10.2 Biomass Gasification in Hydrothermal Media
10.2.2 Thermochemistry of Hydrogen Production from HTG
Hydrothermal gasification of biomass to hydrogen involves a series of complex reaction chemistries. Biomass is a multicomponent feedstock comprising of cel- lulose, hemicellulose, and lignin. Some biomass can also have high ash contents.
The possible interactions of the biomass components and their possible degrada- tion products, significantly increases the complexity of hydrothermal reactions of biomass [16]. However, studies with biomass model compounds indicate that the reactions of carbohydrate-type biomass compounds such as cellulose and glucose can be followed [20]. Ultimately, the overall reaction equation for HTG of biomass can be represented by equation (10.1), which involves the reaction of biomass (represented by glucose).
C6H12O6ỵ6H2O!12H2ỵ6CO2 DH298Kẳ ỵ362 kJ/mol ð10:1ị As shown by equation (10.1), the idealized reaction for the production of hydrogen from biomass in the presence of gaseous water under hydrothermal conditions is endothermic. Hence, external heat input is required for the reaction but this is however, not peculiar to the HTG process alone. Hydrogen production from biomass via HTG as represented by the idealized reaction (10.1) underlies the importance of water as a reactant in hydrogen production [18,19]. The produced hydrogen gas comes from both the biomass and the stoichiometric amount of water used up during the reaction.
Using glucose as biomass feedstock, the maximum yield of hydrogen gas based on reaction (10.1) is 66.7 mol/kg or 133.3 g/kg. This amount of hydrogen is 200 % of the hydrogen content of glucose, showing that the reacting water provides the balance. Evidence of the participation of water in hydrogen production can be found in literature. Table10.1 compares biomass gasification (including HTG) with other existing and developing processes for hydrogen production [30].
Although, the efficiency and theoretical yield of hydrogen via HTG of biomass are lower compared to conventional processes such as steam reforming of natural gas, it has the advantage of being carbon-neutral process based on renewable resource.
In addition, its efficiency can be improved through future research and this will impact positively on the overall cost of the process.
Hydrothermal reactions for hydrogen production must be carried out at rea- sonably high temperatures. In practice, temperature ranges from 673 K up to 873 K are often used [20, 31–33]. However, equilibrium yields have been cal- culated at temperatures of up to 1273 K [16]. High temperature favors hydrogen
production; however a compromise between hydrogen yield and associated costs must be evaluated for sustainable conversion of biomass to hydrogen.
Hydrogen production from HTG of biomass and biomass model compounds is seldom a one-step reaction. There are many elementary component-reactions involved which constitute the idealized equation (10.1) and play significant roles in hydrogen production from biomass. These reactions can often be involved in competing reaction pathways, which can lead to the formation of products other than hydrogen or products whose presence would suppress hydrogen production.
Figure10.3shows a schematic diagram involving the different reaction processes leading towards hydrogen production. In Fig.10.3, all other reaction pathways to Table 10.1 Comparison of various hydrogen production methods [9,30]
Current hydrogen production methods
Status Maximum hydrogen yield (mol/kg)
Efficiency (%)
Cost relative to SMR Steam reforming of methane/
natural gas (SMR)
Mature 333 70–80 1
Partial oxidation of heavy oil Mature 143 70 1.8
Coal gasification Mature 100 60 1.4–2.6
Partial oxidation of coal Mature 100 55 –
High-temperature electrolysis of water
R & D 55.5 48 2.2
Biomass gasification (incl. HTG) R & D 66.7 45–50 2.0–2.4
Primary Gas Products
Biomass Hydrolysis & Degradation
Products (e.g. glucose) Simple Organic Acids (e.g. acetic acid, formic acid)
CO CH4
Hydrogen
Primary reforming Secondary
reforming Tertiary reforming
Increasing Process Temperature Subcritical waterNear-Critical WaterSupercritical water
Fig. 10.3 Reaction schemes for hydrogen production from biomass via HTG
unwanted products are assumed to be suppressed in order to follow the idealized equation for hydrogen production. Particularly, the major competing reaction pathway involving the formation of 5-hydroxymethylfurfural (5-HMF) inhibits gas formation and hence hydrogen production [34–37].
In this section, each possible reaction capable of producing hydrogen gas or its precursors will be treated as ideal, i.e., competing reactions will be ignored. The main chemical reactions involved include biomass reforming to relevant inter- mediate liquids and gaseous species, reforming reactions of the intermediates to carbon monoxide and finally the all-important water-gas shift reaction. It has been shown that the important intermediates responsible for gas formation include small acid and aldehyde molecules such as acetic acid, formic acid, lactic acid, levulinic acid, acetic aldehyde, and formic aldehyde [20,31].
10.2.2.1 Biomass Reforming to Produce Intermediates
C6H12O6ỵ6H2O!6HCOOHỵ6H2 DH 298Kẳ ỵ170 kJ=mol ð10:2ị C6H12O6ỵ6H2O!4HCOOHỵ2CO2ỵ8H2 DH 298Kẳ ỵ234 kJ=mol
ð10:3ị C6H12O6ỵ4H2O!4HCOOHỵCH3COOHỵ4H2 DH298 Kẳ ỵ54 kJ=mol
ð10:4ị C6H12O6ỵ2H2O!2HCOOHỵ2CH3COOHỵ2H2 DH 298Kẳ 68 kJ=mol ð10:5ị C6H12O6ỵ2H2O!2CH3COOHỵ2CO2ỵ4H2 DH 298Kẳ ỵ2 kJ=mol
ð10:6ị Each of these possible reactions above, except reaction (10.5), has a positive standard enthalpy change (DH) values, indicating that external heat input will be required for them to occur. Although, theDH values are not the only thermodynamic properties needed to predict the spontaneity of these reactions, they give some indications about the effect of temperature. Indeed, it is expected that the formation of gas products from solid biomass and liquid water, will lead to a dramatic increase in the entropy change (DS) of the system. A negative or positive but smallDH value and a large and positiveDS value will guarantee a negative Gibb’s Free Energy (DG), which is a prerequisite for spontaneous reactions. It is well known that the hydrolysis of biomass is spontaneous in subcritical water, which requires heat input.
For instance, biomass reforming into simple carboxylic acids such as acetic acid has been well reported in literature during hydrothermal processing of biomass [38–41]
under subcritical water conditions. This suggests that hydrothermal reforming reactions of biomass must include acetic acid as a product [20,28].
The values ofDH for reactions (10.2) and (10.3), suggest that a large amount of heat energy will be required for biomass to be reformed only into formic acid. On the other hand, the presence of acetic acid in reactions (10.4–10.6) gives either negativeDH value, or a positive but smallDH value. Reactions (10.5) and (10.6) are very similar except that in the case of reaction (10.6), the formic acid in reaction (10.5) has been converted to hydrogen gas and carbon dioxide. The difference in DH values for reactions (10.6) and (10.5) is due to the extra heat energy released for the conversion of formic acid to H2and CO2via a subsequent water-gas shift reaction. Experimental evidence [30] suggests that the complete reforming of formic acid, in the form of sodium formate, to hydrogen gas occurs around 673 K. This supports the formation and thermal stability of these simple carboxylic acids within the subcritical water region according to literature [39,41].
In addition, thermodynamic calculations have shown that the yield of hydrogen gas under subcritical water conditions is usually low [16, 22]. This therefore suggests that reaction (10.5) is more probable than reaction (10.6) as a plausible representation of the idealized reforming of carbohydrate biomass in low tem- perature hydrothermal media leading to primary products [31].
10.2.2.2 Reforming of Intermediates
HCOOH!COỵH2O DH 298Kẳ ỵ73 kJ=mol ð10:7ị CH3COOH! CH4ỵCO2 DH 298Kẳ ỵ15:1 kJ=mol ð10:8ị CH4ỵH2O!COỵ3H2 DH 298Kẳ ỵ206 kJ=mol ð10:9ị The reforming reactions of the intermediate products as represented by the reaction equations (10.7–10.9) are also all endothermic in nature. The formation of methane from hydrothermal biomass gasification is thermodynamically favored in near-critical water conditions, which can be attributed to the decarboxylation of acetic acid under such reaction conditions [22]. The formation of hydrogen under such conditions appears to be due to the direct aqueous reforming of biomass. In addition, the separate reforming of formic acid (DH298 K= +32 kJ/mol) to hydrogen and carbon dioxide occurs just above the critical point of water [31].
Compared to the combination of reactions (10.8) and (10.9), which represent aqueous reforming of acetic acid (DH298 K= +268 kJ/mol), reaction (10.8) with a relatively lowDH value, would be favored. In fact, experimental evidence [30]
and thermodynamic equilibrium calculations [42] indicate that the formation of hydrogen via the methane-reforming occurs well into the supercritical water region. The important reactions leading to hydrogen production under hydro- thermal conditions are those involving the formation of carbon monoxide. These are equations (10.7) and (10.9) involving the hydrothermal decarbonylation reactions of formic and acetic acids.
10.2.2.3 Hydrogen Gas Production
COỵH2O!H2ỵCO2 DH 298Kẳ 41 kJ=mol ð10:10ị It has become clear from literature that the production of hydrogen gas can occur at two distinct regions of hydrothermal media. The negativeDH value of the water-gas shift reaction in equation (10.10) indicates that this reaction can readily be accomplished under any region of the hydrothermal space, once CO has been formed. Catalytic water-gas shift reactions can be carried out at temperatures as low as 433 K [43]. Incidentally, much more energy is required to accomplish the reactions leading to the formation of CO than its reaction with water to form hydrogen gas. On this evidence, water-gas shift reaction is unlikely to be the rate- determining step in hydrothermal hydrogen production. While the decarbonylation of formic acid requires less heat energy, methane-reforming reaction with a much larger DH value of +206 kJ/mol, will require more energy input to be accom- plished. In fact, it is almost certain that the formation of CO and its precursors from biomass are the key reactions controlling the eventual formation of hydrogen.
More importantly, much of the hydrogen gas produced during HTG of biomass comes from the complete methane-reforming process (reaction10.12), which still has a large and positiveDH value of+165 kJ/mol. By comparison, the complete reforming of acetic acid produces four times more hydrogen than that of formic acid.
HCOOH!CO2ỵH2 DH 298Kẳ ỵ32 kJ=mol ð10:11ị CH4ỵ2H2O!CO2ỵ4H2 DH298Kẳ ỵ165 kJ=mol ð10:12ị The above outline of a selection of the possible reaction schemes involved in an idealized HTG of biomass for hydrogen production may give an insight into the development of a sustainable process to maximize the potential of the process.
Such optimization and intensification processes can benefit from altering the reaction chemistries to favor hydrogen production. For instance, since hydrogen production from formic acid is less energy-intensive, the process can be designed to convert biomass mainly into this intermediate, while suppressing acetic acid formation. In contrast, the process can be designed to lower the activation energy required for acetic acid reforming, since it appears to be the default daughter product of hydrothermal biomass conversion. Such process designs may require the use of homogeneous or heterogeneous catalysts; a combination of both catalyst types is also possible.