Decarbonisation Technology November 2026 issue

lignin bio-oil. Although this converts the solid into a liquid intermediate, the resulting bio-oil is typically unstable and oxygen-rich, leading directly to the refining challenges discussed in the next section (see Figure 1 ). Refinery does not see lignin as crude oil Traditional distillation units and fluid catalytic crackers (FCC) are designed primarily for

Supports circular bioeconomy

Cellulose

Paper & packaging

Black liquor

Lignin

K raft pulping (Pulp mill)

Lignin separation

Thermochemical conversion

Bridging two industrial systems

Lignin-derived bio-oil

Wood /biomass

Stabilisation + HDO

Renery

Renery co-processing

Fuels (e.g. gasoline, diesel, marine fuels)

Aromatics / chemicals (e.g. BTX, platform chemicals)

Figure 1 From Kraft lignin to the refinery

Extracting lignin from the Kraft process requires specific unit operations. In a standard Kraft mill, lignin is dissolved in black liquor, a highly alkaline mixture of cooking chemicals and wood components. To separate it without disrupting the chemical recovery cycle, commercial technologies such as LignoBoost or LignoForce can be used ( Tomani, 2010; Benali, Kouisni, & Paleologou, 2014 ). Injecting carbon dioxide into the black liquor lowers the pH, causing the dissolved lignin to precipitate. A filtration and acidic washing sequence then removes residual sodium and sulphate ions, yielding a solid lignin cake. Beyond providing a source of aromatic carbon for further conversion, lignin extraction can also reduce the lignin load on the recovery boiler and may provide additional operational flexibility for the pulp mill. The Kraft process produces a solid lignin cake, but petroleum refineries are hydraulically designed for liquids. Solid lignin cannot be pumped into standard pipelines, fractionation columns, or catalytic reactors. To make it processable, the solid lignin must undergo thermochemical liquefaction. Methods such as catalytic fast pyrolysis (CFP) or hydrothermal liquefaction (HTL) can thermally depolymerise the solid macromolecule. During fast pyrolysis, heating lignin to around 450-550°C in an oxygen-free environment breaks the weaker linkages in the structure ( Paasikallio, ve diğerleri, 2014 ). The vaporised fragments are rapidly quenched to form a pumpable liquid known as

hydrocarbon feeds with relatively predictable molecular weight distributions. Lignin bio- oils, however, possess fundamentally different chemical properties that can make them challenging to process in existing catalytic conversion units in their raw state. Furthermore, unlike the relatively consistent nature of crude oil, lignin-derived feeds can show significant variation depending on the wood species, Kraft pulping conditions, and the subsequent conversion process, making stable refinery operation more challenging. Crude oil contains virtually no oxygen by mass, whereas lignin bio-oils can contain substantial amounts of oxygen, with values of up to around 28% reported depending on the feedstock and conversion route ( Figueirêdo, Vanderbosch, Heeres, & Deuss, 2020 ). Processing such a feedstock therefore requires significant deoxygenation, which can increase hydrogen consumption and place an additional demand on a refinery’s hydrogen balance. In addition, thermal conversion of lignin-derived feeds can produce water and light gases, potentially increasing the load on downstream sour water stripping and amine treating systems. The extent of these effects will depend strongly on the composition of the bio-oil and the degree of upgrading achieved before it enters the refinery (see Figure 2 ). The catalyst in the FCC, including HZSM-5- based catalyst systems, is optimised primarily for hydrocarbon-rich feeds and can show reduced performance when oxygenated