Assessing the suitability of various feedstocks for biomass gasification

Abstract

Ten different types of feedstocks available in Louisiana were assessed for their suitability to produce SYNGAS in a down-draft biomass gasifier. The feedstocks tested for this research include: pine pellets, hardwood pellets, cypress mulch, pine bark nuggets, corn stover pellets, switchgrass pellets, sugarcane bagasse pellets, dairy manure pellets, and poultry litter pellets. The feedstocks were first analyzed for volatile & ash content, high heating value, moisture, and mass density. Feedstocks that met the analytical criteria and available in a form that is suitable for the down-draft gasifier at LSU were tested for gasification. The temperature profile within the gasifier and exiting oxygen concentrations were monitored for each of the tested feedstocks. Results indicated that pine and hardwood pellets had moisture contents of 6.04 ± 0.5% and 5.39 ± 0.22%, respectively, which was considered optimum for gasification. However, corn pellets had higher moisture content (13.3 ± 0.44%) and had to be dried to 7 ± 1% moisture for successful gasification. Results also indicated that low ash and high volatile solids contents were critical for gasification. Pine pellets and hardwood pellets had the least ash (0.37 ± 0.1% and 0.85 ± 0.2%) and highest volatile solids (99.62 ± 0.1% and 99.14 ± 0.5%), therefore, performed best during gasification runs. Poultry litter and dairy manure pellets had more than 39% ± 0.8% ash and less than 62 ± 0.8% volatile solids, which made them unsuitable for gasification. Four feedstocks (Alfalfa, switchgrass, bagasse, and corn) had moderate levels (12.16% - 3.28 %) of ash contents, with alfalfa having the highest ash content. Out of these feedstocks with moderate ash contents, alfalfa pellets failed to gasify consistently. Cypress mulch and pine bark nuggets, although had the necessary properties for gasification (low ash, high volatile solids, acceptable high heating values, and low moisture), the mass density was too low and required continuous feeding. Although these feedstocks gasified, the frequent valve openings and closings and constantly varying volumes of biomass inside the gasifier caused major temperature fluctuations. The actual suitability of these feedstocks can be tested either after densification (pelletization) or by incorporating an automated feeding system for the gasifier. The dairy manure pellets, switchgrass pellets, bagasse, and chicken litter pellets could not be tested in the gasifier due to unavailability in these pellets in the market in bulk volumes. The in-house hammermill and pelletmill were not found to be undersized for large scale production of pellets.