**2.7 Performance metrics**

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

(HE). GE is defined as the ratio of the total mass of the gaseous product to the initial mass of the feedstock, expressed mathematically as:

$$GE(\text{\%}) = \frac{\mathbb{X}\_{H\_2} + \mathbb{X}\_{CO} + \mathbb{X}\_{CO\_2} + \mathbb{X}\_{CH\_4} + \mathbb{X}\_{Gas, other}}{\mathbb{X}\_{feed; clock}} \* \mathbf{100} \tag{1}$$

the operating conditions required to upgrade heterogeneous biomass into high-

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

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

Methanation serves to reduce H2 yields by converting it to methane, via the

Overall, both the WGS and methanation reactions are highly important to the

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

the reaction temperature and the concentration of aromatics.

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

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

*CO* þ *H*2*O* \$ *CO*<sup>2</sup> þ *H*<sup>2</sup> (5)

*CO* þ 3*H*<sup>2</sup> \$ *CH*<sup>4</sup> þ *H*2*O* (6) *CO*<sup>2</sup> þ 4*H*<sup>2</sup> \$ *CH*<sup>4</sup> þ 2*H*<sup>2</sup> (7)

value fuels, such as "green" hydrogen.

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

*Gasification Kinetics in Continuous Supercritical Water Reactors*

specified otherwise.

following two pathways:

final gaseous product composition.

**3.1 Aromatic compounds**

surrogates.

**119**

expressed as:

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 to moles of carbon in the feedstock:

$$\text{CE}(\%) = \frac{n\_{\text{CO}} + n\_{\text{CO}\_2} + n\_{\text{CH}\_4} + \infty n\_{\text{C}\_4H\_y}}{n\_{\text{C}\_4 \text{feed stock}}} \ast 100 \tag{2}$$

A less frequently used metric is HE, defined as the ratio of moles of hydrogen in the gaseous product to moles of hydrogen in the feedstock:

$$HE(\%) = \frac{2\varkappa\_{H\_2} + 4\varkappa\_{CH\_4} + \mathcal{yn}\_{C\_xH\_y}}{n\_{H\_2\text{feed stock}}} \ast 100\tag{3}$$

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 gas via reaction of CO with water.

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 experimental temperature.

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* curve with the following expression:

$$
\ln\left(k\right) = \ln\left(A\right) - \frac{E\_A}{RT} \tag{4}
$$

This linear curve fit yields the activation energy (*EA*) and pre-exponential factor (*A*) for the first-order decomposition reaction.
