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Improving the quality of crop residues by the reduction of ash content and inorganic constituents Peng, Yige


Fuel pellet producers in Canada have started to consider alternative feedstocks that include agricultural crop residues. Yet, the higher ash content and inorganic constituents of crop residues can impose problems on the equipment, the quality of products, and the environment due to air emission. In this study, mechanical size fractionation in combination with leaching was conducted to improve the quality of crop residues by reducing the ash content to target value of 6% db following the applicable standard. Experimental results of mechanical size fractionation suggest that the finest fraction of ground crop residues had much higher ash contents than the coarser fractions. Results also indicate that size fractionation can reduce the need for leaching as ash removal technique, and it is most effective for corn stover when compared to canola straw and wheat straw. For the leaching tests, the leaching performance is affected by the leaching conditions whereas particle size can have some impact. Under the same leaching conditions, non-fractionated ground crop residues with larger average particle size had higher ash removal efficiency than the finest fraction of crop residues. Among the three species of crop residues, canola straw was found to have the best leaching performance, regardless of whether mechanical size fractionation was used prior to leaching. Canola straw had the highest ash removal efficiencies, K₂O removal efficiency (greater than 90%) and SiO₂ removal efficiency (more than 50%). The results also demonstrate that the effect of leaching temperature on K₂O removal efficiency is negligible while SiO₂ removal efficiency increases significantly as water temperature increases from 25 to 45 °C upon leaching for 12 h. A preliminary cost analysis was performed to estimate the total production cost (TPC) of agro-pellets based on different process designs with respect to the ash removal techniques. TPC for the base case of pellet production without ash removal pretreatment was estimated to be US$102/dry tonne (dt) pellets, and it would increase by 30% to US$133/dt for Option 1 (pretreatment by mechanical size fraction plus water leaching) and increase by 66% to US$169/dt for Option 2 (pretreatment by water leaching only).

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Attribution-NonCommercial-NoDerivatives 4.0 International