1. Bio-waste potential (WP1)
Unused biogas is 3000 PJ (20% of EU NG), classified into small (5/50 kWe) and medium/large scale (500 kWe and larger). They reside in agriculture (manure and crops residues, in a proportion of 35%-65%); the OFMSW-biogas potential is just 2% of that in agriculture.
53% is for exploitation on 50 kWe scale per site.
For CH, D, F, IT, 107'000 units of 50 kWe SOFC could be deployed (5 GWe) on now unused agrowaste, considering only 50% of the potential,
2. Biogas cleaning (WP2)
Agro-biogas = sulfur contaminants: 100s ppm H2S, and ppm of CH3-S-CH3 (DMS dimethyl-sulfide), CH3-SH, COS.
The other critical contaminant is siloxanes, in landfill (116 PJ), and WWTP (59 PJ).
21 solid sorbents were selected, sourced and tested for S-removal in 3 labs, in variable conditions (gas matrix, humidity, O2 presence, flow velocity). Sorption capacity is sensitive to all parameters. A solution was identified by removing H2S from wet biogas, chilling the gas, then a polish with dry gas to remove the S-rest. COS is the most difficult contaminant, followed by DMS. Sorption capacities = 200 g H2S, 15 g DMS, 6 g COS, per kg of sorbent. Depending on gas quality, the cost of sorbents is few 100€ per kWe per year.
A cleaning kit was built from this and deployed on a Swiss farm biogas site where a 1 kWe SOFC was installed (>50% elec. eff.). A field test is planned with this pilot where sorbents can be exchanged.
A full scale cryocleaning plant was designed, built and tested on a large biogas site in Lithuania, to remove contaminants with a Linde cycle (-100°C). H2S was partially removed, VOC to 80%. The main issue was icing. The unit was shipped back for improvements, and the technology will be developed further. In both cases, OPEX of the cleaning adds 1 ct€ per kWhe.
3. Biogas testing on SOFC (WP3,WP4)
8400 biogas-SOFC-system configurations computed => 4 gas compositions for testing, 2 for SolydEra and Sunfire SOFC technologies, 1 with dry reforming, 1 with mixed CO2/steam reforming. Elec. eff. of 57-64% result. A small scale CO2 capture scheme (carbonate solution looping) was integrated into system calculations, to still reach 57% net el. eff.
3 reforming catalysts were run in tests for 11'400 h. Catalysts withstand poisoning up to 10 ppm S for 100 h and can be partially regenerated. Catalyst activity can be affected after a few days of a 1 ppm S leak. Protection of this is straightforward.
Cells tests cumulated 27'000 h, showing resilience to 3-5 ppm S with 1-2% performance loss. DMS poisoning is fully reversible in low levels, H2S has a part of irreversible performance degradation. Vulnerability is worse in dry reformed biogas because S blocks the RWGS reaction. Performance regeneration is easier in H2-richer gas, and slow, but complete. This was confirmed on stacks tests cumulating 22'000 h.
Techno-economic analyses were performed for 20-200 kWe systems => LCOE of 9-14 cts€/kWhe, TCO of 10 k€/kWe (with stacks of 4-5 yrs life time). Biogas generation cost remains dominant (50%), SOFC 35% and cleaning 15%. Sensitivity analyses show stack cost and stack lifetime as important factors. LCA showed a clear benefit for agro(manure) biogas SOFC; recommendation for cleaning is to limit the use of steel (vessel) and activated carbon (high sorption capicity is needed).
4. Dissemination, management (WP5,WP6)
Project website and templates were set up, 2 flyers and 2 newsletters circulated, and W2W presented at a biogas seminar and 2 open door day events. 12 oral presentations at conferences were given and 10 journal articles published, with 6 more papers in preparation. Detailed data have been collected on sorbents, biogas compositions and trace concentrations, and on the bio-waste resource potential. A biogas market entry for SOFC was carried out, considering the replacement of ICEs on existing biogas installations until 2030: 40’000 units of 50 kWe SOFC or 2 GWe.
