Investigation of the physical (rheological) properties of sewage sludge and digestate in the context of their processing in supercritical water gasification

Abstract

Supercritical water gasification (SCWG) is a thermochemical conversion process in which wet biomass is converted into gaseous products as methane, carbon dioxide and hydrogen. For SCWG both the temperature and pressure of the medium are increased beyond the critical point of water (373.95⁰C and 220.64 bar). The supercritical water acts as an active reactant and increases the conversion efficiency. Moreover, energy-intensive drying of the feedstock can be omitted since wet biomass can be directly introduced to the process.
An important issue related with the wet waste feed is upstream pumpability due to clogging in the pipelines. To understand this behaviour a detailed analysis of the physical parameters responsible for the pumpability of feedstock is imperative. These parameters involve particle size distribution (PSD), dry matter (DM), viscosity and homogeneity. The aim of this research was to identify the physical (rheological) properties of two feedstocks, secondary and digested sludge, and to propose a pre-treatment which could improve system performance.
To identify the physical (rheological) properties experiments have been performed in the domain of rheological methods. The PSD has been determined via laser diffraction with the Microtrac S3500. The rheology has been analysed with the Anton Paar rotational rheometer. The effects of particle size, DM content, temperature and shear rate on the shear stress and viscosity have been investigated. The acquired data provided essential information on suitable pre-treatment methods for feedstock preparation.
The results revealed that the PSD of secondary and digested sludge was below the maximum tolerated particle size for most pumping systems. The rheology data indicated shear thinning behaviour of the sludge, since viscosity decreased upon increased shear rate. Higher temperatures were also found to decrease viscosity and shear stress of the sludge.
For commercial application, the low DM concentrations of secondary sludge (4-6 wt%) and digested sludge (3-4 wt%) need to be further increased. Addition of polymers is discouraged, since it limits pumpability. Secondary sludge (< 5wt% DM) showed a logarithmic correlation between viscosity and DM. Consequently, when DM content is further increased the viscosity will become the limiting factor for pumpability.
The issue of pumpability limited by the sludge viscosity could be overcome by performing a pre-treatment with a continuously stirred tank at elevated temperatures. The viscosity is decreased when the fluid is constantly in motion and temperature will further lower the viscosity.