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Over decades anaerobic digestion (AD) has been successfully established as technology to treat organic wastes. The perspective of turning organic wastes into biogas, a source of renewable energy and profit, through a low-cost process, has gained increased interest around this technology. The AD as biological process is strongly dependent on the environmental conditions such as temperature, pH, presence of inhibitors, and nutrients content, that, in particularly unfavourable situations, can attribute to undesirable drop in performance and even for detrimental failures. Therefore, several strategies such as feedstock pre-treatment and process optimization have been developed to secure AD performance. However, these approaches do not remove inhibitor from the process, which may result in accumulation of the inhibitor and further destabilization of the AD system. From this point of view, it is beneficial to develop methods that remove and/or reduce the mobility and bioavailability the inhibitor within the digestion process. In this study, char derived from lignocellulosic wastes was added to anaerobic digestion to elucidate their roles on process performance and efficiency. The results of this analysis suggest that changes of AD performance (i.e. biogas production and intermediates concentrations) are strongly correlated to biochemical characteristics of char materials. Furthermore, the impact of changes in inhibitor concentrations on the digester’s performance was observed, and found that inhibition tolerance of AD was improved by the presence of selected char materials. However, contrary to the initial hypothesis based on previous studies in related hydrochar application, the addition of RH hydrochar led to reduction in biogas production but an enhanced methane content in biogas was observed. Except from batch AD tests, a larger-scale AD system (semi-continuous stirring tank reactor) was used to determine the possibility of biochar on enhancing digestate structure and reduce the sensitivities towards environmental factors such as pH and other inhibitors. The further outcome from scale-up study revealed that biochar had improved ammonium tolerance (1.59 mg NH4-N/kg) by recovering 12-16.4% biogas production. This study also investigated effects of iron impregnated-thermal activation on physiochemical properties of the resultant biochar. Effects of these modified biochars on gas production and volatile fatty acids concentration was also measured and significant changes was observed. Overall, char material had a positive effect on AD system stability and promoted bio-methanation during the long run operations.

Intensification of agricultural practices is necessary to support a growing global population. Water movement and storage in soils are very crucial for successful intensification of agriculture. They are important for nutrient delivery to plant and overall crop productivity. Climate change and population growth have been predicted to limit water supply especially in arid regions. Therefore, there is an urgent need to proffer solutions that would help maintain or even increase soil moisture retention. The use of biochar for improving soil hydraulic properties is a current and growing area of research. Biochar is a stable form of charcoal gotten from heating natural organic materials in a high temperature and low oxygen process known as pyrolysis. Biochar physical and chemical properties vary due to the pyrolysis process conditions, the type of feedstock, and the ratios of lignin, cellulose, and hemicellulose in the biomass. These change the structure of the biochar and will invariably affect to what extent it can improve soil water retention. To understand how biochar affects soil water properties we must understand the specific characteristics of biochar that influence these changes. Understanding the mechanisms is important for easy prediction of when and by how much biochar will improve soil water properties. The aim of this thesis is to provide new and extensive insight into 4D structural changes of biochar produced from different feedstock and pyrolysis conditions and how these affects soil moisture characteristics of different soil textures. The first aspect of the thesis was to carry out a meta-analysis of the available literature to quantify the effect of biochar on soil hydraulic properties. To enable matching of biochar to soil constraints and application needs, a thorough understanding of the impact of biochar properties on relevant soil parameters is necessary. This meta- analysis of the available literature for the first time quantitatively assessed the effect of not just biochar application, but different biochar properties on the full sets of key soil hydraulic parameters. The review shows that the key factors influencing biochar performance were particle size, specific surface area and porosity indicating that both soil-biochar inter-particle and biochar intra-particle pores are important factors. Next, the role of biochar particle size and hydrophobicity in controlling soil water movement and retention was assessed. Softwood pellet biochar in five particle size ranges was used for the experiment. These particle sizes were tested on two soil types at four different application rates in the laboratory. The results clearly show that both biochar intraporosity (pores inside biochar particles) and interporosity (pore spaces between biochar and soil particles) are important factors affecting amended soil hydraulic properties. Biochar interpores affect mainly hydraulic conductivity; both interpores and intrapores control soil water retention properties. Our results suggest that for a more effective increase in soil water retention of coarse soils, the use of hydrophilic biochar with high intraporosity is recommended. The third aspect of the thesis was to assess the 4D structural changes and pore network model of standard biomass feedstock during pyrolysis. Biochar properties are highly variable and are dependent on the biomass feedstock and production conditions. Despite these variabilities, there has been no study that uses a single sample to study the development of microstructural properties of biomass and biochar through the full range of pyrolysis temperatures. In this study, synchrotron x-ray micro-tomography (SµCT) was used to visualize the internal structure and characterise pore structure of biochar from several feedstock during pyrolysis (50 - 800 °C). The results show a wide range of variation in the pore structural characteristics of the biochar depending on feedstock and pyrolysis temperature, with observed porosity in the range of 7.41 – 60.56 %. The results from this study are not only important for the use of biochar as soil amendments but also a range of other applications relying on biochar pore characteristics, such as biochar as habitat for microbes, water, and wastewater treatments, as an absorbent for the removal of acid gases, or an additive in construction or engineering materials. In addition, the in-depth insights into changes in biomass structure during heating are valuable for research and applications related to fire safety. Finally, the data set from the SµCT was used to produce statistical models enabling the prediction of the effect of biochar on soil water holding capacity from biochar microstructural properties. The results of this thesis, therefore, clearly establish the mechanisms with which biochar improves soil hydraulic properties. It also demonstrates that by appropriately matching microstructural properties of biochar to those of the target soil, it is possible to achieve considerable improvement in soil properties using relatively low application rates of biochar. The relationship between biochar structural properties and its effect on soil moisture characteristics of specific soil texture, is an attractive pathway towards development of precisely tailored biochars aimed to enhance water use efficiency and provide low-dose, high-efficiency benefits.

Carbon dioxide removal technologies present an opportunity to alleviate some of the worst consequences of climate change by actively removing CO2 from the atmosphere while the decarbonization of our economies progresses. Biomass valorization through the production of biochar is a unique carbon negative technology as it comprises benefits beyond carbon sequestration, from the utilization of agricultural waste, the co-production of renewable energy to the use as an adsorbent. However, widespread adoption of biochar is still lacking due to economic obstacles in monetizing its various environmental benefits and high production prices compared to fossil materials. I address these obstacles by evaluating current biochar systems and developing an alternative concept to increase the economic competitiveness of biochar. I argue that the price of biochar can be significantly reduced by changing the perception of biochar as a product. Utilization systems specifically designed to monetize biochar’s multifunctional properties are capable to reduce its price without requiring lower production costs. The developed concept of sequential biochar systems is based on the perception of biochar as a carrier for environmental services rather than a product. Within this service-oriented concept, biochar is rented out and recycled rather than sold, enabling an iterative valorization of its properties while retaining its carbon sequestration value. This approach centers around the novel concept of biochar recycling to effectively decouple the price of biochar from its production costs by distributing these costs over several applications. To evaluate the prospects of sequential biochar systems, I conducted an experimental proof-of-concept trial assessing a sequence of three applications connected by two recycling steps. Engineered biochars were designed with a parallel focus on increased application capacity and recycling performance to provide synergies throughout the sequence. The applications targeted the use of biochar in wastewater and gas filtration as high value applications with emerging use scenarios for biochar. Recycled biochars showed comparable performance to commercial materials, and recycling displayed environmental advantages to current methods by retaining carbon throughout the sequence. The exploratory work in this thesis provides various starting points for further research on effective biochar utilization strategies, which are urgently needed to accelerate the development of biochar into an economically self-sustaining carbon dioxide removal technology.

Lignocellulosic biomasses are considered promising feedstocks for the next generation of biofuels and chemicals; however, the recalcitrance of lignocellulose remains a barrier to its utilisation over conventional sources. Pyrolysis is the heating of biomass to several hundred degrees Celsius in the absence of oxygen, which can thermally depolymerise lignocellulose. Products of pyrolysis are a solid biochar, liquid bio-oil and syngas. Biochar has roles in both carbon sequestration and soil amendment however bio-oil has no defined use, despite a high concentration of fermentable sugars. Bio-oil is a complex organic microemulsion with a host of biocatalyst inhibitors that makes its microbial degradation a challenge. In this work, the use of aerobic cultures using microbial communities isolated from natural environments saw limited potential; however, the use of anaerobic digestion (AD) successfully generated a higher volume of biogas from reactors with bio-oil than controls. Biogas yield test reactors were set up with anaerobic digestate from a wastewater treatment plant as the substrate for degradation and conversion of bio-oils. Next-generation 16S rRNA gene sequencing was utilised to characterise the communities in the reactors while the ultrahigh resolution mass spectrometry technique of Fourier transform ion cyclotron resonance (FT-ICR) was used for characterisation of the chemical changes occurring during AD. Both sets of high-resolution data were additionally combined for multivariate analysis and modelling of the microbial genera that correlated best with the changes in digestate chemistry. This represents a novel analysis method for the microbial degradation of complex organic products. Bio-oil from common lignocellulosic feedstock was the most easily degradable by the AD communities, with significant inhibition observed when bio-oils from anaerobic digestate and macroalgae were used. Additionally it was found that the inclusion of biochars that were pre-incubated in anaerobic digestate prior to use in AD were capable of significantly reducing the lag time observed for biogas production in bio-oil-supplemented reactors. The addition of biochars that were not pre-incubated had no effect on biogas production. Specific inhibition of methanogenesis was also capable of causing the digestates to accumulate volatile fatty acids (VFAs) as a product of greater value than biogas. Scale-up experiments will be required to confirm the precise practicalities of the addition of bio-oil to AD as well as to establish the potential for isolation and purification of VFAs.

For widespread use of biochar in agriculture and horticulture, it must be ensured that application will neither adversely affect soil and plants, nor exceed legislated contaminant concentrations. The most relevant groups of contaminants in biochar are potentially toxic elements (PTEs), polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOC). In this thesis, the concentrations of these groups of contaminants were analysed in 90 different biochars produced by slow pyrolysis. Subsequently, the concentrations were compared to legislation/guideline threshold values and linked to production conditions. The risk these contaminants pose to plant growth was also assessed, to give recommendations on production of safe biochar. PTEs can neither be formed nor destroyed, which means their presence in biochar is predominantly determined by feedstock type. However, significant levels of Cr, Fe and Ni were introduced into biochar from the furnace steel, whilst PTEs with low boiling points, such as As, Cd and Zn, partially evaporated during pyrolysis. PTEs were not responsible for phytotoxic effects observed for PTE-rich biochars despite biochar’s exceedance of available and total PTE threshold values for soil and soil amendments. Although initial tests were promising, the risk that PTE-rich biochars as amendment for soil and growing media pose, needs further investigation. The PAH concentration in biochar was markedly reduced by increasing carrier gas flow rate, and the type of feedstock also influenced the PAH content. However, there was no clear dependence of pyrolysis temperature on PAH concentrations, which was attributed to PAHs being increasingly formed and evaporated at higher pyrolysis temperatures. Ultimately, condensation of pyrolysis vapours and deposition on biochar was identified as the main risk for biochar contamination with PAHs, as this resulted in elevated concentrations of high-risk, higher molecular weight PAHs. Weaknesses in the pyrolysis unit design, such as cold zones, resulted in elevated concentrations of VOCs, as well as PAHs, in biochar. Comparing concentrations and phytotoxic potential of both compound groups, it was concluded that observed toxic effects were much more likely caused by VOCs in biochars containing both contaminants. Overall, formation of VOCs and PAHs cannot be prevented, but their presence in biochar resulting from retention and deposition can be minimised.

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.