Continuous and reliable feeding of biomass is essential for successful biofuel production. However, the challenges associated with biomass solids handling are commonly overlooked. In this study, we examine the effects of preprocessing (particle size reduction, moisture content, chemical additives, etc.) on the flow properties of corn stover. Compressibility, flow properties (interparticle friction, cohesion, unconfined yield stress, etc.), and wall friction were examined for five corn stover samples: ground, milled (dry and wet), acid impregnated, and deacetylated. The ground corn stover was found to be the least compressible and most flowable material. The water and acid impregnated stovers had similar compressibilities. Yet, the wet corn stover was less flowable than the acid impregnated sample, which displayed a flow index equivalent to the dry, milled corn stover. The deacetylated stover, on the other hand, was the most compressible and least flowable examined material. However, all of the tested stover samples had internal friction angles >30°, which could present additional feeding and handling challenges. All of the "wetted" materials (water, acid, and deacetylated) displayed reduced flowabilities (excluding the acid impregnated sample), and enhanced compressibilities and wall friction angles, indicating the potential for added handling issues; which was corroborated via theoretical hopper design calculations. All of the "wetted" corn stovers require larger theoretical hopper outlet diameters and steeper hopper walls than the examined "dry" stovers.

Early lignocellulosic biorefineries have been plagued with numerous issues that involve feedstock handling problems and variations in conversion efficacy that stem from feedstock variability and complexity in dimensional, physical, chemical, and mechanical attributes. Feedstock ash and moisture content vary considerably in corn stover harvested from farms for bioconversion, and their effects on preprocessing (grinding/milling) and subsequent chemical and enzymatic conversion to fermentable sugars is systematically explored here using pilot-scale hammer mill grinders and a chemical hydrolysis reactor. Corn stover with high ash content due to contamination from soil was found to (1) consume higher power during grinding resulting in reductions of processing rates and (2) produce a larger fraction of fines in the feedstock that were lost to dust mitigation systems causing higher mechanical wear rates. Corn stover feedstock coming from fields with a high residual moisture content resulting in bale degradation due to self-heating caused a more pronounced drop in preprocessing throughput due to grinder overloads and process upsets leading to equipment downtime. Conversion yield to sugars was not affected, although differences in fermentation performance on these sugar streams was not examined. The overall process throughput was only 40−70% of nameplate capacity due to preprocessing problems.

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Many potential biochemical pathways producing advanced hydrocarbon fuels from renewable lignocellulosic biomass require hydrolysates with high sugar concentrations and low toxicity, enabling flexible fermentation strategies, such as fed-batch fermentations capable of producing high product titers and production rates during fermentation. High sugar concentrations also increase the osmotic pressure in the hydrolysates, thus helping to decrease contamination issues. We have shown the production of high sugar concentrations directly from biomass without the need for energy/ capital intensive concentration, conditioning, and/or purification steps. In our previous work, we successfully demonstrated high biomassderived sugar concentrations (over 230 g/L fermentable sugars) in enzymatic hydrolysis from high solid (>20 wt % insoluble solids) digestions of dilute alkali deacetylated and mechanically refined (DMR) corn stover slurries. The goal of this work was to understand the effects of initial solid loadings on differently pretreated corn stover substrates on the rheological property changes of enzymatic-hydrolyzed slurries as well as the rates and yields of the enzymatic hydrolysis reactions. We performed high-solid enzymatic hydrolysis using deacetylated and disk refined (DDR), DMR, and dilute acid pretreated (DAP) corn stover substrates at four different initial total solid loadings, 17, 22, 27, and 32%. Slurry samples were collected at regular intervals for measurements of monomeric and oligomeric sugar concentrations (glucose and xylose), particle size distributions, viscosities, and yield stresses. We produced over 270 g/L of fermentable, monomeric sugars (157 g/L of glucose and 114 g/L of xylose) at 32 wt % total insoluble solid during enzymatic hydrolysis of the DMR substrates. The pumpabilities of the digested, high-solid enzymatic hydrolysates were shown to be comparable to other commercially relevant slurry streams, such as honey, peanut butter, and ketchup from the food industry. The conversion and rheological results indicate that high-solid enzymatic hydrolysis (∼33 wt %) of DMR substrates is a promising and scalable technology for producing high sugar concentration slurries from lignocellulosic biomass.

Residence time is a critical parameter that strongly affects the product profile and overall yield achieved from thermochemical pretreatment of lignocellulosic biomass during production of liquid transportation fuels. The residence time distribution (RTD) is one important measure of reactor performance and provides a metric to use when evaluating changes in reactor design and operating parameters. An inexpensive and rapid RTD measurement technique was developed to measure the residence time characteristics in biomass pretreatment reactors and similar equipment processing wet-granular slurries. Sodium chloride was pulsed into the feed entering a 600 kg/d pilot-scale reactor operated at various conditions, and aqueous salt concentration was measured in the discharge using specially fabricated electrical conductivity instrumentation. This online conductivity method was superior in both measurement accuracy and resource requirements compared to offline analysis. Experimentally measured mean residence time values were longer than estimated by simple calculation and screw speed and throughput rate were investigated as contributing factors. A semi-empirical model was developed to predict the mean residence time as a function of operating parameters and enabled improved agreement.