Thermodynamical Material Networks for Modeling, Planning and Control of Circular Material Flows

Waste production, carbon dioxide atmospheric accumulation and dependence on finite natural resources are expressions of the unsustainability of the current industrial networks that supply fuels, energy and manufacturing products. In particular, circular manufacturing supply chains and carbon control networks are urgently needed. To model and design these and, in general, any material networks, we propose to generalize the approach used for traditional networks such as water and thermal power systems using compartmental dynamical systems thermodynamics, graph theory and the force-voltage analogy. The generalized modeling methodology is explained, then challenges and future research directions are discussed. We hope this paper inspires to use dynamical systems and control, which are typically techniques used for industrial automation, for closing material flows, which is an issue of primary concern in industrial ecology and circular economy.Comment: Perspective paper in preparatio

Beyond carbon and energy: the challenge in setting guidelines for life cycle assessment of biofuel systems

Life cycle assessment (LCA) is one of the most suitable tool for a uniform assessment methodology of biofuels’ sustainability. However, there are no binding guidelines for LCA of biofuel systems. Published LCAs use a range of methodologies, different system boundaries, impact categories and functional units, various allocation approaches, and assumptions regarding by- and co-products, as well as different reference systems to which the biofuel system is compared. The European Renewable Energy Directive and the US Renewable Fuel Standard focus on greenhouse gas (GHG) emissions. However, previous LCAs of biofuel systems have shown that a reduction of GHG emissions does not lead automatically to a decrease in other environmental impacts, and might in fact be associated with an increase in impacts such as acidification, eutrophication, and land use change. In order to enable effective comparison of biofuel systems, the authors propose a framework for biofuel LCA. System boundaries should be expanded to include the life cycle of by- and co-products. Results should be reported using more than one functional unit. Burden shifting can be avoided by considering an array of impact categories including global warming potential and energy balance, along with eutrophication and acidification potential, and a land use indicator

Life cycle assessment of a short-rotation coppice willow riparian buffer strip for farm nutrient mitigation and renewable energy production

Publication history: Accepted - 13 January 2022; Published online - 2 February 2022.As agricultural activity intensifies across Europe there is growing concern over water quality. Agricultural run-off is a leading cause of freshwater degradation. Simultaneously there is a continually increasing drive to promote renewable energy and reduce greenhouse gas emissions. Willow coppice planted as a riparian buffer has been suggested as a solution to help mitigate these problems. However, there is limited research into the use of such a system and several key knowledge gaps remain, such as, the energy ratio of the system is not known, and a fully harvested site has yet to be analysed in the literature. The aim of this research is to fill these knowledge gaps to help inform agri-environmental policy. To do this a life cycle assessment was carried out on an established willow buffer system, considering the global warming potential, eutrophication potential, acidification potential and cumulative energy demand impact categories, alongside the calculation of the energy ratio. To our knowledge it is the first site to be fully harvested and for which a full life cycle assessment has been carried out. The willow was combusted to fuel a district heating system. Key results showed emissions of 4.66 kg CO2eq GJheatout -1 and 0.01 kg SO2eq GJheatout -1, both of which are significant reductions compared to an oil heating system (95% reductions for both impact categories). The system also resulted in the permanent nutrient removal of 55.36 kg PO43-eq ha-1 yr-1 and had an energy ratio of 17.4, which could rise to 64 depending on the harvest method.This Bryden Centre project is supported by the European Union’s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB). The work was also supported by Queen’s University Belfast and the Agri-Food and Biosciences Institute in Northern Ireland

Optimising mechanical separation of anaerobic digestate for total solids and nutrient removal

Publication history: Accepted - 16 June 2023; Published - 28 June 2023.Mechanical separation of anaerobic digestate has been identified as a method to reduce pollution risk to waterways by partitioning phosphorus in the solid fraction and reducing its application to land. Separators have adjustable parameters which affect separation efficiency, and hence the degree of phosphorous partitioning, but information on how these parameters affect separation performance is limited in the literature. Two well known technologies were investigated, decanter centrifuge and screw press, to determine the most efficient method of separation. Counterweight load and the use of an oscillator were adjusted for the screw press, while bowl speed, auger differential speed, feed rate and polymer addition were modified for the decanter centrifuge. Separation efficiency was determined for total solids, phosphorus, nitrogen, potassium, and carbon, and the total solids content of resulting fractions was measured. The decanter centrifuge had higher separation efficiency for phosphorus in all cases, ranging from 51% to 71.5%, while the screw press had a phosphorus separation efficiency ranging from 8.5% to 10.9% for digestate of ~5% solids (slurry/grass silage mix). Separation by decanter centrifuge partitioned up to 56% of nitrogen in the solid fraction leaving a reduced nitrogen content in the liquid fraction available for land spreading; this nitrogen would most likely need to be replaced by chemical fertiliser which would add to the cost of the system. The decanter centrifuge is better suited to cases where phosphorus recovery is the most important factor, while the screw press could be advantageous in cases where cost is a limiting factor.This project was supported by The Bryden Centre. The Bryden Centre project is supported by the European Union’s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB). The work was also supported by Queen’s University Belfast and the Agri-Food and Biosciences Institute in Northern Ireland

An economic analysis of anaerobic digestate fuel pellet production: can digestate fuel pellets add value to existing operations?

Publication history: Accepted - 9 April 2021; Published online - 16 April 2021.Anaerobic digestion provides renewable energy through waste valorisation, but the digestate by-product is underutilised and presents a risk to water quality. Mechanical separation partitions phosphorous into the solid fraction and further processing into a fuel pellet can provide an additional source of energy and revenue. Previous economic analyses looked only at aspects of the system (e.g. operational costs solely) and the system requires further investigation to determine viability. In this paper, an economic assessment of digestate fuel pellet production at farm-scale anaerobic digestion plants was carried out. The significance of this work is to provide a comprehensive assessment of the energy, phosphorous, and economic balances involved in digestate fuel pellet production at existing anaerobic digestion plants. The aim of this paper is to determine the financial viability of digestate fuel pellet production with objectives to compare two mechanical separation technologies: screw press, and decanting centrifuge. Economies of scale hold true for digestate pellet production and the available digestate in typical UK farm-based anaerobic digestion plants ( 500 kWe) is insufficient for profitability, with pellet production costing from £176/t (decanting centrifuge) to £215/t (screw press), compared to a typical wood pellet sale price of £185/t. Increasing digestate quantity by collaboration of plant operators can reduce the cost of pellet production to between £95/t and £121/t, improving financial viability and increasing the profit per head of cattle by 9–20% on a typical dairy farm utilising anaerobic digestion. The system has potential to aid rural development while also protecting the environment and contributing to the diversification of energy supply.This project was supported by The Bryden Centre. The Bryden Centre project is supported by the European Union’s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB). The work was also supported by Queen’s University Belfast and the Agri-Food and Biosciences Institute in Northern Ireland

Effect of anaerobic digestate fuel pellet production on Enterobacteriaceae and Salmonella persistence

Publication history: Accepted - 10 June 2022; Published online - 7 July 2022.Production of digestate pellets for fuel has been identified as a promising circular economy approach to provide renewable energy and additional income to farms, while at the same time presenting the potential to divert raw digestate from nutrient-saturated land and reduce the risk to water quality. Although previous research has investigated the feasibility of pellet production, there has been little focus on the bio-safety aspects of the system. Little is currently known about the persistence of bacteria present in the digestate and the potential impacts on human health for those handling this product. The aim of the present research was to determine the effect that each step in the pellet production process has on bacteria numbers: anaerobic digestion, mechanical separation, solid drying, and pelletisation. Enterobacteriaceae enumeration by colony count method was used to quantify bacteria, and the presence of Salmonella at each stage was determined. The Enterobacteriaceae count reduced with each stage, and the final pelletisation step reduced bacteria numbers to below detectable levels (<10 colony forming units/g). Salmonella was only detected in the starting slurry and absent from digestate onwards. Storage of the pellets under winter and simulated summer conditions showed no reactivation of Enterobacteriaceae over time. The pelletisation process produces a digestate product with Enterobacteriaceae counts below the maximum threshold (PAS110 specification) for transport off the source farm, but care must still be taken when handling digestate pellets as complete sterilisation has not been confirmed.This project was supported by The Bryden Centre. The Bryden Centre project is supported by the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). The work was also supported by Queen's University Belfast and the Agri-Food and Biosciences Institute in Northern Ireland

Production pathways for profitability and valuing ecosystem services for willow coppice in intensive agricultural applications

Publication history: Accepted - 14 January 2023; Published online - 20 January 2023Increasing agricultural sustainability is a key challenge facing the globe today. Energy crops, planted as riparian buffers are one way to support this, simultaneously mitigating water quality degradation and climate change. However, the economics of implementing such riparian buffer systems is under researched. Hence this work conducted a bottom-up economic analysis of willow coppice riparian buffers on a Northern Irish dairy farm, which is indicative of agricultural intensification across Europe. This work includes an economic assessment of a willow coppice riparian buffer strip, using harvested yield data from an established willow buffer site for the first time. It also considered the impact of harvesting technology on the economic performance of a willow coppice riparian buffer strip for the first time. The analysis considered three willow production pathways: 1) direct chip harvesting, 2) full-stem harvesting, and 3) a scenario with a guaranteed purchasing contract for fresh chip. Economic performance was considered using net present value over a 25-year plantation lifetime. The full-stem scenario provided the highest economic return over its lifetime with an average yearly net present value of £497 ha−1 (in £ sterling). This system was then considered for integration into a typical dairy farm, assuming 5 % land usage and including government grants for establishing riparian zones. The result was a drop in value of £28 ha−1 yr−1 compared to a dairy-only scenario; however, per litre of milk the farm employing willow coppice riparian buffer strips outperformed a typical dairy farm both environmentally and economically. Further analysis considered a novel approach that included payments for ecosystem services in the economic analysis. This analysis found that the implementation of government payments for ecosystem services (nutrient removal) increased the economic return of the willow coppice riparian buffer system by £400 ha−1 yr−1, resulting in minimal impact on the return from dairy land.This Bryden Centre project is supported by the European Union's INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). The views and opinions expressed in this paper do not necessarily reflect those of the European Commission or the Special EU Programmes Body (SEUPB). The work was also supported by Queen's University Belfast and the Agri-Food and Biosciences Institute in Northern Ireland. The authors would like to thank David Gilliland of Organic Resources and Alan Hegan of Hegan Biomass for the expert insight readily provided for this research