Experimental process development and techno-economic assessment of in-situ biomethanation of carbon dioxide in biowaste anaerobic digesters

Biomethanation of carbon dioxide has recently emerged as a competitive technology for upgrading biogas produced from the anaerobic digestion of biowastes. In this configuration, carbon dioxide in biogas is reduced to methane through a biological reaction with hydrogen, resulting in several benefits: increased carbon efficiency, decarbonisation of the natural gas grid and transport, and potentially storage of renewable electricity in the form of high-energy-density fuel. In-situ biomethanation combines conventional biogas production from organic matter with the addition of H2 to produce a higher quality biomethane gas through retrofitting existing anaerobic digestion (AD) infrastructure. However, process engineering challenges remain to the uptake of in-situ biomethanation especially surrounding its continuous operation, control, and economic viability, which are addressed by this research using lab-based and techno-economic studies. An automated rig for the study of continuous in-situ biomethanation, including a monitoring and control system for control of the H2 injection rate to maintain process stability, was designed, commissioned and operated across a series of experiments to study the dynamics and performance of in-situ biomethanation alongside AD of sewage sludge (SS) and food waste (FW). This configuration would correspond to the retrofit of existing industrial-scale AD reactors, maintaining the typical plants' operational strategies and gas storage characteristics (size and pressure). Effects investigated experimentally included: H2 injection systems, gas recirculation and stirring intensity, and variation of the organic loading rate (OLR). All results highlighted the rate-limiting effect of H2 gas-liquid mass transfer, implied by a lack of evidence of biological limitation or inhibition and a relatively high equilibrium H2 content (11-36 % vol.) in the headspace appearing requisite to the gas-liquid transfer. Using a porous sparger as an improved injection system, a higher gas recirculation rate and mechanical stirring intensity improved the H2 conversion and methane evolution rate (MER). In the case of sewage sludge, the highest MER achieved was 0.16 LL-1day-1, with H2 conversion at 75 %, while in the case of FW, the highest MER was 0.23 LL-1day-1 with H2 conversion at 66.3 %. The importance of the OLR on the achievable H2 conversion was also highlighted and rationalised by its relationship with the gas Retention Time (RT). When operating the digesters at a reduced OLR of 1 gVS L-1 day-1, higher H2 conversion was achieved, up to 94 % for SS and 87 % for FW. However, that resulted in a trade-off with lower MER values, at 0.1 LL-1day-1 for SS and 0.16 LL-1day-1 for FW. The techno-economic work modelled the economic viability of in-situ biomethanation in the current market conditions. The analysis considered a hypothetical consumer tariff linked to the wholesale (variable) electricity price, allowing scheduling of hydrogen production from lower-cost electricity. Scenarios were investigated using different electrolyser technologies, varying electrolyser sizes, and biomethane end-use. The economic assessment showed the sale price of biomethane ranging from £94-110 /MWh across all scenarios in current market conditions, with electricity price and electrolyser costs being the most influential parameters on the results.