Table 3.
Options for delivery of bio‐H2 into power, all via electro‐photofermentation (Figs 1 and 2; M.D. Redwood, R.L.Orozco and L.E. Macaskie, unpublished work)a
| Feedstock (upstream) | Power (downstream) | Comments |
|---|---|---|
| Fermentation of food wastes | Fuel cell electricityb or combined heat and powerc | Food wastes (FW) required (tonnages). Anaerobic digestion (AD) has monopoly on FW. Bio‐H2 can power a fuel cell directly. |
| Fermentation of cellulosic wastes | Fuel cell electricity or CHP | Comminution/maceration energy demand adversely affects overall energy balancee. Upstream hydrolysis is required. |
| OAs obtained from anaerobic digestion (AD) | ‘Hythane’: mix of CH4 (AD) + bio‐H2; CHP | AD interrupted at acetogenesis stage; organic acids diverted into a bolt‐on photofermentation. Overall AD residence time is reduced. This increases process complexity but gives a higher energy output. Gas is compatible with current infrastructure. Scenario 1: 20% more powerd. Scenario 2: 70% more powerd |
| OAs used directly from wastes (e.g. wastewaters) or CHP | Fuel cell electricity | Organic acid waste streams (tonnage scale) are (e.g.) vinasse (from bioethanol production) and municipal wastewater treatment plants (see text). |
Calculations were made independently of incentivization schemes as these tend to be ephemeral and skew the longer term picture. Likewise, increasing/decreasing feed‐in tariffs would complicate economic assessments.
Fuel cell technology is still emergent at large scale, and FCs fail prematurely (see Rabis et al., 2012).
Combined heat and power (CHP: well‐established technology). In this scenario, the methane stream from anaerobic digestion can be supplemented with photofermentatively derived H2 to make ‘hythane’ for CHP.
Scenario 1: diversion of 10% of the organic acids into photofermentation and use of hythane in CHP. Scenario 2: diversion of 80% of the organic acids into photofermentation and use of AD‐methane in CHP plus use of the photofermentation H2 in a fuel cell would give 70% more power (R.L. Orozco, unpublished). The proportion of flow diverted from the acetogenesis step of anaerobic digestion (via electroseparation) could be simply ramped in response to incident light intensity to feed the photofermentation; at night the flow would pass to the methanogenic reactor as normal. By combining the two processes, the residence time in the system would also be reduced as compared to traditional anaerobic digestion due to reduced flow entering the methanogenesis reactor daily.
Using Miscanthus as an example, the energy demand of comminution to 4 mm particles is 184 kJ kg dry matter−1; energy from H is 10 kJ l−1 (at 1 atm and 125°C); that from the dark fermentation was only 110 kJ kg cellulose; hydrolysate; hence the PF (~4 times the H2 as the dark fermentation) is key to a positive energy balance from complex substrates.