Catalytic depolymerisation and conversion of Kraft lignin into liquid products using near-critical water

Highlights

• We set up a continuous plant for the conversion of lignin in near-critical water.

• We used phenol (co-solvent) and ZrO₂ + K₂CO₃ (catalysts), operating at 350 °C.

• LignoBoost Kraft lignin can be converted into bio-oil and water-soluble organics.

• Main 1-ring aromatic products are anisoles, alkylphenols, guaiacols and catechols.

• The yield of 1-ring aromatic products increases from 17% to 27% with K₂CO₃.

Abstract

A high-pressure pilot plant was developed to study the conversion of LignoBoost Kraft lignin into bio-oil and chemicals in near-critical water (350 °C, 25 MPa). The conversion takes place in a continuous fixed-bed catalytic reactor (500 cm³) filled with ZrO₂ pellets. Lignin (mass fraction of approximately 5.5%) is dispersed in an aqueous solution containing K₂CO₃ (from 0.4% to 2.2%) and phenol (approximately 4.1%). The feed flow rate is 1 kg/h (reactor residence time 11 min) and the reaction mixture is recirculated internally at a rate of approximately 10 kg/h. The products consist of an aqueous phase, containing phenolic chemicals, and a bio-oil, showing an increased heat value (32 MJ/kg) with respect to the lignin feed. The 1-ring aromatic compounds produced in the process are mainly anisoles, alkylphenols, guaiacols and catechols: their overall yield increases from 17% to 27% (dry lignin basis) as K₂CO₃ is increased.

Introduction

After cellulose, lignin is the second most abundant natural organic polymer found on earth, being the first among those composed of aromatic units (phenylpropane monomer units). A vast amount of lignin is extracted from wood in the Kraft pulping process and thereby ends up in the black liquor. Today this liquor is concentrated by means of evaporation and used as fuel in the production of heat and power for the pulping process itself. So far only a small proportion of the lignin (approx. 1%) has been used as a raw material in the production of materials and chemicals with a higher value than fuel. A new process called “LignoBoost” has recently been developed with the aim of extracting part of the lignin in the black liquor at a high purity level, thus exploiting the energy surplus of Kraft pulp mills. The LignoBoost process has recently gained commercial status; therefore, in years to come, a huge amount of LignoBoost Kraft lignin is expected to be available for valorisation [1], [2].

Lignin is an amorphous biopolymer whose primary aromatic units are linked together randomly by different types of ether (COC) and carbon–carbon (CC) bonds [3]. Its chemical structure suggests that this biopolymer could be a good source of liquid fuel additive and commercially important chemicals, phenolics in particular, if it could be broken down into smaller molecular units that retain the oxy-aromatic nature [4], [5]. In the quest to reach this goal, several processes for depolymerisation and conversion of different types of lignin have been investigated for decades, such as pyrolysis, which uses thermal treatment in the absence of air, hydrogenolysis, which is a thermal process in the presence of gaseous hydrogen or of a hydrogen-donating solvent, and oxidative cracking, which occurs in the presence of oxidising agents such as hydrogen peroxide [5].

More recently, it has been demonstrated that lignin can be depolymerised and converted into smaller molecular units in supercritical water. This process was, for example, carried out by Saisu et al. [6] at 400 °C without using a catalyst: they found that the yield of monomeric phenolic compounds was lower than expected. This was probably due to the re-polymerisation of monomers, resulting in the formation of char. Many authors [6], [7], [8] have however found that adding phenol to the reacting system decreases the formation of char. The effect of phenol in particular was studied in detail by Fang et al. [8], who decomposed lignin in supercritical water at temperatures between 400 °C and 600 °C in a micro-reactor. More specifically, according to these authors, phenol allows the system to become homogeneous at the reaction conditions and prevents the re-polymerisation reactions, which lead to char being formed. The fact that the addition of phenol was found able to prevent the formation of char is also very much in line with the results obtained by Aida et al. [9], who carried out the depolymerisation of coal in phenol–water mixtures at 400 °C.

One promising alternative for processes in supercritical water is the catalytic conversion in near-critical water, where the bonds of the lignin network are cleaved at mild temperatures (i.e. 280–370 °C), thus allowing the aromatic nature of the building block molecules to be retained to a greater extent. In this regard, some authors have focused on lignin depolymerisation and conversion in near-critical water using strong bases as a catalyst [10], [11], [12], [13], but high yields of heavy fractions and char are reported. In another research work, Karagöz et al. [14] compared the effect of different base solutions (i.e. K₂CO₃, KOH, NaOH and Na₂CO₃) in the hydrothermal treatment of wood biomass at 280 °C; they reported K₂CO₃ as being the most efficient catalyst in increasing the liquid-product yield and decreasing the solid residue. In a following work on the same topic, Karagöz et al. [15] observed that an increase in the concentration of K₂CO₃ leads to a decrease in char formation and an increase in liquid-product yield, together with an increase in catechol and a decrease of guaiacol in the water-soluble products. These results on the effect of K₂CO₃ are in line with those reported by Bhaskar et al. [16] pertaining to the same process applied to different wood biomasses.

Moreover, Andersen [17], [18] disclosed a process whereby an organic material is converted into small molecular units operating in water at temperatures from 270 °C to 380 °C, pressures from 15.2 MPa to 27.4 MPa and in the presence of the catalyst ZrO₂. In these patents, the important role of ZrO₂ in reducing the formation of char is emphasised. Furthermore, Hammerschmidt et al. [19] and Toor et al. [20] carried out the catalytic conversion of different types of sludge waste streams from food industry and machinery work [19] or distiller's grain with solubles [20], operating in near-critical water (>300 °C, 25 MPa) in the presence of both K₂CO₃ and ZrO₂ catalysts. These works show not only the feasibility of the conversion of different slurries of organic substances using this catalytic system but also good results in terms of oil yield. Since these applications are very different compared to conversion of pure lignin, it is of interest to test the feasibility of a process based on ZrO₂ + K₂CO₃ on this peculiar biomass. Furthermore, the previous attempts of converting lignin in near-critical or supercritical water were carried out on small laboratory batch units and they typically led to high amount of solid residues, if phenol was not added to the reacting system. A high production of solid residues may lead to serious operational problems in the development of the process to a continuous mode of operation on a large scale.

Therefore, the objective of the present work is to study the feasibility of a continuous process for converting LignoBoost Kraft lignin into transportation bio-fuels and aromatic chemicals in near-critical water (350 °C, 25 MPa), using ZrO₂ and K₂CO₃ as the catalysts and phenol as the co-solvent. The set-up of the plant and the development of the analytical procedure of the reaction products are described in details. In addition, preliminary results focusing on the effect of the K₂CO₃ mass fraction are presented and discussed.