This study sought to better understand the torrefaction process, and more specifically, how torrefaction affects the physical and chemical properties of corn stover biomass. The work done to accomplish this was divided into three sections that map to three research objectives. First, effect of torrefaction residence time, temperature and untreated biomass moisture content on chemical properties of torrefied corn stover was addressed. Second, effect of torrefaction process condition on physical characteristics of torrefied biomass, namely hydrophobicity was assessed. In addition, resistance to microbial degradation as a result of torrefaction and increased hydrophobicity was investigated. Third, influence of gas residence time and biomass particle size on chemical characteristics of torrefied corn stover was studied.

Corn stover biomass at three moisture contents (30, 45, and 50% wet basis) was torrefied at three different temperatures (200, 250, and 300 °C), and at three reaction times (10, 20, and 30 min). In each of the 17 treatments elemental and proximate compositions of the torrefied stover was determined, along with the composition of released gaseous and liquid products. Using these data, the mass and energy balance of each torrefaction was quantified. The energy balance accounted only for energy contained in the biomass. As torrefaction process temperature increased, an overall increase (2-19%) in the energy density of torrefied biomass and decrease (3-45% and 1-35% respectively) in mass and energy yield was observed. At 200 ºC, mass and energy losses increased with an increase in the initial biomass moisture content. The difference in both mass and energy losses between biomass of 22% and 41% initial moisture content was about 10 percentage points at 200 ºC. The liquid phase condensed from the stream of volatiles was composed primarily of water, followed by acetic acid, methanol, hydroxyacetone, and furfural. The yield of condensables increased with torrefaction temperature. Permanent gas released in the process was mainly composed of carbon dioxide and carbon monoxide, with traces of hydrogen and methane present only at 300 ºC.

The equilibrium moisture content (EMC) of raw corn stover, along with corn stover thermally pretreated at three temperatures, was measured using the static gravimetric method at equilibrium relative humidity (ERH) and temperature ranging from 10 to 98% and from 10 to 40 °C, respectively. Five isotherms were fitted to the experimental data to obtain the prediction equation which best describes the relationship between the ERH and the EMC of lignocellulosic biomass. Microbial degradation of the samples was tested at 97% ERH and 30 °C for period of 30 days. Fiber analyses were conducted on all samples. In general, torrefied biomass showed an EMC lower than that of raw biomass, which implied an increase in hydrophobicity. The modified Oswin model performed best in describing the correlation between ERH and EMC. Corn stover torrefied at 250 and 300 °C had negligible dry matter mass loss due to microbial degradation. Fiber analysis showed a significant decrease in hemicellulose content with the increase in pretreatment temperature, which might be the reason for the hydrophobic nature of torrefied biomass. This is probably due to loss of polar hydroxyl groups that serves as binding sites for water molecules.

The effects of particle size and gas residence time on the torrefaction of corn stover were investigated via torrefaction of different stover fractions: stalk shell, pith, and corn cob shell, and particle sizes, in a form of whole corn stalk and ground corn stover. Three levels of the purge gas residence times (1.2, 12 and 60 sec) were employed to assess the effects of volatiles and torrefied biomass interaction. Elemental analyses of all the samples were done, and the obtained data was used to estimate the energy contents and energy yields of different torrefied biomass samples. Particle densities, elemental composition, and fiber composition of raw biomass fractions were also determined. The dry matter losses, higher heating values, and energy yields for different torrefied corn stover fractions were significantly different. This was probably due to the differences in particle densities, hemicellulose quantities, and the chemical and physical properties of the original biomass samples. Gas residence time did not have a significant effect on the aforementioned parameters.