Simulation of biomass gasification for hydrogen production

Abstract

This thesis proposed and simulated a process to produce hydrogen from municipal solid waste (MSW) gasification, by combining methane reforming, two-stage Water Gas Shift conversion and pressure swing adsorption purification by simulation software Aspen Plus. The Peng-Robinson equation of state with nonconventional MSW property models (HCOALGEN and DCOALIGT) was used in the simulation, which proceeded through sequential drying (RStoic), decomposition (RYield), gasification (RGibbs), reforming, WGS (REquil), and PSA stages. Different gasification parameters like air to MSW ratio (AMR), steam to MSW ratio (SMR) and gasification temperature were investigated through a sensitivity analysis at a fixed MSW feeding rate of 1000 kg/h to evaluate the effects on syngas composition, hydrogen production efficiency and CO₂ emissions.

The base case (750°C, SMR = 0.5, ER = 0.2) produced 101.81 kg/h of hydrogen at 66.94% efficiency, with syngas containing 23.66 mol% H₂ and 22.86 mol% N₂. The gasification temperature showed positive effect on hydrogen production up above 1000°C before it began to level-off, and SMR had a positive effect on the yield up to 0.7, while ER had a negative effect due to oxidation losses and dilution by nitrogen. With optimization conditions (1000°C, SMR = 0.7, ER = 0.2), 119.16 kg/h of hydrogen was achieved at 78.35% efficiency, which is a 17% gain, WGS CO conversion was increased from 96.7 to 97.5%. CO₂ emissions increased by only 0.27% (from 1351.6 to 1355.3 kg/h), indicating improved carbon utilization efficiency with negligible environmental penalty. The study also led to the conclusion that a realistic prediction of methane and the need for using restricted equilibrium approaches in the RGibbs reactor were required for the air-blown gasification systems.