Biochar – synergies between carbon storage, environmental functions and renewable energy production
Growing concerns about climate change and the inevitable depletion of fossil fuel resources have led to an increased focus on renewable energy technologies and reducing GHG emissions. Limiting the atmospheric level of CO2 is essential to prevent the most damaging effects of climate change. Among renewable energy resources, biomass combustion has the largest potential to contribute to global energy demands, however it is considered to be a carbon neutral solution and so only limits CO2 concentrations rather than reducing them. Through pyrolysis rather than combustion, biomass can lead to carbon negative liquid, gaseous and solid fuels while also offering a route for long term carbon storage in the form of biochar. Biochar is a carbonaceous material which has shown potential for improving soil fertility, reducing GHG emissions and most importantly long term C storage in the environment. However many questions still remain unanswered with regard to biochar, especially the influence that process conditions can have on its performance in soil as well as any potential trade-offs between soil amendment, C sequestration and heat/power generation. This thesis is therefore focused on assessing the influence that process conditions and feedstock selection have on biochar properties related to carbon stabilisation, improving soil fertility (functional properties) as well as the distribution of energy amongst the pyrolysis co-products. To achieve this, a systematic set of biochar samples was produced, using a wide range of pyrolysis parameters (highest treatment temperature (HTT), heating rate, residence time, carrier gas flow rate and feedstock type), and analysed for physicochemical and functional properties. Pyrolysis HTT consistently showed a dominant influence on determining the final yields and properties of biochar, while the effect of other production parameters was varied. In this thesis the candidate first studied the effect that process conditions had on the long term stability of biochar, as an important indicator of its ability to sequester carbon. While increasing the HTT resulted in a decrease in biochar yield, overall the yield of stable-C increased with temperature. This meant that by applying a higher HTT during pyrolysis a higher C sequestration potential for biochar was achieved. Next to be examined was the influence that process conditions had on other functional properties (labile-C yield, biochar pH, extractable nutrients and cation exchange capacity (CEC)) was then examined. The labile-C yield of biochar decreased with increasing HTT due to the release of volatile matter, while the CEC and concentration of extractable nutrients tended to be higher in biochar produced at 450oC rather than greater HTTs. Biochar pH was also highly alkaline at elevated HTT. This indicated that while high HTT favoured C sequestration and biochar pH, lower HTT may be more favourable for other functional properties. Furthermore by assessing the mass and energy distribution amongst the solid, liquid and gaseous fractions, it was possible to determine the energy balance of the process and through this evaluate the trade-off between the C sequestration potential of biochar and the energy output of the liquid and gas fractions. As the severity of pyrolysis was raised, the total energy stored within the liquid and gaseous co-products increased at the expense of the energy content of biochar, therefore increasing the available energy output of the system and reducing the energy lost when using biochar for carbon storage rather than for bioenergy. This also demonstrated that the pyrolysis process could be fine-tuned to increase the amount of stored C while also improving the heat/power generation of the system. The higher energy content of the gas stream at elevated HTT was also seen to contain sufficient energy to sustain the pyrolysis process, which would free up the solid and liquid fractions for higher value applications while reducing the necessity for external fuel sources. Finally, the data set was used to produce statistical models enabling the prediction of biochar stable-C yield as well as the heating value of biochar. The results of this thesis therefore demonstrate that through applying high HTT the potential energy output of the pyrolysis system can be increased while producing a biochar product with high C sequestration potential and positive functional properties for soil amendment. Due to potential trade-offs, the final choice of process conditions and feedstock would then be made based on the specific requirements of a selected site for biochar application. Understanding the influence that production conditions have on the functional properties of biochar as well as the energy balance of the system is critical to developing specifically engineered bespoke biochar, be it for agricultural use, carbon storage, energy generation or combinations of the three.