Biochar Analysis Methods

Background to Biochar and Pyrolysis

Numerous strategies are being implemented to achieve a neutral carbon footprint, but biomass is one of the few alternatives that have the potential to sustainably meet energy and material needs while being, potentially, carbon-negative.

One route for this is by transforming biomass through pyrolysis, a thermochemical modification process where biomass is broken down in an oxygen-free atmosphere into biochar, bio-oil, and combustible gases. Biochar is a carbon-rich material with typically 70 to 80% of fixed carbon (on an ash-free basis) that has a extensive array of potential applications, whereas the combustible gases are commonly used to fuel the pyrolysis process.

During pyrolysis the release of volatiles components present in biomass provides biochar with a characteristic honeycomb structure with, what can be, a relatively high surface area. This porous nature potentially allows biochar to be suitable for a range of applications such as: a soil supplement for improving plant growth; an adsorbent to decontaminate air and water; and as a catalyst during the production of biodiesel. Additionally, biochar can be used as: a water filter for domestic use; a clean-burning source of bioenergy; an additive for upgrading biogas production; a stabilizer in composting; a carbon sequestration instrument in the building sector; and even for producing engineering parts for electrical devices.

The pyrolysis process is carried out by optimizing parameters such as residence time, temperature, heating rate, inert gas flow rate, and particle size. For example, a particle size of 0.8 mm and a heating rate of 10 °C/min can improve the biochar yield. In contrast, smaller particles and faster heating rates prompts the release of more volatiles, which results in higher bio-oil and gas yields. However, the temperature is considered to be the most critical parameter because this influences biochar porous configuration and yield. For instance, biochar surface area can decrease when the reaction temperature increases (i.e., 400 to 600 °C) due to the melting of its porous structure. Moreover, high temperatures (greater than 400 °C) also lead to thermal cracking, increasing liquid and gas components and decreasing biochar yield.

Nevertheless, the most porous biochar is derived from physical or chemical activation using high temperatures (450–900 °C). Physical activation releases volatiles trapped in the biochar pores meaning that the surface area of physically activated biochar is larger and more microporous (less than 2 nm) rather than mesoporous (2–50 nm). Air is one of the most common activation methods because the activation energy is less compared to carbon dioxide or steam, which improves the economics of the process. However, physical activation using air can result in partial combustion, leading to a high ash content and a lower biochar yield if the method is not properly controlled.

During chemical activation biochar reacts with the chosen set of chemicals at temperatures of around 450 to 900 °C. While chemical activation is less common than physical activation, it has several advantages like lower energy input, a better carbon yield, higher microporosity and significantly increased surface area (over 3,000 m2/g). Nevertheless, chemical activation requires regular equipment maintenance because the methods can lead to chemical corrosion.

Important Parameters for Biochar

Surface Area and Porosity

The surface area of biochar can vary significantly and, with it, the potential range of applications for the biochar. 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.

Our reports are comprehensive assessments and provide a user-friendly interpretation of the pore analysis results. We also provide application tests and consultation services based on the analysis data to enable clients to determine the most suitable application of their biochar samples.

Click here to read in more detail about the techniques we use for surface area and porosity analysis and to see the types of data that our reports provide.

Suitability for Soil Amendment

The amendment of soil with biochar has attracted attention for two main reasons. Firstly, biochar is an inert form of carbon that is highly recalcitrant to microbial degradation, particularly when compared against the feedstocks it has been produced from. Hence, carbon, initially taken from the atmosphere to grow the feedstock used for biochar production, then becomes locked into biochar which is then added to the soil - a form of carbon sequestration.

Additionally, biochar can help improve the fertility of the soils, allowing for enhanced plant growth and resilience. Biochar can benefit plant growth as a result of several complementary effects including: increasing carbon stocks, increasing nutrient availability, and allowing for an improved environment for Arbuscular mycorrhizal fungi to proliferate. These fungi exist in a symbiotic relationship with plant roots, allowing for the update of increased amounts of nutrients by the plants.

Celignis personnel have prior experience in the evaluation of biochar as a plant growth promoter. Images are provided of a prior nationally-funded research project which looked at the effects of soil amendment using biochars from different feedstocks. It was found that the production over an initial 21 day period was significantly greater than the no-biochar control and that the size of the increase depended on the feedstock from which the biochar was produced. There was also evidence that the plant roots orientated towards the biochar, as shown in the photos here.

At Celignis we can undertake such plant-growth pot trials and we can measure various important productivity parameters, including Time to Germination, Mean Shoot Length, Shoot Weight and Root Weight. We can also measure a range of other properties relevant to soil amendment, including: Electrical Conductivity, Water Holding Capacity, Cation-Exchange Capacity, Liming, and the contents of 18-different Polycyclic aromatic Hydrocarbons (PAHs).

We can also take Scanning Electron Microscopy (SEM) images to evaluate the ultrastructure of the biochar, a useful companion piece of data alongside surface area analysis, to see how suitable the biochar can be as a substrate within which Arbuscular mycorrhizal fungi can proliferate.

Thermal Properties

Biochar has several advantages as a fuel over the original biomass feedstocks. For example, the carbon and hydrogen contents are greater whilst the oxygen contents are lower, leading to increased calorific values. The loss of highly volatile components during the pyrolysis process also allows for biochar to be a cleaner burning fuel.

Many of Celignis's analysis methods and packages designed to evaluate biomass as a combustion feedstock are also relevant for biochar. Click here to read more about these methods.

Additionally, there are number of other analyses that may be of particular interest for biochars. These include the determination of the inorganic carbon content, the ash content at 815 °C (compared with the 550 °C traditionally used for biomass), the inherent moisture content and the thermogravimetric analysis (TGA) of the sample. An example TGA thermogram and derivative thermogram are provided in the video above, extracted from results on the Celignis Database of the internal analysis of a polysaccharide sample for one of our research projects. Click here to read more about TGA analysis at Celignis.

About Celignis

We are dedicated to the development of innovations for our clients related to the use and valorisation of biomass and wastes. This is enabled through our state-of-the-art laboratories, equipment, and highly qualified personnel. Whether you need sample analysis or the development of a full process for the production of commercially relevant products, our top-class multi-disciplinary team of scientists and business leaders can help.

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