Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification

Fuel 89 (2010) 1784–1795

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Review article

Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification Chunbao (Charles) Xu a,*, Jaclyn Donald a,b, Enkhsaruul Byambajav b, Yasuo Ohtsuka b,* a b

Department of Chemical Engineering, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1 Research Center for Sustainable Materials Engineering, IMRAM, Tohoku University, Sendai, Japan

a r t i c l e

i n f o

Article history: Received 7 August 2009 Received in revised form 7 February 2010 Accepted 8 February 2010 Available online 18 February 2010 Keywords: Biomass gasification Hot-gas cleanup Tar Ammonia Catalysts

a b s t r a c t Biomass gasification produces a low to medium-BTU product gas (or syngas) containing primarily CO2, H2, CO, CH4 and (C2 + C3), as well as some contaminants such as tars, NH3, H2S and SO2. In order to achieve better efficiencies of the syngas applications, these contaminants must be removed before the syngas is used for internal combustion, gas engines, and in particular for fuel cells and methanol synthesis. Compared with the wet scrubbing technology, hot-gas cleanup technology to remove tar, ammonia and other contaminants at the ‘‘hot” state is more advantageous with respect to energy efficiencies. This paper provides an overview on recent advances in catalysts for hot-gas removal of tar and ammonia from biomass gasification. The review focuses on the recent development and applications of dolomite catalysts, ironbased catalysts, nickel and other metal supported catalysts, and the novel carbon-supported catalysts for hot-gas tar removal and ammonia decomposition. The barriers in applications of hot-gas cleanup processes and catalysts for full-scale biomass gasification, and areas for future research, are also discussed. Ó 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysts for hot-gas tar removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Dolomite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Iron-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Nickel and other metal supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Carbon-supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysts for hot-gas ammonia decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Dolomite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Iron-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Nickel and other metal supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Carbon-supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barriers to full-scale application of hot-gas cleanup for biomass gasification and areas for future research . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction In biomass gasification, the biomass fuel is partially oxidized/ gasified in an oxidizing atmosphere of air, oxygen and/or steam * Corresponding authors. Tel.: +1 807 343 8761; fax: +1 807 343 8928 (C. (Charles) Xu). E-mail addresses: [email protected] (C. (Charles) Xu), [email protected]. ac.jp (Y. Ohtsuka). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.02.014

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to form a low to medium-BTU product gas (also called producer gas or syngas) containing primarily CO2, H2, CO, CH4 and (C2 + C3), which can be used for heat and electricity generation, or for synthesis of liquid fuels and methanol [1–5]. In oxygenor steam-blown gasifiers, lower heating values (HHV) of 10– 14 MJ/Nm3 can be attained for the product gas, compared with only 4–8 MJ/Nm3 in a typical air-blown process [6,7], as shown in Table 1.

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C. (Charles) Xu et al. / Fuel 89 (2010) 1784–1795 Table 1 Typical gas composition and yields from gasification of pine wood chips in a bubbling fluidized bed with different gasifying agents (summarized from Gil et al. [6]). Gasifying agent

Air Steam Steam + O2 a b

T (°C)

780–830 750–780 785–830

Gas composition (vol.%, dba)

Yields b

H2

CO

CO2

CH4

C2Hn

N2

H2O

5–16 38–56 14–32

10–22 17–32 43–52

9–19 13–17 14–36

2–6 7–12 6–8

0–3 2 3–4

42–62 0 0

11–34 52–60 38–61

Tars (g/kg)

LHV (MJ/Nm3)

4–62 60–95 2–46

4–8 12–14 10–14

On a dry basis. Moisture content in the producer gas.

In addition to syngas, biomass gasification also produces some contaminants in the product gas, such as tars, NH3, H2S, HCl and SO2 [1–5]. These contaminants must be removed before the syngas is used for internal combustion, gas engines, and in particular for fuel cells and methanol synthesis, in order to achieve better efficiencies in syngas applications. Tar, representing all organics with a molecular weight greater than that of benzene [8], is highly undesirable because of the detrimental problems associated with condensation of tar in the process equipment, pipe lines as well as in end use applications such as engines and turbines. The allowable limits for tar in the producer gas depend on the process and end use applications. Milne and Evans [9] reported tar tolerance limits for various end use devices: for example, less than 50– 500 mg/Nm3, 50–100 mg/Nm3 and 5 mg/Nm3 is recommended for compressors, internal combustion systems, and direct-fired industrial gas turbines, respectively. For methanol synthesis, the contents of tar and ammonia are required to be <0.1 mg/Nm3, and <10 ppm, respectively [2,4,5]. The content of NH3 in the product gas, typically 1000–5000 ppm [10], is dependant on the type of biomass used, as well as the gasifier parameters and operating conditions. Most of the nitrogen that is contained within biomass or coal would end up mainly as NH3, N2, and some HCN, HNCO, and NOx during the processes of gasification and pyrolysis [11–13]. As much as 60–80% of the nitrogen in biomass would convert to NH3 during the gasification process [14], and as much as 50–90% of the NH3 would form NO, through a complex radical mechanism illustrated in Fig. 1, in gas turbines when the gas is combusted to produce power [15]. A large number of studies were reported on

Fig. 1. Formation of NOx from NH3 in combustion (modified from Miller et al. [15]).

the formation of tar and ammonia during gasification. It has been generally shown that increasing the air to fuel ratio decreased the ammonia and tar in the product gas [6,16,17], a high temperature decreased the formation of tar but a longer residence time could increase ammonia formation [10]. The tar, ammonia and other contaminants (e.g., HCl and H2S) in the product gas can be cleaned by the wet scrubbing technology, widely adopted in the existing biomass gasification processes. Compared with the wet scrubbing technology, hot-gas cleanup technology, preferably employing catalysts, to remove these contaminants at the ‘‘hot” state is more advantageous with respect to energy efficiencies as it eliminates the needs of cooling the product gas and re-heating again for the syngas applications (internal combustion, gas engines, fuel cells and methanol synthesis, etc.). Very recently, Ohtsuka et al. [18] overviewed some recent progress in Japan on the hot-gas cleanup of HCI, H2S and NH3 in raw fuel gas for coal-based, combined cycle power generation technologies. The light hydrocarbons and tars formed during the gasification process could be removed through steam/dry reforming or cracking/hydro-cracking reactions as shown below [19]:

 m Cn Hm þ nH2 O $ nCO þ n þ H2 2 m Dry reforming : Cn Hm þ nCO2 $ 2nCO þ H2 2 m H2 Cracking : Cn Hm $ nC þ 2   4n  m Hydro-cracking : Cn Hm þ H2 $ nCH4 2 Steam reforming :

ð1Þ ð2Þ ð3Þ ð4Þ

Without the use of catalyst, steam reforming of tars occur appreciably at >900 °C (due to the endothermic nature of these reactions and a high activation energy >250–350 kJ/mol). Steam and dry reforming reactions with the use catalysts have shown to be a promising way to remove the tar components from the gasification gas at a lower temperature [20]. Dolomite and VIII metals such as Ni, Fe, and Co, were the most common catalysts for the tar removing reactions [21]. For instance, the use of NiMo/Al2O3 and dolomite could significantly reduce the activation energy of the steam reforming reaction of toluene from >250–350 kJ/mol to 56–123 kJ/mol at 650–850 °C [22]. The decomposition of benzene at 900 °C through steam reforming without a catalyst led to only 2% conversion, while the addition of a dolomite catalyst increased the conversion significantly, reaching 40% at 950 °C and 80% at 1000 °C [23]. In addition, dolomite catalysts were found to be active for tar cracking. The ammonia in the producer gas can be reduced by hot-gas cleanup either through catalytic decomposition or selective oxidation, as shown in the following reactions (Eqs. (6) and (7)) [24]. For some applications such as hydrogen fuel cells, complete decomposition of ammonia into H2 and N2 is desirable. In catalytic decomposition of ammonia, dehydrogenation occurs forming some reaction intermediate or N and H on the surface of a catalyst, which subsequently form N2 and H2 through surface reactions on the catalyst. Other gas components (CO, CO2, H2, etc.) present in the bio-

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syngas may have a negative effect on the decomposition of ammonia, as they would compete for the adsorption and active sites on the catalyst, and they could also react with NH3 to form HCN [24]

2NH3 ! 3H2 þ N2

ð5Þ

4NH3 þ 3O2 ! 2N2 þ 6H2 O

ð6Þ

The ammonia decomposition reaction (to form N2 and H2) over an iron-based catalyst was found to be first-order with respect to the partial pressure of ammonia whose rate expression was shown as follows [25]:

r ¼ k0 expðEa =R=TÞpNH3

ð7Þ

where Ea is the activation energy of ammonia decomposition. The value of Ea was found to be 96 kJ/mol for the Fe catalyst with potassium as a promoter, and 87 kJ/mol for the catalyst without potassium. Chellappa et al. [26] investigated ammonia decomposition using Ni–Pt/Al2O3 catalyst, and the reaction proved to be first-order reaction too, with the activation energy of about 200 kJ/mol. From the above discussion, it is clear that development of effective and stable catalysts for tar and NH3 removal are extremely critical for the development of viable hot-gas cleanup technologies. This is because the reactions for steam/dry reforming or cracking/ hydro-cracking of tars and ammonia decomposition are highly endothermic and require high activation energies in the absence of a suitable catalyst. As such, this review will focus on the recent developments and applications of catalysts for hot-gas removal of tar and ammonia from biomass gasification. Table 2 Characteristics of the common model tar compounds [9,22]. Compound

Characteristics

Benzene

A stable aromatic structure in tars formed with hightemperature processes A stable aromatic structure apparent in tars formed with hightemperature processes. With toluene, catalyst deactivation due to charring can be less severe and the hydrocarbon conversion to gases is much higher in comparison with real tar The major tar compound from processes at temperatures lower than 800 °C The major single compound in tars. The thermal reactivity follows the order of toluene > naphthalene > benzene

Toluene

Phenol Naphthalene

2. Catalysts for hot-gas tar removal The biomass-derived tars consist of a wide range of condensable hydrocarbon and oxygen containing compounds, which are mostly aromatics and complex poly-aromatic hydrocarbons (PAHs) [19]. The complex composition of biomass tars makes it difficult to understand the reaction mechanisms. Therefore, most studies used model tar compounds such as benzene, toluene, phenol, and naphthalene in the research. The properties of these model compounds are shown in Table 2. It was observed that in the fluidized-bed airblown gasification of peat, the major component is benzene, representing 50–75% of the total tar, followed by toluene and naphthalene [27]. Naphthalene is a popular model tar compound for most research on tar decomposition, since it is likely the most stable and difficult to decompose among all the compounds in the biomassderived tars. There has been extensive research on catalysts that have the potential for decomposing tars generated in biomass gasification. Alkali metal catalysts have been studied as primary catalysts to enhance the gasification reactions in the gasifier, but have been found to be less active for carbon conversion and hard to recover. Materials such as dolomite, calcium–magnesium carbonates and Ni-based catalysts have been used widely as the secondary catalysts to minimize tar in gasification product gas [28]. Some typical catalysts and their performance for decomposition/reforming of biomass tar and model compounds are summarized in Tables 3 and 4. The mechanism of tar cracking (assuming toluene as the model compound) may be illustrated as follows in Fig. 2 [16,29– 33]. The tar molecule absorbs on the surface of the catalyst, and forms radicals or other intermediate compounds (active or inert species). CO2, O2 and H2O in the fuel gas absorb on the surface of catalyst, and dissociate into CO (desorbing into the gas phase) and free radicals of O, OH and H, followed by desorption of the free radicals. Reactions of free radicals form CH4, CO2 and H2 as well as a small amount of benzene [34].

2.1. Dolomite catalysts Several studies using calcined dolomites (MgO–CaO), obtained by the decomposition of dolomite CaMg(CO3)2 at high temperatures from 800 to 900 °C, as the secondary catalysts have shown great success for minimizing tar in the product gas [28]. The in-

Table 3 Summary of some typical catalysts used for decomposition/reforming of biomass tar. Catalysts

Chemical composition of the catalyst

GHSV (h1)

Temperature (°C)

Feed gas composition

Tar conversion (%)

Reference

Various dolomites

(18–21) wt.% MgO–(30–32) wt.% CaO–(0–3) wt.% SiO2–(0–0.8) wt.% Fe2O3–(0–1) wt.% Al2O3

10,600– 12,000 11,000– 12,000 13,000– 16,000 12,000

794–850

Gas from air-blown biomass gasifier

84–91

[16]

Chinese dolomite Chinese dolomite modified with Fe2O3 ICI-46–1a a

Z4091

Ni/dolomite Metallic iron Fe Fe2O3 a

20 wt.% MgO–31 wt.% CaO–0.7 wt.% SiO2–0.5 wt.% Al2O3 Dolomite with 5% Fe2O3 24 wt.% NiO–13 wt.% MgO–13 wt.% CaO–14 wt.% SiO2–29 wt.% Al2O3 5 wt.% Fe2O3–22 wt.% NiO–11 wt.% MgO–13 wt.% CaO–11 wt.% SiO2–23 wt.% Al2O3 n.a. n.a. n.a.

Ni-supported commercial steam reforming catalyst.

814–923

94–97

805–810

71–92

650–850

N2 bubbled through melted biomass tar

43–95

[29]

44–97 87–99 82–98

n.a.

600–800

Gas from fluidized-bed gasification Swedish birch

84–98 10–90 6–22

[30]

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C. (Charles) Xu et al. / Fuel 89 (2010) 1784–1795 Table 4 Summary of some typical catalysts used for decomposition/reforming tar model compounds. Catalysts

Chemical composition of the catalyst

GHSV (h1)

Temperature (°C)

Feed gas composition

Tar conversion (%)

Reference

Fe2O3/SiO2 North Carolina Olivine Calcined Austrian Olivine Austrian Olivine

55 wt.% SiO2–45 wt.% Fe2O3 6 wt.% Fe2O3–31 wt.% MgO– 19 wt.% SiO2 7 wt.% Fe2O3–28 wt.% MgO– 21 wt.% SiO2 7 wt.% Fe2O3–28 wt.% MgO– 21 wt.% SiO2 6 wt.% Fe2O3–29 wt.% MgO– 18 wt.% SiO2 57 wt.% Fe2O3–2 wt.% SiO2– 1 wt.% Al2O3 2 wt.% Ni–7 wt.% Mo–1.4 wt.%P 90 wt.% FeOOH–4.2 wt.% SiO2– 5.8 wt.% Al2O3

90,000 1166

650 800–900

300 ppm C6H6, 10% H2, N2 balance 16% H2, 8% CO, 12.0% CO2, 16% H2O, 4% CH4, balance N2, 400 ppm C10H8

100 75–98

[31] [32]

Washington Olivine Australian limonite NiO–MoO2/Al2O3 South American limonite

32–92 90–100 60–97 0.5 LHSV 8200

440–460 450–460 650

80 wt.% –vacuum residue–20 wt.% decant oil

1400 ppm C6H6, 9.6% H2, 13% CO, 9.5% CO2, 2.5% CH4, 15% H2O, He balance

54–83

[33]

64–77 95

[35]

material and its pore diameter [43]. In another study, powdered dolomite was modified by mixing with Fe2O3 powders to increase its Fe2O3 content, and it was observed that the addition of Fe2O3 powders led to a slight increase in tar conversion at 850 °C [29]. Although calcinated dolomite catalysts have shown to be very active for decomposition of phenols and oxygenated compounds, typically formed in steam gasifiers, the dolomite catalysts were less effective for removal of PAHs, formed in air-blown gasification [44,45]. Another problem with using calcined dolomite is that it can be easily eroded as it is a soft and fragile material, limiting its use in some types of reactors such as in a fluidized-bed reactor [19]. 2.2. Iron-based catalysts

Fig. 2. Mechanism of tar (e.g., toluene) cracking over a catalyst (e.g., Ni supported on Al2O3 or carbon).

creased surface area and oxide contents on the surface make it more active than the un-calcined dolomite for tar decomposition [36,37]. Corella et al. [38] studied calcined dolomites as both primary and secondary catalysts, and they observed that calcined dolomites can be equally effective as either a primary or secondary catalyst. Simell et al. [39,40] studied tar removal using secondary catalysts containing CaO and MgO, and found that the catalytic activities were ranked in the following sequence: CaO > CaO– MgO > MgO > NiMo/c–Al2O3, suggesting that the presence of CaO in dolomite might be responsible for its activity in tar conversion. Using calcined dolomites as secondary catalysts could increase the H2 content and the H2/CO ratio in the product gas owing to cracking and reforming of the tars [23,41]. Hu et al. [42] compared a calcined dolomite (by calcination at 900 °C) with an un-calcined dolomite as well as a calcined olivine and raw olivine (a mineral containing magnesium, iron and silicon) as the secondary catalysts for apricot stone gasification [42], where it was found that among all the catalysts tested the calcined dolomite was the most effective catalyst for increasing the H2 content in the gas. For tar conversion, however, it was demonstrated that the activity of dolomite increased with increasing Fe2O3 content in the

Olivine is a natural mineral containing magnesium, iron and silicon in the form of either Fe2SiO4 or Mg2SiO4. In a study by Kuhn et al. [32], olivine catalysts of different origins were comparatively tested for tar decomposition using naphthalene and toluene as tar models. With a simulated gas of 400 ppm C10H8 diluted in 16%H2– 8%CO–12% CO2–16% H2O–4% CH4–N2 (balance), an Austrian olivine showed the greatest activity of 100% tar conversion at 900 °C with a GHSV = 1166 h1. Calcination of olivine (at 1600 °C) could deactivate the catalyst: the tar conversion at 800 °C with the calcinated Austrian olivine decreased from 90% to 32%. In another study by Devi et al. [19], however, it was found that calcined olivine was more active than un-calcined olivine. The calcinations were carried out at 900 °C in the presence of air, and with various lengths of time ranging from 1 to 10 h. They found that increasing the calcinations time from 1 to 10 h increased the activity for naphthalene decomposition from 62% to 81%, respectively, compared with a naphthalene conversion of only 47% with the un-calcined olivine. Using the XPS technique, it was found that the content of Fe (III) on the surface of the catalysts increased with increasing calcination time. In the study by Hu et al. [42], discussed earlier, it was also found that calcined olivine increased the gas yield from 37% to 47%, and therefore was more active for tar cracking than the un-calcined olivine. The above contradictory results on the effects of calcinations observed from different studies might be due to the different calcinations conditions. For example, the calcination temperature in the study by Kuhn et al. [32] was very high (1600 °C), compared with the temperature (900 °C) used by Devi et al. [19] and Hu et al. [42]. Hence, it might be concluded that high calcination temperatures for olivine is not desirable for tar decomposition. Limonite is a naturally occurring iron ore with a high iron content that has been used in tar decomposition studies. Limonite is an attractive alternative to the commercial Ni catalysts, as it is less

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expensive and abundant resource, and the spent limonite catalyst waste can be used directly for iron and steelmaking [46]. In the study by Li et al. [46], Indonesian limonite was used for the decomposition of coal volatiles from a coal carbonization process. The limonite ore was composed mainly of goethite (FeOOH) with an iron content of 41.4%. They studied the effects of reduction temperatures on the catalyst, and found that at temperatures above 400 °C FeOOH was reduced to Fe3O4, and metallic iron was dominant at temperatures above 600 °C. For the tar decomposition experiments, the samples were reduced at temperatures above 650 °C to obtain samples with mainly metallic iron. The tests were carried out in a two stage reactor with hydro-pyrolysis of coal occurring in the first stage and the catalyst bed in the second stage using a 50 vol.% H2 gas and a temperature of 750 °C. The Indonesian limonite showed a high conversion of tar resulting in only 0.3 wt.% of the carbon remaining in the tar and the liquid fractions after the catalytic reactor [46]. The effects of the catalyst bed temperature on the decomposition of coal-derived volatile were also investigated. Without a catalyst placed in the second-stage reactor, the yields of CH4 and C2–C4 gases increased with increasing temperature as expected, while in the presence of limonite the yield of CH4 decreased with increasing temperature, accompanied with an increase in CO yield. This result might imply that limonite catalyzed the steam reforming reactions in the second-stage reactor [46]. Matsumura et al. [33] tested a slurry catalyst containing 30 wt.% Yandi Yellow Australian limonite with an Fe content of 57 wt.% for the decomposition of asphaltene from a vacuum residue, and compared it with a conventional NiO–MoO2/Al2O3 (NiMo) catalyst. The asphaltene conversions obtained with the limonite slurry were slightly higher than with the NiMo catalyst. At a LHSV of 0.5 h1, the use of the Australian limonite led to an asphaltene conversion of 83% at 460 °C, which was higher than that with the NiMo catalyst, being about 77% at the same temperature [33]. In another study by the same authors, Brazilian limonite, with an iron content of 46 wt.%, was examined for asphaltene decomposition, and its activity was also higher than the NiMo catalyst, while the activity was lower than that of the Australian limonite [47]. Besides olivine and limonite, other iron containing catalysts that have been studied for tar reduction include iron oxides (FeO, Fe2O4, Fe3O4, and Fe2O3), ankerite (CaFe(CO3)2 as a ferrous dolomite), sintered iron ore, and pelletized iron ore. Nordgreen et al. [31] gasified Swedish birch in a fluidized bed, where pre-reduced hematite (Fe2O3) was used in a downstream reactor and almost 100% tar decomposition was achieved at 900 °C. In contrast, other iron oxides, e.g., FeO, Fe2O4, and Fe3O4 had very little catalytic activity towards tar decomposition. Leppalahti et al. [48] compared the effects of ferrous dolomite and a sintered iron ore, and found that the sintered iron ore had a very low activity for tar conversion, although the sintered iron ore had a larger content of Fe (59.2 wt.%) than the Fe-dolomite (4.6 wt.%). In this study, the Fe-dolomite was calcined at 900 °C. A possible reason why the Fe-dolomite showed a higher activity than the sintered iron ore could be that the dolomite had a higher calcium content of 19.2 wt.% compared with 5.4 wt.% for the sintered iron ore, while CaO in dolomite was believed to be a highly active component in tar conversion [39,40]. 2.3. Nickel and other metal supported catalysts Nickel-based catalysts, used extensively in the petrochemical industry, have shown high activities for tar reforming/decomposition in coal and biomass gasification. When used as the secondary catalysts, the supported nickel catalysts could attain nearly complete decomposition of both tar and ammonia decomposition [49,50]. As primary catalysts, however, Ni catalysts were less effective for tar conversion due to the severe coke formation, leading to deactivation of the catalysts [28].

Simell and Bredenberg [51] studied the effectiveness of 11 wt.% Ni/Al2O3 and several other catalysts including dolomite, activated alumina, silica–alumina, and silica carbide for tar decomposition. It was found the Ni/Al2O3 almost completely decomposed the tar and light hydrocarbons at 900 °C. Sutton et al. [52] compared several Ni-supported catalysts, prepared either by wet-impregnation or co-precipitation, for tar reduction from peat pyrolysis. A variety of supports including Al2O3, ZrO2, TiO2, SiO2 were used with a nickel loading of 5 wt.%. For the co-precipitated catalysts, a nickel to aluminum ratio of 1:6 and 1:3 were used. Among all the wetimpregnated catalysts, the Ni/TiO2 catalyst was the most active catalyst, attaining a tar conversion of 98.1% at 800 °C, followed by Ni/ZrO2 with a 95.2% conversion. Under the same reaction conditions, the co-precipitated catalysts were slightly less effective, obtaining about 92% tar conversion. In another study by the same authors [53], the supports were tested without the addition of Ni, and the tar conversion over the supports alone were significantly lower than with the addition of Ni, for example the TiO2 and ZrO2 alone obtained a conversion of only 79.5% and 78.4%, respectively, at 800 °C [53]. The addition of nickel to dolomite and olivine has been found to significantly improve the activity towards tar conversion [32,54,55]. The olivine alone showed almost no activity for steam reforming of toluene at 750 °C whereas the Ni/olivine had a toluene conversion of 85%. In addition, the Ni/olivine showed good selectivity to H2, CO, and CO2 [53]. For conversion of naphthalene, the addition of Ni to calcinated olivine remarkably enhanced the conversion, almost double that of the olivine alone [19,56,57]. The addition of Ni to dolomite drastically increased the tar conversion at 650 °C from 43% to 84% [29]. Dou et al. [50] studied tar cracking catalysts including aluminasupported NiMo, Y-zeolite, alumina, lime and silica, using 1-methylnaphthalene as the tar model component, and found that the NiMo/Al2O3 (4 wt.% NiO, 14.3 wt.% MoO3, 82.7 wt.% Al2O3) and Yzeolite (34.1 wt.% Al2O3, 44.5 wt.% SiO2, 14.5 wt.% CaO, 6.9 wt.% Na2O) were the most effective catalysts, decomposing almost 100% tar at temperatures above 500 °C. The use of the aluminasupported NiMo catalyst initially led to a high conversion of nearly 100% but it dropped to less than 80% after 9 h on-stream, due to catalyst deactivation. Silica showed much less activity for tar reduction compared to the other catalysts. The long-term durability of alumina-supported catalysts remains the biggest challenge in application of this type of catalyst. 2.4. Carbon-supported catalysts Activated carbon is a non-graphitic, microcrystalline form of carbon. Its structure is more disordered than that of graphite, and contains crystallites only a few layers thick and less than 10 nm wide [58]. The porous areas between the crystallites constitute the large surface area of the activated carbon. The porosity can vary from micropore (<2 nm), to mesopores (2–50 nm) to macropore (>50 nm). These pores contribute to the large internal surface areas and pore volumes of activated carbons and can range from 250 to 2500 m2/g and 0.1 to 1.0 ml/g, respectively [59,60]. Due to their highly porous textural structures, activated carbons or charcoals (derived from biomass or coal) have also been widely used as catalyst supports for conversions of hydrocarbons and tar cracking, not only because their macropores and mesopores would greatly improve the dispersion of metal ions, but facilitate transport of reactant molecules (e.g., toluene, 0.68 nm molecular size) into the internal surfaces of the catalyst, as illustrated in Fig. 3. It has been demonstrated that activated carbon has a high affinity and adsorption selectivity to hydrocarbon compounds such as asphaltenes [61]. Particularly the mesopores of the activated carbon was thought to play an important role for effective conversion

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vated carbon derived from coconut shells by Takaoka et al. [64], H2O2 and HNO3 were used as an oxidizing agent to modify the AC for decomposition of pentachlorobenzene. The catalysts were prepared by mixing the AC with oxidizing agents for either 3 or 24 h. It was observed that by oxidizing the AC, a greater number of pores were formed with larger pore diameters. In the tar decomposition tests, the temperatures varied from 300 to 400 °C, and GHSV maintained at 400 h1 for 30 min. The HNO3-treated AC samples (for 24 h) had high conversions of pentachlorobenzene at 98% in air and 86% in N2 at 300 °C. At 400 °C all catalyst samples including the untreated AC showed 100% conversion of pentachlorobenzene. 3. Catalysts for hot-gas ammonia decomposition

Fig. 3. Roles of micropores/mesopores/micropores of a carbon-supported catalyst in tar cracking.

of heavy hydrocarbon compounds into lighter fractions, while restricting the carbon/coke formation [60,61]. In addition, carbon-supported catalysts, due to their neutral or weak base properties, were found to be superior to the solid acid catalysts with respect to the resistance to deactivation by deposition of coke and heavy metals [60,61]. Abu El-Rub et al. [62] compared the activity of a biomass char to other catalysts that are commonly used for tar decomposition, including calcined dolomite (21 wt.% MgO, 30 wt.% CaO, 0.2 wt.% Fe2O3), olivine (50 wt.% MgO, 42 wt.% SiO2, and 7 wt.% Fe2O3), and a nickel catalyst (70 wt.% NiO, 12 wt.% Al2O3 and 7 wt.% SiO2). The biomass char was produced by pyrolysis of pinewood at 500 °C. Phenol (8–13 g/ N m3) and naphthalene (40 or 90 g/ N m3) were used as tar models and the tests were carried out in the presence of CO2 (6 vol.%), H2O (10 vol.%) and N2 (balance) at 700 and 900 °C. At 900 °C all the catalysts showed high activities, and phenol was completely converted. At 700 °C the Ni catalyst had the highest phenol conversion, and the biomass char was less active, but still more effective than the olivine. The naphthalene conversion tests were carried out at a temperature of 900 °C. The biomass char achieved nearly complete conversion of 90 g/Nm3 naphthalene, whereas the dolomite obtained only 55 wt.% conversion of 40 g/Nm3 naphthalene. Being slightly inferior to the Ni catalyst, the biomass char was found to be more active for naphthalene conversions than the other catalysts tested (olivine and dolomite) [62]. AC derived from coconut shells was used in a study by Lu et al. [63] for toluene decomposition. The AC was used as a support for three copper precursors, i.e., copper nitrate, copper acetate and copper sulfate, and the effects of Cu loadings and reaction temperatures were examined. The tests were carried out using 200 ppm toluene in a N2 gas containing 10% O2, and at a space velocity of 40,000 h1. The copper nitrate precursor produced a catalyst with the highest toluene conversion at lower temperatures. The effect of the Cu loading on the toluene decomposition was investigated using the AC the copper nitrate and copper acetate precursors at 270 °C and a space velocity of 158,720 h1. The copper loadings were varied from 1 to 5 wt.%. An increase in Cu loadings for the Cu (nitrate)/AC led to an increase in its activity, approaching to 73% at 5 wt.% Cu loading, but for the Cu (acetate)/AC catalysts, its activity peaked at copper loading of 3%. A higher loading of Cu on the AC-supported catalysts might cause agglomeration of the copper particles, leading to large particles on the surface of the AC, therefore decreasing its activity. In another study using acti-

The activation energy required for ammonia decomposition is high, and the typical temperatures in fluidized-bed gasifiers, ranging from 800–900 °C, is not effective for ammonia decomposition without the presence of a suitable catalyst [11,27]. A catalyst is required to reduce and even eliminate ammonia completely from the product gas at a relatively low temperature, and secondary catalytic processes are most commonly used. Many of the same catalysts used for tar decomposition, such as dolomite catalysts [23] have also been used to minimize the ammonia in the syngas by decomposing it into N2 and H2. The most common catalysts used in NH3 decomposition studies are metals of Ni [40,65–68,74], Ru [65,66,69–73,75,76] and Fe [25,40,65,78–81], as summarized in Table 5. 3.1. Dolomite catalysts In previous studies, it was found that ammonia could partially decompose on CaO, MgO and on dolomites at temperatures of about 800 °C in inert gas atmospheres, but the presence of other gas species such as CO, CO2, N2 and H2 would interfere with the reaction [23]. It was also found that calcined dolomite was more active than CaO or MgO alone in inert helium gas, but with the presence of other gases the decomposition of NH3 was hindered [11,27,77]. Compared to other catalysts (e.g., ferrous materials and nickel catalysts), dolomite was less effective for ammonia decomposition. When being used as a primary catalyst in biomass gasification, dolomite, as well as limestone, actually increased the ammonia content of the gas product at temperatures below 1000 °C. Corella et al. [45] also observed that the addition of dolomite as a primary catalyst to the gasifier, for the purpose of tar decomposition, increased the amount of ammonia of the product gas. It was suggested that this might be because dolomite has a high activity for tar cracking, thus releasing the tar-bound nitrogen to ammonia. Therefore, although ammonia can decompose over dolomite, the presence of other competing components, such as tar, will actually decrease its activity towards ammonia decomposition [45]. 3.2. Iron-based catalysts Very few studies have been done using olivine for the purpose of ammonia decomposition, as olivine was found to be less active. In contrast, high conversions of NH3 to N2 were achieved using an Australian limonite with a high content of a-FeOOH in a study by Tsubouchi et al. [78]. In an NH3 gas stream diluted with high purity helium, almost complete conversion of ammonia (P99%) was obtained at a temperature of 500 °C and a space velocity of 45,000 h1 with an Australian limonite. The limonite catalyst was reduced with H2 at 500 °C prior to the ammonia decomposition experiments. In a later study by the same authors [79], the effi-

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Table 5 Summary of some typical catalysts used for NH3 decomposition. Catalysts

Chemical composition of the catalyst

GHSV (h1)

Temperature (°C)

Feed gas composition

Ammonia conversion (%)

Reference

Ru/Al2O3 Ru/SiO2 Ru/TiO2 Ru/MgO Ni on Ce0.9La0.1O2 Ru–Ni/Al2O3

10 wt.% Ru 10 wt.% Ru 4.8 wt.% Ru 2.8 wt.% Ru 10 wt.% Ni (2–5) wt.% Ru–(2–5) wt.% Ni

30,000

450 400 400 400 750 900

Pure NH3

32 35 27 41 97 90

[71]

Ru/SiO2 Ni/SiO2 Ni/Fumed SiO2 Ru/Fumed SiO2 Ni/Al2O3 Ni–Ce/Al2O3 146 (Johnson Matthey) Ni monolith (Ni/A12O3) Iron containing dolomite Australian limonite Australian limonite

10 wt.% Ru 10 wt.% Ni 5 wt.% Ni 5 wt.% Ru 1.2 wt.% Ni 1.2 wt.%-1% Ce 0.5% Ru/Al2O3

30,000

12,000

500–700

Pure NH3

64 10 93 97 38 72 7–84

[66]

30,000

500 500 700 550 500

n.a.

2500

900

4400 ppm NH3, in 11% CO, 14% CO2, 5% CH4, 10% H2, 12% H2O, 0–500 ppm H2S, 3200 ppm toluene, N2 balance

100

[40]

2000 ppm 2000 ppm balance 2000 ppm 2000 ppm

NH3 in He NH3 in 20% CO, 10% H2, 3% H2O, He

99 90

[78] [79]

NH3 in 100 ppm H2S, He balance NH3 in He

99 100

[80] [81]

Australian limonite Coal-charsupported Fe

30,000 30,000 100,000 20,000

n.a. 90 wt.% a-FeOOH 90 wt.% a-FeOOH

45,000 45,000

500 750

90 wt.% a-FeOOH 2–6 wt.% Fe

45,000 45,000

750 650

ciency of the Australian limonite was tested for ammonia decomposition in an inert gas and a simulated gas typical of an air-blown gasification process, containing CO, H2, CO2, and H2O. In the presence of fuel gas (20%CO/10%H2) the ammonia conversion was found to decrease significantly at lower temperatures, accompanied with a higher conversion to HCN. At higher temperatures (>750 °C), however, there was no deactivation of the limonite by the presence of CO and H2. Also, it was found that the addition of 10% CO2 or 3% H2O, helped to restore the ammonia conversion to 90% at 750 °C, and by increasing the temperature further the conversion approached 100% [79]. Limonite also showed a higher activity than other reference Fe oxides (Hematite (a- Fe2O3) and magnetite (Fe3O4)) for ammonia conversion [70]. More interestingly, the catalyst of Australian limonite was found to be resistant to H2S, according to another study by Tsubouchi et al. [80], where the limonite showed a stable activity of almost complete conversion of 2000 ppm NH3 in the presence of 100 ppm H2S (He balance) for 50 h on stream at 750 °C. Leppälahti et al. [48] studied several inexpensive catalysts including iron sinter, iron pellet, ferrous dolomite, dolomite and limestone, inert silicon carbide and a commercial nickel catalyst

Pure NH3 Pure NH3 500 ppm NH3 in H2, CO, CO2, CH4 1040 ppm NH3 in 10.5% H2, 28.4% CO, 3.6% CO2, 3.1% H2O, N2 balance Pure NH3

15% NH3–85% He

[72] [73] [74] [75]

[67] [76]

53

as the reference, for decomposition of ammonia in the gas product from peat gasification. The research demonstrated that the ferrous materials and the commercial nickel were more effective than dolomite and limestone for decomposing ammonia. Ohtsuka et al. [81] were able to obtain complete conversion of 2000 ppm NH3 by using an Fe and Ca supported on brown coal chars at 750 °C and a space velocity of 45,000 h1. They found that the 2– 6 wt.% Fe catalysts were actually more effective than using an 8 wt.% Fe catalyst loaded on commercial activated carbon, owing to the high dispersion of the iron particles on the coal char catalysts. Cycle mechanisms involving the formation of N-containing intermediate species and the subsequent decomposition to N2 were proposed for the Fe and Ca catalysts in NH3 decomposition, as depicted in Fig. 4 [81]. In a study by Xu et al. [82], pyrolysis chars from low rank coals (brown and sub-bituminous coals) were tested as catalysts for the conversion of NH3 to N2. It was found that the inherently present Fe and Ca minerals in the coal chars were responsible for their activities towards ammonia decomposition. More recently, the group of Ohtsuka and co-workers [83] discovered for the first time that the presence of phosphorous in the carbon-supported Fe cat-

Fig. 4. Cycle mechanisms for the Fe and Ca catalysts in NH3 decomposition.

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alyst could greatly promote catalytic decomposition of ammonia into N2 and H2. The catalyst was prepared using a meso-porous activated carbon (AC) support derived from a Canadian peat by H3PO4 activation. The newly developed AC-supported 13 wt.% Fe catalyst contained a significant amount of phosphorous (at a P/C molar ratio of 0.02 on the surface). The conversion of 2000 ppm NH3 diluted in helium over the phosphorous-promoted AC-supported Fe catalyst reached as high as 90% at 750 °C and a space velocity of 45,000 h1, compared with less than 30% for an Fe catalyst supported on a commercial activated carbon at the same conditions reported previously by Ohtsuka et al. [81]. In addition, the new phosphorous-promoted AC-supported Fe catalyst showed superior performance with respect to its resistance to catalyst deactivation. The following cycle mechanism, involving both metal phosphides and metal nitrides, was proposed for the NH3 decomposition reactions over the P-promoted Fe catalyst [83]

2Fe þ P ! Fe2 P

ð8Þ

6Fe þ 3P þ 2NH3 ðgÞ ! 3Fe2 P þ N2 ðgÞ þ 3H2 ðgÞ

ð9Þ

Fe2 P þ 0:5NH3 ðgÞ þ 0:75H2 ! 0:5Fe4 N þ PH3 ðgÞ

ð10Þ

PH3 ðgÞ þ 2Fe ! Fe2 P þ 1:5H2 ðgÞ

ð11Þ

Fe4 N ! 4Fe þ 0:5N2 ðgÞ

ð12Þ

4Fe þ NH3 ðgÞ ! Fe4 N þ 1:5H2 ðgÞ

ð13Þ

3.3. Nickel and other metal supported catalysts Mojtahedi and Abbasian [84] studied the activity of Ni/Al2O3 catalysts with different nickel loadings of 2, 7.5 and 15 wt.% in a gas mixture containing CO, CO2, CH4, H2, H2O, N2 and varying NH3. The ammonia conversion was found to be strongly dependent on the Ni content of the catalyst, and the largest ammonia conversion was obtained with the 15 wt.% Ni/Al2O3. Nassos et al. [74] studied the effect of Ni loading on various supports with varying space velocities. Ni loadings of 5 and 10 wt.% were used on the supports Ce0.9La0.1O2, Ce0.9Zr0.1O2, and Al2O3. For the Ni supported on Ce0.9La0.1O2, and Ce0.9Zr0.1O2, they found that the ammonia decomposition efficiency was directly related to the Ni loading, increasing with increased Ni content. It was also found that the catalytic activities differed greatly with Ni loadings at lower temperatures of 500–600 °C, while this difference was minimized at higher temperatures. More interestingly, there was a remarkable effect of catalyst support on the catalyst’s activity for ammonia decomposition. For instance, the 10 wt.% Ni loaded on the Ce0.9La0.1O2 and Ce0.9Zr0.1O2 supports achieved ammonia conversion of 97% and 94%, respectively, at 750 °C, compared with only 32% conversion with the Al2O3-supported 10% Ni at the same temperature. Simell et al. [39] were able to convert 80% of ammonia using a nickel monolith catalyst (NiMo/c–Al2O3) and completely decompose tar at temperatures greater than 900 °C. In a study by Wang et al. [85], nickel catalysts were used to reduce both tar and ammonia in the product gas from a pilot-scale fluidized-bed gasifier, where 95% of the ammonia removal was obtained with the Ni catalysts at 874 °C at a GHSV of 1200 h1. In addition, some commercial Ni-based catalysts, such as G43-A (United Catalyst), 2800 Raney Nickel (Grace Davison), 146 (Johnson Matthey) were tested for pure ammonia decomposition for the PEM fuel cell application, and they showed good activities (ammonia conversions of >80%) at temperature above 600–700 °C, but they were less effective (ammonia conversion <30%) at <600 °C [76]. However, a Ni–Pt/ Al2O3 catalyst exhibited a very high activity for pure ammonia decomposition, leading to almost compete conversion of NH3 (97.4%) at 560 °C (at 28.44 g cat. h/g mol NH3) [76]. In the same study by Mojtahedi and Abbasian [84], a comparison was made between Ru/Al2O3 and Ni/Al2O3 catalysts. The Ru/

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Al2O3 catalyst was much more effective than the Ni catalyst. For example, the Ru/Al2O3 catalyst obtained ammonia conversions of 95.3%, compared with 90.4% for the Ni catalyst at 850 °C. When toluene was added to the gas feed, the Ni and Ru catalysts were slightly deactivated towards ammonia decomposition. In another study by Li et al. [66], 10 wt.% Ru supported on SiO2 was also found to be much more effective than the 10 wt.% Ni/SiO2 catalyst. In a 50 ml/min flow of pure ammonia at a GHSV of 30,000 h1, the Ru/SiO2 catalyst achieved ammonia conversion of 64%, compared to only 10% conversion with the Ni/SiO2 catalyst. Ru catalysts at a loading of 3–10 wt.% supported on Al2O3 [71], SiO2[71], TiO2 [72] and MgO [73] have been widely tested for pure ammonia decompositions for fuel cell applications, and these Ru catalysts attained 30–40% conversions at a low temperature (400 °C). Moreover, Al2O3-supported Ru or Ru–Ni catalysts (6–9 wt.% total metal loading) have shown to be effective for decomposition of ammonia 1000 ppm in a simulated coal-derived syngas (10 vol.% H2, 28 vol.% CO, 54 vol.% N2, 3.6 vol.% CO2 and 3 vol.% H2O), leading to 90% ammonia conversion at 900 °C and a GHSV of 20,000 h1 [75]. However, the major problem for these Al2O3-supported Ni or Ru catalysts for hot-gas removal of ammonia or tar is associated with the deactivation by fouling of the catalyst due to the carbon deposition and by H2S [86,87]. The problem would be serious for tarry fuel gases from gasification and pyrolysis of coal or biomass solid fuels, containing contaminants of tar, CO, CH4 and H2S. To prevent the carbon deposition and hence the catalyst deactivation, effective strategies include introduction of steam in the product gas so as to gasify the accumulated coke/carbon [20], and using non-alumina (MgO or CeO2) as catalyst supports or promoters [88]. It was reported that, a Rh/CeO2/SiO2 catalyst was able to maintain its activity for tar decomposition even in the presence of a high concentration of H2S (180 ppmv) in the gas stream [89]. 3.4. Carbon-supported catalysts Activated carbons (ACs), produced physically and chemically from various carbonaceous materials, have larger surface areas and higher porosities, which make them good adsorbents for removal of some specific contaminants from gas/liquid streams. Commercial ACs alone and the commercial AC-supported metal catalysts were found to be much less active towards NH3 decomposition compared to other catalysts such as dolomite, olivine, ironbased and Ni-based catalysts, as well as chars derived from coals [81,82]. In a study by Fortier et al. [90], commercial ACs derived from coconut shells with and without impregnation of ZnCl2 were tested to adsorb a variety of vapours including ammonia, cyclohexane, nitrogen and water. The research showed that the AC alone had a very low adsorption capacity for ammonia, 0.011 ± 0.002 mmol NH3/g AC, and the capacity was improved by impregnation of ZnCl2 up to a loading of 3.5 mmol ZnCl2/g AC. The low ammonia adsorption capacity of the commercial AC towards ammonia might be a key factor responsible for its low activity for hot-gas ammonia decomposition. Although AC and ACsupported catalysts are less active for ammonia and tar decomposition than those supported on other materials such as Al2O3, SiO2, TiO2, MgO, and CNTs, more research effort is worthy to be made on development effective AC-supported catalysts because activated carbon materials have high surface areas and relatively low costs. In another study by Ohtsuka et al. [81], a coal-char-supported Fe catalysts was found to be very effective for ammonia decomposition: an iron content of 6 wt.% on an Australian brown coal char attained a conversion of nearly complete conversion of 2000 ppm ammonia (diluted in inert gas) at 750 °C and a GHSV of 45,000 h1. It was further found that the inherently present Fe and Ca minerals in the coal chars were also highly active towards

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ammonia decomposition [82]. As discussed before, the group of Ohtsuka and Xu [83] developed a new carbon-supported Fe catalyst promoted by phosphorous – the newly developed P-promoted 13 wt.% Fe/AC catalyst obtained 90% conversion of 2000 ppm ammonia in helium at 750 °C and a space velocity of 45,000 h1, compared with less than 30% for an Fe catalyst supported on a commercial activated carbon under the same conditions [81]. Yin et al. [91,92] demonstrated that the use of carbon nanotubes (CNTs) as a catalyst support for Ru in ammonia decomposition studies. The catalytic performance of Ru catalysts was strongly dependent on support materials: under similar reaction conditions, NH3 conversion decreased in the order of Ru/ The CNTs > Ru/MgO > Ru/TiO2 ffi Ru/Al2O3 ffi Ru/ZrO2 > Ru/AC. excellent catalytic performance of Ru/CNTs was believed to be related to the high dispersion of Ru on the CNTs. Moreover, Yin et al. [65,91] proposed that the conductivity of the support might also be an important factor for catalytic activity. A conductive support is beneficial for the transfer of electrons from promoter and/or support to Ru, which would facilitate desorption of surface N atoms to form N2. It was further demonstrated by Yin et al. [65,90] that a support of high acidity is unsuitable for NH3 decomposition. Accordingly, CNTs (of high conductivity due to the graphitization of carbon atoms) combined with a basic support (MgO) may lead to enhanced activities for supported Ru catalysts. As a matter of fact, this has been evidenced by another study by Yin et al. [92], where the Ru/MgO–CNTs catalyst with an equal weight of MgO and CNTs exhibited a catalytic activity higher than that of Ru/ MgO or Ru/CNTs.

4. Barriers to full-scale application of hot-gas cleanup for biomass gasification and areas for future research Due to the increasing interest in alternate fuels (e.g., F–T diesel and methanol) and integrated combined cycle (IGCC) for heat/ power generation, the worldwide gasification capacity in synthesis gas generation has climbed rapidly in the recent years. The syngas production was projected to increase from 40,000 MWth syngas in 2000 to 70,000 MWth syngas in 2006 [93]. The major commercialized gasifiers include (a) the Lurgi gasifier – the moving bed gasifier introducing coarse solids at the top of the gasifier while the oxidant and steam are introduced at the bottom of the gasifier, (b) the air/steam-blown Kellog Rust Westinghouse gasifier – the fluidized-bed gasifier introducing dried feed at the bottom of the gasifier, and (c) the Texaco and Shell gasifiers – the entrained bed gasifier introducing finely ground solid feed in the form of water slurry or dry powers from the gasifier top. However, industrial-scale gasification is so far limited to the synthesis gas production from coal. To date, there is no biomass gasification facility operating on a full-scale, although there has been significant global interest in building demonstration plants (in a capacity of 8– 12 MWe electricity output) for biomass gasification since 1990s. Typical demonstration plants were mainly built in Europe, such as ARable Biomass Renewable Energy (ARBRE) plant (8 MWe) in UK, Energy Farm (EF) project (12 MWe) and Chianti plant (7.7 MWe) in Italy, and a newly developed steam biomass gasification plant in Güssing, Austria. However, most of these demonstration projects were ended or cancelled due to the unfavourable economic situation and the difficulty in obtaining a sufficient amount of reasonably priced biomass, as well as the operating difficulty associated with gas cleaning [94]. The major obstacles for industrial application of biomass gasification are related to the scattered distribution of the biomass resources and the inferior properties of biomass compared with coal (e.g., a lower bulk density and a lower ash melting point, potentially causing slagging/ corrosion problems in a boiler). More importantly, the high tar for-

mation may be the key technical barrier to the large-scale implementation of biomass gasification technology, as it demands very costly gas cleaning processes. A typical biomass gasification process consists of a biomass gasifier, gas cleaning/conditioning units and the end application units for the syngas (gas burners, gas turbines or catalytic synthesis processes). For almost all the biomass gasification processes, the syngas cleanup and conditioning is the largest cost component because of the stringent requirements of the end application units for syngas. For example, the allowable limit for tar in the producer gas is less than 5 mg/Nm3 for a direct-fired gas turbine [9], and for some fuel synthesis processes the contents of tar and ammonia are required to be <0.1 mg/Nm3 and <10 ppm, respectively [2,4,5], in order to protect catalysts and the downstream equipment and to improve the overall efficiency and economics. The wet scrubbing technology has been adopted in the existing biomass gasification processes for the removal of tar, ammonia and other contaminants (e.g., HCl and H2S) in the product gas, it has proved to be a very expensive approach due to its high capital cost and energy loss in the process. The hot-gas cleanup technology is advantageous and has attracted increasing interest as it can utilize the sensible heat in the syngas, benefit CO shifting reaction, lower the capital cost with simpler cooling systems, and enhance the overall energy efficiency of the process [18]. The ARBRE plant in UK as discussed before was an integrated system coupling the gasifier with a gas turbine, a steam turbine and a tar cracker with hot-gas catalysts [94]. Some demonstration processes on hot-gas cleanup (particularly tar removal) have been developed, e.g., in the Carbona Project at Skive, Demark and the Thermochemical Process Development Units (TCPDU) at NREL, USA [95]. From the industrial point of view, however, the only commercial success has been the ‘‘hot” or ‘‘warm” gas cleanup of particulate using ceramic and metallic barrier filters [96] or granular moving bed filters [97]. There has been poor performance and limited use of hot-gas cleanup catalysts and processes (except the particulate cleanup) in full-scale biomass gasification facilities, although some demonstration processes have been conducted [98]. The major challenges/barriers for industrial application of hot-gas cleanup technologies may be summarized below: (1) There is a lack of research in hot-gas cleaning on pilot/largescale biomass gasification facilities. As discussed before, almost all the reported work on hot-gas cleaning of tar and ammonia is based on lab-scale studies with model compounds or a simulated gas under well controlled conditions. The actual conditions of the producer gas differ far from that of the lab-scale research, e.g., depending on feedstock the producer gas contains alkali-based fly ash, Hg, As, HCl, HCN and H2S, all of which might deactivate the hot-gas cleaning catalysts. Thus, more research shall be conducted on a pilot/large-scale biomass gasification facility, where hot syngas is in situ generated as the feed for the hot-gas cleaning reactor. (2) There is a lack of research in the hot-gas cleaning of HCl, HCN, H2S and COS, and their effects on the tar reforming or ammonia decomposition catalysts. Most of the studies in the field of biomass gas cleaning targeted at abatement of tars and ammonia. Other contaminants (sulfur and chlorine) have received much less attention probably because of their small concentrations in biomass gasification gas. However, concentration of HCl and H2S might be high in the gasification of some feedstock, e.g., in gasification of peat, the concentration of these contaminants could be as high as 200–500 ppmv in the product gas [24]. Such high concentrations of HCl and H2S would pose a severe corrosion threat to the downstream equipment and the poisoning

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effects to the catalysts. More research on this aspect is needed. (3) The chemical reactor engineering aspect of the hot-gas cleanup technology has been overlooked. Most the studies were performed at a lab-scale employing a fixed-bed or pack-bed reactor under a relatively low space velocity, where the bed pressure drop and the mechanical properties of the catalysts were not investigated. For full-scale application of a hot-gas cleanup process for high throughput operation, the catalyst bed pressure drop and the catalyst’s mechanical strength would be the key factor for the continuous operation of the process. When a fluidized-bed reactor is employed, some active catalysts such as dolomite, limonite and the novel carbon-supported catalysts will not be suitable due to their weak mechanical strength, leading to significant attrition loss in the fluidized-bed process. A technical measure to tackle this problem may be to load these fragile catalysts on monolith support materials of improved mechanical strength. (4) Development of active, inexpensive and stable catalysts for hot-gas cleaning will still be a critical task for the development of viable hot-gas cleanup technologies for biomass gasification. For instance, the solid acid-supported metal catalysts (nickel/iron/ruthenium) have proved to be very active for both tar reforming and ammonia decomposition, while the major problem of these catalysts is their weak resistance to coke deactivation and sulfur poisoning. Considerable loss of catalytic activity is typically observed for these catalysts after a few hours on the stream. Thus, more research (e.g., through the addition of promoters such as MgO) shall be conducted to enhance the resistance of these supported metal catalysts to deactivation. (5) The economics of hot-gas cleaning process shall be performed. For instance, tar cracking and mitigation has been widely identified as the highest priority gas cleanup technical barrier, while the economics of removing tars on a fullscale have not been evaluated. 5. Conclusions

(1) Biomass gasification produces a low to medium-BTU product gas (or syngas) containing primarily CO2, H2, CO, CH4 and (C2 + C3), as well as some contaminants such as tars, NH3, H2S and SO2. In order to achieve better efficiencies of the syngas applications, these contaminants must be removed before the syngas is used for heat and electricity generation, or in particular for synthesis of liquid fuels and methanol. (2) Tars and ammonia contaminants from the gasification processes could be removed by hot-gas cleanup technology through catalytic steam/dry reforming or catalytic cracking/hydro-cracking reactions of tar, and through catalytic decomposition of ammonia to form N2 and H2. (3) Calcined dolomites can be equally effective as either the primary or secondary catalysts for minimizing tar in the product gas. The presence of CaO in dolomite might be responsible for its activity in tar conversion. Compared to other catalysts (e.g., ferrous materials and nickel catalysts), dolomite is less effective for ammonia decomposition. The major problem with using calcined dolomite is that it is a soft and fragile material, which would limit its use in some types of reactors such as fluidized-bed reactors. (4) Fe-based natural mineral catalysts, including olivine (Fe2SiO4 or Mg2SiO4), limonite (composed mainly of goethite, FeOOH), hematite (Fe2O3), ankerite (as a ferrous dolo-

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mite), are active for tar decomposition. Olivine is less active for ammonia decomposition, but an Australian limonite with a high content of a-FeOOH was found to be very effective for ammonia decomposition. Cycle mechanisms involving the formation of N-containing intermediate Fe species were proposed for the Fe catalysts in NH3 decomposition. (5) Nickel-based supported catalysts, used extensively in the petrochemical industry, have shown high activities for tar reforming/decomposition and ammonia decomposition in coal and biomass gasification. When used as secondary catalysts, the supported nickel catalysts could attain nearly complete decomposition of both tar and ammonia decomposition at >800 °C. The addition of nickel to dolomite and olivine could significantly improve the activity towards tar conversion. The Al2O3 or SiO2-supported Ru catalyst was demonstrated to be much more effective than the Ni catalysts. However, the major problem for these Al2O3-supported Ni or Ru catalysts for hot-gas removal of ammonia or tar is associated with the deactivation by fouling of the catalyst due to the carbon deposition and by H2S. The strategies to prevent carbon deposition and hence the catalyst deactivation include introduction of steam in the product gas so as to gasify the accumulated coke/carbon, and using non-alumina (MgO or CeO2) catalyst supports or promoters. (6) Carbon-supported catalysts including pyrolysis chars of low rank coals, coal-char-supported Fe and Ca catalysts and carbon nanotubes supported Ru catalysts, showed high activities towards ammonia decomposition. More interestingly, phosphorous could be an effective promoter in activated carbon-supported Fe catalysts for decomposition of ammonia into N2 and H2. (7) Hot-gas cleanup catalysts and processes (except the particulate cleanup) have poor performance and limited use in fullscale biomass gasification facilities. More research is needed.

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