Gasification of biomass and other forms of waste carbon can be used to produce a gaseous product that is a mixture of carbon monoxide and hydrogen known as synthesis gas (syngas). The use of gasification for the production of chemicals from syngas is a very significant global technology; billions of tonnes per year of basic and specialty chemicals are manufactured worldwide from syngas. To date, most commercial development has focused on use of fossil feedstocks such as coal and natural gas for the manufacture of syngas. Figure 1 presents a high-level overview of the basic processing steps involved in converting carbonaceous feedstocks to chemicals via gasification.
Feedstocks containing carbon are first converted (gasified) at temperatures ranging from 800°–1,200°C to make a raw synthesis gas containing carbon oxides (CO and CO2), H2, and a variety of other undesirable chemical species such as tars (C6+ organics), various contaminants such as sulphur and nitrogen oxides and other compounds that can severely poison catalysts, and/or biological organisms that are used in the downstream chemicals synthesis step. The type of feedstock and gasifier technology employed has a dramatic impact on the composition and amount of these undesirable components. Commercial unit operations and technologies such as wet scrubbing, absorption, and catalytic tar cracking/reforming are often used to reduce the levels of these contaminants. Catalysts used for chemicals synthesis have a wide range of tolerance for contaminants. For production of hydrocarbons, FT catalysts typically require a very clean syngas (sulphur- containing compounds at parts-per-million or even parts-per-billion levels), while catalysts used to make methanol are much more tolerant of sulphur and other contaminants.
Biological reactors used for syngas fermentation require very low levels of certain contaminants (parts per billion or trillion) to maintain viability for the organisms that perform the chemicals synthesis. Regardless of the upstream or downstream unit operations and requirements, gas cleaning and tar mitigation are essential processing steps. The clean syngas is then conditioned to provide the correct H2:CO ratio for the downstream synthesis step; for example, synthesis of methanol (CH3OH) requires a molar H2:CO ratio of 2, while the ratio for the raw clean syngas is usually closer to 1–1.5. This adjustment in composition often accomplished by use of the WGS reaction, which produces additional hydrogen but also additional CO2:
CO + H2O → H2 + CO2
The shifted clean raw syngas generally will contain too much CO2 for optimal performance of the chemical synthesis reactor, so removal of this compound is usually required.
Processes for CO2 removal from gaseous streams such as pressure-swing absorption (PSA) are well developed and used worldwide on large scales. Current practice is to vent the CO2 from PSA, which imparts a large carbon footprint. Depending on the final product, a wide range of catalysts and reactor types—ranging from high-temperature, gas-phase, fixed bed shell-in-tube systems to ambient-temperature, aqueous-phase fermenters—are employed in the final synthesis step.
The global chemicals industry is one of the major economic drivers in the world economy. In 2019, the chemicals sector contributed $5.7 trillion to the world’s gross domestic product (GDP) through direct, indirect, and induced impacts, equivalent to 7% of the world’s GDP and supporting over 120 million jobs worldwide.1 The carbon footprint of the chemicals industry is correspondingly very large; according to IEA, 2020 direct CO2 emissions from primary chemicals production were 923 metric tonnes of CO2, or roughly 18% of all industrial CO2 emissions.2 This places chemicals as the third largest industrial contributor (behind iron and steel and cement) in terms of direct CO2 emissions. Figure 2 shows a breakdown of greenhouse gas emissions from various industry sources in the United States.
Production of chemicals from syngas is a robust commercial enterprise dating back more than 100 years to early experiments by Fischer and Tropsch on catalytic conversion of syngas to hydrocarbons. Production pathways are many and varied depending on the final products that are desired, but can generally be summarized as4:
- Products from direct catalytic conversion of syngas
- Products from biological conversion of syngas
- Products derived via a methanol intermediate
- Products derived via a hydrogen
These products are shown schematically in Figure 3. Methanol is one of the most common commercial chemicals manufactured from synthesis gas:
CO + 2H2 → CH3OH
A wide range of other basic or platform chemicals can be manufactured from methanol, as shown schematically in Figure 3. In addition, many specialty chemicals can be made from syngas; Figure 4 shows the array of chemicals produced by BASF from syngas at its major chemicals facility in Ludwigshafen, Germany.
As illustrated, a wide variety of chemicals can be derived from syngas with currently available technologies. For most of the products of current commercial interest, catalytic synthesis of intermediates and final products from syngas is the preferred pathway. This means that products synthesized from biomass-derived syngas can serve as direct replacements of their fossil-derived counterparts, which are produced from feedstocks such as natural gas, coal, or petroleum refinery waste gases. However, opportunities for functional replacements do exist through emerging biological conversion processes such as syngas fermentation.
Impact of Gasification Technology
Biomass gasification processes utilize several different reactor types and technology pathways. Reactor technology categories include fixed and fluid bed, entrained flow, high temperature (plasma), and supercritical water. Selection of the appropriate gasification technology and accompanying gasifier type is usually done by considering the nature of the feedstock; however, there are several additional key parameters that can influence selection of the gasification technology, including pre-processing requirements, reactor capital expenses and operating expenses, and the desired product. For example, gasification of dry biomass (<10% moisture) for production of heat and power can be accomplished using either directly or indirectly heated fixed and fluid bed systems with air or steam, accompanied by simple gas cleaning to reduce tars and particulates. If, however, the desired products are fuels and chemicals, then direct-heated gasifiers such as oxygen- blown pressurized fluid beds or entrained flow systems are preferred. Extensive gas cleaning is usually required to reduce contaminates to very low levels to protect downstream catalysts. Feedstock properties such as ash fusion temperature and moisture can also dictate what kind of technology is best. For wet waste (50% or more water), supercritical gasification technologies are generally preferred. Comprehensive reviews of gasifier types and technologies can be found in review articles on biomass gasification and gasifier types.7
Processes for chemicals from fossil-derived syngas are well developed and practiced commercially at large scale worldwide. The carbon footprint of these processes is often quite large because feedstocks are not renewable and are generally used for both process energy and raw material. Concerns over climate change and the impact of carbon pollution on the environment have led to increased interest in production of chemicals from renewable resources such as biomass and other forms of waste carbon. The U.S. Department of Energy estimates that over 1 billion tons of sustainable and renewable carbon in the form of biomass can be obtained annually in the United States alone.8 The inherently low carbon intensity (CI) of these feedstocks—when compared to their fossil counterparts—provides a pathway for production of chemicals with lower CI when compared with petroleum counterparts. In addition, reduction of carbon pollution in the atmosphere by enabling CCS and, in some cases, utilization/reuse of captured CO2 in the process provides an additional mechanism for further reducing the product CI with the possibility of manufacturing strategies that produce carbon-negative chemicals.
An opportunity exists for developing a new paradigm for production of low-CI chemicals from syngas involving switching to a renewable carbon source—such as biomass—and employing carbon capture, utilization, and storage (CCUS) to greatly reduce or even eliminate greenhouse gas emissions. A general schematic showing biomass gasification coupled with CCUS for production of low-carbon chemicals is shown in Figure 5. Capture and either sequestration or recycle/reuse of CO2 enables this transition from our current high-CI chemicals to low-, zero-, or negative-CI chemicals produced from renewable resources.
Current Commercial Developments
Examples of commercial applications of chemicals from fossil syngas include the Eastman Chemical Company plant (Kingsport, Tennessee, USA), which produces more than 500 KTA of acetyl chemicals including acetic acid and acetic anhydride from coal-derived syngas. A second example is the Huayi Group’s coal-to-chemicals plant in Shanghai, China, which produces more than 800 KTA of methanol and 500 KTA of glacial acetic acid from syngas produced via goal gasification. BASF’s Ludwigshafen chemical complex produces a wide variety of chemicals from fossil syngas (Figure 4).
Examples of commercial production of chemicals from syngas derived from biomass or waste carbon are relatively few today, but the field is growing rapidly in response to efforts to defossilize and attain sustainability goals. Notable examples include the following companies and projects.
Enerkem: Since 2016, Enerkem’s Alberta Biofuels plant (Canada) has processed MSW from the city of Edmonton to produce methanol and ethanol. Current capacity is 38 million litres from 100,000 tons of MSW per year. The primary conversion technology is based on Enerkem’s proprietary pressurized fluidized bed gasification technology, followed by catalytic conversion of syngas to methanol. Other projects include construction of a 125- million-litre-per-year biofuels plant in Varennes (Quebec). Varennes Carbon Recycling will produce biofuels and renewable chemicals made from 200,000 tonnes per year of non- recyclable residual materials and wood waste, and extensive integration of renewable hydrogen and oxygen is anticipated. Commissioning is planned for 2023.
Synova/TNO/Technip: Synova and Technip Energies announced in 2021 that they had entered into a joint development and cooperation agreement to commercialize Synova’s advanced plastic-waste-to-olefins technology, in conjunction with Technip Energies’ steam cracking technology. Synova’s patented thermochemical recycling technology is based on the MILENA gasifier and takes dirty and mixed plastic waste and breaks it down to olefin monomers and coproducts to produce circular plastics. The technology was invented by the Netherlands Organization for Applied Scientific Research (TNO).
Fulcrum: Fulcrum’s Sierra Biofuels project will produce 11 million gallons of FT hydrocarbon wax and naphtha from 175,000 tons of processed MSW per year. The project, located near Reno, Nevada (USA), employs steam-reforming gasification technology from ThermoChem Recovery International and FT technology from Johnson Matthey. Wax will be refined to products (jet/diesel) by Marathon. The plant began startup operations in early 2022.
LanzaTech: LanzaTech has worked to develop commercialization of biomass-derived syngas fermentation based on the Sekisui (Japan) gasifier to produce ethanol and other specialty chemicals (e.g., 2,3-butanediol, acetone). As of 2022, several major commercialization projects have been announced using gasification from a variety of different technology providers.
7 See, for example: S. Misra and R. K. Upadhyay. 2021. “Review on Biomass Gasification: Gasifiers, gasifying mediums, and operational parameters.” Materials Science for Energy Technologies 4: 329–340. https://doi.org/10.1016/j.mset.2021.08.009.; J. Hrbek. 2016. Status report on thermal biomass gasification in countries participating in IEA Bioenergy Task 33. https://www.ieabioenergy.com/wp- content/uploads/2017/12/Status-report-Task-33-2016.pdf.