hydrocracking or deep hydrotreating units. While pretreatment removes part of the oxygen and other contaminants, further hydroprocessing can saturate remaining reactive groups and convert the lignin-derived molecules into more stable hydrocarbon products. Depending on the severity of
Fluid catalytic cracking (FCC)
Petrochemical aromatics (BTX)
FCC
Gasoline-range pool
Zeolite catalyst (HZSM-5 / Y-Zeolite
Sustainable aviation fuel (SAF) Renewable diesel
Hydroprocessing units (Hydrocracking / hydrotreating)
Upgraded lignin bio-oil
H H
Marine fuels
Hydrogen
Figure 4 Integration of lignin-derived bio-oil to refinery
hydrogenation and the resulting boiling range, this route may produce middle-distillate-range hydrocarbons with potential applications in renewable diesel or sustainable aviation fuel pathways. Operationally, this approach is attractive because it offers the possibility of using existing middle-distillate processing infrastructure, although hydrogen demand, catalyst stability, and product quality still need to be demonstrated for each specific feed. Industrial symbiosis: Two-way material and energy integration Beyond direct product benefits, lignin utilisation creates the foundation for a two-way industrial symbiosis. This approach links two large-scale industries that have traditionally operated in separate value chains: the Kraft pulp sector and petroleum refining. Rather than a simple, one-way shipment of bio-oil, co-locating or closely integrating a Kraft mill and a petroleum refinery unlocks mutual operational and thermal synergies: • Heat and energy exchange: Extracting 15- 20% of dissolved lignin reduces the heating value of the remaining black liquor ( Tomani, 2010 ). If this creates a steam or heat deficit in the pulp mill, excess high-grade heat or steam generated by the nearby refinery’s exothermic catalytic units (for example, FCC regenerators or hydroprocessing reactors) can be piped to the pulp mill to balance its thermal requirements. • Hydrogen and gas network integration: The HDO of lignin bio-oil requires substantial hydrogen supply ( Lv, ve diğerleri, 2022 ). An integrated refinery provides existing high-
capacity hydrogen production (SMR) and gas- treating facilities (amine treating and sour water stripping). Conversely, light biogenic off-gases (such as CH 4 and CO) produced during lignin thermochemical conversion can be recycled into the refinery’s fuel gas system or utilised as SMR feed supplement. • Balanced recovery operations: By extracting a targeted fraction of dissolved lignin, thermal stress on the recovery boiler is mitigated ( Tomani, 2010 ). This frees up recovery boiler hydraulic and thermal capacity, allowing the mill to process additional wood chips and increase pulp output without requiring capital-intensive boiler retrofits ( Benali, Kouisni, & Paleologou, 2014 ). Regulatory frameworks such as the EU’s RED III and CORSIA may also influence the value of renewable fuels through carbon-intensity and sustainability requirements. However, the actual environmental and commercial value of lignin- derived products will depend on the feedstock pathway, processing energy, hydrogen source, product certification, and the specific regulatory framework. Understanding the scale: Mill yield vs refinery throughput To understand the practical scale of this opportunity, it is necessary to compare the daily lignin output of a modern pulp mill with the feedstock volumes processed by a petroleum refinery. • Typical Kraft pulp mill scale: A modern, world-scale Kraft pulp mill producing 2,000- 3,000 air-dried metric tons (ADMT) of pulp
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