**3. Gasification kinetics of model compounds in continuous supercritical water reactors**

Model compounds serve as useful surrogates for studying the reaction chemistry of biomass constituents in supercritical water. Current interest in converting wet biomass waste into useful fuel has prompted studies of aromatic compounds as model compounds for lignin [22, 29, 30]; glucose and fructose, as cellulose surrogates [7, 14, 31–38]; alcohols, as model compounds for common liquid gasification intermediates and by-products [26, 39–44]; and amino acids—as model compounds for protein [15, 45–50]. Overall, these studies serve as the basis for approximating

*Gasification Kinetics in Continuous Supercritical Water Reactors DOI: http://dx.doi.org/10.5772/intechopen.90503*

the operating conditions required to upgrade heterogeneous biomass into highvalue fuels, such as "green" hydrogen.

For most studies, reaction mechanisms, pathways, kinetics, and yields are determined by varying the temperature, feedstock concentration, and residence time. Few studies investigate the effect of pressure on reaction chemistry; however, no significant pressure-related trends have been observed. The only time pressure that significantly impacts reaction chemistry is near the critical point, where pressure change can affect the thermophysical properties of SCW, such as density and ion product. For all studies reviewed here, the pressure is taken at 25 MPa unless specified otherwise.

Prevailing reaction mechanisms that deserve mention are the WGS reaction and the methanation reactions. The WGS increases H2 yields by converting CO to CO2, expressed as:

$$\text{CO} + \text{H}\_2\text{O} \leftrightarrow \text{CO}\_2 + \text{H}\_2\tag{5}$$

Methanation serves to reduce H2 yields by converting it to methane, via the following two pathways:

$$\text{CH} + \text{3H}\_2 \leftrightarrow \text{CH}\_4 + \text{H}\_2\text{O} \tag{6}$$

$$\rm{CH}\_{2} + 4H\_{2} \leftrightarrow \rm{CH}\_{4} + 2H\_{2} \tag{7}$$

Overall, both the WGS and methanation reactions are highly important to the final gaseous product composition.

### **3.1 Aromatic compounds**

(HE). GE is defined as the ratio of the total mass of the gaseous product to the initial

CE is another metric used to quantify completeness of gasification; it is especially relevant if solid or liquid carbonaceous compounds are formed as refractory gasification products. It is defined as the ratio of moles of carbon in the product gas

> *nCO* þ *nCO*<sup>2</sup> þ *nCH*<sup>4</sup> þ *xnCxHy nC*,*feedstock*

A less frequently used metric is HE, defined as the ratio of moles of hydrogen in

2*xH*<sup>2</sup> þ 4*nCH*<sup>4</sup> þ *ynCxHy nH*,*feedstock*

HE and GE values from SCWG can be well above 100%, due to a prominent role of the water-gas shift (WGS) reaction during gasification, which can produce H2

For determining rates of molecular decomposition in SCW, first-order reaction behavior is commonly assumed. This assumption is typically valid for pyrolysis or hydrolysis reactions or monomolecular decomposition reactions. However, this assumption is not valid for free radical reactions where radical induction and radical pooling behavior are present; more complex reaction modeling is required. The first-order decomposition rate (*k*) is determined by fitting an exponential decay curve to the reactant concentration varying with residence time, at a given experi-

Once a range of first-order decomposition rates (*k*) is determined at various temperatures, Arrhenius parameters can be determined by fitting the *ln(k)* vs. *1/T*

ln ð Þ¼ *<sup>k</sup>* ln ð Þ� *<sup>A</sup> EA*

**3. Gasification kinetics of model compounds in continuous supercritical**

Model compounds serve as useful surrogates for studying the reaction chemistry of biomass constituents in supercritical water. Current interest in converting wet biomass waste into useful fuel has prompted studies of aromatic compounds as model compounds for lignin [22, 29, 30]; glucose and fructose, as cellulose surrogates [7, 14, 31–38]; alcohols, as model compounds for common liquid gasification intermediates and by-products [26, 39–44]; and amino acids—as model compounds for protein [15, 45–50]. Overall, these studies serve as the basis for approximating

This linear curve fit yields the activation energy (*EA*) and pre-exponential factor

*xH*<sup>2</sup> þ *xCO* þ *xCO*<sup>2</sup> þ *xCH*<sup>4</sup> þ *xGas*,*other xfeedstock*

∗ 100 (1)

∗ 100 (2)

∗ 100 (3)

*RT* (4)

mass of the feedstock, expressed mathematically as:

*CE*ð Þ¼ %

the gaseous product to moles of hydrogen in the feedstock:

*HE*ð Þ¼ %

*GE*ð Þ¼ %

*Advanced Supercritical Fluids Technologies*

to moles of carbon in the feedstock:

gas via reaction of CO with water.

curve with the following expression:

(*A*) for the first-order decomposition reaction.

mental temperature.

**water reactors**

**118**

One of the most recalcitrant biomass constituents is lignin, a heterogeneous organic polymer with numerous aromatic rings. In order to gain insight into lignin decomposition in SCW, phenol, benzene, and guaiacol have been proposed as lignin surrogates.

Huelsman and Savage [22] gasified phenol in an SCW batch reactor at 500–700° C; the authors identified major reaction products as H2, CO, CO2, CH4, benzene, phenol, PAHs, and char. The presence of benzene and phenol as products indicates two competing reaction mechanisms are at play: aromatic ring growth and ring cleaving. The relative importance of the two mechanisms is highly dependent on the reaction temperature and the concentration of aromatics.

Yong and Matsumura [29] gasified phenol and benzene (separately) in a continuous SCWR at 370–450°C in the residence time range of 0.5–100 s. Observed products from each reagent include benzene, phenol, catechol, naphthalene, char, TOC in the liquid phase, and gaseous products. Catechol and naphthalene are indicative of the aromatic ring growth pathways leading to char formation. Increasing temperature and residence time led to increased yields of gas, TOC, and char. Generally, free radical mechanisms have been thought to be responsible both for decomposition to gaseous products and for ring growth to char. First-order decomposition was assumed, and Arrhenius parameters for general disappearance of phenol and benzene were proposed, as shown in **Table 1**. Yong and Matsumura [30] also gasified guaiacol, another aromatic model compound for lignin, at 300– 450°C and residence times of 0.5–40 s. Again, yields of benzene, phenol, catechol, gas, TOC, and char were reported. Char formation was so significant that the initial guaiacol concentration had to be limited to 0.1 wt% to prevent reactor plugging. The higher temperatures and residence times increased yields of gas, TOC, and char. In this temperature range, hydrolysis, pyrolysis, ionic, and free radical


These studies show good agreement in reported product yields in the region of

SCWG of glucose has also been studied at higher temperatures. Goodwin and Rorrer [35] used a microchannel reactor to gasify glucose at 650–750°C; complete conversion was achieved after 2 s at 750°C. Intermediate products were identified as acetic and propanoic acids, 5-HMF, 2,5-hexanedione, phenol, lactic acid, formic acid, and furfural. Small inner reactor diameters were credited with improving heat transfer to the reaction environment and enhancing gasification, likely due to an increased catalytic wall effect. Hendry et al. [36] gasified 10–15 wt% glucose for residence times of 4.0–6.5 s and 750–800°C. Higher temperatures and lower initial concentrations improved conversion rates, which were used to calculate the Arrhenius parameters. There is significant discrepancy between the Arrhenius parameters proposed by Promdej and Matsumura [34] vs. Hendry et al. [36], which is likely due to the different temperature ranges tested. Near the critical point ionic chemistry is dominant, while free radical mechanisms are favored at temperatures well above

Both Castello et al. [8] and Weiss-Hortala et al. [51] gasified glucose/phenol mixtures to study the effect of phenol on the gasification of glucose. Broadly, it was found that the presence of phenol inhibited H2 production while promoting CH4

Fructose has also been studied as a model compound for waste fruits and vegetables. Kabyemela et al. [37] subjected fructose to SCW at 300–400°C for 0.02–2 s, to determine decomposition pathways and kinetic rates. Major liquid products were identified as dihydroxyacetone, glyceraldehyde, erythrose, pyruvaldehyde, acetic acid, formic acid, and 5-HMF. The yields and kinetic rates were very similar to those obtained during glucose gasification at the same conditions [36]. Aida et al. [38] also studied SCWG of fructose, at temperatures up to 400°C and residence times from 0.14 to 0.78 s. In agreement with Kabyemela et al. [37], liquid yields of glyceraldehyde, dihydroxyacetone, pyruvaldehyde, lactic acid, and 5-HMF were

Nanda et al. [7] gasified fructose at higher temperatures from 550 to 700°C, residence times of 30–75 s, and initial concentrations of 4–10 wt%. Broadly, higher temperatures and lower concentrations increased gasification efficiency. Higher concentrations of phenolic compounds were observed at 700°C, possibly indicating that char-forming pathways are enhanced at higher temperatures. Finally, residence times past 60 s increased CH4 yields due to the consumption of H2 via methanation

Several studies have investigated methanol decomposition in SCW, due to its chemical simplicity and prevalence as a refractory intermediate gasification product. Boukis et al. [39] reformed methanol in a continuous SCWR a T = 400–600°C, for residence times of 3–100 s, and initial methanol concentrations from 5 to 64 wt %. Gaseous yields of H2, CO, CO2, and trace CH4 were detected, with higher temperatures and residence times increasing conversion rates. Bennekom et al. [40] also gasified methanol in a continuous reactor at T = 450–650°C and residence times of 6–173 s. H2, CO, and CO2 were reported as main products, with trace yields of methane, formaldehyde, and formic acid. The results show that methanol reforms

Limited investigations are available related to the conversion of ethanol to gaseous products in SCW. Schanzenbacher et al. [41] subjected ethanol to SCW in a continuous reactor at temperatures from 433 to 494°C, for residence times from 2

to H2 and CO in SCW, with CO converted to CO2 by the WGS reaction.

enhanced ionic chemistry around the critical point.

*DOI: http://dx.doi.org/10.5772/intechopen.90503*

*Gasification Kinetics in Continuous Supercritical Water Reactors*

production, with an overall decrease in gas yield.

the critical point.

reported.

reactions.

**121**

**3.3 Alcohols**

### **Table 1.**

*First-order Arrhenius parameters of model compound gasification.*

reaction mechanisms were all thought to be active. First-order Arrhenius parameters for guaiacol decomposition into intermediate products can be found in **Table 1**.

### **3.2 Glucose and fructose**

Glucose has received considerable attention as a model compound due to its natural prevalence in biomass. Early studies of SCWG by Modell [32] investigated the decomposition of glucose, and early results showed that glucose could be completely converted to gaseous products without significant char formation after 20 s at 600°C [31].

Kabyemela et al. [33] investigated SCWG of glucose at 300–400°C and residence times of 0.02–2 s. Short residence times and low temperatures allowed for the identification of significant intermediate products, such as fructose, saccharinic acids, erythrose, glyceraldehyde, dihydroxyacetone, 1,6-anhydroglucose, pyruvaldehyde, and 5-hydroxymethylfurfural (5-HMF). The presence of numerous molecules containing furan rings, such as 5-HMF, illustrates the potential to form char via ionic mechanisms near the critical point. In a similar study, Aida et al. [14] gasified glucose at 350–400°C and residence times of 0.2–1.7 s with the explicit goal of optimizing furfural and 5-HMF yields. At these relatively low temperatures and short residence times, products were identified as fructose, erythrose, glycolaldehyde, glyceraldehyde, hydroxyacetone, 5-HMF, and furfural. Promdej and Matsumura [34] gasified glucose in the 300–460°C range, proposing Arrhenius parameters for the decomposition reaction (**Table 1**). Reported products include 5- HMF, furfural, and char, with subcritical temperatures favoring char formation.

### *Gasification Kinetics in Continuous Supercritical Water Reactors DOI: http://dx.doi.org/10.5772/intechopen.90503*

These studies show good agreement in reported product yields in the region of enhanced ionic chemistry around the critical point.

SCWG of glucose has also been studied at higher temperatures. Goodwin and Rorrer [35] used a microchannel reactor to gasify glucose at 650–750°C; complete conversion was achieved after 2 s at 750°C. Intermediate products were identified as acetic and propanoic acids, 5-HMF, 2,5-hexanedione, phenol, lactic acid, formic acid, and furfural. Small inner reactor diameters were credited with improving heat transfer to the reaction environment and enhancing gasification, likely due to an increased catalytic wall effect. Hendry et al. [36] gasified 10–15 wt% glucose for residence times of 4.0–6.5 s and 750–800°C. Higher temperatures and lower initial concentrations improved conversion rates, which were used to calculate the Arrhenius parameters. There is significant discrepancy between the Arrhenius parameters proposed by Promdej and Matsumura [34] vs. Hendry et al. [36], which is likely due to the different temperature ranges tested. Near the critical point ionic chemistry is dominant, while free radical mechanisms are favored at temperatures well above the critical point.

Both Castello et al. [8] and Weiss-Hortala et al. [51] gasified glucose/phenol mixtures to study the effect of phenol on the gasification of glucose. Broadly, it was found that the presence of phenol inhibited H2 production while promoting CH4 production, with an overall decrease in gas yield.

Fructose has also been studied as a model compound for waste fruits and vegetables. Kabyemela et al. [37] subjected fructose to SCW at 300–400°C for 0.02–2 s, to determine decomposition pathways and kinetic rates. Major liquid products were identified as dihydroxyacetone, glyceraldehyde, erythrose, pyruvaldehyde, acetic acid, formic acid, and 5-HMF. The yields and kinetic rates were very similar to those obtained during glucose gasification at the same conditions [36]. Aida et al. [38] also studied SCWG of fructose, at temperatures up to 400°C and residence times from 0.14 to 0.78 s. In agreement with Kabyemela et al. [37], liquid yields of glyceraldehyde, dihydroxyacetone, pyruvaldehyde, lactic acid, and 5-HMF were reported.

Nanda et al. [7] gasified fructose at higher temperatures from 550 to 700°C, residence times of 30–75 s, and initial concentrations of 4–10 wt%. Broadly, higher temperatures and lower concentrations increased gasification efficiency. Higher concentrations of phenolic compounds were observed at 700°C, possibly indicating that char-forming pathways are enhanced at higher temperatures. Finally, residence times past 60 s increased CH4 yields due to the consumption of H2 via methanation reactions.

### **3.3 Alcohols**

reaction mechanisms were all thought to be active. First-order Arrhenius parameters for guaiacol decomposition into intermediate products can be found in **Table 1**.

<sup>300</sup>–<sup>374</sup> 4.0 <sup>10</sup><sup>1</sup> 39.7 [5]

<sup>374</sup>–<sup>430</sup> 3.6 <sup>10</sup><sup>12</sup> <sup>175</sup> [5]

Glucose has received considerable attention as a model compound due to its natural prevalence in biomass. Early studies of SCWG by Modell [32] investigated the decomposition of glucose, and early results showed that glucose could be completely converted to gaseous products without significant char formation after

Kabyemela et al. [33] investigated SCWG of glucose at 300–400°C and residence times of 0.02–2 s. Short residence times and low temperatures allowed for the identification of significant intermediate products, such as fructose, saccharinic acids, erythrose, glyceraldehyde, dihydroxyacetone, 1,6-anhydroglucose,

pyruvaldehyde, and 5-hydroxymethylfurfural (5-HMF). The presence of numerous molecules containing furan rings, such as 5-HMF, illustrates the potential to form char via ionic mechanisms near the critical point. In a similar study, Aida et al. [14] gasified glucose at 350–400°C and residence times of 0.2–1.7 s with the explicit goal of optimizing furfural and 5-HMF yields. At these relatively low temperatures and

glycolaldehyde, glyceraldehyde, hydroxyacetone, 5-HMF, and furfural. Promdej and Matsumura [34] gasified glucose in the 300–460°C range, proposing Arrhenius parameters for the decomposition reaction (**Table 1**). Reported products include 5- HMF, furfural, and char, with subcritical temperatures favoring char formation.

short residence times, products were identified as fructose, erythrose,

**3.2 Glucose and fructose**

**Compound Temperature**

*Advanced Supercritical Fluids Technologies*

**range (°C)**

*First-order Arrhenius parameters of model compound gasification.*

**Pre-exponential factor "A" (s<sup>1</sup> )**

Phenol <sup>370</sup>–<sup>450</sup> 7.72 <sup>10</sup><sup>1</sup> 53.06 [29] Benzene <sup>370</sup>–<sup>450</sup> 2.78 104 91.16 [29] Guaiacol <sup>300</sup>–<sup>450</sup> 6.52 101 32.40 [30] Glucose <sup>300</sup>–<sup>460</sup> 6.9 <sup>10</sup><sup>7</sup> 95.54 [34] Glucose <sup>750</sup>–<sup>800</sup> 1.2 <sup>10</sup><sup>3</sup> <sup>70</sup> [36] Methanol 450–650 Not reported 191 [40] Glycerol 445–600 Not reported 145 [44] Glycerol 450–650 Not reported 196 [40] Glycine <sup>200</sup>–<sup>340</sup> 3.51 <sup>10</sup><sup>13</sup> <sup>166</sup> [48] Glycine <sup>250</sup>–<sup>450</sup> 3.6 <sup>10</sup><sup>11</sup> <sup>160</sup> [15] Alanine <sup>200</sup>–<sup>340</sup> 2.65 <sup>10</sup><sup>12</sup> <sup>154</sup> [50] Alanine <sup>250</sup>–<sup>450</sup> 1.4 <sup>10</sup><sup>12</sup> <sup>156</sup> [15] Serine <sup>200</sup>–<sup>340</sup> 9.85 <sup>10</sup><sup>12</sup> <sup>149</sup> [50] Aspartic acid <sup>200</sup>–<sup>340</sup> 5.40 <sup>10</sup><sup>13</sup> <sup>148</sup> [50] Formic acid <sup>320</sup>–<sup>420</sup> 1.58 <sup>10</sup><sup>6</sup> 85.8 [52]

**Activation energy "EA" (kJ/mol)**

**Source**

20 s at 600°C [31].

**120**

Formic acid (subcritical)

Formic acid (supercritical)

**Table 1.**

Several studies have investigated methanol decomposition in SCW, due to its chemical simplicity and prevalence as a refractory intermediate gasification product. Boukis et al. [39] reformed methanol in a continuous SCWR a T = 400–600°C, for residence times of 3–100 s, and initial methanol concentrations from 5 to 64 wt %. Gaseous yields of H2, CO, CO2, and trace CH4 were detected, with higher temperatures and residence times increasing conversion rates. Bennekom et al. [40] also gasified methanol in a continuous reactor at T = 450–650°C and residence times of 6–173 s. H2, CO, and CO2 were reported as main products, with trace yields of methane, formaldehyde, and formic acid. The results show that methanol reforms to H2 and CO in SCW, with CO converted to CO2 by the WGS reaction.

Limited investigations are available related to the conversion of ethanol to gaseous products in SCW. Schanzenbacher et al. [41] subjected ethanol to SCW in a continuous reactor at temperatures from 433 to 494°C, for residence times from 2

to 12 s. Maximum conversion of 16.5% was observed, with acetaldehyde identified as the only reaction by-product. Pinkard et al. [26] gasified ethanol in a continuous SCWR at 560°C for residence times of 3–8 s, identifying reaction products as H2, CO, CO2, ethylene, ethane, and acetaldehyde.

with the work of Klinger et al. [15], which identified major reaction products as lactic acid, ethylamine, acetaldehyde, and CO2 from alanine decomposition at 250– 450°C. Arrhenius rate parameters from both studies are presented in **Table 1** and show good agreement, likely due to the similar reactor configuration and tempera-

Several studies also investigated the gasification kinetics of more complex amino acids, such as valine, leucine, proline, serine, and aspartic acid to determine the effect of the amino acid functional group on conversion rates and final product yields. Arrhenius parameters for the decomposition of serine and aspartic acid are presented in **Table 1**. The activation energies determined for all amino acids are within a similar range, showing that functional group has only a minor effect on the

Some intermediate SCWG products are common across a range of feedstocks, notably aldehydes and organic acids. Formic acid has been identified as an intermediate product from SCWG of glucose [35], fructose [37], and glycine [48]. Yu and Savage [52] studied formic acid gasification in a continuous SCWR to understand the decomposition pathways and rates. Temperatures were varied between 320 and 500°C, pressures between 18 and 30.7 MPa, and residence times between 1.4 and 80 s. Major products were consistently identified as H2 and CO2, with minor yields of CO. Overall, this suggests the dominance of a decarboxylation pathway with a minor dehydration pathway. Arrhenius parameters for formic acid decomposition are presented in **Table 1**. Zhang et al. [53] also gasified formic acid in a continuous SCWR at temperatures between 550 and 650°C for residence times between 16 and 46 s. Again, H2 and CO2 were present as dominant reaction products, with minor CO yields. Trace yields of formaldehyde and methanol were also

Pinkard et al. [5] gasified formic acid in a continuous SCWR at T = 300–430°C and residence times in the range of 4–65 s. In situ Raman spectroscopy was used to calculate kinetic rates and Arrhenius parameters. It was found that the transition across the critical point increased the reaction rate, favoring the production of H2 and CO2 via the decarboxylation reaction pathway. The notable change in reaction rate across the critical point indicates the importance of both ionic and free radical reactions to the overall decomposition of formic acid in SCW. Arrhenius parameters for formic acid decomposition in subcritical and supercritical water are presented in

It is beyond the scope of this chapter to review the body of work investigating various catalysts for SCWG. We direct the reader to the comprehensive reviews on the subject [2, 3]. However, it is worth mentioning some broad findings from this

Numerous alkali metal compounds (some naturally present in biomass) have been investigated as suitable gasification catalysts. Broadly, these tend to enhance decomposition and increase gaseous product yields. A major disadvantage to adding salts is the persistent corrosion issue, as salts are insoluble in SCW, leading to precipitation of a molten or solid salt layer on the reactor walls. This scaling rapidly corrodes reactor components, increasing the frequency of component replacement. Noble metals are effective gasification catalysts, with nickel and ruthenium generally accepted as the most promising and effective across a wide range of

ture range used in the two studies.

*DOI: http://dx.doi.org/10.5772/intechopen.90503*

overall conversion rates of amino acids in SCW.

*Gasification Kinetics in Continuous Supercritical Water Reactors*

**3.5 Other relevant model compounds**

reported at the highest tested temperatures.

**Table 1**.

**123**

**3.6 Gasification catalysts**

extensive area of research.

Glycerol has been extensively studied due to its abundance as a by-product of biodiesel production. Reforming of this low-cost and widely available feedstock could be a source of inexpensive renewable H2. Buhler et al. [42] reported intermediate products from glycerol gasification to include methanol, acetaldehyde, allyl alcohol, propionaldehyde, acrolein, ethanol, formaldehyde, and standard gaseous products. Non-Arrhenius decomposition behavior was observed, which was attributed to competing ionic and free radical reaction pathways. May et al. [43] reported acetaldehyde, hydroxyacetone, and acetic acid as major products from glycerol gasification in a continuous SCWR. Guo et al. [44] gasified glycerol at T = 445–600° C, reporting high conversion rates after 9 s at 600°C. The activation energy for decomposition was determined and is presented in **Table 1**. Bennekom et al. reported the activation energy for glycerol decomposition, for temperatures of 450– 650°C, residence times of 6–173 s, and initial concentrations from 5 to 20 wt%.

### **3.4 Amino acids**

Hydrothermal decomposition of amino acids has been studied in the context of valorizing protein-rich wastes from agriculture and seafood processing. Glycine, alanine, and leucine have all been identified as intermediate products from fish waste decomposition in subcritical water [45]. Islam et al. [46] also identified glycine and alanine as intermediate compounds from the decomposition of more complex amino acids, making them ideal model compounds for protein-rich waste.

Samanmulya and Matsumura [47] gasified glycine from 500 to 650°C with 1.0, 3.0, and 5.0 wt% initial concentrations. Higher temperatures and lower initial concentrations increased gasification efficiency; higher yields of char and tar were thought to decrease carbon conversion at higher concentrations.

Sato et al. [48] measured the decomposition of glycine at subcritical temperatures of 200–340°C. Gaseous yields were low, but liquid products were identified as ammonia, methylamine, glycolic acid, and formic acid. At subcritical conditions, ionic mechanisms likely facilitated the decomposition reactions. Klinger et al. [15] gasified glycine at subcritical and supercritical temperatures from 250 to 450°C. Strong temperature dependence was observed, with key liquid products identified as methylamine, diketopiperazine, glycolic acid, and formaldehyde. The decomposition rates for both studies are presented in **Table 1**. The slight discrepancy in observed products and reported reaction parameters is likely due to the different experimental temperature ranges; at supercritical temperatures, ionic mechanisms are suppressed, while free radical mechanisms are enhanced.

Samanmulya et al. [49] gasified alanine from 500 to 650°C with 1.0, 2.0, and 3.0 wt% initial concentrations. Results showed no change in gasification efficiency for varied initial concentrations, a positive indication of first-order reaction kinetics. Carbon conversion rates were found to be similar to those determined for glycine gasification, indicating that both amino acids likely react via similar free radical reaction mechanisms in high-temperature SCW. However, the gaseous yield from glycine was found to be rich in H2 and CO2, while the gaseous yield from alanine contained much more CO and CH4. This can be attributed to the methyl (CH3) group present in the alanine molecule, which likely reacts to form CH4.

Sato et al. [50] investigated the decomposition of alanine at subcritical temperatures from 200 to 340°C major reaction products were identified as ammonia, ethylamine, carbonic acid, lactic acid, and pyruvic acid, which is in good agreement *Gasification Kinetics in Continuous Supercritical Water Reactors DOI: http://dx.doi.org/10.5772/intechopen.90503*

with the work of Klinger et al. [15], which identified major reaction products as lactic acid, ethylamine, acetaldehyde, and CO2 from alanine decomposition at 250– 450°C. Arrhenius rate parameters from both studies are presented in **Table 1** and show good agreement, likely due to the similar reactor configuration and temperature range used in the two studies.

Several studies also investigated the gasification kinetics of more complex amino acids, such as valine, leucine, proline, serine, and aspartic acid to determine the effect of the amino acid functional group on conversion rates and final product yields. Arrhenius parameters for the decomposition of serine and aspartic acid are presented in **Table 1**. The activation energies determined for all amino acids are within a similar range, showing that functional group has only a minor effect on the overall conversion rates of amino acids in SCW.

### **3.5 Other relevant model compounds**

to 12 s. Maximum conversion of 16.5% was observed, with acetaldehyde identified as the only reaction by-product. Pinkard et al. [26] gasified ethanol in a continuous SCWR at 560°C for residence times of 3–8 s, identifying reaction products as H2,

Glycerol has been extensively studied due to its abundance as a by-product of biodiesel production. Reforming of this low-cost and widely available feedstock could be a source of inexpensive renewable H2. Buhler et al. [42] reported intermediate products from glycerol gasification to include methanol, acetaldehyde, allyl alcohol, propionaldehyde, acrolein, ethanol, formaldehyde, and standard gaseous products. Non-Arrhenius decomposition behavior was observed, which was attributed to competing ionic and free radical reaction pathways. May et al. [43] reported acetaldehyde, hydroxyacetone, and acetic acid as major products from glycerol gasification in a continuous SCWR. Guo et al. [44] gasified glycerol at T = 445–600° C, reporting high conversion rates after 9 s at 600°C. The activation energy for decomposition was determined and is presented in **Table 1**. Bennekom et al. reported the activation energy for glycerol decomposition, for temperatures of 450– 650°C, residence times of 6–173 s, and initial concentrations from 5 to 20 wt%.

Hydrothermal decomposition of amino acids has been studied in the context of valorizing protein-rich wastes from agriculture and seafood processing. Glycine, alanine, and leucine have all been identified as intermediate products from fish waste decomposition in subcritical water [45]. Islam et al. [46] also identified glycine and alanine as intermediate compounds from the decomposition of more complex amino acids, making them ideal model compounds for protein-rich waste. Samanmulya and Matsumura [47] gasified glycine from 500 to 650°C with 1.0,

3.0, and 5.0 wt% initial concentrations. Higher temperatures and lower initial concentrations increased gasification efficiency; higher yields of char and tar were

Sato et al. [48] measured the decomposition of glycine at subcritical temperatures of 200–340°C. Gaseous yields were low, but liquid products were identified as ammonia, methylamine, glycolic acid, and formic acid. At subcritical conditions, ionic mechanisms likely facilitated the decomposition reactions. Klinger et al. [15] gasified glycine at subcritical and supercritical temperatures from 250 to 450°C. Strong temperature dependence was observed, with key liquid products identified as methylamine, diketopiperazine, glycolic acid, and formaldehyde. The decomposition rates for both studies are presented in **Table 1**. The slight discrepancy in observed products and reported reaction parameters is likely due to the different experimental temperature ranges; at supercritical temperatures, ionic mechanisms

Samanmulya et al. [49] gasified alanine from 500 to 650°C with 1.0, 2.0, and 3.0 wt% initial concentrations. Results showed no change in gasification efficiency for varied initial concentrations, a positive indication of first-order reaction kinetics. Carbon conversion rates were found to be similar to those determined for glycine gasification, indicating that both amino acids likely react via similar free radical reaction mechanisms in high-temperature SCW. However, the gaseous yield from glycine was found to be rich in H2 and CO2, while the gaseous yield from alanine contained much more CO and CH4. This can be attributed to the methyl (CH3) group present in the alanine molecule, which likely reacts to form CH4. Sato et al. [50] investigated the decomposition of alanine at subcritical temperatures from 200 to 340°C major reaction products were identified as ammonia, ethylamine, carbonic acid, lactic acid, and pyruvic acid, which is in good agreement

thought to decrease carbon conversion at higher concentrations.

are suppressed, while free radical mechanisms are enhanced.

CO, CO2, ethylene, ethane, and acetaldehyde.

*Advanced Supercritical Fluids Technologies*

**3.4 Amino acids**

**122**

Some intermediate SCWG products are common across a range of feedstocks, notably aldehydes and organic acids. Formic acid has been identified as an intermediate product from SCWG of glucose [35], fructose [37], and glycine [48].

Yu and Savage [52] studied formic acid gasification in a continuous SCWR to understand the decomposition pathways and rates. Temperatures were varied between 320 and 500°C, pressures between 18 and 30.7 MPa, and residence times between 1.4 and 80 s. Major products were consistently identified as H2 and CO2, with minor yields of CO. Overall, this suggests the dominance of a decarboxylation pathway with a minor dehydration pathway. Arrhenius parameters for formic acid decomposition are presented in **Table 1**. Zhang et al. [53] also gasified formic acid in a continuous SCWR at temperatures between 550 and 650°C for residence times between 16 and 46 s. Again, H2 and CO2 were present as dominant reaction products, with minor CO yields. Trace yields of formaldehyde and methanol were also reported at the highest tested temperatures.

Pinkard et al. [5] gasified formic acid in a continuous SCWR at T = 300–430°C and residence times in the range of 4–65 s. In situ Raman spectroscopy was used to calculate kinetic rates and Arrhenius parameters. It was found that the transition across the critical point increased the reaction rate, favoring the production of H2 and CO2 via the decarboxylation reaction pathway. The notable change in reaction rate across the critical point indicates the importance of both ionic and free radical reactions to the overall decomposition of formic acid in SCW. Arrhenius parameters for formic acid decomposition in subcritical and supercritical water are presented in **Table 1**.

### **3.6 Gasification catalysts**

It is beyond the scope of this chapter to review the body of work investigating various catalysts for SCWG. We direct the reader to the comprehensive reviews on the subject [2, 3]. However, it is worth mentioning some broad findings from this extensive area of research.

Numerous alkali metal compounds (some naturally present in biomass) have been investigated as suitable gasification catalysts. Broadly, these tend to enhance decomposition and increase gaseous product yields. A major disadvantage to adding salts is the persistent corrosion issue, as salts are insoluble in SCW, leading to precipitation of a molten or solid salt layer on the reactor walls. This scaling rapidly corrodes reactor components, increasing the frequency of component replacement.

Noble metals are effective gasification catalysts, with nickel and ruthenium generally accepted as the most promising and effective across a wide range of

compounds and reaction regimes. Typically, metal catalysts are impregnated in a support compound (e.g., activated carbon, γ-Al2O3), after which they are crushed and loaded into a packed bed reactor. Both nickel and ruthenium are effective at cleaving C-C bonds, reducing char formation. However, issues exist with catalyst stability, longevity, and process economics. Nickel suffers from sintering and deactivation as carbon layers tend to accumulate on the catalytic surface, while ruthenium catalysts are expensive and can be poisoned by the presence of sulfur. More research is needed toward an economically viable catalyst for SCWG.

### **4. Discussion and opportunities for further exploration**

In general, conversion rates for gasification of organic compounds in SCW can be improved by increasing the temperature and the residence time and by decreasing the initial feedstock concentration. Amino acids, carbohydrates, and simple organic acids are the compound classes with the fastest decomposition rates, with aromatic compounds and alcohols being the most recalcitrant compounds. Arrhenius plots with all mentioned compounds are presented in **Figures 2** and **3** for comparison between compound classes.

Additional studies are needed to understand chemical reactions in supercritical water. It is likely that key functional groups will behave similarly under SCW conditions and further experimentation and interpretation are needed to describe the reactions routes and rates. In situ product identification has the potential to provide accurate data for characterization of decomposition pathways. Special attention should be given to char-forming compounds, in order to understand the mechanisms leading to char formation and the conditions required to promote gasification. Additionally, the role of both homogeneous and heterogeneous catalysts in affecting reaction rates, pathways, and mechanisms should be explored and quantified.

**5. Conclusions**

**Figure 3.**

**Acknowledgements**

work.

water.

**125**

reaction behavior of more complex feedstocks.

*Decomposition rates of model amino acids in sub- and supercritical water.*

*Gasification Kinetics in Continuous Supercritical Water Reactors*

*DOI: http://dx.doi.org/10.5772/intechopen.90503*

Supercritical water gasification promises to revolutionize the processing of waste streams to value-adding gaseous fuels. Technical barriers remain between the current state-of-the-art and widespread industrial adoption of the technology, several of which can be addressed through studying the chemistry of model compounds in supercritical water. The knowledge gained in these studies can be applied toward developing reaction pathways and mechanisms, lending insight toward

The authors would like to recognize funding provided by the DOD Defense Threat Reduction Agency (DTRA), Grant HDTRA1-17-1-0001. Special thanks to the University of Washington for providing resources toward the completion of this

Additional thanks to David Gorman, Kartik Tiwari, Elizabeth Rasmussen, Vedant Maheshwari, Anmol Purohit, Stuart Moore, Eric Molnar, Justin Davis, and other members of the Novosselov Research Group at the University of Washington who contributed to advancing knowledge of chemical reactions in supercritical

**Figure 2.** *Decomposition rates of model compounds in sub- and supercritical water.*

*Gasification Kinetics in Continuous Supercritical Water Reactors DOI: http://dx.doi.org/10.5772/intechopen.90503*

**Figure 3.** *Decomposition rates of model amino acids in sub- and supercritical water.*
