Transforming biorefinery designs with ‘Plug-In Processes of Lignin’ to enable economic waste valorization

Lignocellulosic biorefineries are essential for a sustainable bioeconomy by simultaneously deconstructing and valorizing cellulose, hemicellulose, and lignin. Lignin, the second most abundant terrestrial polymer, has high potential for fuels, chemicals, and materials, yet current carbohydrate-first biorefineries leave lignin underutilized. Natural ligninolytic microbes such as Pseudomonas putida can funnel heterogeneous aromatics to valuable products like polyhydroxyalkanoates (PHAs), offering environmental benefits through biodegradable plastics and improved refinery economics. However, classical pretreatments designed to optimize carbohydrate release often generate lignin with poor processibility for bioconversion due to insufficient fractionation, high molecular weight, condensation, and low solubility. Acidic pretreatments can break β-O-4 linkages but also cause lignin coalescence and repolymerization; AFEX preserves lignin polymer structure within biomass; alkaline pretreatments solubilize lignin but vary in downstream bioprocessibility. Lignin-first strategies stabilize lignin to reduce condensation but are not tailored for microbial conversion and may compromise carbohydrate processing and increase capital costs. The key challenge is achieving efficient lignin bioconversion without substantial changes to existing biorefineries and without sacrificing carbohydrate utilization. This study introduces ‘plug-in processes of lignin’ (PIPOL)—a solubilization, conditioning, and fermentation module integrated with five leading pretreatments—to improve lignin solubility/reactivity, enhance PHA production by engineered P. putida KT2440, synergize carbohydrate hydrolysis, and evaluate techno-economics for commercial relevance.

The work builds on prior insights that sustainable biorefineries must valorize all biomass fractions, with lignin offering a route to fuels, chemicals, and materials. Biological lignin valorization leverages microbial ‘funneling’ pathways to process heterogeneous aromatics; Pseudomonas putida is a prominent chassis for converting lignin-derived streams to PHAs. Classical pretreatments (acid, steam explosion, liquid hot water) can depolymerize lignin via β-O-4 cleavage but also induce lignin coalescence, increased guaiacyl and condensed phenolic units, and repolymerization at high temperatures, reducing bioprocessibility. AFEX cleaves ester and lignin-carbohydrate bonds but retains overall lignin polymer structure within biomass; alkaline pretreatments solubilize lignin and partially hemicellulose by cleaving ester and glycosidic side chains. Lignin-first fractionation reduces condensation and preserves β-O-4 linkages but yields higher-molecular-weight lignin less suited for microbial uptake and may require substantial process redesign, potentially compromising carbohydrate processing. The literature emphasizes the need for approaches that tailor lignin chemistry (lower molecular weight, increased hydrophilicity) to improve biological conversion while maintaining or improving carbohydrate processing and capital efficiency.

Pretreatments: Corn stover underwent five leading pretreatments. DSA: 0.2% w/w H2SO4, 10% solids, 150 °C, 10 min (2.0 L Parr reactor), followed by solid-liquid separation. SEP: biomass pre-soaked (L/S 5:1, 30 min), adjusted to 60% moisture, treated at 200 °C for 10 min in a 2.7 L steam explosion reactor. LHW: heated to 160 °C (0.8 °C/min), 5 min hold, cool-down ~1 °C/min for 1 h in a 5 L digester. AFEX: 140 °C, 30 min, ammonia loading 1.0 g/g dry biomass; collected and stored at -4 °C. SHP: 1% NaOH w/w, 10% solids, 150 °C, 10 min; solid-liquid separation; conditioned liquid lignin stream used for fermentation.

PIPOL solubilization and conditioning: For each pretreated solid, additional lignin solubilization used 1% NaOH w/w at 10% solids in a 1 L bottle, 121 °C for 30 min (steam sterilizer). Vacuum filtration separated soluble lignin from solids. The lignin-containing liquid was conditioned (neutralized to pH ~7.0) for fermentation. PIPOL consists of solubilization, conditioning, and fermentation integrated into the existing biorefinery process.

Enzymatic hydrolysis: Conducted on solids from pretreatment and from post-PIPOL solubilization. Low-solids hydrolysis at 1% w/w in 0.05 M citrate buffer (pH 4.8) with Cellic CTec2 (5 FPU/g glucan) and HTec2 at 9:1 volumetric ratio, 50 °C, 200 rpm, 168 h; samples at 12–168 h for sugar analysis. High-solids hydrolysis at 10% w/w with same enzyme loading and conditions for 168 h; post-centrifugation and washing, liquids analyzed for sugars.

PHA fermentation: Engineered Pseudomonas putida KT2440 used soluble lignin streams as carbon source. Media prepared by adjusting soluble lignin to pH 7.0 (1.0 M H2SO4), adding 10X Basal salts and 100X Mg/Ca/B1/goodies to 100 ml. Fed-batch in 250 ml flasks, 100 ml working volume, 28 °C, 200 rpm, 24 h. Biomass harvested, washed, lyophilized for cell dry weight and PHA content determination. PHA quantified by GC-MS after methanolysis (15% v/v H2SO4 in methanol, chloroform, 100 °C, 140 min), phase separation, and analysis on GC-MS-QP2010SE.

Lignin characterization: Corn stover native lignin (CSNL) prepared via ball milling, enzymatic hydrolysis, and 96% dioxane extraction. Lignin molecular weights determined by GPC. Structural features analyzed by 2D 1H–13C HSQC NMR; hydroxyl and carboxyl functional groups quantified by 31P NMR.

Techno-economic analysis: Conceptual process design embedded PHA fermentation module into NREL 2011 ethanol biorefinery base model. Aspen Plus used for process modeling; materials/energy balances, capital and project costs estimated via Peters and Timmerhaus factors. Discounted cash flow analysis determined minimum PHA selling price (MPSP) at 10% after-tax IRR. Key economic drivers included PHA yield and titer, chemical and lignin treatment costs. Cost contributions (raw materials, utilities, operating, fixed capital, taxes/insurance, overhead, labor) assessed across scenarios.

Analytical methods: Composition analyses per NREL LAPs. Sugars quantified by HPLC (Aminex HPX-87P, RI detector, water mobile phase 0.6 ml/min). PHA yields calculated based on consumption of lignin and residual glucose. Mass balances performed across fractionation and conversion steps.

• PIPOL substantially improved lignin dissolution across all pretreatments, yielding soluble lignin (liquid phase) of 70.2% (DSA), 65.6% (SEP), 56.2% (LHW), 81.5% (AFEX), and 48.7% (SHP). Lignin content in solids after PIPOL decreased by 39.1% (DSA), 38.2% (SEP), 40.0% (LHW), and 61.2% (AFEX) relative to corresponding pretreated solids.
• Carbohydrate hydrolysis was enhanced by PIPOL. Final glucan conversion after 168 h on post-PIPOL solids reached 89.7% (DSA), 94.9% (SEP), 80.1% (LHW), 96.8% (AFEX), and 84.2% (SHP). Initial glucan conversion rates increased markedly (e.g., by 71.7% for DSA, 56.7% for SEP, 50.4% for AFEX). At 10% solids, PIPOL increased glucan conversion by 13.3% (DSA), 11.8% (SEP), 21.6% (LHW), and 10.6% (AFEX). Overall sugar yields were highest for AFEX-PIPOL, followed by SEP, DSA, SHP, and LHW.
• Microbial conversion to PHAs improved with PIPOL-derived soluble lignin, with performance depending on pretreatment. Fed-batch fermentations yielded PHA concentrations of 2.2 g/L (DSA), 2.3 g/L (SHP), 3.2 g/L (LHW), 3.6 g/L (AFEX), and 4.5 g/L (SEP). PHA content in cells followed DSA < SHP < AFEX < LHW < SEP (0.22, 0.24, 0.28, 0.31, 0.38 g/g dry cell). Soluble lignin from AFEX and SEP was consumed more extensively, supporting higher cell growth (up to 12.7 g/L CDW for AFEX-PIPOL and 11.9 g/L for SEP-PIPOL).
• Mechanistic insights: PIPOL further depolymerized lignin, lowering molecular weights; maximum decreases in Mw and Mn reached 81% and 76% (SEP-PIPOL). Fermentation increased apparent Mw of residual lignin, consistent with preferential consumption of lower-MW fractions. PIPOL reduced β-O-4 and β-5 linkages and increased hydrophilic functional groups (phenolic hydroxyl and carboxyl), particularly for SEP and AFEX; these streams were enriched in H- and G-type units, aligning with P. putida metabolic capabilities.
• Techno-economics: Minimum PHA selling price (MPSP) varied by configuration: $6.18/kg (AFEX-PIPOL), $6.82/kg (SEP-PIPOL), $8.35/kg (LHW-PIPOL), $9.58/kg (DSA-PIPOL), and $11.99/kg (SHP-PIPOL). Higher lignin and PHA yields/titers reduced MPSP; operating costs (47–58%) were the largest contributors, followed by raw materials (31–44%), with fixed capital and taxes/insurance comprising the next largest shares.

Integrating PIPOL into existing pretreatment-based biorefineries resolves the lignin- versus carbohydrate-first tradeoff by simultaneously enhancing lignin bioconversion and carbohydrate hydrolysis. By solubilizing and depolymerizing lignin to lower-molecular-weight, more hydrophilic fractions, PIPOL improves microbial processibility and PHA production without sacrificing, and often enhancing, enzymatic saccharification efficiency. The strongest synergy occurs with AFEX and SEP, which, when combined with PIPOL, deconstruct lignin–carbohydrate complexes, remove/deposit and then remove lignin, increase carbohydrate accessibility, and tailor lignin chemistry to match P. putida pathways (H/G enrichment, fewer β-O-4/β-5 linkages). Techno-economic analysis confirms that embedding PIPOL within a lignocellulosic ethanol plant reduces capital intensity relative to stand-alone lignin-first schemes and can achieve competitive MPSP when yields and titers are high. These findings demonstrate a viable path to transform biorefineries toward improved carbon efficiency, product diversification (PHAs as coproduct with ethanol), and better overall economics.

The study introduces ‘Plug-In Processes of Lignin’ (PIPOL) as an add-on module that integrates lignin solubilization, conditioning, and microbial fermentation into established biorefinery pretreatments. PIPOL increases lignin solubility and reactivity (lower molecular weight, more hydrophilic functional groups), leading to enhanced PHA production by Pseudomonas putida KT2440, while concurrently improving carbohydrate hydrolysis and sugar yields. The best-performing integrations (AFEX-PIPOL and SEP-PIPOL) achieved PHA titers up to 4.5 g/L and the lowest projected MPSP ($6.18–$6.82/kg). Mechanistic analyses link improved performance to reduced interunit linkages and tailored functional groups and S/G/H composition. Future work should optimize process conditions to further boost PHA yields/titers, reduce enzyme loadings, expand to other lignin-derived products, and refine TEA with scale-up data to drive down operating and raw material costs.

• Performance depends strongly on pretreatment-PIPOL configuration; not all pretreatments yield equally processible lignin (e.g., SHP showed lower soluble lignin yield and fermentation performance). This can limit generalizability across feedstocks and processes.
• Fermentations were conducted in flask-scale fed-batch with 24 h runs; scale-up dynamics, oxygen transfer, and process control effects on PHA titer/yield remain to be validated.
• TEA relies on modeled integrations with NREL 2011 assumptions; projected MPSP is sensitive to PHA yield and titer, raw material, and operating costs, which may vary at commercial scale.
• Residual sugars and lignin chemistries vary by configuration, requiring conditioning (pH adjustment) and potentially further detoxification/optimization for robust microbial performance.
• Enzymatic hydrolysis improvements were demonstrated at specific enzyme loadings; broader enzyme cost sensitivity and potential for enzyme recycling were not fully explored.