Oceanic Algal Growth for CO₂ Removal: Why the Concept Excels on Paper but Falls Short in Practice

Abstract

Marine algal cultivation has often been presented as a natural extension of blue carbon strategies and an apparently limitless route to atmospheric carbon dioxide (CO₂) removal. While the biological principles are sound - photosynthetic uptake of dissolved inorganic carbon (DIC) and conversion into organic biomass - the translation of these processes into genuine, certifiable greenhouse gas removals remains unresolved. This paper reviews the underlying mechanisms, the scientific and engineering constraints, and the reasons why open-ocean or sub-aquatic algal systems are unlikely to deliver verifiable CO₂ removal at scale under current frameworks.

1. Introduction

The idea of using marine algae to mitigate climate change is intuitively appealing. Oceans cover more than 70 per cent of the planet, macroalgae such as kelp can grow at extraordinary rates, and seawater already contains dissolved carbon that can be photosynthetically fixed. A growing number of start-ups and research initiatives have proposed large-scale algal farming, open-ocean fertilisation, or sub-aquatic installations as potential carbon dioxide removal (CDR) pathways.

However, when the chemistry, physics, and verification requirements of greenhouse gas removal are examined in detail, most of these approaches face fundamental barriers. The challenge is not that algae cannot grow — but that growth alone does not equal atmospheric CO₂ removal.

2. Mechanism of carbon uptake

Algae — micro or macro — do not absorb gaseous CO₂ directly from the atmosphere. Instead, they draw on the dissolved inorganic carbon present in seawater, primarily bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.

When algae photosynthesise, they lower the local concentration of DIC and hence the partial pressure of CO₂ (pCO₂) in the surrounding water. In theory, this local drawdown can cause additional CO₂ to diffuse across the air–sea interface to restore equilibrium. The degree to which this occurs depends on temperature, salinity, wind mixing, and other factors that govern gas exchange.

In practice, only a fraction of the carbon fixed by marine algae represents new atmospheric CO₂ removal; the rest simply redistributes carbon within the ocean’s existing inorganic pool.

3. Why the concept looks attractive

From a systems perspective, oceanic algal growth has several features that appear favourable:

• Abundant area: vast ocean surfaces theoretically available for cultivation.

• No land conversion: avoids direct competition with agriculture.

• Low input medium: seawater provides the growth environment and major nutrients can, in principle, be recycled.

• Visual simplicity: the notion of “growing carbon sinks” is easy to communicate.

These advantages make oceanic algae an attractive research topic and a recurring theme in popular discussions of negative emissions. Yet they mask the chemical and logistical realities of converting oceanic photosynthesis into measurable removals.

4. Why it fails as a greenhouse gas removal method

4.1 Not direct atmospheric CO₂ removal

Marine algal photosynthesis draws on DIC rather than directly capturing atmospheric CO₂. The subsequent equilibration between seawater and air is partial and slow, and depends on environmental conditions that are highly variable. Quantifying how much atmospheric CO₂ is actually displaced therefore requires complex coupled ocean–atmosphere modelling.

Without this quantification, the process cannot meet the accounting standards required for inclusion in regulated GGR or carbon credit systems.

4.2 Uncertain permanence

Even when marine biomass production is high, the carbon is seldom stored permanently. If algae decompose near the surface or are consumed by marine organisms, the carbon quickly returns to seawater and, ultimately, to the atmosphere.

Long-term storage would require either:

• deliberate sinking of biomass to depths greater than 1 000 m, or

• conversion of harvested material into stable forms such as biochar or bio-oil on land.

Both options introduce energy, cost, and environmental risks. Deep-ocean deposition, in particular, raises ecological and legal concerns under the London Convention on ocean dumping.

4.3 Poor scalability and resource intensity

Modelling studies suggest that to remove even 1 Gt CO₂ yr⁻¹ through macroalgal sinking, vast ocean areas — on the order of hundreds of thousands of square kilometres — would be required, along with large nutrient inputs and extensive infrastructure for mooring, harvesting, or towing. The logistical footprint and cost exceed most realistic deployment scenarios.

4.4 Lack of certifiable measurement, reporting, and verification (MRV)

Under voluntary and emerging compliance markets, a removal pathway must demonstrate additionality, verifiability, and permanence. For open-ocean systems, none of these can currently be guaranteed.

Monitoring air–sea CO₂ fluxes and deep-ocean carbon burial requires continuous chemical and isotopic measurements that are infeasible at operational scale. The UK Government’s Independent Review of Greenhouse Gas Removals (2025) acknowledges that “marine CDR remains scientifically intriguing but is not yet suitable for inclusion in national inventories” due to unresolved MRV and ecological impact issues.

5. Sub-aquatic installations and near-shore concepts

Several organisations are developing sub-aquatic algae installations within ports, harbours, or coastal zones, often with additional aims such as water quality improvement or biodiversity enhancement. These systems may provide local environmental benefits, but from a carbon accounting standpoint they remain subject to the same limitations:

• Uptake is from seawater, not directly from the atmosphere.

• Biomass decomposition typically returns carbon within short timeframes.

• Quantitative verification of net CO₂ removal is not yet possible.

Such projects therefore contribute to mitigation or adaptation rather than to certified carbon removal.

6. Pathways toward credible alternatives

Progress in this field is not without merit. Marine biology and coastal restoration play important roles in ecosystem health and carbon cycling. The key lesson is that controlled, measurable systems are required for true removals accounting.

Closed or semi-closed photobioreactor systems — operating on seawater media but with defined process boundaries and stable end-of-life carbon storage — offer a more defensible route to permanence. These approaches maintain the advantages of marine feedstocks while avoiding the uncertainties of open-ocean operations.

7. Conclusions

Oceanic algal growth represents an elegant concept that leverages nature’s largest carbon reservoir, yet it does not currently satisfy the scientific or regulatory criteria for greenhouse gas removal. The process is indirect, difficult to measure, and unlikely to provide permanent storage without significant intervention.

Until measurement technologies and regulatory frameworks advance substantially, oceanic algae should be regarded as a valuable component of marine ecosystem management, not as a scalable or certifiable pathway for carbon dioxide removal.

References

1. Boyd, P. W. et al. (2024). Macroalgae open-ocean mariculture and sinking (MOS) as a potential CO₂ removal pathway: constraints and uncertainties. Earth System Dynamics, 14, 185–210.

2. Krause-Jensen, D. & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Science, 354(6319), 70–74.

3. Department for Energy Security and Net Zero (2025). Independent Review of Greenhouse Gas Removals. GOV.UK, March 2025.

4. Keller, D. P. et al. (2023). The status of marine carbon dioxide removal research and readiness. Annual Review of Marine Science, 16, in press.

5. MBARI (2024). Measurements of oceanic CO₂ uptake following phytoplankton bloom development. Technical memorandum.

Policy Review: The Independent Review of Greenhouse Gas Removals and the Role of Bioengineered Systems

1. Overview

In March 2025, the UK Government published the Independent Review of Greenhouse Gas Removals (GGRs), a major assessment of how engineered and nature-based carbon removal can contribute to achieving net zero. The Review emphasises that GGRs must move from conceptual to operational scale through clarity of regulation, credible monitoring, and strong domestic innovation. It calls for clear definitions of permanence, high environmental integrity, and equitable regional deployment to ensure that removals deliver genuine national benefit.

The Review situates GGRs as an essential complement to emissions reduction, recognising that even the most ambitious decarbonisation pathways will leave residual emissions from heavy industry, agriculture, and transport. Engineered removals such as direct air capture (DAC), bioenergy with carbon capture and storage (BECCS), and biochar are expected to fill this gap.

2. The relevance of engineered biochar and algal systems

Among the portfolio of removals reviewed, biochar is identified as a low-regret option with strong co-benefits for soil health and circular economy outcomes. However, the Review highlights uncertainty in the permanence of carbon storage, variation in standards, and the need for rigorous monitoring, reporting, and verification (MRV).

These are areas where bioengineered algal systems can make a significant contribution. Integrating biological CO₂ capture with controlled thermochemical conversion enables traceable, high-integrity removals. In particular, systems that operate within defined process boundaries — from capture to sequestration — can address concerns about additionality, leakage, and double counting.

3. Nellie Technologies’ input to the Review

Nellie Technologies was invited to provide technical evidence to the Independent Review, reflecting its position as one of the UK’s leading developers of integrated CO₂ removal systems. The company submitted data from its operational pilot site in South Wales, detailing energy use, carbon capture efficiency, and biochar stability characteristics.

This contribution helped inform the Review’s discussion of engineered biological pathways and the role of algal feedstocks in the UK’s GGR landscape. Nellie’s evidence demonstrated that purpose-grown microalgal biomass can deliver consistent carbon capture without reliance on waste inputs or volatile commodity feedstocks.

4. Alignment with the Review’s key themes

Permanence and integrity

The Review calls for stronger definitions of permanence and for clear differentiation between temporary and long-lived carbon storage. Nellie’s system addresses this directly. Carbon fixed biologically is converted to stable biochar through pyrolysis and then incorporated into soils, where independent testing confirms high carbon stability.

Regulatory clarity

The Review recommends that Government clarify how biochar-based removals will be regulated and recognised within the UK’s GGR framework. Nellie supports this approach and advocates for standardisation that balances scientific robustness with accessibility for smaller modular operators.

Integration and control

Where the Review identifies risks associated with fragmented supply chains and uncertain feedstock sourcing, Nellie’s approach provides a solution. The company operates as a fully integrated project developer, growing its own proprietary biomass, processing it on-site, and managing sequestration directly. This closed-loop model avoids market volatility and ensures complete control over system boundaries and carbon accounting.

Equitable deployment and regional balance

A recurring theme of the Review is the need for GGR projects to support regional economies and avoid concentration in the South East of England. Nellie’s deployment in South Wales directly aligns with this principle. The company regenerates brownfield and post-industrial land for productive use, creating skilled engineering roles and contributing to regional decarbonisation infrastructure.

5. Resource and land use considerations

The Review notes that GGR expansion must not exacerbate competition for land, water, or biomass resources. Nellie’s model inherently avoids these pressures. Biomass is produced internally using controlled photobioreactors, requiring no agricultural land conversion and minimal freshwater input. Sites are co-located with existing infrastructure and carbon sources, demonstrating compatibility with the UK’s principles for sustainable land use and resource efficiency.

6. Economic and policy outlook

The Review’s economic chapter highlights Contracts for Difference (CfDs) as a potential mechanism for early deployment of engineered removals. For operators with reliable, measurable outputs, CfDs could provide the financial certainty needed to scale domestic GGR capacity. Nellie Technologies is well placed to participate in such mechanisms due to its quantifiable carbon yields and predictable operational performance.

By integrating biomass production, carbon capture, and permanent storage into a single system, Nellie offers a pathway that aligns with Government priorities: durable removals, UK-based innovation, and verifiable environmental integrity.

7. Conclusion

The Independent Review of Greenhouse Gas Removals confirms that engineered solutions will be indispensable for the UK’s net zero transition. It also recognises the value of modular, regionally distributed projects that combine technological rigour with environmental responsibility.

Nellie Technologies’ operational model embodies these principles. By linking microalgal CO₂ capture with permanent biochar sequestration, and by delivering this through fully integrated, locally anchored systems, Nellie provides a blueprint for the kind of secure, scalable, and regionally balanced GGR infrastructure envisioned in the Review.

References

1. Department for Energy Security and Net Zero (2025). Independent Review of Greenhouse Gas Removals. GOV.UK, March 2025.

2. UK Government (2025). Independent Review of Greenhouse Gas Removals – Full Report. ISBN 978-1-5286-5365-9.

3. Nellie Technologies (2025). Technical Data Submission to the GGR Independent Review: Algal Biomass Carbon Removal and Biochar Stability. Submitted January 2025.

Technical Review: Innovation Pathways in Algal Biomanufacturing

Technical Review: Innovation Pathways in Algal Biomanufacturing

Author: Nellie Technologies

Date: October 2025

Abstract

This technical review analyses “Algae Biotech Part III: Innovation to Facilitate Commercialisation” by Vitivise (2025). The original article argues that biological potential is no longer the limiting factor for algal production. Instead, the barriers to commercialisation lie in system design, environmental control, and cost optimisation. Nellie Technologies evaluates these arguments from the perspective of its operational experience in carbon dioxide removal (CDR) and biomass valorisation. Drawing on pilot-scale data from photobioreactor (PBR) systems and downstream biochar conversion, the review discusses the technical and economic parameters that govern productivity, strain stability, and process environment control.

1. Introduction

The Vitivise paper provides a clear overview of the challenges facing algal biotechnology as it transitions from research to industrial production. Its central thesis is that improvements in biology alone cannot drive large-scale deployment. Instead, scalability depends on integrated solutions that combine robust engineering, adaptive control, and cost efficiency.

This position aligns with current techno-economic data across the CDR and biomanufacturing sectors. At Nellie Technologies, real-world reactor trials show that stable operation at high productivity depends primarily on environmental and mechanical optimisation. The biological ceiling for growth is well understood. The task now is to maintain that performance in variable outdoor conditions at acceptable cost.

2. Geometric Productivity and Economic Sensitivity

Vitivise identifies geometric productivity as the most significant variable influencing unit cost. Tredici (2016) reported an average productivity of 15 g m⁻² day⁻¹ across the warmest months, equivalent to 30–40 tonnes of dry biomass per hectare per year. These figures are comparable to terrestrial energy crops and indicate that photosynthetic efficiency is not the main constraint. The difficulty lies in sustaining this rate throughout the year.

Geometric productivity depends on light path, gas-liquid mass transfer, and residence time. Reactor geometry determines irradiance distribution, CO₂ dissolution, and oxygen stripping. Environmental variability causes these parameters to fluctuate continuously. The Vitivise analysis correctly notes that most current systems cannot maintain equilibrium across changing light and temperature regimes.

Nellie’s pilot systems confirm that productivity loss often results from inconsistent CO₂ delivery and pH imbalance rather than strain performance. Implementing predictive feedback control based on environmental monitoring has been shown to stabilise growth and reduce downtime. This supports the conclusion that environment, not biology, dictates achievable productivity in continuous outdoor systems.

3. Strain Optimisation and Environmental Adaptability

The discussion of Wlodarczyk et al. (2020) illustrates how strain selection can significantly improve volumetric productivity. The Synechococcus sp. PCC 11901 strain achieved a three-fold increase in growth rate under laboratory conditions. However, as Vitivise notes, the scalability of such results is uncertain. Many laboratory strains fail to replicate performance outdoors due to fluctuating light intensity, nutrient gradients, and gas exchange limitations.

For climate-relevant applications, strain selection must balance three factors. The first is productivity under variable light and temperature. The second is tolerance to elevated CO₂ and intermittent nutrient supply. The third is compatibility with downstream conversion processes such as drying, pyrolysis, or nutrient recovery.

Nellie’s operational data indicate that incremental improvements in tolerance, achieved through adaptive evolution rather than full genetic modification, can improve consistency without introducing regulatory complications. However, strain development cycles remain long and resource-intensive. Engineering toolboxes often lag behind discovery, delaying commercial implementation. Vitivise’s argument therefore holds: progress in strain biology must be matched by equal progress in system engineering if meaningful cost reduction is to be achieved.

4. Reactor Design and Process Environment Control

Vitivise highlights the 2016 CellDEG work on membrane-based gas-exchange reactors, which achieved volumetric productivities up to 5 g L⁻¹ day⁻¹. These results demonstrate that controlled gas delivery and oxygen removal can dramatically improve cell density. The limitation, as Vitivise acknowledges, is scalability. Systems requiring complex membranes or high-pressure gas distribution may struggle to operate economically at hectare scale.

At Nellie Technologies, similar challenges arise when balancing light penetration, mixing energy, and gas transfer efficiency. Optimisation of these parameters determines both productivity and energy cost per tonne of biomass produced. Computational modelling suggests that modest improvements in hydrodynamic design can yield significant energy savings.

External environmental variation is the next major factor. Continuous outdoor operation must accommodate fluctuating temperature and irradiance. Models that incorporate weather forecasting and dynamic inoculation strategies, such as those discussed by De Luca (2017) and Long (2022), show strong potential. This is an area of active research at Nellie, where predictive algorithms are being developed to modulate aeration and nutrient delivery in anticipation of environmental change.

5. Techno-Economic Scalability

The Vitivise paper notes that only a few continuous operations have published cost data. Tredici (2016) estimated biomass costs north of 10 USD kg⁻¹, primarily due to high capital and labour inputs. For algal systems to compete with sub-1 USD kg⁻¹ fossil-derived commodities, both CAPEX and OPEX must fall by an order of magnitude.

Automation, modular construction, and process continuity are therefore critical. Nellie’s approach to scaling emphasises mechanical simplicity and modular replication. Reducing downtime between harvest cycles has proven to be one of the most effective ways to lower cost per tonne of carbon removed. Energy recovery through the coupling of drying and pyrolysis further reduces total system emissions, supporting a net-negative carbon balance.

6. Implications for Carbon Dioxide Removal

While Vitivise’s focus is general biomanufacturing, the implications for engineered carbon removal are clear. If algal productivity can be maintained year-round at or above 30 t ha⁻¹ yr⁻¹, and biomass can be stabilised through pyrolysis into durable carbon products, the pathway to scalable biological sequestration becomes realistic.

Nellie’s PhycoTank™ system integrates microalgal growth with downstream carbon fixation in the form of PhycoChar®. This operational model directly addresses the environmental and economic constraints identified by Vitivise. By controlling gas supply, light geometry, and automated harvesting within modular units, the system achieves both stability and replicability.

The central insight from the Vitivise analysis, that biological limits are secondary to environmental control, aligns with the engineering direction of CDR-focused algae systems. Scalable carbon removal depends not only on photosynthetic performance but on the continuous maintenance of optimal conditions and the valorisation of every process output.

7. Conclusions

The Vitivise paper offers a valuable synthesis of the technical innovations required for algae to transition from niche applications to bulk commodity production. Its argument that biology is no longer the main constraint is persuasive. The limiting factors are reactor geometry, environmental stability, and cost of operation.

From Nellie Technologies’ perspective, the review reinforces the importance of integrated engineering, predictive control, and modular system design. Each incremental improvement in productivity, energy recovery, and process automation contributes to the viability of large-scale CDR.

The future of algal biotechnology lies in convergence. High-resolution biological understanding must combine with robust mechanical design, advanced data analytics, and transparent MRV frameworks. Together, these will allow algae to move from laboratory promise to climate-relevant impact.

References

1. Vitivise (2025). Algae Biotech Part III: Innovation to Facilitate Commercialisation. Substack, September 2025. Available at: https://vitivise.substack.com/p/algae-biotech-part-iii-innovation

2. Tredici, M.R. (2016). Photobiology of microalgae cultivation. Journal of Applied Phycology, 28(6), 15–30.

3. Wlodarczyk, A. et al. (2020). High-rate cultivation of Synechococcus sp. PCC 11901 under industrial conditions. Bioengineering Reports, 3(4), 101730.

4. Ruiz, J. et al. (2016). Challenges and opportunities in microalgal biofuel production. Energy & Environmental Science, 9, 1036–1048.

5. De Luca, S. (2017). Weather-adaptive optimisation of algal bioreactor systems. Journal of Process Control, 55, 44–52.

6. Long, T. (2022). Predictive control in photobioreactor inoculation strategies. Nature Communications, 13, 27665.

7. Bähr, L. et al. (2016). Membrane-based bioreactors for high-density algal cultivation. Algal Research, 18, 200–212.

Attribution

This article reviews and comments on Vitivise (2025) “Algae Biotech Part III: Innovation to Facilitate Commercialisation”, originally published on Substack. Reproduced under fair-dealing provisions for the purpose of critical review.

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

1. Cavalloni, F. C., Strassburg, J., Lustenberger, D., & Griffin, T. (2025). Oxidative Pyrolysis for Variable Heating Output with Wood Pellets. Energies, 18(7), 1702.  

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. 

6.Ke, L., Zhou, N., Wu, Q., et al. (2024). Microwave catalytic pyrolysis of biomass: a review focusing on absorbents and catalysts. npj Materials Sustainability, 2, 24. 

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

8.Sun, W., Yan, Y., Wei, Y., Ma, J., Niu, Z., & Hu, G. (2023). Catalytic Pyrolysis of Biomass: A Review of Zeolite, Carbonaceous, and Metal Oxide Catalysts. Nanomaterials, 15(7), 493.  

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. 

16.(Anonymous) (2024). The pyrolysis of biosolids in a novel fluidized bed heat exchanger reactor: pilot plant trials, biochar properties, gas emissions testing, and fate of PFAS. (Published Nov 2024) 

Technical Review: The Chonkus Cyanobacterium as a High-Density, Fast-Settling Bioproduction Chassis

Abstract

A recently described cyanobacterium nicknamed Chonkus (UTEX 3222, Cyanobacterium aponinum) was isolated from CO₂-rich volcanic seeps off Vulcano Island, Italy. Peer-reviewed work reports rapid growth to very high culture densities and unusually fast sinking behaviour. These traits could reduce harvesting energy and open new options for high-density photobioreactors and solid-liquid separation strategies. This review synthesises the primary literature and authoritative summaries, evaluates engineering implications for industrial photobioreactors, and identifies key knowledge gaps relevant to Nellie’s bioengineered carbon removal systems.

1. Origin and taxonomic assignment

Chonkus derives from shallow marine sites where volcanic seeps deliver near-continuous CO₂, driving low pH conditions in Baia di Levante, Vulcano, Sicily. The strain is catalogued as UTEX B-3222 and taxonomically placed within Cyanobacterium aponinum. The UTEX entry records the collection site conditions and recommended artificial seawater medium for culture. These details suggest innate tolerance to high dissolved inorganic carbon, variable salinity, and warm surface waters.

2. Key reported phenotypes

The primary paper by Schubert et al. reports two traits of direct process interest. First, robust high-density growth under laboratory conditions compared with model cyanobacteria. Second, rapid settling relative to other fast-growing strains, which the authors quantified as accelerated sedimentation in static assays. Public summaries emphasise large cell or colony size and high intracellular carbon storage, with images showing dense, cohesive pellets after short quiescent periods.

Several institutional and media explainers are consistent with the paper. They note fast growth in CO₂-rich conditions, tolerance to variable pH and salinity, and a tendency to form dense aggregates that sink quickly. While secondary sources are not substitutes for the article, they help clarify potential use-cases and have been cross-checked against the peer-reviewed text.

3. Engineering relevance

3.1 Harvesting and dewatering

Rapid sedimentation is uncommon in planktonic cyanobacteria. If maintained at scale, Chonkus could enable gravity thickening, lamella settlers, or low-g centrifugation instead of energy-intensive dissolved air flotation or high-g disk stacks. The potential to operate photobioreactors at higher optical densities with periodic quiescent settling would alter downstream energy balances and capital choices. Validation requires size distribution, floc strength, and hindered settling kinetics under relevant ionic strength and shear, since shear in loop reactors often disrupts flocs.

3.2 High-density growth

The reported high-density cultures suggest advantages for land use and reactor footprint. In closed photobioreactors, light path and mixing become limiting at high optical density, so maintaining productivity requires either short path reactors, strong internal circulation, or light dilution strategies. If Chonkus maintains photosynthetic efficiency at higher densities than common strains, geometric productivity could improve. Comparative areal productivity data across light spectra and temperatures are needed to quantify this advantage.

3.3 Gas transfer and pH control

Chonkus’ native environment is CO₂-rich and acidic. In industrial media, elevated CO₂ delivery depresses pH and increases dissolved inorganic carbon availability, improving Rubisco kinetics. The trade-off is increased inorganic carbon stripping if gas transfer is not optimised. Process design should pair mass transfer coefficients with buffering and pH control to exploit the strain’s tolerance without incurring excessive CO₂ losses.

3.4 Compatibility with downstream valorisation

Reports highlight conspicuous intracellular carbon storage. The biochemical form of this storage matters. If carbon is in glycogen or storage polysaccharides, hydrothermal and catalytic routes will differ from lipid-rich strains. If cells are larger and more carbon dense, pyrolysis mass yields and char quality may shift. Bench tests should quantify elemental C, H, N, S, O, ash, and H/C_\text{org} at typical harvest moisture to model char yield and stability in Nellie’s pyrolysis units.

4. Cultivation options and control strategies

4.1 Suspension growth with triggered settling

A logical mode is standard suspension growth with mixing strong enough to prevent continuous sedimentation, followed by periodic quiescent phases for gravity separation. Control variables include superficial velocity, mixing Reynolds number, and cycle timing to balance light exposure with settle-and-harvest efficiency. This approach suits tubular or flat-panel photobioreactors with side-stream settlers.

4.2 Biofilm or attached growth

Chonkus’ cohesive behaviour suggests potential for biofilm or trickle-bed modes. However, attached growth changes light fields and mass transfer. Bench studies should compare surface attachment tendencies on hydrophilic and hydrophobic carriers, including whether fast settling translates into strong biofilm formation. If biofilm is viable, hydraulic shear and backwash cycles must be tuned to avoid sloughing losses.

4.3 Open systems and risk

Given Chonkus was isolated from natural seawater, open raceways might appear attractive. In practice, contamination pressure, predators, and weather variability argue for enclosed systems until more data are available. The strain’s robust growth in high CO₂ environments does not guarantee competitiveness in open ponds at ambient conditions.

5. Data gaps and experimental plan for Nellie

• Kinetics under industrial light and temperature. Replicate laboratory growth curves under Nellie-relevant photon flux densities, diel cycles, and temperatures. Compare to a well-characterised fast cyanobacterium as control. Track biomass concentration, specific growth rate, photosynthetic efficiency, and pigment content.

• Sedimentation and rheology. Measure isokinetic settling rates, floc strength versus shear, and sludge rheology at target harvest densities. Determine whether lamella settlers or inclined plate clarifiers will meet throughput. Quantify how nutrient levels and ionic strength affect settling.

• Gas transfer and carbon use efficiency. In flat-panel and short-path reactors, map k_\text{La}, pH, and dissolved inorganic carbon under high CO₂ sparging. Optimise CO₂ utilisation by coupling gas composition, superficial velocity, and residence time to minimise stripping.

• Biomass composition and downstream fits. Perform proximate and ultimate analysis, carbohydrate and storage polymer assays, and then pyrolyse wet and dried biomass to compare char yield and H/C_\text{org}. This will establish whether Chonkus improves carbon retention or energy integration in pyrolysis versus current strains.

• Genetic tools and stability. Confirm availability of transformation or genome tools for UTEX 3222. The AEM paper discusses genomics and biochemical composition. If engineering is planned, tool availability will influence timelines.

6. Opportunities and constraints for policy audiences

• Potential energy savings in harvesting: If rapid settling at scale is confirmed, process energy for separation could fall significantly. That would improve net energy balance and operating costs for algal bioproduction facilities. The magnitude depends on actual settling velocities and achievable thickening without polymer aids.

• Land and water footprint: High-density growth offers better land productivity. If productivity scales with culture density, facilities could be more compact, which supports brownfield siting near CO₂ sources. Confirmation requires year-round areal productivity data.

• Responsible deployment: Open water release is not advised. The discovery team itself highlights the need for safeguards and controlled systems. Enclosed photobioreactors with well-defined containment are compatible with UK environmental protection principles while enabling rigorous study.

7. Conclusions

Chonkus combines two rare and industrially attractive traits. It grows to high density and it sinks quickly. Together these traits could reduce harvesting costs and reshape reactor operations if reproduced outside the laboratory. The strain’s origin in CO₂-rich, low-pH volcanic seeps aligns with industrial conditions that feed elevated CO₂ into reactors. The engineering case is not yet proven at scale. The next step is a focused bench-to-pilot programme that quantifies growth and settling under industrial conditions, characterises biomass for downstream thermal conversion, and establishes whether the advantages outweigh control challenges. On current evidence, Chonkus merits near-term laboratory evaluation within Nellie’s photobioreactor platform.

References

1. Schubert, M. G. et al. Cyanobacteria newly isolated from marine volcanic seeps display rapid sinking and robust high-density growth. Applied and Environmental Microbiology 90(11), e00841-24, 2024.

2. Wyss Institute. Newly discovered cyanobacteria could help sequester carbon from oceans and factories. News release, 29 Oct 2024.

3. UTEX Culture Collection of Algae. UTEX B-3222 Cyanobacterium aponinum entry. Accessed Oct 2025.

4. Science News. Meet Chonkus, the mutant cyanobacteria that could help fight climate change. 7 Nov 2024.

5. ScienceDaily. Cyanobacteria newly isolated from marine volcanic seeps display rapid sinking and robust, high-density growth. 29 Oct 2024.

6. EcoWatch. Newly discovered bacteria Chonkus offers potential for carbon capture. 5 Nov 2024.

7. Interesting Engineering. New CO₂-hungry algae found, could eat carbon in oceans. 29 Oct 2024.

8. SynBioBeta. Meet Chonkus: the algae that eats carbon and sinks like a rock. 29 Oct 2024.