Waste gasification for hydrogen production

Abstract

Hydrogen, vital in sectors like chemical production, polymer processing, and energy, is expected to see a demand surge to 450 million tonnes by 2050. Its importance in industry, transport, and power is growing, but current production, primarily from fossil fuels (95 million tonnes, 96% of total), significantly contributes to CO2 emissions (3% of global annual total). To reduce emissions, research is shifting to low-carbon methods like renewable energypowered water electrolysis and gasification processes. Challenges include the variability of solar and wind energy, nuclear energy's public acceptance and safety, and waste gasification's efficiency and heterogeneity. Effective waste gasification involves pre-treatment and a complex multi-stage process, with separate pyrolysis and gasification reactors improving syngas quality and gasifier efficiency. Addressing the increasing solid waste, responsible for 4.5% of global CO2 emissions, is crucial, making waste gasification a key focus. The aim of the PhD project was to investigate the effects of operating conditions on performance of two-stage gasification (TSG) of waste. Key parameters considered include steam to carbon ratio (S/C), stage temperature, heating rate, CO2 concentration in the carrier gas, and residence time. Hydrogen production from waste via the TSG was also assessed for its environmental impact and sustainability to provide insights into the overall ecological footprint. Combining CO2 and steam as gasifying agents resulted in 30% increase in hydrogen content in the producer gas compared to theoretical amount of hydrogen in waste wood. In addition, CO2/steam TSG reduced up to 93% tar (high molecular weight compounds) in the gas stream compared to steam gasification at the same operating condition (900°C for 1st stage and 1100°C for the 2nd stage at S/C of 5.7. These can be explained due to the synergy between Boudouard and water-gas shift reactions. Based on mass balance analysis, up to 34 wt% CO2 in the gasifying agent was utilised in the process. The optimization of the refuse-derived fuel pellets (RDF) TSG, leveraging the response surface methodology (RSM) for enhanced efficiency. This led to the discovery of optimal conditions for RDF-TSG: 1st stage temperature of 587°C, 2nd stage temperature of 924°C, S/C of 2.6, and a CO2 flow rate of 100 cm³/min. Under these conditions, it achieved a remarkable hydrogen yield of 93.4±1.2 mg/g RDF and a process efficiency of 67.0±0.7%. The life cycle assessment (LCA) study revealed that RDF yielded 4-7 g CO2 eq/kg H2 through TSG. Local waste wood TSG resulted in emissions between 0.9-2.0 kg CO2 eq/kg H2. However, the origin and transportation of waste wood, especially from countries like the USA, Canada, Latvia, and the Netherlands, increased emissions to around 5 kg CO2 eq/kg H2, challenging its classification as UK low carbon hydrogen. With carbon capture and storage (CCS) technology, emissions from waste wood TSG were reduced to approximately -1.3 kg CO2 eq/kg H2, and RDF TSG to 0.4 kg CO2 eq/kg H2 in the UK context. This thesis establishes a benchmark for multi-stage gasifier plant advancements, merging fundamental concepts with practical outcomes. It highlights the necessity for future research on blending RDF with waste wood to reduce environmental impacts and increase CO2 utilization in carrier gas. It also encourages detailed studies on scaling up these technologies. The included LCA provides vital information for decision-makers in policy, industry, and academia, aiding in comparative analyses and shaping future hydrogen production strategies.