Advanced alternative fuel pathways: Technology overview and status
Significant greenhouse gas savings are possible by transitioning from first-generation, food-based biofuels to advanced alternative, non-food based fuels. Advanced biofuels are produced by converting additional chemicals contained in biomass feedstocks, specifically hemicellulose and cellulose, which are more difficult to extract and convert into transport-grade liquid fuels. For this reason, more complex conversion techniques are required. Advanced fuels also can include non-biological pathways, such as power-to-liquid or power-to-gas, which are generically referred to as PtX fuels or electrofuels.
Advanced alternative fuels pathways are at varying stages of development. A critical barrier to widespread commercialization of advanced alternative fuels is the issue of scaling: Across pathways, reactors for larger-scale facilities may need to adopt different designs than what has been done already at a demonstration scale. Another common obstacle is the supply of raw materials, which may be dispersed, leading to high transportation costs, or highly heterogenous, potentially requiring complex and expensive collection. Finally, pretreatment is a significant cost in advanced alternative fuels production. Although many waste and lignocellulosic feedstocks are cheap to acquire, preparing them for conversion can be a capital-intensive, expensive, and complex process.
Several obstacles could be overcome through the deployment of systems that allow for material and energy recovery or otherwise take advantage of economies of scale. Recycling energy and other inputs, as well as co-processing of bio-oil and bio-crude along with conventional petroleum, could also reduce costs as long as these can be executed efficiently.
Summary of the current status and obstacles facing the conversion technology pathways and upgrading processes
|Technology||Status in 2019 for producing advanced fuels||Obstacles to commercialization|
|Cellulosic Ethanol||Early commercial||Recalcitrance of lignin and hemicellulose||High-solids loading; cost of enzymes and other chemicals;C5 sugar content|
|Gasification||Early commercial, but demonstration for transport fuels production||Grinding, milling, and drying the feedstock||Tar production||Syngas cleaning|
|Gas fermentation||Demonstration||Maintaining activity rate of microorganisms is energy intensive; bacterial activity is inhibited by by-products; bacterial contamination|
|Fischer-Tropsch||Commercial, but demonstration for low-carbon fuels production||
|Optimization of gasification technology for use with FT; low catalyst selectivity|
|Fast pyrolysis||Early commercial for on-site combustion, but demonstration for transport fuels production||Grinding, milling, and drying the feedstock||Heating the feedstock to an appropriate temperature is expensive||High water and oxygen content; bio-oil is corrosive|
|Hydrothermal liquefaction||Demonstration||Expensive alloy materials are necessary to prevent corrosion of system components; high pressure can damage system components; moving and stirring large volume of biomass slurry may create technical problems and creates high volumes of wastewater|
|Hydroprocessing||Commercial (using fats and oils)|
|Power-to-X||Demonstration||High electricity input costs in many regions|