Enhancing Green Hydrogen Production during One and Two Stage Catalytic Pyrolysis of Biomass with Mechano-chemically Prepared Char Supported Iron Catalyst

Abstract

Thermo-chemical conversion of biomass is one route to create a pathway to renewably sourced energy, including hydrogen; and precursor products for the manufacture of transport fuels and chemicals. While this prospect is tantalising, the three realities of economics, complexities of processing, and securing feedstock supplies currently hinder the transition. The aims of this work were to hasten the use of biomass conversion by improving its economics. Two key considerations were the catalyst system and the type of thermochemical process, pyrolysis or gasification, as both have the potential to not only influence reaction rates and yields, but also both capital and operating costs. Pyrolysis was selected as the process. A new catalyst system was devised based on a mechano-chemical preparation method, suitable for direct addition of the catalyst to the biomass. The catalyst components were iron, being plentiful, low cost and an established catalyst in biomass processing; and char, generated internally during pyrolysis. Almond residues were the primary biomass used. Using thermogravimetric analysis the impact of this catalyst on reaction kinetics was assessed over a range of pyrolysis temperatures (450 – 750°C) with catalyst iron loading of 2%, 4% and 6%(wt./wt. biomass). Char yields were reduced, the major ligno-cellulose components had lower onset and peak decomposition temperatures, increased total mass loss and reduced apparent activation energies. The extent of each of these changes reflected catalyst loadings (Chapter 2). A one-stage laboratory fixed bed reactor was used to conduct a series of catalytic pyrolysis trials over a similar range of temperature and catalyst loadings, using almond residue and pine chips as biomass. Overall product gas volumes and hydrogen production were higher with increasing pyrolysis temperature and catalyst loadings. Notably, char yields reduced as hydrogen production increased. The impact of the catalyst on different biomass was essentially the same. Commercial iron ore (CIO) and waste iron ore tailings (IOT) were selected as alternatives to replace iron oxide prepared from AR grade ferric nitrate (LFN). The mechano-chemical preparation method was progressively simplified for each of CIO and IOT. At the lower catalyst loadings and pyrolysis temperatures each catalyst type produced similar yields. Above 650°C yields from CIO and IOT types were lower by around 15% with a 30% divergence evident at peak temperature and catalyst loading. Nevertheless, at these conditions, hydrogen production by all catalyst types was over 175% of that from baseline pyrolysis at 600°C. A two-stage catalytic pyrolysis process was developed, with both stages operating at the same temperature. Product from the first stage passed to the second, where char residue, containing the catalyst, supported reforming reactions. Total gas and hydrogen production, was essentially double that achieved in equivalent one-stage processing. Increases (up to 60%) in syngas production were also assisted by higher CO production at higher pyrolysis temperatures, bringing improved H2/CO ratios. IOT proved equally as effective as LFN in these experiments. This further enhances the development of this process for producing green hydrogen through recycling of waste ore.