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Recent Advances in Supercritical Water Gasification of Pulping Black Liquor for Hydrogen Production

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Changqing Cao, Lihui Yu, Wenhao Li, Lanjun Liu and Peigao Duan

Submitted: 09 May 2022 Reviewed: 26 May 2022 Published: 07 December 2022

DOI: 10.5772/intechopen.105566

Clean Energy Technologies IntechOpen
Clean Energy Technologies Hydrogen and Gasification Processes Edited by Murat Eyvaz

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Clean Energy Technologies - Hydrogen and Gasification Processes

Edited by Murat Eyvaz, Yongseung Yun and Ahmed Albahnasawi

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Abstract

Black liquor is the wastewater produced from paper-making-pulping industry, which has great threats to the environment and human health. Its pollution handling and resourcing attracted much attention, but conventional method showed several drawbacks, including low energy efficiency, emission of secondary pollutants, and safety and operating issues. Supercritical water gasification (SCWG) is a promising method for organic wastes, especially for those with high moisture content, so it has been widely investigated. A previous study showed that SCWG of pulping wastewater can realize pollution elimination and hydrogen production simultaneously. In this chapter, the recent development of this technology in past decade will be reviewed, including the gasification performance, the influence of the main operating parameter, the catalysts used in this process, the synergetic effect in co-gasification with other energy sources, and the evaluation of integrated system of SCWG of black liquor with pulping process. These progresses have been made will boost the industrial utilization of SCWG of black liquor in pulping industry.

Keywords

  • black liquor
  • supercritical water gasification
  • hydrogen
  • catalysts
  • co-gasification

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.

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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 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.

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].

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.

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].

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 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. 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.

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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+ 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+ content dropped to 86 mg/L. Cooling fluid in the range of 360–200°C achieved 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.

Figure 3.

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

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4. Catalysts used in SCWG of black liquor

To improve the gasification efficiency and lower the reaction temperature, the addition of catalysts can be considered. Catalysts for SCWG can be classified into alkali salts, metals, and metal oxides [24]. They have different advantages in SCWG of biomass due to their different catalytic properties. In particular, alkali salts are effective catalysts for SCWG of biomass [25]. However, some catalysts, including alkali salts, may be inapplicable in SCWG of black liquor due to its unique composition and properties (Table 1).

AdvantagesDisadvantages
AlkaliHomogeneous, strong promotion on water-gas shift reaction and C∙C bond cleavageHard to recycle and regenerate, weak catalytic effect
MetalHigh catalytic activity, strong selectivity, thermal stabilityExpensive, deactivation, hard to regenerate
Metal oxideStable, low-cost, recyclable and regenerate, unlimited loadingLess active, hydrogen consumption

Table 1.

Comparison of different types of catalysts in SCWG of black liquor.

4.1 Alkali

Alkalis are widely used for enhancing hydrogen production in SCWG due to their strong promotion of the water-gas shift reaction and C–C bond cleavage [24, 26, 27]. With NaOH, the fraction of H2 can reach over 50%, and the H2 volume increased to 80% in SCWG of glucose [27]. Compared with noncatalytic gasification, the H2 yield from SCWG of sugarcane bagasse can be increased to over 10 times with the catalysis of five alkali salts [28]. The highest H2 yield of 75.6 mol/kg was obtained under the catalysis of KOH at 800°C. Comparatively, the catalytic effect of alkali salts on SCWG of black liquor is weaker. Casademont et al. [12] studied SCWG of black liquor (0.81 wt%, pH = 11.12) in a continuous tubular reactor with NaOH as the catalyst. The gasification efficiency slightly improved from 36.5% to 37.0% with the addition of 0.6 wt% NaOH, but the H2 yield decreased from 24.58 to 23.34 mol/kg at 600°C. Moreover, when the NaOH loading increased to 1.2 wt%, the H2 yield decreased to 22.27 mol/kg, while the gasification efficiency increased slightly to 44.3%. When the temperature was raised to 700°C, the H2 yield increased to a maximum of 38.68 mol/kg with 0.9 wt% NaOH from 33.62 mol/kg without catalyst. They attributed the weak effect of alkali to the abundant alkali contained in black liquor, which made the addition of alkali ineffective. Guo et al. [29] also studied the effect of NaOH on SCWG of glycerol in a continuous reactor and found that the gasification efficiency increased slightly with over 0.1 wt% alkali. They proposed that excess alkali will not dissolve in SCW to improve the catalytic effect but will accelerate plugging due to salt deposition. In addition, the absorption of CO2 by OH− increased the chemicals consumption or regeneration time from Na2CO3 to NaOH. The addition of potassium will lower the melting point of salts and cause operating and safety problems [30]. Therefore, alkali was not a suitable catalyst for SCWG of black liquor for the high content of alkalis in itself.

4.2 Metals

Some metals, such as Pb, Pt, Rh, Ni, and Ru, are effective catalysts in SCWG of biomass and organic wastes [31, 32]. However, they were prone to be deactivated with the presence of sulfur for poisoning. Peng et al. [33] studied the performance of Ru/C catalyst in SCWG of sulfur-containing (0.53 wt%) microalgal and found that the BET surface of the bottom catalyst decreased from 1254 m2/g to 56 m2/g after 55 h. The activity of the catalyst was severely reduced. The other notable change was that the amount of S contained in the catalyst significantly increased from 151 mg/kg to 1156 mg/kg. They proposed that sulfur poisoning was the main cause of catalyst deactivation. Osada et al. [34, 35] investigated SCWG of lignin catalyzed by Ru, Rh, Pt, and Pd in the presence of sulfur and found deactivation of the catalysts due to sulfur poisoning. It was found that sulfur will combine with Ru and chemically adsorb on the catalyst surface, which will decrease the active sites and reduce the gas yield. Black liquor obtained from Kraft pulping contains a high fraction of NaOH and Na2S [1, 23, 36], which will deactivate the metal catalyst and increase the operating cost.

Appropriate supports can improve the stability of metal catalysts in harsh environments and increase the dispersion of active components, such as using oxides, zeolites, and carbons as the support [37]. However, the supports may be unstable in SCW. For example, γ-Al2O3 was converted to α-Al2O3 in a hydrothermal environment, and zeolites undergo structural collapse in SCW [38, 39]. The phase transformation and structural collapse of the support can reduce catalytic activity.

4.3 Metal oxides

Compared with alkali and metal catalysts, metal oxides show unique advantages and can be used in SCWG of black liquor. Metal oxides are easier to recover than alkali salts, and they are more stable and lower-cost than metals. It allows metal oxides to be stored and transported for a long time. Unlike metal catalysts, metal oxides can act as sulfur fixers and exhibit excellent activity [33, 40]. Ates et al. [41] studied the role of ZnO and MoO3 in SCWG of model feed (dibenzothiophene-hexadecane). The conversion and sulfur removal of dibenzothiophene greatly increased with the metal oxides. The catalysts used consisted of metal oxides and sulfided metal oxides. Moreover, sulfur-fixing metal oxide can be regenerated by a facile calcination treatment, whereas metal catalysts cannot [42].

Some metal oxide exhibit high catalytic activity in SCWG of biomass. Park et al. [43] studied RuO2-catalyzed SCWG of organic components and achieved nearly complete gasification of organic compounds. They proposed that the redox cycle between RuIV and RuII was the mechanism of the catalytic impact. Especially, some transition metal oxides can catalyze water-gas shift reaction to improve hydrogen production. The gasification efficiency of glucose and cellulose under the catalysis of ZrO2 was found to be approximately twice that obtained under noncatalytic condition [44]. Seif et al. [45] used Co3O4, CuO, and MnO2 to catalyze SCWG of industrial wastewaters for hydrogen production in a batch reactor. The catalysts increased the hydrogen yield by promoting water-gas shift reaction. The largest improvement in H2 yield was from 0.46 mol/kg without catalyst to 1.25 mol/kg with 40 wt% Co3O4 catalyst at 350°C. The order for increasing hydrogen production was Co3O4 > CuO > MnO2. In our previous study, 12 transition metal oxides were screened for SCWG (600°C, 25 MPa, and 10 min) of 10 wt% glucose for hydrogen production in quartz capillary reactors [46]. Most of the metal oxides improved gasification efficiency. The highest carbon gasification efficiency of 87.11% was obtained by Cr2O3 compared with that without catalyst (68.27%). The highest H2/CO ratio up to 19.61 and 14.55 was obtained with the promotion of CuO and MnO2 on water-gas shift reaction.

The application of metal oxide catalysts in SCWG of black liquor has also been studied. Boucard et al. [8] used nano-CeO2 to enhance the conversion of 10 wt% black liquor in subcritical (350°C and 25 MPa) and supercritical (450°C and 25 MPa) water in a batch reactor. The composition of the gas product was H2, CO, CO2, and light hydrocarbons. The catalyst improved gasification efficiency and hydrogen production. The H2 yield increased from 322 to 345 μmol under supercritical conditions. CeO2 promoted the splitting of SCW into hydrogen and oxygen, where the hydrogens combined with each other to produce H2, and complete oxidation could be achieved with oxygen. Then, the degradation of black liquor was promoted, and the color changed from dark brown to yellow and was almost transparent. Under subcritical condition, the H2 yield increased by 10 times with the catalyst but was lower than that under supercritical condition. The catalyst promoted water splitting with lower efficiency under this condition, so a low H2 yield and partial oxidation were achieved [8].

In our previous study [11], hydrogen production by SCWG of 12.6 wt% black liquor with 14 common metal oxides (ZnO, TiO2, SnO2, V2O5, WO3, MoO3, Fe3O4, Fe2O3, MnO2, Cr2O3, CeO2, CuO, ZrO2, and Co2O3) was studied at 600°C. The gasification efficiency of black liquor was 77.7% without catalyst, and the gasification efficiency was increased to over 100% with over half of these catalysts. As the gasification efficiency was defined as the ratio of the mass of the gas product to the mass of black liquor solids, it indicated that the presence of these catalysts promoted the involving of water in the reaction, which contributed more in hydrogen production. The gasification efficiency obtained with WO3, Co2O3, and V2O5 was 111.84%, 111.82%, and 110.06%, respectively. All the catalysts improved the carbon gasification efficiency, among which WO3, Co2O3, and V2O5 exhibited the highest activity, and the carbon gasification efficiency increased from 50.85% without catalyst to 71.05%, 70.92%, and 67.98%, respectively. Most metal oxides also increased hydrogen production by promoting the water-gas shift reaction. The presence of SnO2, ZnO, and Co2O3 increased the H2 fraction from 30.76% to 56.79%, 54.9%, and 54.8%, respectively. The highest H2 yields of 21.67 mol/kg and 21.03 mol/kg were obtained with Co2O3 and ZnO, respectively. From the characterization of the metal oxides before and after being used in SCWG, the authors proposed that catalysts may provide oxygen for reactions and promote degradation of black liquor (Figure 4). The high activity of V2O5 in improving gasification efficiency may be due to its highest oxygen content, which also led to the highest CO2 production. The activity of Fe2O3 was higher than that of Fe3O4, and the yield of CO2 was higher. TiO2, as the catalyst with the oxygen content second to V2O5, also increased the CO2 fraction. The effect of catalyst loading on gasification was also investigated. Increasing the amount of catalyst from 0 wt% to 180 wt%, the gasification efficiency continued to increase without an upper limit. It seems to be a unique advantage of metal oxides compared with alkali salts and metal catalysts [47, 48, 49]. The maximum carbon gasification efficiency (80.11%) and gasification efficiency (149.90%) increased by nearly 1.6 times and two times, respectively. Sufficient metal oxides reduced the H2 fraction and increased the CO2 fraction. This may be related to the catalytic mechanism of metal oxides in SCW. Metal oxides can supply oxygen species for reactions and enhance the degradation of black liquor. Excessive oxygen may consume hydrogen and reduce H2 production. Residual low-valence metal oxides or metals still have catalytic activity, which can promote water-gas shift reaction to improve gasification and H2 production.

Figure 4.

Catalytic mechanism of metal oxides in SCWG of black liquor [11].

The synergistic and complementary catalysis brought by bimetallic oxides is remarkable [50, 51]. It can also play a well catalytic role in SCWG of biomass. Mastuli et al. [52] studied SCWG of oil palm frond biomass with the catalysis of NiO, CuO, and ZnO supported on MgO and found that the catalysts increased gas yield from 55.4 mol/L with MgO to 72.7, 81.1, and 118.1 mol/L under the addition of NiO/MgO, CuO/MgO, and ZnO/MgO, respectively. The H2 yield was improved from 35.4 mol/L without catalyst to a maximum of 118.1 mol/L under the promotion of ZnO/MgO. They proposed that Zn-based catalyst was more effective than Ni- and Cu-based catalysts in promoting water-gas shift reaction. The catalytic effect of bimetallic oxides on lignin, one of the organic components of black liquor, was also studied. We used CeO2-ZrO2 as catalyst in SCWG of lignin and cellulose at 500°C and 600°C [53]. The gasification efficiency of lignin increased from 57.71% to 70.43% with catalyst at 600°C and the highest H2 yield increased to 20.39 mol/kg. The catalytic effect of CeO2-ZrO2 at a lower temperature was more significant. This may be because at lower temperatures, the gasification intermediate products were mainly distributed in the liquid, which can be fully contacted with the catalyst. In SCWG of cellulose, the catalyst also increased gasification efficiency and hydrogen yield. The H2 yield increased nearly 2.6 times from 3.29 mol/kg without catalyst to 8.5 mol/kg at 500°C and increased to 19.39 mol/kg at 600°C. Similarly, the catalysis of CuO-ZnO and Fe2O3-Cr2O3 in SCWG of lignin and cellulose was investigated [54]. In SCWG of lignin by CuO-ZnO, the highest gasification efficiency of 96.24% and the highest H2 yield of 29.62 mol/kg was obtained at 600°C. The order of catalysts in enhancing hydrogen production from SCWG of lignin is CuO-ZnO > Fe2O3-Cr2O3 > CeO2-ZrO2. In the gasification of cellulose, the catalytic effect of Fe2O3-Cr2O3 on H2 yield was more significant than that of CuO-ZnO, reaching a maximum of 18.74 mol/kg.

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5. Co-gasification of black liquor with other energy substances

SCWG is a promising technology to convert black liquor into hydrogen-rich gas, but our previous study [55, 56] showed that the energy produced from SCWG of black liquor cannot fully meet the requirement of the pulp mill. To produce more energy and realize the energy self-sufficiency of the pulp mill, supplementary energy resources can be mixed and co-gasified with black liquor. Co-gasification also has a series of advantages, such as reducing the reaction conditions by synergistic effects, improving the gasification efficiency (GE) of black liquor and other energy substances, increasing the hydrogen content, and reducing the cost of hydrogen production. In response to this situation, we studied the co-gasification of black liquor with different energy substances including coal, biomass, and waste plastics (Figure 5) and studied the interaction between two different materials in co-gasification.

Figure 5.

Schematic diagram of co-gasification of black liquor and other energy substances.

5.1 Co-gasification with coal

Coal is an important energy source with huge reserves in China, with approximately 162.29 billion tons accounting for approximately 13% of the world’s total reserves [57]. Coal can be used as a stable supplementary energy source for pulp mills, and clean and efficient conversion of coal can be realized in SCWG at the same time [58, 59]. Inherent alkali in black liquor is an efficient catalyst in SCWG of the organics that can suppress the formation of tar and coke and increase hydrogen production, so co-gasification of black liquor and coal can utilize the inherent alkali in black liquor to enhance coal gasification.

Co-gasification of coal and black liquor in SCW was studied by thermodynamic analysis and experiments [16]. Thermodynamic analysis showed that the mixing of coal with black liquor can enhance H2 production in the equilibrium state. The experiments performed in a fluidized-bed reactor at 550°C and 25 MPa showed that the presence of coal and the ratio of coal/black liquor influenced the gasification (Figure 6). With increasing black liquor fraction, the H2 fraction and GE increased, while the CO fraction decreased. When the black liquor fraction was increased from 0 to 50 wt%, GE increased from 15% to above 60%; the H2 fraction increased from 34 to 45%; and the CO fraction decreased from 20% to below 1%. We proposed that the improvement in the GE of coal with the addition of black liquor was due to the alkali and lignin contained in black liquor. The alkali in black liquor not only catalyzed the decomposition of coal but also accelerated the water-gas shift reaction [59], so the gasification efficiency and hydrogen product were improved with increasing black liquor fraction. On the other hand, the lignin in black liquor may also promote coal gasification during SCWG. Lignin will decompose before coal due to unique components and molecular structure, so the generated phenolic compounds from lignin will facilitate the extraction of organics and decomposition of coal [60].

Figure 6.

Experimental results of SCWG of coal-black liquor mixtures with different black liquor fractions: (a) gas fractions; (b) GE (T = 550°C; P = 25 MPa; total concentration = 10 wt%) [16].

We also investigated the co-gasification performance of coal and black liquor at high temperatures (600–750°C) [61]. The variation in the temperature greatly changed the gasification efficiency. The CE and GE increased from 36.88% and less than 80% to 79.46% and over 180%, respectively. The highest CE (79.46%) was obtained for the co-gasification reaction of coal with black liquor with a mixing ratio of 50:50 at 750°C. With increasing temperature, the PAHs can be further decomposed, thereby improving CE and GE. In addition to increasing temperature, the use of fluidized-bed reactors and catalysts can also improve GE at lower temperatures. Therefore, higher gasification efficiency can be obtained with the combination of the proper reaction conditions, catalyst, and advanced reactor.

5.2 Co-gasification with biomass

As biomass is the raw material for pulp production, which is readily available in pulp mills and is also inexpensive and renewable. Therefore, biomass was considered as an available supplementary energy source to co-gasify with black liquor to provide more energy. As wheat straw is an important pulping raw material in China, we studied the co-gasification of black liquor with wheat straw in a batch reactor at 500–750°C with different mixing ratios [62]. The addition of black liquor and the change in the mixing ratio significantly influenced the gasification performance. As the black liquor fraction increased from 25 wt% to 100 wt%, both the H2 yield and CO2 yield decreased by approximately 7 mol/kg. The CE and GE of co-gasification were higher than the theoretical value calculated from the separate gasification of black liquor and wheat straw, indicating that a synergistic effect existed in co-gasification. The synergistic effect was probably because the inherent alkali in black liquor had a catalytic effect on wheat straw, and the presence of wheat straw can utilize the alkalis to the maximum extent. When the mixing ratio was 50:50, the highest CE and GE of close to 90% and 140% were obtained, respectively, indicating that it is the optimal mixing ratio for co-gasification under this reaction condition.

Comparatively, the synergistic effect of black liquor with wheat straw [62] was smaller than that of black liquor with coal [16, 61]. At a mixing ratio of 50:50, the GE and H2 yields of co-gasification of black liquor and wheat straw were approximately 30% and 20% higher than those in co-gasification of black liquor and coal. The difference was mainly attributed to the different compositions and properties of the organic matter. Compared with coal, wheat straw was more similar to black liquor because it was the source of black liquor. The larger difference between black liquor and coal may lead to the generation of different reaction intermediates and environments, which is more beneficial for the interaction of their gasification and synergistic effect.

5.3 Co-gasification with waste plastics

Besides, the municipal solid wastes (MSW) generated from nearby communities were also a good option for the energy supplement. In MSW, plastic waste is an important component, and its treatment is one of the important issues [63]. Co-gasification of plastic wastes and black liquor not only improves the energy production from SCWG of black liquor but also achieves efficient and clean conversion of waste plastics into hydrogen-rich gas. Hence, we studied the co-gasification of different plastic wastes (PE, PC, PP, ABS) and alkali lignin, which is the main component of black liquor.

The co-gasification of alkali lignin with various plastics (PE, PC, PP, and ABS) in SCW was investigated in a batch reactor [64]. The gasification efficiencies of the four plastics were different due to their different degrees of polymerization and chemical structures, and the order of magnitude was PE > PC > PP > ABS. In the separate gasification, the CE and GE (58.59% and 109.90%) of alkali lignin were higher than those of polyethylene (53.27% and 91.38%), but both were lower than the values obtained in the co-gasification. It indicated that there was a synergistic effect in co-gasification of alkali lignin with polyethylene. This was also supposed to be related to their different chemical structures and properties of different organic substances (Figure 7). During co-gasification, alkali lignin can promote the depolymerization of polyethylene into different fragments, and the generated intermediates will further be decomposed into large amounts of CH4, C2H4, and C2H6. Alkali lignin can also catalyze steam reforming of these gaseous products to H2 and CO2. In addition, polyethylene had a higher H/C ratio than lignin, which can provide more hydrogen radicals and inhibit the cross-linking and repolymerization of aromatic compounds during lignin decomposition. Therefore, co-gasification of alkali lignin with polyethylene can not only inhibit the formation of char, but also promote the gasification of each other.

Figure 7.

Interaction mechanism of alkali lignin and PE in supercritical water gasification [64].

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6. System analysis on SCWG of black liquor

Compared with conventional treatment methods, the hot and compressed products from SCWG of black liquor include combustible gases such as H2, CH4, and CO and contain considerable chemical energy and sensible heat, which was necessary to be recovered to improve the energy efficiency. On the other hand, pulping is a complex process involving multiple steps, including raw material preparation, pulping, bleaching, chemical recovery, and pulp drying, which consume various types of energy, including power, medium-pressure (MP) and low-pressure (LP) steam [55]. Therefore, it is necessary to integrate SCWG of black liquor and the pulping process to make full use of the energy and improve the energy efficiency, which can also provide a reference for process designing for its industrialization.

An integrated system of SCWG of black liquor coupling with pulping process to mainly produce hydrogen as well as electric power and steam for the pulping plant was proposed in our previous study (Figure 8) [55]. The high-temperature and high-pressure gas products were used to produce power and steam through steam turbines and heat exchangers. Hydrogen was purified through PSA, and the residual gas and supplementary natural gas were burned to provide heat for SCWG reaction. In this study, a pulp mill with a daily output of 1000 ADt (Air Dry ton) pulp was selected as a reference, and the main data about the energy consumption were collected from the literatures [65, 66]. The reaction temperature and pressure of SCWG were set at 700°C and 25 MPa, respectively. From this integration system, 37,126 Nm3/h high-purity hydrogen, 53,053 kg/h MP steam, and 72,446 kg/h LP steam can be produced. The generated MP and LP steam could fully meet the requirements of the pulping process, and some excess heating could also be exported to the nearby community. However, some electric power needs to be imported to fulfill the requirements of the plant. The difference between using air and oxygen as oxidants in the burner of natural gas and exhaust gas products was evaluated and found that using air was more energy-efficient, but the introduction of nitrogen would generate pollutants (NOx), and the tail gas needs to be treated. The comprehensive energy consumption of pulp production from the pulping process coupling with SCWG of black liquor was calculated. For 1 ADt pulp produced, 221.48 kgce energy can be produced by using air as oxidant, and 288.01 kgce energy was consumed when using oxygen as oxidant. These values were much lower than the energy consumption level of China for pulp production in 2015 (approximately 370 kgce/ADt) [67].

Figure 8.

Conceptual diagram of SCWG of black liquor coupled with evaporation. The green dot dash line and blue dashed lines represent the heat and power flow between the units, respectively [55].

An advantage of SCWG over conventional treatment is that black liquor can be gasified directly without energy-intensive evaporation. However, concentrating black liquor to a certain extent can reduce the system scale and related heating loss. To balance the evaporation energy consumption and heating loss reduction, the necessity of black liquor evaporation was evaluated based on energy and exergy analysis of the above system (Figure 8). This evaluation used the steam and power generated from SCWG of black liquor for evaporation to improve the energy efficiency. In different pulping processes, the concentration of weak black liquor varied in the range of 10–20 wt% for different pulping processes [10]. And the influence of this concentration on the system efficiency was studied firstly [56]. With weak black liquor concentration increasing from 10 wt% to 20 wt%, the energy efficiency decreased, and the exergy efficiency increased. When coupled with black liquor evaporation and the concentration was increased from 15 wt% to 40 wt%, the energy efficiency decreased due to energy consumption of evaporation, and the exergy efficiency first increased and then decreased. The maximum exergy efficiency of 41.95% was obtained when the black liquor concentration was 21.82 wt% (optimal concentration). For the black liquor with different initial concentrations, the influence of evaporation was also different. The dilute black liquor needed to evaporate more water to reach the optimal concentration, and the efficiency improvement caused by evaporation was greater. The effect number of the multieffect evaporators was also studied, and it was found that the optimal concentration increased from 19.93 wt% to 23.13 wt% when the effect number increased from 4 to 7.

Some scientists noticed the high energy loss in hydrogen separation by PSA and used the syngas chemical looping (SCL) coupled with SCWG of black liquor to improve the energy efficiency. Taking advantage of the SCL to produce H2 and CO2, it can not only produce hydrogen and power but also avoid additional CO2 separation processes. Ajiwibowo et al. [68] designed a system composed of SCWG of black liquor, SCL, and power generation process to produce hydrogen and power (Figure 9). SCL was used to reduce the syngas product by Fe2O3. The generated steam and CO2 were used to produce power by steam turbine, and the generated Fe and FeO were oxidized by steam to generate hydrogen and Fe3O4. The steam and hydrogen flows were also used to produce power by steam turbine, and high-purity hydrogen could be separated after cooling. Then Fe3O4 reacted with oxygen in a furnace to regenerate Fe2O3, which released heat for SCWG reaction. The effects of temperature (375, 500, and 600°C) on gasification products and system efficiency were studied. The highest energy efficiency of 73% was obtained at 600°C. At higher temperature, the concentration of hydrogen was lower, while the concentrations of CO, C2H6, and CH4 were higher. Darmawan et al. [69] further discussed this system. The maximum efficiency was 82%, which was higher than that of traditional gasification system (69.1%), and the CO2 capture of the system was 75%. In addition, the influence of pressure in SCL was also studied. When the pressure increased from 2.5 MPa to 3.5 MPa, there is no significant change in energy efficiency, but the consumed power in pump and compressor increased and the net power generation was decreased.

Figure 9.

Schematic diagram of SCWG of BL, chemical looping, and power generation [68].

To evaluate the impact of SCWG of black liquor on pulping process in terms of raw material recovery, energy utilization, and economic benefits, Magdeldin et al. [70] designed an integrated process of SCWG and Nordic pulp mills. In this study, the process of combining SCWG with recovery boiler was proposed, and the influence of SCWG product on the performance of the recovery boiler was studied. The energy efficiency of SCWG system reached 83% and 80% at 450°C and 600°C, respectively, which was basically consistent with above studies [55, 56]. The gas products were used as the alternative fuel of heavy fuel oil in causticizing plant, and the surplus was sold to the public society. They also studied the impact of integrating SCWG system on the capacity of pulping plant and found that the pulping capacity can be increased by 75% on the premise of meeting the power and steam demand of the pulping plant. If it is considered to purchase power from outside and separate 75–77% of the black liquor into the SCWG system, the pulping capacity can be tripled. The economic benefits of the integrated gasification system were calculated. Compared with the reference Nordic pulp mill, the lowest selling price per 1ADt pulp can be reduced by 22%.

To explore the technoeconomic feasibility of SCWG of black liquor in sulfate pulp mills, Özdenkçi et al. [15] evaluated different integrated schemes using different reactor materials for hydrogen, heat, and power generation. Compared with stainless steel, the use of Inconel reactor was found to be able to obtain better economic benefits and reduce the cost of hydrogen production. Subsequently, Özdenkçi et al. [71] studied the effects of different process conditions. The main process conditions included: stainless steel and Inconel reactor, temperature (600–750°C), short (133–162 s) and long (300 s) residence time. It showed that using Inconel reactor helped achieve the highest cold gas efficiency (87.8%) and hydrogen yield (24.92 mol/kg) at 750°C with a long residence time (300 s). Through the technical and economic evaluation, it was concluded that the longer residence time at 700°C and the shorter residence time at 750°C could be used as alternative conditions.

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7. Conclusions

SCWG is an innovative and potential treatment method for black liquor from pulping processes. Several researchers have studied this topic, and many progresses had been made, such as:

  1. SCWG is a promising handling method for black liquor, where both hydrogen production and pollution handling can be realized simultaneously. After treatment at 600°C, the COD concentration decreased from 95,000 mg/L to 2160 mg/L. The increase in temperature enhanced gasification, and the highest carbon gasification efficiency of 94.10% and hydrogen fraction of nearly 70% were achieved at 750°C. The inherent alkali, prolongation of reaction time, diluting the black liquor, and the use of Inconel-625 reactor favored gasification.

  2. Metal oxide was a better choice to be used as catalyst for SCWG of black liquor than alkali and metal catalysts due to the alkali and sulfur content of black liquor. With the catalysis of metal oxide, the oxygen species can promote black liquor degradation, and the reduced metal can also serve as the catalyst. The combination of different metal oxides, such as CuO-ZnO, CeO2-ZrO2, and Fe2O3-Cr2O3, can further improve catalyst activity.

  3. Some accessible energy sources, including coal, biomass, and plastics, can be co-gasified with black liquor to produce more energy for pulp mill to realize the energy self-sufficiency. The synergistic effect was found in their co-gasification because the alkali in black liquor can promote the gasification of other substances, and the addition of other substances can take more fully use of the abundant alkali in black liquor.

  4. The utilization of hot-compressed products of SCWG of black liquor is the main optimization direction of the integrated system. At present, the main utilization methods in the system analysis include: PSA separation of hydrogen, production of hydrogen and power by chemical looping, as alternative fuel in causticizing plants. Considering the difficulties in the storage and transportation of hydrogen and the energy consumption in the separation process, CHP is a more practical utilization method that should be further analyzed and optimized.

Though great progress has been made in SCWG of black liquor, several challenges still need to be handled before its industrial utilization. For example, reactor corrosion can be affected by the operating parameters, reactor materials, and properties of the feedstock. The alkali salts in black liquor were found to be able to aggravate the reactor corrosion. In addition, reactor plugging was another challenge to be overcome for continuous running of the system. Both the char generated for incomplete gasification and alkali salt precipitation in SCW can result in reactor plugging. Furthermore, though the distribution of the alkali salts in SCWG was revealed, the development of the proper method and equipment for the alkali recovery was still needed to improve the economic efficiency of this technology.

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Acknowledgments

The authors are grateful for the financial support from Fundamental Research Funds for the Central Universities (xzy012020072).

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Written By

Changqing Cao, Lihui Yu, Wenhao Li, Lanjun Liu and Peigao Duan

Submitted: 09 May 2022 Reviewed: 26 May 2022 Published: 07 December 2022

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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