MINISTERIODE CIENCIAE INNOVACIÓN

1233Junio, 2011

Informes Técnicos Ciemat

GOBIERNODE ESPAÑA Centro de Investigaciones

Energéticas, Medioambientales

y Tecnológicas

Optimisation of ShiftReactor Operating Conditionsto Maximise HydrogenProduction

J. M. SánchezM. MaroñoE. Ruiz

Informes Técnicos Ciemat 1233Junio, 2011

Departamento de Energía

Optimisation of Shift Reactor Operating Conditions to Maximise Hydrogen Production

J. M. SánchezM. MaroñoE. Ruiz

Toda correspondencia en relación con este trabajo debe dirigirse al Servicio de In-formación y Documentación, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Ciudad Universitaria, 28040-MADRID, ESPAÑA.

Las solicitudes de ejemplares deben dirigirse a este mismo Servicio.

Los descriptores se han seleccionado del Thesauro del DOE para describir las ma-terias que contiene este informe con vistas a su recuperación. La catalogación se ha hecho utilizando el documento DOE/TIC-4602 (Rev. 1) Descriptive Cataloguing On-Line, y la cla-sificación de acuerdo con el documento DOE/TIC.4584-R7 Subject Categories and Scope publicados por el Office of Scientific and Technical Information del Departamento de Energía de los Estados Unidos.

Se autoriza la reproducción de los resúmenes analíticos que aparecen en esta pu-blicación.

Depósito Legal: M -26385-2011ISSN: 1135 - 9420NIPO: 471-11-024-4

Editorial CIEMAT

Catálogo general de publicaciones oficialeshttp://www.060.es

CLASIFICACIÓN DOE Y DESCRIPTORES

S08GASIFICATION; HYDROGEN PRODUCTION; WATER GAS PROCESSES;CHEMICAL REACTIONS; EXPERIMENTAL DATA; CATALYSTS

Optimisation of Shift Reactor Operating Conditions to Maximise Hydrogen Production

Sánchez, J. M.; Maroño, M.; Ruiz, E.33 pp. 19 ref. 9 figs. 2 tables

Abstract:This report compiles the results of the work conducted by CIEMAT for Task 6.5 “Shift reaction” of the FLEXGAS project “Near Zero Emission Advanced Fluidised Bed Gasification”, which has been carried out with financial support from the Research Fund for Coal and Steel, RFCR-CT-2007-00005. The activity of an iron-chromium-based catalyst for the water gas shift reaction is studied. Results about WGS experiments conducted by CIEMAT on laboratory scale under different operating conditions are presented. The influence on the activity of the catalyst of main operating para-meters- temperature, pressure, excess steam, and space velocity and gas composition - is evaluated and discussed.

Optimización de las Condiciones de Operación de un Reactor Shift para Maximizar la Producción de Hidrógeno

Sánchez, J. M.; Maroño, M.; Ruiz, E.33 pp. 19 ref. 9 figs. 2 tablas

Resumen:Este informe recoge los resultados del trabajo realizado por CIEMAT en la tarea 6.5 “Reacción Shift” del Proyecto FLEXGAS “Near Zero Emission Advanced Fluidised Bed Gasification”, el cual se ha realizado con financiación de los Fondos para la Investigación del Carbón y del Acero, RFCR-CT-2007-00005. Se estudia la actividad de un cata-lizador de base hierro-cromo para la reacción de gas de agua, en inglés WGS. Se presentan resultados de los estudios WGS realizados a escala de laboratorio, empleando diferentes condiciones experimentales. Se evalúa la influencia en la actividad del catalizador de las principales variables de operación, temperatura, presión, exceso de vapor, velocidad espacial de gas y composición del gas de alimentación.

CONTENTS

SUMMARY ...............................................................................................................................5

1 Overview of the FLEXGAS Project .......................................................................................7

2 Water-Gas-Shift Reaction: Background & State of the Art....................................................9

3 Experimental .........................................................................................................................12

3.1 Test Rig..........................................................................................................................12

3.2 Catalyst ..........................................................................................................................13

3.2 Test Programme .............................................................................................................13

3.3 Experimental Procedure.................................................................................................14

4 Results and Discussion..........................................................................................................16

5 Conclusions ...........................................................................................................................21

6 References .............................................................................................................................22

3

LIST OF FIGURES

Figure 1. Laboratory test rig for WGS studies.........................................................................12

Figure 2 Picture of fresh catalyst .............................................................................................13

Figure 3. Effect of temperature on CO conversion (SV=4715 h-1, P= 20 bar, Gas composition

(%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %, R=3) ....................................................16

Figure 4. Effect of space velocity on CO conversion Gas composition (%v/v): N2 13%, CO

60%, H2, 23%, and CO2, 4 % ...................................................................................................17

Figure 5. Effect of pressure on CO conversion Gas composition (%v/v): N2 13%, CO 60%,

H2, 23%, and CO2, 4 % ............................................................................................................17

Figure 6. Effect of steam to CO ratio on CO conversion Gas composition (%v/v): N2 13%,

CO 60%, H2, 23%, and CO2, 4 %, P=10 bar , SV=2885 h-1 ....................................................18

Figure7. Effect of steam to CO ratio on CO conversion Gas composition (%v/v): N2 13%, CO

60%, H2, 23%, and CO2, 4 %, P=10 bar, SV= 4715 h-1 ...........................................................18

Figure8. Effect of gas composition and CO2 content on CO conversion SV=4715 h-1, R=3,

P=10 bar ...................................................................................................................................19

Figure 9. Hydrogen content in the outlet gas for different shift reactor operating conditions

(Inlet gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %, P=10 bar) ..........20

4

LIST OF TABLES

Table 1. Catalyst specifications ...............................................................................................13

Table 2. WGS catalytic activity tests: Gas composition (% v/v dry basis) .............................14

5

SUMMARY

This report compiles the results of the work conducted by CIEMAT for Task 6.5 “Shift

reaction” of the FLEXGAS project “Near Zero Emission Advanced Fluidised Bed

Gasification”, which has been carried out with financial support from the Research Fund for

Coal and Steel, RFCR-CT-2007-00005. Task 6.5 is part of Workpackage 6 which addressed

the optimisation of gas composition, gas cleaning and shift conversion of syngas produced by

oxy-gasification of mixtures of coal, biomass and waste as feedstock.

The shift reactor converts CO and steam to a gas richer in CO2 and H2. This operation was

studied to optimize the operating conditions to maximise production of H2 and ensure

catalyst’s life. This stage is important in order to increase hydrogen content and produce a

CO2-rich gas for subsequent capture, or to adjust H2/CO ratio for instance for chemical

synthesis for instance is usually converted into hydrogen and carbon dioxide by means of the

water gas shift reaction.

The water gas shift (WGS) reaction has been used for many years in many industrial

processes to produce a hydrogen-rich gas from CO and H2O.

CO + H2O ? CO2 + H2 (? Hº =-41 kJ/mol)

The present practice in industry e.g. for production of high-purity hydrogen for ammonia

synthesis, is to carry out the water gas shift reaction in two steps with two types of catalysts:

first a high temperature WGS reaction (300-400 ºC) that is generally performed over Fe-Cr

catalysts and then, a Cu-Zn catalyst, that is primarily used in the temperature range of 200-

300ºC. Based on technology transfer, the use of iron-chromium catalysts is most frequent

nowadays for the shift reaction of syngas. According to this, CIEMAT studied the activity of

an iron-chromium-based catalyst for the water gas shift reaction. It is important to highlight

that despite the fact that the catalytic water-gas-shift reaction is amply used in the chemical

industry, the operating conditions should be tuned-in almost on a case to case basis when

applying it to syngas upgrading due to the remarkable differences in gas composition

compared to that of the application of the water-gas shift reaction to hydrogen for ammonia

synthesis.

Thus, this report presents results about WGS experiments conducted by CIEMAT on

laboratory scale under different operating conditions. The influence on the activity of the

catalyst of main operating parameters- temperature, pressure, excess steam, and space

velocity and gas composition - is evaluated and discussed.

For the experimental studies, base case gas composition was set according to the composition

expected for an oxygen-blown coal & waste gasifier: N2 13.4%, CO 60.7%, H2, 22.3%, and

CO2, 3.6 %. The effect of gas hourly space velocity GHSV (2000-5000 h-1), steam to CO

6

ratio R, (1-3), pressure P (1-20 bar), and temperature (250-500ºC) has been determined. In

addition to the above mentioned gas composition, in order to study the effect of CO2 on the

activity of the WGS catalyst, which is one objective of the FLEXGAS oxy-gasification

project, the following gas mixtures were evaluated: CO 60%, N2 balance and CO 60.7%, H2,

22.3%, and CO2, 17 % and CO 60%, and CO2, 40 %.

The work accomplished included:

- Review of the state of the art on water gas shift reaction applied to gasification gases

- Design of the experimental plan to laboratory scale level

- Adaptation of the experimental facility to carry out the tests

- Experimental campaigns

- Analysis and Discussion of Results

The catalyst shows very good performance at intermediate temperature, 350-400ºC, having

potential for producing a gas rich in hydrogen and carbon dioxide. Despite the high CO

content in the feed gas, by choosing the right shift reactor conditions, CO concentration at the

reactor outlet reached values below 3%, whereas H2 increased up to above 50% v/v (dry

basis). Increasing space velocity leads to lower CO conversion and a higher temperature is

required to achieve a given CO conversion. The activity of the catalyst depends strongly on

the amount of steam available during reaction. In order for the reaction to proceed

significantly an excess of steam is needed. A higher H2O/CO ratio gives higher carbon

monoxide conversion, and with better selectivity to production of hydrogen. Pressure does

not affect much the activity of the catalyst nor does gas composition, particularly CO2

content.

7

1 OVERVIEW OF THE FLEXGAS PROJECT

In the 1970’s and 1980’s the development of fluidised bed gasification technologies stopped

at pilot/demonstration scale for economic reasons but the interest in its use with biomass

grew for several reasons (fuel flexibility, wide range operation scales suitability, etc.).

In the FLEXGAS project the way to overcoming the potential disadvantages of fluidised bed

gasification and its use to process biomass/waste together with coal (at different operation

scales and applications) and the technology for CO2 capture/reduction are investigated. The

effects of different fuels, composition and gasification medium on the quality of the producer

gas are also evaluated. Particular focus is on novel technologies for gasification coupled with

CO2 sequestration and for producing hydrogen rich syngas from coal and biomass/waste.

The FLEXGAS project “Near Zero Emission Advanced Fluidised Bed Gasification”, funded

by the Research Fund for Coal and Steel (project No. RFCR-CT-2007-00005) is intended to

contribute to sustainable coal development by improving the potential for the application of

fluidised bed technology to coal gasification and co-gasification with biomass/wastes under

near zero emissions. From the onset the project aimed to achieve the following targets:

- Adapting the concept of oxy-fuel firing for PF combustion to fluidised bed gasification

- Considering the potential of fluidised bed technology to produce H2 rich gas and CH4

- Investigating the co-gasification of coal with biomass and/or waste in fluidised beds and to

consider the impact of scale of operation

- Considering the impact of the above developments on the release of tar, fuel N and S

- Investigating alternative gas cleaning/separation options (e.g. for CO2 sequestration)

- Promoting the exploitation and utilisation of energy source that are available within EU

- Promoting the project findings with potential developers

- Supporting the European and local policy on energy concerns

To fulfil the above, the project was organized in seven work packages that were mutually

interconnected

- Work Package 1 (WP1): Coordination and management

8

- Work Package 2 (WP2): Selection and characterization of materials for co-gasification

- Work Package 3 (WP3): To adapt and test the concept of ‘oxy-fuel firing’ to fluidised bed

gasification

- Work Package 4 (WP4): Fundamental research and system analysis on co-gasification of

coal with biomass/waste in a fluidised

- Work Package 5 (WP5): Experimental investigations and scale up of ICFB gasification

process with coal/biomass mixtures

- Work Package 6 (WP6): Optimisation of gas composition, gas cleaning and shift conversion

- Work Package 7 (WP7): Data collection and dissemination of results.

Workpackage 6 has dealt with different approaches to optimise gas composition, gas cleaning

and upgrading. One of the tasks in WP6 has been the optimisation of shift reactor operating

conditions to maximise the production of hydrogen and catalyst life. CIEMAT was

responsible for that task and the outcome of the study is presented in this report.

9

2 WATER-GAS-SHIFT REACTION: BACKGROUND & STATE OF THE ART

The water gas shift (WGS) reaction is a well-known reaction used in many industrial

processes to produce a hydrogen-rich gas from CO and H2O.

CO + H2O ? CO2 + H2 (? Hº =-41 kJ/mol)

Some traditional examples of processes involving this reaction include coal gasification, H2

production for ammonia synthesis or other industrial processes such as hydro treating of

petroleum stocks. In recent years, production of a hydrogen-rich gas from fossil fuels, waste

and renewable sources such as biomass is attracting interest in power generation processes

especially if coupled to CO2 capture. The approach is the so-called “pre-combustion fuel

decarbonisation”, in which the carbon of the fuel is removed prior to combustion by partial

oxidation or gasification, followed by steam reforming and water-gas shift (WGS) so that a

CO2 and H2-rich gas is produced. Both components are subsequently separated and CO2 in

the exhaust gas is captured using chemical or physical solvents (e.g. amine scrubbing). The

water gas shift (WGS) reaction is, therefore, a key step on production of hydrogen and CO2-

capture-ready streams from gasification.

The WGS reaction is mildly exothermic and conversion of CO is limited by chemical

equilibrium. The reaction is thermodynamically favoured at low temperatures, whereas it is

kinetically favoured at high temperatures. In order for the reaction to proceed with a relevant

conversion at intermediate temperature, a catalyst is often required. Depending on operating

conditions -temperature, pressure and feed composition- different sorts of catalyst may be

needed for the reaction.

According to literature, WGS catalysts can be classified in three main groups:

High temperature sweet catalysts

Iron oxide is the primary catalyst for the high temperature WGS reaction. The catalytically

active phase is magnetite (Fe3O4) that usually comes from the partial oxidation of haematite

(Fe2O3). Pure magnetite catalysts suffer from sintering which reduces rapidly its activity. A

stabiliser, Cr2O3, is commonly added and the combination of Fe3O4 and Cr2O3 gives

commercially stabilised catalysts that can operate for several years before replacement.

Deactivation of the catalysts is, however, a continuous process and many studies are being

addressed to the identification of promoters that can increase catalytic activity over larger

range of temperature. To prolong catalyst life numerous promoters have been studied and

added to catalyst formulation. Other times completely new catalysts have been proposed. A

few are highlighted here:

- Addition of promoters: Copper was the first to be added [1], [2] but there are other

10

metals such as rhodium, platinum, nickel, cobalt, manganese, palladium, [3].

- Formulation of Cr-free catalysts: Replacement of chromium by aluminium,

manganese, cobalt [4]

High temperature acid catalysts

During co-gasification, part of the sulphur contained in the fuels is released in the product

gas, mainly as H2S. Depending on the sulphur content of the feedstock, H2S concentration in

the gas can vary between 50 to several hundreds ppmv. In very extreme cases (such as coal

gasification) values up to 10000 ppmv are reported. Since the current industrial high

temperature catalysts are extremely sensitive to sulphur, the conventional approach to solve

the problem with sulphur poisoning has been its removal before the water gas shift reactor.

However, it is inevitable that trace amounts of H2S remain in the gas stream and cause

catalyst deactivation. In order to overcome that sulphur-resistant have been developed. Some

of the already explored catalysts include those based on transition metal sulphide [5], the use

of Pt/ZrO2 materials [6], [7] or the use of carbides as promising candidates such as molybdenum

carbide [8]. All these efforts show that the development of sulphur-tolerant catalysts for the

water gas shift continues to be a challenge.

Low temperature catalysts

For processes carried out at temperatures below 300 ºC, commercial WGS catalysts are

typically based on a Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3 structure, where zinc oxide and/or

chromium oxide are generally used as structural stabilizers and promoters. For processes

carried out at temperatures below 300 ºC commercial WGS catalysts are typically based on a

Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3 structure, where zinc oxide and/or chromium oxide are

generally used as structural stabilizers and promoters.

R&D efforts deal with the development of advanced low temperature catalysts for CO

conversion, for instance transition metal supported-ceria (CeO2), including platinum,

rhodium, palladium or gold [9-14]. Recent investigations have found that in the temperature

range of 150-450 ºC noble metal catalysts (M= Pt, Rh, Ru, Pd) highly dispersed on TiO2

carriers with small TiO2 crystallite size are promising candidates for use in low-temperature

WGS reactors for fuel cell applications [15]. Research in this field is still on going, trying to

find catalysts that work well in a broader temperature range.

The present practice in industry e.g. for production of high-purity hydrogen for ammonia

synthesis, is to carry out the water gas shift reaction in two steps with two types of catalysts:

first a high temperature WGS reaction (300-400 ºC) that is generally performed over Fe-Cr

catalysts and then, a Cu-Zn catalyst, that is primarily used in the temperature range of 200-

11

300ºC.

Apart from enhancing performance of existing catalysts or developing new ones, state of the

art review shows that current R&D efforts on WGS focus also on promotion of WGS

reaction thermodynamics by adding an excess of steam, displacement of the reaction to the

right side by a continuous removal of the products, as for example combination of the WGS

reaction and CO2 capture by a solid adsorbent [16] or implementation of catalytic membrane

reactors, where the WGS reaction is combined with H2 separation by means of H2 selective

membranes [17] leading to more compact high-temperature gas upgrading and separation

technologies for CO2 capture.

12

3 EXPERIMENTAL

3.1 Test Rig.

CIEMAT has studied the activity of a commercial WGS catalyst at lab-scale in a

Microactivity Pro Unit, in the facility shown in Figure 1. It is an automatic and computerised

laboratory rig for the study of catalytic reactions. The maximum operating gas flow rate is 4.5

Nl/min. The unit can work at up to 700ºC and 30 bar.

Figure 1. Laboratory test rig for WGS studies

The gas mixture is produced synthetically using mass flow controllers (Hi-Tech). Deionized

water is metered by a piston pump (Gilson 307) and vaporized before entering the reactor.

Dry gas and water are preheated separately, in two independent loops. To this aim the entire

set-up is housed in a forced air circulation oven maintained at 190ºC. Dry gas and steam are

mixed before entering the reactor, which is a stainless steel tubular reactor manufactured by

Autoclave Engineers. It has an internal diameter of 9.2 mm, and it is 300mm long. The

reactor is placed in a one single zone SS304 oven, which is able to heat the reactor up to

700ºC. Gas temperature in the reactor is measured by a 1.5 mm thermocouple, directly in the

catalyst bed. The reactor can be passed by and isolated- for instance to analyse the inlet gas

composition- by means of a six-way valve that connects reactor inlet and outlet. For the

WGS tests the reactor was operated in fixed-bed, down-flow mode. Inlet and outlet gas

composition were analysed using a 5890 Series II Hewlett-Packard gas chromatograph

equipped with a thermal conductivity detector (TCD). Two packed 530 µm columns – a 6 Ft

Porapak Q, 80/100 19001A-00 and one 6 Ft Molecular Sieve 5Å 60/80 19001A-MA2-

13

connected in series were used to provide good gas components separation. A specific

software (HP Chemstation A.6.03) was used for controlling the gas chromatograph and to

analyse GC data.

A detailed description of the facility and the chromatographic analysis method is described in

detail elsewhere [18].

3.2 Catalyst

For this project, a commercial high temperature shift catalyst -KATALCOJM 71-5M- was

selected for water gas shift studies. The catalyst was supplied by Johnson Matthey Catalysts.

A picture of the catalyst is shown in Figure 2.

Figure 2 Picture of fresh catalyst

As reported in the technical brochure [19], the main characteristics of the catalyst are summarised in Table 1

Table 1. Catalyst specifications

Trade name KATALCOJM 71-5M Size mini Composition iron/chrome/copper Form pellets Nominal sizes Diameter (mm) 5.4 Height (mm) 3.6 Typical loaded density (kg/m3) 1220

3.2 Test Programme

Optimisation of shift reactor operating conditions to maximise selective conversion of CO to

CO2 and production of hydrogen while ensuring catalyst life has been studied. The effect of

main operating parameters on the activity and selectivity of the catalyst has been determined.

For the study the following range of operating parameters was considered:

14

- Temperature, (T): From 250ºC to 380ºC

- Gas hourly space velocity (GHSV): From 2500 h-1 to 5000 h-1

- Pressure, (P): 1, 10 and 20 bar

- Steam to CO molar ratio (R) between 1 and 3

Table 2. WGS catalytic activity tests: Gas composition (% v/v dry basis)

Component M1 M2 M3 M4

H2 23 23 - - CO 60 60 60 60 CO2 4 17 - 40 N2 13 - 40 -

WGS tests were performed using different gas compositions as shown in Table 2. M1

composition was set according to the gas composition expected for a process based on

entrained flow oxygen gasification. M2 is the composition expected for oxygen-carbon

dioxide gasification in fluidised bed, which is the main approach in the FLEXGAS projects.

As can be seen M2 is similar to M1 but it contains more CO2 what makes it possible to look

into the effect of this component on the activity of the catalyst. For comparison purposes, M3

and M4 consist of binary mixtures of CO in N2 and CO2 respectively.

3.3 Experimental Procedure.

In a typical run, the test procedure followed is described below.

The reactor is filled with the amount of catalyst set for the specific run, and water flow-rate is

entered in the pump control unit according to the steam to carbon monoxide ratio to get. Due

to the presence of toxic and flammable gases, the system is checked to ensure that there are

not gas leaks. Then, N2 is passed through the reactor until the desired test temperature and

pressure are reached. This is the so-called preheating stage.

When stationary conditions have been attained in the reactor, the pump is switched on and

the gas valve is opened to let the gas mixture flow in.

Feed gas composition shall be determined in each test in order to calculate the CO conversion

accurately. This is usually done by passing the reactor by and running the analytical method

only for the feed dry gas. Stationary conditions should be reached before the analytical

15

method is run. Three or four analyses are run to provide an exact, correct gas measurement.

Once the feed gas has been analysed, gas is led through the reactor and the reaction proceeds.

A slipstream of the gas leaving the reactor is diverted to the gas chromatograph and analysed.

For each temperature, usually five to seven measurements of gas composition at the reactor

outlet were taken under steady state conditions what means that the catalytic performance of

the catalyst at every temperature was evaluated for 60 to 90 minutes. The procedure is

repeated for three or four different values of temperature, within the range of interest so that

gas composition profile is obtained as a function of temperature. Four to six temperatures

were investigated for every single test, and thus the total duration of each catalytic test was

240-540 minutes.

The activity of the WGS catalyst is expressed as the percentage of CO converted (% mol)

XCO calculated according to (1),

( ) ( )

( ) 100(%) ×−

=inCO

outCOinCOCO F

FFx (1)

where (FCO)in, and (FCO)out are the molar flow rate of carbon monoxide at the reactor inlet

and outlet, respectively. This way of expressing the catalytic activity of a catalyst is widely

used. Performance of the catalyst is also established in terms of hydrogen production.

16

4 RESULTS AND DISCUSSION

Experimental measurements show that the catalyst starts to be active around 280-300ºC,

depending on operating conditions and it does not require any activation or becomes active

on-stream.

300 320 340 360 3800

10

20

30

40

50

60

70

80

90

100

CO

Con

vers

ion

(% m

ol)

Temperature (ºC)

20bar

Figure 3. Effect of temperature on CO conversion (SV=4715 h-1, P= 20 bar, Gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %, R=3)

As Figure 3 shows, in which catalytic activity at 20 bar was determined, CO conversion

increases with increasing temperature and reaches a maximum between 350ºC and 380ºC. In

all tests, despite the high CO content in the feed gas -60% v/v dry basis- the catalyst has

shown very good performance at intermediate temperatures 320ºC-380ºC rising hydrogen

content in syngas typically 25-27 percentage points, that is from 23%v/v (d.b.) at the reactor

inlet to around 48-50%v/v at the reactor outlet and reducing CO concentration at the reactor

outlet to less than 3% v/v.

Regarding the influence of space velocity, higher GHSV values lead to lower CO conversion

and a higher temperature is required to achieve a given CO conversion as shown in Figure 4.

Conversely, lower space velocity results in higher CO conversion at lower temperature.

WGS reaction is in principle not affected by the reaction pressure in a traditional packed bed

reactor, because the overall number of moles does not change. Experimental results show that

in the high temperature bracket, pressure certainly does not have a significant influence on

CO conversion, as Figure 5 shows. Some differences were observed, however, at low

pressure but this might be attributed to changes in gas linear velocity rather than to due to

pressure.

17

240 260 280 300 320 340 360 380 400 420-10

0

10

20

30

40

50

60

70

80

90

100

CO

con

vers

ion

(% m

ol)

Temperature (ºC)

CO conversion for different GHSV values. Steam/CO=2

SV= 2885 h-1; R= 2

SV= 4715 h-1; R= 2

Equilibrium R=2

Figure 4. Effect of space velocity on CO conversion Gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %

240 260 280 300 320 340 360 380 400 4200

20

40

60

80

100

Space velocity SV= 4715 h-1; Steam to CO ratio H2O/CO=3

10bar 20bar 1bar

CO

con

vers

ion

(% m

ol)

Temperature (ºC)

CO conversion versus pressure

Figure 5. Effect of pressure on CO conversion Gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %

The effect of steam to carbon monoxide ratio was also evaluated. According to the

stoichiometry of the water gas shift reaction, at least a steam to CO ratio of 1 is required to

convert all CO into CO2. It is known that the WGS reaction is thermodynamically

unfavourable at elevated temperatures and that running the reaction at stoichiometric steam

to CO ratio can promote undesirable secondary reactions, such as Boudouard reaction or

methane formation. In order to prevent secondary reactions and to drive the reaction

18

thermodynamically to the products side achieving high conversion, excess steam is often

used, though the energy efficiency of the process is consequently lower. In this study steam

to CO ratio was varied from 1 to 3.

240 260 280 300 320 340 360 380 4000

10

20

30

40

50

60

70

80

90

100

Gas space velocity SV = 2885 h-1

CO

con

vers

ion

(% m

ol)

Temperature (ºC)

Effect of excess steam on CO conversion

R= 1

R=2

Equilib R=2

Figure 6. Effect of steam to CO ratio on CO conversion Gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %, P=10 bar , SV=2885 h-1

240 260 280 300 320 340 360 380 4000

20

40

60

80

100

Space velocity 4715 h-1 P= 10 bar

CO

con

vers

ion

(% m

ol)

Temperature (ºC)

R= 2 R= 3

Effect of excess steam (R= H2O/CO).

Figure7. Effect of steam to CO ratio on CO conversion Gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %, P=10 bar, SV= 4715 h-1

At low space velocity a noticeable enhancement of catalytic activity is gained on doubling

steam to CO ratio as Figure 6 shows. Secondary reactions such as methane formation was not

19

found to be taking place, even when steam to CO ratio was set at the stoichiometric value

(R=1). However at higher space velocities (SV> 4715 h-1) and low steam to CO ratio, R=2,

carbon built on the surface of the catalyst and methane formation was measured by GC, so

that a steam to CO ratio above 2 is required. A further increase of the steam to CO ratio did

not result in significantly higher CO conversion as observed in Figure 7.

As shown in Figure 8, increasing the content of CO2 in the feed gas does not have a

significant influence on CO conversion above 350ºC. However, below that temperature,

where thermodynamics controls the reaction, the presence of higher content of CO2 in the

feed gas -CO2 is a product of the WGS reaction- resulted in a decrease in the conversion of

CO

300 320 340 360 380

40

50

60

70

80

90

100

SV= 4715 h-1, R H2O/CO=3, P= atmospheric

xCO

(% m

ol/m

ol)

Temperature (ºC)

CO 60%; N2 40%

CO 60%; H2 23%; CO

2 4%; N

2 13%

CO 60%; H2 23%; CO

2 17%

CO 60%; CO2 40%

Influence of feed gas composition

Figure8. Effect of gas composition and CO2 content on CO conversion SV=4715 h-1, R=3, P=10 bar

Production of hydrogen under different WGS operating conditions is depicted in Figure 9.

Starting from a gas containing around 23% H2 v/v (d.b.) it is possible to produce a H2-rich

gas of approximately 50%v/v (d.b.) by optimising shift reactor conditions. Gas leaving the

shift reactor has high hydrogen and carbon dioxide content and also contains excess steam

not consumed by the reaction and a low amount of unconverted CO, as well as inert gas

components such as N2.

20

240 260 280 300 320 340 360 380 4000

10

20

30

40

50

60

70

80

90

100

Hyd

roge

n co

nten

t WG

S re

acto

r out

let (

% v

/v d

.b.)

Temperature (ºC)

SV=2885 h-1 R=2

SV=4715 h-1 R=2

SV= 2885 h-1 R=1

H2 production under different operating conditions

Figure 9. Hydrogen content in the outlet gas for different shift reactor operating conditions (Inlet gas composition (%v/v): N2 13%, CO 60%, H2, 23%, and CO2, 4 %, P=10 bar)

21

5 CONCLUSIONS

The activity for the water gas shift reaction of an iron-chromium-based catalyst has been

studied on laboratory-scale. The experimental study was conducted under realistic conditions

typical of oxy-gasification. The effect of temperature, space velocity, steam to carbon

monoxide ratio, and gas composition on the performance of the catalyst has been evaluated.

The catalyst shows very good performance at intermediate temperature, 350-400ºC, having

potential for producing a gas rich in hydrogen and carbon dioxide. Despite the high CO

content in the feed gas, by choosing the right shift reactor conditions, CO concentration at the

reactor outlet reached values below 3%, whereas H2 increased up to above 50% v/v (dry

basis). Increasing space velocity leads to lower CO conversion and a higher temperature is

required to achieve a given CO conversion. The activity of the catalyst depends strongly on

the amount of steam available during reaction. In order for the reaction to proceed

significantly an excess of steam is needed. A higher H2O/CO ratio gives higher carbon

monoxide conversion, and with better selectivity to production of hydrogen. Pressure does

not affect much the activity of the catalyst nor does gas composition, particularly CO2

content.

22

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