Encyclopedia of Catalysis || Synthesis Gas Generation-Industrial

SYNTHESIS GAS GENERATION—INDUSTRIAL

Introduction

Synthesis gas or shortly syngas is a term used for a mixture of hydrogen andcarbon monoxide. Synthesis gas is an important building block for a number ofpetrochemicals as well as an important feedstock for liquid fuels via the FischerTropsch route or as reducing gas for iron ore reduction (Fig. 1).

At present, natural gas and light hydrocarbons are the predominant feed-stocks for the production of synthesis gas. Synthesis gas can also be producedfrom heavy hydrocarbons or coal, but in that case noncatalytic partial oxidationprocesses will be used (see the section Hydrocarbon Steam Reforming).

In the past decade, increased environmental legislation (lower sulfur levelsin transportation fuels, use of GTL fuels), and the use of heavier feedstocks (re-quiring more hydrogen in the refinery) have resulted in a relatively strong growthof the hydrogen demand in refineries. In addition, several developments have setoff the road to the so-called hydrogen economy in relation to the use of hydrogenin clean energy systems, including fuel cells.

Another development in the past decade has resulted in an increase in maxi-mum plant capacity for hydrogen generation of more than 250,000 Nm3/h (equiv-alent to more than 200 million scfd). This is a result of more accurate and ad-vanced simulation tools, in particular for the flow modeling (CFD) in the largereactor furnaces that are needed for these huge capacities.

An additional feature that is closely related to syngas generation is the de-velopment of carbon capture and sequestration (CCS) that is initiated by theincreased CO2 concentration in the atmosphere, and the increased understand-ing about increased CO2 levels that are contributing to the increased averagetemperature in the atmosphere.

The technology for CO2 capture will therefore be described as well (see thesection CO2 Capture).

Table 1 lists the typical syngas requirements for major petrochemicals.Of the petrochemicals consuming CO, the largest use is for methanol

(>50%), followed by oxo-alcohols (15%), and acetic acid (10–15%). Minor use ofCO is for herbicides and pharmaceuticals.

Next to petrochemicals, synthesis gas is being produced in refineries as afirst step to produce fuels in a refinery. A special case here is the Fischer Tropsch(FT) process that converts synthesis gas to clean diesel. The syngas manufacturein a FT plant is responsible for up to 60% of the capital investment. The syngas

1

Encyclopedia of Catalysis. Copyright c© John Wiley & Sons, Inc. All rights reserved.

2 SYNTHESIS GAS GENERATION—INDUSTRIAL

Table 1. Syngas Requirements for Major World-Scale Petrochemicals

Required H2:CO Typical world scale Syngas requiredProduct (mole/mole) capacity MTA Nm3/h

Methanol 2:1 160,000 – 1,275,000 48,000 – 190,0000Acetic acid 0:1 275,000 – 545,000 18,000 – 36,000Acetic anhydride 0:1 90,000 3,500Oxo alcohols 2:1 overall 115,000 – 275,000 12,000 – 25,000Phosgene 0:1 45,000 – 160,000 ∗ 3,500 – 12,000Formic acid 0:1 45,000 3,500Methyl formate 0:1 9,000 600Propionic acid 0:1 45,000 – 68,000 2,400 – 3,500Methyl methacrylate 1:1 45,000 4,7001,4 Butanediol 2:1 45,000 4,700aWorld scale capacity of MDI and TDI plants shownbCourtesy of Gunardson, Abrardo (1).

Natural gas Oil Coal

Synthesis gasH2, CO

AmmoniaNH3

MethanolCH3OH

Fischer–TropschLiquids n[-CH2]

Reducing gasfor iron ore

Various chemical applications

Nitrogen

Hydrogen

Fig. 1. Synthesis gas manufacture and applications.

technology aspects in relation to the FT process are described by Rostrup-Nielsen(2).

In the following sections, the applications for synthesis gas will be summa-rized, the chemistry and thermodynamics related to the production of synthesisgas will be described and after which the various industrial processes for themanufacture of synthesis gas will be highlighted. The emphasis will be on cat-alytic routes, but thermal (partial oxidation) routes will be briefly described aswell. Carbon capture and sequestration (CCS) is added, as this is an increasinglyimportant issue for the use of synthesis gas as a source for hydrogen as an energyvector.

Chemistry and Thermodynamics

The catalytic conversion of hydrocarbons into synthesis gas involves a num-ber of chemical reactions, which may take place. Catalytic conversion of

SYNTHESIS GAS GENERATION—INDUSTRIAL 3

hydrocarbons with steam to produce synthesis gas does occur to the followingreactions:

The steam reforming of higher hydrocarbons (reaction 2) such as naphthais carried out with a different type of Ni-based catalyst than used for the steamreforming of natural gas. The steam-reforming step converts the hydrocarbonfeedstock to a synthesis gas with a methane slip of 4–10%. This methane slipis dictated by the catalyst, the reactor pressure, and the reactor tube exits tem-perature. The exit temperature may be as high as 950◦C (3). Depending on thelevel of hydrocarbons in the natural gas feed stream, a separate prereformingstep may be added prior to the steam reforming of natural gas. The prereformingstep is discussed in more detail in the section Hydrocarbon Steam Reforming. Thewater–gas shift reaction (4) converts CO in CO2 and hydrogen using two differentcatalysts. First, the bulk of the CO is converted in a high temperature shift cat-alyst that has Fe3O4 as the active phase to a CO content of approximately 1–2%.The conversion is limited by the equilibrium that shifts to the reactant side atincreasing temperatures. Subsequently the gas is cooled and enters into the lowtemperature shift reactor, which contains a Cu–Zn catalyst and reduces the COcontent to approximately. 0.2% (4).

�H◦

298, (kJ/gmol)

Steam reforming of CH4 + H2O ↔ 3H2 + CO (1) 206natural gas

Steam reforming of CnHm + H2O ↔ (m/2 + n) H2 + nCO (2) For C7H16 +1108higher hydrocarbons

CO2 reforming CH4 + CO2 ↔ 2H2 + 2CO (3) 247Water-gas shift CO + H2O ↔ H2 + CO2 (4) −41Methanation CO + 3 H2 ↔ CH4 + H2O (5) −206

The reforming reactions with steam are strongly endothermic; the shift re-action is moderately exothermic. In case CO2 is available, a low H2/CO synthesisgas can be produced in a reaction, which is indicated as CO2 reforming. The over-all reaction to produce synthesis gas is still strongly endothermic. This calls fora reactor where the heat management is extremely important, as will be furtherexplained in the next section.

In addition to the reactions mentioned, oxygen may be also take place in theconversion, in which case some additional reactions are involved:

�H◦298, (kJ/g mol)

Oxidation, partial CH4 + 12 O2 ↔ 2H2 + CO (6) -36

CH4 + 3/2 O2 ↔ 2H2O + CO (7) –520Oxidation, total CH4 + 2O2 ↔ 2H2O + CO2 (8) –803

These reactions are, of course, strongly exothermic. The reaction with oxy-gen, only to produce synthesis gas, is usually indicated as partial oxidation (orshortly POx) and is a noncatalytic reaction. The noncatalytic thermal processes


will be briefly described in the overview of industrial processes (see the sectionOverview).

In the catalytic routes (1–4) to produce synthesis gas, many reactions occursimultaneously; the composition of the synthesis gas product is hence determinedby the thermodynamics of the corresponding-equilibrium reactions (1–4).

The catalysts used in the synthesis gas production process are presented inTable 2.

During the steam-reforming reactions, undesired carbon deposits on the cat-alyst may also be formed as a result of the following reactions:

�H◦298 (kJ/gmol)

Boudouard reaction 2 CO ↔ C + CO2 (9) −172Methane cracking CH4 ↔ C + 2H2 (10) +75CO reduction H2 + CO ↔ C + H2O (11) −131

The values of the heat of reaction in (1–11) can be easily derived from theheat of formation of the species involved.

Whether carbon formation takes place, it is a function of kinetics, processconditions, and reactor design. It has been found (5) that the measured equi-librium constants for reactions 8–10 do not correspond to the values obtainedfrom thermodynamics, assuming the carbon to be graphite. The morphology ofthe carbon-catalyst deposit is such that it is possible to operate a reformer with-out carbon formation when theoretically predicts it is that carbon will be formed.

Furthermore, the values of the equilibrium constants for reactions 8–10 ap-pear to depend upon the composition of the catalyst. For example, the methane-cracking reaction 9 at 645◦C has the following equilibrium constants:

(1) K9 (graphite) = 4.0 bar (calculated on the basis of graphite)(2) K9 (Ni catalyst) = 3.3 bar (measured in the laboratory with nickel catalyst)(3) K9 (Pt catalyst) = � K9 (Ni catalyst)

Furthermore it is important to consider reaction kinetics in relation to thetendency of carbon formation.

Catalytic reactions can be generally described as these have taken placeduring the following steps:

(1) Diffusion of reactions from the bulk stream into catalyst’s pore,(2) Reaction (eg, methane steam and shift reactions) inside the catalyst pore,(3) Diffusion of products from the catalyst pores into the bulk stream.

The kinetics of steam reforming has been widely studied, and there are anumber of rate equations available in the literature (6).To design a steam re-former, equations expressing the intrinsic reaction rates in the following formare required:

Intrinsic rate = (Kinetic term)×(Driving force term)/(Adsorption term)

Tab

le2.

Cat

alys

ts,T

emp

erat

ure

Win

do

wan

dV

end

ors

.

Ope

rati

ng

Com

posi

tion

win

dow

Maj

orve

ndo

rs

Pre

refo

rmin

gN

i/Al 2

O3

250–

650◦ C

Hal

dor

Top

søe,

Joh

nso

nM

atth

ey,S

ud

Ch

emie

Pri

mar

yst

eam

refo

rmin

g(n

aph

tha/

arom

atic

s)N

i/MgO

;Ni/M

gAl 2

O4

500–

850◦ C

Hal

dor

Top

søe,

Joh

nso

nM

atth

ey,S

ud

Ch

emie

Pri

mar

yst

eam

refo

rmin

g(m

eth

ane)

Ni/C

aAl 2

O4;N

i/α-A

l 2O

360

0–87

0◦ CH

aldo

rT

opsø

e,Jo

hn

son

Mat

they

,Su

dC

hem

ieC

atal

ytic

part

ialo

xida

tion

(CP

O)

Su

ppor

ted

prec

iou

sm

etal

s90

0–12

00◦ C

Cu

stom

cata

lyst

byva

riou

sve

ndo

rsH

igh

tem

pera

ture

wat

er-g

assh

ift

(HT

S)

Fe 3

O4/C

r 2O

332

0–52

0◦ C

Hal

dor

Top

søe,

Joh

nso

nM

atth

ey,S

ud

Ch

emie

Low

tem

pera

ture

wat

erga

ssh

ift

(LT

S)

Cu

/Zn

O/A

l 2O

320

0–30

0◦ C

Hal

dor

Top

søe,

Joh

nso

nM

atth

ey,S

ud

Ch

emie

a Cou

rtes

yof

Bar

thol

omew

and

Farr

outo

(4).

5


The intrinsic rate is measured in the laboratory with small catalyst par-ticles, which do not exhibit any diffusion limitations. For a given catalyst, in-tegral reactor tests provide enough information to determine the temperature-dependent coefficients in the intrinsic rate equation. However, since diffusion-freesmall catalyst particles cannot be used in large reformers because of excessivepressure drop considerations, the impact of diffusion limitations on industrialcatalysts must be taken into account as well:

Actual rate = (Intrinsic rate) × (Effectiveness factor)

The effectiveness factor is a global multiplier of the intrinsic reaction ratethat accounts for the severe diffusion limitations encountered in industrial re-formers. Effectiveness factors of industrial Ni-based catalysts for the methane-steam and shift reactions are of the order of 0.02; this means that the actualreaction rate experienced by the bulk fluid in the reformer is typically only aboutfew percents of the reaction rate measured in the laboratory under the same con-ditions but with very small catalyst particle sizes.

The significance of this small effectiveness factor is the following: The chem-ical reaction rate occurring inside the catalyst pores is much faster than the rateat which the reaction components can enter and then leave the catalyst pores.This means that the composition inside the pores can be at equilibrium the bulkgas composition is quite far from equilibrium. Therefore, when checking to seewhether a reforming mixture has a tendency to form carbon, it is necessary tocheck the bulk composition of the gas as well as the equilibrium compositionof the gas. As mentioned earlier, steam reformers can be said to be “heat flux-limited,” which means that the reactor is usually limited by heat transfer consid-erations and not by reaction kinetics. In other words, once the reformer has beenconfigured in terms of the number of tubes and their dimensions to achieve thedesired heat flux profile, there should be enough catalyst volume in the tubes toachieve the desired level of conversion.

Despite the rather good heat transfer characteristics of a well-designed re-former, the heat fluxes are so high that a significant film temperature drop ex-ists between the inside reformer wall temperature and the bulk gas temperature(Fig. 2). This means it is also necessary to evaluate coking tendencies at the re-former at wall temperature.

To evaluate the potential of carbon formation in a steam reformer, it is there-fore essential to have a rigorous computer model, which contains kinetic modelsfor the process side (reactor), as well as heat transfer models for the combustionside (furnace). The process and combustion models must be coupled together toaccurately calculate the process composition, pressure, and temperature profiles,which result from the complex interaction between reaction kinetics and heattransfer. There may also be a temperature difference between bulk fluid, cata-lyst surface, and catalyst interior. Lee and Luss (7) have derived formulas forthis temperature difference in terms of directly observable quantities: The Weiszmodulus and the effective Sherwood and Nusselt numbers based on externalvalues (8).

In Table 3, the composition of a steam reformer operating at a steam-to-carbon ratio of 1.5 with full CO2 recycle is shown at a position part way down thereformer tube:


Table 3. Partial Pressure

Bulk EquilibriumComponent T = 645◦C T = 645◦C

CH4 6.910 6.839CO 0.746 0.766CO2 2.579 2.588H2 5.889 6.029H2O 10.191 10.092

Total 26.314 26.314PCO2/PCO

2 4.640 4.407PH2

2/PCH4 5.019 5.315

Fig. 2. Temperature profile over catalyst tube.

Inside the catalyst pores, the composition of the gas is at the equilibriumcomposition shown above. The values of PCO2/PCO

2 and PH22/PCH4 of the bulk

fluid and the equilibrium composition are greater than the values (based on Nicatalyst) measured for K8 (Boudouard reaction) and K9 (methane cracking reac-tion):

(PCO2/PCO2 = 4.64)bulk>(PCO2/PCO2 = 4.41)equil>(K5 = 2.3bar−1)Ni-catalyst at 645◦C

(PH2/PCH4 = 5.32) equil > (PH22/PCH4 = 5.02)bulk > (K6 = 3.3 bar)Ni-catalyst at 645◦C

This means that carbon will not form in either the bulk fluid or in the cat-alyst pores at this temperature. However, checking the equilibrium constantsat the reactor wall temperature, it is found that carbon could form from the


methane-cracking reaction (but not the Boudouard reaction):

(PCO2/PCO = 4.64)bulk > (PCO2/PCO = 4.41)equil > (K5 = 0.3 bar-1)Ni-catalyst at 744◦C

(K6 = 12.1 bar)Ni-catalyst at 744◦C > (PH2/PCH4 = 5.32)equil > (PH2/PCH4 = 5.02)bulk

This comparison shows that there is a potential to form carbon from themethane-cracking reaction in the inside of the reformer wall at this location inthe reformer tube. Detailed, proprietary kinetic expressions for the reactions 8–10 indicate that while reaction 9 will form carbon, the coke will be gasified bysteam and CO2 (reactions 8 and 10), so there is no net accumulation of carbon inthis example. However, as the steam-to-hydrocarbon feed ratio is reduced further;there will be a point where there is an accumulation of carbon because the cokingrate of reaction 9 will be greater than the combined gasification rates of reactions8 and 10.

A correct simulation of the tendency to form carbon deposits is, therefore, ofutmost importance for the design of reactors for the synthesis gas production forthe design of reactors for synthesis gas production.

Industrial Processes for Syngas Production

Overview. A number of different technologies are available for the pro-duction of synthesis gas and have been described in the literature. An importantparameter is the so-called balance ratio or stoichiometric number (SN), which isdefined as

SN = [H2]−[CO2][CO]+[CO2]

The process route selected is strongly dependent on this SN number.In Figure 3, an overview is given on the various schemes, which can be

employed to produce synthesis gas.The various technologies referred to in this figure will be described in more

detail in the next sections.Hydrocarbon steam reforming (HSR) refers to the reaction of hydrocarbons

with steam to produce synthesis gas. In the case of heavier feedstocks than nat-ural gas (eg, naphtha), a so-called prereforming step is also employed, to con-vert the heavy hydrocarbon feedstock into a methane-rich gas, which is then fur-ther converted with steam by using the Steam methane reforming (SMR) process.For the steam-reforming process, several types of reactors are possible, such asthe conventional widely used top-fired or side-fired reformer or, usually appliedfor smaller capacities, a heat exchange reformer (HER) or a compact reformer.Another mode of operation is described as adiabatic reforming or autothermalreforming (ATR), which converts hydrocarbons with steam and oxygen, whereoxygen supplies the heat of reaction for the subsequent endothermic-reformingreactions, such that the overall process is autothermal.


ADIABATICREFORMING

(ATR)

HC STEAMREFORMING

(HSR) HERPRE

REFORMING

COMPACT REFORMING

COMBINEDREFORMING

HSR+ATR

PRE-REFORMING+ATR

ATR+HER

HSR+ATR+HER

GASIFICATION

FIXED BED

ENTRAINED FLOW

FLUIDIZED BED

Gas to heavy naphtha + steam Gas to heavy naphtha + oxygen + steam

Gas to coal+ oxygen + steam

Fig. 3. Synthesis gas generation, overview.

H2 - CO - SYNGAS PLANT

HC FEED

SYNGASGENERATION

ACID GASREMOVAL

CO2ABSORBER

PSA

MEMBRANE

COLDBOX

METHANATOR

CO2 VENT

STEAM

BFW O2

H2

CO

Low Purity H2

TECHNOLOGY and ROUTE SELECTION

AIRSEPARATION

UNIT

CO SHIFTCONVERSION

Syngas

H2 rich

AIR

CO2

POWERPLANT

AIRCOM PRESSOR

CO2COM PRESSOR

SYNGASCOMPRESSOR

OXYGENCOMPRESSOR

COCOMPRESSOR

PURGE GASCOMPRESSOR

HYDROGENCOMPRESSOR

PERMEATECOMPRESSOR

CO rich

CO rich

H2 rich

Fig. 4. Synthesis gas technology and route selection.

HSR and ATR can also be combined in several ways. This is indicated asCombined reforming in Figure 3.

Finally, the noncatalytic route of partial oxidation or gasification can alsoproduce synthesis gas. This can be applied for feedstocks ranging from naturalgas to coal.

The synthesis gas generation is one of the various unit operations, whichmay be employed to obtain the final product (Fig. 4).

The syngas generation will require hydrocarbons as feedstock and fuel andBFW (boiler feed water to produce steam) in the case of HSR. In the case of


Feed

Desulfurizer

Reformer

Stack

Heat recovery(Convection section)

Process steam

H.T. shift

PSASystem

(Radiantsection)

FuelHeat

recovery and coolingHydrogenproduct

PSA purgegas drum

Fig. 5. Flow scheme HSR plant (with PSA) for hydrogen production (top fired configura-tion).

ATR, oxygen is also needed, which is being produced in an air separation unit.The raw synthesis gas has to be purified further in various unit operations. CO2is being removed in the acid gas removal unit using a physical or chemical ab-sorption process and the resulting CO2 may be recycled to the syngas generationunit. After the acid gas removal the synthesis gas composition is further tunedto the required CO to H2 ratio. In the case of pure (>95–99.99 vol%) hydrogen apressure-swing adsorption (PSA) unit is mostly used to produce the pure hydro-gen. The operating conditions in the different unit operations of the syngas plantwill be described later.

For low purity hydrogen product (eg, for refinery applications), a methana-tion unit, to convert unreacted methane, may be present as well. In the case ofpure CO, the CO is separated from the hydrogen by a cryogenic separation stepor a cold box. In addition, a membrane unit may be employed to tune the synthe-sis gas composition to the required ratio. Various compression steps may also bepresent to deliver the product at the desired pressure.

Hydrocarbon Steam Reforming

The principle flow sheets for steam reforming for hydrogen production (9,10) andreforming for syngas production (11–13) are indicated in Figures 5 and 6, respec-tively.

The feedstock, natural gas, or a light hydrocarbon fuel usually contains sul-fur components, which have to be removed to levels below 1 ppm to avoid poison-ing of the reforming catalyst. The reformer itself usually contains 40–400 tubes,typically 6–12 m long, 70–160 mm in diameter, and 10–20 mm in wall thickness.These tubes are vertically placed in a rectangular furnace, the so-called radiantsection. Typical reactor pressure inside the reactor tubes varies between 30 and40 barg. For small-scale plants, the reaction pressure may be 1 barg. The reac-tor tubes contain catalyst, usually in the form of small cylinders or Rashig rings.


Feed

Desulfurizer

Reformer

Stack

Heat recovery(Convection section)

Process steam

(Radiantsection)

Fuel

(Import CO2)

CO2Removal

Coldbox

PSA

Drying

CO Product

Hydrogen product

Reformer

Process gas gaswaste heat boiler

Fig. 6. Flow scheme HSR plant for CO/H2 production (side fired configuration).

The reactor tubes are fired by burners, which may be located at the bottom, atthe side, or at the top of the furnace. Combustion of the fuel takes place in theradiant section of the furnace. After the flue gas has been supplied all the reactorduty, it passes into the convection section where it is further cooled by heatingother streams such as process feed, combustion air and boiler feed water, as wellas producing steam. The product gas, leaving the reformer at a temperature of850–950◦C, is cooled in a process gas waste heat boiler (PGWHB), which producesprocess steam for the reformer.

In the reformer, the reactor tube diameter is determined by mechanical con-siderations and the heat flux applied. This leaves the space velocity in the steam-reforming catalyst fixed with a low catalyst utilization, which is reflected in a loweffectiveness factor of less than 10% (14).

Depending on the level of higher hydrocarbons present in the feed, an ad-ditional conversion prior to the HSR may be present. This step is called prere-forming (15) and is aimed to convert all the heavy hydrocarbons in the feed tomethane, hydrogen, and carbon oxides. These higher hydrocarbons, being morereactive than methane, would otherwise lead to generation of carbon on theHSR catalyst and thus deactivate the catalyst quickly. This prereforming pro-cess unit will absorb part of the heat of reaction and may allow the reduction ofthe steam/carbon ratio of the HSR unit. Prereforming is in particular attractivewhen a plant needs to operate with a variety of feedstock compositions (16–18).

Depending on the downstream process (H2, CO, oxosynthesis, ammonia,etc.), the product gas goes to the different unit operations as described in the sec-tion Overview. In the case of hydrogen production, this usually involves a shiftreactor and a pressure-swing-absorption reactor (PSA) (Fig. 4). For CO produc-tion, the scheme involves a CO2 removal section and a low temperature separa-tion unit (so-called cold box). CO2 is recycled to the reformer. If in this case, H2product is also desired, a PSA unit may also be present (Fig. 5).


The production of low synthesis gas with a low ratio of H2 to CO or CO isfavored by a

(1) low steam-to-carbon ratio (S/C);(2) high reformer outlet temperature.

Economics of the process requires high pressure operation and high heatfluxes, which push thermodynamics toward (undesired) carbon formation. A lowS/C ratio will shift the equilibrium of the water gas shift reaction to producemore CO and less CO2 and H2. This will also reduce the size of the CO2 removalunit and CO2 recycle compressor.

At lower S/C ratios, the methane conversion will be reduced at constantoutlet temperature. Therefore, the reformer outlet temperature is increased toachieve methane conversion of >96%. In plants for CO or synthesis gas, theprocess outlet temperature is typically selected between 900 and 960◦C. Thesesevere conditions require the use of a specially designed process outlet system,which transfers the product to the process gas waste heat boiler.

The heat flux to the catalyst tubes has a major effect on the size of the re-former and thus on its capital investment. For both HSR for H2 production aswell as for CO/H2 or CO, heat flux of 100,000 kcal/m2·h or greater is applied.

Autothermal Reforming and Partial Oxidation

In the autothermal reforming process (ATR), a conversion of hydrocarbon feed-stock with oxygen (as oxygen, air, or enriched air) takes place in combination withthe conversion of hydrocarbons with steam (Fig. 7). ATR is basically a combina-tion of HSR and partial oxidation (POx) technology (19).

The feedstock, which may range from natural gas to naphtha, is heated anddesulfurized (usually in a conventional ZnO/CoMoX bed ) to levels below 1 ppmafter which steam is added and the combined feed is introduced into the ATR.The upper part of the reactor basically consists of a burner, mounted on the re-actor shell; the burner itself is the key item of the autothermal reactor and iscritical for the reactor operation. In this part of the reactor, temperatures until1,400◦C are reached. The burner design is a proprietary design of the technol-ogy supplier. Flame stability, in combination with a sootless operation, is the keyrequirement for proper burner operation. This is accomplished with the correctratio of feedstock, oxygen, and steam.

The reaction with oxygen delivers the endothermic heat of reaction of the re-forming reaction. The overall reaction is auto thermal. In Figure 7, the reactionwith oxygen takes place in the upper part of the reactor, whereas the steam-reforming reaction takes place in the fixed bed. Typical reactor outlet tempera-tures are 800–950◦C (20).

As mentioned before, autothermal reforming can also be achieved via non-catalytic partial oxidation. As an example, the scheme presented in Figure 8shows a partial oxidation route to produce synthesis gas.


Hydrocarbon feed

OXYGEN or AIR

FEEDGASHEATER

AUTO THERMAL REACTOR (ATR)

DESULFURIZER

Raw Syngas

BFW

STEAM DRUM

STEAM

STEAM

Fig. 7. Auto thermal reforming.

In the partial oxidation process, the feedstock may range from natural gasto heavy hydrocarbons such as refinery residue to solid feedstocks like coal. Forthese heavy feedstocks the partial oxidation reactor is usually indicated as a gasi-fication reactor. Licensors for the partial oxidation or gasification route includeShell GE Power (originally developed by Texaco), Sasol-Lurgi, and ConocoPhillipsE-gas (originally developed by DOW) (21–23). Of these three processes, the Shell,GE, and ConocoPhillipps E-gas process are entrained flow gasifiers and are char-acterized by a high gasifier outlet temperature (1400–1500◦C) to avoid soot (car-bon) in the flame. The Shell technology uses a dry feed system and a membrane-lined gasification chamber, whereas the GE technology and the PhillipsConocotechnology use a wet feed system and a refractory-lined gasification chamber.

Combination Reforming

The technology for HSR and ATR can also be combined (20) in one configurationas shown in Figure 9.

This combination of technologies was originally developed for ammoniaplants in which e air is used in the autothermal process, to introduce nitrogeninto the system. Nowadays this concept is also used for production of synthesisgas. The HSR reactor in combination with a downstream located auto thermalreformer is often selected to achieve an optimum balance between capital in-vestment and operating costs. The choice for combined reforming technology is


Feedstock

Raw SyngasBFW

condensate

OXYGEN

FEEDGASHEATER

CW DRUM

QUENCHWATER DRUM

BURNERCW PUMPS

SYNGASSCRUBBER PUMPS

SYNGASSCRUBBER

NOZZLESCRUBBER

CW

PARTIAL OXIDATION REACTOR

Fig. 8. Partial oxidation process.

RAW SYNGAS

BFW

STEAM

STACK

STEAM DRUM

REFORMER

PREREFORMER (OPTIONAL)

DESULFURIZER

NATURAL GAS

FUEL AIR FUEL AIR FUEL AIR

FEEDGAS BYPASS

OXYGEN or A IR

STEAM

AUTO THERMAL REFORMER

Fig. 9. HSR reforming in combination with ATR.

strongly dependent on capacity and price of oxygen (as utility) or the investmentof an air separation plant.

The first reaction step takes place in the HSR, which is operated at moderatetemperature (upto 850◦C) and partial conversion. In the second ATR, reactor themethane-rich synthesis gas is further converted to achieve the desired methane


HSRreactor(~70%

efficiency)

EHTR Syngas productNatural gas+ steam

Fig. 10. Principle of EHTR.

conversion through the reaction with oxygen or air. The combination of HSR withATR is often called combination reforming or shortly combireforming. The syn-thesis gas at the exit of the auto thermal reactor is cooled in a process gas boilerto convert the sensible heat present in the synthesis gas into steam. Alternatively,the heat present in the synthesis gas exit ATR may be used to serve the heat ofreaction of the steam-reforming reaction. This principle has been applied as heatexchange reforming and will be further detailed in the section Heat ExchangeReforming.

Heat Exchange Reforming

Basic Concept. In the basic concept of heat exchange reforming (HER),a part of the heat of reaction needed for the endothermic steam-reforming processis delivered by convective heat transfer from hot syngas product and/or flue gas.Several reactor concepts have been developed, which make use of this convec-tive heat transfer concept. Plants that use this concept produce much less exportsteam, because much more heat integration takes place in the reactor itself. Thevarious concepts of heat exchange reforming, which have been developed, includethe following

Enhanced Heat Transfer Reformer. The enhanced heat transfer re-former (EHTR) was invented and developed by Air Products in the mid-1980s (24)and has primarily been developed for natural gas conversion. Basically EHTRutilizes the high heat content of the syngas produced in a conventional steamreformer (HSR) to reform additional natural gas to synthesis gas, instead to uti-lize this heat content to produce steam in a process gas boiler.The block schemeshown in Figure 10 summarizes the basic process step involved:

The EHTR concept can conveniently be combined with an existing HSRplant to increase capacity to about 30%. The EHTR reactor is a pressurized-refractory-lined vessel containing reactor tubes filled with catalyst. These tubesare supported from the bottom tube sheet. The tubes may expand on the top endwhere they are freely located (Fig. 11).

Gas-Heated Reformer. The gas-heated reformer (GHR) concept (25,26)is another process, which uses the heat content present in the synthesis gas,


CastableRefractory

To ProcessHeat Recovery

FromPrimary Reformer

Cr–MoShell

CatalystTubes

Reformed GasCollection

Natural Gasand Steam Inlet

Fig. 11. EHTR configuration.

which, in the case of GHR, is being produced by an ATR. Instead of the ATR,(Fig. 12) the synthesis gas may also be produced in the HSR.

A gas and steam mixture is fed to the catalyst tubes of the gas-heated re-former. The reaction takes place, and the partially reformed gas is then fed tothe ATR, which is fired by oxygen or air. In the ATR, the reforming reaction iscompleted and the resulting synthesis gas with high heat content is passed to theShell side of the GHR where it supplies the heat for the reforming reaction. Forstart-up purposes, a start-up ignition unit is included between the two units. TheGHR technology has been patented and developed by ICI Katalco (later namedSyntax and nowadays incorporated in Johnson Matthey Catalysts) in the early1980s and was originally developed for ammonia applications, in which case thesecondary reformer is fired with air. A plant was built in the UK (Severnside)for ammonia. In 1994, a plant with a GHR was commissioned in Australia formethanol application.

The GHR reactor consists of a number of catalyst-filled tubes each with acentral bayonet tube. The annular space between these concentric tubes is filledwith catalyst. The feed gas (natural gas and steam) enters the top of the reactorvessel and flows through the catalyst-filled annular space and then back throughthe central tube, giving off heat to the incoming feed gas. The gas then passes onto the ATR or secondary reformer. The outside surface of the outer tube has an


RAW SYNGAS

OXYGEN or AIR

REFORMER FEED GAS

BFW

STEAM

STEAM

NATURAL GAS

REFORMEREFFLUENT

REFORMEREFFLUENT

DESULFURIZER

FIRED HEATER STEAMDRUM

RTARHG

Fig. 12. GHR configuration with ATR.

extended surface through a finned surface, thereby increasing the heat transfercoefficient.

Combined Autothermal Reforming. In the combined autothermal re-forming (CAR) process developed by Uhde (27), the HSR process and the ATRprocess are combinedin to one reactor (Fig. 3).

In the CAR process, the natural gas feed is mixed with steam and intro-duced into the CAR reactor via a tube sheet to the catalyst-filled tubes in whichreforming to synthesis gas takes place. The natural gas is partially converted,and the slip methane is allowed in the lower chamber where partial oxidationtakes place. In this lower section, temperatures are about 1300–1400◦C. The re-sulting hot synthesis gas then passes upward and supplies heat to the primaryreforming reaction inside the catalyst tubes. An important element in the CARreactor is the tube sheet, which acts as a feed stream distributor to the reformertubes. In addition, there are enveloping tubes around the catalyst tubes, whichconstrict the flow of the autothermal product gas, thereby increasing the convec-tive heat transfer coefficient. The CAR reactor, due to the high temperatures, isalso jacketed with water.

Pressure Swing Reformer. The pressure swing reformer (PSR) processwas suggested by Hershkowitz and co-workers (28) from ExxonMobil Researchand Engineering. Figure 14 provides a schematic of the cyclic mode of operationof the PSR process.

The authors claim that the PSR reformer yields syngas at high efficiencyand with the compactness of an ATR. PSR is a cyclic, reverse-flow reactor thatalternates combustion steps to heat the catalyst bed with reforming steps thatcool the bed. During these steps, the center of the catalyst bed remains at


REFORMER FEED GAS

STEAM

NATURAL GAS

DESULFURIZER

BFW

STEAM

FIRED HEATER STEAM DRUM

UHDE - CAR PROCESS

FEEDGAS BYPASS

RAW SYNGAS

OXYGEN or AIR

OXYGEN or AIR

Fig. 13. Combined auto thermal reforming.

Fig. 14. Pressure swing reformer principle (28).

temperatures approaching 1200◦C, enabling rapid and high conversion. Heat ex-change within the packed-bed reactor results in relatively cool products, resultingin a high thermal efficiency.

Convective Reforming. Haldor Tøpsoe AS developed the concept of con-vective reforming (HTCR) (29,30), as shown in Figure 15.

The HTCR reactor consists of a number of bayonet reformer tubes and com-bines basically the radiant section and the convection section of a conventionalHSR in a single piece of equipment. The reaction heat is provided by the flue gasflowing on the outside of the reformer tubes and by reformed gas flowing in anupward direction in the bayonet tubes. This results that is about 80% of the firedduty is utilized in the process, and steam export is minimized.

The convective reactor concept is especially attractive for small-sized syn-thesis gas plants (<5,000 Nm3/h). The reactor can be prefabricated on a skidsuch that construction costs can be minimized through shop fabrication.

Advanced Reforming Technology. The advanced reforming technology(ART) reformer concept (31) developed by KTI SpA (now Technip) is a convectivereformer and is characterized by utilizing up to the maximum extent of the wasteheat available in the steam-reforming process for the process itself and not for theproduction of export steam. This results in a reduced operating and investmentcosts.

The ART design is based on the use of regenerative catalyst tubes (Fig. 17).


Burner Air Burner Fuel

ProcessGas Inlet

ProcessGas Outlet

Flue Gas Outlet

ReformerTubes

Fig. 15. Haldor Tøpsoe convective reforming.

Figure 16 shows a conventional reactor tube configuration as present in asteam reformer furnace. The preheated hydrocarbon feedstock passes throughthe catalyst tube in which it reacts producing an equilibrium mixture of hydrogenand carbon oxides; the reformer effluent, at a temperature, which is in the rangeof 800–950◦C, is then sent to the process gas boiler where steam is generated.

In the ART regenerative catalyst tube configuration (Fig. 17), the effluentflows upward from the bottom to the exit of the catalyst tube inside the regener-ative tube. With this arrangement, more than 25% of the heat required for theendothermic steam-reforming reaction can be provided by the reforming effluent.As a result, the amount of fuel is reduced and the temperature of the reformereffluent decreases.

Figure 18 shows the schematic longitudinal view of the ART tube. The hy-drocarbon feedstock, introduced at the top of the catalyst tube, passes throughthe catalyst, which is located between the tube and the internal riser and is con-verted to hydrogen and carbon oxides. At the bottom of the catalyst tube, where


Fig. 16. Conventional reactor tube.

Fig. 17. ART regenerative tube.

the equilibrium conversion is reached, the process gas inverts its direction and itflows through the internal riser. At the exit from the catalyst tube, the tempera-ture of the process gas is reduced to such an extent, that it depends on the heatexchange with the feedstock.


Fig. 18. Longitudinal section of ART catalyst tube.

The heat recovery from the reformer effluent can decrease its temperatureto about 350◦C.

Kellogg Reforming Exchange System. The Kellogg reforming ex-change system (KRES) developed by Kellogg (32) is a hybrid configuration be-tween the ICI GHR concept and the combined reforming principle. The KRESsystem replaces the standard reformer in a combination process of an ATR witha heat exchange reformer (Fig. 19).

The advantages claimed by Kellogg for the KRES system include

(1) Elimination of the conventional HSR,(2) Flexibility in tuning the H2 to CO ratio, and(3) Higher thermal efficiencies and reduced NOx emissions.


Natural Gasplus Steam

~ 600–640OC

PROCESSHEATER

AUTO THERMAL REFORMER

Enriched Air

REFORMINGEXCHANGER

~ 600 – 640 O C

~ 500–550 O C

~ 660– 680 O C

~ 40 kg/cm2g

~ 1000OC

Fig. 19. KRES system.

As with the other combination technologies, which include an autothermalreforming step, the cost of the KRES system is strongly dependent on the cost ofoxygen, either as a utility or as the cost of an air separation plant.

Fluid Bed Reforming

Exxon has developed the fluid bed synthesis gas (FBSG) unit (33) as part of alarge program on gas to liquids (GTL) to convert natural gas to refinery feedstocksvia a Fischer Tropsch route. Exxon has built a large pilot unit for this FBSG, anda simplified flow scheme is presented in Fig. 20.

Core of the unit is the fluid bed reactor in which reforming and partial oxi-dation reactions take place simultaneously. The reaction is controlled by the feedcomposition, oxygen feed rate, and outlet temperature. Typically, the reaction iscontrolled at 950–1050◦C, and a pressure up till 40 bar.

The product gas is sent to a cyclone to separate any entrained catalyst par-ticles, which are returned to the fluid bed reactor. The synthesis gas is then sentto a heat recovery train. The first exchanger is a process gas waste heat boiler,which has to cool the synthesis gas rapidly to prevent the Boudouard reaction toform carbon deposits on equipment walls and resulting in possible metal dusting(see also the section Application of Synthesis gas).

Applications for Synthesis Gas

Overview. As the name synthesis gas implies, synthesis gas is a buildingblock for a number of important petrochemicals, as well as fuels. In Fig. 21 (34),


Steam

NaturalGas

DESULFURIZATION

PREHEAT

Oxygen

FBSGREACTOR

GASCLEANUP

Steam

Cond.

HEAT RECOVERY

Steam

BFW

Syngas product

Fig. 20. Exxon fluid bed synthesis gas unit.

the syngas applications are summarized. Syngas is used as feedstock for bothchemicals and transportation fuels.

It is beyond the scope of this article to describe the technology of these syn-gas applications in more detail and the reader is referred to the existing literatureon this topic. The catalytic aspects of these applications are described by Stitt andco-workers (35), and Steynberg and Dry (36)

One particular emerging trend in applications is related to the use of syn-thesis gas in the context of a possible hydrogen economy. This will be the topic ofthe next paragraph in this article.

Synthesis Gas and The Hydrogen Economy. Syngas also plays animportant role as a bridging technology for a sustainable energy system.

Hydrogen plays a pivoting role in all strategies to lower CO2 emissions, im-prove the air quality of urbanized areas, and increase the possibilities of coveringthe energy demand with energy sources other than petroleum. In the EuropeanUnion, the Hydrogen, and Fuel Cell Platform has set up a Strategic ResearchAgenda (37), aiming at the development of technologies needed for hydrogen pro-duction, storage, transport, and application in stationary and mobile systems. Inaddition, a deployment strategy (37) has been outlined for introduction of thesetechnologies for industries. A program of more than 2 billion euros is foreseen inthe 7th Framework Programme. In the United States, the Freedom Car Initiativeand the FutureGEN (38) projects are just two examples of a huge program on hy-drogen and fuel cell technologies. The Department of Energy has a well-organized


Fig. 21. Syngas applications (34).

Fig. 22. The hydrogen bridge.

program in which clear technology development targets are set and the progressis frequently assessed. In California, demonstration fleets of fuel cell vehicles areon the road and state legislation on emissions pose a strong driving force to cleanvehicles. In Japan, being completely dependent on fossil fuels imports, a long tra-dition exists in developing stationary and mobile fuel cell applications. Programsfunded by the NEDO and METI departments have already resulted in hundredsof small-scale micro Combined Heat and Power Units as well as fuel cell vehiclesrunning in road demonstrations (39). In most of the technologies to produce hy-drogen for energy applications, synthesis gas is produced first and provides thebridging function as illustrated in Figure 22. This will be the case for the comingdecades, during which period technologies to produce hydrogen from sustainablesources such as solar, wind, and biomass, will also be developed.


eElec.

Power

700-900°CC7H14 + O2 + 10 H2O ->

4CO + 17 H 2 + 3CO2

400°C -> 250°CCO+H2O->CO2+H2

300°CH2 2->H2O

PEMFC

PROX LTS HTS

After

burner

PEMFC ATR Fuel

Exhaust

Air

80–250°C2->CO2

70–90°CH2 2->H2O

Fig. 23. Fuel processor configuration with a low temperature PEMFC fuel cell (courtesyECN).

In combination with fossil fuels, a fuel processor converts the hydrocarbonfuel into synthesis gas, using one of the technologies as described in the sectionIndustrial Processes for Syngas Production, albeit at a much smaller capacity,for example, in the application for microCHP, using natural gas as fuel, or forapplication as APU (auxiliary power unit) in automotive applications.

Figure 23 gives a simplified configuration of a fuel processor with a PEMFCfuel cell configuration, emphasizing the catalytic reactors that are present.

For this application, the reaction pressure in the system is usually justabove atmospheric, enough to overcome the pressure drop within the system. Inthis case, the fuel that is used is a diesel fuel that is converted first in the ATR.The resulting syngas is then purified in a number of steps before the reformate isfed to a PEMFC fuel cell. This configuration is envisaged in an APU applicationon board of cars, boats, or recreational vehicles.

The reformate composition that enters the anode compartment of thePEMFC stack is dependent on the hydrocarbon feed. For natural gas feed, thetypical reformate composition is, in mol%: H2 ∼50%, CH4 <1%, H2O ∼1.5%, CO2∼15 %, N2 ∼32% (for Dutch natural gas), CO <10 ppm (ECN, internal communi-cation. The CO content in the reformate is the most critical parameter, becausethe anode catalyst in the PEMFC, a Pt-based catalyst, will be poisoned by CO.

CO2 Capture

The capture and geological storage of carbon dioxide is one of the routes to pro-vide carbon-lean electricity and is seen as a technology that may bridge the gapbetween the current CO2-intensive electricity production and a future where re-newable sources provide the bulk of the electricity (40). For the power generationsector, several options are in development for the capture of CO2. PostcombustionCO2 capture removes the CO2 from the flue gases of a power plant, using chem-ical solvents. Alternatively, combustion of coal or natural gas can be carried out


using pure oxygen or oxygen-rich air, to increase the partial pressure of CO2 inthe flue gas.

The third option, precombustion CO2 capture, involves the conversion ofthe fuel into synthesis gas, shifting the syngas to a hydrogen and CO2 mixtureand separating the two compounds prior to combustion. Subsequently, the CO2is compressed and made ready for transport and storage. The hydrogen is usedfor power generation in a gas turbine combined cycle. The hydrogen productionroute can in principle be similar to the technologies described above: hydrocarbonreforming or coal gasification followed by water-gas shift and CO2 separationusing chemical or physical solvents. There are, however, a few considerationsthat make the precombustion CO2 capture different from conventional hydrogenproduction. A major consideration is that in a natural gas combined cycle (NGCC)or an integrated gasification combined cycle (IGCC), the steam produced from thewaste heat boiler is converted into extra electrical power in the steam turbines.So for the syngas production, the water-gas-shift step, and the CO2 separationstep, the steam use must be kept as low as possible (41).

All the state-of-the-art technologies cause a considerable reduction of thepower production efficiency and an increase in power production costs (40). Majorcontributors to the loss of efficiency are the steam use, the compression of the CO2for transport and storage, and losses by heating and cooling of the syngas. Muchresearch is being devoted to improving solvents, catalysts, and the developmentof entirely new processes. Important improvements compared to the state of theart would include

(1) Hydrogen fuel being available at the right temperature and pressure forintroduction in the combustion chamber of the gas turbine, to prevent heatexchangers;

(2) CO2 being available at pressure to reduce compression work;(3) Low steam use for shift catalysis and solvent regeneration; and(4) Reduction of the number of process steps to reduce costs.

This article describes a number of technologies in development that are re-lated to the syngas technologies described above, but obviously produce a sepa-rate stream of hydrogen and CO2.

Membrane Reforming

The principle of membrane reforming is illustrated in Figure 24. Natural gasand steam enter the middle compartment, in which a steam-reforming cata-lyst converts the mixture into hydrogen and CO. The catalyst also catalyzes thewater-gas shift reaction, but at the temperatures used in steam reforming (above550◦C), the shift equilibrium lies at the reactants side. Simultaneously, the hy-drogen is withdrawn from this compartment through the membrane. Removinghydrogen from the reaction zone through the membrane pulls the equilibriumof both the reforming and shift reactions to the product side. The gas leaving themiddle compartment, the retentate, mainly consists of CO2 and steam, with small


CO2, H2O,(H2, CO, CH4)Membrane

CO+H2O CO2+H2

CH4+H2O CO+3H2Retentate

Permeate (incl. H2)

H2 H2H2H2

Sweep

Natural gas + H2O

Fuel + air Flue gasCombustionHeat Heat

Fig. 24. Operating principle of a membrane reactor for steam methane reforming.

amounts of unconverted CH4 and CO and not-permeated hydrogen. The heat forthe endothermic steam methane-reforming reaction is provided by combustionof a fuel (natural gas or hydrogen) in the top compartment. In the bottom com-partment, hydrogen that has permeated through the membrane is harvested, thepermeate stream. In many cases, sweep gas is used to reduce the partial pressureof the permeate and increase the driving force over the membrane.

In the past years, the development of membrane-integrated hydrogen pro-duction has been pursued by many. Currently, Tokyo Gas has demonstrated theworld’s largest scale membrane reformer with a rated H2 production capacityof 40 Nm3/h on natural gas. The membrane units are made by rolling sheets ofPd-alloy to a thickness of about 20 µm. A main drawback has been the cost ofthe membrane material, which is connected to the thickness (>20 µm) of exist-ing commercial membranes (42). It is thus necessary to develop membranes withreduced thickness of the Pd layer. Thin layer (approximately 2 µm) Pd mem-branes always need a stainless steel or ceramic support to provide mechanicalstrength for the membrane. The membranes used in this process are dense pal-ladium membranes or Pd alloyed with silver or copper. The palladium acts as acatalyst in dissociation of the hydrogen molecule and transports the hydrogen ina solution–diffusion mechanism. The Pd-alloys are used to improve stability ofthe membranes in relation to phase changes that occur in hydrogen-loaded purePd membranes at certain temperatures (42).

These thin supported membranes are currently being produced at 0.5–1 mlength and have been tested in membrane reforming and water-gas shift (43).One of the advantages of applying membrane reformers in precombustion CO2capture is that several steps (reforming, shift, CO2 separation) are combined inone reactor. Also CO2 becomes available at elevated pressure, and hydrogen be-comes available at approximately the inlet temperature of the gas turbine. Thismakes CO2 capture using the membrane reformer more efficient and less expen-sive compared to a base case consisting of ATR, water-gas shift, and chemicalCO2 solvents (43).

Sorption-Enhanced Reforming

In sorption-enhanced reforming (SER) reactors, one of the products is extractedfrom the reaction zone, thus shifting the reaction equilibrium to the productside. In SER, the methane steam-reforming catalyst is mixed with a CO2 sorbent(“acceptor”). The CO2 produced during the reaction is adsorbed and the reverse


CH4+ H2O H2 +CO2

CH4+ H2O H2

CO2

CO2CO2

CO2

CO2

CO2

sorbent sorbent

CO2

Ordinary steam reforming Sorption-enhanced steam reforming

catalyst catalystcatalyst catalyst

CH4+ H2O H2 +CO2

CH4+ H2O H2

CO2

CO2CO2

CO2

CO2

CO2

sorbent sorbent

CO2

catalyst catalystcatalyst catalyst

Fig. 25. Sorption-enhanced reforming.

reaction cannot occur (Fig. 25). The hydrogen stream also contains unconvertedmethane and CO. For production of pure hydrogen, an extra hydrogen purifi-cation step is needed; for application of SER in precombustion CO2 capture forpower generation, the gas can be fed directly to the gas turbine. The two ma-jor sorbents examined for sorption-enhanced reforming are CaO-based materialsand hydrotalcites (44).

SER using hydrotalcite sorbents was pioneered by Air Products in the 1990s(45). It was shown at lab-scale and at atmospheric pressure that around 98%methane conversions can be reached at 400◦C using hydrotalcite CO2 sorbents.At higher pressures, however, thermodynamics causes the conversion to decreaseand the temperature needs to be higher to reach high methane conversions (46).

Calcium oxide can act as a CO2 acceptor at higher temperatures than hydro-talcites. It reacts with methane and steam to form hydrogen and calcium carbon-ate (Reaction 11). Between 600 and 650◦C, the CaO–CaCO3 equilibrium lies atthe carbonate side, which means that high methane conversions can be reached.To regenerate CaCO3, temperatures of 900◦C (at atmospheric pressure) to 1100◦C(at 15 bar) are necessary (44). The process is carried out in circulating fluidizedbed reactors.

�H◦298 (kJ/mol)

CH4 + 2 H2O +CaO ↔ 4 H2 + CaCO3 (12) 1.4

Reaction 12 shows that the hydrogen production is almost thermally neu-tral. The sorbent regeneration step does require energy, but it has been reportedthat an energy saving of 20–25% compared to conventional steam methane re-forming can be reached with CaO-SER (44).

Chemical Looping Reforming Chemical looping reforming (CLR) draws fromthe rapid development of a related, but different, process that is, chemical loop-ing combustion, in which a solid oxygen carrier is circulated between two beds. Acombustor in which a metal oxide (typically NiO) is reduced by natural gas un-der formation of steam, CO2 and a hot flue gas and a regenerator in which thedepleted oxygen carrier is regenerated using air (47). Whereas chemical loopingcombustion aims at full conversion of the fuel, chemical looping reforming aimsat synthesis gas production, that is, partial conversion of the fuel.


Fig. 26. Schematic representation of CLR(s) concept. (1) Air reactor/riser, (2) Cyclone, (3)Fuel reactor, (4) fluidized bed heat exchanger/reformer, (5) Shift Reactor, (6) CO2 separa-tion. Courtesy Chalmers University of Technology, Sweden.

Two concepts have been described. In autothermal chemical looping reform-ing (CLR(a)), mainly Ni-based oxygen carriers are used to produce synthesis gas.The air-to-fuel ratio is kept low to prevent full oxidation of the fuel (48). In chem-ical looping steam reforming (CLR(s)), a chemical looping combustion system isused to provide heat to a steam methane–reforming reactor (see Fig. 26). In a con-ventional steam reformer, heat is provided by burning fuel with air, which leadsto dilute CO2 emissions. With the CLR(s) concept, the fuel is combusted with oxy-gen, which gives a pure CO2 stream. Both the CLR(a) and CLR(s) reactor shouldbe followed by a water-gas-shift section and a CO2 absorption section to convertthe produced synthesis gas into separate streams of hydrogen and CO2.

Oxygen-Selective Membranes for Syngas Production

Autothermal reforming can be carried out inside a membrane reactor withoxygen-selective membranes. Air Products–who use the term ion transport mem-branes, ITM–developed this concept in a consortium named ITM syngas (49). InFigure 27, the process is shown schematically. Air is fed to one side of the mem-brane and permeates through the membrane in the form of oxygen ions. On theother side of the membrane, the ATR reaction takes place, producing syngas. Theperovskite-type membranes used for this process must be able to withstand ox-idizing conditions on one side and reducing conditions on the other. To produce


Fig. 27. Syngas production in an oxygen-selective membrane reactor. Courtesy Air Prod-ucts..

separate hydrogen and CO2 streams, a water–gas shift and CO2 separation stepis needed.

Engineering Issues

Reactor Design. The HSR reactor for catalytic steam reforming is car-ried out in large tubular reactors, mounted in a large firebox (see the sectionHydrocarbon Steam Reforming). These reactor tubes are typically about 10 min length and 5 in. inside diameter. Because the overall reaction is strongly en-dothermic, the reaction is in most cases heat flux limited. That is, on the basisof average heat flux in (expressed in kcal/m2/h1 or in kW/m2), the required (ex-ternal) reactor tube surface is calculated. The reactor tubes are connected to asupport system of variable springs or constant load hangers. A reformer fireboxmay contain several hundred of tubes, mounted in several tube lanes (each 30–50tubes) with rows of burners mounted on top, between these tube rows, or at thesides of the furnace. Figure 28 gives a typical layout sketch of a top fired reformerfurnace.

The right-hand side of this figure shows the radiant reformer box, and theleft-hand side show the convection section and the flue gas stack.

Heat flux, operating material limits, and combustion engineering are keyissues for the reactor design of steam-reforming units. Since about 50% of theheat of combustion is delivered in the radiant section of the furnace to the re-actor tubes, the remaining 50% of combustion heat has to be recovered in theconvection section of the furnace, through preheating of feed and combustion air,boiler feed water, raising steam. A clear trend is present in reformer design to


Fig. 28. Reformer layout (top-fired), (courtesy Technip Benelux B.V.).

increase the heat flux, which will allow a reduction in the number of tubes and areduction in firebox volume. Currently, the maximum heat flux applied is about1,00,000 kcal/m2/h1. Another trend is a reduction in tube wall thickness, whichwould allow reducing the outside metal temperature, resulting in significant en-ergy savings.

Autothermal Reforming. In the autothermal process for syngas produc-tion, the heat of reaction is being delivered by partial oxidation of the feedstock,in most cases natural gas.

The ATR reactor is basically a reactor vessel, filled with reforming catalystand a burner mounted on top. The key engineering item of this ATR reactor isthe burner design, which should give a stable, soot-free flame. Today, computa-tional fluid dynamics (CFD) is a powerful tool in the design of these burners. Thematerial of the burner tip (nozzle) is selected on the requirement of high tem-perature operation and the need to minimize burner replacement. Burner designshould allow soot-free operation and allow reasonable turndown. In this respect,it is critical that reactants and intermediate products are well mixed to ensurethat the heat of reaction of the oxidation will be used in the subsequent reformingreactions. Residence time in the burner is short; 1–3 s are typical.

Tube Design and Material Issues. Most of the synthesis gas reactors asdescribed in this article contain catalyst-filled reactor tubes. These reactor tubes,fabricated from high alloy materials, constitute an important percentage of thetotal reactor cost. These tubes are exposed to high temperatures and a reaction


pressure of up to 30–40 bars, whereas the pressure at the heat supply side maybe close to ambient pressure.

As such, the reactor tube is a cylindrical vessel exposed to significant in-ternal pressure and stress and temperature (both inside and outside the reactortube). The high temperature reactor tubes present in the steam-reforming fur-nace are the most critical in terms of design and overall plant economics andrequire, therefore, a separate discussion. The materials used in the other lowertemperature reactors in the synthesis gas production process are much less criti-cal.

Materials for catalyst tubes are selected in combination with the processconditions employed. Alloys with high chromium and nickel content are used forthe reactor tubes in a steam-reforming furnace. The first centrifugally cast tubessuch as HK 40 contained 25% Chromium and 25% Nickel. Today, tube materialcontaining 25% Chromium and 35% nickel, niobium, and traces of zirconium andtitanium are used (so called HP alloys) (50). The HP alloys are more expensivebut allow a higher tube design temperature and have a better creep strength andoxidation as well as carburization resistance.

The selection of the tube material and tube dimension is based on the tem-perature and pressure profile calculation. Based on the temperature profile andthe strength of the material (determined as the yield or strength (in the elas-tic temperature range of tube materials)) as the stress-to-rupture strength for a100,000-h service time), the tube thickness is calculated. Frequently, this processis an economic optimization between process conditions, material choice, and costof bulk materials.

The catalyst tubes are located with a specific spacing between the tubes(tube pitch).

The main concern in proper reformer design and tube design is the supplyof the same heat flux profile over each catalyst tube, ensuring that the heat fluxsupplied to the process gas inside the tube is in balance with the required reactionheat in that tube. This results in a specific temperature profile inside the tubefrom the inlet to outlet of the tube. This temperature profile is also reflected inthe tube metal.

As an example, the flue gas heat duty profile (in W per m2 of catalyst tubesurface) is shown in a top-fired steam reformer furnace, which was designed withthe use of CFD, in combination with a kinetic model (Fig. 29). The top part ofthis picture shows the burners, the bottom part shows the position of the flue gasdischarge ducts, where the flue gas exits the reformer box.

For the design of the firebox and reactor tubes, it is further essential tominimize hot spots in the reactor which may lead to much faster creep rupture,and catalyst problems (sintering or plugging). Another important element is theguarantee of an even process feed flow over the catalyst tubes in the reactor.

Several design standards may be used for the design of reactor tubes, forexample, the guidelines AP530 of the American Petroleum Institute (51), whichgives guidelines for creep rupture stress calculations.

Flue Gas Discharge. As outlined in the section Application of Synthe-sis Gas, the combustion management of a synthesis gas reactor is of paramountimportance. The heat flux required is, in externally fired units, supplied by hotflue gas, resulting from combustion. The flow of this flue gas in the combustion


Fig. 29. Flue gas heat duty profile in a reformer furnace (courtesy Technip Benelux BV)(Units in W/m2).

Tube Rows

A B C D E F

Fluegasdischargeduct

Fig. 30. Flue gas discharge channels (tunnels).

chamber, the firebox, is determining how the heat flux profile and resulting tem-perature profiles are established (52).

In steam reformer units, the flue gas in the firebox is discharged via spe-cially designed flue gas channels, also indicated as tunnels (Fig. 30). This tunneldischarge is needed to ensure a “plug flow” of flue gas inside the firebox.

The flue gas is discharged through holes, present in the discharge duct tothe convection section of the reactor. In case of the absence or incorrect design


(a) With fluegas ducts (b) Without fluegas ducts

Tubes Fluegas duct

Fluegas flow patterns

Fig. 31. Flue gas flow distribution: (a) correct discharge design and (b) absence dischargechannels.

of these discharge channels, a significant flue gas maldistribution can take place(Fig. 31).

This flue gas maldistribution, in turn, may lead to overheating of reactortubes and wide distributions of reformer outlet temperatures.

Proper design of flue gas tunnels becomes even more important for verylarge firebox designs (up to 200,000 Nm3/h of synthesis gas product per furnace).

Catalyst Engineering. Since most synthesis gas reactors are heat fluxlimited, the catalyst present in the reactor tube has to ensure that this heat fluxfrom the reformer tube wall is effectively transported into the process fluid, whichis converted on the catalyst surface.

The trend in for example, steam reformer units, to operate at higher heatfluxes, has to be met by catalysts with adequate activity and heat transfer prop-erties.

In addition, the macroscopic shape of the catalyst should allow a pressuredrop of maximum 2–3 bar.

Catalyst manufacturers have addressed these issues and have produced var-ious shapes of catalysts to achieve maximum activity and maximum heat transferwhile minimizing the pressure drop. These catalysts typically exhibit a lifetimeover more than 5 years. There are a number of catalyst vendors for synthesis gasprocesses, including Johnson Matthey Catalysts (formerly Synetix), Sud Chemie,Umicore, and Haldor Tøpsoe.

Revamping. Revamping of a synthesis gas production unit is defined asa capacity increase of an existing unit through modification of the plant, keep-ing in place the main reactors. There are various degrees of revamping possiblein a synthesis gas product unit. The Selection of the most optimum solution for


revamping is an optimization of various possibilities (53). Revamping possibili-ties, in increasing order of degree of complexity (and costs) include

(1) replacement of reactor tubes with tubes of a more advanced material,which allows operation at a higher heat flux, can increase the capacity bysome 10–20%;

(2) replacement of the catalyst with a more active catalyst, which allows anincreased heat transfer, may increase the capacity by 10–20%; and

(3) an add-on unit to the primary reactor may add 20–40% of capacity. Forexample, addition of a prereformer prior to a HSR or addition of a HER toa HSR.

Metal Dusting. Metal dusting is a phenomenon of catastrophic carbur-ization of metals and alloys, which are exposed to carburizing atmospheres (54).This exposure can lead to disintegration of the metals into metal particles andcarbon. As such, this phenomenon is a very dangerous one and can lead to leaksin plant equipment and piping.

Whether or not metal dusting will occur does depend on

(1) the carburizing activity of the atmosphere and(2) the protection of the metal against metal dusting.

The first condition is related to the thermodynamic activity of carbon in theatmosphere. This value is calculated by using the CO reduction reaction:

CO reduction H2 + CO ↔ C + H2O (10)

with: ac = K2PCOPH2

PH2O

where ac = 1 for equilibrium with graphite, K2 is the equilibrium constant for theCO reduction reaction and PCO, PH2 and PH2O are the partial pressures, respec-tively.

The first step in metal dusting is the formation of unstable carbide, followedby its decomposition:

Me3 C → 3 Me + CThis reaction is in particular fast for low alloy steals. For high alloy steals,

a protective layer of chromium oxide. However, even in this case, small defects inthe metal surface can lead to metal dusting of the metal. Another protective mea-sure, which helps to reduce metal dusting, is the presence of sulfur compoundssuch as H2S, dimethylsulfide (DMS), or dimehyldisulfide (DMDS) (55). These sul-fur compounds intervene in the mechanism of metal dusting and suppress someof the steps in this mechanism.

The production process for synthesis gas can indeed satisfy the conditions toproduce an atmosphere with ac >> 1, and thus a strong carbonizing atmosphere.This is in particular the case at temperatures in the range of 600–750◦C and lowsteam dilution ratios.


In the synthesis gas production process areas of particular attention withregard to metal dusting, include the reactor outlet headers and the process gaswaste heat boiler, where the synthesis gas is cooled to temperatures where themetal dusting activity is significant.

New Developments. New developments, which may influence the tech-nology of synthesis gas production, are multifold.

The following developments are considered to have an important impact onthe next decade:

More advanced tube materials:This issue will have an impact on the selection of reactor tubes and is in

fact a more or less continuous improvement overtime. Better alloys for new tubematerials have creep-rupture strength, which is more than double as that of thetraditional HK-40 tube material.

(1) Better catalytic materials More advanced catalysts may play an importantrole to achieve syngas manufacture at lower costs. An increase of heattransfer capability and an increase in activity while maintaining accept-able pressure drop are key elements (56,57).

(2) Catalysts, which will allow operating at more severe process conditions,are expected to improve the technology as well. For example, developmentof coke-resistant catalysts for 1HSR that are based on noble metals, canallow operating the synthesis gas reaction at much lower steam ratios (58).

(3) A new horizon in synthesis gas production may be the process of catalyticpartial oxidation (CPO) (59–61), of hydrocarbons, for example, the selectiveoxidation of methane

2CH4+O2→2CO+4H2

For this reaction, the most important requirement is to avoid the complete oxida-tion reaction, which will initiate a runaway situation.

A lot of advancement has been made for the CPO reaction, which has been,demonstrated over monolith reactors or noble metal gauzes (62,63). An impor-tant consideration for this type of reaction is the operation outside the explosionlimits.

(1) Another advancement in the technology of synthesis gas reaction is touse selective membranes, (64–66) which would allow performing theequilibrium-limited reactions at much lower temperatures by removing oneof the products (usually hydrogen) continuously. For example, the steam-reforming reaction would be performed at 650◦C, rather than 900◦C byremoving hydrogen continuously by a selective membrane. Critical hereis the component mass flux over the membrane. Another approach in thisarea is the use of mixed conducting membranes, where oxygen is perme-ated through a mixed conducting membrane and reacts with the partiallyconverted gas to produce additional hydrogen and carbon monoxide.(67)


(2) The dry reforming of methane (also indicated as CO2 reforming) provides amethod to play a similar role as steam in the SMR process. The dry reform-ing reaction is given in the section Chemistry and Thermodynamics andgives a lower H2 to CO ratio than the similar reaction with steam and thusgives a syngas that is suitable as feed to FT or to manufacturing of CO. Inthe catalysis of CO2 reforming, the catalyst carrier plays an important role(68). The CO2 reforming process may also be used in conjunction to CO2recycle in the CCS process.

(3) The incorporation of CCS technology within the process schemes for syngaswill be implemented in the next decade.

Conclusions

Production of synthesis gas can be achieved by a number of technologies, of whichsteam reforming of hydrocarbons, notably natural gas, is the predominant route.Oxygen-based conversion production processes for synthesis gas will play an in-creasing important role, in particular when cheaper methods of air separationwill be developed. In the coming decades, syngas technologies will be designed toinclude carbon capture and sequestration (CCS). Also routes to produce hydrogenon a sustainable feedstock will be increasingly important in the long term (fromca. 2030 onwards). The role of hydrogen as an energy vector will significantlyboost the syngas requirement.

Engineering issues are focused on the increase of the conversion efficiencyand thermal efficiency through heat integration and improvements of economicsthrough reduction of costs by applying more advanced materials and integrationof equipment units. Advanced numerical tools as CFD and modeling tools willenable to increase capacity of syngas and hydrogen production plants.

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Encyclopedia of Catalysis || Synthesis Gas Generation-Industrial