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

Abstract

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 kW e ) 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. • Energy balance, phosphorous flows, and cost analysed for digestate fuel pellets. • Fuel pellet production redirects phosphorous from land and runoff to waterways. • Pellet breakeven price of £95 ​t −1 to £121 ​t −1 compared to £186 ​t −1 market price. • Profits of up to £200,000 yr −1 achievable for 3 plant collaboration. • Process can improve profitability of dairy cows by 9–20%.