**1. Introduction**

Supercritical water (SCW) exhibits unique physiochemical properties beneficial for oxidation or gasification of organic (carbon-containing) compounds ranging from simple molecules to complex heterogeneous waste. Near the critical point (374°C, 22.1 MPa), water exists in a high-temperature, dense fluid phase with high concentrations of H<sup>+</sup> and OH ions. These conditions serve to facilitate enhanced ionic chemistry and acid-catalyzed reactions [1]. At temperatures above the critical point, the density, viscosity, and ion product of water drop significantly. In this high-temperature, low-density phase water exhibits superb mass transfer properties, and organic compounds become fully miscible and/or soluble [2]. However,

ionic reactions are no longer favored. Instead, pyrolysis, hydrolysis, and free radical reaction mechanisms dominate in this higher-temperature region. These two overlapping reaction regimes explain why SCW is of interest as a reaction medium for applications related to the thermochemical conversion of organic waste into heat and/or gaseous fuel.

improved, heat recovery can be utilized, and opportunities exist for in situ process

Continuous SCWRs are nearly always manufactured from nickel-base alloys (e.g., Inconel 625, Hastelloy C-276), which offer excellent corrosion resistance and good material strength at high temperatures. However, it is important to mention that nickel-base alloys also provide a catalytic surface for gasification reactions. For this reason, reaction rate parameters determined using batch SCWRs are not applicable for continuous SCWRs. DiLeo and Savage demonstrated this by gasifying methanol with and without a nickel wire in a batch quartz capillary reactor. The nickel wire increased methanol conversion from 20% after 2 h to 90% after 5 min (both at 550°C) [4]. Continuous reactors can enhance the catalytic effect even

It should be noted that the catalytic effects within continuous reactors are dependent on reactor geometry and "aging" of the reactor components. Smaller diameter reactors with high surface-to-volume ratios (S/V) show increased catalytic effects due to increased molecular interaction between the reagents and the reactor wall. Also, reactor aging leads to the decreased catalytic activity over time, due to the formation of carbon layers on the reactor wall, leaching of metals, sintering,

For these reasons, this section will primarily focus on common designs of continuous reactors at the lab-scale. A representative schematic of a continuous SCWR

The most reliable way to achieve independent pressure and mass flow control in a continuous SCWR is to operate a constant flow rate pump(s) in series with a back pressure regulator (BPR). Spring-loaded or dome-loaded BPRs are simple to use and reliable [3, 5]. High-performance liquid chromatography (HPLC) pumps are often used for pumping liquid reagents to high pressures with precise flow rate control in

*Representative schematic of a continuous supercritical water gasification reactor with post-critical reagent*

monitoring and control.

further, especially in turbulent flow regimes.

*Gasification Kinetics in Continuous Supercritical Water Reactors*

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

catalytic deactivation, and other effects.

is provided in **Figure 1**.

**Figure 1.**

**115**

*injection and* in situ *monitoring.*

**2.2 Heating and pressurization**

An understanding of chemical reaction rates, pathways, and mechanisms involved in decomposing model compounds in SCW sheds important insight into the reaction chemistry of complex organic molecules in SCW. Interest in supercritical water gasification (SCWG) for industrial-scale applications is growing due to increased interest in generating low-cost "green" H2 from renewable feedstocks. However, there are a number of technical barriers in developing large-scale plants; these include (i) controlling char formation, (ii) limiting salt precipitation which rapidly corrodes reactor components, (iii) identifying the optimal process parameters for high conversion efficiency (CE), (iv) identifying suitable gasification catalysts, and (v) designing an effective heat exchanger for waste heat recovery. Studies of model compounds can aid in addressing some of these challenges.

This chapter summarizes previous studies investigating reaction chemistry in continuous supercritical water reactors (SCWRs). Common reactor designs used to investigate reaction chemistry are discussed, enabling the researchers to replicate or to extend the knowledge of the previous studies. A synthesis of these studies yields important insights into common reaction mechanisms, pathways, and decomposition rates of certain compound classes. Opportunities for further investigations are described, and the practical value of these studies is highlighted.

## **2. Continuous supercritical water reactors for investigating reaction chemistry at the laboratory-scale**

Achieving high temperatures and pressures, mitigating corrosion of reactor components, rapid heating and quenching of the reagent, acquiring accurate experimental data, and strategies for achieving a well-mixed, uniform flow must all be considered in the design of a supercritical water reactor for studies of reaction chemistry. Solutions for mitigating some of these challenges have been reported in the literature, but open questions remain regarding the best methods to mitigate char formation and salt precipitation in reactors designed to process complex feedstocks [3].

### **2.1 Batch vs. continuous reactors**

Studies of reaction chemistry in SCW have been conducted using both batch and continuous reactors at the lab-scale. Batch reactors offer a unique opportunity to study reaction kinetics, mechanisms, and pathways of model compounds in the absence of a catalytic surface. The reactor can be constructed using a number of materials, including quartz capillaries and stainless steel tubing, which are filled with reactants and heated to reaction temperatures in a fluidized bath or electric furnace. Reactions occur at fixed conditions for the desired residence time, after which the reaction is quenched, and products are recovered for ex situ analysis. One limitation of batch reactors is that mass transfer (and, therefore, molecular interaction) is limited by molecular diffusion rates.

Continuous reactors are more complicated and expensive to fabricate. For industrial applications of chemical processes, a continuous setup is preferred over a batch setup. Process throughput is much higher, energy efficiency is significantly

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

ionic reactions are no longer favored. Instead, pyrolysis, hydrolysis, and free radical reaction mechanisms dominate in this higher-temperature region. These two overlapping reaction regimes explain why SCW is of interest as a reaction medium for applications related to the thermochemical conversion of organic waste into heat

An understanding of chemical reaction rates, pathways, and mechanisms involved in decomposing model compounds in SCW sheds important insight into the reaction chemistry of complex organic molecules in SCW. Interest in supercritical water gasification (SCWG) for industrial-scale applications is growing due to increased interest in generating low-cost "green" H2 from renewable feedstocks. However, there are a number of technical barriers in developing large-scale plants; these include (i) controlling char formation, (ii) limiting salt precipitation which rapidly corrodes reactor components, (iii) identifying the optimal process parameters for high conversion efficiency (CE), (iv) identifying suitable gasification catalysts, and (v) designing an effective heat exchanger for waste heat recovery. Studies

This chapter summarizes previous studies investigating reaction chemistry in continuous supercritical water reactors (SCWRs). Common reactor designs used to investigate reaction chemistry are discussed, enabling the researchers to replicate or to extend the knowledge of the previous studies. A synthesis of these studies yields important insights into common reaction mechanisms, pathways, and decomposition rates of certain compound classes. Opportunities for further investigations are

**2. Continuous supercritical water reactors for investigating reaction**

Achieving high temperatures and pressures, mitigating corrosion of reactor components, rapid heating and quenching of the reagent, acquiring accurate experimental data, and strategies for achieving a well-mixed, uniform flow must all be considered in the design of a supercritical water reactor for studies of reaction chemistry. Solutions for mitigating some of these challenges have been reported in the literature, but open questions remain regarding the best methods to mitigate char formation and salt precipitation in reactors designed to process complex feed-

Studies of reaction chemistry in SCW have been conducted using both batch and continuous reactors at the lab-scale. Batch reactors offer a unique opportunity to study reaction kinetics, mechanisms, and pathways of model compounds in the absence of a catalytic surface. The reactor can be constructed using a number of materials, including quartz capillaries and stainless steel tubing, which are filled with reactants and heated to reaction temperatures in a fluidized bath or electric furnace. Reactions occur at fixed conditions for the desired residence time, after which the reaction is quenched, and products are recovered for ex situ analysis. One limitation of batch reactors is that mass transfer (and, therefore, molecular interac-

Continuous reactors are more complicated and expensive to fabricate. For industrial applications of chemical processes, a continuous setup is preferred over a batch setup. Process throughput is much higher, energy efficiency is significantly

of model compounds can aid in addressing some of these challenges.

described, and the practical value of these studies is highlighted.

**chemistry at the laboratory-scale**

**2.1 Batch vs. continuous reactors**

tion) is limited by molecular diffusion rates.

stocks [3].

**114**

and/or gaseous fuel.

*Advanced Supercritical Fluids Technologies*

improved, heat recovery can be utilized, and opportunities exist for in situ process monitoring and control.

Continuous SCWRs are nearly always manufactured from nickel-base alloys (e.g., Inconel 625, Hastelloy C-276), which offer excellent corrosion resistance and good material strength at high temperatures. However, it is important to mention that nickel-base alloys also provide a catalytic surface for gasification reactions. For this reason, reaction rate parameters determined using batch SCWRs are not applicable for continuous SCWRs. DiLeo and Savage demonstrated this by gasifying methanol with and without a nickel wire in a batch quartz capillary reactor. The nickel wire increased methanol conversion from 20% after 2 h to 90% after 5 min (both at 550°C) [4]. Continuous reactors can enhance the catalytic effect even further, especially in turbulent flow regimes.

It should be noted that the catalytic effects within continuous reactors are dependent on reactor geometry and "aging" of the reactor components. Smaller diameter reactors with high surface-to-volume ratios (S/V) show increased catalytic effects due to increased molecular interaction between the reagents and the reactor wall. Also, reactor aging leads to the decreased catalytic activity over time, due to the formation of carbon layers on the reactor wall, leaching of metals, sintering, catalytic deactivation, and other effects.

For these reasons, this section will primarily focus on common designs of continuous reactors at the lab-scale. A representative schematic of a continuous SCWR is provided in **Figure 1**.

### **2.2 Heating and pressurization**

The most reliable way to achieve independent pressure and mass flow control in a continuous SCWR is to operate a constant flow rate pump(s) in series with a back pressure regulator (BPR). Spring-loaded or dome-loaded BPRs are simple to use and reliable [3, 5]. High-performance liquid chromatography (HPLC) pumps are often used for pumping liquid reagents to high pressures with precise flow rate control in

### **Figure 1.**

*Representative schematic of a continuous supercritical water gasification reactor with post-critical reagent injection and* in situ *monitoring.*

the range of 0.01–30 mL/min [6–9]. Diaphragm, syringe, and piston pumps can be employed when higher flow rates are needed or when pumping a slurry [7, 10].

is a need to study the effect of feedstock type and concentration on char formation

*Gasification Kinetics in Continuous Supercritical Water Reactors*

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

For studying reaction chemistry in a lab-scale SCWR, the mixing strategy used to introduce the reagent into the SCW environment should be carefully considered. Mixing can be achieved by (i) premixing water and reagent before heating to supercritical conditions or (ii) injecting reagent directly into supercritical water. If chemical kinetic rates are sought, post-critical injection is a preferred mixing strategy, as it rapidly heats the reagent to reaction temperatures and establishes a

Premixing is required if the feedstock is solid or viscous and must be pumped as an emulsion or when high reagent loading is considered. Premixed reagents should be rapidly heated, as char and tar formation can be significant when reagents are

The vast majority of SCWG studies rely on ex situ product analysis to quantify yields and determine reaction pathways. Several ex situ techniques exist for analyzing gaseous, liquid, and solid products; for properly characterizing full reaction networks and kinetic rates, all reaction products must be identified and quantified for each experimental condition. Gaseous products (H2, CO, CO2, CH4) are often identified and quantified using gas chromatography (GC) with a thermal conductivity detector (TCD) and flame ionization detector (FID). Liquid products may be identified using HPLC, nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, or Raman spectroscopy. However, some researchers prefer to report the total organic carbon (TOC) concentration in the liquid phase by using a TOC analyzer, which is sufficient for calculating carbon CE. Occasionally solid products are analyzed ex situ, using scanning electron microscopy (SEM), proton-induced X-ray emission (PIXE), Raman spectroscopy, or FTIR

In-line effluent analysis methods are available, such as GC and TOC analysis. These methods may lack the sensitivity and specificity required for the determination of chemical rates, but they can provide real-time input for process control. Alternatively, in situ product analysis greatly speeds the collection of the experimental data. In situ Raman spectroscopy is one of the most promising in situ analysis methods, as it is particularly well-suited for analyzing aqueous mixtures [25]. Water has a strong fluorescence and infrared signal, but a weak Raman signal, allowing product species to be identified and quantified [5]. For example, immersion in situ Raman spectroscopy was used to analyze formic acid decomposition [5], the conversion of ethanol to fuel gas [26], and the oxidation of methanol and isopropyl alcohol in SCW [27, 28]. Note that any spectroscopic methods are sus-

From a system-level perspective, effective SCWG is best described as a complete conversion of the mass and energy content of the original feedstock into gaseous products. Three performance metrics are commonly used to quantify this conversion: (i) gasification efficiency (GE), (ii) carbon CE, and (iii) hydrogen efficiency

ceptible to fouling of the optical access point in the system.

rates and char morphology.

**2.5 Reagent mixing strategies**

definite reaction start time [24].

**2.6 Reactor monitoring and data acquisition**

heated slowly [6, 16].

spectroscopy [3].

**2.7 Performance metrics**

**117**

Reagent heating is important to consider as water can exhibit complex heat transfer characteristics near the critical point. Enhanced or deteriorated heat transfer can occur due to a combination of rapidly changing thermophysical properties and factors such as reactor geometry and the ratio of mass flux to heat flux. Generally, deteriorated heat transfer can be avoided by installing a downwardoriented heating section, which takes advantage of buoyancy effects for more efficient heating. A coiled heating section with a small diameter for high S/V is also desirable for improved heat transfer near the critical point [11, 12].

Resistive heaters, electric furnaces, and immersive fluidized baths have been used to reach the desired reaction temperatures [5, 7, 10, 13–15]. Resistive cartridge heaters are attractive options for preheating, as the tubing can be tightly wound around the cartridge to minimize heat loss. Electric furnaces offer precise control, are well-insulated, and are easy to install. A fluidized bath is a great option for maintaining isothermal conditions in the reactor section but can be expensive and bulky. Some combination of these heating methods is generally sufficient to achieve (a) rapid and efficient heating past the critical point and (b) isothermal reactor conditions.

### **2.3 Corrosion mitigation**

Corollary to its ability to rapidly decompose organic compounds, supercritical water is extremely corrosive to most metals and metal alloys, especially if alkali metals or halogens are present. Thus, corrosion mitigation strategies need to be considered during SCWR design. Many studies have focused on corrosion control methods in SCWRs and SCW heat exchangers [16–18]. Generally, four corrosion mitigation strategies have been proposed and are thoroughly discussed in a review by Marrone et al. [16]. These are (i) preventing corrosive species from interacting with the reactor surface, (ii) forming a corrosion-resistant barrier, (iii) manufacturing the reactor from materials resistant to corrosion, and (iv) tuning operating conditions to minimize severe corrosion conditions. For reactors used to study reaction chemistry of organic compounds that do not contain heteroatoms, it is generally sufficient to rely on the corrosion resistance of the reactor material.

### **2.4 Mitigating char formation**

Char has been reported as a common recalcitrated product formed during the gasification of aromatic compounds or homogeneous biomass components, such as lignin and cellulose [19–23]. Char can rapidly clog reactors, and it should be avoided or suppressed if possible. Broadly, char yields are known to decrease in the presence of certain metal catalysts (such as nickel and ruthenium), which are thought to effectively cleave C–C bonds in the aromatic rings of polycyclic aromatic hydrocarbons (PAHs). Many open questions remain surrounding the exact mechanisms responsible for char formation in SCW. Multiple studies have confirmed that char formation rates are highly dependent on temperature and the initial feedstock concentration. The literature suggests that ionic mechanisms near the critical point are responsible for charring and coking from compounds such as glucose, fructose, and cellulose [20], while free radical mechanisms form char during SCWG of aromatic compounds at higher temperatures, such as phenol, benzene, and lignin [21, 22]. While the industrial implementation of SCWG would require a method for suppressing char formation at high feedstock loadings, researchers can circumvent this issue by performing experiments with low feedstock concentrations [19]. There is a need to study the effect of feedstock type and concentration on char formation rates and char morphology.
