Abstract
The goal of this work is to understand the gasification process for Miscanthus briquettes in a double-stage downdraft gasifier, and the impact of different Equivalence Ratios (ER) on syngas, biochar, and tar characteristics. The optimal ER was found to be 0.35, which yielded a syngas maximum heating value of 5.5 MJ/Nm3 with a syngas composition of 20.29% CO, 18.68% H2, and 0.86% CH4. To better understand the observed behavior, an equilibrium reaction model was created and validated using the experimental data. The model showed that the heating value decreased with increasing ER, and that hydrogen production peaked at ER = 0.37, while methane (CH4) became negligible above ER = 0.42. Tar and particle content in the gas produced at a certain temperature can now be predicted. To assess the biochar characteristics, surface structure image analysis and a surface area porosity analysis were carried out. Employing images from a scanning electron microscope (SEM), the biochar cell bonds and pore structures were examined and analyzed. By using the Brunauer-Emmett-Teller (BET) analysis of the surface porosity, the surface area to be 186.06 m2/g and the micro pore volume was calculated to be 0.07 m3/g. The final aspect of the analysis involved an evaluation of tar production. Combining current and prior data showed a logarithmic relationship between the amount of tar produced and the gasifier bed temperature, where the amount of tar produced decreased with increasing bed temperature. This results in very low tar levels, which is one of the known advantages for a double-stage downdraft gasifier over a single-stage system.
Highlights
Today, 85% of the world’s energy demand is reached using conventional fossil fuel, which releases56.6% of anthropogenic greenhouse gas emissions [1]
The results suggest that a reduction of the CH4 concentration in a double stage downdraft gasifier is associated with the decreases of the tar content in the produced gas
The model was developed based on stoichiometric equilibrium, known as the Gibbs free energy minimization method
Summary
85% of the world’s energy demand is reached using conventional fossil fuel, which releases56.6% of anthropogenic greenhouse gas emissions [1]. The development of renewable energy has been the focus of attention for several decades, because of its promise of energy independence and sustainability. A forecast in global demand predicts a 40% increase in energy demand around the world by 2030, as average standards of living rise. This is especially true for areas such as Sub-Saharan Africa, Energies 2018, 11, 3225; doi:10.3390/en11113225 www.mdpi.com/journal/energies. Energies 2018, 11, 3225 where currently only 290 million of 915 million inhabitants have access to electricity [2]. It is clear that different sustainable technologies have to be developed to achieve clean energy production. Biomass gasification is one of the possible routes through which carbon-neutral energy can be produced
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