Physical and Chemical Structure of Biochars Produced From Different Feedstocks and Under a Variety of Pyrolysis Conditions

      Effective carbon sequestration must be based on sustainable processes that provide safe, stable carbon sinks with enough capacity to sequester a substantial fraction of anthropogenic carbon dioxide emissions. Soil amendment with biochar made by pyrolyzing biomass is a promising new approach with the potential to sequester large amounts of atmospheric carbon. At the same time, strong evidence suggests that amending soils with charcoal may increase soil fertility, improve soil drainage and help manage nitrogen and phosphorus nutrient pollution.

      Our group has undertaken a systematic study to determine the pyrolysis conditions that lead to highly stable biochars with good carbon sequestration capacity and the ability to improve soil fertility.  We report here results from our recent experiments with two biomass feedstocks: corn stover and apple wood. Particles of various sizes from both feedstocks were pyrolyzed in a computer-controlled fixed bed reactor under flowing nitrogen.  The custom-built computer software combining feedforward and feedback control allowed us to accurately program the temperature history of pyrolyzing biomass samples. Heating rates from 0.1 to 1°C/s, final heat treatment temperatures (HTT) between 450 and 600°C and various exposure times to the final temperature were used for these experiments.

      The chemical and physical structure of the produced biochars were characterized with a battery of analytical techniques that included ¹³C NMR, XPS and BET pore surface analysis. The NMR studies revealed that the produced biochars consist of condensed aromatic ring clusters whose size was strongly affected by the final heat treatment temperature (HTT).  The clusters in biochars produced at a HTT of 450°C contained about ten carbon atoms, while the average number of carbon atoms per cluster increased to about 25 for biochars produced at a HTT of 600°C.  The size of aromatic ring clusters was not affected by any other pyrolysis condition (heating rate, particle size etc.).  XPS measurements were also carried out to quantify the chemical composition of the external surface of the biochar particles.

      Our BET measurements showed that the surface area of the smaller pores was primarily determined by the biomass feedstock, with the biochars produced from corn stover having significantly smaller BET areas (5-70 m²/g) than biochars produced from apple wood chips (70-340 m²/g).  The final HTT also had a strong influence on the BET areas, while the pyrolysis heating rate had a much smaller effect.

      Images obtained with scanning electron microscopy show that the produced biochars have an extended network of larger pores with sizes of the order of a few microns. Such large pores cannot be quantified by BET analysis. However, they play a significant role in determining the ability of biochars to absorb and retain water and water-soluble nutrients.  In order to study the accessibility of the small and large pores from the exterior of biochar particles, we carried out a systematic study of biochar combustion with air in a thermogravimetric reactor.  The patterns of biochar reactivity vs. conversion were computed form combustion temperatures ranging from the regime of kinetic control (325-400°C) to the regime of intraparticle pore diffusion control (500-700°C).  Significant differences in the reactivity patterns obtained for the two regimes were observed.  While the reactivity in the kinetic control regime reaches a maximum at conversions between 30-40%, the reactivity in the diffusion control regime continues to increase until conversion levels as high as 80% are reached and drops rapidly as the combustion reaction approaches completion. A careful analysis of these reactivity patterns using well-established theoretical models provides strong evidence that the biochars produced under a wide variety of conditions have an extensive network of interconnected large pores. 

      Finally, we will discuss the implications of these findings to connect the chemical and physical structure of biochars to their ability to absorb, retain and release water and water-soluble nutrients.