(6c) Investigation of Sludge Conversion during Gasification Process Using Electrical Capacitance Tomography   

Waste management and energy consumption are two major challenges throughout the world nowadays. Sewage sludge, for example, is generated from wastewater treatment plants and usually contains a large portion of organic matters that could be further recovered to produce energy. Recently, converting solid waste to energy through gasification technology is attracting more and more focus, as through gasification, toxic gases such as dioxin could be prevented comparing with traditional incineration process ^[1]. During gasification process, carbon content in the solid waste decreases gradually as it is decomposed and gasified to CO, CO₂, CH₄and other gases with smaller quantities. However, gasification is a highly complex process that is difficult to model without very simplifying assumptions, and experiments are limited by the partial information that can be measured. Therefore, the process efficiency can be improved if more theoretical information is recognized. In order to achieve such improvement, the complex fluid dynamics, including reaction rates and reaction zones should be identified. One step toward this target is detecting the profiles of compositions during gasification.

The main goal of this work is therefore to investigate a new method to monitor online the solid waste decomposition along the gasifier, and consequently understand what are the solid and gas compositions at these positions. This identification can also help to recognize what is the local mass transfer rates inside the gasifier from an experimental point of view, which was not achieved before. The potential use of such measurement is not only to measure the local composites, but to be able to produce data sets for quality validations of numerical models that are typically validated by temperature profiles and output composition, rather than full composition analysis throughout the system.

The methodology of the current study is to use Electrical Capacitance Tomography (ECT) technique to correlate the solid waste conversion with solid permittivity. ECT is a well-known method that has been widely used in pneumatic transport to monitor solid volume fraction cross sectional distribution in enclosed pipes or vessels by measuring the variations in the dielectric properties of the materials ^[2]. Because the particles composition is expected to change through the gasifier, dielectric properties of the material is changing as well, and the measured permittivity should also change accordingly. By correlating this property with carbon conversion, gasification efficiency and syngas production can be monitored easily and fast during industrial operations and controlled experiments with a non-intrusive technique.

[1] Ong Z, Cheng Y, Maneerung T, et al. Co‐gasification of woody biomass and sewage sludge in a fixed‐bed downdraft gasifier[J]. AIChE Journal, 2015, 61(8): 2508-2521.

[2] Zhang W, Cheng Y, Wang C, et al. Investigation on hydrodynamics of triple-bed combined circulating fluidized bed using electrostatic sensor and electrical capacitance tomography[J]. Industrial & Engineering Chemistry Research, 2013, 52(32): 11198-11207.