Are there carbon benefits to incorporating biochar into building materials?

Ever since the IPCC highlighted it in its 2022 report (AR 6^[1]), biochar is considered a carbon storage solution. Carbone 4 conducted a study for Vicat & Soler to determine whether the use of biochar in building materials offers carbon benefits and, if so, to quantify those benefits. This article aims to present the methodology used and the main findings.

Scope of the Study

Our study focused on the potential of biochar in six building materials, under two future scenarios: in 2030 and in 2050

To characterize the potential carbon benefits of biochar incorporated into building materials, we studied the resulting emissions and carbon sequestration of six building materials: cement used in concrete, plaster, plastic, steel, paint, and adhesives, sealants, and resinss^[2]. We estimated the volumes of materials used by considering both the new construction and renovation markets, for two time horizons: 2030 and 2050. Only the use of biochar in building materials was studied, Other uses were not considered (for example, as fertilizer, industrial biochar, or an energy source). This study therefore does not rank the various uses of biochar in order of merit, nor does it compare the benefits of biochar in construction materials with those of other alternative uses for biochar.

Definition: What is biochar?

Biochar is a stable, carbon-rich material produced by the pyrolysis of biomass (wood, agricultural residues, organic waste) in the absence of oxygen or in an oxygen-poor environment, which enables long-term carbon sequestration.

This thermal process transforms organic matter into stable charcoal, capable of storing carbon for at least hundreds of years. Without pyrolysis, the carbon captured during photosynthesis would be released as carbon dioxide or methane through natural decomposition or combustion. There are several processes for producing biochar, each requiring varying amounts of energy for pyrolysis. Our study focuses on biochar produced using the Soler process, which is highly energy-efficient and consumes 2.4 metric tons of dry wood to produce 1 metric ton of biochar.

The sequestration^[3]carbon is capture and long-term storage a significant amount of carbon dioxide. By cross-referencing various authoritative sources (Net Zero Initiative^[4], GHG Protocol Land Sector, Verified Carbon Standard, Carbon Removal Certification Framework^[5]), Carbone 4 sets the minimum absorption period required for a product to be considered a permanent carbon sink at 100 years.

The Scientific Bibliography^[6]This suggests that the majority of the carbon contained in biochar is permanently stored—specifically, 820 kg of carbon per metric ton of biocharif the biochar has not degraded by the end of its lifespan. We therefore sought to determine whether biochar incorporated into building materials would degrade before reaching this 100-year threshold.

Only by incorporating biochar into cement and plaster can carbon be permanently stored

To ensure that carbon storage via biochar is permanent—and thus that carbon sequestration continues for at least 100 years—we assessed the lifespan of biochar when incorporated into building materials. We therefore studied the lifespans and end-of-life scenarios for the materials under consideration.

The service life of cement in concrete used in calculations is sometimes less than 100 years (particularly for buildings^[7]), but its end-of-life options—recycling through crushing (approximately 75%) and disposal in an inert landfill—allow for carbon storage in an inert material for more than 100 years. As for gypsum, its average lifespan is only 50 years, but its three possible end-of-life options are conducive to CO₂ sequestration₂ : recycling through combustion at a temperature low enough to prevent the release of carbon stored in the biochar, disposal in a landfill for non-hazardous waste, and reuse in the cement industry through grinding. In terms of the lifespans and end-of-life stages of materials, carbon could be permanently stored in the cement used in concrete and plaster.

Based solely on the average lifespans and end-of-life characteristics of these other materials, we cannot draw a positive conclusion regarding their ability to store carbon over the long term. We did, however, quantify the carbon potential of biochar in these materials and concluded that incorporating biochar appeared to offer little or no benefit from a carbon emissions perspective. For example, plastichas a lifespan of less than 100 years. It is therefore necessary to study these possible end-of-life scenarios to determine whether biochar incorporated into plastic could remain stable for at least 100 years. However, only landfill disposal is conducive to long-term carbon storage, and this is not an end-of-life option that should be promoted for plastic. Incineration, on the other hand, would release CO₂. Furthermore, the technical feasibility of incorporating biochar has not yet been proven, and the potential for emissions sequestered and substituted, as calculated in the study, is low due to a smaller market volume compared to the cement or plaster markets.

To obtain a range of results for the carbon potential of biochar in building materials, we considered two scenarios (high potential and low potential) with two datasets for the following values:

• market volume projections;
• biochar incorporation rates in materials^[8] ;
• Projections for the market penetration rate of the material containing biochar.

The results also depend on the following assumptions (all assumptions are available in the appendix):

• the percentage of carbon permanently sequestered in biochar;
• the carbon footprint of materials.

Before presenting the results, it is important to review certain concepts related to induced and sequestered emissions and the possibility of summing them:

• The sum of induced and sequestered emissions does not account for the difference in timing between these two types of emissions or for the potential short-term warming that could result if the induced emissions are short-term. The Net Zero Initiative distinguishes between these two types of emissions within its three pillars:
  • Pillar A – Reduction of emissions generated directly by the organization;
  • Pillar B – Contributing to global decarbonization through projects that reduce emissions both within and outside the organization’s value chain;
  • Pillar C – Increasing carbon sinks within and outside the organization’s value chain.
• In this study, the timing of emissions and carbon sequestration differs: initial carbon sequestration occurs through photosynthesis before timber harvesting, while additional carbon sequestration takes place during forest regeneration. The assurance of additional sequestration depends on forest management: it is possible only if the amount of timber harvested does not exceed forest growth.

In the event of a reduction in the forest carbon sink at the regional or national level, or a reversal of that sink, emissions from the forest must be reallocated to wood products. These are then no longer carbon sequestrations but induced emissions! The draft version of the Land Sector and Removals Guidance^[9] The GHG Protocol recommends tracking carbon stocks over time to report emissions released or sequestered in accordance with actual forest conditions. It recommends separating emissions released and sequestration from products.

The carbon sequestration potential amounts to 3 MtCO₂e/year in 2050, at the cost of a very slight increase in cement-related emissions, which is due to the additional volume of biochar-based binder required to achieve the same properties as cement without biochar.

The carbon intensity of cement depends on that of the clinker and the additives, such as limestone. When incorporated into cement, biochar can replace all or part of the limestone, as well as a portion of the clinker.

• The carbon intensity of biochar is higher than that of limestone^[10]. Thus, when biochar replaces only limestone (in whole or in part), it increases the carbon intensity of cement.
• On the other hand, when it replaces a portion of clinker—which has higher emissions—it reduces cement emissions.
• When biochar replaces both limestone and a portion of the clinker, the effect on emissions depends on the rate of biochar incorporation.

Replacing limestone and a portion of the clinker with biochar can thus reduce the carbon intensity of the biochar-containing binder compared to conventional cement without biochar.

However, the properties of the binder containing biochar differ from those of CEM II/A cement. To maintain similar performance and incorporate it into concrete, it is necessary to increase the binder volume by 10 to 15 percent compared to the volume of CEM II/A cement required. This difference in volume results in additional emissions.

The rate at which biochar is incorporated into cement is limited in order to preserve the properties of the binder when biochar is added. The potential for reducing emissions through clinker substitution does not offset the additional emissions resulting from the extra volume of binder required for incorporation into concrete. With current production methods, the use of biochar in cement results in additional emissions.

However, incorporating biochar into cement makes it possible to sequester CO₂, with the following results:

These results are calculated based on the following assumptions:

• Total French cement market volumes estimated at between 8.4 and 9 Mt in 2050
• binder penetration with biochar at 25% for low potential and 50% for high potential
• an addition of 10 to 25% biochar to the binder.

The incorporation of biochar into plaster offers potential carbon benefits, but the industry strategy does not appear to be compatible with its integration. Further investigation is needed to determine the carbon sequestration potential of biochar in plaster. For a market in 2050 of ~2.1 Mt of plaster, with a 25% penetration rate of plaster containing 20% biochar, we have estimated the potential at 350 ktCO₂ eq sequestered and 17 ktCO₂ substituted eq. However, incorporating biochar into plaster involves technical constraints that are not yet well understood and runs counter to the sector’s objectives^[11].

The contribution of biochar in cement is significant in relation to national carbon sequestration targets, but raises questions about the availability and allocation of wood and biochar resources.

According to this study, incorporating biochar into cement contributes significantly to achieving national carbon sequestration goals. Incorporating biochar into cement would enable the cement sector to achieve 23% to 51% (depending on the scenario) of its 2050 carbon sequestration target (in its central scenario), though this would result in additional induced emissions (+0.6–1.3% in 2050, depending on the scenario) when the sector has a 90% emissions reduction target^[12]. The incorporation of biochar into cement will contribute 3.1%–5.5% (depending on the potential considered) toward the SGPE’s carbon sink targets by 2030^[13], and 3.4%–7.4% of the carbon sink targets for wood products defined by SNBC 2^[14] by 2050. Given that the solutions needed to achieve these national goals are—and must be—diverse, we consider the contribution of incorporating biochar into cement to be significant. In addition, biochar is estimated to increase the carbon sequestration potential identified for the forestry and wood industry by a factor of 1.5 to 2^[15], by sequestering CO₂ made from wood that could not be used in long-lasting wood products.

However, given the quantities of wood and biochar required in the defined scenarios, it is necessary to take into account a potential conflict over the use of wood and biochar (for example, through the production of particleboard from degraded wood resources, or the generation of energy from wood fuel). The amounts of biochar required (ranging from 0.2 Mt in 2030 under the low-potential scenario to 1.1 Mt in 2050 under the high-potential scenario) are not currently available in Europe, but appear to be achievable based on projections for the growth of the European market^[16]. However, Hans-Peter Schmidt identified 55 possible uses for biochar in 2012^[17]. To prioritize these, several criteria can be considered, such as environmental impact, economic viability, technological accessibility, or local needs (for example, in agriculture, waste management, or pollution control). This constraint is all the more significant given that biochar is a product intended for the local market (transportation must be minimized due to its low density). These competing uses must therefore be analyzed at the local level based on available wood resources.

The quantities of lower-quality wood needed in 2050 to enable the incorporation of biochar into cement (3 Mm3 to 6 Mm3, depending on the scenario) appear to be attainable, given the additional resources estimated to be available by that time according to Carbone 4^[18]. However, they account for a significant portion of this increase (40% to 80%), which is equivalent to 6% to 12% of the current annual national harvest. It should be noted that these estimates are based on the yield of the Soler process, which is currently more efficient than other existing biochar production processes. It is therefore essential to ensure that biochar production relies on a sustainable source of biomass—specifically, one that does not depend on practices contributing to deforestation.

The SGPE has prioritized the uses of biomass and classified carbon sinks derived from wood and forestry products as a priority, placing a priori Wood converted into biochar and incorporated into construction products is among these priority uses. However, biochar uses low-quality wood intended for energy and industrial purposes^[19], for which the standard order of priority is not clear. This wood can, for example, contribute to the decarbonization of high-temperature industries or district heating systems—uses that are also classified as priorities by the SGPE. In reality, this is not incompatible with biochar production, as current pyrolysis technologies can generate heat for heating networks and industry, in addition to contributing to carbon sequestration. In the case of the Soler biochar production process, the co-produced synthesis gas is used to generate electricity, which provides an additional benefit beyond the use of biochar itself.

Conclusion

Among the six materials, we identified promising carbon sequestration potential only for cement used in concrete; however, this raises questions regarding the allocation of wood and biochar resources.

We have thus quantifiedsome We found positive carbon benefits from incorporating biochar into cement, but did not reach a positive conclusion for the other building materials considered. The sequestration potential identified in cement is significant, with 3 MtCO₂emissions sequestered in the high-scenario projection for 2050. The Integration of Biochar would nevertheless result in induced emissions, which remain low compared to the sequestration potential. Carbon sequestration relies on volumes of wood and biochar that appear achievable but must be tracked to prevent the development of agricultural practices or industrial processes that would result in additional greenhouse gas emissions. The allocation of wood and biochar resources will also need to be clarified using priority rankings to avoid competition among different uses.

Furthermore, the implementation of a carbon sequestration solution remains dependent on its valuation within the voluntary carbon market. Mechanisms for valuing carbon sequestration do exist, including voluntary certification frameworks such as the Label Bas Carbone in France or the Carbon Removal Certification Framework in Europe, but these do not currently address the specific case of biochar.

APPENDIX

This study and this article were funded by Vicat.