(363c) Thermodynamic Investigation of Selected Production Processes of Hydrogen from Biomass

It is widely acknowledged that the solution to the global energy problems would be to replace the existing fossil fuels by hydrogen as the universal energy carrier. The main advantages of hydrogen systems are that both production and utilization of hydrogen can be emission-free and hydrogen can be obtained from a variety of feedstocks. However, hydrogen currently is produced mostly from fossil fuels, by steam reforming of natural gas (SMR) and naphtha or coal gasification.

The future hydrogen energy systems must be based on hydrogen production from renewable energy sources. Biomass is the most versatile non-renewable resource that can be used for sustainable production of hydrogen. Renewable hydrogen technologies based on low value waste biomass as feedstock have great potential to become a cost-effective energy production.

This paper presents a second law analysis of hydrogen production processes from a variety of biomass feedstocks by thermochemical methods (gasification of wood, straw, vegetable oil, manure, sludge) as well as biochemical methods (fermentation and anaerobic digestion of organic wastes). These biomass feedstocks vary greatly in chemical composition, energy content, ash and moisture content. The question is whether all the biomass type can be converted to hydrogen with reasonable exergetic efficiency. The relatively high efficiencies are very desired as the hydrogen content in biomass is low (approximately 6 wt% versus 25% for methane) and energy content is also low due to high oxygen content (about 40 wt% of biomass).

The detailed exergy analysis of thermochemical H2 production has been performed for different biomass feedstocks. All processes have been simulated with a computer model using the flow-sheeting program Aspen Plus. The main process steps are biomass drier, biomass gasifier, syngas purification and compression, high and low temperature water gas shift reactors, water separation, and pressure swing adsorption. Exergy analysis is performed changing various operating conditions such as the gasifier temperature and the moisture content in the biomass leaving the drier. The largest exergy losses occur in the biomass gasifier, followed by water separation and syngas compression. The overall exergetic efficiency increases with decreasing gasifier temperature. It is found that overall exergetic efficiency depends strongly on the original moisture content in the biomass feed. The highest efficiency is found for gasification of vegetable oil which is moisture free, followed by straw and wood (moisture content 13-20 wt%), and the lowest efficiency is for sludge and manure (moisture content 30-40 wt%).

The exergetic efficiency of hydrogen production by gasification of more dry feedstocks, such as vegetable oil, wood and straw (66 ? 79%) is comparable to that of the current hydrogen production by steam methane reforming SMR (78%) based on fossil fuels. However, SMR is not a sustainable process and if the additional sequestration of produced CO2 would be taken into account to make SMR more sustainable, than the exergetic efficiency of this process will be lower then the reported value of 78%.

Finally, the exergetic efficiency of hydrogen production by biochemical processes has been evaluated based on the literature data. In general, biochemical processes for H2 production are small-scale with a laboratory of pilot-scale status. The most promising methods are the combination of dark and photo fermentation and the anaerobic digestion, followed by steam methane reforming. However, the exergetic efficiency for hydrogen production using these methods is lower compared to that of biomass gasification of drier biomass.