Plasma gasification of water hyacinth

Abstract

This study evaluates the feasibility of water hyacinth as a carbon fuel source for energy production, as a waste valorization method. The H-PlasWEn system at NECSA was evaluated as a possible plasma waste-to-energy (PlasWEn) process for hyacinth (H). Hyacinths collected from the Hartbeespoort dam were prepared and characterized through chemical and elemental analysis, calorific value (CV) tests and devolatilization. The WHW sample can be chemically described as 45.8% C, 5.9% H, 2.7% N, 45.4% O and 0.2% S on moisture and ash free basis, with a HCV of 14.3 MJ/kg. Various plasma gasification experiments were performed where the O:C molar ratio, specific power input (kWh/kg) and reactor temperature (°C) were varied. The produced gas is referred to syngas in this study. The averaged results for the syngas, from Run A with 0.9 O:C, is 12.6% H2, 12.5% CO, 0.3% CO2, 0.05% CH4, 0.09% C2H2, 0.02% O2 and remaining a 74.4% N2, and for Run B is with 1.2 O:C, 10.3% H2, 6.4% CO, 0.02% CO2, 0.03% CH4, 0.03% C2H2, 0.06% O2 and 80.7% N2. The carbon conversion efficiency (CCE) for Run A and Run B were determined to be 95.6% and 63.2% respectively. The experimental results, simulated using FactSage, and the plasma generated gas composition approached chemical equilibrium, specifically at high temperatures. The results deviated more from equilibrium as the O:C ratio increased, and also as the specific power input was decreased. Finally, a techno-economic evaluation (TEE) was conducted on the HPlasWEn system based on the syngas composition of Run A, where a syngas LCV value, of 14.7 MJ/kg was derived using the experimental gas composition of H2, CO, CO2, CH4, C2H2 and O2 on kg/kg feed basis. The PlasWEn techno-economic analysis considered two scenarios: one involving the conversion of both syngas and heat energy into electricity, sold as a single product (Scn1), and another where only the syngas was converted into electricity and sold independently from the heat energy, resulting in two distinct products (Scn2). A PlasWEn plant size of 20 tpd was chosen as it is sized according to the largest plasma gasification reactor built in South Africa. The total capital investment of MR 66 is required of which the total fixed capital investment is MR 54.9 and operational costs are MR 3.4/year (75% attributed to labour). The plant requires 5 551.3 kWh/d and is self sufficient once operational, and generates 23 371 kWh/d electricity for sale. A 20 tpd system show an 20.8% IRR,12.7% RIRR, 43.6 MR NPV, 5.7 years SPP and 9.3 years DPP and determined economically viable and a promising waste-to-energy project.