Thermochemical Conversion Processes

Bio-Oil Pathways R&D

Bio-Oil Pathways R&D focuses on technology to convert biomass to fuels, including transportation fuels and heating oils; chemicals; and power via direct liquefaction-based processes, such as fast pyrolysis, catalytic and non-catalytic vapor phase upgrading, hydrothermal liquefaction, solvent liquefaction, hydropyrolysis, and other alternative processes.  Process intermediates from liquefaction technologies primarily include bio-oil, a liquid product from pyrolysis or liquefaction; bio-char, a solid product from liquefaction or gasification; and gases that may be used as fuel gas or for reforming. Bio-oil intermediates may be upgraded to products such as renewable gasoline, renewable diesel, renewable jet fuel, heating oil, chemical products, or high-purity hydrogen, or even used directly for heat and power generation. R&D efforts focus on direct liquefaction-based processes, as well as the upgrading of bio-oil intermediates, to produce direct substitutes for fossil fuel-based intermediates and products that are compatible with existing fossil fuel processing and distribution infrastructure.

Bio-oil pathways have the potential to maximize biomass resource utilization to produce biofuels because they can convert biomass resources with high lignin fractions, such as woody feedstocks and the lignin-rich non-fermentable residues from biochemical conversion processes, as well as algae-based feedstock at high-moisture contents. Advanced conversion technology scenarios rely on considerable liquid fuel yield per ton of biomass and enable higher overall energy efficiencies by allowing integration of high-efficiency heat and power production systems.

Figure illustrating Thermochemical Platform Integration for Pyrolysis Conversion

 

Bio-Oil Pathways Conversion Processes

A simple bio-oil pathway for converting biomass to transportation fuels such as renewable gasoline, jet fuel, and diesel, or to power applications such as heating/fuel oil is shown in Figure 2-18 below. Process details for producing renewable gasoline and diesel from woody biomass via fast pyrolysis with bio-oil stabilization and upgrading are available in a 2009 design report.

Feed Processing and Handling: The feedstock interface for the bio-oil pathway addresses the main biomass properties that affect the long-term technical and economic success of liquefaction conversion processes: moisture content, elemental composition, impurity concentrations, particle size, particle porosity, and ash content. High moisture and ash content reduce the usable fraction of delivered biomass. Most liquefaction processes require dry feedstocks, while hydrothermal and solvent liquefaction approaches can use biomass at high moisture percentages, such as algae.

Liquefaction: Liquefaction is the thermal or chemical decomposition of biomass to produce a bio-oil intermediate. Fast pyrolysis is typically performed at 400–550°C and primarily produces liquid products together with some gases and bio-char. Catalytic fast pyrolysis employs a catalyst to produce a bio-oil with lower oxygen content than conventional fast pyrolysis bio-oil. Other liquefaction technologies include hydropyrolysis, hydrothermal liquefaction, and solvent liquefaction. Each technology produces bio-oils with varying characteristics and properties for oxygen content, water content, or viscosity that depend on the processing conditions.

Bio-Oil Stabilization and Upgrading: Bio-oil stabilization and upgrading involves mitigating reactive compounds to improve storage and handling properties. This encompasses the removal of water, char, and ash particulates, and destabilizing components such as metals and oxygenated species. Hydroprocessing and similar thermal catalytic processing techniques reduce the total oxygen and acid content, thereby increasing stability. This processing is required before a bio-oil intermediate can be processed under conventional hydroprocessing conditions (e.g., high temperature or pressure) in a stand-alone biorefinery or before it can become a suitable feedstock for a petroleum refinery. 

Fuel Processing: Hydroprocessing converts the stabilized bio-oil to hydrocarbons by eliminating oxygen. After such processing, the total fuel may be separated into renewable gasoline, jet fuel, diesel, or co-products, such as heating oil, using conventional technologies such as those employed by petroleum refiners. The hydroprocessing and separation of fuel cuts may leverage the economies of scale and the capital investments of the petroleum industry.

Balance of Plant: This encompasses the entire site and significant contributions are derived from the hydrogen generation and air- and water-operation. Cost reductions are attained through more efficient hydrogen usage and better usage of power, water, and process recycle streams.

Gasification

In gasification conversion, lignocellulosic feedstocks such as wood and forest products are broken down to synthesis gas, primarily carbon monoxide and hydrogen, using heat. The feedstock is then partially oxidized, or reformed with a gasifying agent (air, oxygen, or steam), which produces synthesis gas (syngas). The makeup of syngas will vary due to the different types of feedstocks, their moisture content, the type of gasifier used, the gasification agent, and the temperature and pressure in the gasifier.

Figure illustrating Thermochemical Platform Integration for Gasification Conversion.

Gaseous Intermediate Conversion Process

A simplified gaseous intermediate process flow to convert biomass to biofuels is shown in Figure 2-22.  This biomass indirect liquefaction process encompasses various methods to produce a high-quality gaseous intermediate that is suitable for upgrading to liquid fuels or chemical products. While further R&D is needed for several of the process steps, the Program will focus efforts on those that have the highest impact (as appropriations allow).  Each process block is discussed below.

Feed Processing and Handling: Cost and quality of biomass feedstock can greatly affect the economics and quality of the gaseous intermediates and the final fuel or product. The feedstock interface examines and optimizes the main biomass properties that affect the long-term technical and economic success of a gaseous intermediate conversion process: moisture content, fixed carbon and volatiles content, impurity concentrations, and ash content. For example, high moisture and ash content reduce the usable fraction of delivered biomass in high-temperature gaseous intermediate conversion processes. Solids concentration, pH, and feedstock composition can affect the efficiency of low-temperature gaseous intermediate conversion processes.

High-Temperature Production of Gaseous Intermediates: Clean gaseous intermediates are produced by deconstructing biomass (e.g., gasification, catalytic gasification,) followed by gas cleanup and conditioning. As an example, biomass gasification is a high-temperature conversion process that begins with the rapid thermal decomposition of a lignocellulosic feedstock. This is followed by partial oxidation or reforming of the biomass with a gasifying agent—usually air, oxygen, or steam—to yield raw syngas.  This all occurs in the same reactor within seconds. The raw gas composition and quality are dependent on a range of factors, including feedstock composition, type of gasification reactor, gasification agents, stoichiometry, temperature, pressure, and the presence or lack of catalysts. Gas cleanup is the removal of contaminants from biomass-derived synthesis gas. It involves an integrated multi-step approach that varies depending on the intended end use of the product gas. However, gas cleanup normally entails removing or reforming tars and acid gas, ammonia scrubbing, capturing alkali metal, and removing particulates. Typical gas conditioning steps include sulfur polishing (to reduce levels of hydrogen sulfide to acceptable amounts for fuel synthesis) and water-gas shift (to adjust the final hydrogen-carbon monoxide ratio for optimized fuel synthesis).

Low-Temperature Production of Gaseous Intermediates: Low-temperature gaseous intermediate conversion processes include biological (e.g., landfill gas, anaerobic digestion) and catalytic deconstruction processes. While the deconstruction mechanisms may differ widely, many low-temperature deconstruction processes yield lower British thermal units, compared to natural gas, synthesis, or methane, , in addition to creating lower concentrations of other gases such as carbon monoxide and carbon dioxide. As with high-temperature production of gaseous intermediates, some gas cleanup and conditioning may be required for catalytic or biological synthesis of fuels and products. Catalytic hydrothermal gasification, applicable to a wide range of organic-in-water mixtures, is a useful means to recover the energy value of the organics as a fuel gas and allows for reuse of the water and dissolved nutrients. It is an energy efficient process that can be used with various wet biomass feedstocks and by-product aqueous stream from biomass conversion processes.

Fuels Synthesis: Clean gaseous intermediates may be converted to fuels via biological organisms (e.g., syngas fermentation) or catalytic processes (e.g., Fischer-Tropsch synthesis). The production of fungible liquid transportation fuels from these intermediates also yields high-value biobased by-productby-products and chemicals. Since catalytic fuel synthesis is typically exothermic, heat recovery is essential to maximize the process efficiency.

Balance of Plant: Balancing the plant encompasses the entire site and its need for integrated and effective energy, heat, steam, and water usage. Pinch analysis is used to analyze the energy network of the process and optimize energy integration of the process. Cost reductions are attained through better usage of the waste heat stream.