First-generation biofuels are produced mainly from sugar or starch crops (e.g., wheat straw, corn stover) (ethanol) and oilseeds (biodiesel).
Second-generation biofuel technologies have been developed to utilize the whole plant as well as biogenic residues, expanding the volume and variety of sources available for biofuel production. Municipal waste and waste products from agriculture, forestry, and manufacturing industries, as well as new energy crops such as fast-growing trees and grass, can potentially be used, reducing land competition with food and feed production.
Gasification is an efficient technology for the production of second-generation biofuels. The gasification process converts biomass or waste into a gaseous chemical energy carrier, the synthesis gas, which can be further used for synthesis of a variety of fuels such as synthetic long-chain hydrocarbons like Fischer-Tropsch (FT) diesel, gasoline, and kerosene or oxygenates like methanol, ethanol, and dimethyl ether (DME). The production of biomethane (SNG) (Subtask 2.2), hydrogen (Subtask 2.5), and chemicals (Subtask 2.4) via gasification are dealt with in other subtasks.
The IEA Bioenergy Task 41 report Advanced Biofuels – Potential for Cost Reduction provides information on conversion efficiencies:
“The overall conversion efficiency on an energy basis is the product of the individual conversion efficiencies of the four main process steps [of the biomass-to- liquid process chain]. Overall energy conversion efficiency (from fuel as received to ready-for-delivery product) is typically in the range of 40-65% on an energy basis (based on Low Heating Value – LHV). Efficient utilisation of by-products like steam and heat can increase the overall energy efficiency of the plant by up to 5-10%, when integrated with district heating or combined with heat and power production. There are some special biorefinery applications, such as gasification of black liquor in pulp mills, where the overall marginal conversion can reach around 70% and even higher when the black liquor is used for biofuels instead of for generating internal energy (steam/power). The production of liquid hydrocarbons which can be used as drop-in replacements for fossil fuels, e.g. FT diesel and kerosene or gasoline, have the lowest yield from feedstock to product, and at the same time the highest specific investment costs. In contrast, the production of biomethane (and hydrogen) have higher overall conversion efficiency and relatively low investment. The difference in yield and specific investment cost between these two extremes is quite significant.”1
Figure 1 shows the process steps for the production of fuels from biogenic and anthropogenic waste streams. This section covers the production of the following biofuels:
- Oxygenates like methanol, ethanol, and
- Synthetic long-chain hydrocarbons such as FT diesel, gasoline, or kerosene.
The main process steps are pretreatment, gasification, gas cleaning, conditioning, and the synthesis to biofuels. A wide range of carbon-containing feedstock from dry biomass to mixed waste streams can be used, which undergo a physical pretreatment in a first step, whereby the feedstock is dried and grinded. Fluidized bed or EFGs are especially suitable for the production of biofuels, converting the fuel to syngas at temperatures ranging from 800°–900°C for fluidized bed gasifiers and up to 1,200°C for EFGs. Only the EFG is operated under pressurized conditions. Air, oxygen, or steam is used as the gasification agent. Fluidized bed gasifiers can be operated with solid feedstock in the size range up to 50 mm, such as wood chips. EFGs require solid feedstock pulverized to a size below 200 µm, but they can also be operated with liquid or suspension fuels produced from the original feedstock by pyrolysis. Depending on feedstock composition and operating conditions, varying amounts of undesirable by-products like methane, higher hydrocarbons, tar, soot, H2S, and HCl are formed in addition to the main syngas components CO, CO2, and H2. If air is used as the gasification agent, the syngas also contains nitrogen.
For an efficient synthesis process, the syngas must fulfil several requirements. Most synthesis processes require an H2:CO ratio of 2, which can be adjusted in a separate catalytic reactor, shifting some CO to H2 by the WGS reaction. The concentration of inerts like nitrogen should be as low as possible, as they reduce the partial pressure and thus conversion. Methane and higher hydrocarbons are also normally inert in the synthesis reactor. However, even in low concentrations they reduce the overall conversion from the feedstock to the final product. These species are mainly produced in fluidized bed gasifiers and are generally converted to H2 and CO in an additional reformer. Species containing sulphur act as a catalyst poison and thus must be removed to very low levels.2
The purified and conditioned syngas consisting of CO and H2 can be converted to liquid hydrocarbons applying FT synthesis. By adjusting temperature, the product can be shifted from a higher fraction of higher-boiling hydrocarbons (above 360°C) to products with shorter chains and methane, olefin, and aromatic production. Conversion rates and formation of the desired long-chained alkanes can also be improved by increasing the pressure. Methanol can be produced from the syngas by hydrogenation of CO applying a suitable catalyst. Gasoline, olefins, or DME can be produced from methanol, and DME in particular can replace fossil gasoline or diesel fuels.2
After passenger transport has been fully electrified, there will still remain a large demand for molecule-bound fuels for aviation, shipping, and long-distance truck transport, as well as the stock fleet of older vehicles with combustion engines. For these applications, a fuel with high energy density is mandatory.
The syngas produced by gasification also contains CO2, which can be captured and either stored (i.e., carbon capture and storage [CCS]) or used in chemical processes to produce materials (i.e., carbon capture and utilization [CCU]). Thus, by avoiding emission of CO2 to the atmosphere, bioenergy with CCS or CCU contributes to reduce the carbon footprint of the process. Integrating the gasification process into chemical production sites enhances the efficiency of the process, as synergies between the different plants can be directly used. Carbon efficiency can be increased by adding green hydrogen produced by electrolysis, shifting the water–gas equilibrium to CO and H2O.
Current Commercial Developments
The following sections summarize projects and plants in operation, on hold and planned, or under construction. There are multiple processes in operation, of which 8 apply fluidized bed gasifiers and 4 entrained flow gasifiers. Five processes are under construction or planned.
Entrained Flow Gasification
BioTfueL (France) (technology readiness level [TRL] 7–8)
BioTfueL developed an innovative process for converting lignocellulosic biomass into biofuel. The raw biomass first undergoes a pretreatment step (drying and torrefaction) before being grinded and fed to a PRENFLO® direct quench gasifier. The produced syngas is then cleaned to remove gaseous impurities by solvent and catalytic processes, and H2/CO ratio is adjusted for the FT synthesis. Afterward, biodiesel and biojet fuels are produced using GASEL® technology—a combination of FT synthesis, hydrotreatment and hydrocracking.
Six partners are involved in this project: Avril, Axens, CEA, IFP Energies nouvelles, Thyssenkrupp Uhde, and TotalEnergies.
The biomass pretreatment demonstrator is located at Avril’s Venette site (Oise, northern France). Gasification, purification, and synthesis demonstration units are situated at TotalEnergies Etablissement des Flandres near Dunkirk (north France). The project finished in 2021, and the process chain is now industrial and commercialized.
bioliq® process (KIT, Germany, Karlsruhe) (TRL 6–7)
The bioliq plant operated at KIT consists of the complete process steps to convert dry lignocellulosic residues into customized fuels. A 5-MW high-pressure slagging entrained gasifier is operated at 40 to 80 bar with biosyncrude, a suspension of pyrolysis oil and char produced by fast pyrolysis. The produced syngas is cleaned from particles and gaseous by- products like HCl and H2S via hot gas cleaning. After CO2 and water separation, the purified syngas is first converted to DME and then to gasoline. The gasifier has been operated for more than 2,300 hours in test campaigns feeding model and technical fuel suspensions.
LTU Green Fuels Pitea (Sweden, CHEMREC) (TRL 6–7); this process is currently mothballed
In Pitea, Sweden, a 3-MW high-pressure refractory-lined EFG was operated for more than 25,000 hours using Kraft black liquor as feedstock. The produced syngas is first cooled down in a quench, where droplets and particles are separated, and then purified from H2S. The clean gas is either used as fuel or further synthesized to DME.
Fluidized Bed Gasification
Advanced methanol process (Netherlands, Gidara Energy) (TRL 8)
In the advanced methanol Amsterdam project, a high-temperature Winkler (HTW) 2.0 gasifier produces syngas from non-recyclable pelletized waste, which is currently landfilled or incinerated. The syngas is partly synthesized to methanol used for fuel blending. The CO2 produced during gasification is separated from the syngas and fed to greenhouses. The plant has been in operation since 2021, and production capacity averages 87,500 t of methanol per year.
A second plant will be built at Rotterdam starting in 2025, which will produce 90,000 t methanol/year.3
Since 2016, Enerkem’s Alberta Biofuels (Canada) plant has processed MSW from the city of Edmonton to produce methanol and ethanol. Current capacity is 38 million litres per year from 100,000 tons MSW per year. More information is provided in Section V: Biomass and Waste Gasification for the Production of Chemicals.
Waste2Value (Austria, Vienna, BEST) (TRL 6–7)
In Vienna, Austria, a new pilot plant is under construction and startup, which will demonstrate the conversion of waste materials into eco-friendly and carbon-neutral fuels. At the site of a hazardous waste incineration plant in the urban area of Vienna, Bioenergy and Sustainable Technologies (BEST) will operate a novel process chain to generate and utilize a hydrogen-rich synthesis gas on an industrial scale. The plant was built by the SMS Group. Biogenic residues and waste are converted to syngas in a 1-MW pilot fluidized bed gasifier. After syngas cleaning and conditioning, fuels are produced via FT synthesis.4
Processes Under Construction or Planned, all TRL 8–9
LanzaTech has worked to develop commercialization of biomass-derived syngas fermentation based on the Sekisui (Japan) gasifier. More information is provided in Section V: Biomass and Waste Gasification for the Production of Chemicals.
Fulcrum’s Sierra Biofuels project will produce 11 million gallons per year of FT hydrocarbon wax and naphtha from 175,000 tons per year of processed MSW. More information is provided in Section V: Biomass and Waste Gasification for the Production of Chemicals.
JV controlled by ENI (NextChem)
In this process, waste is fed to a high-temperature melting gasifier. The ash fraction is collected as inert granulate. The syngas is cleaned, purified, conditioned, and finally synthesized to methanol.
For this process, forest residues are at first chipped, dried, and pelletized. The wood pellets are then converted to syngas in an HTW gasifier under pressurized conditions with oxygen as the gasification medium. After purification and adjusting the CO-to-H2 ratio, fuel-grade biomethanol is synthesized.
1 A. Brown, L. Waldheim, I. Landälv, J. Saddler, M. Ebadian, J.D. McMillan, A. Bonomi, and B. Klein. 2020. Advanced Biofuels – Potential for Cost Reduction. IEA Bioenergy. https://www.ieabioenergy.com/wp- content/uploads/2020/02/T41_CostReductionBiofuels-11_02_19-final.pdf.
3 D. Chafia. 2022. “High Quality Syngas from Non-Recyclable Waste: A Pilot-Scale Study Based on HTW 2.0 Gasification Technology.” Presentation at TC biomass, April 2022. https://www.gti.energy/training-events/tcbiomass/tcbiomass-conference-proceedings/tcbiomass-2022-conference-proceedings/.