Assessing the potential of biochar from crop residues to sequester CO2: Scenarios to 2100

Amongst the suite of current or developing climate change mitigation tools, biochar is discussed within the literature as a method for long-term carbon sequestration (CS). The biochar field is rapidly developing, though there are uncertainties and limitations for which understanding could be improved. The aim of this thesis was to assess the potential of biochars from crop residues to sequester carbon, under the land-use pathways of the Representative Concentration Pathways, to 2100. Eight crop residue feedstocks and their biochars were fully characterised to examine the effects of feedstock and process conditions on biochar characteristics. Biochar yield, carbon content and recalcitrance values from this experimental work were utilized in the global modelling of scenarios exploring future carbon sequestration potential. Biochars produced were Class B or Class C, using the recalcitrance classification of Harvey et al. (2012), and classed as moderately or highly degradable. Recalcitrance increased with increasing pyrolysis temperature. The recalcitrance index of Harvey et al. (2012) may underestimate recalcitrance in high alkali metal content biochars. The carbon sequestration (CS) potential of the biochars was affected by the yield and content of stable carbon content of the biochar and predicted to be between 21.3 % and 32.5 %. The feedstock carbon remaining in the biochars decreased with increasing pyrolysis temperature although carbon stability increased with temperature. Biochar CS potential decreased with increasing pyrolysis temperature, despite increased stability and is due to the decreasing yields observed. A new equation was developed, using feedstock volatile content, as an alternative to the CS equation of Zhao et al. (2013). The Representative Concentration Pathways (RCPs) were used alongside the experimental results for biochar yield, carbon content and recalcitrance, and secondary data such as future crop yield and crop residue trends to project the CS potential of crop residues from 2005 to 2100. Scenarios of biochar production and carbon storage were developed, built around the RCPs, investigating biochar potential under changing crop land area and exploring parameters such as biochar characteristics and biochar systems. Scenario 1 used the mean or most likely values from experimental data and literature, Scenarios 2 to 7 explored parameter assumptions and Scenarios 8 and 9 explored the impact of climate change on crop yields and subsequent biochar CS potential. Global biochar production in Scenario 1 for the four RCPs over 95 years (2005 to 2100) was: RCP 2.6, 138.4 Gt biochar; RCP 4.5, 132.3 Gt biochar; RCP 6, 173.2 Gt biochar and RCP 8.5, 217.9 Gt biochar. Although the carbon mitigation potential of biochar in the scenarios generally increased from RCP 2.6 to RCP 8.5, the quantity of emissions requiring iv mitigation also increased. Scenario 1 saw 49.0, 45.8, 60.9 and 77.2 GtC sequestered over the 95 year period for the four RCPs respectively. These are reductions of 11 %, 5 %, 5 % and 4 % on the RCPs carbon emissions pathways. The maximum and minimum carbon emission mitigation potentials achievable under the assumptions of scenarios 1 to 7 were 22.5 %, 10.8 %, 10.0 %, 8.3 % and 4.7 %, 2.2 %, 1.9 %, 1.5 % for the four RCPs respectively. Climate change generally resulted in a decreasing carbon sequestration potential from RCP 2.6 up to RCP 8.5. This negative impact also increased over time. The maximum impact on mitigation potential in 2100 was - 0.14 GtC yr-1 for RCP 2.6, this increased to - 0.72 GtC yr-1 for RCP 8.5. Biochar has the potential to sequester carbon in all of the scenarios explored, however the magnitude of this sequestration potential is dependent on a number of factors of which many are currently subject to large amounts of uncertainty. Reduction in these areas of uncertainty would be a valuable area of further work following this study.