Influence of feedstock properties and pyrolysis conditions on biochar carbon stability as determined by hydrogen pyrolysis
Introduction
Biochar is pyrogenic carbonaceous material (PCM) produced purposefully from waste organic feedstocks by controlled pyrolysis for use as a soil amendment [1]. Research over the past decade has demonstrated that biochar has considerable potential as a sustainable tool for carbon sequestration, soil amelioration, greenhouse gas emissions reduction and fertilizer runoff reduction, as well as waste management [1], [2].
A basic requirement for the use of biochar as a carbon sequestration tool is that the carbon in the biochar is stable, meaning that a substantial fraction of the carbon sequestered is not re-mineralized on at least centennial timescales [3]. However, a variable component of the carbon in many biochars is degradable on annual to decadal timescales and hence does not contribute to long-term carbon sequestration [4], [5]. In order to predict the long-term carbon sequestration capacity of biochar, a deeper, more predictive, understanding is needed of the biochar characteristics that confer resistance to environmental degradation and re-mineralization [3]. The stability of biochar is likely to be related primarily to feedstock type and pyrolysis conditions in the first instance, followed by secondary environmental factors (temperature, rainfall, soil type) at the site of incorporation into the soil.
A number of approaches have been used to assess biochar stability. Incubation studies have directly measured carbon re-mineralization rates up to 8.5 years. Such studies have demonstrated the existence of a labile fraction (general <5%) in most biochars that is re-mineralized over weeks to months, and a ‘stable’ fraction with an apparent residence time commonly, although not exclusively, measured in centuries [6], [7], [8], [9]. It is possible that the ‘stable’ fraction identified in incubation studies actually represents at least two pools, a ‘semi-labile’ pool with a residence time measured in years to decades and a ‘stable’ pool with a residence time measured in centuries to millennia [10]. The ability to determine the proportion of carbon in each of these longer turnover pools is pivotal to being able to predict the proportion of carbon in a biochar that is likely to have a minimum residence time of a century [10].
Biochar carbon in both the semi-labile and stable pools is dominated by polycyclic aromatic carbon compounds (PAC) [11]. At lower pyrolysis temperatures, the number of fused rings in an aromatic cluster is small, as shown by both BPCA analysis [12], [13] and the “ring current” NMR spectroscopy method [14], [15]. As temperature increases, the average number of rings in a cluster increases, leading ultimately to the development of ordered microcrystalline graphitic sheets at high temperature [11]. It has been demonstrated that PACs with a ring size <7 are degradable by micro-organisms [16] and hence are likely to form a component of the semi-labile pool and unlikely to contribute to a significant degree to the stable biochar carbon pool.
Moreover biochar's composition and structure can vary widely depending upon pyrolysis conditions and source feedstock and it is this variability that is likely to have a significant effect on their chemical stability in soils [17]. Some feedstocks can contain considerable amounts of organic nitrogen which becomes incorporated into the biochar's structure as thermally altered N (black nitrogen) in the form of heterocyclic aromatic N [18]. There has been a growing interest in how these structures will influence the stability of biochar in soils where more recent studies have shown that these structures may not be as recalcitrant as formerly thought [18], [19], [20]. Thus when considering mechanisms of stabilization of biochar in the environment both C and N aromatic structure's need to be taken into consideration.
A number of techniques have been proposed to determine the size of the stable carbon pool in biochars, on the assumption that the labile and semi-labile pools are small in most cases. Zimmerman [21] and Enders et al. [22], have suggested that decreasing volatile matter and O:C or H:C can be used to infer relative increases in stability, though this technique does not differentiate between semi-labile and stable pools, and hence cannot directly be used to quantify the relative sizes of these pools. An alternative technique uses hydrogen peroxide oxidation to accelerate the ‘aging’ and hence the oxidative loss of carbon from biochar. In combination with O:C or H:C, this technique has been used to estimate the proportion of biochar that will be stable over ca. one hundred year period (The ‘Edinburgh Stability Tool’) [10], [23]. The results of the accelerated ageing experiments have demonstrated chemical behaviours for a range of biochars that are consistent with the hypothesis that biochars produced at higher temperatures exhibit more resistance to oxidative degradation, however separate quantification of a semi-labile and stable pool has yet to be achieved. There is currently no technique that has been shown to directly measure the stable pool of biochar carbon.
Hydrogen pyrolysis (hypy) has been demonstrated to be a technique of considerable utility for pyrogenic carbon isolation and quantification across a range of environmental matrices [24], [25], [26]. These studies have shown that the analytical window for hydrogen pyrolysis is PAC with the average ring cluster size greater than ca. 7 (coronene). Given that smaller PAC's are known to be degradable [16], this analytical window holds the promise of being able to directly measure the stable polycyclic aromatic carbon (SPAC) pool in biochars.
In recent years there has been a large number of thermosequence studies but many of these have been limited by the number of feedstocks and generally have a narrow pyrolysis temperature range (usually ca. 300 °C–600 °C; see Table S1 in supplementary information for references). These studies have employed a wide variety of methods to characterize different biochars and, moreover, have introduced a number of different methods for comparing the relative stability of different types of biochars. However the chemical heterogeneity of biochar has made it difficult to establish a single method suited to assessing biochar stability. In this study we produced biochar thermosequences from eighteen commonly-used feedstocks at ten temperatures from 300 to 900 °C under identical laboratory conditions. We have quantified the mass and carbon yield for each of these samples and used hypy to directly determine the size of the SPAC pool in each in order to better define the relationships between feedstock, pyrolysis temperature and the amount of carbon in each sample that is likely to contribute to long-term carbon sequestration.
Section snippets
Biochar feedstock
Eighteen feedstocks were selected on the basis that they are in common use for biochar production and are deemed a waste product. These feedstocks have been divided into four categories; woods (hard, oil mallee, pine), residues (bagasse, corn stover, green waste, hay, paper-mill waste, peanut shells, rice husk, sugarcane trash, wheat chaff), manures (biosolids, dairy, feedlot, poultry) and macroalgae (saltwater and freshwater). Further feedstock source details and initial properties are
Biochar yield
The char yield (termed char fraction) from each feedstock thermosequence are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5. For each feedstock, increasing pyrolysis temperatures resulted in a lower char yield, as commonly reported [30], [31]. The initial decline in char yield within the biomass feedstocks, at the lower temperature range (300–450 °C), is mainly due to the thermal decomposition of cellulose and hemicellulose followed by the conversion of other organic materials with increasing
Conclusions
The aim of this study was to examine the stability of the carbon in biochar in the context of its carbon sequestration potential, taking advantage of the fact that hypy isolates the carbon component that is most likely to be stable in the environment on centennial timescales. The eighteen feedstocks examined in this study cover the range of materials commonly used for biochar manufacture, with a wide range of initial carbon, nitrogen and ash contents. This study has the advantage that all
Acknowledgements
This work was supported by funding from the Australian Government Department of Agriculture, Fisheries and Forestry, Carbon Farming Initiative Biochar capacity Building Program. We thank the follow for supply of feedstocks: Anthony Fermo of Biochicken Pty Ltd., Bill Knowles of Bill Knowles Beef Feedlot, Marlin Cove Waste Water, Aaron Palmer of Palmers Tree Zone, Catherine Reid of Australian Paper, Karen Siepen of Renewable Carbon Resources Australia, Ian Standley of Rainbow Bee Eater group and
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