Abstract
As a part of developing a hydrothermal liquefaction (HTL) process to valorise lignin, it is important to consider integration possibilities with existing infrastructures in order to obtain an overall positive economic impact. One obvious example is to integrate the HTL process with the kraft pulp mill: transport and storage costs is reduced, the temperature levels on process streams can be matched (energy integration) and the recovery/use of alkali can be made efficient. In this study, softwood kraft lignin was depolymerised using sub-critical water (623 K; 25 MPa) in a continuous, small pilot unit with a flow rate of 2 kg/h. ZrO2, K2CO3/KOH and Na2CO3/NaOH were used as catalytic system, and phenol as the capping agent. The influence of the ratio between sodium and potassium in the feed on the yield and composition of the product stream was investigated. The results showed that bio-oil, water-soluble organics (WSO) and char yields were not remarkably influenced by shifting the catalytic system from potassium to sodium. Moreover, the yields of most phenolic compounds did not change significantly when the sodium fraction was varied in the feed. The amounts of suspended solids in the bio-oil produced showed, however, a diminishing trend, (decrease from 10.8 to 3.8%) when the sodium fraction was increased in the feed, whilst the opposite trend was observed for the heavy oil, which increased from 24.6 to 37.6%.
Highlights
Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Technology, Göthenburg, SwedenEfforts have been made to reduce the worldwide consumption of petroleum in response to increased awareness of the longterm environmental impacts caused by its utilization [1]
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
In order to keep the inlet pH fixed, to cover the whole range from only K to only Na, and to emphasise Na to K ratios similar to those commonly found in pulp mill streams, the added amounts of the individual alkali salts were varied as follows: run A is the reference test with 100% potassium ion (0.3% KOH and 1.6% K2CO3), the replacing of K ions by Na ions was done in a way that there was an equivalent number of mole of Na for each mole
Summary
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. 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]. 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
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