Conversion of Residual Biomass into Liquid Transportation Fuel: An Energy Analysis

An energy balance, in broad outline, is presented for the production of a high-quality liquid transportation fuel from residual crop biomass. The particular process considered is comprised of (1) harvesting surplus biomass (such as crop residue); (2) locally pyrolyzing the biomass into pyrolysis oil (PO), char, and noncondensable gas (NCG); (3) transporting the PO to a remote central processing facility; (4) converting the PO at this facility by autothermal reforming (ATR) into synthesis gas (CO and H-2); followed by, at the same facility, (5) Fischer-Tropsch (FT) synthesis of the syngas into diesel fuel. In carrying out our calculations, we have made several assumptions about the values of the process parameters. These parameters, of course, can be modified as better input data become available. The material and energy balance has been incorporated into an Excel spreadsheet. The scope and our approach to the energy budget using a widely available spreadsheet hopefully provides greater transparency, as well as ease of scenario manipulation than has generally been found in the literature. The estimated energy efficiencies computed with the spreadsheet are comparable to those obtained with Aspen software. A spreadsheet is offered as a tool for further analysis of the energy budget of this and related processes. The Excel spreadsheet can be used as a nimble scouting tool to indicate promising avenues of study in advance of using a more comprehensive analysis such as that afforded by Aspen software. The process considered, in which a portion of the char and noncondensable gas are used to supply heat to the drying and pyrolysis steps and under the assumptions made, was found to have an energy efficiency to liquid fuel on the order of 40%. That is, 40% of the initial energy in the biomass will be found in the final liquid fuel after subtracting out external energy supplied for complete processing, including transportation as well as material losses. If the energy of the remaining char and NCG is added to that in the product diesel oil, the total recovered energy is estimated to be similar to 50% of the initial energy content of the biomass. If char and NCG are not used as a heat source in the process, the energy efficiency of the produced diesel drops from 40% to 15%. It must be realized that the distribution of energy content among the fast pyrolysis products PO, char, and NCG is similar to 69%, similar to 27%, and similar to 4%, respectively. Therefore, using char and NCG to provide fuel for the drying and pyrolysis steps is very critical in maintaining high energy efficiency of the product fuel. The weight of diesel fuel produced is estimated to be similar to 13% of the initial weight of biomass, implying that it of biomass (30% moisture) will produce 1.0 barrels of diesel oil. The pyrolysis of biomass to PO, char, and NCG is estimated to have an intrinsic energy efficiency of similar to 90%. For the model considered, trucking biomass to a central facility without first converting it to PO is estimated to reduce energy efficiency by similar to 1%.