Leaching Pretreatments for Improving Biomass Quality: Feedstocks, Solvents, and Extraction Modeling

In this research, a systematic study was conducted to quantify the inorganic and organic compounds leached from rice straw, wheat straw, corn stover, switchgrass, Jose Tall Wheatgrass, Douglas fir, and Miscanthus with water, and to evaluate the feedstock quality and characteristics of leached solids for thermal process applications. Leaching feedstocks with water at ambient temperature with a 20 L/kg (dry matter) ratio for 2 hours greatly increased the ash fusion temperature of rice straw (from 1050°C to above 1550°C) and wheat straw (from 900°C to 1250°C), but the treatment only increased the ash fusion temperature of corn stover from 900°C to 950°C. Miscanthus had relatively good initial feedstock quality and leaching may not prove necessary for this feedstock in thermal systems. Leaching also changed the combustion kinetics of biomass by increasing the initial degradation temperature of most feedstocks from originally between 165°C and 186°C to between 180°C and 250°C depending on feedstock. Moreover, leaching increased the maximum rate of weight loss of feedstock by 11% to 54% and increased the corresponding temperatures for peak loss up to 34°C. Leaching removed a sizeable fraction of organic compounds (between 2% and 12% of dry matter). These organic extracts were identified as mostly sugars and acids which might be valuable co-products. Moisture contents of feedstocks after leaching were typically high, ranging between 68 and 81% wet basis. A dewatering step is generally required prior to using the leached biomass for thermochemical conversion.;Solvents with ability to dissolve ion-exchangeable, organically associated, and acid soluble metals can further remove non-water soluble metals from biomass and may also improve feedstock quality. In a solvent evaluation, corn stover and wheat straw were leached with water, 1M ammonium acetate, 1M HCl, 100% methanol, 50% methanol, 100% ethanol, and 50% ethanol, and leached solids and leachate were characterized. Among these solvents, 1M HCl improved the corn stover feedstock quality better than other solvents investigated while water worked better for wheat straw. The results show that 1M HCl increased the ash fusion temperature of corn stover from 900 to 1300°C and above 1550°C for water-treated wheat straw. Leachability of each ion extracted from corn stover and wheat straw with different solvents was determined, and the results may be useful in studying the solubility of different elements in a complex ionic solution like leachate.;A parallel diffusion model was developed from Fick's second law of diffusion to describe the mass transfer of compounds extracted from biomass to the bulk liquid phase. The model describes two leaching steps with two effective diffusion coefficients, the first step being an initial washing and fast diffusion ( Dfast) followed by a second slower intraparticle diffusion (Dslow). Mass concentration profiles of anions, cations, and sugars leached from wheat straw with water were developed and were used to estimate parameters in the model and for model validation. Results show that Dfast increased as leaching temperature increased while the effects of leaching ratio on Dfast and Dslow were inconclusive. The calculated Dfast and Dslow for leaching ions and sugars from wheat straw did not follow an Arrhenius behavior, suggesting that the effective diffusion coefficients were different depending on batch leaching conditions. In general, the model was able to predict the concentrations of selected anions, cations and total sugars leached from wheat straw with R2 between 0.85 and 0.99 from linear regression analysis of the experimental data versus predicted data. However, the predictability of the model significantly decreased when employing as low a leaching ratio as 15 L/kg due to the high solids concentration of the slurry and resulting inability to apply sufficient agitation. The model tended to overestimate the concentrations of ions in leachate and underestimate the total sugars during the initial 6 minutes of leaching, possibly due to undetermined characteristics of feedstock rehydration. (Abstract shortened by UMI.).