Pyrolysis and Gasification of Biomass and Waste Plastics under Novel Catalyst for H2 Production in the context of Carbon Capture and Utilisation

Energy demand is rising due to worldwide population increase and improvement of living standard. Excessive utilisation of fossil fuels results in problems increasing global CO2 emission. The technology of pyrolysis/gasification of biomass and waste plastics for H2 production is a promising solution to solve these two problems simultaneously. This thesis aims to study pyrolysis/gasification of biomass and waste plastics for H2 production plus carbon capture and CO2 utilisation (CCU) through experiments and modelling/simulation studies. Catalyst is key to in pyrolysis/gasification process. A novel dual-support catalyst Ni-CaO-C was developed. Three catalyst preparation methods (i.e. wet impregnation method, rising pH method and sol-gel method) were evaluated through comparing the performance of gas production. Experimental studies indicated that the catalyst prepared by rising pH method has the highest H2 yield. Therefore, this method was selected to prepare catalyst Ni-CaO-C for the further experimental studies. Through lab experiments, the optimal catalyst compositions of catalyst Ni-CaO-C were studied by changing Ni load and the support ratio of CaO and activated carbon. The catalytic activity and CO2 adsorption capability of catalyst were used as key performance indicators. The optimal operating conditions of catalyst Ni-CaO-C were studied in different lab experiments through adjusting feedstock ratio, pyrolysis stage temperature, reforming stage temperature and water injection flowrate. Results indicated that the new catalyst Ni-CaO-C possesses high catalytic activity and CO2 adsorption capability simultaneously. The optimal catalyst compositions are Ni load 10 wt% and support ratio (CaO:C) 5:5. Under the optimal operating conditions, the H2 production under Ni-CaO-C is at very high level at 115.33 mmol/g and 86.74 mol%. Use of catalyst Ni-CaO-C for different plastics and biomass was studied experimentally. Three different plastics (i.e. HDPE, PP and PS) were mixed with pine sawdust respectively. Experimental studies of pyrolysis/gasification under situations of no catalysts and using different catalysts (i.e. Ni-Al2O3 and Ni-CaO-C) were carried out. The influences of various operating conditions (e.g. feedstock ratio, reforming temperature and water injection flowrate) on the gas production using catalyst Ni-CaO-C to treat different feedstocks were also studied. Results indicated that catalyst is needed to improve H2 production and catalyst Ni-CaO-C has better performance of than catalyst Ni-Al2O3. The effect of catalyst Ni-CaO-C on different plastics ranks as HDPE > PP > PS. The plastics content in feedstocks is suggested to be 30 ~ 40 wt% to avoid the inhibition on H2 production under excessively high plastics content. PS requires the highest reforming temperature and highest water injection to achieve the lowest acceptable H2 production. To achieve further reduction of CO2 emissions, CCU was applied for pyrolysis/gasification process using Aspen Plus®. Process analysis is carried out based on the validated model to investigate the influence of recycling captured CO2 on the gas production and CO2 conversion when changing various operating conditions (i.e. amount of recycle CO2, reforming temperature and steam to feed (S/F) ratio). This is to achieve high H2 production and to promote CO2 conversion can be found. Simulation results indicated that the following findings: (i) Applying CCU for pyrolysis/gasification inhibits H2 and CO2 production but promoting CO production; (ii) The H2/CO ratio of gas products can be controlled flexibly after recycling CO2 to reforming stage; (iii) Increase of CO2 recycle amount and S/F ratio results in lower CO2 conversion while increase of reforming temperature improve the CO2 conversion; (iv) It is suggested to add solid carbon (e.g. bio-char or carbon-based catalyst) in the reforming stage and changing the operating conditions (i.e. relatively high reforming temperature (e.g. 600 ~ 700 °C) and low S/F ratio (e.g. 3~4)) simultaneously to protect H2 production and achieve a high CO2 conversion. The findings presented in this thesis should be very useful for future large-scale commercial deployment of pyrolysis/gasification process to achieve high H2 production and low CO2 emission.