What is it about?

An indicative appraisal has been undertaken of a combined Anaerobic Digestion and Steam Methane Reforming process to produce sustainable hydrogen (H2) from organic waste. The anaerobic digestion plant was based on the plant in Tilburg (The Netherlands), and was modelled from the kerbside organic waste collections through to methane production. Data on biogenic waste was obtained from a collection trial in a municipal area in the UK. This was scaled-up to match that of a Tilburg-like anaerobic digestion plant. The waste collection trials enabled the catchment area for an anaerobic digestion plant on a commercial scale to be estimated. A thermodynamic evaluation of the combined process included energy and exergy analysis in order to determine the efficiency of each process, as well as to identify the areas that lead to inefficiencies. The overall energy efficiency is 75% and the overall exergy efficiency is 60%. The main energy losses were associated with compressor inefficiencies. In contrast, the main exergy consumption was found to be due to the fermentation in the digestion tanks. Other H2 process efficiencies vary from 21% to 86%, with the higher efficiencies belonging to non-renewable processes. However, the sustainable H2 produced comes from entirely renewable sources (biogenic waste) and has the benefit of near-zero carbon emissions in contrast to fossil fuels. Finally, the case study included an indicative financial assessment of the collection to processing chain. A discounted payback period of less than 20 years was estimated with a modest annual charge for householders.

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Why is it important?

Commercial hydrogen (H2) production is almost entirely via the steam reforming of natural gas at the present time. Sustainable production of H2 may follow either a direct biological route, biological production of methane (CH4) followed by reforming, or gasification of biomass. Other possible sustainable sources are electrolysis using electricity from wind or solar sources and photocatalytic splitting of water. However, the main priority of the present study is the biological routes. The aim here was to provide an indicative appraisal of a H2 production plant using thermodynamic and other methods of analysis of the biochemical process. Thermodynamic (energy and exergy) analysis gives rise to differing insights into the relative performance of various process chains. The thermodynamic property known as ‘exergy’, for example, reflects the ability of undertake ‘useful work’, but does not represent well heating processes within an energy sector. This was followed by a financial appraisal of the technology on a discounted cash flow (DCF) basis, and a brief qualitative review of its likely environmental burdens.

Perspectives

There are a number of technological bottlenecks associated with anaerobic digestion and steam methane reforming plants. A the Royal Society [RoySoc] H2 expert study (2018) concluded that anaerobic digestion plants are feasible now at a laboratory or small scale; notwithstanding the fact that the Tilburg facility demonstrates the practicality on a much larger scale. The RoySoc experts argued that anaerobic digestion plants might have greatest impact if used to produce high value chemicals in conjunction with a biorefinery. Achinas et al. (2017) suggested that there were important technical, economical, and ecological barriers. Costs are particularly prohibitive when utilising multi-stage reactors. Optimising key elements (such as micro-organism species, pretreatment methods, purification technologies, and substrate properties) are the main challenge to cost-effective methane production. The US Office of Energy Efficiency & Renewable Energy (2019) recently noted that gas-liquid mass transfer also provides a bottleneck, due to the low solubility of CH4 and results in cost challenges to gas phase fermentation systems. In terms of the steam methane reforming technology, the RoySoc (2018) suggested that it was already a commercial route, but was not low carbon. They therefore recommended that it would have to be coupled with carbon capture technologies. However, this H2 expert group did not consider the option of a combined anaerobic digestion - steam methane reforming facility of the type studied here that potentially delivers low carbon, renewable H2.

Professor Emeritus Geoffrey P Hammond
University of Bath

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This page is a summary of: Indicative energy technology assessment of hydrogen processing from biogenic municipal waste, Applied Energy, September 2020, Elsevier, DOI: 10.1016/j.apenergy.2020.115329.
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