**1. Introduction**

Hydrogen is a clean energy carrier and favorable in relieving pollution, which gained much attention. Hydrogen production from biomass and organic wastes has great potential for sustainable development. Black liquor is the wastewater from the paper-making pulp industry and mainly contains inorganic pulping chemicals (NaOH and Na2CO3), organic components (lignin, hemicellulose, and their derivatives), and water [1]. It has the properties of high alkalinity, high chemical oxygen demand (COD) content, offensive odor, dark caramel color, and high moisture, so it will bring great threat to the environment and human health if not treated properly. Approximately 170 million tons of black liquor solids are generated annually [2]. Traditionally, black liquor is treated by combustion or conventional gasification [3, 4]. Both methods require an energy-intensive evaporation process, which leads to the waste of energy and production of secondary pollutants, such as NOx, SO2, and fine particles. Alternatively, the electrochemical treatment of black liquor is a clean method, but it requires high consumption of valuable electric power [5]. Therefore, more efficient and cleaner technology is desired as an alternative for handling black liquor.

Supercritical water gasification (SCWG) is a promising technology that can convert biomass and wastewater into hydrogen-rich gas. Relying on its unique physicochemical properties, SCW provides an excellent reaction environment for the gasification of biomass and organic wastes. Compared with traditional handling methods of black liquor, this technology has several advantages. For example, it reduces the energy dissipation of evaporation and allows wastewater with high moisture as feedstock, the alkali present in the black liquor can be served as a catalyst, no pollutants are generated, and the products are easy to separate and purify. Therefore, it has attracted the attention of many researchers and much progress has been made in this decade, and many studies have been published on this topic.

In this chapter, recent advances in SCWG of black liquor in recent decades will be reviewed. First, the effect of operating parameters, including temperature, concentration, and wall material, is discussed. In addition, the recovery of alkali salts in black liquor is introduced. Second, the catalysts used in this process and synergetic effect in its co-gasification with coal, biomass, and plastics are presented. Third, the poly-generation system of SCWG of black liquor is evaluated.

### **2. Gasification performance of black liquor**

#### **2.1 Influence of reaction temperature**

Temperature is a key parameter affecting the gas product composition and gasification efficiency in SCWG of black liquor. From the thermodynamic aspects, high temperatures tend to increase gasification efficiency and hydrogen production, while methane dominates the gas products at lower temperatures [6]. We performed thermodynamic analysis of SCWG of black liquor at different temperatures (300–800°C) with a pressure of 25 MPa and found that the temperature can greatly affect the gas composition [7]. CH4 and CO2 were the main components at low reaction temperatures, but their content decreased with increasing temperature. The fraction of H2 increased with temperature, which was above 50% at temperatures over 650°C, and the maximum equilibrium H2 fraction of 61.34% was obtained at 800°C. Gasification efficiency higher than 100% was attributed to the participation of water, which was also increased with temperature. The decrease in HHV (higher heating value) of the gas product with increasing temperature was because the fraction of H2 (HHV = 12.75 MJ/Nm3 ) increased, while the fraction of CH4 (HHV = 39.82 MJ/Nm3 ) decreased.

The experimental study also showed that temperature is a critical factor on the gas composition and yield. Boucard et al. [8] studied the gasification of 10 wt% black liquor at different temperatures (350–450°C, 25 MPa, and 60 min) and found the C content in the gas phase increased from less than 1% at 350°C to more than 25% at 450°C. The H2 yield increased from 2 mol/kg under subcritical conditions to 10–40 mol/kg under supercritical conditions. Previously, we studied SCWG of 9.5 wt% wheat straw black liquor at 400–600°C and 5 kg/h in a continuous flow system [9]. When the temperature was increased from 400°C to 600°C, the gas yield *Recent Advances in Supercritical Water Gasification of Pulping Black Liquor for Hydrogen... DOI: http://dx.doi.org/10.5772/intechopen.105566*

nearly doubled, and gasification efficiency and H2 yield increased from 28.05% and 6.82 mol/kg to 67.89% and 11.26 mol/kg, respectively. The COD concentration and pH decreased from 95,000 mg/L and 11.3 in the raw black liquor to 2160 mg/L and 7.0–7.8 at 600°C, respectively. In addition, the color also changed from dark caramel to clear as pure water (**Figure 1**). SCWG of black liquor at higher temperatures (600–750°C) was investigated in a batch reactor [7]. The H2 fraction increased from 55% to nearly 70% as the temperature increased from 600°C to 750°C. And the maximum carbon gasification efficiency of 94.10% was obtained at 750°C and 30 min.

#### **2.2 Influence of black liquor concentration**

The concentration of weak black liquor produced from pulping varied in the range of 10–20 wt% [10], which can influence the gasification performance. **Figure 2** showed the influence of black liquor concentration on the equilibrium composition, yield, and HHV of the gas product. The effect of concentration on gas composition was almost opposite to temperature. Increasing the concentration greatly decreased the H2 fraction but increased the fraction of carbonaceous gas (CO2, CH4, and CO), so increasing the concentration increased the heating value of the gas product.

In the experiments with both continuous systems and batch reactors, high H2 yields and fractions were obtained from gasification of diluted black liquor. The gas yield

#### **Figure 1.**

*The color changes of black liquor after SCWG in a tubular reactor at different temperatures (concentration = 9.5 wt%; pressure = 25 MPa) [9].*

#### **Figure 2.**

*Equilibrium gas composition (A) and GE and HHV of the gas product (B) of wheat straw black liquor with change of concentration (pressure = 25 MPa; concentration = 9.5 wt %) [7].*

almost doubled when the concentration decreased from 9.5 wt% to 3 wt% at 550°C [7, 11]. Casademont et al. also found that the gas yield and H2 yield increased more than fourfold as the black liquor concentration decreased from 2.43 wt% to 0.81 wt% in SCWG at 600°C [12]. At higher concentrations, reducing the concentration from 20 wt% to 10 wt% increased hydrogen in black liquor converted to H2 from 7.5 to 9.8% at 650°C [1]. Moreover, the H2 yield obtained by gasification of low-concentration black liquor (below 3 wt%) can reach over 24 mol/kg, while the gas products from gasification of above 10 wt% black liquor were mainly composed of C1–C4 hydrocarbons [1, 9, 12]. Therefore, high concentration black liquor is more difficult to gasify. However, a lower concentration of black liquor will improve the system scale and energy loss, so a proper concentration needs to be selected through further detailed investigation.

#### **2.3 Influence of reactor wall material**

In SCWG, water will fill the entire space of the reactor and Ni was an effective catalyst in SCWG [13], so the material of the reactor wall can also affect the gasification performance. Casademont et al. [12] obtained different gasification efficiencies of black liquor in SCWG from the literatures [7, 9], and they attributed the difference mainly to the different reactor materials. They proposed that the reactor made of Inconel 625 was more favorable to gasification than other material. De Blasio et al. [14] investigated SCWG of black liquor in stainless steel 316 and Inconel 625 reactors. It was found that the carbon gasification efficiency obtained was similar in these two reactors, while the hydrogen gasification efficiency obtained in Inconel 625 was much higher than that of the stainless steel 316 reactor at both 600 and 700°C. Hydrogen production with the Inconel 625 reactor was also much higher than that with stainless steel 316 reactor at 600°C. Inconel 625 was slightly better than stainless steel 316 in hydrogen production at 500°C and close to each other at 700°C. Besides, the hot gas efficiency obtained with Inconel 625 reactor was higher than that with stainless steel 316 at 600–700°C, and the maximum value exceeded 80% at 700°C. In a word, the treatment of black liquor in the Inconel 625 reactor can increase hydrogen production and inhibit the formation of tar and char. Özdenkçi et al. [15] investigated the technoeconomic feasibility of SCWG of black liquor in Inconel 625 and stainless steel 316 reactors. It showed that Inconel 625 outperformed stainless steel 316 in terms of energy production, hydrogen production, resistance to pulping chemical losses, and changes in energy prices.

#### **2.4 Influence of alkali in black liquor**

The role of alkali contained in black liquor in the gasification process was investigated by comparing the effect of black liquor and lignin as additives on SCWG of coal [16]. It was found that the fraction of CO decreased significantly, while the content of H2 and CO2 increased when the additive was changed from lignin to black liquor. It was probably because the alkali in black liquor promoted the water-gas shift reaction. The total gas yield of coal with black liquor (12.07 mol/kg) was higher than that of coal with lignin (6.90 mol/kg). Hawangchu et al. [17] investigated the effect of inherited alkali on product distribution by comparing the gasification performance of soda black liquor, kraft black liquor, and lignin compound in SCW. Hydrogen from soda black liquor was always higher than that of lignin, confirming the promotion of alkali on the water-gas shift reaction. They proposed that both the dissolution of organic substances and inhibition of coke production by alkalis promoted gas production.

*Recent Advances in Supercritical Water Gasification of Pulping Black Liquor for Hydrogen... DOI: http://dx.doi.org/10.5772/intechopen.105566*

In addition, Rönnlund et al. [18] studied the effect of the addition of alkalis (KOH, K2CO3, NaOH) and black liquor on SCWG of paper sludge. Similarly, the addition of black liquor improved the hydrogen production and gasification efficiency of paper sludge gasification. Furthermore, the promotion extent of black liquor on gasification was similar to that of alkali salts.

### **3. Recovery of alkali in SCWG of black liquor**

Alkali salts are another main component of black liquor, including cooking chemicals (NaOH and Na2S), as well as derived salts such as Na2CO3, Na2SO4, and Na2SO4 [19]. The organic components in black liquor can be converted in SCWG into gas products, while the inorganic salts still reside in the reactor. Thus, the salts can be recovered and reused in the pulping process as cooking chemicals to reduce the pulp cost. In conventional treatment, the recovery of alkali can be realized by burning the organic components in black liquor [20]. Depending on the unique physicochemical properties of SCW, the alkali can also be recovered during SCWG. The literature showed that the dielectric constant of water decreases sharply with increasing temperature around the critical point [21], which reduces the solubility of inorganic substances and enables the recovery of alkali salts.

Alkali was the main cooking chemical (NaOH), so the recovery of Na+ salts in SCWG of black liquor was investigated [22, 23]. We studied the distribution of alkali during SCWG in a fluidized-bed reactor using glucose as the model compound and attempted to recover the alkali salts [22]. For the extremely low solubility, the Na+ salts were mainly precipitated in SCW and distributed in the reactor during gasification, which were not carried out of the reactor by the fluid in the form of ions [22]. While the fluid temperature in the reactor was reduced close to the critical point after the gasification, the alkali was dissolved in the water and flowed out as the reaction effluent. As a result, the Na<sup>+</sup> content and pH value of the effluent changed dramatically with the temperature in the cooling process of the reactor (**Figure 3**). The Na+ content of the effluent was 10–30 mg/L when the temperature was higher than 360°C, and it increased sharply when the temperature dropped to 355°C and reached a maximum value of 1815 mg/L at 335°C. With flushing of the reactor for a certain time, the Na<sup>+</sup> content dropped to 86 mg/L. Cooling fluid in the range of 360–200°C achieved

**Figure 3.** *The Na+ content of effluent varies with the temperature of the fluid [22].*

a Na+ recovery of 81.07%, and the losses may be attributed to salt entrainment. Based on this result, we proposed an alkali recovery method during SCWG of black liquor, in which the alkalis can be recovered by cooling the reactor after operating for a certain time. And for the fluidized-bed reactor, the velocity of the fluid was important to constrain the alkali solids in the reactor and reduce the alkali salt entrainment and losses. Besides NaOH, the inorganic components of the effluent also contained Na2CO3 and NaHCO3 generated by the reaction of NaOH with CO2. Regeneration from Na2CO3/NaHCO3 to NaOH can be achieved by a conventional method (causticization reaction) with CaO [22]. Some methods can be used to improve the recovery of NaOH in SCWG of black liquor. Fedyaeva et al. [23] studied SCWG of Na2CO3-free black liquor and calculated the mole ratio of 2NaOH/Na2CO3. The ratio could reach 4 mol% without water flow. The amount of produced Na2CO3 can be controlled by pumping SCW through the reactor to carry out CO2 and reduce the residence time of SCW. The ratio can reach 14–18% at 710–750°C and 30 MPa. It is a simple and effective method to achieve highly economic recovery of alkali salts without the addition of chemicals.
