WORKING PAPER 2019-13 Advanced alternative fuel pathways: Technology overview and status Authors: Chelsea Baldino, Rosalie Berg, Nikita Pavlenko, Stephanie Searle Date: July 19, 2019 Keywords: Thermochemical conversion; biochemical conversion; non-food feedstock; lignocellulosic biomass; biofuels; renewable fuel; electrofuels Introduction and summarized in this paper. For Fischer-Tropsch (FT) synthesis and this figure, as well for all the follow- methanation, before it can be used as Policymakers recognize the signifi- ing figures in this paper that illustrate a transportation fuel. Hydrothermal cant greenhouse gas (GHG) savings the individual conversion technology liquefaction and fast pyrolysis are that are possible by transitioning from pathways in more detail, yellow boxes other primary conversion technolo- first-generation, food-based biofuels indicate processes and green boxes gies that process biomass into crude to alternative, non-food based fuels, represent an input to the process. oils, which are then fed into hydropro- i.e., advanced alternative fuels. First- Blue boxes represent the desired cessing or other upgrading technolo- generation biofuels are produced by fuel output, whereas gray boxes are gies to produce drop-in fuel. converting readily available biomass intermediary products. Bold arrows chemicals—sugars, starch or oils—into indicate primary steps in the conver- This paper synthesizes the best avail- transportation fuels such as ethanol sion process; narrow arrows indicate able information on the feedstocks and fatty acid methyl esters (FAME). less common, or less important, steps. that are or could be suitable for each Advanced biofuels, on the other hand, The black, dashed box groups pro- pathway. Overall, these feedstocks are those produced by converting cesses that can be applied to the include materials such as lignocel- additional chemicals contained in oxygen and hydrogen that is produced lulosic biomass, municipal or indus- the biomass feedstocks, specifically in the PtX pathway. PtX refers to elec- trial waste streams, and waste oils hemicellulose and cellulose, which are trolysis followed by upgrading. and fats. Lignocellulosic biomass is more difficult to extract and convert organic material with a high cellulose into transport-grade liquid fuels. For This paper reviews primary conver- and lignin content. It includes agri- this reason, more complex, advanced sion technology pathways, such as cultural residues such as wheat straw conversion techniques are required. electrolysis and gasification, which and corn stover, forestry residues, Advanced fuels also can include non- convert water and biomass, respec- and purpose-grown energy crops biological pathways, such as power- tively, into gases, as well as cellulosic such as switchgrass and poplar. In to-liquid or power-to-gas, which are ethanol conversion. In some cases, the case of PtX, the feedstocks are such as in the production of cellulosic generically referred to as PtX fuels or electricity and carbon dioxide (CO2). electrofuels. This paper provides an ethanol, the finished product from Also described is any pretreatment, overview of some of the most promis- the pathways addressed in this paper chemical or physical, necessary to ing advanced alternative fuels produc- can be used directly as transporta- prepare the feedstock for the conver- tion pathways. tion fuel. In most cases, however, the sion processes. liquid or gas carrier that is produced Figure 1 presents the conversion tech- from the pathway requires further Although technical assessments for nology pathways that are assessed processing and upgrading, such as each of these conversion technology Acknowledgements: This work was generously supported by the David and Lucile Packard Foundation. Thanks to helpful reviews from Nic Lutsey, Jacopo Giuntoli, Adam Christensen, Bruno Miller, Rob Conrado, Dave Newport, Arne Roth, Alex Menotti, and Steven Gust. © INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION, 2019 WWW.THEICCT.ORG ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS CO2 or CO Legend Process Oxygen Methanol Methanol Synthesis Water Electrolysis Input or Intermediary Hydrogen Methanation Methane Product or Wax Desired FT Product Synthesis Biomass Other Gasification Syngas or Wastes Hydro-carbons Gas Fermentation Cellulosic Ethanol Ethanol Conversion Hydro- thermal Bio-Crude Liquefaction Hydro-processing Fast Drop-in Bio-Oil and Other Pyrolysis Fuels Upgrading Waste Oils & Fats Figure 1: Simplified overview of the conversion pathways reviewed in this paper. Primary steps are highlighted with bold arrows. The black, dashed box groups processes that can be applied to the oxygen and hydrogen that is produced in the PtX pathway. pathways are available in the litera- Cellulosic Ethanol polymer chains that form the physical ture, there is no study that the authors Conversion structure of plants. The process of know of that includes all of these converting lignocellulosic biomass pathways together. This overarching to ethanol is called cellulosic ethanol OVERVIEW OF CONVERSION study summarizes how these tech- conversion. Cellulose is trapped inside PROCESS nologies convert biomass and wastes the lignin, making it more difficult into fuel and assesses their com- Ethanol is conventionally made primar- to convert to ethanol compared to mercial potential, citing examples, ily from sugars and starches obtained first-generation ethanol production if they exist. It also addresses the from food crops, such as corn, wheat, from starches or sugars. Cellulose and major costs associated with these sugarcane, and sugar beet. These hemi-cellulose are first broken down pathways, when information is avail- sugars and starches can be easily into monomeric sugars by enzymes, able. Obstacles that might inhibit converted into ethanol by microbes, and the sugars are then fermented commercialization are reviewed for such as yeast. Lignin and cellulose, to ethanol by yeasts. Lignin is a by- each of the technologies. in contrast, consist of long organic product of this process that is typically 2 INTERNATIONAL COUNCIL ON CLEAN TRANSPORTATION WORKING PAPER 2019-13 ADVANCED ALTERNATIVE FUEL PATHWAYS: TECHNOLOGY OVERVIEW AND STATUS combusted on-site for electricity gen- There are different feasible pretreat- route, and is described in the sections eration, although the use of lignin to ment methods for various kinds of on Gasification and Gas Fermentation. extract chemical products with higher biomass due to differences in physical The biochemical route uses a com- added-value is being explored in bio- structure, lignin content, and other bination of either enzymes or acids refinery concepts. considerations (Maurya, Singla, & Negi, to convert the pretreated cellulosic 2015). In particular, the pretreatment biomass to ethanol; this pathway Feedstocks utilized process for woody biomass differs includes three steps: hydrolysis, fer- substantially from that for agricultural mentation, and distillation. Within lignocellulosic biomass, the cel- biomass (Limayem & Ricke, 2012). lulose is bundled into structures, called Figure 2 illustrates the steps neces- Various pretreatment techniques have microfibrils, that are surrounded by sary to convert biomass inputs into been developed, each with its own lignin and attached to each other by cellulosic ethanol combustion fuel: benefits and drawbacks, as addressed hemicellulose. Typical proportions are pretreatment, hydrolysis, fermen- in the Obstacles to Commercialization 40%–50% cellulose, 25%–30% hemi- tation, distillation and drying, and section below. These include the cellulose, and 15%–20% lignin (Menon separation of distillation. Cellulosic application of physical processes & Rao, 2012). Cellulose is a polymer ethanol production requires external (e.g., particle size reduction through of glucose, which means it is made heat and energy, but in this figure grinding or steam explosion), chemi- of many glucose molecules bound we show only where there is poten- cals (e.g., sulfuric acid), physicochemi- tightly into chains. Hemicellulose also tial for recycling or export of energy. cals (e.g., liquid hot water combined is a polymer of sugars, but a mixture As shown, along with the primary with ammonium fiber explosion, of sugars, including six-carbon sugars product of ethanol, a secondary, where high-pressure, liquid ammonia (such as glucose) and five-carbon desired product—biogas, which is is applied and then the pressure is sugars (such as xylose). shown in blue—can also be produced. explosively released), and biological Pretreatment agents (e.g. white-rot or brown-rot Hydrolysis breaks down cellulose fungi and bacteria), or combinations into free sugars in order to make the Pretreatment is the first step in the of these (Bensah & Mensah, 2013). glucose within the cellulose acces- conversion of lignocellulosic biomass sible for fermentation (see Figure 2). into cellulosic ethanol (see Figure 2). Chemistry of the conversion This process is also called sacchari- Pretreatment alters the lignin and fication or cellulolysis. Hemicellulose Lignocellulosic biomass can be con- hemicellulose structures, exposing has more complex sugar polymers as verted into ethanol through either the cellulose to the action of enzymes well, but the use of enzymes to cleave thermochemical or biochemical and reducing particle size to maximize these sugar polymers has been found pathways. In both routes, the recal- surface area to mass ratio; this mini- to be cost-prohibitive (Limayem