Energy recovery from anaerobic digestate combined with nutrient management

Abstract

Anaerobic digestion is the process of converting organic materials such as animal slurries and energy crops into biogas for energy provision, while producing anaerobic digestate as a by-product. This thesis investigates the management of anaerobic digestate, by mechanical separation, to reduce the risk to water quality while providing a potential new source of energy in the form of pelletised digestate solids. In regions with large livestock agriculture industries, management of organic fertilisers is of importance as excess phosphorus can wash into waterways. In Northern Ireland, the livestock industry produces digestate and slurry with P content exceeding agronomic requirements by 20% . The aim of this research was to determine: the suitability of digestate solids as a fuel, and how mechanical separation can impact nutrient management during the pellet production process. The economic feasibility of a pellet production system and safety concerns regarding a product of animal waste origin were also considered. The digestate studied in this thesis came from a small scale (230 kW_e) farm anaerobic digester processing 89% dairy cattle slurry with 11% grass silage by mass. Mechanical separation of the anaerobic digestate was carried out with two commonly used separators: decanter centrifuge, and screw press. Operational parameters of the separators were varied to determine the most efficient method of operation for each and achieve the best separation efficiency. Separation by decanter centrifuge was shown to partition up to 71.5% phosphorus in the solid fraction, leaving the liquid fraction to be spread as fertiliser with reduced phosphorus content. The screw press showed reduced phosphorus partitioning ability (up to 10.9% phosphorus in the solid fraction) in comparison but was also at a lower cost. The solid fraction produced by the screw press had a lower nitrogen content (2.6 g/kg) compared to the solids produced by decanter centrifuge (4.2 g/kg), leaving a liquid fraction with higher nitrogen concentration (1.2 g/kg vs 0.8 g/kg) and higher value as an organic fertiliser. The digestate pellets produced from screw press separated solids had an energy content of 15.8 MJ/kg, compared to 16.7 MJ/kg for commercial wood pellets. When combusted in a small-scale multi-fuel boiler (50 kW_th) the NOx and particulate matter emissions exceeded the UK Renewable Heat Incentive domestic emissions limits by 381% and 344% respectively; the emissions exceeded the stricter EU domestic limits by 589% and 542% respectively. The high emissions observed rule out the use of anaerobic digestate pellets as domestic fuel, unless blended, but they could have value as a co-firing fuel in power stations where coal or peat may be displaced, reducing reliance on fossil fuel for heat and electricity provision. The economic feasibility of pellet production at farm-scale anaerobic digesters, of a size commonly found in the UK, was determined. It was found that typical 500 kW_e rated anaerobic digesters would not produce enough digestate to make the system economically viable, however, 1 MW_e anaerobic digesters would be able to produce pellets at a cost of £96/t of pellets (decanter centrifuge) or £113/t of pellets (screw press). A cooperative enterprise, where three small anaerobic digester operators (~500 kW_e) each pool together digestate, was evaluated and showed that a profit of up to £160,000-198,000 per year could be achieved overall. The presence of Enterobacteriaceae and Salmonellae bacteria was determined through each stage of the pellet production system, from slurry to finished pellet, to assess whether additional safety measures would need to be taken by pellet producers and consumers. The research found that Salmonellae was reduced to below detectable levels in the anaerobic digestion process, while Enterobacteriaceae numbers were reduced at each stage of the process and were below detectable levels in the finished pellet. Novel aspects of the research in this thesis include: analysis of demonstration scale digestate separation; an economic analysis considering a cooperative enterprise of anaerobic digester operators; analysis of combustion emissions with comparison to legislative limits; and investigation into the survival of potentially pathogenic bacteria during the production and storage of anaerobic digestate fuel pellets. In summary, separation of digestate and production of pellets from the resulting solid fraction can reduce the impact of nutrient loading on soils, but the emissions produced during their combustion rule out their use as an alternative to wood pellets in domestic solid fuel boilers. The pellets may have value as a co-fuel in fossil fuel power stations in particular, to reduce reliance on coal/peat.

Thesis is embargoed until 31 December 2024.

Date of Award	Dec 2023
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	The Bryden Centre
Supervisor	Beatrice Smyth (Supervisor), Chris Johnston (Supervisor), Gary Lyons (Supervisor), Simon Murray (Supervisor), Christina Forbes (Supervisor) & David Rooney (Supervisor)