• Surface Area and Porosity
    Analysis in Biochar
    At Celignis Biomass Lab

Background to Surface Area and Porosity Analysis

Biochar and other carbonaceous materials obtained after thermochemical modification processes like pyrolysis, torrefaction, hydrothermal carbonization or gasification typically develop a honey-comb porous configuration. This very complex network of pores allows these carbonaceous materials to be potentially used to produce: soil amendment; activated carbon; carbon molecular sieves; catalysts during the production of advanced biofuels; or as media for upgrading the production of biogas.

However, not all pyrolysis processes result in a competitive porous material. This is because there are numerous variables that can impact the porosity of the sample. These include: the type of feedstock, the pretreatment, process conditions, the presence of ash or condensates clogging the pores, and the selected activation method. Therefore, a sample obtained after pyrolysis-like processes needs to be analysed to determine its surface area and pore size distribution in order to reveal the true porous nature of the sample and its most suitable application. We determine the surface area and pore size distribution of carbonaceous samples using a Quantachrome NOVA-e Series 2200e analyser which has been designed to satisfy the procedures outlined in EBC (2012-2022) 'European Biochar Certificate - Guidelines for a Sustainable Production of Biochar.' European Biochar Foundation (EBC), Arbaz, Switzerland. Version 10.1 from 10th Jan 2022.

The surface area of the samples is determined by the Brunauer-Emmett-Teller (BET) method which is the most well-known technique for accurate determination of the surface area of carbonous materials, together with a micropore BET Assistant AI tool that helps us to improve the accuracy of the analysis.

We also determine the pore size distribution of the samples by the Quenched Solid Density Functional Theory (QSDFT) which is a method that, coupled with computer simulations, provides an accurate description of micro- and mesoporous carbons based on the sorption and phase behaviour of fluids in narrow pores on a molecular level. Furthermore, we provide an interpretation of the sample isotherm for supporting the data obtained from the surface area and pore size distribution analysis.

Click here to read in more detail about these different analysis methods.

Biochar Applications Based on Surface Areas and Porosities

Low Surface Areas

  • Defined as a surface area of less than 250 m2/g.
  • Soil Amendment - Biochar as a soil additive can improve the fertility, chemistry, and microbial community of the soil. Biochar also contains major and minor elements that are beneficial for plant growing such as nitrogen, potassium, calcium and phosphorus. In addition, biochar can moderate acidic soils, and can also improve the cation exchange capacity of the soil.
  • Combustion - Virgin biomass has low heating values due to its high moisture, low energy density, high bulk density, and hygroscopic nature. However, pyrolysed biomass like biochar has higher calorific value and therefore can be used as a solid biofuel for heat and electricity generation.
  • Construction Material - Such biochar can be used as a material additive for the construction sector and has the potential to both reduce the high carbon footprint of construction materials as well as to upgrade the physical properties of common construction materials such as thermal conductivity, low flammability and chemical stability.
  • Batteries - Biochar with low surface area have the potential to be used for lithium-ion batteries or for the production of supercapacitors. Moreover, amorphous carbons like biochar can be used for making anodes in silicon batteries. Likewise, graphitic carbon nanosheets that are a high added-value product used in electronic devices.

Moderate Surface Areas

  • Defined as a surface area of between 250 and 500 m2/g.
  • Plant Growth Promotion - Biochar mixed with soil and compost reduces the bulk density of the soil and increases soil porosity and water holding capacity which improves plant growing rates and ultimately crop yield. Moreover, moderate to high surface area biochar holds water for a longer time reducing the irrigation cycles needed per season.
  • Composting - Biochar as a compost additive enhances the water holding capacity and aeration of the compost which accelerates the compost production time. Furthermore, biochar has also been used for reducing the gas emissions from the compost which significantly reduces the bad odours associated with domestic compost production.
  • Water Treatment - Such biochar can also be used for purifying domestic water in developing countries lacking safe and continuous water supplies. Additionally, biochar can be used to remove pollutants from water, including bacteria, methylene blue, organic waste and inorganics like phosphates and nitrates.
  • Tar Removal - During pyrolysis recalcitrant pollutants known as tars are produced. These tars represent a problem since they can cause blockages in the downstream pipelines of the pyrolysis equipment. However, biochar can potentially be used as a catalyst for reducing the formation of such tars.

High Surface Areas

  • Defined as a surface area of more than 500 m2/g.
  • Pollution Cleanup - Biochar with a high surface area has an adsorption capacity comparable to activated carbon, which makes it suitable as a remediation agent for removing heavy metals such as iron, lead, copper, zinc, and cadmium. However, it should be considered that biochar's absorption capacity not only depends on its surface area because its chemical structure defines its chemical affinity which it's a critical factor for the efficiency of the application.
  • Air Cleanup - Biochar can be used as a purification agent in air purification systems for heavy indoor pollution like in the case of industrial activities.
  • Capture of CO2 - High surface area biochar can also be used for capturing and storing carbon dioxide through physisorption by making use of biochar for designing carbon nanotubes or carbon molecular sieves.
  • Biodiesel Production - Biochar can also be used as a catalyst during the transesterification and esterification process of turning waste oil into biodiesel for alleviating system contamination from pollutants such as sulphur deposition.
  • Anaerobic Digestion - High surface-area biochar can also be used in anaerobic digestors for removal of H2S from the combustible gases matrix. Removal of H2S is critical for upgrading the production of biogas. In addition, biochar can improve the hydrogen and methane percentage in the biogas. On the other hand, biochar can also be used in anaerobic digestion for controlling the presence of heavy metals which can represent a threat for the microbial community of the digestor.

Analysis of Biochar Surface Area and Porosity at Celignis

We have a variety of different analysis packages for determining the surface area and pore size distribution of biochar samples. These packages differ in the number of datapoints collected and in the gas(es) used.

The smallest number of datapoints that can be collected are 5 (in analysis package P360) which is insufficent to determine the pore size distribution of the sample. However, package P364 uses 20 datapoints and package P366 uses 40 datapoints, allowing for the analysis of porosity, with an increasing number of datapoints providing improved accuracy in the surface area and porosity analysis.

The analysis can also be done using CO2 as the absorbate for getting an even higher resolution description of the smallest micropores (less than 1.5nm) which are commonly present in carbonaceous materials, such as biochar or activated carbons.

At Celignis we provide highly comprehensive reports for the surface-area and porosity analysis of samples. These reports are provided online, on the Celignis Database, as soon as we obtain the results, as well as in detailed Excel and pdf reports that are provided once the order has been completed. The Celignis Database includes interactive charts, like the examples provided on this page, as well as detailed summary statistics. The final reports also incldue these results along with interpretations of the data, personally written by trained members of the Celignis team.

Our packages that quantify surface area in biochar are listed below:

Additional Information on Surface Area and Porosity Analysis in Biochar

Feel free to get in touch with us if you have any questions about our analytical services regarding the determination of surface area and pore size distribution in biochar samples. Relevant members of the Celignis biochar team will be happy to assist. Those team members with the most experience with undertaking these tests and interpreting the resulting data are listed below.

Edgar Ramirez Huerta

Biochar Project Developer


Has taken a major role in developing Celignis's capabilities for biochar analysis and project development. His thesis covered the evaluation of high value applications for high-carbon materials.

Kassiani Pliatsika

Plant-Growth Applications


Passionately believes in biochar for a sustainable bioeconomy. Involved in the development of our tests for assessing the soil-amendment potential and plant-growth effects of biochar.

Other Celignis Tests and Services for Biochar

Global Recognition as Biomass and Biochar Experts

Celignis provides valued services to over 1000 clients. We understand how the focus of biochar projects can differ between countries and have advised a global network of clients. We also have customs-exemptions for samples sent to us allowing us to quickly get to work no matter where our clients are based.

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Feedstock Evaluation

Our analysis packages can screen biochar feedstocks. We can estimate biochar yield and quality using feedstock chemical composition and can estimate biochar composition using the ultimate and major/minor elements analyses of the feedstock. With TGA analysis we can also monitor feedstock behaviour under pyrolysis conditions.

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Biochar Production

We can produce biochar samples from your feedstocks using a wide range of temperatures, heating rates, and residence times. We can formulate a Design of Experiments (DoE) to study the effects of varying process parameters on biochar yield and quality and can optimise these outputs according to your desired biochar market applications.

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Biochar Analysis

We have an extensive array of analysis packages to evaluate the suitability of biochar for a range of applications. These analyses cover properties relevant to combustion, soil amendment, feed, and biomaterials. Our reports compare the results against internationally-recognised limits for using the biochar in specific end-products.

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Biochar Combustion Properties

Biochar can be a superior fuel versus virgin biomass due to its greater carbon content and energy density. We offer a wide array of analysis packages to fully evaluate biochar as a fuel. For example, we can determine both organic and inorganic carbon and can monitor the behaviour of the biochar ash over wide temperature ranges.

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Soil Amendment & Plant Growth Trials

We can test biochar for several properties (e.g. water holding capacity, electrical conductivity etc.) relevant to its use in soil amendment. We can also grow plants in biochar-amended soils and assess the impacts of this approach on germination, plant growth, plant health, and soil biology.

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Analysis of PAHs in Biochar

Polycyclic aromatic hydrocarbons can be formed during the pyrolysis of biomass and accumulate in biochar, leading to potential risks to the environment. We can accurately quantify a range of different PAHs and determine if their concentrations exceed regulatory limits. We can also develop strategies to reduce the amount of PAHs in biochar.

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Thermogravimetric Analysis of Biochar

TGA is a powerful analytical technique for the study of biochars because it allows us to examine the thermal stability of the material as a function of temperature. The thermal stability of biochars is an important factor to consider when evaluating their potential use as a soil amendment or for carbon sequestration.

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Biochar Upgrading

There are several different methods (covering physical, chemical and biologial routes) by which we can upgrade your biochar in order to increase its value and make it more suitable for the desired market applications. We are able to fully characterise the changes in physicochemical properties associated with upgrading.

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Biochar for Carbon Sequestration

Biochar's efficacy as a means for sequestering carbon depends on a range of factors (e.g. feedstock and pyrolysis conditions). We can undertake a range of analytical tests to help you determine the stability of your biochar's carbon. We can also suggest alternative approaches to improve carbon sequestration potential.

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Technoeconomic Analyses of Biochar Projects

Our TEA experts work with you to evaluate the economic prospects of your biochar facility, considering various scale, technology, and feedstock options. We apply accurate costing models to determine CAPEX/OPEX of simulated and pilot scale processes which are then used to determine key economic indicators (e.g. IRR, NPV).

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Research Project Collaborations

Celignis is active in a number of important research projects focused on biomass valorisation. Biochar is a key component in some of these ongoing projects as well as in several prior projects. We are open to participating in future collaborative research projects where our extensive infrastructure and expertise in biochar can be leveraged.

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