Abstract
The urgency of global decarbonization requires the development of scalable renewable hydrogen and methane production technologies. Processes based on lignocellulosic biomass gasification could offer a scalable solution in this space. The existing literature includes many studies where such an opportunity has been investigated both through experimental and modeling approaches. Hydrogen minimum selling prices and plant efficiencies achievable in biomass-to-hydrogen processes were reported in a range between 1.17 $/kg and 3.63 €/kg and 45.6% − 70%, respectively. Experimental research has also evaluated the gasification of low-cost residues for hydrogen-rich syngas in a variety of setups, demonstrating clear positive effects of higher temperatures and steam flow rates on hydrogen yields and concentrations. Biological methanation has been demonstrated a robust scalable technology that can represent an alternative to complex catalytic methanation processes, especially with the recent success of trickle-bed bioreactors. However, as opposed to techno-economic evaluations of concepts based on catalytic methanation, only a single study considering the biological methanation route was available at the start of the present work. In the experimental part of this PhD, a two-step pyrolysis and char-steam gasification fixed-bed setup at different temperatures and steam flow rates was, and the possibility to reach syngas concentrations compatible with Fischer-Tropsch synthesis and corresponding to 78 – 80% of the hydrogen requirements of a biomethanation system was demonstrated. Moreover, the co-fermentation of food waste and syngas for biomethane production in a continuously stirred tank reactor was tested under a high syngas loading rate of 9.5 l/l/day and an immediate intoxication of non-adapted cultures was observed, confirming previous evidence. The study also highlighted the strong solids destruction performance delivered by the intoxicated culture as well as an initial adaptation after three weeks of exposure, thus pointing to the potential for a co-fermentation bioreactor to achieve culture adaptation and waste mass reduction simultaneously. The extensive techno-economic modeling activity carried out in this thesis on biomass-to-biomethane returned a range of minimum selling prices of 1.63 – 2.74 €/Nm3 and indicated a Power-to-Gas integrated process with electrically-heated tar reforming as the most economic (1.63 €/Nm3), highest-yield (0.48 Nm3/kgdb) case. The findings showed that Power-to-Gas integration delivers a clear economic and efficiency benefit to biomass-to-biomethane processes, while seasonal carbon capture and storage services depress process efficiency and profitability at current carbon credit prices. Similarly, I assessed biomass-to-hydrogen under several scenarios, reporting minimum selling prices between 5 and 17.51 €/kg and identifying a Power-to-Gas integrated oxygen gasification process as the most economic, highest-efficiency (73%) process, and steam gasification as the least economic, least efficient (29%) process. The enhancement of the large-scale potential of biomass-to-biomethane and biomass-to-hydrogen systems relies on additional technological developments, including scalable gasification technologies treating low-cost biomass, co-fermentation processes and oxygen gasification technologies. Such developments, though, will need the support of an adequate set of policies that include higher biomethane subsidies, the suspension of electrical grid tariffs and long-term minimum carbon pricing.