Experimental Analysis and Modeling of Biomass Gasification using a Downdraft Gasifier

Abstract

The call for the use of renewable energy is an urgent one, for not the future but the present. Concerns over depletion of fossil fuels and their negative impact on the global climate have led to much research in this sector. Gasification of biomass for energy production is not only environmentally sustainable but also economically beneficial as it reduces the dependence on imported fuel. Through this thermo-chemical process, biomass is converted into a mixture of combustible gases known as syngas (a.k.a. producer gas) which can be directly used for heat or power, and also synthesized into liquid fuels. The major objective of this study was to understand the effect of different biomass feedstock on the quality of syngas produced through gasification in a downdraft gasifier. Hardwood, loblolly pine, switchgrass, yellow pine and torrefied pine pellets, pine chips and bark were used as the feedstocks for experiments. Compositions of major gases (H2, CO, CO2 and CH4) and contaminants (tar and H2S) were determined, and mass, energy and exergy analyses were performed to substantiate the experimental results. Switchgrass had the highest ash content of 4.66% (d.b.) followed by pine bark (1.59% d.b.) while other feedstocks had lower ash content. Carbon content ranged from 47 to 56% (d.b.), hydrogen from 6.50 to 7.50% (d.b.) and sulfur from 0.32 to 0.40% (d.b.) for all the feedstocks. The higher heating values (HHV) of biomass types ranged from 19 MJ/kg (switchgrass) to 23 MJ/kg (torrefied pine). Syngas obtained from yellow pine showed the highest hydrogen (17.35%) and carbon monoxide (25.05%) fraction and the highest HHV (6 MJ/Nm3), while switchgrass and loblolly pine had significantly lower concentration of H2 and CO as well as lower HHV. Loblolly pine showed the highest total tar concentration (2.54g/m3) along with higher concentration of condensable tar compound, indene (>0.1 g/m3). The hydrogen sulfide concentration was found to be above 70 ppmv for all the feedstocks, which is higher than the tolerable limit for many syngas applications. Furthermore, biomass feedstocks that have higher ash content were blended with other lower ash content biomass (switchgrass w/ yellow pine and pine bark w/ pine chips) and similar gasification experiments were carried out. Owing to reduced ash content, gasification of switchgrass/yellow pine blends did not show any ash agglomeration. The hydrogen and carbon monoxide concentrations in syngas obtained from yellow pine/switchgrass blends were higher compared to switchgrass alone, while those from pine chips/bark blends were not significantly different from individual runs. The 75:25 and 50:50 blends of yellow pine and switchgrass showed a total tar concentration of 1.97 g/m3 and 1.86 g/m3, respectively, while the concentrations for the 75:25 and 50:50 blends of pine chips and bark were 1.66 g/m3 and 1.57 g/m3, respectively. The hydrogen sulfide concentration was found to be above 65 ppmv for both the yellow pine/switchgrass and pine chips/bark blends. In addition, commercial software, Comprehensive Simulator of Fluidized and Moving Bed Equipment (CeSFaMB), was applied to simulate the gasification process. It was able to reproduce the syngas composition within 5-10% deviation for all the major gases except methane. Moreover, a parametric study was conducted to evaluate the effect of factors such as mass flow rate and moisture content of feeding fuel, injected gas/air flow rate, elemental composition and proximate analyses of biomass feedstocks on the program.