Indian Journal of Engineering & Materials Sciences Vol. 6, April 1999, pp. 73-86

Hydrogen generation from natural gas and methanol for use in electrochemical energy conversion systems (fuel cell)

N S Arvindan, B Rajesh, M Madhivanan & R Pattabiraman

Central Electrochemical Research Institute, Karaikudi 630 006,India

Received 8 January /998; accepted 23 December /998

Hydrogen is the 'preferred fuel for use in the present day fuel cell systems developed for various commercial applications. Of all the potential sources of hydrogen, natural gas and methanol have many advantages since the process of conversion of these primary fuels into hydrogen has been described as a simple, most efficient and economically viable route yielding higher volume percentage of hydrogen. This paper reviews the state of the process , the catalyst materials for the reforming reaction and the reaction conditions.

Fuel cell systems

About fuel ceUs

Fuel cells are electrochemical energy conversion systems converting the chemical energy of a fuel directly into d.c . electricity. They differ from other power generation systems in that the efficiency of energy conversion process is very high compared to the other conventional methods of power generation namely thermal and gas turbines and the pollutant emissions from these fuel cell power plants are also less'. They are silent and· noise free and can be sited at locations where the fuel source is available in plenty.

Fuel ceU subsystems

The major subsystems of a fuel cell power plant are

HEAT AKJ STEAM

indicated in Fig.l. It essentially consists of a fuel processor unit, electrochemical cell stack and power conditioner unit. Since hydrogen is the only reactable fuel in a fuel cell, all primary fuels such as coal, natural gas, naphtha are converted into hydrogen in the fuel processor unit.

The electrochemical cell stack produces the d.c. electricity by converting the chemical energy of hydrogen into electrical units. Essentially a typical fuel cell consists of two electrodes and the electrolyte is sandwiched in a matrix and positioned between the electrodes as shown in Fig.2. The fuel from the fuel processor namely hydrogen is fed into the anode compartment and the oxidant oxygen/air to the catho'de chamber. The electrochemical reactions take

FLEL CELL STACK

DC AC POWER

FOR Fl£L PRXESSGl STEAM TLRBINE I

(().;tBlNED CYCLE

AC POWER EXHAUST GASES

COGENERATION

Fig. I-Major subsystems of a fuel cell power plant

74 INDIAN 1. ENG. MATER. SCI., APRIL 1999

e LOAD

, . 5 1>.=-------1 ,

I _OXYGEN

~ H:t +-In· 1

~ Hz~~WATER CATMODC

A~.~DE~~EL=E=CT=RO=LV~T~E~~~

Fig. 2-Schematic of an acid electrolyte fuel cell

place on the electrodes and there is a flow of electrons from the anode to the cathode through the external circuit. The overall reaction in the electrochemical cell IS

. . . (1)

The electrical energy output (E) is rerated to the fuel energy (Ml), according to the relation E = -/1G/2F, where ~.G is the free energy change for the reaction, determined by the equation

... (2)

where, T is the operating temperature, /).S is the entropy change and F is the Faraday constant = 96500 IN equivalent. The theoretical (ideal) efficiency of the electrochemical energy conversion step is /1G//1H and is about 83%. Individual cell produces a voltage of about 1.0 V, but a stack consisting of large number of cells produces a higher output.

The third important unit is the power conditioner, which converts the d.c. into a.c . electricity suitable for onsite applications. The overall efficiency of energy conversion from fuel to electricity depends on the efficiency of these various subsystems. The fuel cell by itself has a maximum fuel utilization efficiency of 75-80%.

Types of fuel cells The fuel cell systems are regarded as the clean

energy system and hence considered as the power generation systems for the next century. There are about fi ve major types of fuel cells under development worldwide. These types differ among themselves in their operating temperature and the nature of

electro lyte·. These fuel cell s are (i) low temperature alkaline fuel cell s (AFC) with KOH as the electrolyte operating below 100 DC, (ii ) polymer membrane electrolyte fue l cells (PEMFC) with a polymer sulphonic ac id membrane as the electrolyte which also operate below 100 DC, (iii) medium temperature fuel

cells with phosphoric acid (PAFC) as the electrolyte in SiC matrix operating in the temperature range 180-210 DC, (iv) high temperature fuel cells with molten

. carbonates (MCFC) as the electrolyte embedded in LiAI02 matrix operating at 650 De. and (v) very high temperature fuel cells employing stabilized zirconia as the ionic conducting electrolyte (SOFC) operating at 1000 De.

The characteristics of these types are described in Table I (Ref.2).

Importance of fuel processor

The fuel processor is an integral unit of the overall fuel cell system. The low and medium temperature fuel cell systems work with hydrogen of high purity and cannot tolerate the presence of CO or CO2 in the fuel stream. However, high temperature fuel cell systems namely MCFC and SOFC can tolerate the presence of impurities like CO and CO2 to a certain extent. In addition, the high operating temperature also favours direct oxidation of fuels such as CO in MCFC and CH4 in SOFe. But for the most practical fuel cell systems currently under use for various commercial applications (PAFC and PEMFC) , it is necessary to produce hydrogen of high purity from the available primary fuel source. This must be an ~:nergy efficient, economical and cost effective method producing higher yields of hydrogen. This step also includes process optimization, purification and gas cleanup columns.

Fuel sources for hydrogen

Hydrogen is also an important conunodity in many industrial processes as both a raw material and an energy source. Hence, its production has been identified as a major subject of R&D since the past decade and the major processes have been described earlier'-? The processes include (i) coal as a primary source _. by steam iron process or coal gasification procedures4

, (ii ) hydrocarbons as a source - by catalytic steam reformjng of natural gas and light weight hydrocarbons) or catalytic decomposition of natural gas and partial oxidation of heavy oils6 and (i ii ) non-hydrocarbon based processes such as

electrol ytic, thermo-chemical, photochemjcal and photo-electrochemical methods 7.

The economics of hydrogen production ·by vari ous processes have been compared by various authors8

-11

•

Of the above processes, steam reformati on of hydrocarbons has been reported to be the most efficient and economical process .

-4

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ARVINDAN et 01.: HYDROGEN GENERATION FROM NATURAL GAS AND METHANOL 75

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Hydrogen Production by Refonning of Natural Gas

About natural gas Steam refonnation of natural gas is a well known

and widely used technology. Natural gas is reported to be abundant around the world. The world annual production is estimated to be about 2000 BCu.MI2. It can be easily transported as gas through pipelines and also as a liquid (LNG).

Natural gas (NG) is cheapest of all primary fuels and is the most environmentally acceptable in tenns of its products of combustion. The principle component is methane. The composition varies from 76 to 94 % at different countries. The reminder being lightweight hydrocarbons, ethane (3-12 %), propane (0.5-3.0 %), butane (0.1-1.0 %), higher molecular weight (0.5 % max.), nitrogen (4%) and the rest being carbondioxide. The natural gas may contain a small amount of sulfur in the fonn of H2S, COS and mercaptans. Hence, the steam reforming reaction is simply the steam methane reforming (SMR) reaction .

Steam methane reforming (SMR) The process basically consists of three stages. The

basic steps are shown In Fig.3a. First, the liquid hydrocarbons are vapourised in pre-heaters . The heat for this step is provided by using auxiliary heaters or by heat exchange with hot streams generated by the process. The first stage is the catalytic conversion methane and steam to produce hydrogen and carbon oxides (synthesis gas).

CH4 + H20 ~ CO + 3 H2 t:ili = + 206.8 kllmol

... (3)

The required heat for this step is supplied by the combustion reaction of a part of the fuel and the steam for the reaction is produced in the boilers .

CH4 + 2 O2 ~ CO2 + 2 H20 -Mf = -800 kllmol

.. . (4)

In the second stage, an additional quantity of hydrogen is generated, by reacting the synthesis gas (SYN gas) in one or more shift converters .

CO + H20 ~ CO2 + H2 6H = - 41 .2 kllmol

. . . (5)

Both the reforming reaction (3) and water gas sh ift reac ti on (5) are carried out over nickel catalyst at 500 °C. The shi ft gas is cooled by heat exchange with

O\----E . . l (Sulptu)

Steam

fuel

H2 (Proo.x:t)

Me1tcnation

Fig; 3a-Flow sheet for the steam reformation of natural gas (conventional)JO

the incoming feed water and the condensed water is collected and returned to the system via a dearator.

The third stage is the gas purification. The off gas from shift converters is purified by CO2 removal, methanation and cooling and enriched in hydrogen . Essentially the chemistry and conditions of the above steps are elaborated below as described elsewhere 13

•

Purifitation of natural gas To protect the catalyst in the hydrogen plant, the

feedstock hydrocarbons have to be desulfurized before being fed into the reformer. When the catalytic SMR is used to provide fuel for fuel cell, it is essential that sulphur compounds are removed by various available processes, since'S' is a poison for Ni steam reforming catalysts and Ni or Pt electrodes employed as anode in fuel cells. The high temperature fuel cells especially MCFC and SOFC use Ni electrodes. The sensitivity of Ni electrodes to S poisoning is a function of time, temperature and amount of S level in the fuel. SOFC because of its high operating temperature is able to tolerate a high concentration of S (3 ppm) than MCFC (0.2 ppm)14. Hence, the sulphur removal step has become essential prior to the SMR process.

The vapourised NG is mixed with a small quantity of hydrogen (2-5%) and passed through a catalyst system containing Co or Ni (3-4%) with molybdenum oxide (13.5-15%) on alumina (76-80%) support to convert the S compounds into H2S at 350 - 400° C with a space velocity of about 1000/h.

Steam

CH

(SuIpt-ur) FU!'I

c.o,-

f . Hz --; MethanatlOrl

(Proc1Jct) .... ' ----......

Fig. 3b-Flow sheet for the steam reformation of natural gas NG (PSA modified)1O

RSH + H2 ~ RH + H2S and RSR + H2 ~ RHR + H2S

(6)

(7)

The H2S produced is then removed by direct adsorption on a bed of ZnO catalyst. This process is described hydrodesulfurization (HDS) .

... (8)

Chemistry of methane conversion

The thermodynamics SMR process is described by RosenI 5-17• The desulfuri~ed feed stock is then mixed with high pressure steam and fULther heated and passed into the reactor, where the reforming reaction (3) takes place at temperatures above 500 °c, typically at 800 - 900 °C at 2.17 - 2.86 MPa (300-400 psi) over a Ni catalyst supported on AhO). The higher hydrocarbons in NG also react with steam in a similar fashion . CnHm + n H20~ n CO + (n + m/2) H2 .. . (9)

A certain amount of unreacted methane and steam exist in the hot gases leaving the processor.

Since the reactions inside the reformer are reversible, steam is normally added in excess of the stochiometric requirement. Low pressure favours the reforming reactions . High pressure however has certain economic advantages in that it reduces subsequent purification, heat recovery and compression costs. The high steam to C ratio is

.".

ARVINDAN et at. : HYDROGEN GENERATION FROM NATURAL GAS AND'METHANOL 77

employed to ensure minimum concentration of CH4 in the SYN gas and maximum conversion of CH4 (>80 %) to oxides of carbQns. Typically the optimum conditions are set at steam to C ratio at 3 to 5, operating temperature at 815°C, pressure at 508 psi 16.

A maximum of 95% of CH4 is reformed in this step. The typical composition of the SYN gas at 100 psi from the reformer is· reported in Table 2 [Ref. 9]. A comparison has also been made with the other processes.

Carbon monoxide shift reaction After the reformer, the process gas mixture passes

through a heat recovery step and .then through additional reactors where 94 % of CO is converted to H2 via the exothermic water gas shift reaction at 200 -400 DC. Normally two stages of conversions are utilized for this step.

High temperature shift reaction (HTSR) The same catalyst NiJAh03 or Cr on AhO) is used

for this reaction at 370° CiS. The gas exits the HTSR step at 220°C, and preheats the incoming boiler and methanator feeds.

Low-temperature shift reaction (LTSR) Over a low-temperature catalyst of Cu-ZnO/AhO),

83 % of remaining CO in the raw gas is reacted via the shift reaction at 180-200 DC, with a space velocity of 4000/h. The gas exit at the LTSR has a maximum purity of 77% H2, 0.3% CO, 18% CO2 and 4.7% C~.

It exits at 150 °C and preheats incoming feed water. Here also excess steam is employed to favour the shift the reaction towards more CO2. This is to avoid the ' carbon deposition through the Boudard reaction, which is catalyzed by Ni.

lCO ~C02 +C

Gas purification Systems

CO2 removal

... (10)

The off gas from the reformer after heat exchanger is compressed to 3.5 MPa. The cold gas is passed through gas purification units to remove CO2. Two specific processes were recommended. One method is the wet scrubbing process using mono-ethanolamine (MEA), hot K2C03 or sulfinol.

Methanation Steam preheats the methanator feed to 350°C and

2.4 Mpa. The CO in the stream is further converted to CH4 by a methanation reaction on a '15.48% Ni/Si0 2

catalyst at 315°C with a space velocity of 60001h19.

... (11)

Table 2-Compositions of SYN gas and product gas by various processes

SI. .No. Process H2 (%) CO(%) CO2(%) CH4(%) N2(%) Others Ref (%)

(a) SMR 74.0 8.0 6.0 2.0 8 (b) (SYN gas) 76.7 12.0 10.0 I.3 13 (c) LTSR 77.0 0.3 18.0 4.7 24 (d) Purified 97.0 0.4 0.9 1.7 8 (e) PSA modified 99.7 0.3 24

2. POX of heavyoils 46.0 46.0 6.0 1.0 1.0 8 (SNG)

3 (a). POX-NG 60.0 34.0 4.0 1.0 1.0 8 (SNG)

(b) Purified 800 2.0 14.0 3.0 24 4 CMR (SNG) 37.8 45 .1 8.1 8.7 25 5. POX of coke oven 60.0 6.5 22.5 6.5 4.5 24

gas (SYN) 6. Coal gasi fier 34.0 48 .0 17.0 1.0 10

(a) TEXACO gasifier (b) Kopper Totak 29.0 60.0 10.0 1.0 \0

7 (a) Melhanol 41.0 21.0 38 .0 65 48 .0 20.0 21.0 11.0 65

(b) POX 63 .0 22.0 4.0 11.0 67 (c) Reformer 74.0 3.0 23.0 61 (d) before puri fication 80.0 2.0 14.0 1.0 3.0 24 (e) purified 99.9 0.1 61

8. Thermal cracking 93.3 0.1 6.50 0.10 \0

78 INDIAN J. ENG. MATER. SCI., APRIL 1999

Cooling The hot product gas preheats the feed water and is

cooled to 25 DC with cooling water. Water is separated and the product. containing 97-98 wt.% hydrogen exits at 25 DC and at 2.4 MPa. The final CO2 content is 0.1 wt %. The rest is about 1.8 % CHt.

The main overall chemical reaction for SMR can be represented as CH4 + 2 H20 --') CO2 + 4 H2 ... (12)

Pressure swing adsorption (PSA)

An alternate technology to wet scrubbing is the use of pressure swing adsorption (PSA) technique2o. The PSA procesS has been traditionally used to produce one high purity gas stream from a gas mixture. One of the most common uses of this technology is in the production of ultrahigh purity hydrogen from various gas streams such as steam methane reformer (SMR) off-gas. Process cycle steps for the PSA process are illustrated by SMR off-gas fractionation for the production of hydrogen and carbon dioxide21 . The capital and power savings of this process as well as other advantages compared with the previous technology are also discussed.

In this process the CO2 and other impurities, except hydrogen are adsorbed in beds of molecular sieves and activated carbon at ambient temperatures . An excellent review is presented on the use of activated carbons for gas separation and purification22. Single, binary, temary and quatt;rnary adsorption equilibrium of CO2, CO, CH4 and N2 on molecular sieve SA and activated carbon were reported over a pressure range from 10-4 to 10 MPa, a temperature range from 303 to 363 K and at various compositions23. The CO2 and CH4 adsorbed from the cold SYN gas are released by the solid adsorbent upon reduction of pressure into a purge gas stream, which is suitable for in the reformer to generate the heat for the process step. By using mUltiple adsorbent vessels, which are periodically and automatically switched for adsorption at high pressures to desorption at low preSsures, the production of hydrogen is carried out in a continuous manner.

The process flow sheet after PSA modification is simpler as shown in Fig. 3b. This process reduces the number of unit operations and complexity of these steps by replacing the steps L TSR, CO2 removal and methanation24. The purity of hydrogen from this process can be as high as 99.7% and can thus be more suited for fuel cells.

Partial oxidation (POX) This process basically involves conversion of steam, oxygen and CHt to hydrogen and carbon oxides. The reaction proceeds at moderate pressures; which may be catalyzed or non-catalyzed (150-1500 DC). The catalytic POX reaction works at 590 DC at 3-8 MPa (435-1160 psi) . The POX is normally used when the feed stock is a heavy hydrocarbon, which can not readily be reacted over a catalyst.

... (13)

The POX is also attractive for the conversion of CH4 or oils (n = 1 and m = 1.2).

CHt + 1/2 O2 ~ CO + 2 H2 Mi = 38.8 kllmol

... (14)

Typical gas compositions of SYN gas leaving the POX reactor is also shown in Table 2. The SYN gas from this reactor can also be purified either by conventional or PSA modified steps. Compared to the SMR, the POX requires additional facilities like air separation columns to provide oxygen as well as larger shift and purification columns. The process details and flow sheets are also discussed by Rosen lO

• The final product composition of POX is 80% H2, 2.0% CO, 14% CH4, 1% N2 and 3% CO2.

Thoe combined SMR and POX is occasionally practiced and can provide an economic advantage over either by itself. The combined technology is widely u·sed in the ammonia industry. However, since POX of CH4 produces lesser hydrogen per mole of CHt than SMR, fuel cells employing the hydrogen from POX reactors will have lower electrical generation efficiencies than those employing SMR off gas.

CO2 reforming of methane (CMR) Steam may be replaced completely or in part by

CO2. and the reactions occur in the temperature range 500 - 600 DC, which produces a SYN gas with a low H 2/CO ratio, which is desired for certain specific applications .

CH4 + CO2-4 2 CO + H2 Mi = 247 kJ/mol

or 2 CH4 + CO2 + H2 ~ 3 CO + 5 H2 Mi = 480 kJ/mol

(15)

... (16)

This reaction also has very important environmental implications since both methane and carbon dioxide contribute to the greenhouse effect. Converting these

ARVINDAN et at.: HYDROGEN GENERATION FROM NATURAL GAS AND METHANOL 79

gases into a valuable feedstock may significantly reduce the atmospheric emissions of CO2 and C~. A comprehensive review of the thermodynamics, catalyst selection and activity, reaction mechanism, and kinetics of this reaction is discussed by Wang25

.

Group - VIII metals supported on oxides are used as catalysts. However, carbon deposition causing catalyst deactivation is the major problem inhibiting the industrial application of the C02/CH4 reaction. Ni­based catalysts impregnated on certain supports show carbon-free operation and thus attract much attention.

Thermal cracking

Under conditions of stochiometric CO2 reforming, carbon deposition occurs by Boudard reaction 2CO~C02 +C ... (16) !1H = 172 kllmol or due to methane cra~king .

CH4 ~ C + 2 H2 !1H = 75 kJ/mol

. . . (17)

. .. (18)

Thermal cracking occurs in the absence of air and can be catalyzed by metals. Thermal decomposition of C~ occurs at 650 - 980 °C at 1-2 atm (10.1-20.3 kPa.). In principle these reactions produce very pure hydrogen since no carbon oxides are formed . The final composition of the product gas is 93 .3% H2, 6.5% CH4, 0.1 % CO and 0.1 % N2. However, such advantage is outweighed by the problem of C deposition. The carbon collected is used to provide the heat for the reactor. Higher hydrocarbons produce more C than eH4 . Thermal or steam cracking of hydrocarbons occurs at 600 - 65(} °C even in the absence of Ni catalysts. Such e deposition is detrimental to the life

of the reformer catalyst employed in MCFC or SOFe. To avoid this, pre-reforming of NG is done to remove C2 + higher hydrocarbons before, the gas is fed to the main reformer. Pre-reforming is conveniently carried out in the temperature range 250-500 °C26

.

Catalyst development for the steam reforming reaction Several metals have been found to catalyze the

steam reforming reaction, but Ni is the generally preferred catalyst on cost grounds. In commercial steam reforming catalysts, the Ni is supported on a refractory oxide such as Ah03, MgO, or mixed oxide ceramics, which can tolerate high temperatures . The Table 3 indicates various other catalysts reported for the reformation of CH4 and other hydrocarbons . Porous U30 8 was reported as a support for Ni and Rti catalysts for SMR at P = I atm and T = 600 - 700° e 27

.

The hydrogen production rate was as high as 17-18 cm3/s per gram of the catalyst.

The Ni catalysts are prone to deactivation and sintering at low temperatures . The excess pressure of steam oxidises the Ni catalyst. A Ni (6.8 wt.%)1 alpha­AI 20 3 catalyst showed a continuous and irreversible deactivation over 430 h due to sintering of Ni in the presence of 2% H2 addition at 600 °C28

. At 744 °C, the catalyst also showed an activity decrease with 2% H2 addition, but the activity was stabilized at a certain level over 130 h and it recovered after the H2 addition was stopped.

Development of highly active catalysts for SMR and POX of methane to produce hydrogen with high rates was reported29.Jo. The effect of Ru combination with Ni/La20 3 on SMR and POX on the composition

of SYN gas are also described. ANi-based three­component catalyst such as Ni-La20rRu was reported to be suitable for both the reactions. The catalyst

Table 3-Catalysts for steam reformation of methane

SI.No Fuel Catalyst Support Temp.("C) Ref. I. CH4 Ni A~03 18 2. CH4 Ni-Ru U30 R 600-700 27 3. CH4 Ni (6.8 wt%) ~O3 744 28 4. CH4 Ni/Ni-Ru La203 600 29 S. CH4 Ni/Ni-Pt,Ni-Ru La203,Ce02 600 30 6. CH4 Ni-Ru AI 20 3 600 31 7. CH4 Pt-Pd-Ru AI20 3 500 32 8. Ethane Li/Li-Na MgO 600 35 9. Propane Pt-Rh CeO/AI20 3 >350 33 10. Butane Ru-Rh zr02 500 34 II. C+S HYLt"O- Ni Hexaluminates

carbons Ni BaAI20 9 600 36 12. CH4 Ni -CaO CeO/AI20 3 750 37

8b INDIAN 1. ENG. MATER. SCI., APRIL 1999

composition was set at 10 wt. % Ni, 5.6 wt. % La203, and 0.57 wt. % Ru. The precious metal enhanced the reaction rate very markedly, and this synergisfic effect was ascribed to the hydrogen spillover effect. A marked enhancement in the reaction rate of CMR was observed by the modification of a low concentration Rh to the Ni- Ce20 ,-Pt catalyst. Very high space-time yields of H2 (i .e., 8300 molJl h in POX of methane at 600 °C with a methane conversion of 37.5%, and 3585 molll h in CO, reforming of methane at 600 °c with a methane conversion of 58%) were also reported . A similar effect of the promotion effect of Ru on Ni/AIzO, catalyst was also reported, l.

Similarly a wide variety of catalysts have been reported for SMR of CH/2

, and other hydrocarbons33.

36 and CMR reaction:n . The hexaluminates can retain the hrgh surface area >20 m2/g up to 1300 DC, which is quite desirable for catalytic combustion'6. These supports not only suppress the sintering of Ni particles but also inhibit the C deposition .

Steam reforming technology

The conventional catalytic steam reforming technology has been reviewed periodicall/8

.39

•

Reformer plants vary in size with a hydrogen production capacity ranging from 100 NM' /h to 14000 NM ' /h. For integration with fuel cells medium size refo rmers are generally required . Several reformer technologies have been identified for fuel cell power plant applications and reported by Dicks l2.

The directly fired atmospheric tubular reformer is the main workhorse. Depending upon the product and process requirement a number of alternate reforming methods have been developed . These include membrane reformers, heat exchange reformers, intensified combined reactors, partial oxidation reformers, auto-thermal or secondary reformers and adiabatic pre-reformers.

Compact reformers The reformer is a direct fired chamber containing

single or mUlt iple rows of high nickel alloy tubes HK 40 or Incoloy 800. Tubes are normally 72-110 mm dia and 10-13 m long. The catalyst is 5-25 wt. % Ni on AL20 .1, or CaAlz04 or MgAI 20 4 . The inlet steam to carbon rati o is 3 to 5. The reaction proceeds with a space velocity of 5000 - 8000/h at 2.16 - 2.51 MPa.

The outlet gas temperature is 800-870 DC. Two famous designs of this tubular type are the UTC design developed by United Technologies Corporation38 and KTI compact reformer40

.

The IGT gas generation process employs a packed bed reactor with Girdler G-56 B catalyst, reformed a feed gas of CH4 to steam (I :3) at 800 °C41

. The shift converter employed Girdler G-66 catalysts at 175 -300 DC. The product gas was 76% Hz, 19.7% CO2,

0.3% CH4, 1 % H20 and 8 PPM CO. They have been designed for fuel cell systems and they offer high thermal efficiency, compactness and ability to withstand thermal cycling.

A reformer using a circulati ng fluidized bed has been reported for the production of hydrogen and SYN gas42

.4,. A catalyst is circulated between two

reactors for the catalyt}c decomposition of methane to hydrogen and carbon, and for the gasification of carbon deposited on the catalyst. Catalytic activities of NilSi02 for both CH4 decomposition and CO2

gasification of deposited carbon were investigated at 600-750 °c in a periodically operated fixed bed reactor.

IHI and British Gas have developed plate type reformers44

, but the tubular and spiral type45 are the widely used. The configuration of the spiral type is shown in Fig.4 . The feed gas enters an internal heat exchanger, where its temperature is raised to 800 °C by the process heat. The catalyst ( [ - 4 mm size) is located in the spiral shaped passage ways. The feed gas enters into the catalyst bed from its outer section. The reformed gases [eave the catalyst chamber at

FUJE GAS -NET

HEAT RE'SlSTAICE

Ir ~fr \ lMG

IL I

~ I I

I Fll£ GAS

' - OUTlET

HEAT - EXCHA/(jf

\ REACTION

- Gl.S OUTlET

I PRX:ESS GAS IN.£T

Fig. 4-Spiral steam reformer for natural gas l2

ARVINDAN el at.: HYDROGEN GENERATION FROM NATURAL GAS AND METHANOL 81

800 °c and pass to the heat exchanger when they are cooled to about 310°C. The advantages of this design are the high heat transfer area and high catalyst effectiveness.

Membrane reformers This has been demonstrated in the laboratory level,

where hydrogen produced by SMR is selectively removed from the reactor using a Pd membrane46

.

These metal dispersed alumina membranes were also used in a membrane reactor for ' methane steam reforming at low temperature. In the temperature range of 300 - 500 DC, the membrane reactor attained a methane conversion twice as high as the equilibrium value of the packed bed' catalytic reactor system as a result the selective removal of hydrogen from the

. 47 reactIOn system .

Partial oxidation reformers Both SHELL, U.K. and TEXACO, USA have

developed POX reactors called HYTEX which is non catalytic and works at 1200 - 1350 °C48

. The SYN gas is thfn purified by PSA method. Catalytic partial oxidation reactors operate at lower temperatures. ASHCROFT employs Ln2Ru20 7 as a catalyst at 775 °C. Methane conversion was found to be 97%. British Gas employs Ni promoted with 1% Ru, 1% Rh and 1% Pd on AI20 J as catalyst. Johnson - Mathey, U.K employs Pt-Cr20 , on a ceramic oxide support.

Auto-thermal reformers In auto-thermal reactors, the heat generated by the

exothermic POX is used in the SMR. For low temperature fuel cell applications auto-thermal reactors have the advantage over previous reformers in that they do not require external heat source. They have a faster startup and can be started without steam addition.

The auto-thermal conversion of methane to hydrogen has been reported as a function of the configurations of an oxidation and a steam reforming catalyst. It was necessary to pre-heat the bed to 300 °C when methane oxidation was initiated over a supported platinum catalyst. A twin bed system was found to be inferior in performance to one bed containing two mixed catalysts . Optimal performance (60-65 %) conversion of methane with 80-85 % selectivity to hydrogen) was obtained when both catalysts were located on the same support49

.

In combined auto-thermal reactors (CAR) both the

partial-oxidation and steam reforming reaction&. take place in the same vessel. CAR is conveniently carried out at 925-1000 °C over a Ni catalyst in a refractory lined outer vessel. The high level heat is recovered downstream by generating steam at high pressures. These plants are more popular for producing SYN gas suitable for the manufacture of NH3, CH30H with a wide variety of feed stocks ranging from NG, LPG, naphtha, coke-oven gas. This may not be suitable for fuel cell systems. Engelhard, USA was reported to have developed an auto-thermal reactor especially for fuel cell applications50

.

CO2 methane reformers (CMR) may not be suitable for fuel cell systems, since they also produce less pure hydrogen (LOW BTU Fuel).

Economic and energy analysis of the various processes

The energy and exergy analysis of the conventional and PSA modified process were discussed by MosenI 6

.17. The analysis has indicated that the

principal exergy loss occurring in the reformer is due to the irreversibility associated with the combustion and heat transfer steps . Overall efficiencies an~

determined to range widely, from 21 to 86% for energy efficiencies, and from 19 to 83% for exergy efficiencies. Energy losses associated with emissions account for 100% of the total energy losses, while exergy losses associated with emi ssions account for 4-10% of the total exergy losses. The remaining exergy losses are associated with the internal consumption . The results may prove to be useful to the improvement of existing and design of future hydrogen production processes .

The economic analysis of the above SMR and POX processes are compared with other techniques and presented in Table 4 [Ref. 10, 13] . The table indicates that the cost of hydrogen produced by PSA route is about 10% less than the conventional steps. For high purity H2 pl ants PSA has become the preferred route, whereas the classical or conventional route is used mainly due to lower operating and capital costs, efficient heat recovery and increased reliability .

Steam reformation of methanol Steam reformation is also a principal route for the

production of hydrogen. Liquid methanol is a suitable fuel for coupling with fuel cells for transportation applications. PEMFC are the 'best suited for transport applications, since it works at ambient temperatures

82 INDIAN 1. ENG . MATER. SCI., APRIL 1999

Table 4--Characteristi cs and efficiencies and cost analysis of hydrogen production processes 10.13.24

SI.No. Characteristics SMR SMR POX TEXACO Water conventional PSA Gasifier Electrolysis

I. Source NG NG Oil Coal Coal 2. Thermal Eff. (%) 98.5 96.5 76.8 63.2 27.2 3. Byproduct Steam Steam Sul fur Su lfur Oxygen 4. H2 Purity (%) 97.0 99.9 80.0 99.9 5. Capital cost (x 106 $) 80.0 83 .2 204.5 3 16.4 13 1. 8 6. Processing cost ($/100 M3

) 7.35 1l.l 8 15.54 23.46 7. Hydrogen Cost ($/1 00 M3

) 7.1 9 I 1.15 15.46 .22.63 8. Hydrogen Cost ($/1 00 SCF) 2.0 1.9 3. 1 4.19 5.74 9. Process Ranking I 2 3 4

The cost analys is have been compared and reported on the basis with a hydrogen production capacity of 2.80 x 106 M 3/day (l00 x 106 SCF/day) at 2 1-42 kg/cm2 ( 1.5 - 3.0 atm)

below 100 °C, and can deliver higher output (kW/kg) . However, the state of art PEMFC can tolerate only 10 PPM CO in the fuel gas that is fed into the anode compartment. For thi s reason, the development of a compact reformer based on methanol is being pursued .

Methanol is a good transportable fuel. Methanol is produced from SYN gas using Zn , Cr, Cu catalysts at 200 - 400 DC at pressures, higher than 10 MPa (100 atm)

.. . (19)

Chemistry of the methanol reforming reaction

The methanol water mixture is converted into H2 and CO2 in a heterogeneously catalyzed reactor51 .

CH,OH + H20 -~ CO2 + 3 H2 MI = 58.4 kllmal

(20)

Hydrogen is produced by the following partial ox idation step also .

Usuall y, steam in excess of the stochiometric quantity is used . The endothermic reforming act ion takes place above 200 - 300 °C on Cu-Zn or Zn/CaOI AI 20 1 catalysts at pressures I - 5 bar. The product SYN gas composi tion from the reformer is also indicated in Table 2. The reac tions during partial ox idation are:

CH,OH + 1/2 O2 ~ 2 H2 + CO2 I1H = 154.6 kllmol

CH,OH (I) + H20 (I) ~ 3 H2 + CO2 I1H = 131 kllmol

CH,OH (g) + H20 (g) ~ 3 H2 + CO2 I1H = 49.5 kllmol

... (22)

.. . (23)

... (24)

The partia l ox idation reaction (POX) is exothermic, while the steam reformation reaction (SRR) is endothermic. The endothermic SRR IS also accompanied by CH,OH decomposition and produc ing CO in the product stream.

CH,OH ~ CO + 2 H2 I1H = 97.8 kllmol

... (25)

This CO can also be further steam reformed in shift converters as in SMR di scussed earlier. The thermal splitting reaction without excess steam in the feed gas leads to hi gher amounts of CO (> 30%), which is not suitable for low temperature fuel cells . liang has di scussed the thermodynamics of the reactions52. The mechani sm of the methanol reforrrting reaction has been described e lsewhere5' -55.

Catalysts for the methanol refonuing reaction

Various supported oxides of Cu and Zn have been used most often for the methanol steam reforming reaction , although additions of AhO/6

, Fe and Cr, and alkaline earth metals have been employed successfull / 7

. A combination of ox idati on catalys t, e.g., NiO-Zr02, NiO-AI 20,-CaO, and Cu-Pd-Si02 and reforming catalysts such as NiO-AhOr MgO and Ca­Si02 is used for the POX reactions58.

Kinetic studies were reported using a Mn promoted co-prec ipitated Cu-AI catalyst at reaction temperatures in the range 170-250 °c and space time ranging from 0.1 to 2.5 g cat hlmol CH,OH to examine the influence of catalyst and reaction temperature methanol steam reforming process for the production of hydrogen59. Gold on Ti02 has also been recommended for the POX of CH30H . They have selective capabilities to oxidise CO alone. AuIMn02 is especially effective in the selective CO removal from hydrogen60

.

ARVINDAN et al.: HYDROGEN GENERATION FROM NATURAL GAS AND METHANOL 83

Partial oxidation vs steam reforming

Product gas composition Both in POX and SRR, the product gas contains

residual CO. For use in PAFC,this CO must be reduced to a level less than 1.0%. For use in PEMFC, the residual CO must be reduced to trace amounts. The concentration of this residual CO can be reduced by injecting H20 in the POX reactor, use of excess steam in the reformation process, selective CO methanation, selective CO oxidation, adsorption process and membrane separation. The first three steps will convert the CO to a level below 100 ppm and the last two steps will permit the purity of hydrogen to be as high as 99 .99%. The Pd-Ag membranes have been recommended for the last stage.

In POX reforming of CH:I)H, the product gas normally consists of 41 % H2, 21 % CO2 and 38% N2, if all the CO is oxidized to CO2. Using excess H20 for POX and selective oxidation of residual CO. the product gas contains 48% H2, 20% CO2, II % H20 and 21 % N2. In SRR with 50% excess H20 and selective oxidation of residual CO, the product gas contains 63% H2, 22% CO2, 11 % H20 and 4% N2.

Energy calculations POX produces excess thermal energy, which can be

used to vaporize the methanol and water to heat the reformed product gas . SRR requires input of external energy. If the energy for the vaporizing and reforming steps are obtained from the fuel cell stack waste heat and combustion of spent fuel leaving the stack, the efficiency can be increased.

Dynamic rC.I"J.'Ollse and start up

The dynamic response and startup of the POX reformer can be more superior to that of SRR. This is possible because of POX lI ses direct heat transfer, which provides very high heat flux. In contrast the

AIR

, Fig. 5-Flow sheet for the methanol refonnation~2

SRR uses indirect heat transfer which limits the heat

flux. The POX reformer is an intrinsically simple device due to the absence of burner, heat transfer surface, and combustion manifolding ducts. In comparison, the SRR is relatively complex, because it contains one or more burners, heat transfer devices, <:ombustion air and other exhaust ducts and perhaps even a process gas recirculator. The process flow sheet for the SRR is shown in Fig. 5. Th(! POX reactors are more compact than the SRR of equal methanol processing capacity. Since the combustion products form part 0,£ the reformed gas itself, there is no separate reformer combustion exhaust stream for the POX reformer.

The reformer characteristics

Cataly tic burners

The des ign of methanol reformer has been reported6 1

.63 The essential component of the reformer

is the burner itse lf as sho.wn in Fig.6. The fuel feed mixture flows in radial direction through the centre. The catalytic burner consists of a fibrou s support structure, which is surrounded with wire meshes. Pt supported on Al 20 1 is used as a catalyst. The fibrou s support structure equalizes the speed of the fuel/air mixture into the burner. Moreover, it is also used as thermal insulator between the hot reaction zone and the cool feed gas. The burner is cooled by a water jacket and flue gas heat exchanger. The mixture is ignited electrically. The typical composition of the

Sampling Unit

Water out

Consumer

Recuperotor

Catalyst

Heat Remowl System

Fibrous Support Structure Fuel Gas Fuel/Air Mixture

Fig. 6-Catalyiic burner for refonning methanol62

84 INDIAN 1. ENG. MATER. SCI., APRlL 1999

product gas is reported to be 74% H2, 3% CO and 23% CO2 with refor~er efficiency bf 95%64.

Deve lopment of a bench scale POX reformer, was reported by Argonne National Laboratory65. A design study of a compact hydrogen generating plant based on meth anol decomposition intended for fuel cells was also reported by Ms.Kellog and Co., USA 66.

Auto-thermal reactors To improve the production economjcs, a novel

concept in vo lvin g the coupling of the oxidation of the methanol (a strongly exothermic step) to the steam reforming r~action is an intuitively attractive operation for energy minimi zation .

The methanol ox idati on over a PtlAI20 J catalyst is ex tremely rapid even at sub ambient temperatures and accompli shed by large heat re lease according to the foll ow ing.

C H,OH + 3/2 O2 ~ CO2 + 2 H20 t1H = 667 kllmol

... (26)

Depending upon the air to CHJOH ratio, temperatures above 600 K, may be reached throughout the entire catalyst beds.

Different arrangements for the catalysts, in the reac tor were reported to influence ' the overall performance by affecting the mass and heat transfer in the system67

. li ang67 et al. Have used a dual feed flow

reactor to study the auto-thermic steam reforming of methanol. Water added to the feed was found to enhance heat transfer between beds.

Adiabatic dual-bed catalytic reactor systems with cy lindrical and spherical geometry that may be used to promote internal heat exchange for the coupled reaction network68

. Analysis have shown that, while the coax ial cy lindrical system and the dual-bed single tubular reactor generally need optimal water-to­methanol feed ratios of about 3-4, · the spherical arrangement always requ ires a ratio less than I for equi va lent or even better performance. While the spherical reactor system, had the most prorrusing performance in terms of reactor efficiency (about 80%) and H2 production (125 m3 gas)/(m' reactor)/s, the coaxial reactors exhibited the poorest efficiency (less than 10%) with a H2 production rate of 19.5 m3

gas/em' reactor)/s. Thus the spherical reactor with an inner oxidation catalyst bed is the most attractive configuration for this auto-thermal process in terms of product maxirruzation, feed and energy rrunimization .

Internal reforming in fuel cells When the reforming action is done in a separate

reactor outside the fuel cell stack as described by the above methods, and the off feed from the reactor after purification is fed into the anode compartment of the fuel cell stack, then the method is called external ' reforming action. Since the reforming temperature is about 800 °C and the MCFC and SOFC generate waste heat at temperatures exceeding 650 °C, the waste heat from the fuel cell can be utilized in reformer. In such cases the reformer is in close proxi rru ty with the electrocherrucal cell stack . Such a concept is referred as internal reforming (IR).

Internal reforming lowers the requ irement for cell cooling and by e liminating the external reformer, capital costs are also reduced. It gives natural gas a big advantage over other fue ls. There are two methods of internal reforming which are usually referred to as direct (DIR) and indirect internal reforrrung (IIR). The details of the above two are depicted in F ig.748

. With

DIR, the reforming reac tion takes place over a catalyst located within the fue l cell anode channel. Heat and steam are supplied direc tl y from the electrochemical ce ll s. The ce ll reac ti on drives the reformation reaction to completi on by removing the hydrogen as it is formed. Thus, DIR will result in high cell effi cienc ies and fuel to e lectri city energy convers ion efficiency even at the operating temperature of the MCFC as indicated in Table I . With IIR, the reformjng reaction takes place in a separate catalyst chamber, which is in c lose thermal contac t with the cell. This method gives the advantages of good heat transfer from the cell. But it is not as effi c ient as DIR, since there is no direct removal of hydrogen or steam produced to help the reforming reacti on forward. However there ex ists a di sadvantage in DIR th at the reformjng catalysts can get deactivated in the presence of carbonate e lec trolyte, which moves out of the cell.

The severe conditi ons inside the MCFC, however, require that the catalysts must have a very high stability . Suitable types of nickel catal ys t a lpha-Al20 3

and a co-precipitated Ni/A IzO, were reported69. Only a

co-precipitated nicke llalumina catalyst with high alumina content showed satisfactory residual activities . Addition of magnes ium or lanthanum oxide to a co-precipitated nickellalurruna catalyst decreased the stability. Ni/MgO and NilLiAIOz catalysts have a lso been tried .

Energy Research Corporation (ERC), USA and Mitsubishi Electric Co. (MELCO), Japan are working

ARVINDAN et at. : HYDROGEN GENERATION FROM NATURAL GAS AND METHANOL 85

OtRECT INTERNAL REFORMINQ INOIRECT INTERNAL REFORMING EXTERNAL REFORMINQ

WI. TIll)( 00. i WI. TJIIX 00. i CA1l<ODE CA1l<ODE CH.- CA1l<ODE

H.O -- i -- -- i -- -- --t t t i I ~ I~.) I ~ I~.) I ~ I~.) .00. .00.

Fig. 7-Types of reforming configuration48

on the development of MCFC stacks with DIR catalysts.

In SOFC, the Ni-Zr02 anode rnaterial itself will catalyze the direct reformation of CH4 at its operating temperature of 1000 0e. Most of the work in this area have been performed by Westinghouse Electric. Co., USA . But still pre-reforming and sulfur removal is required as the sulfur is found to be a poison for the Ni-Zr02 catalyst, which otherwise will affect the fuel cell performance.

Conclusions PEMFC requires 99.99% pure hydrogen in the fuel

feed and best suited for transportation applications. The fuel to e lectricity conversion efficiency is also as high as 85 % in PEMFC compared to PAFC or AFt. Only CH,OH reformation coupled with membrane purification can yield H2 of that required purity . Hence. deve lopment of compact reformers to suit the fuel cell's fue l utilization requirements is a desirable factor. The energetic and cost of the methanol reformer depends on its size, nature of the catalyst and the number of purification steps employed.

On the other hand , methane reformers have been scaled to yield higher production rates of hydrogen for use in NH3 and CH)OH plants. The off feed from these plants are also suited for high temperature fuel cells (MCFC and SOFC); where the tolerance limit for CO and CO2 is better than the PAFe. Among the SMR and POX methods the SMR will be the preferred one after PSA modification, which can yield almost 99.7% pure hydrogen . The cost of production of hydrogen by SMR (PSA) is also reported to be less than the other steps and any other conventional processes like coal gasification and water electrolysis.

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