Periodic Reporting for period 2 - REBOOT (Resource efficient bio-chemical production and waste treatment)
Période du rapport: 2021-07-01 au 2022-12-31
The current state of the art solution for wet waste management such as sewage sludge or manures is anaerobic digestion. This solution still leaves a digested sludge which is often applied to land to recover fertilizers or incinerated. Due to pollutant concerns, land application is becoming less common practice and valuable nutrients are lost when wastes are incinerated. Hydrothermal liquefaction (HTL) is an emerging alternative solution to anaerobic digestion where wet wastes are heated in water, under pressure to around 350°C for 15 minutes. This mimics the natural formation of fossil fuel creation and forms a renewable bio-crude. This bio-crude can then be refined, using conventional refining technology into sustainable aviation or marine fuels. The REBOOT project addresses this technology and specifically targets the recovery of phosphorous for nutrient applications. Phosphorus is a finite resource and hence important to recycle and reuse. The project aims to develop new separation technologies that can recover inorganics in the hydrothermal liquefaction reactor while producing a bio-crude with minimal metal content which is beneficial for further upgrading to finished fuels. The project also aims at developing improved catalysts for the technology to increase the quality of bio-crude, this would mean the refining step would be easier and cheaper, increasing the overall viability of the technology. Finally, the project addresses the process water that is produced during the reaction. As HTL uses wet wastes and no prior drying is carried out, a significant amount of wastewater is produced. This process water contains high levels of organics and needs to be cleaned before discharge. Novel technologies are utilized to valorize the process water and integrate the solution into the overall process by either producing hydrogen or heat for the process. Overall the project will mature the HTL technology and develop novel strategies to improve the performance and sustainability. The impacts for society could be very beneficial; firstly the HTL technology destroys all types of pathogen and bioactive compounds typically found in wet wastes, ensuring they do not end up in the food chain which will have negative effects on heath. Secondly phosphorous is recycled with results in a lower greenhouse gas emission footprint for fertilizers, decreasing our dependency on foreign imports of nutrients and reducing their cost which could result in more sustainable and affordable food production. Finally the production of sustainable fuels for the hard-to-electrify transportation system will ensure society can continue travelling and transporting goods in the future without emitting additional greenhouse gases.
Initially a study was carried out to evaluate the effect of temperature on the partitioning of phosphorous into the different product phases. Diverse wet wastes were employed such as manure, sewage sludge, a digestate from anaerobic digestions and slaughterhouse waste. It was shown that the HTL technology indeed recovers over 95% P into the solid fraction which is the target for the novel separation technologies which are implemented in continuous flow reactors. For this purpose a dedicated continuous HTL reactor was designed and constructed where novel in-line separation devices can be tested and optimized. Initially manure has been evaluated using this setup and the temperature varied to assess if the decreased solubility at higher temperatures in subcritical water leads to precipitation of salts which can be recovered inline. These findings have been compared to the common practice in literature where batch reactors are employed and solids are separated from the cold reaction mixture where salts are more soluble than under pressure and high temperature. The results have indeed shown that inline recovery is beneficial for HTL processing.
Work has also been carried out finding catalysts suitable for HTL. Catalyst ssupports TiO2, ZrO2 and TiO2+ZrO2, were doped with promising transition metals, Ni, Co and Fe and subsequently screened for activity and stability. All supports were shown to be stable under hydrothermal conditions and increased bio-crude yields could be obtained. In parallel catalytic upgrading of bio-crude has been carried out for manure biocrudes in a fixed bed hydrogenation reactor to produce diesel and kerosene.
The process water challenge in HTL has been addressed by two technologies: wet-oxidation and electrocatalytic oxidation. The former converts soluble organics to heat and CO2, which can be integrated in the main HTL process to provide the heat demand for the HTL reactor while cleaning the water. Electro-oxidation has shown to be effective in converting the organics to hydrogen using electricity which could be used to upgrade the bio-crude by hydrogenation. Both technologies have been experimentally validated in continuous flow and published in peer review literature.
Work has also been carried out finding catalysts suitable for HTL. Catalyst ssupports TiO2, ZrO2 and TiO2+ZrO2, were doped with promising transition metals, Ni, Co and Fe and subsequently screened for activity and stability. All supports were shown to be stable under hydrothermal conditions and increased bio-crude yields could be obtained. In parallel catalytic upgrading of bio-crude has been carried out for manure biocrudes in a fixed bed hydrogenation reactor to produce diesel and kerosene.
The process water challenge in HTL has been addressed by two technologies: wet-oxidation and electrocatalytic oxidation. The former converts soluble organics to heat and CO2, which can be integrated in the main HTL process to provide the heat demand for the HTL reactor while cleaning the water. Electro-oxidation has shown to be effective in converting the organics to hydrogen using electricity which could be used to upgrade the bio-crude by hydrogenation. Both technologies have been experimentally validated in continuous flow and published in peer review literature.
The most significant progress against the state of the art lies in the achievements in the HTL process water management and valorization. The use of wet-oxidation has proven to be effective, low cost and beneficial for the overall process. As the reaction is exothermic, the produced heat is envisioned to be used for the HTL reaction itself, while cleaning the process water. We have shown high carbon conversion efficiencies, nitrogen conversion to ammonium and residual production of volatile fatty acids which could hold additional value. We have published the results on wet-oxidation of HTL process water from sewage sludge showing the effect of temperature and residence times. This was used to design the experiments in continuous flow, where heat integration using heat exchangers has been employed. These are important contributions to advance the state of the art and bring the HTL technology process water management closer to implementation. We envision that further optimization and integration of the wet-oxidation process with the HTL reaction will allow close to autothermal operation of the combined process and could become the state of the art in HTL for process water management.
In terms of phosphorous recovery and inorganic separation a new continuous flow reactor has been built where the P recovery can be optimized using different process conditions and separation devices. Early results have already shown that there are significant differences in the recovery of inorganics hen comparing state of the art continuous reactors to batch reactors. It is envisioned that with this newly built reactor the most optimal separation condition and devices can be identified and optimized to allow their installation, testing and validation in a continuous pilot scale reactor. This would set the state of the art in terms of reactor design for future HTL commercial plants.
In terms of phosphorous recovery and inorganic separation a new continuous flow reactor has been built where the P recovery can be optimized using different process conditions and separation devices. Early results have already shown that there are significant differences in the recovery of inorganics hen comparing state of the art continuous reactors to batch reactors. It is envisioned that with this newly built reactor the most optimal separation condition and devices can be identified and optimized to allow their installation, testing and validation in a continuous pilot scale reactor. This would set the state of the art in terms of reactor design for future HTL commercial plants.