Antecedents and scientific foundations of OSB
Metzger and Benford (2001) proposed depositing vegetation biomass in the form of bales of excess crop residues with weights attached in the ocean as a way of avoiding the CO2 that would otherwise enter the atmosphere if the vegetation was allowed to decay. They cited studies, e.g., Hoffert et al. (1980), that indicate CO2 originating on the sea floor several thousand meters deep, such as that from vegetation decay, would exhibit vertical transport measured in millennia, and if this vegetation were covered by ocean sediments, vertical transport could be indefinite.
Keith (2001) challenged Metzger and Benford saying that instead of wasting the energy value locked up in the vegetation biomass by depositing it in the ocean, it would be better to burn the vegetation and produce (almost) net-zero emission power. Keith (2001) essentially argued that burning fossil fuels for power to substitute for the sacrificed energy of vegetation deposited in the sea would produce the same net-zero emissions but would be more costly than burning the vegetation for power, i.e. due to the cost of furnishing the fossil fuels.
Metzger and Benford (2001) responded saying that Keith’s (2001) “argument only would be true if the carbon per unit of primary energy through combustion (C/E), was the same for all organic fuels. In fact this is not nearly so.” Metzger et al. (2002) went on to provide the scientific explanation through chemistry and thermodynamics that because natural gas has a much lower carbon-to-energy production ratio, it produces about half as much atmospheric emissions in a power plant compared to biomass fuel for an equivalent amount of power production. “This strongly suggests that rather than burning crop residue for power generation a combination of crop residue sequestration and power generation from lower carbon emission sources is a wiser optimum use of crop residues.” Metzger et al. (2002) continued saying that Keith (2001) “…assigns a dollar cost to the two methods, rather than following the tradeoff between carbon and power.”
In a final response, Keith and Rhodes (2002) conceded that sequestering vegetation biomass and replacing its sacrificed energy with lower-carbon sources such as natural gas would produce lower emissions than the zero net-emissions resulting from burning the vegetation; however, Keith and Rhodes maintained that “In the real world the costs of managing carbon do matter and the choice between these and other options will be determined strongly by economics.”
Strand and Benford (2009) extended Metzger and Benford (2001). They provided cost estimates and emissions benefits of depositing in the ocean bales of excess agricultural wastes wrapped in plastic and ballasted with rocks.
The concept of OSB presented in this paper emerged from the realization that depositing Black Pellets in the ocean would offer the prospect of resolving the obvious impracticalities present in Strand and Benford (2009), namely, Black Pellets would not require a wrapper; the pellets would sink on their own, not requiring ballasting; and the form of vegetation biomass as Black Pellets would not be subject to the same microbial degradation as raw vegetation. Significantly, Strand and Benford’s idea just pertains to a limited form of vegetation feedstocks—excess raw crop residues—whereas the oceanic deposition of Black Pellets is much more general, allowing for the thermochemical transformation of a wide variety of vegetation feedstocks, especially woody biomass.
Reconciliation of economics versus energy efficiency perspectives for making Black Pellet bury-versus-burn decisions
The exchanges between Metzger et al. and Keith and Rhodes constituted a debate over whether costs or emissions per unit of energy produced should be the critical factor in vegetation biomass bury-versus-burn decisions. Keith and Rhodes argued that because cost was the dominant consideration, the burning of vegetation for power was favored, even though it has less negative emissions benefit than sequestering the vegetation biomass in the sea and substituting energy from natural gas. However, Keith and Rhodes’ approach to cost comparisons was incomplete which could lead to wrong conclusions.
Just comparing in isolation the cost of burning vegetation for power versus the cost of burying the vegetation and substituting energy overlooks that the use of vegetation biomass would be but one of a portfolio NETs that would be needed to mitigate climate change.
Finding the most economic (lowest cost, least resource-demanding) mix of NETs becomes a portfolio optimization problem that is constrained by the need to not only meet the 2100 goals of the Paris Agreement and beyond but also by the need to meet given global energy demands. In a portfolio of NETs, an increase in the quantity of negative emissions from any of the NET alternatives—say switching from using Black Pellets for energy to OSB with lower-carbon substitute energy—would have the effect of reducing the need for negative emissions from the most costly NET in the portfolio.
Due to its large need for power and other resources, among the most discussed NETs, DACCS appears to be the most costly, capital-intensive, resource-demanding NET on the horizon. Assuming so, optimization models would show that an increase in negative emissions from Black Pellets for OSB versus for power would result in a commensurate reduction in the need for negative emissions from DACCS. Thus, the economic value of using Black Pellets for OSB would be to reduce the use of DACCS resulting in a less costly, less resource-demanding pathway to meeting the 2100 Paris Agreement goals than using the pellets for power generation.
The National Academies of Sciences, Engineering, and Medicine (2019, pg. 23) report forecasts that beyond 2100, in a carbon-free power generating future, NETs still would be needed for an indefinite time. In such a future where low-cost carbon-free energy is readily available, OSB would provide negative emissions and geologic sequestration benefits compared to using Black Pellets for power. Assuming that OSB would produce negative emissions at less cost than that from DACCS, depositing Black Pellets in the sea would reduce the need for DACCS. In such a future, the mindset regarding the use of vegetation biomass to mitigate climate change would be as a direct source of negative emissions rather than, as it has generally been viewed heretofore, as a source of (almost) net-zero emissions energy.
Alternatively, in circumstances where the availability of low-cost carbon-free power is limited, it might be superior to use Black Pellets for BECCS where BECCS and DACCS facilities are coupled, the BECCS plant producing energy for, and sharing the geologic sequestration infrastructure with, the DACCS plant.
The potential benefit of OSB displacing a need for the more costly and resource-demanding DACCS does not mean that OSB must be concurrent with DACCS. The amount of DACCS needed to meet the Paris Agreement goals toward the end of century will depend on the amount of atmospheric CO2 that accumulates until then. A slow start in reducing atmospheric CO2 would cause damaging overshoot in global temperatures and the sooner we get started with reducing atmospheric CO2 the better. As it is based on fairly mature technologies (Negi et al. 2020), oceanic deposition of biocoal as Black Pellets may be a NET relatively ready-to-go. A fast start with OSB would reduce the eventual amount of negative emissions needed toward the end of the century, thereby lessening the eventual need for DACCS.
Logistics—optimal temporal and spatial deployment of Black Pellets
Temporal deployment of Black Pellets
The generalized path to meeting the Paris Agreement goals of 2100 and meeting the world’s demand for power assumed in this paper can be characterized as the global evolution of power generation decarbonization in different phases: (1) starting with coal, (2) ramping up with carbon-free sources through a transition period in which natural gas makes up for a shortfall in the availability of carbon-free sources, (3) the mid-century large-scale introduction of CCS, and (4) ultimately reaching entirely carbon-free power generation. The results in Table 3 lead to the following conclusions about the optimal use of Black Pellets in these four phases of global power migration to an entirely carbon-free power generating future:
Phase 1: Coal power generation. Where power is generated from coal, it would be better to use Black Pellets as a coal substitute rather than OSB. Compared to using the pellets for power, OSB would add the cost of an extra supply chain without net emissions benefits.
Phase 2: Natural gas power generation before CCS. Where low-leakage natural gas is available to generate power, it would be better to use Black Pellets for OSB than to use them to displace natural gas power generation. Although adding the cost of natural gas, OSB plus natural gas power would mean less CO2 in the atmosphere and, thereby, would reduce the future need for more costly NETs such as DACCS.
Phase 3: Natural gas power generation with CCS. Where natural gas is available for power generation and can be coupled with CCS, there would be advantages and disadvantages to using Black Pellets for OSB rather than for power generation with CCS (BECCS). OSB plus natural gas with CCS may produce somewhat less negative emissions; however, using the pellets for OSB would mean approximately 58% less need for geologic sequestration compared to BECCS.
Phase 4: Power production from carbon-free sources. Where carbon-free sources of power are readily available, Black Pellets should be used for OSB rather than to displace carbon-free power generation. Both a relative negative emissions benefit and an elimination of the need for geologic sequestration would result from OSB compared to using the pellets for BECCS.
In the foregoing conclusions, the availability of natural gas in the transition from coal to an entirely carbon-free power generating future should not be taken as a prediction of this paper. There is increasing awareness of significant harmful methane leakage in many natural gas supply chains, but where OSB is examined in this paper with natural gas as a substitute energy source, it is just illustrative of the results for natural gas from sources that meet stringent leakage controls, if available. This paper is in the spirit of the National Academies of Sciences, Engineering, and Medicine (2019, pg. 2) study, presenting what needs to be done to meet the Paris Agreement goal of controlling atmospheric CO2 . From cost and technological standpoints, fixing natural gas supply chains is considered low-hanging fruit in mitigating climate change (IEA 2017) and it would be inconsistent to contemplate a future with BECCS and DACCS and not presume that low-leakage natural gas would be public policy.
Two key conclusions emerge from the bury-versus-burn analyses described in “Section 3.3”:
To the extent that OSB is used, it would result in a commensurate decrease in the worldwide need for geologic sequestration both from replacing BECCS with natural gas CCS and reducing the ultimate use of Direct Air Capture which requires CCS, and
The faster humankind can achieve an entirely carbon-free power generating future, the larger would be the benefits from OSB.
Spatial deployment of Black Pellets
The foregoing principles of the optimal deployment of Black Pellets represent a generalized decarbonization evolution as applied to the world as a whole. However, viewing things spatially, there will be differences between countries concerning their decarbonization progress that would affect their individual decisions as to using Black Pellets for OSB or power.
The choice of how best to use Black Pellets—for energy or oceanic deposition—will depend on specific circumstances and that can change over time. For example, a country might abandon its coal dependence in favor of importing LNG, thereby switching the use of its pellets from being co-fired with coal for power generation to OSB. If circumstances warrant, the Black Pellets used for energy applications could readily be redirected to oceanic deposition or vice versa. From this emerges a key point: OSB is a negative emissions option that would provide a needed flexibility in meeting the 2100 Paris Agreement goals and beyond. The availability of OSB as an option would reduce investment risks and would add a degree of freedom in optimizing the global system for using vegetation biomass to mitigate climate change.
However, if each country acted independently, it could be witnessed that coal-equivalent biocoal was being dumped in the sea by some countries while elsewhere on the planet, coal was still being mined to generate power. In short, if each country on their own used the logistics principles described in “Section 4.4.1,” it would lead to a globally suboptimal result. The global optimal usage of vegetation biomass to mitigate climate change would require international cooperation.
Ideally, instead of each country deciding on the allocation of its own Black Pellets for OSB, the pellets would be regarded as a dual-purpose commodity for global allocation by an intergovernmental clearinghouse. Individual countries could meet their negative emissions commitments by contributing their pellets destined for OSB to the central body, irrespective of where on the planet the countries produced those supplies. The intergovernmental body would make allocation decisions—for oceanic deposition or burning for power—based on complex considerations regarding what then is best from a global climate change perspective. It might grant or sell pellets on a subsidized basis to, say, less developed counties as a coal substitute until when and if such countries could catch up with the rest of the world.
Envisioned herein is an international convention created to control the use of OSB. An intergovernmental body would be the permitting authority for all OSB activities. All dumping proposals would be examined and monitored on an ongoing basis to assure that the pellets meet established standards as to sustainable feedstock sourcing and chemical and physical characteristics. Specific marine environments for deposition would need approval after environment-specific scientific studies were conducted. The governing body would issue credits based on the quantities and qualities of deposited pellets in approved areas.
The potential deployment of OSB would raise novel issues that are the concern of many existing international laws, particularly:
The UN Framework Convention on Climate Change (UNFCCC)
The Paris Agreement
The United Nations Convention on the Law of the Sea (UNCLOS)
The Convention on Biodiversity (CBD)
Substantive discussion of OSB herein with respect to these laws would be premature. With this paper’s objective being to propose research to answer threshold viability questions concerning OSB, immediately relevant to this objective are the London Convention and the London Protocol (LC/LP). As discussed in “Section 4.7,” because OSB is a technology that involves dumping materials in the sea, the provisions of LC/LP directly pertain to the types of research on OSB and their staging that would be permissible.
Oceanic environmental safety and sequestration persistence of OSB
Here, unique aspects of Black Pellets are discussed regarding their chemical and physical properties which suggest that OSB offers the prospect of environmentally satisfactory long-term sequestration of atmospheric CO2. Different marine environment candidates for OSB are discussed in terms of their potential capacities to sequester Black Pellet material.
Chemical properties of Black Pellets and their oceanic environmental implications
Black Pellets show resistance to decay in terrestrial environments. Under torrefaction conditions, the hemicellulose component of vegetation decomposes to a recalcitrant form. The lignin component, a highly recalcitrant substance, increases as a percentage of the torrefied mass but remains largely unchanged chemically and plasticizes to become the key binding material during the pelletization process (Negi et al. 2020). It is a conjecture subject to research that Black Pellets as terrestrial material would be no more subject to microbial decay in the relatively cold, oxygen-poor deep sea as it is on land.
The rate of mixing of dense deep-sea waters with ocean-surface waters means that CO2 released from deep horizons is extremely slow. To the extent that remineralization of Black Pellet material into CO2 on the deep-sea floor occurs, models and observations indicate that venting of the CO2 to the atmosphere would be measured in centuries or millennia (Caldeira et al. 2005; Hoffert et al. 1980), the time generally increasing with depth. Deposition depth, rather than the less-certain rate of remineralization, is the key variable ito assure extremely long sequestration persistence through OSB.
The closest natural material to torrefied Black Pellets is lower-ranked coal but unlike some coals, Black Pellets made from raw vegetation should generally have less potential for containing leachates of concern such as heavy metals and sulfur. Feedstock selection and treatment methods are available to control the levels of harmful chemicals. Specific chemistry standards including for trace elements and nutrients would be imposed for Black Pellets intended for oceanic deposition just as chemistry standards are now specified for solid biofuels for use in energy applications.
Physical properties of Black Pellets and their oceanic environmental implications
Black Pellets are densified today to meet durability and safety requirements for industrial handing and transportation to overseas power plants. Their durability can be a matter of densification research and design with the objective of the pellets maintaining their integrity while sinking to the ocean floor.
Depositional control of the pellets would be a function of their rate of descent. The speed of descent of the Black Pellets to the sea floor can be influenced by their design, as this speed is a function of the pellet’s size, shape, and specific gravity.
With months-long immersion in water, torrefied Black Pellets exhibit a mechanical breakdown into small particles having the absolute density and negative buoyancy of the original pellets and which form deposits with mud-like consistency (Thrän et al. 2016; Kymäläinen 2015). Such material mixing with seabed sediments or forming self-sealing deposits could suppress respiration effects but have the potential to envelope seabed lifeforms. The degree of concern over seabed disturbances depends on the thickness and areal extent of the deposited material which leads to the question of the scale of the deposits relative to the capacities of different ocean environments.
Oceanic capacity for OSB
Many IAM scenarios that meet the 2100 goals of Paris Agreement rely on CO2 removal and sequestration through all NETs starting in the 2030’s, ramping up to about 10 GtCO2 per year in the 2050’s and reaching 20 Gt per year globally by the end of the century (National Academies of Sciences, Engineering, and Medicine 2019). Estimates of the cumulative need for NETs to meet the Paris Agreement goals vary widely, from 100 to as much as 1,000 GtCO2 (1 GtCO2—1 billion, 109, metric tons of CO2) by 2100 (IPCC 2018). Thereafter, there would be a need for large-scale negative emissions over an indefinite period.
For pellets containing 1.54 tonnes CO2e per tonne of pellets (see “Section 2.4”), one GtCO2e would be sequestered by depositing approximately 649.4 million tonnes of pellets in the ocean. With a specific gravity of approximately 1.2 (Peng 2012), a tonne of Black Pellets would occupy 0.833 m3 on an absolute density basis (not bulk density). Therefore, each GtCO2e would be contained in 541.0 million m3 of pellets in the sea, volumetrically equal to a cube 814.8 m on edge.
Rivers of the world deliver about 18 Gt of sediments to the global ocean per year including embedded particulate terrestrial vegetation materials (Milliman and Farnsworth 2013). It is likely that the shelf sediment deposits of major rivers have capacity to embed meaningful amounts of Black Pellet material for indefinitely long sequestration, although the amounts and their fate in these environments would require much study.
The flat areas of mid-ocean abyssal plains are approximately 100 million km3 in extent, or approximately 28% of the global sea floor (Voelker 2016). Ranging from 3,500 to 6,000 m below the sea surface, they are relatively devoid of benthic biota reflecting the lower detritus flux and laterally transported terrestrial organic materials as nutrients compared to the ocean’s continental margins. The equivalent of a billion tonnes of CO2 as Black Pellet material spread as a 1 cm thick layer deposited on abyssal plains would cover an area of 54,094 km3, approximately 5 ten-thousandths of the flat areas of the world’s abyssal plains. Studies may show that an area thus disturbed may exhibit minor transient effects and may be reusable for further deposition in intervals of a given number of years.
The impact of OSB on the ocean’s biota habitats and the persistence of sequestration of the Black Pellet material would be a function of its seafloor areal coverage and deposition thickness. Depositing the material in thick layers in depressions would not only reduce the areal extent of its seafloor disturbance but also suppress remineralization of the material in the lower layers.
As defined and mapped by Harris et al. (2014), there are many major seafloor depressions worldwide such as 57 trenches and 167 troughs. V-shaped subduction zone trenches near continental margins extend from seabed surfaces at 6,000 m deep to more than another 4,000 m deep. They are typically narrow—up to 100 km wide—and can extend more than 5,000 km long. At the bottom of some trenches, considerable microbial activity has been found feeding on trapped organic material runoff from nearby land and detritus from productive coastal ocean areas (Glud et al. 2013). Perhaps more promising for OSB from the standpoints of depositional control and sparser biotic activity are the geologically diverse flat-bottomed abyssal plain troughs at depths 4,000 to 5,000 m below the ocean surface.
The volumetric capacity of the world’s deep ocean trenches and troughs to store CO2 is vast. By way of illustration, in an analysis of the capacities of very deep ocean trenches to store CO2 as a liquid, the calculations of Goldthorp (2017) indicated that 1,000 GtCO2—the extreme estimated cumulative need from all NETs through 2100—could be contained as liquid CO2 in just 5% of the volume of a single trench, the Sunda, and that other trenches exist with similar capacities. Moreover, at depths where liquid CO2 would be compressed to a density equal to that of Black Pellet material, for a given volume, Black Pellet material as “concentrated” CO2e would sequester approximately twice the amount of CO2e that would be sequestered if in the form of liquid CO2 (CO2e:CO2 = 1.91:1 see “Section 2.2”).
Social and legal barriers to deployment
Intolerances to sacrifice loom large as deployment barriers to climate change solutions that appear to threaten peoples’ economic well-being or ways of life. Where NETs involving the geologic sequestration of CO2 would overlap with population concentrations, people have shown concern about infrastructure intrusions associated with CO2 transportation and about real and imagined threats to their health and safety from geologic sequestration including seismic and leakage possibilities. Social acceptance has been deemed to be a significant barrier to the scale of deployment of BECCS that is considered necessary in many IAM scenarios to reach the goals of the Paris Agreement (Vaughan and Gough 2016).
The opportunities and their potential burdens associated with the various terrestrial NETs do not uniformly occur around the globe. Experience has shown that uneven terrestrial burdens can give rise to feelings of disproportionate sacrifices and antagonism among classes of people and among countries. Using the global ocean as “common ground” under joint governance should foster feelings of equity and cooperation. Nonetheless, surveys have consistently found more opposition to ocean-based versus terrestrial NETs (Bertram and Merk 2020; Cox et al. 2021). With much of the ocean still being a mystery, oceanic solutions to climate change are considered high risk with potential for unintended consequences. A strong aversion to “tampering with nature” leads many to feel we should leave the ocean alone.
Sober analysis, however, recognizes that humankind “has already been conducting a global intervention [affecting the ocean] through the rapid and sustained release of anthropogenic CO2” (GESAMP 2019). Tipping point scenarios cannot be dismissed. To save the ocean and the rest of the Earth from catastrophic change, it may be necessary to look to the ocean as part of the solutions.
A need has been recognized for contingency planning to develop and compare oceanic climate change solutions. The Convention on the Prevention of Marine Pollution by Dumping Wastes and Other Matters (London Convention 1972) and the London Protocol (LC/LP) spell out what types of research on and deployment of those ocean-based solutions might be allowed based on the precautionary principle. To be consistent with the intent of LC/LP, approval of OSB deployment would require it to be deemed “dumping of uncontaminated organic material of natural origin, subject to satisfactory assessment.” Nonetheless, “legitimate scientific research,” especially laboratory research, is accepted as being appropriate.
Comparison of OSB to other ocean-based NETs
GESAMP (2019) was a major study undertaken to understand the potential environmental, social, and economic impacts of different ocean-based approaches to control climate change and to provide advice to the London Protocol Parties to assist them in identifying those approaches that might be acceptable to pursue. This study identified from the literature 27 such ocean-based approaches for preliminary evaluation and ranking, including the following NETs that, like OSB, would use the ocean to sequester terrestrially captured CO2:
Depositing crop wastes in the deep ocean
Injection and dissolution of liquid CO2 in mid/deep ocean waters
Placing liquid CO2 on or within the deep seabed
Mineralization of CO2 in rocks below the seabed
Relative to the foregoing approaches, OSB offers the following prospective advantages:
OSB avoids the problems that make the ocean sequestration of crop wastes of limited applicability and impractical (see “Section 4.1”).
Black Pellet material deposited in the deep ocean offers the possibility of extremely long sequestration persistence as a consequence of it being the sequential combination of slow remineralization of a recalcitrant substance on or in the seabed followed by vertical transport over millennia of any CO2 that might be produced.
With long-lasting sequestration in deep-ocean levels, OSB should avoid the ocean acidification issues associated with direct injection and dissolution of CO2 at shallower depths in the sea (Goldthorp 2017).
OSB offers high sequestration efficacy, as Black Pellet material contains approximately twice the amount of CO2e as the same volume of liquid CO2 on or in the deep seabed. Depositing Black pellet material would result in approximately half the volumetric intrusion in the ocean and seabed compared to an equivalent amount of CO2 sequestered in liquid form.
The costs and energy requirements of sequestration through OSB should be relatively low.
Black Pellets material, as a reduced form of a natural substance (vegetation) and without unnatural additives, has characteristic that arguably meet the intent of the LC/LP regarding the acceptance of materials for dumping in the ocean. OSB may be seen to involve less tampering with the ocean, have less potential for unintended consequences, and be more “natural” than other approaches, akin to the processes that created coal deposits.
It would appear that OSB offers a set of characteristics that, if confirmed by research and design, would rank OSB favorably against the other leading ocean-based NET concepts. Consequently, discourse on OSB is encouraged, and a program of initial laboratory research is suggested.
Suggested Black Pellet laboratory testing and optimization for oceanic deposition
The pelletization stage of the manufacture of Black Pellets has design variables—pellet size, shape, and density—that would be tuned to affect the speed and drift of the pellets in their descent to the sea floor. The durability of the pellets of different types would be tested under a range of high-pressure conditions, including those that were pelletized prior to torrefaction and steam explosion pellets which have shown greater resistance to mechanical breakdown with prolonged immersion in water (Kymäläinen 2015). Mechanisms and systems would be investigated for precision deposition including adapting existing ultra-deepwater drilling technologies to deliver Black Pellet material to the sea floor as slurries through vertical pipes. Earth system and integrated assessment models would be used to predict environmental, economic, and other societal impacts over numerous what-if scenarios. If these threshold design activities and laboratory experiments produce favorable results, LC/LP petitions would be made to investigate depositional methods, the effects on ocean chemistry, disturbances of lifeform habits and habitats, and Black Pellet degradation in actual marine environments.