What is biochar and what are the main parameters affecting its quality?

Within the H2020 project RUSTICA, a mixture of fertilizer building blocks is made from food waste streams aimed at replacing artificial fertilizers in the near future. TNO was concerned with the production of Biochar as one of the building blocks of the RUSTICA fertilizer mix. The other building blocks are shown in Figure 1.

The biochar production process involves the heating of ligno-cellulosic bio-residues to temperatures in the range 350-650 °C, and for a period of 30 to 60 minutes. Heating is under oxygen-deficient concentrations in a gasification process. Approximately 20-30% of the bio-residue is maintained as a very porous, carbon-rich product that is defined as biochar. The gasses released form a burnable gas that can replace natural gas in bioenergy generation. In total 18 biochar were produced from 15 different bio-residue feedstock, to be used as light-weight and structural element in the fertilizer mix as well as a form of carbon sequestration due to its very high carbon stability. However, a large variation exists, based on the feedstock, concerning the potential fertilizer elements brought to the mixture.

When biochar is aimed to be part of a fertilizer mixture, biochar quality and the leachable element concentrations must be known to avoid a high salinity. At the same time it can be seen that the desired volumes of biochar, necessary to replace artificial fertilizers, requires a more practical and general approach to the classification of every individual biochar. It is costly and impractical to categorize every feedstock source and biochar made from it separately. Availability over the seasons changes and the economic viability of a biochar installation depends on the amount of operating hours/year. From both literature and many years of experience in making biochar, I conclude that once the knowledge on how to make a good biochar is incorporated and set into the technological procedure, all emphasis shifts to the feedstock as the prime concern for quality.

The 18 RUSTICA biochars were grouped based on the main elemental composition in three groups. These were1) the Woody biochars which consisted of biochars made from Apple clippings, Pear clippings, Almond tree clippings, Pistachios clippings, Olive tree clippings, Pinot and Cabernot Sauvignon wine clippings and Kiwi plant clippings; 2) Shells and husks which were made from Cocoa pods, Cocoa shells and Coconut husks and 3) the highly fertigated from fertigated crops which consisted of sweet pepper and egg-plant stems and leaves (residues after harvesting). Only for the highly fertigated feedstock, leaching tests are necessary to avoid high instant salinity in a fertilizer mixture. Based on these, it was concluded to only use 10% of the highly fertigated feedstock in a woody feedstock mix for plant trials. The variation in fertigating elements in the woody feedstock could be neglected and hence gives a high degree of freedom for bulk volumes and easier logistics.