Recent Lab-Scale Advances in Biomass Pyrolysis for Biochar Production

Introduction

Biomass pyrolysis is a thermochemical process that converts organic feedstocks into biochar (solid carbon-rich residue), bio-oil (liquid hydrocarbons), and syngas, in the absence of oxygen. It is gaining attention as a carbon removal and energy technology, since stable biochar can sequester biogenic carbon in soils while co-producing renewable fuels. Conventional pyrolysis is endothermic and typically requires external heating, which can limit scale-up due to heat transfer constraints. Recent research (2024–2025) has focused on innovative laboratory-scale reactor designs and process modes to intensify pyrolysis, improve energy efficiency, and tailor product yields and quality. This review highlights four core technological advances: autothermal pyrolysis, microwave-assisted pyrolysis, catalytic co-pyrolysis, and other lab-scale reactor innovations. For each, we summarise recent experimental findings, reactor configurations, operating conditions, and performance metrics such as temperatures, product yields, biochar properties (e.g. elemental H/C ratios), and feedstock/catalyst materials. The discussion is confined to technical developments and results, without addressing monitoring or policy mechanisms. All units are reported in SI (temperatures in °C, etc.), and UK English spelling is used.

Autothermal Pyrolysis

Autothermal pyrolysis (also known as oxidative pyrolysis) is an advanced mode where the reaction’s heat is supplied internally by controlled oxygen injection rather than external heaters. In practice, a small airflow is introduced (e.g. as the fluidising gas in a reactor), causing partial combustion of pyrolysis products in situ. This internal exothermic heating eliminates the external heat transfer bottleneck and intensifies the process, allowing higher throughput and simpler, scale-up-friendly reactor designs. For example, researchers at Iowa State modified a fluidised-bed fast pyrolyser to operate autothermally using air-blown corn stover feed, achieving several-fold higher biomass feed rates than allothermal (externally heated) operation

Recent lab-scale studies demonstrate the performance and controllability of autothermal systems. Cavalloni et al. (2025) investigated a 15 kW oxidative pyrolyser for wood pellets over a range of air flow rates. By varying the sub-stoichiometric air flux from 0.03 to 0.14 kg·m−2·s−1, they could tune the reactor’s useful heat output between ~4 and 30 kW while maintaining stable operation . The biochar yield ranged from about 12 wt% up to 24 wt% of feedstock, with higher air inputs (and thus higher peak pyrolysis temperatures around 700 °C) giving lower char yields. Even at the lowest char-yield conditions (~12%), the resulting char met stringent quality standards (European Biochar Certificate “Feedstock Plus” grade). Notably, at lower temperatures (~520 °C) the process retained up to ~40% of the biomass energy in the biochar, whereas at higher temperatures around 720 °C roughly 26% was retained in char with the rest released as heat . The biochars produced at the high-temperature end were highly carbonised (approaching anthracite-like composition with O/C <0.03 and H/C <0.35 molar ratios), indicating a very stable carbon form. These conditions still produced quality char with minimal polycyclic aromatic hydrocarbon (PAH) contaminants (e.g. ~1.6 mg/kg, well below EBC limits). Autothermal operation thus can yield a range of biochar fractions, all with high carbon stability, while supplying useful energy. Furthermore, it has been shown that the process can be controlled rapidly via the air feed: adjusting the primary air can ramp reactor power output up or down within minutes, without upsetting biochar quality. Lab and pilot experiments in Europe have confirmed that oxidative pyrolysis scales from small fixed beds up to ~36 kg/h continuous units without loss of performance. In summary, autothermal pyrolysis is a promising intensification approach that simplifies reactor design (no external heaters), improves energy efficiency, and produces high-quality biochar in a single step.

Microwave-Assisted Pyrolysis

Microwave-assisted pyrolysis (MAP) uses electromagnetic radiation (typically 915 MHz or 2.45 GHz microwaves) to heat biomass volumetrically from within. This approach bypasses the slow conductive/convective heat transfer of conventional reactors, enabling rapid and uniform heating of feedstock particles. A key challenge is that many dry biomasses have poor dielectric loss (inefficient microwave absorption) on their own. Recent advances address this by using microwave absorbers (susceptors) and hybrid heating strategies. For example, adding a modest amount of a microwave-absorbing medium like silicon carbide (SiC), activated carbon, or char can dramatically improve heating rates and temperature uniformity. Ke et al. (2024) reviewed that using carbonaceous absorbents not only accelerates heating but can also act as a catalyst, reforming pyrolysis vapours to increase aromatic oil yields . The absorber type and loading allow tuning of the product distribution: in one comparative test, an inert SiC bed led to a high biochar yield (~60 wt%) and lower bio-oil (~28 wt%) under slow heating, whereas replacing SiC with activated carbon increased oil aromatic content above 180 g·L−1 (with a corresponding drop in char yield). Another advantage of microwave heating is reduced feed preparation requirements; unlike conventional pyrolysers that often require fine grinding, microwave systems can process larger particles with no loss in efficiency. Corn stover particles between 0.5 and 4 mm were found to produce similar volatile yields under microwave pyrolysis, obviating the need for energy-intensive milling.

Researchers in Brazil recently developed a lab-scale hybrid microwave reactor that combines microwave heating with resistive heating for precise temperature control. Their bench-scale MAP system, built from a modified 2 kW microwave oven with an integrated SiC-packed chamber, was automated via Arduino control. This design achieved a controlled heating rate of ~31.9 °C/min and maintained the reactor temperature within ±19 °C of the setpoint. The use of hybrid heating eliminated cold spots and “hot spot” interference from susceptors, yielding consistent pyrolysis products. Leite et al. (2024) validated this system by pyrolyzing sugarcane bagasse at various temperatures (250–550 °C). As expected, lower temperatures gave higher solid yields – at 250 °C, biochar output was high (exact yield not given in abstract) with correspondingly less liquid, whereas at 550 °C the biochar yield dropped but the char had the highest surface area (~25.1 m2·g−1) and more microporosity. The char’s pore structure was confirmed by BET and SEM analyses, showing a porous morphology that could be beneficial for soil applications or adsorption uses. Meanwhile, higher temperatures favoured pyrolysis liquids: at 550 °C the bio-oil fraction increased, containing a range of low-molecular compounds (aldehydes, ketones, phenols, etc.) identified by GC–MS. An interesting finding is that microwave heating can produce very carbon-rich char even at moderate temperatures. Due to rapid in situ heating and possible plasma effects, lignin-rich components tend to form char with a high degree of graphitisation. In one study, char derived from lignin had ~89.5% graphitic carbon content and enhanced electrical conductivity; this suggests microwave chars may have unique properties (e.g. for electrode or material use) alongside their carbon sequestration role. Overall, microwave-assisted pyrolysis has emerged in 2024–2025 as a fast and flexible lab-scale technique. The latest systems demonstrate improved control and efficiency, producing biochars with tailored surface properties and providing high heating rates without thermal runaways. Continuing research is addressing scale-up challenges (e.g. magnetron efficiency and energy demand) and the integration of microwave pyrolysis into continuous feed systems.

Catalytic Co-Pyrolysis

Catalytic pyrolysis involves introducing catalysts or co-feeds to the biomass during pyrolysis to influence reaction pathways. The goal is often to improve the quality of vapours (and hence bio-oil) by cracking heavy compounds and removing oxygen, or to increase desirable products like aromatic hydrocarbons. In recent years, catalytic co-pyrolysis of biomass with waste plastics has gained attention as a strategy to both enhance biofuel yields and dispose of plastics. Biomass is rich in oxygen, whereas plastics (polyolefins) can act as hydrogen donors during co-pyrolysis, mitigating the formation of oxygenated tars and promoting hydrocarbon production. A 2025 laboratory study by Mishra et al. examined co-pyrolysis of low-value wood (Jungle Cork Tree) with varying proportions of non-recyclable PET plastic in a semi-batch reactor at 500 °C. They found that blending 50 wt% PET with biomass increased total volatile yields (more oils/gases) at the expense of char: the char yield dropped by about 9 percentage points compared to pure biomass pyrolysis. However, the char that did form was of significantly higher quality – at 50% plastic, the biochar’s fixed carbon content was ~22.6% higher and its higher heating value ~6.17 MJ·kg−1 greater than char from biomass alone. The blended char also showed lower oxygen content (by ~18%) and a more ordered carbon structure (higher Raman I_D/I_G, indicating increased graphitic domains). These improvements suggest co-pyrolysis char may be more stable and energy-dense. On the vapor side, TGA-FTIR analysis confirmed that adding 30–80% PET sharply increased hydrocarbon release while suppressing the formation of CO2 and CO from biomass decomposition. Essentially, the plastic provides additional hydrogen that helps convert biomass oxygenates into H2O rather than CO/CO2, and yields more oil-phase organics. This synergy produces a bio-oil with lower oxygen fraction and higher calorific value, addressing a key limitation of pure biomass pyrolysis (which often yields acidic, oxygen-rich oils).

Besides plastics, numerous catalysts have been tested in lab pyrolysers to upgrade bio-oil or target specific products. Zeolite catalysts (like HZSM-5) are widely used to crack pyrolysis vapours, increasing aromatic hydrocarbons (BTEX – benzene, toluene, xylenes) and deoxygenating phenolic compounds . Metal oxides (e.g. CaO, MgO) can capture or react with oxygenates, raising the bio-oil’s hydrogen-to-carbon ratio and reducing corrosive acids. One recent review noted that introducing catalysts can significantly decrease the yield of reactive oxygenates (like furans, aldehydes) in bio-oil, instead favoring the formation of stable aromatics and aliphatics. Catalytic fast pyrolysis is also reported to lower coke deposition on reactor surfaces by promoting continuous volatilisation of heavy fragments. A striking result from co-pyrolysis research comes from Nie et al. (2024): using a spent fluid catalytic cracking (FCC) catalyst with a 4:1 polyethylene to pine sawdust blend, they achieved up to ~71% of the carbon converted into liquid hydrocarbons (petrochemical-range products) at ~600 °C. This exceptionally high carbon conversion to oil underscores how effective catalysts plus co-feeding of hydrogen-rich material can be in driving pyrolysis toward fuel production. However, such conditions yield very little biochar. In contexts where biochar is the desired product for carbon sequestration, catalytic co-pyrolysis is more useful for tuning char properties or reducing contaminants than for maximising char quantity. For instance, catalysts have been used to immobilise or remove pollutants in the char when pyrolysing waste feedstocks. A late-2024 study of sewage sludge pyrolysis in Australia employed a novel two-stage (pyrolysis–gasification) reactor with catalysts to destroy PFAS (per- and polyfluoroalkyl substances) in situ. The resulting biosolid-derived biochar was cleaner (lower in persistent organic pollutants) and met safety criteria while capturing the sludge’s carbon in solid form. Such developments show the versatility of catalytic approaches in tailoring both bio-oil and biochar outcomes from pyrolysis, depending on whether the priority is fuel production, carbon retention, or contaminant mitigation.

Lab-Scale Reactor Innovations

Beyond specific process modes, researchers have introduced various innovative reactor designs at laboratory scale in the past two years. These designs aim to improve heat transfer, residence time control, and scalability of biomass pyrolysis. One example is the staged free-fall pyrolysis reactor reported by He et al. in 2024. This novel unit consists of a vertical tube divided into sequential zones: a preheating zone for drying/pretreatment, a high-temperature free-fall pyrolysis zone, a char collection section, and a downstream vapour upgrading section with a catalyst bed. Biomass (or biomass-plastic mixtures) can be fed semi-continuously (~50 g·h−1) at the top; it then travels by gravity through the stages and is pyrolysed in a plug-flow like manner. This setup allows very fast heating of feed (simulating flash pyrolysis in the free-fall section) while also enabling ex situ catalytic treatment of the vapours in a separate hot zone. The reactor can be operated under inert gas at atmospheric pressure or even pressurised up to ~50 bar, providing a wide range of testing conditions in a single apparatus. Using this system, the authors obtained high liquid yields – for example, non-catalytic fast pyrolysis of pine sawdust at 475 °C gave about 63 wt% bio-oil (on biomass input). When they pyrolysed a biomass with high ash content (paper sludge), the oil yield was lower (~51 wt%) due to mineral interference, but they could then demonstrate the reactor’s upgrading capability: running the paper sludge pyrolysis at 475 °C followed by an ex situ HZSM-5 catalyst at 550 °C produced a biphasic liquid (≈25.6 wt% aqueous phase, 11 wt% organic phase) enriched in light aromatics. In another trial, the free-fall unit was used for polypropylene plastic pyrolysis; it achieved ~77 wt% liquid fuels yield at 500 °C in thermal mode, and when a catalyst was applied in situ, it produced BTEX aromatics corresponding to ~7.8 wt% of the plastic feed. These results illustrate the flexibility of staged reactor designs to perform combined fast and slow pyrolysis or integrated vapour upgrading in one system. Such lab reactors accelerate development of pyrolysis processes by allowing quick reconfiguration and testing of catalysts, pressures, and feed mixtures in a controlled environment.

Another notable direction is the integration of external renewable heat sources for pyrolysis. Researchers are revisiting allothermal reactor designs wherein the endothermic heat is supplied by electricity or concentrated solar thermal energy, rather than by combusting part of the biomass. The motivation is to maximise carbon retention in products (especially in biochar) by avoiding oxidising any of the feedstock’s carbon to CO2. One cutting-edge concept is solar-driven pyrolysis. Amjed et al. (2024) modelled a 10 MW solar-powered pyrolysis plant using a falling-particle solar tower to heat a continuous biomass flow. They found that a design combining solar heating with a backup biochar combustor could achieve ~83–90% overall carbon efficiency (fraction of feed carbon ending in biochar + bio-oil), significantly higher than a conventional case (~74% efficiency) that burns some char for process heat. The trade-off was cost: the fully solar option had a bio-oil production cost ~20% higher due to the capital for solar receivers and thermal storage. However, hybrid configurations narrowed this cost gap. On a lab scale, practical demonstrations of solar pyrolysis have been conducted using concentrated light from Fresnel lenses or solar simulators. Joardder et al. achieved direct solar pyrolysis of biomass in a fixed-bed reactor, reporting ~33% reduction in fuel costs and ~32% less CO2 emissions compared to a conventional heated reactor, for the same bio-fuel output. Solar-thermal reactors can readily reach the 400–600 °C range needed for pyrolysis, and with thermal storage, they can operate beyond daylight hours. While solar pyrolysis is still experimental, these studies show it can produce high-quality biochar and bio-oil with a very low carbon footprint for the process energy.

Elsewhere, researchers are addressing feedstock-specific challenges with new reactor configurations. For wet and heterogeneous materials like sewage sludge or municipal solid waste, combined pyrolysis–gasification or multi-stage reactors are being tested to improve stability and handle contaminants. A novel closed coupled pyrolysis and gasification pilot reactor was reported in late 2024 for wastewater biosolids. It utilised a fluidised-bed heat exchanger design to uniformly pyrolyse the sludge, then gasify a portion of the char and tar in a second stage. This approach successfully processed ~1100 kg·h−1 of sludge in autothermal mode (self-sustaining on its own gas energy) and produced a pathogen-free, PFAS-decontaminated biochar. Similarly, two-stage pyrolysis has been applied to plastic waste to optimise liquid fuel outputs: e.g. a recent design used a first-stage vacuum pyrolysis followed by a second-stage catalytic reformer to crack plastic waxes, achieving high oil yields with low char residue. Although such systems extend beyond pure biochar production, they contribute to the broader advancement of thermochemical reactor engineering at lab scale. By improving heat integration, mixing, and multi-phase reaction control, these innovations ultimately benefit biomass pyrolysis as well.

In summary, the period 2024–2025 has seen significant progress in laboratory reactor technology for pyrolysis. Autothermal reactors prove that feeding a bit of air can scale up throughput and still deliver high-yield, high-carbon biochar. Microwave reactors offer precision and speed, producing unique char materials. Catalytic and co-pyrolysis strategies open new pathways to upgrade products or handle challenging feedstocks. And inventive reactor designs – from staged drop-tube units to solar-assisted plants – are expanding the boundaries of how and where pyrolysis can be deployed. These advances at lab scale lay the groundwork for the next generation of pilot and commercial systems aimed at efficient biochar production and negative-emission bioenergy.

Conclusion

Recent laboratory innovations have markedly enhanced biomass pyrolysis techniques, with direct implications for biochar production and carbon removal. Autothermal (oxidative) pyrolysis has demonstrated self-heating reactors that intensify throughput and yield biochars with very low H/C and O/C (approaching coal-like stability) . Microwave-assisted systems offer an electrically-driven route to fast pyrolysis, enabling uniform heating of unprocessed biomass and giving fine control over temperature profiles and product selectivity . Catalytic and co-pyrolysis approaches, using zeolites, metal oxides or co-feedstocks such as plastics, have shown the ability to tailor bio-oil composition (e.g. higher aromatics, lower oxygen) while also improving char properties or addressing feedstock contaminants  . In parallel, novel reactor configurations at lab scale – including staged free-fall reactors, fluidised-bed heat exchangers, and solar-thermal pyrolysers – are overcoming traditional design limitations and integrating pyrolysis with upgrading or sustainable heat sources  . Collectively, these breakthroughs point to a future generation of pyrolysis systems that are more efficient, controllable, and feedstock-flexible than before. For stakeholders in the UK concerned with carbon removal and bioenergy, such technical advances are crucial. They suggest that higher carbon yields (in biochar), greater energy recovery, and cleaner operation (via in-process oxygen or catalyst use) are all achievable in pyrolysis units coming out of the lab today. The challenge ahead lies in scaling up these innovations from bench to pilot scale without loss of performance. Ongoing research is addressing scale-up issues like heat management in larger autothermal reactors, continuous feeding in microwave units, catalyst longevity, and integration of renewable heat. As these technologies mature, policy-makers and industry can expect more cost-effective and sustainable pyrolysis solutions for carbon sequestration and bioenergy production. The recent lab-scale results provide a strong evidence base; with quantitative performance metrics – to inform future deployment and support for pyrolysis-based carbon removal initiatives.

References

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2. Cavalloni, F. C., et al. (2025). Ibid. 

3. Bioeconomy Institute – Iowa State University. (n.d.). Autothermal Pyrolysis (webpage)  

4. Leite, J. C. S., Suota, M. J., Ramos, L. P., Lenzi, M. K., & Luz, L. F. L. (2024). Development of a Microwave-Assisted Bench Reactor for Biomass Pyrolysis Using Hybrid Heating. ACS Omega, 9(23), 24987–24997. 

5.Leite, J. C. S., et al. (2024). Ibid. 

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7.Ke, L., et al. (2024). Ibid. 

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9.Mishra, R. K. (2025). Co-pyrolysis of low-value wood sawdust and non-recyclable plastics into char: effect of plastic loading on char yield and its properties. RSC Sustainability, 3(4), 1774–1787.  

10.Mishra, R. K. (2025). Ibid. 

11.Nie, J., et al. (2024). Catalytic Co-pyrolysis of Biomass and Different Plastics. Energy & Fuels, 38(??), 8740–8748. 

12.He, S., Osorio Velasco, J., Strien, J. R. J., et al. (2024). Novel Staged Free-Fall Reactor for the (Catalytic) Pyrolysis of Lignocellulosic Biomass and Waste Plastics. Energy & Fuels, 38(10), 8740–8748.  

13.He, S., et al. (2024). Ibid. 

14.Amjed, M. A., Sobic, F., Romano, M. C., Faravelli, T., & Binotti, M. (2024). Techno-economic analysis of a solar-driven biomass pyrolysis plant for bio-oil and biochar production. Sustainable Energy & Fuels, 8, 4243–4262.  

15.Amjed, M. A., et al. (2024). Ibid. 

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