Assessment of novel utilisation pathways for biogas and nitrogenous waste streams at wastewater treatment plants

A combination of process modelling, numerical modelling, economic analysis and experimental techniques have been used to analyse novel utilisation pathways for biogas and nitrogenous waste streams at wastewater treatment plants. An assessment of a large wastewater treatment plant serving a population equivalent of 750,000 people was carried out including compositional analysis of various streams at the facility. This facilitated three key findings that function as the bedrock for the rest of study: the facility’s greenhouse gas footprint, its energy balance and its digestate liquor ammonia concentration.
Aspen Plus process modelling software was used to develop a system that recovers ammonia in a way that prepares it for thermochemical decomposition to hydrogen and nitrogen. Sensitivity analysis showed that air stripping was energetically preferable to steam stripping as the base recovery technology. This was proceeded by an absorption step that uses a water-only solvent and finally a distillation step that was found to be energetically preferential to flash separation. The modelling showcased an ability to recover 91% of ammonia contained in the digestate liquor. Ordinarily, the wastewater treatment plant would recycle the liquor back into its conventional process. By recovering the ammonia, and diverting it away from conventional treatment, it is proposed that the plant will experience significant reductions in energy consumption and GHG emissions.
Aspen Plus was used to develop a process model that combines the recovery of ammonia with the operation of an internally reforming solid oxide fuel cell stack, which uses a blend of biomethane and ammonia as its fuel. A numerical model was developed that precisely calculates its power production potential, based on a commercially available solid oxide fuel cell stack. It was found to operate at a net electrical efficiency of 48% and if implemented at the referenced wastewater treatment plant, would increase the site’s power production by 45%. It was also proposed that the site’s lifecycle GHG emissions would reduce by 7.7% due to a combination of ammonia diversion and reduced grid electricity consumption. An economic study showed that it would be financially viable to implement this technology at the site with a positive net present value facilitated after eight years of operation.
A process model was developed which utilises recovered ammonia and biomethane as feedstock for a thermochemical H2 production system. Steam to carbon ratios of 2, 3 and 4 were analysed to assess their impact on H2 production, energetics and financial viability. The scenario with a steam to carbon ratio of 3 showcased the best economic potential with net present value becoming positive during its 14th year of operation. It was proposed that if the H2 produced was used as a vehicle fuel for bus transportation the process implementation would reduce the facility’s lifecycle GHG emissions by 25%.
An H2-rich syngas was generated experimentally using ammonia, methane and steam feeds in a fixed-bed reactor holding a conventional Ni-Al catalyst. Ammonia, methane and carbon monoxide conversions were less than predicted via equilibrium calculations. However, the general selectivity of products closely resembled that of equilibrium equivalents – showcasing an ability to combine the steam reforming of methane and the decomposition of ammonia in a single reactor.