Direct and Standard Injection of Sewage Sludge in a Supercritical Water Gasification System - Optimization of the Energy Efficiency Using Pinch Analysis

Supercritical Water Gasification is a promising technology to convert wet biomass into syngas. One of the challenges of this technology is the coke formation during heating-up of the wet biomass to its supercritical condition. Literature studies show that coke is the result of polymerization reactions that takes place at sub-critical conditions. It also states that a fast heat up of the wet biomass can minimize this unwanted coke formation. An option to achieve a fast heating rate is by direct injection of the relatively unheated wet biomass feed into supercritical water. Incomplete heating up of the wet biomass increases the waste heat in the product gas which will significantly reduce the energy efficiency. Parametric studies of some operating parameters are done to investigate the influence of this direct injection on the process performance. Results from this investigation show that a SCWG process with direct injection will have an efficiency reduction of approximately 7~16% in comparison to the standard (premixed) injection system. Using a feed with 8 wt% and 20 wt% dry matter at 600 °C and 25MPa, the thermal efficiencies of the system are 13.8 % and 65.5 respectively. Preliminary pinch analysis shows that these numbers can be improved to around 20 % and 70 %.