The use of different types of catalyst for hydrogen production during HTG of biomass offers a huge promise for the economic development of the process. In general, catalyst in this process work in several ways to enhance hydrogen pro- duction; these include (a) conversion of biomass into intermediates that can be easily gasified (b) lowering the activation energy required to achieve complete gasification (c) selectivity of reaction routes toward hydrogen production.
Catalysts used in HTG of biomass can be broadly but vaguely classified as homogenous and heterogeneous. Essentially, homogeneous catalysts are those that are water soluble, while heterogeneous catalyst are not. However, it can be expected that reactions occurring during HTG could be gas-phase, liquid-phase, or solid-phase. Therefore, a water soluble catalyst will become heterogeneous for gas-phase and solid-phase reactions. In this same way, a heterogeneous catalyst becomes a homogenous catalyst for solid-phase reactions.
10.4.1 Catalyst Supports
Due to the reaction conditions of HTG, it has not been possible to literally deploy largely successful conventional gasification catalysts to the hydrothermal process.
Successful catalysts for hydrothermal biomass gasification must adapt to the specific conditions inherent in hydrothermal media. Such conditions include the large presence of water, high pressures, variable ionic product, and dielectric constant. Only a few catalyst supports remain stable under hydrothermal condi- tions. The use of hydrolyzable catalysts and catalyst supports will need to be avoided if catalysts are to be reused. Elliott et al. [18] found that other chemical transformations of catalyst supports could include phase transition and dissolution.
They found that catalyst supports which showed satisfactory stability under hydrothermal conditions include alpha-alumina and zirconia or titania in their monoclinic forms [18].
10.4.2 Alkaline Catalysts/Additives
Nearly in all cases reported in literature, enhanced hydrogen production has been achieved through the use of highly soluble alkaline compounds as homogeneous catalysts. The most commonly used alkaline catalysts include KOH, NaOH, CaO, Trona, Ca(OH)2, K2CO3, and Na2CO3[29,36,47,52,67]. Compared to noncat- alytic HTG of biomass, hydrogen yields have been observed to increase up to four times in the presence of some alkaline catalyst [68]. A recent work by Madenoglu et al. [69] is adapted and shown with permission in Fig.10.8as a good evidence to the enhanced effect of alkaline additives, including alkaline minerals such as Trona, in the production of hydrogen via HTG.
In general, the mechanistic schemes depicting the catalytic activities of these compounds indicate that they are effective in transforming biomass into simple molecules capable of HTG to product hydrogen. Onwudili and Williams [36]
proposed a set of reaction mechanisms for the hydrothermal degradation of glu- cose into sodium formate and sodium acetate in the presence of sodium hydroxide.
Experimental evidence suggests that alkaline additives operate within the sub- critical region of water based on the variety of ionic reactions possible within this region. In acting to degrade biomass, these alkaline compounds alter the selectivity of the default reaction chemistry shown in equation (10.5) in favor of the for- mation of formic acid. This prevents polymerization reactions that could lead to tar formation. Using the gas products obtained from alkaline HTG of biomass, On- wudili and Williams [36] proposed as reaction scheme as follows:
C6H12O6ỵ6NaOHỵ3H2O!C6H6O6:Na6:9H2O ð10:13ị C6H6O6:Na6:9H2O!6NaCOOHỵ6H2ỵ3H2O ð10:14ị
6NaCOOH!6COỵ6NaOH ð10:15ị
6COỵ6H2O!CO2ỵ6H2 ð10:16ị
6CO2ỵ6NaOH!6NaHCO3 ð10:17ị
The overall equation,
C6H12O6ỵ6NaOHỵ6H2O!6NaHCO3ỵ12H2 ð10:18ị Similar reaction schemes were proposed by Sinag et al. [35] which showed that the formation of hydrogen during K2CO3-catalyzed HTG of biomass following the formation of potassium formate as intermediate. The preferential formation of hydrogen at near-critical water conditions indicates that the formate ion becomes the favored product of biomass-reforming reactions under alkaline hydrothermal conditions. However, experimental results indicate that in the presence of NaOH, the gas product consists of about 80–85 % hydrogen and 10–15 % methane [36].
Hence, reaction (10.1) can be rewritten in the presence of sodium hydroxide as follows:
C6H12O6ỵ4H2Oỵ5NaOH!5NaHCO3ỵCH4ỵ8H2 ð10:19ị Further research with pure sodium formate and sodium acetates [31] indicates that there are two possible hydrogen formation ‘‘windows’’ in the presence of alkaline additives. The earlier hydrogen formation ‘‘window’’ is completely accomplished at about 673 K, which corresponds to the hydrothermal reactions of sodium formate, shown in reaction (10.20). The latter hydrogen formation ‘‘win- dow’’ occurs near 773 K, which also corresponds to the reactions of sodium acetate, depicted in reaction (10.14). The formation of methane in reaction (10.21) is a prerequisite for its reforming to hydrogen in reaction (10.12).
2HCOONaỵH2O!2H2ỵCO2ỵNa2CO3 ð10:20ị 2CH3COONaỵH2O!2CH4ỵCO2ỵNa2CO3 ð10:21ị These reactions can all be accomplished fairly quickly within a temperature upper limit of 723 K at the most. In addition, the hydroxides (alkalis, e.g., NaOH and KOH) give much better hydrogen yields than the carbonates (e.g., K2CO3, Na2CO3) as shown by the work of Muangrat et al. [67] shown in Fig.10.9. It has been demonstrated that the alkalis are able to capture the carbon dioxide in the gas produced to form soluble carbonates, which are stable under reasonable temper- atures ranges. The removal of CO2is important in enhancing the efficiency of the water-gas-shift reaction by driving it toward hydrogen production.
0 4 8 12 16
NaOH KOH
Hydrogen yield, mol/kg
Different alkaline catalysts/additives
Ca(OH)2 Na2CO3 K2CO3 NaHCO3 NaOH+ H2O2
Fig. 10.9 Yields of gas products from glucose gasification in relation to the type of alkaline additive [60,67]
For effective hydrogen production, a large quantity of the alkali must be used, usually in a ratio of 1 g biomass/1.2 g alkali [36,60]. This has cost implications for the HTG process. Also, the high water solubility of alkaline additives is seen as a major drawback for their recovery and reuse. This is because the alkalis are converted to their metal carbonates, which are not as effective as the original alkalis. Research into the recausticization of these carbonates may resolve this challenge.
10.4.3 Metal and Metal Oxide Catalysts
The two most significant metals used for HTG of biomass for hydrogen are nickel and ruthenium on various catalyst-supports [33, 70–80]. The reduced metals are significantly more effective than their corresponding metal oxides for gas production [16, 79]. The metal catalysts often work best in supercritical water medium for gas production, possibly due to a combination of gas-phase and solid- phase radical reactions. High carbon gasification efficiencies of 99.9 % and over have been achieved with ruthenium catalyst of supports such as alpha-alumina, gamma-alumina, carbon and rutile [33,75].
Hydrogen has been reported as the major gas during catalytic HTG of biomass with metal catalyst in continuous reactors [81,82]. This also suggests that reac- tions with high heating rates and short residence times are likely to produce high yields of hydrogen, possibly due to the absence of secondary reactions. However, the use of alkaline compounds as catalyst or co-catalysts has been reported to increase the yield of hydrogen in the product gas [33]. In addition, the use of Raney-nickel or a combination of nickel and sodium metal gives increased hydrogen gas yield [42,70].
One of the challenges of using metal catalysts in HTG of biomass is that the effective metals such as Ni, Ru and Pd can catalyze both steam reforming reactions of hydrocarbons to hydrogen as well as methanation reaction, which consumes hydrogen [75, 76]. This is similar to the Sabatier reaction. Hence, rather than hydrogen or methane, both gases are obtained during catalysis with ruthenium catalyst as shown in Table10.5. A cursory look at the table indicates that the selectivities of ruthenium-based catalysts toward hydrogen or methane in a batch reactor could be influenced by reaction temperatures and residence times as well as the catalyst supports. It appears that lower temperatures and longer residence times favor hydrogen production, while methane selectivity increases at higher tem- peratures [33]. Recently, Zhang et al. [83] showed that hydrogen generation from glucose increased in the presence of ruthenium-modified nickel catalyst. Such catalyst combination is proposed to favor hydrogen production against methana- tion reaction.
10.4.4 Carbon Catalyst
Carbon, usually in its activated form has been used for hydrothermal biomass gasification both as a catalyst and as catalyst support [66, 84]. It is clear from literature that the catalytic activity of carbon for HTG is mostly limited to high temperature applications, i.e., within the supercritical region of water with tem- peratures above 873 K as shown in Fig.10.10. This in itself poses a challenge with respect to the mechanisms of carbon-catalyzed high temperature HTG, since high temperature alone can give comparable yields of hydrogen [85]. However, carbon shows relatively high stability in supercritical water conditions and can be recovered [86].
In addition, it appears that the catalytic activities of activated carbons depend on the biomass from which they are sourced. As shown in Fig.10.10, the yields of hydrogen from the supercritical water gasification of 1.2 M glucose solution varied considerably with different types of activated carbons. This could be attributed to the ash contents of the activated carbons; for instance, a high content of alkaline metals may promote hydrogen formation. In addition, it has also been shown that the carbon catalyst itself may be gasified under certain supercritical water con- ditions to form hydrogen [87].
Table 10.5 Effect of ruthenium-based catalysts on hydrogen and methane yields during HTG of biomass in batch reactors [33]
Ruthenium-based catalysts
Feed Feed loading
wt %
Temperature (C)
Residence time (min)
H2yield (mol/kg)
Ru/TiO2 Sugarcane bagasse 3.3 400 15 3.2
Ru/C Sugarcane bagasse 3.3 400 15 1.9
Ru/TiO2 Cellulose 3.3 400 15 2.8
Ru/c-Al2O3 Glucose 2.0 380 15 18
Ru/c-Al2O3 Glucose 2.0 380 60 33
Ru/C Glucose 2.0 380 15 17
Ru/C Glucose 2.0 380 60 26
Ru/c-Al2O3 Cellulose 2.0 380 60 34
Ru/C Cellulose 2.0 380 60 22
Ru/C P. tricornutum
(alga)
2.5 400 60 2.5
Ru/C P. tricornutum
(alga)
5.1 400 67 1.9
Ru/a-Al2O3 Glucose 6.7 550 10 10.8
Ru/a-Al2O3/ NaOH
Glucose 6.7 550 10 21.1
Ru/a-Al2O3/CaO Glucose 6.7 550 10 14.7
Ru/a-Al2O3/CaO Cellulose 6.7 550 10 9.10
Ru/a-Al2O3/CaO Xylan 6.7 550 10 10.7
Ru/a-Al2O3/CaO Sawdust 6.7 550 10 10.4
Adapted from Ref. [33], Copyright 2013, with permission from Elsevier
The activities of carbon catalysts can also depend on the types of biomass and feed concentrations as shown in Fig.10.11. As mentioned earlier, lower biomass concentrations and carbohydrate-rich biomass types favor hydrogen production.
Figure10.11 indicates that corn starch (10 wt% concentration) could produce hydrogen of up to 31 mol/kg, whereas sewage sludge (3 wt% concentration) produced only 11 mol/kg using the same coconut shell activated carbon as cata- lyst, within similar supercritical water conditions.