BIOCHAR - definition, production, and application

widok biowęgla

International Biochar Initiative definition
Biochar: A solid material obtained from thermochemical conversion of biomass in an oxygen-limited environment. Biochar can be used for a range of applications as an agent for soil improvement, improved resource use efficiency, remediation and/or protection against particular environmental pollution and as an avenue for greenhouse gas (GHG) mitigation. In addition, to be recognized as biochar, the material has to pass a number of material property definitions that relate both to its value (e.g., H/Corg ratios relate to the degree of charring and therefore mineralization in soil) and its safety (e.g., heavy metal content).

European Biochar Certificate definition
Biochar is a heterogeneous substance rich in aromatic carbon and minerals. It is produced by pyrolysis of sustainably obtained biomass under controlled conditions with clean technology and is used for any purpose that does not involve its rapid mineralisation to CO2 and may eventually become a soil amendment.
Torrefaction, hydrothermal carbonisation and coke production are further carbonisation processes whose end products cannot however be called biochar under the above definition.


Biochar seems to be a promising product with numerous potential applications. The most important ones are associated with application of the biochar as:

  • Renewable fuel for heat and power generation,
  • Renewable fuel for carbon fuel cells,
  • Tool for carbon sequestration,
  • Soil amendment and fertilizer,
  • Substitute of activated carbon,
  • Sorbent for the capture of mercury and other pollutants,
  • Substrate for fuel emulsions.

There are numerous technologies focused on efficient processing and carbonization of biomass, the most common ones are based either on gasification, or pyrolysis, or hydrothermal carbonization (HTC) treatment and focus mainly on the maximizing of the yield of gas and bio-oils, or the ‘destruction’ of problematic biomass.
Although promising (like gasification and HTC processes), they are unfortunately endothermal and still face operational difficulties associated particularly with the formation of soot and tar in the piping system that bring about the blockage of the installation and the necessity of an emergency shutdown.

widok biowęgla

An interesting method for biochar production is a process developed in our Department of Energy Engineering. The technology has been under development since 2003 and so far various horizontal- and vertical-type reactors have been constructed.
The carbonization process that can be operated in an autothermal mode is presented and shown as a possible way to minimize the tar and soot-related problems, and enable significant reduction of the atmospheric CO2 emission. Instead of gas and bio-oils the process is focused on the production of a solid residue (biochar) and autothermal operation of the carbonization reactor can be maintained due to its specific construction design providing the possibility to immediately burn the pyrolytic gases evolved during thermal treatment of the fuel and thus to avoid the formation of any soot and tars, and the occurrence of endothermic reactions. Part of the heat evolved during the combustion of the gases is used to maintain the process, while the remaining enthalpy of the flue gases can be efficiently used in other processes (e.g., for the production of electricity, or heat and/or cold, or the sequestration in soil to avoid its oxidation and thus the associated emission of CO2

Taking into consideration the properties and parameters of the produced biochar it seems that its production may become an interesting option for the sequestration of carbon dioxide. Combination of the biochar production via the technology presented above with simultaneous use of the enthalpy of the flue gases for, e.g., power generation and with the sequestration of the biochar in soil would become an interesting option and a cheap alternative for ‘classical’ and very expensive CCS technologies that are planned to be implemented soon to European power plants.

Publications about our biochar research (in Polish):

Biochar as a fuel for Direct Carbon Fuel Cell (DCFC))

DCFC

The direct carbon fuel cell (DCFC) is a power generation device converting the chemical energy of carbon directly into electricity. The energy is converted via direct electrochemical oxidation of the fuel and without any combustion or gasification processes. The basic structure of DCFC is to other cell types, such as molten carbonate fuel cell (MCFC) or solid oxide fuel cell (SOFC). However, in the DCFC technology solid carbonaceous fuels (e.g. hard coals, charred biochars, active carbons, carbon black, graphite, coke, etc.) are used and directly oxidized at the anode surface and no clean and gaseous fuels (e.g. H2 or CO) are required as it is in the case of MCFC or SOFC. Compared to other technologies the solid carbon fuelled fuel cells have several unique features and advantages offering higher thermodynamic efficiency and lower emission of carbon dioxide per unit of the generated electricity. Furthermore, the fuel for the DCFC does not require any sophisticated preparation since the solid carbon can be easily obtained from various resources such as coal, petroleum coke, biomass (e.g. grass, woods, nut shells, corn husks), or even organic garbage.

The direct carbon fuel cell with molten hydroxide electrolyte, developed in Department of Energy Engineering, is considered as to be the most promising type of DCFCs, due to its advantages, such as high ionic conductivity, higher electrochemical activity of carbon (higher anodic oxidation rate and lower overpotentials) and higher efficiency of carbon oxidation due to the lower operating temperature (the dominant product of carbon oxidation is CO2 vs. CO). Accordingly, the DCFC may be operated at lower temperatures (roughly 673-873 K) and thus cheaper materials may be used to manufacture the cell.

widok biowęgla

Based on the results of preliminary studies and modifications, the DCFC prototype III (made of nickel) was chosen for further investigations due to stability and reproducibility of measured values of electrical parameters (excluding the impact of corrosion processes) under the same conditions in subsequent intervals. That model could also be fed with various fuels containing elemental carbon, mainly coal and biochar obtained in the thermolysis of the biomass. In addition, the special test setup has been completed to allow the measurement of current and voltage as well as the preparation of the necessary voltage-current characteristics of the cells while maintaining the requirement of high accuracy and repeatability.

So far, most of the research has been focused on the possibility to apply various forms of carbonaceous fuels to the prototype DCFC. The promising results of initial experiments with a solid graphite electrode encouraged the authors to investigate the possibilities of using crushed coal and biochar (i.e. the product of biomass carbonization) as fuel. The use of particulate carbon fuel without any additional preparation or compressing resulted in the development of an original separator that enabled direct contact of fuel with the electrolyte and served as a current collector but also prevented the uncontrolled transport of fuel particles to the electrolyte.
Various carbon fuels (i.e. graphite, carbon black, hard coal, biochar produced from the carbonization of various biomass types: apple tree chips, sunflower husks, pine and energetic willow shavings) have been tested.
Next stage of the work was to determine the effects of various process parameters as well as the influence of composition of NaOH-KOH and NaOH-LiOH electrolytes on the fuel cell electrical parameters. Studies which have been performed included, among others, fuel particle size, amount of air supplied to the cathode, chemical composition and temperature of the electrolyte.

Publications: