HTL Co-Processing: Biomass & Polymer Conversion

• HTL co-processing is a method that converts lignocellulosic biomass and synthetic polymers under subcritical water conditions into biocrude oil with synergistic interactions.
• The process leverages unique chemical pathways, especially with amine-rich polymers like PUR and PA-6, to enhance depolymerization, oil yield, and energy/carbon recoveries.
• Scalable batch and continuous reactor systems, operating at 300–350°C and controlled residence times, optimize feedstock blending for circular-economy valorization.

Hydrothermal liquefaction (HTL) co-processing involves the simultaneous conversion of lignocellulosic biomass and synthetic polymers under subcritical or near-critical water conditions to produce biocrude oil, with enhanced mass, energy, and carbon recovery. Distinct from standalone HTL, co-liquefaction strategies leverage potential chemical interactions and complementary reactivity between polymeric and biological feedstocks, aiming to synergistically upgrade low-value waste streams into refinery-compatible intermediates. Recent studies focusing on Miscanthus spp. and various polymers, notably polyurethanes, polyamides, and polyolefins, provide quantitative, mechanistic, and process-scale insights into the efficacy, pathways, and scalability of these hybrid approaches (Passos et al., 2021, Passos et al., 2021).

1. Experimental Systems and Process Conditions

Bench and pilot-scale HTL co-liquefaction has been conducted in both batch and continuous configurations. Typical batch experiments use 20 mL stainless-steel bomb reactors with 1.00 g dry feedstock blended in defined ratios (e.g., 0.5 g Miscanthus Giganteus and 0.5 g polymer) with deionized water (8–8.5 g), with reaction at 300–350 °C and residence times of 20–30 min under autogenous pressure (≈200–250 bar). No external catalyst is added; all transformations proceed via thermal and in situ autogenic catalysis (Passos et al., 2021, Passos et al., 2021).

Continuous flow pilot plants utilize high-solids (≈13.8 wt %) slurries of milled feedstocks, typically pumped through pre-heated tubular reactors with controlled residence times (e.g., 18–19 min at ≈316 °C), followed by phase separation and energy/utility monitoring to support mass and energy balances. Slurry rheology, pumpability (often enhanced by 0.5 wt % CMC), and rigorous reactor heat integration (e.g., shell-and-tube exchangers) are critical for sustained operation (Passos et al., 2021).

2. Metrics, Yields, and Synergy Quantification

HTL co-processing performance is evaluated using several key metrics:

• Oil yield: $Y_{oil} = \frac{m_{oil}}{m_{feed}} \times 100\%$
• Carbon recovery to oil: $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$
• Chemical energy recovery: $$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$
• Energy return on investment (EROI): $EROI = \frac{E_{oil}}{E_{external\,input}}$

Synergistic effects are formally quantified with the synergy index (SE):

$$SE = \frac{Y_{co}}{X_{misc} Y_{misc} + X_{poly} Y_{poly}}$$

where $Y_{co}$ is experimental yield from co-processing, $Y_{misc}$ and $Y_{poly}$ are yields from single-feed HTL, with $X_{misc}$ and $X_{poly}$ the respective mass fractions. SE > 1 indicates positive synergy; SE < 1 denotes antagonism (Passos et al., 2021).

3. Polymer-Dependent Interactions and Mechanisms

Distinct chemical pathways are observed depending on polymer structure:

• Polyolefins (LDPE, HDPE, PP): Major resistance to depolymerization at 350 °C results in negligible direct oil from the polymer itself. The molten polyolefin phase modestly alters the miscanthus oil composition, notably by increasing alkyl-phenols and ethers—radical-driven alkylation of lignin-derived phenolics at the polymer–biomass interface is implicated (Passos et al., 2021).
• Polystyrene (PS), Acrylonitrile Butadiene Styrene (ABS): Limited monomeric aromatic fragments (e.g., styrene, ethylbenzene) appear in the oil with enrichment of aromatic content, though absolute yields remain low.
• Polyvinyl Chloride (PVC): Hydrolytic dechlorination yields HCl, catalyzing acid-promoted dehydration reactions; results in high char/solid formation and polyaromatic-rich but modest oil.
• Polycarbonate (PC), Epoxy Resin: Backbone hydrolysis releases bisphenol-A–like phenols, producing oil enriched in low-molecular-weight monomers. Secondary condensation with lignin fragments reduces O/C ratio.
• Polyethylene Terephthalate (PET): Subcritical hydrolysis mainly forms terephthalic acid (solid phase) and minor oil rich in phenols, acids, and ketones, with limited interaction synergy.
• Polyamides (PA-6, PA-6/6): Hydrolysis liberates amines and amides, producing NH₃/NH₄⁺, which can catalyze biomass depolymerization. These blends exhibit substantial positive synergy in oil yield.
• Polyurethane (PUR): Hydrolysis of urethane linkages yields primary/secondary amines and isocyanates which engage in nucleophilic attack on biomass-derived carbonyls, giving rise to novel N-heterocyclic aromatics, and drastically increasing both oil yields and energy/carbon recoveries.

A mechanistic summary for these transformations is provided in the following table:

Polymer Class	Dominant Pathway(s)	Oil Effect
Polyolefins/PS	Radical alkylation, no scission	Minor oil compositional shift
PC/Epoxy	Backbone hydrolysis	Bisphenol-A phenols, lower O/C
PA-6, PA-6/6	Amine/amide hydrolysis	N-rich oils, enhanced decomposition
PUR	Urethane bond hydrolysis	High N-heterocycles, strong synergy
PVC	Acid-catalyzed dehydration	Char, polyaromatics, low oil
PET	Hydrolysis, little synergy	Low-MW phenols/acids, limited effect

4. Quantitative Outcomes: Yields, Energy and Carbon Efficiencies

Oil yields, carbon recoveries, and energy recoveries show distinct trends by polymer:

• Pure Miscanthus HTL at 350 °C delivers $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$0, $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$1, and $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$2 in batch.
• PUR/Miscanthus co-HTL achieves $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$3 (batch, 1:1 ratio) with $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$4, $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$5 (Passos et al., 2021). In optimized pilot (PUR:M = 0.78:0.22, 316 °C), $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$6, $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$7, $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$8 are observed, with total energy efficiency $$C_{rec} = \frac{w_C^{oil} \cdot m_{oil}}{w_C^{feed} \cdot m_{feed}} \times 100\%$$9 and $$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$0 (Passos et al., 2021).
• Other polyamides (PA-6, PA-6/6): $$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$1–$$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$2 (batch), with positive synergy (SE = 1.35–1.62).

For polyolefin, PET, and PVC blends, oil yields remain close to the weighted average of single-feed experiments ($$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$3) or are antagonistically reduced due to incomplete polymer conversion or char formation. Only co-feeding of amine-rich polymers (particularly PUR, PA-6, PA-6/6) consistently elevates oil yield and energy/carbon recoveries above pure biomass processing.

5. Oil Chemistry and Product Characterization

Comprehensive chemical analysis of co-HTL oils employs GC–MS, CHNS analysis, FTIR, TGA, and high-resolution FTICR-MS. Critical observations include:

• Biocrude from PUR co-HTL contains high fractions of nitrogen heteroaromatic compounds (indoles, naphthalenediamines, phenanthrolines, pyrazoles), as revealed by FTICR-MS, with even $$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$4 families dominant and double bond equivalence often >8. FTIR identifies strong OH/NH stretches (3030–3660 cm⁻¹) and C–O–C ether signatures (≈1090 cm⁻¹). Polyol oligomers (CₙH₂ₙ₊ₓOₘ) are prevalent. TGA shows a single volatility maximum at ≈400 °C, indicating low char residue (<5 wt %) (Passos et al., 2021).
• Elemental trends: Blending N-containing polymers increases biocrude N/C; H/C rises for PUR and polyamides, implying more hydrogen-rich oils with high higher heating value (HHV), except for PET (which increases O/C and lowers HHV).
• Feedstock signature: Biocrudes resembling the parent polymer dominate at high polymer loading, but oil from synergistic blends displays unique product distributions absent in either feedstock alone.

6. Synergy, Feedstock Selection, and Optimization

The synergy index (SE) reveals that only polymers with accessible amine groups deliver true positive interaction—PA-6 (SE≈1.35), PA-6/6 (SE≈1.62), and PUR (SE≈1.54) at 1:1 mass ratio. Mechanistically, in situ formation of ammonia and reactive amines catalyzes biomass depolymerization, dehydration, and N-heterocycle formation, resulting in more than doubling the oil-phase carbon recovery compared to pure biomass or polymer (Passos et al., 2021, Passos et al., 2021).

Optimization guidelines include:

• Co-feeding 10–50 wt % amine-rich polymers (e.g., PUR, PA-6 variants) to maximize catalytic enhancement without excessive dilution.
• Avoiding high PET or PVC fractions or pretreating these to reduce oxygenation or charification.
• For aromatic monomer recovery, integrating longer residence times or solvent extraction, especially for PC/Epoxy blends.
• Recirculating aqueous phases in continuous systems to retain catalytic NH₃ and further promote biocrude yield (Passos et al., 2021).

7. Process Integration, Scale-Up, and Industrial Implications

HTL co-processing is amenable to scale-up via numbered high-pressure tubular reactors, as demonstrated in continuous pilot-plant campaigns with throughputs >7 kg h⁻¹ (dry solids). Mixing rigid thermoset polymers with biomass not only enhances yields but also confers favorable slurry pumpability and feedstock flexibility (Passos et al., 2021).

Heat integration with counter-current exchangers and moderate utility requirements (≈7.9 kW for 55 kg h⁻¹ feed) drive overall process efficiencies (η_tot ≈ 61 %) and energy return ($$E_{rec} = \frac{HHV_{oil} \cdot m_{oil}}{HHV_{feed} \cdot m_{feed}} \times 100\%$$5) that exceed most conventional thermochemical valorization techniques for mixed waste. The non-requirement for exogenous catalysts reduces cost and complexity. However, the high nitrogen content in PUR- or PA-derived biocrudes implies that downstream hydrotreating or specific refinery co-processing will be essential for fuel blending.

A plausible implication is that HTL co-processing, optimized for synergistic polymer/biomass blends, provides a rigorous, feed-flexible approach for circular-economy-aligned valorization of mixed plastic–lignocellulosic solid waste streams into liquid chemical intermediates compatible with petroleum refining infrastructure (Passos et al., 2021).