AbstractHydrogen could replace fossil fuels for power generation and transportation and contribute to a low carbon economy. However, current methods of producing hydrogen e.g. steam methane reformation of natural gas are not sustainable and also contribute to COi emissions.
Dark fermentation of carbohydrate-rich waste organics and energy crops to hydrogen using mixed microflora could contribute to the mix of technologies for producing hydrogen sustainably. Naturally available mixed microflora can be enriched e.g. by heat treatment to select for hydrogen producers, typically clostridia. Fermentation endproducts from the hydrogen-producing stage could be fed to a second anaerobic digestion stage to recover more energy as methane and to stabilise the effluent. Although anaerobic digestion is well established, fermentative hydrogen production is not.
This work evaluated the feasibility of hydrogen production from two energy crops, grass and fodder maize in batch culture without pretreatment; investigated the effect of increase in substrate (sucrose) concentration, attractive from an energy point of view, on the yield and stability of hydrogen production in continuous culture; examined the performance of a mesophilic high rate anaerobic digester treating effluent from a continuous hydrogen-producing bioreactor; demonstrated the possibility of changeover of substrate - sucrose, starch and xylose - during continuous hydrogen production and evaluated the effect of sparging with CO2 , a process gas, on hydrogen production.
It was demonstrated for the first time that hydrogen production from grass and fodder maize by direct fermentation in batch culture (2.3 L reactor, 35°C, pH 5.2-5.3) is possible, with hydrogen yields of 75.6 ml/g dry matter wilted perennial rye grass and62.4 ml/g dry matter of fodder maize. In continuous culture (pH 5.2-5.3, 35°C, 12 hour hydraulic retention time (HRT)), stable hydrogen production was achieved up to 40 g/L sucrose concentration - with decreasing hydrogen yields, from 1.7±0.2 mol/mol hexose added at 10 g/L to 1.2±0.3 mol/mol hexose at 40 g/L - beyond which the system became unstable. The decrease in hydrogen yield and lack of stability at higher substrate concentrations was attributed to feedback inhibition by volatile fatty acids (VFAs). Effluent from the hydrogen reactor was readily degraded in an upflow anaerobic filter up to an organic loading rate of 10 gCOD/L/d (2 d HRT) and/or a sodium concentration of 1.87 g/L. Reduction of sodium levels in the methane reactor by using calcium hydroxide as alkali in the hydrogen reactor was found to extend the efficiency of degradation of VFAs; overall COD reduction for the two stage system fed with 20 g/L sucrose increased from 83% (with NaOH as alkali) to 91% with Ca(OH)2 . It was easier to switch from starch to sucrose and vice versa during continuous hydrogen production; however switching from sucrose or starch to xylose was slower, requiring operation for about 1 day in batch culture before continuous operation could commence. Sparging with CO2 improved hydrogen yield from sucrose by at least 71% and appeared to inhibit homoacetogenesis from starch. This work verifies the potential technical feasibility of generating hydrogen, a clean energy carrier, sustainably from carbohydrate-rich waste organics and energy crops.
|Date of Award
|Freda Hawkes (Supervisor), Richard Dinsdale (Supervisor), Alan Guwy (Supervisor), Dennis Hawkes (Supervisor) & Giuliano Premier (Supervisor)
- Hydrogen as fuel