Solving the problem of locally available wet residue streams by recovering the energy content and extracting the valuable elements from the inorganic part using a small scale biorefinery concept based on the SCWG process

Background During the SUPERVALUE project we worked to answer the need for knowledge about the possibilities to use a small-scale biorefinery concept to solve the problem of locally produced wet organic residues. Drying of these waste streams is economically not feasible and energetically irresponsible, while the most common and established conversion technology (anaerobic digestion, AD) has non-negligible operational issues and also produces a waste stream itself: digestate. Moreover, AD does not reduce certain bio-active organic compounds, e.g. antibiotic residues. As an alternative a biorefinery concept based on supercritical water gasification (SCWG) was proposed and investigated, focusing on the achievable process efficiency, possibility of the recovery of resources and the economic aspects (required investment and the payback time). Goals and approach The project had the following main goals: • investigate the boundaries of SCWG process optimization by means of mathematical modelling and simulations, and propose an optimum process layout of an SCWG plant; • investigate the recovery of Phosphorus and other elements from SCWG residues, i.e. char and the effluent. SCWG experiments have been performed on a laboratory scale with real substrates (SCWG with a feed rate 1 ml min-1), followed by the recovery experiments; • prepare a spreadsheet-based decision support tool to evaluate different gas valorisation pathways; • perform four case studies at companies producing different waste streams: fruit&vegetable residues (FVR), cow manure, sewage sludge and cheese whey. A case study on FVR was performed at our industrial project partner. The goal was to calculate the payback time of an investment in SCWG-based conversion system for wet organic residues and to identify a gas valorisation pathway optimally matching the company’s energy consumption pattern. Results & recommendations Simulations showed, that compared to AD the fraction of methane in SCWG product gas is lower, below 50% by volume. Process temperature of 650°C instead of 500°C favours the carbon conversion efficiency, but the amount of gas produced is lower. The maximum process efficiency in terms of chemical energy transfer calculated for FVR with 20% dry mass content was 57%. Simulation results were in reasonable agreement with the experimental results, although more and longer tests at larger scale are needed to check the repeatability. The Phosphorus recovery experiments using acid showed efficiencies up to 90% P-recovery, however long (24h) residence times are needed to achieve this. Less successful were the experiments where added value products (fertilizers) like hydroxyapatite (HAp) were to be synthesised. HAp was not identified in the synthesis products. However, as simulations predict its existence, tuning the process should make synthesis possible. Case studies showed an economic viability of SCWG-based systems with prognosed payback times of less than 10 years for the FVR and sewage sludge case, and 10 to 17 years for the manure and whey case, depending on the selected valorisation pathway. The highest uncertainty in this analysis is however the investment cost of SCWG and the reliability of the installation. Nonetheless, with biochar instead of digestate at the process outlet, and the destruction of bacteria and antibiotics during the SCWG process, this method


Dr. Marcin Siedlicki

Research and Innovation Centre Pro-Akademia, Poland


Project partners: 

Technische Universiteit Delft, Netherlands

Fresh World Int. Sp. z o.o., Poland

Material from EUBCE 2019

Poster from EUBCE 2019