DOI: 10.3303/CET2292110

Paper Received: 3 December 2021; Revised: 28 March 2022; Accepted: 27 April 2022
Please cite this article as: Tacconi A., Savuto E., Di Giuliano A., Di Carlo A., 2022, Laboratory-scale Study of Nickel-catalyst Pellets
Performance for Tar Steam Reforming Obtained from Biomass Gasification, Chemical Engineering Transactions, 92, 655-660
DOI:10.3303/CET2292110

CHEMICAL ENGINEERING TRANSACTIONS

VOL. 92, 2022

A publication of

The Italian Association
of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti

ISBN 978-88-95608-90-7; ISSN 2283-9216

Laboratory-Scale Study of Nickel-Catalyst Pellets

Performance for Tar Steam Reforming Obtained from

Biomass Gasification

Alessandra Tacconi*, Elisa Savuto, Andrea Di Giuliano, Andrea Di Carlo

Department of Industrial and Information Engineering and Economics-University of L’Aquila, Piazzale E. Pontieri 1,

Monteluco di Roio, 67100 L’Aquila

alessandra.tacconi@graduate.univaq.it

Biomass steam gasification is a promising solution to obtain an H2-rich syngas to be exploited in high

temperature SOFC to produce electricity with high efficiency and a reduced environmental impact. The aim of

the H2020 BLAZE project is to integrate a dual fluidized bed biomass gasifier with a 25 kWe SOFC.

Tars, in any case, have been identified as one of the major impurities in biomass gasification fuel gas, causing

degradation of the anodes of the SOFC due to carbon deposition. Hot gas cleaning and conditioning using

ceramic filter candles, filled with an annular packed-bed of commercial nickel-based catalyst, has been identified

as the best solution to convert tars and reduce them to few hundreds of mg Nm-3.

This work reports the results obtained in one task of the BLAZE project, in which two different commercial

catalysts supplied by Johnson Matthey (catalyst A and catalyst B) have been tested at micro-reactor scale to

evaluate their activity for the steam reforming of tar with the presence of sulfur compounds, which act as catalyst

deactivators, and at temperatures comparable with those obtainable in biomass steam gasification. The main

objective of this work was to evaluate which catalyst could be used inside the filter candles in the BLAZE power

plant and which are the best operating conditions to have a conversion of tar as required for the SOFC. Twelve

tests were conducted, six for each catalyst, varying both the temperature (750, 800 and 850 °C) and the

concentration of thiophene in the pseudo-tar solution fed, a compound used to simulate the sulfur compounds

deactivators of the catalyst (one with 50 ppm of thiophene, the other with 100 ppm). The results show that the

hydrocarbons conversion increases (reaching 100%) with increasing temperature and decreasing thiophene

concentration. Furthermore, catalyst A showed better performance than B.

1. Introduction

Gasification of biomass is a very promising process to produce energy from agricultural and woody waste

materials, but the main pollutants produced during the process, tar, and particulate, have to be removed from

the product syngas in order to make it exploitable.

The H2020 BLAZE project (https://cordis.europa.eu/project/id/815284) aims at feeding a SOFC with the syngas

produced by biomass steam gasification in an innovative dual fluidized bed reactor (Di Carlo et al., 2019) for

this reason, the syngas has to undergo several steps of cleaning and conditioning in order to remove the

compounds that could damage the SOFC or compromise its operation (Ouweltjes, 2019). Furthermore, the

presence of tar among the products of gasification reduces gas yield and conversion efficiency. Therefore, the

gas cleaning units play an important role, in order to reach the limits imposed by the downstream units.

Catalytic filter candles inserted directly in the freeboard of the gasifier are an innovative solution for a first step

of hot gas cleaning and conditioning. These components can remove the particulate thanks to their porous

filtering structure, and then their internal cavity can be used to place commercial catalyst pellets in order to

perform catalytic tar steam reforming directly in the freeboard of the gasifier (Heidenreich and Foscolo, 2015).

One of the aspects that must be considered during the catalytic steam reforming of tar is the presence of sulfur

compounds in the gas (H2S, COS) that easily deactivate the catalyst. Ma et al. (2005) found that the sulfur

deactivation of Ni is surely reversible up to H2S concentration of 200 ppmv at process conditions similar to those

655

mailto:alessandra.tacconi@graduate.univaq.it

expected in BLAZE project, as only physical adsorption of H2S on Ni catalytic sites occurs. Depner and Jess

(1999) investigated the tar steam reforming on a commercial Ni-catalyst and determined the upper limit of

reversible H2S deactivation at 0.1 vol% of H2S; for higher concentrations, Ni-sulphide formation was reported.

Along the lines of these findings, in this work it is assumed to deal with a reversible deactivation of the

commercial Ni-catalyst.

The aim of the work was to investigate experimentally the best operating conditions (temperature, space

velocity, sulfur content) and the most suitable commercial catalyst, to obtain a tar concentration outcoming the

gas cleaning and conditioning in line with the limits required for the safe operation of the SOFC; this threshold

values, investigated in D3.2 (Ouweltjes, 2019), are toluene < 250 mg Nm-3 and naphthalene < 25 mg Nm-3.

2. Materials and Methods

In this work two commercial catalysts have been tested (catalyst A and catalyst B), using the same micro-reactor

test rig shown in the work of Di Giuliano et al. (2021). As shown by XRF analyses the catalysts have about 8%

of Ni that is recognized as an efficient metal for the steam reforming of hydrocarbons at high temperature. The

XRD analysis showed that the support of the two catalyst is a calcium aluminate compound. The use of a similar

support is chosen to reduce the effect of carbon deposition and sulfur poisoning (Li, Hirabayashi, & Suzuki,

2009). The main difference between the two catalysts is the shape and the presence on the catalyst B of small

amount of potassium that should further increase the resistance to carbon deposition.

The micro-reactor is a vertical stainless-steel pipe with an internal diameter of 1.6 cm, 0.5 m long, heated by a

cylindrical electrical furnace. The catalyst bed (few grams of catalyst) is placed at middle height of the micro-

reactor, in the central part of the furnace. The temperature control is carried out using a thermocouple with its

tip located inside the catalyst bed. The operating temperature for the tests was varied between 750 °C and 850

°C: this range, in fact, is of interest for the in-situ syngas cleaning by filtering-catalytic candles. Two stainless-

steel pressure syringes, driven by electric engines KDS LEGATO 110, are used to pump water and a synthetic

solution of tar key-compounds into a vaporization chamber at 220 °C. These syringes control the volumetric

flow of water and of the synthetic tar compounds solution. The density of the pumped synthetic tar solution has

been determined by a pycnometer in order to compile mass balances for each test. This solution was made up

by toluene and naphthalene. The toluene/naphthalene molar ratio was 3.7, that is close to the naphthalene

solubility in toluene at ambient temperature; this ensured that the synthetic tar was fed liquid, without any solid

precipitate that could clog the syringe pump. Small amount of thiophene (0.02 or 0.04 molar fraction) was also

added to simulate the presence of sulfur compounds.

More details about the test rig can be found in the work of Di Giuliano et. al. (2021). Thiophene was added to

evaluate the deactivation of Ni-catalyst due to sulfur species in concentrations of 50 and 100 ppm. In addition,

the toluene/naphthalene ratio utilized in this work is close to that found in the product gas of steam biomass

gasification tests, before any catalytic treatment (Rapagnà et al., 2018).

N2 was used as a carrier gas, in order to convey vaporized fluids to the reactor. N2 was selected to simulate the

flow rate of the actual syngas, in order to allow the specific quantification of tar conversion due to catalytic steam

reforming by carbon balance, being an inert gas.

The flow rates for liquids and gases, were chosen to make the content of steam, heavy hydrocarbons and sulfur

species comparable with those of the raw syngas obtained in the biomass gasification tests of Savuto et al.

(2019) and to have contact times between inlet gas stream and catalytic bed as expected in the catalytic filter

candles. The concentration of tar key-compounds was equal to 13 g Nm-3dry. The amount of water injected was

that necessary to obtain a molar concentration of steam in the gas equal to 25%.

The chosen flow rates allowed to have the necessary thiophene in order to develop 50 or 100 ppmv of equivalent

H2S in the inlet stream; the complete conversion of thiophene into H2S was assumed (1:1 atomic ratio of S

between thiophene and H2S). This is ensured by the high excess of steam and the reductive environment inside

the packed-bed rig.

A glass double-pipe condenser is equipped downstream the microreactor to separate the unreacted water and

the condensable hydrocarbons from the product stream. Ethylene glycol at 0 °C was used as the cooling fluid.

A BRONKHORST mass flow meter is used to measure the overall molar flow rate (Ftot,out) of the dried outlet

stream. Finally the gas volumetric percentage (yi,out) of CO, CO2, CH4 and H2 of this outlet stream was analyzed

by an ABB online system. This on-line analyser is equipped with an ADVANCE OPTIMA URAS 14 module for

CO, CO2, CH4 (non-dispersive infrared detector) and an ADVANCE OPTIMA CALDOS 17 module for H2

(thermal conductibility detector). The values of Ftot,out and yi,out were sampled with a period of 5 s. From the

knowledge of Ftot,out and yi,out during time it was possible to calculate the outlet molar flow rates (Fi,out, Eq. (1))

and percentages on dry, dilution-free basis (Yi,out, Eq. (2)) of CO, CO2, CH4 and H2.

656

𝐹𝑖,𝑜𝑢𝑡 =
𝑦𝑖,𝑜𝑢𝑡
100

𝐹𝑡𝑜𝑡,𝑜𝑢𝑡 ; 𝑖 = 𝐶𝐻4, 𝐻2, 𝐶𝑂, 𝐶𝑂2 (1)

𝑌𝑖,𝑜𝑢𝑡 =
𝐹𝑖,𝑜𝑢𝑡

∑ 𝐹𝑗,𝑜𝑢𝑡𝑗
100 ; 𝑖 𝑜𝑟 𝑗 = 𝐶𝐻4, 𝐻2, 𝐶𝑂, 𝐶𝑂2 (2)

From the knowledge of gas carbon species flowrate, the tar conversion was finally obtained using Eq. (3):

𝜒𝐶𝑡𝑎𝑟,𝑜𝑢𝑡 =
∫ (𝐹𝐶𝑂,𝑜𝑢𝑡 + 𝐹𝐶𝑂2,𝑜𝑢𝑡) 𝑑𝑡𝜏

∫ 𝐹𝐶𝑡𝑎𝑟,𝑖𝑛 𝑑𝑡𝜏

(3)

Where FCtar,in is the molar flowrate of the inlet carbons entering with tar.

The catalytic pellets were pre-reduced before any experiment, in order to obtain Ni0, that is the actual catalytic

active phase for reforming. A heating ramp at 10 °C min-1 was set from room temperature up to 900 °C, followed

by a 30 min dwell at 900 °C;150 Nml min-1 of a mixture of H2 and N2 (10 vol% of H2 in N2) was used as reducing

stream through the packed-bed.

Each experiment lasted long enough to observe the stabilization of products formation, in order to have an

adequate amount of data in steady-state conditions and be sure to observe the sulfur deactivation. Test

durations were comparable to those of the gasification tests with the catalytic filter candle carried out by Savuto

et al. (2019).

3. Results and Discussion

The following table summarizes the operating conditions and the conversions obtained in the tests of tar steam

reforming performed with the bench scale packed bed rig. The tests conditions, for both catalysts, are the same,

only the temperatures and the concentration of thiophene change, since the goal is to study how much their

variation can affect the tar conversion.

Table 1: Operating conditions and test results for catalyst A.

TEST Catalyst A 1 2 3 4 5 6

Pressure P [atm] 1 1 1 1 1 1

Temperature T [°C] 850 800 750 850 800 750

GHSV [h-1] 4500 4500 4500 4500 4500 4500

Toluene/Naphthalene [g/g] 70/30 70/30 70/30 70/30 70/30 70/30

Thiophene (H2S equiv) [ppm] 50 50 50 100 100 100

Inlet tar concentration Ctar [g Nm-3dry] 13 13 13 13 13 13

Inlet steam/carbon ratio αin [mol/mol] 19.3 19.3 19.3 19.3 19.3 19.3

CH to COx conversion χCtar, out [%] ∼100 92.5 79.1 ∼100 60.2 52.9

Table 2: Operating conditions and test results for catalyst B.

TEST Catalyst B 7 8 9 10 11 12

Pressure P [atm] 1 1 1 1 1 1

Temperature T [°C] 850 800 750 850 800 750

GHSV [h-1] 4500 4500 4500 4500 4500 4500

Toluene/Naphthalene [g/g] 70/30 70/30 70/30 70/30 70/30 70/30

Thiophene (H2S equiv) [ppm] 50 50 50 100 100 100

Inlet tar concentration Ctar [g Nm-3dry] 13 13 13 13 13 13

Inlet steam/carbon ratio αin [mol/mol] 19.3 19.3 19.3 19.3 19.3 19.3

CH to COx conversion χCtar, out [%] 96.0 64.6 36.2 86.4 57.3 32.0

As the following graphs show, it is evident that catalyst A is more efficient than catalyst B. The conversions

reported are those relating to the final and stable part of the tests.

657

Figure 1: Inlet carbon conversion as a function of temperature for two different thiophene concentrations using

catalyst A.

Figure 2: Inlet carbon conversion as a function of temperature for two different thiophene concentrations using

catalyst B.

Furthermore, higher temperatures and lower amounts of thiophene favor the tar conversion process. As a matter

of fact, a greater quantity of sulfur inhibits in any case the conversion of the inlet hydrocarbons, sooner

deactivating the catalyst: in the worst condition studied, namely at 750 °C (the decrease in temperature

thermodynamically and kinetically disfavors reforming), conversions of 79% and 53% were obtained with

catalyst A, respectively with 50 ppm and 100 ppm of thiophene in the feed solution. On the other hand, 36%

and 32% are obtained with catalyst B. To have a tar conversion close to 100%, the operating temperature must

be close to 850 °C, but only for catalyst A in the presence of both equivalent H2S concentrations. In the previous

work by Di Giuliano et al. (2021), the behavior of the same catalyst A was evaluated at different operating

conditions varying the temperature between 700 and 800 °C, with a GHSV between (10000-12000 h-1) and with

658

an equivalent concentration of H2S between 40 and 100 ppm. In that case the space velocity was 2-3 times

higher than in this work (4500 h-1) and the tar conversion was for this reason lower. Such higher space velocities

were chosen because the main purpose was to derive a lumped kinetics, referred to a pseudo-component

representing tar. In this work the goal was instead to evaluate the best operating conditions to have a tar

conversion close to 100% for two different catalysts (the same catalyst A and another catalyst B, always supplied

by Johnson Matthey) with a GHSV equal to 4500 h-1: such lower space velocity is that expected in a filter candle

as shown in the work of Savuto et. al. (2019). The results obtained in this work are in line with those obtained

in the work of Rapagnà et al. (2018) where catalytic and non-catalytic filter candles were tested in a bench scale

biomass fluidized bed gasifier for different bed inventories (olivine or olivine/dolomite) with air and/or steam as

the gasification media. The comparison of the gasification results with non-catalytic and catalytic filter candles,

using steam as gasification media, demonstrated that at temperatures higher than 800 °C the conversion of tar

varied between 91 to 96 %. These results are comparable with the results obtained in this work with catalyst A,

even though the concentration of H2S in the raw syngas obtained by Rapagnà et al. (2018) was not measured.

To further highlight the deactivating effect of sulfur, the following graph of the conversion as a function of time

is shown.

Figure 3: Inlet carbon conversion as a function of time using catalyst A at 800°C in presence of 100 ppm of H2S.

This is the tar conversion of the test at 800 °C with 100 ppm of sulfur using catalyst A. As is evident, the

conversion at the beginning is equal to approximately 96%, during the test it decreases until it reaches a plateau

of about 60%.

BET and XRF analyses were carried out on the two catalysts before and after tests. The BET analyses showed

that the surface area of the two catalysts were for both around 15 m2/g. The same analysis carried out for the

spent catalysts showed that the surface area of catalyst A was around 11 m2/g, while that for catalyst B was

around 8 m2/g, showing a greater sintering effect for this last catalyst. This could explain the worst performance

of catalyst B. XRF confirmed for both catalysts the presence of S that was evidently absorbed on Ni sites.

4. Conclusions

This work was dealt with studying of tar steam reforming process, in order to obtain, more generally, a syngas

with a higher added value. To do this it is necessary to use catalysts. In this case, two different commercial

nickel-based catalysts were studied and compared, using a laboratory micro-reactor. The main aim of the

experiments was to assess their behavior with varying temperature and sulfur content, which acts as a catalyst

deactivator. Specifically, this research, carried out as part of the European project BLAZE, has as its final

objective the insertion of this catalyst inside the filter candles in the gasification plant on a pilot scale. Particular

attention must be paid to identifying the best operating conditions to have a high tar conversion to make the

syngas usable in its subsequent processing, in this specific case for SOFC.

Twelve different tests were performed, at three different temperatures (750, 800, 850 °C) and with two different

pseudo-tar solutions, containing the same amounts of toluene and naphthalene, but different concentrations of

the sulfur deactivating element. The results showed that the tar conversion process is favored at higher

659

temperatures and in the presence of lower thiophene quantities. In addition, catalyst A was found to perform

better than catalyst B.

Nomenclature

BLAZE – Biomass Low cost Advanced Zero

Emission

BET – Brunauer-Emmett-Teller surface area

analysis

XRF – X-ray fluorescence

SOFC – Solid Oxide Fuel Cell

GHSV – Gas Hourly Space Velocity, h-1

Ci – molar concentration of gas species i, mol m-3

F – molar flowrate of species i, mol h-1

P – pressure, atm

T – temperature, °C

yi – molar fraction of species i, mol mol-1

Yi – molar percentage dry, dilution-free of gaseous

species I, vol%

α – molar steam to carbon ratio, mol mol-1

χi – conversion of species i

in – inlet

out – outlet

Acknowledgments

The authors would like to acknowledge Johnson Matthey to have supplied the catalysts for the tests.

The authors also kindly acknowledge the financial support of R&D Project BLAZE G.A. 815284 funded by the

European Union’s Horizon 2020 research and innovation program.

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Laboratory-Scale Study of Nickel-Catalyst Pellets Performance for Tar Steam Reforming Obtained from Biomass Gasification