Gasification of waste for hydrogen production

Abstract

The transportation sector relies heavily on fossil fuels and contributes up to one third of greenhouse gas emissions. Among available renewable resources, only biomass contains the carbon and hydrogen that can be converted into multiple energy vectors (i.e. heat, electricity, biofuels and chemicals) to reduce the heavy dependency upon fossil fuels and their associated environmental impacts. Advanced thermochemical technologies, i.e. pyrolysis and gasification are considered as potential approaches to convert biomass into fuels for the transportation sector and chemical industry. Although, coal gasification is well developed, gasification of biomass has not been widely commercialised due to a number of challenges including low H2/CO ratios (less than 1), low H2 content (40-60 mol%), low biomass conversion (80-85%), low process efficiency (70-80%), high CO2 content (20-30 mol%) and high tar formation (30-80 g/Nm3). The aim of this study was to develop a modular gasification system to produce H2 from waste biomass/residues to be used as an energy carrier or converted further into synthetic liquid fuels (diesel fuel and gasoline) and valuable chemicals (i.e. methanol, ethanol and alcohol). Gasification of waste biomass was carried out in a two-stage gasification process in which feedstock was decomposed into intermediate products and subsequently gasified using steam. The effect of operating conditions in the pyrolysis and gasification steps on maximising the H2 production and quality of synthetic gas (known as syngas) as well as process efficiency was studied. The synergistic effect of CO2 environment and catalyst (Ni/MRM, Ni/Al2O3 and Ni/HZSM-5) on the gas properties and tar formation was evaluated. A small-scale air-blown throat downdraft gasifier was optimised using Computational Fluid Dynamics (CFD) to obtain high quality syngas/H2 production and validated using experimental and literature data. The pyrolysis temperature has a strong effect on char morphology and the volatiles produced, which in turn affected the syngas properties. Increasing pyrolysis temperature from 600 oC to 900 oC, resulted in an increase in H2 (from 54 mol% to 66 mol%) and CO (from 5 mol% to 10 mol%) and a reduction in CO2 (from 37 mol% to 22 mol%), CH4 (from 5 mol% to 2 mol%) and tar content (from 39 g/Nm3 to 24 g/Nm3) in the gas stream after gasification. Around 67 mol% H2 together with high carbon conversion (94%) and low tar formation (21 g/Nm3) were observed under N2/steam gasification at a pyrolysis temperature of 900 oC and gasification temperature of 1000 oC with steam to carbon in feedstock molar ratio of 5.7. A process efficiency of 84% was achieved in this case. Combining CO2 and steam in the gasification stage produced up to 78 mol% H2 with a low CH4 (0.9 mol%) and tar content (9 g/Nm3) with process efficiency ≤ 97% at a pyrolysis temperature of 900 oC and gasification ii temperature of 1000 oC with steam to carbon in feedstock molar ratio of 3.4. Less than 10 mol% CO2 was generated from CO2/steam gasification compared to 20-30 mol% in N2/steam gasification. Therefore, the use of CO2 in a gasification process could be route to utilise waste CO2 for H2 production from biomass/waste, contributing significantly to the environmental footprint and sustainability of the process. Adding a Ni-based catalyst in the process had no effect on the syngas properties, but reduced by 2-3 times, the amount of tar (particularly heavy PAH compounds). The use of the Ni/MRM catalyst (waste product of bauxite processing) proved to be successful for the removal of naphthalene constituents (58%), the main component in tar, compared to only 50% reduction for the commercial Ni-based catalysts (Ni/Al2O3 and Ni/HZSM-5). For a small-scale air-blown throat downdraft gasifier, a throat to gasifier diameter ratio of 0.4 and the position of the air inlet nozzles at 10 cm above the throat provided high quality syngas/H2 production. The modelling can be used to predict the syngas compositions under the various operating conditions and provides an operating window for the development of a simple, highly efficient and robust gasification unit that can be used for H2 production without major downstream processing to remove impurities.