�Al2O monolith, the methaneconversion exceeds 90% at a Н2 and CO formation selec�tivity that is also above 90% [21].

The intrareactor combustion product heat recupera�tion is used in the filtration combustion processes toincrease the temperature of the initial reagents and toextend the ignition limits. The possibility for the thermo�chemical methane conversion into synthesis gas in therecuperative self�sustaining regime in reactors of differenttypes has been demonstrated. The values of α = 0.29–0.33 are the optimal ones for the attainment of the maxi�mal synthesis gas yield [22].

In works [8–11], the regularities of the gas–air mix�ture combustion in volumetric matrixes under the condi�tions of intensive radiative–convective heat exchangewere investigated and the constructions of newly devel�oped burner devices were described. The development ofthe results of these investigations in the given work hasdemonstrated the possibility of extending the combustionlimits into the region of rich methane–air mixtures in thesurface combustion regime in a volumetric permeablematrix under conditions of locked infrared irradiation.The nearly complete locking of irradiation inside the vol�ume of the matrix, fulfilled in the form of a deep geomet�rically locked cavity, reduces the radiation losses in thecombustion by many times, and the appearing radiativepositive feedback provides for a higher temperature of theinternal surface of the matrix and the appropriate heatingof the gas mixture in its channels.

The internal recuperation of the heat of the reactionproceeds at the expense of the partial combustion heattransfer to the inlet gas through the chain of the flamefront (combustion products)–permeable matrix–gaspassing through matrix channels. Hence, the inlet gas isheated before the flame front owing to the heat releasedduring the combustion of the gas, which was burnt earlierand left the system and has been partially accumulated bythe reactor cavity (matrix) walls. Therefore, such a chem�ical system is open and the thermodynamic calculation ofthe products cannot be applied to it in principle. Theproducts leaving the system have a lower temperature incomparison with that of the adiabatic combustion prod�ucts due to the intensive cooling of the gas by means ofconvective and radiative heat withdrawal to the matrixwalls. The internal matrix surface temperature is causedby the equilibrium between the heat generation by the gascombustion; the radiation heat wastes into the environ�ment; and the heat entrainment by the cold gas, whichpasses through the matrix, becomes heated, and with anappropriate temperature enters the combustion zone.

This temperature may be accepted as the initial one for thegas entering into the flame front, although it can actuallybe higher at the expense of radiation and the convectiveheating of the gas directly before the front. In the flamefront itself, the chemical processes proceed so quickly thatthey could be considered to be adiabatic if it were not forthe intense radiative–convective exchange with thematrix surface.

The initial heating of the gas at the expense of the reac�tion heat recuperation and the reduction of radiationlosses by several times during locking irradiation in thegeometrically locked matrix cavity provide for the expan�sion of the ignition limits and make the combustion ofvery rich mixtures with α ≤ 0.4 possible. Methane com�bustion itself proceeds over millisecond time periods inthe very narrow 1� to 2�mm flame front zone near thematrix surface. The composition of the products is causedsolely by the kinetics of the gas�phase combustion and,undoubtedly, is not equilibrium. Strongly endothermicprocesses, such as the reactions of steam or carbon diox�ide reforming that play a great role in the establishment ofa thermodynamic equilibrium in catalytic systems, can�not produce any appreciable effect on the composition ofproducts at ~700°С in the gas phase over the time of theirresidence in the burner.

The temperature constancy in the volume behind theflame front results not from the fact that the system attainsthermodynamic equilibrium, but from the temperatureprofile smoothing under the action of intensive equilib�rium radiation in the locked matrix cavity. However, evenin the absence of oxygen, at these temperatures behind theflame front the pyrolysis and residual methane condensa�tion processes that produce a great number of differentcompounds from heavier hydrocarbons to soot can pro�ceed at appreciable rates.

In this work, we have shown the possibility of theapplication of a volumetric matrix for the organization ofa stable combustion for rich methane–air mixtures at α ~0.4 and a low (500–700°С) temperature of combustionproducts with the obtainment of hydrogen and carbonoxide concentrations close to the thermodynamic equi�librium values. The exclusion of radiation losses allows forthe realization of the conversion for those mixtures whosecombustion is impossible under conventional conditions.To put this into perspective, it is possible to change the gasmixture composition and the radiation field along thegeometric coordinates of the volumetric matrix, whichleads to the control of the distributed surface combustionregime and the optimization of the composition and yieldof the target products.

The obtained results have shown the possibility ofeffective natural gas conversion into synthesis gas with theclose�to�optimal ratio of H2/CO ≈ 2. When atmosphericair is used as an oxidizer, at the given convertor inlet thehydrogen concentration reaches 22% and that of carbonoxide comes to 11%; these values are both close to thethermodynamic equilibrium ones for these conditions.



The principal possibility of the conversion of methaneinto ethylene with quite a high yield (up to 15%) in such aburner device was demonstrated. The resemblance of theresults obtained in burners with different configurationsand matrixes made from such materials as ceramic andmetal, which are very different in respect to their catalyticproperties, confirms the homogeneous character of theconversion process that proceeds mainly in the gas phaseabove the matrix surface.

Owing to the specificity of the conditions realized in aburner device with a volumetric matrix, the kinetic mod�eling of oxidation methane conversion is extremely com�plicated in it, and adequate results are hardly expected.Nevertheless, the results of the simulation of methaneconversion in the flame of a diffusion air burner [23] arequite comparable to those of our experiment. When per�forming the simulation, we obtained a sharp concentra�tion maxima for Н2 and CO of 21.4 and 14%, respectively,at α = 0.4. At lower α values, these concentrationsdropped sharply. As the calculation was conducted for acertain fixed time of the process, this most likely reflectsthe reduction of the rate of the process with a decrease inthe temperature and the respective drop in the methaneconversion during the given fixed time period. Some indi�cations of an analogous reduction can be discovered in therespective experimental curves in Fig. 4. It is interestingthat at α ≤ 0.4, a sharp reduction in the internal reactortemperature with the simultaneous drop in the yield is alsoobserved in the synthesis of other products, for example,technical carbon [24] by partial natural gas oxidation.Evidently, in this case the decrease in the methane conver�sion also is the main reason.

Generators based on rocket technologies are the clos�est analogs of a synthesis gas generator based on the com�bustion of hydrocarbons in a radiation burner with a volu�metric matrix. However, as distinct from the volumetricmatrix–based radiation burners for which the reductionof radiation losses and the reaction heat recuperation arethe primary factors influencing the expansion of the igni�tion limits, for the generators based on rocket technolo�gies such factors evidently are the increased pressure andhigh flow turbulization degree.

In spite of the formal resemblance to the processes ofcatalytic combustion on grids or honeycomb block cata�lysts, a principal difference exists between them and thecombustion of hydrocarbons in a radiation burner with avolumetric matrix. The main factor providing for the sta�bility of the catalytic conversion process is radicals, whichare formed on the catalytic surface heated to a tempera�ture above 1000°С and leave it for the reaction volume[17]. At the same time, neither the reduction of radiationlosses nor the reaction heat recuperation plays any impor�tant role here. Certainly, for the mixture combustion in aradiation burner with a ceramic or metallic volumetricmatrix, one must not rule out the certain contribution ofcatalytic reactions inside the channels of the heatedmatrix, but the relatively low temperature (~400°С) of the

latter is hardly able to produce any appreciable initiatingeffect, even if it leads to the small conversion of the mix�ture before the flame front.

On the grounds of the obtained results that coincidewith the conclusions of works on the other methods ofmethane conversion into synthesis gas, one can draw theconclusion that the reaction mixture composition that isoptimal for the production of synthesis gas in the partialoxidation processes lies within the interval of α = 0.3–0.4regardless of the conversion method. Although from thethermodynamic standpoint lower values of α seem to bemore favorable, as they give higher equilibrium Н2 andCO concentrations, in most cases these values are practi�cally unattainable. Besides the abundant soot formation atα < 0.3, the quick descent of the temperature with adecrease in α leads to a drop in the methane conversionand, consequently, in the yield of Н2 and CO.

The burner device suggested in the given work is sim�pler than the above�described known synthesis gas pro�duction methods, in particular, catalytic reforming, and atthe same time, it is able to stably provide similar values ofmethane conversion and hydrogen and carbon oxideyield. Its productivity defined by the fuel mixture flow ratecan be changed within wide intervals. And with an allow�ance for the possibility to widely vary the burner powerand the module construction of burner devices, similarhydrogen and synthesis gas generators are able to cover thewhole interval of interesting capacities from small capac�ity and even from microcapacity units to large chemicalplants. At that, taking into account the gas�phase charac�ter of the process, such a scaling will hardly create anyprincipal difficulties and significant changes in the unit’scapital costs.

Besides simplicity, compactness, and a high produc�tivity, among the advantages of a generator based on aradiation burner with a volumetric matrix, one shouldinclude the process of autothermicity, the possibility toconvert hydrocarbon gases with different compositionsand even heavier hydrocarbons, and also the absence ofthe dirtying of the working matrix surface in the case ofpossible soot formation. Note that the conversion can berealized in a radiation burner with a volumetric matrixalso at a high pressure. This makes this method attractiveas a source of synthesis gas for industrial processes thatrequire a high pressure of synthesis gas.

The device can be supplemented with a catalytic con�verter of carbon oxide into hydrogen in the reaction withthe steam, and this will allow for an increase of the hydro�gen yield by 1.5 times. Such hydrogen generators are espe�cially prospective for autonomous small�capacity sys�tems, for example, for distributed energy supply sourcesbased on electrochemical generators (fuel cells) and forhydrogen fuel stations. At present, it is the absence ofcompact autonomous sources that is one of the main fac�tors restraining the progress of hydrogen energetics.

28


ARUTYUNOV et al.

CONCLUSIONS

The possibility of methane conversion in very richhydrocarbon–air mixtures in a stable surface combustionregime in a volumetric permeable matrix under condi�tions of locked infrared irradiation was shown. The possi�bility of controlling the composition of the combustionproducts and realizing the oxidation conversion for a mix�ture whose combustion is practically impossible underconventional conditions was demonstrated. The effectiveconversion of natural gas by atmospheric air into synthesisgas with the ratio of H2/CO ≈ 2 and the concentrations ofhydrogen and carbon oxide of up to 22 and 11%, respec�tively, was performed.

From the standpoint of the technological prospects forthe process, its further development at increased pressureswith the use of enriched air or oxygen as an oxidizer is ofgreat importance. However, for economic and technolog�ical reasons, the application of cheap nitrogen�containingsynthesis gas, which is produced during the oxidation byair and has the parameters close to those obtained in thegiven work (see, for instance, [25]), has good prospects,especially in the field of small�capacity technologies.

It is possible that the above�discussed relatively simpledevices based on the matrix combustion of hydrocarbonsand not connected with the use of cumbersome techno�logical apparatuses and energetic aggregates will be able tonot only fundamentally simplify the conversion processand increase its effectiveness, but be used for the produc�tion of small hydrogen volumes. The practical creation ofsimple and efficient hydrogen and synthesis gas sourcesbased on burner devices with a volumetric matrix for theconversion of gaseous and liquid hydrocarbons can makethe wide application of electrochemical hydrogen�basedcurrent sources with a power of several kilowatts to severalmegawatts realistic for the reliable energy supply ofremote regions and for the autonomous energy supply ofhousing and communal services. The use of compactradiation matrix burner–based sources for the productionof hydrogen from network gas or liquid hydrocarbonsdirectly at fuel stations will allow for the transfer of theproblem of hydrogen motor transport onto a practicalplane.

ACKNOWLEDGMENTS

The authors are grateful to S.N. Podoinitsyn for hisparticipation in performing the experiments.

NOTATION

α—oxidizer excess coefficient;γ = Sc/S —matrix material porosity, which is equal to

the ratio of channel cross�section area Sc to the total sur�face area S;

C—concentration of reaction products, vol %;ΔТр—adiabatic heating for methane–air combustion

products;L—distance from the matrix burner bottom, mm.

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A Generator of Synthesis Gas and Hydrogen Based on a Radiation Burner