Hydrothermal liquefaction of kraft lignin in sub-critical water: the influence of the sodium and potassium fraction

Efforts have been made to reduce the worldwide consumption of petroleum in response to increased awareness of the long-term environmental impacts caused by its utilization [1]. These efforts are reflected in government mandates and incentives and also in research by the development of technologies for the production of liquid transportation fuels, chemicals and materials from biomass. This trend will lead either to the implementation of new technologies or the integration of emerging technologies with existing infrastructures. One infrastructure of particular interest in this context is pulp mills. Despite facing many challenges, such as shifting demand of different pulping products and the energy market situation, pulp mills have a great potential for diversifying their product portfolios and increasing their revenues through new biorefinery concepts [1‐6], where by-products/residues such as black liquor [4, 7] can be valorised.

Various measures and integration investments have been carried out in recent years in pulp mills [2] to ensure an energy-efficient process which makes it possible to integrate new biorefinery concept with the present pulp mills and thereby allowing hemicellulose, extractives and/or lignin to be valorised further to form high-value products. The kraft process is the most common pulping process currently employed, accounting for more than 85% of the global lignin production: it amounts to approximately 63 × 10⁴ tons annually [8], making this type of process of great interest as an obvious part in an integrated forest biorefinery (IFB). In this process, lignin is available in the black liquor, but is usually used as fuel to meet the steam and power needs of the pulping processes. Furthermore, the capacity of a mill’s production is often limited by the thermal flue gas capacity of the recovery furnace. Extracting a portion of lignin from black liquor presents an opportunity for a cost-effective approach for increasing the production capacity of the mill as well as the potential for using the lignin internally or externally, depending on the overall energy balance. The recovery of lignin from kraft pulping is also being pursued actively as a resource from a biopower, biofuels and bio-based chemicals/materials perspective [9]. Indeed, a relatively new method for separating part of the lignin from black liquor is the LignoBoost process, which has recently been commercialized. For instance, Domtar has implemented LignoBoost successfully in their kraft pulp mill in Plymouth, NC, USA [10, 11] and Stora Enso has installed a 50,000 t.p.a. LignoBoost plant at their Sunila mill in Finland [11, 12]. Lignin can be valorised via different routes: pyrolysis, gasification or hydrothermal liquefaction (HTL). Having a high-quality lignin in the pulp mills produced in the LignoBoost process represent a great potential of integrating these emerging technologies of lignin valorisation such as hydrothermal liquefaction (i.e. base catalysed depolymerisation in this case). This integration should consider technical constraints of a pulp mill, i.e. the chemical balance (especially the alkali ratio that should not be disturbed) and the energy balance.

Base-catalysed depolymerisation (BCD) has been investigated extensively as a technique for the conversion of biomass in general and lignin in particular. The BCD process involves the utilization of bases such as K₂CO₃, KOH, Na₂CO₃ and NaOH to promote lignin degradation. Many studies have shown how crucial the choice of base is in lignin degradation [13‐15]. Table 1 shows a summary of various parameters used in several studies on lignin depolymerisation using bases (homogenous catalysts).

Strong bases, such as KOH and NaOH, have been used as homogenous catalysts in several previous studies. Roberts et al. [14], who investigated the degradation of organosolv lignin in a base-catalysed process at 300 °C and 25-MPa pressure and NaOH as base, showed that the product yields are influenced by operating conditions such as temperature, pressure, residence time and concentration of the base: when NaOH increased (from 2 to 4%) in the feed, the oil, oligomers and monomers yield increased. Toledano et al. [15] tested different bases for organosolv lignin processed from olive tree waste and obtained catechol as the main product (0.1–2.4%); they found that the oil yield depended strongly on the base concentration: using NaOH gave higher oil yield than when using KOH (19.7% for NaOH vs 13% for KOH). Furthermore, Wang et al. found that base-catalysed treatment of various types of lignin led to many phenolic products with different yield distributions [13]. Some studies have concluded more generally that using a base is important for promoting lignin degradation [13, 20].

Attention in these investigations was paid mainly to study the influence of various bases on product yields, which is an important part of developing a process that is both efficient and effective. The key question is the economic viability of this type of process: stand-alone technology may be too costly due to the logistics and high investment constraints involved, so a better alternative is to adapt emerging technologies to existing infrastructure. Most of the studies in Table 1 were based on organosolv lignin despite the fact that it hardly can be regarded as an industrial lignin. Thus, the knowledge is still limited regarding kraft lignin which is the major industrial lignin available, not only today but also in the future.

The present investigation is a part of the development of a process for depolymerisation of kraft LignoBoost lignin with near critical water to transportation fuels and/or chemicals. In our previous studies [16, 21‐24], the dependences of potassium carbonate concentration, temperature, pH, recirculation ratio and phenol as capping agent and if methanol could replace phenol have been investigated as well as the storage stability of the produced bio-oil [25].

All the investigations have been performed systematically using the same reference conditions (350 °C, 25 MPa, 5.5 wt% lignin, 4 wt% phenol, and 1.6 wt% potassium carbonate). The influence of temperature was investigated in five steps from 290 to 370 °C [19] and showed that the yield of bio-oil decreased slightly when the temperature was increased. The yield of water-soluble organics (WSO) as well as the char on catalyst increased slightly.

The influence of homogeneous catalyst concentration and pH was studied by varying the amount of potassium carbonate [17] and, to be able to reach higher pHs, by adding different amounts of potassium hydroxide [20]. The investigated pH interval was from 7.1 to 9.7 in the product and 0.8 to 1.5 unit higher in the feed. There was a minimum in the formation of char around pH 8.1 (in the product) and the amount of water-soluble organics increased with the pH, but the most significant result was that the formation of suspended solids (SS) was low until pH 8.1 and then increased considerably by increasing pH.

The storage stability of the produced hydrothermal liquefaction (HTL) lignin bio-oil was investigated for temperatures from ambient up to 80 °C [25]. The stability was shown to be good for the produced lignin bio-oil and if water and suspended solids were separated, the remaining oil showed almost no aging at all.

A natural question is if the addition of 4% phenol in the feed as capping agent can result in an economically viable process. In a recent report [26], a number of emerging technologies, including HTL, for the production of transportation fuels from lignin of forest residues are compared and it was found that the use of 4% phenol in the feed is not likely to give a viable process. However, we investigated if another and lower phenol concentration could be used [24]. The tests showed that it was possible to lower the concentration of phenol to 2% and still get almost the same amount of bio-oil as with 4% but at the expense of obtaining a more viscous oil. The yields of individual compounds were dependent on the phenol concentration but the total yield of bio-oil was not. The fraction of the phenol converted to other products was also lower for the 2% case. Calculations for the 2% case, using the same assumptions as in the report [26], showed that at this phenol concentration, the process was likely to be viable.

In these studies, potassium salts (in the form of K₂CO₃ or K₂CO₃/KOH) were used in order to promote lignin degradation: as explained earlier, the focus was on the potassium ion rather than the sodium because of its high catalytic activity [27]. On the other hand, it is known that the alkaline process streams in a kraft paper pulp mill contain a mixture of K and Na. The reason for this is that the make-up chemical is sodium-based, but wood contains potassium which is dissolved during the cooking operation. The alkali concentration, especially the ratio of sodium to potassium, in the pulp mill is an important parameter to control, and disturbances may affect several operations (e.g. cooking, evaporation and recovery boiler). It means that the alkali concentrations and the Na/K ratio should be kept to a certain level to ensure the steady operation of the pulp mill. In order to make integration of the HTL process with existing pulping process possible, it is important to study the effects of a Na/K mixture on the yields of products in the HTL process. The aim of this investigation is twofold: to study the effects of the Na/K ratios by using different concentrations and to investigate the possibility of using a concentration ratio of Na/K similar to the existing chemical streams in the pulp mill.

Experiments were conducted in a small continuous pilot unit (Fig. 1), comprised of a fixed-bed reactor having a volume of 500 cm³ (Parr 4575), a recirculation, electrical heating, and feed and exit systems. The feed flow rate was 2 kg/h, resulting in an average residence of about 6 min.

The electrical heating system was mounted around the piping, the feed tank and the reactor. The feed was first heated to 50 °C in the feed tank and then to 80 °C in the pre-heating section. The pressure of the feed was increased by using a high-pressure diaphragm pump. As we believe a high heating rate of lignin to be important in order to have high yields, we have chosen a reactor construction with recirculation. The recirculation flow was also powered by a high-pressure diaphragm pump. After being preheated to about 80 °C, the feed was mixed with the recirculation flow and was, thus, heated rapidly. If the mixing is good, this heating will be more or less instantaneous, but even in a worst case scenario (almost no mixing), heating through conduction from 80 to about 300 °C will not take more than about 4 s. This is much more rapid than what is possible (without special measures) to obtain for example in batch reactors. Jiang et al. [28] had, for instance, a heating rate of 10 °C/min in their study of lignin depolymerisation using HTL. In all of the experiments, the reaction temperature was 350 °C the pressure 25 MPa and the ratio of recirculation-to-feed flow was between 4 and 5. Earlier tests had shown that lower ratios gave difficulties obtaining proper heating and higher ratios resulted in increased solids formation.

The runs were carried out in the following steps: First, a lignin slurry was prepared by mixing LignoBoost kraft lignin (5.5%), deionised water and K₂CO₃/KOH and Na₂CO₃/NaOH bases that were used according to the ratios shown in Table 2. This mixture of lignin, water and bases was then dispersed using an Ultra Turrax disperser for approximately 10 min at room temperature. Phenol (4%) was added afterwards, when the slurry was heated to 50 °C in the feed tank. The deionised water was pumped through the system until it reached the operating conditions (350 °C, 25 MPa). Thereafter, a mixture of the slurry containing no lignin, i.e. just phenol, K₂CO₃/KOH, Na₂CO₃/NaOH and deionised water, was used as a feed for 1 h. Then, the lignin slurry in the feed tank was pumped in continuously, and steady-state was established after about 80 min. The period of the steady-state phase was 80 min, followed by a run-down period of 40 min. The liquid products were collected in a sampling bottle that was replaced every 20 min.

The lignin used as the feed material in this investigation was a softwood kraft lignin produced by the LignoBoost process at the Bäckhammar pulp mill in Sweden. Its content of moisture was 32.6%, high heating value (HHV) was 27.7 MJ/kg, sodium was 0.09 mol/kg of lignin and potassium 0.03 mol/kg of lignin; its average molar mass was 17,000 g/mol [29]. The heterogeneous catalyst zirconia (ZrO₂) used in the reactor was supplied by Saint-Gobain NorPro, France length = 3 mm, diameter = 3 mm and BET surface area = 20 m²/g. Potassium carbonate (K₂CO₃, ≥ 99.5%); potassium hydroxide (KOH, ≥ 85%); sodium carbonate (Na₂CO₃, ≥ 99%) and sodium hydroxide (NaOH ≥ 85%) were used as the homogeneous co-catalysts and phenol (crystallized, ≥ 99.5%) was used as both the co-solvent and the capping agent. These components were all sourced from Scharlau Chemicals, Austria, and were used as received.

This investigation aimed at investigating how different ratios of sodium and potassium influence the depolymerisation of softwood kraft lignin. The results obtained for the measured amounts of sodium and potassium in the aqueous phase and the pH of the product are reported in Table 3. The mass balance of potassium and sodium between the feed and product was fairly consistent, with differences that can be attributed to experimental uncertainties. By comparing the measured amounts of sodium and potassium in the aqueous phase (Table 3) and in the bio-oil phase (Table 4), it was found that most of alkali ended up in the aqueous phase, with an average of 94% for potassium and 91% for sodium. Moreover, the Na/(Na + K) ratios in both the feed and the aqueous phase were almost identical. This comparison between the feed and the aqueous phase indicates that the latter could be the main source for recycling potassium and sodium, while the fraction of sodium and potassium remaining in the bio-oil might pose difficulties in its further processing. In the case of there being a mutual exchange of sodium and potassium between an HTL process and the chemical stream in a pulp mill, it might be possible that the streams in the pulp mill can be used as a way of adding alkali to the HTL process.

The pH was measured in the feed and the product; no significant difference was found in the pH of the feed (9.4 ± 0.2) with different sodium fractions in the feed. The same observation was made for the pH in the product (7.5 ± 0.1).

For the conditions investigated, it was found that it is possible to use Na/K ratios similar to those in a kraft pulp mill and to shift the catalytic system from potassium to sodium, keeping the overall yields of the products fairly constant. Moreover, using a greater amount of sodium than potassium in the feed resulted in less suspended solids being formed in the bio-oil. This study opens up for integration with the pulp mill in different ways.