Synthesis gas from agricultural feedstock: a review of possible technical pathwaysBernesson, Sven; Ahlgren, Serina
Syngas, or synthesis gas, is a fuel gas mixture consisting of CO + H2 in different proportions. In some cases, CO2 is also included in the mix. Syngas is an important feedstock when producing many chemical products (e.g. ammonia and methanol), but also gaseous biofuels (e.g. substitute natural gas (SNG) and hydrogen) and liquid biofuels (e.g. Fischer-Tropsch diesel (FTD) and dimethyl ether (DME)). Syngas can also be used in turbines for efficient production of electricity and heat.
Much attention has been given to the possibility of thermal gasification of forest products, but there are other alternatives for syngas production. This report reviews the possibilities of converting agricultural feedstock (crops, manure, residues etc.) to syngas via (1) upgrading of biogas from anaerobic digestion and (2) thermochemical conversion. The focus of the review is on technical conversion systems rather than feedstock and it is based on existing literature, but we also present a rough energy analysis, examining some of the energy inputs and outputs to the system.
According to the literature, there are many different reforming techniques for production of syngas from biogas. The energy analysis indicated that the output energy (as H2, CO and CH4) from the reforming processes was largest from combined steam and carbon dioxide reforming. However, this process also requires high energy inputs due to very endothermic reactions in the reforming phase. If incoming methane is included in the calculations, the energy balance of the different types of reforming methods was very similar. The study also shows that energy for reforming dominate the process energy input, while energy use for upgrading biogas, desulphurisation and production of pure oxygen had marginal effects on net energy yield.
It was difficult to find a consistent method to compare energy balance between gasification and the anaerobic-to-syngas route. However, the anaerobic-to-syngas production route appears to be within the lower range of energy yields reported for gasification. Note that the energy analysis in this report is only indicative. Note also that anaerobic digestion and gasification should not be seen as competing but as complementary technologies, providing the opportunity for a diversity of raw material to be used as feedstock for syngas production.
When evaluating the different technologies scalability and costs are vital, and would be a logical continuation to this project. Another interesting follow-up project would be a more detailed study of energy balances and greenhouse gases for the different technical pathways, where the whole life cycle, including cultivation of feedstock, is included. Extending the system boundaries to include production of biofuels would be also interesting, as it would enable comparison with other types of biofuels. Further research on the technical side could also include studies on the development of small-scale reforming for biogas applications, as well as novel technologies such as plasma reforming.
KeywordsBioenergy, biogas, synthesis gas, thermochemical gasification, energy analysis, agricultural feedstocks
Published inf3 report
2013, number: 2013:24
Publisher: f3 The Swedish Knowledge Centre for Renewable Transportation Fuels
UKÄ Subject classification
Renewable Bioenergy Research
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