PVC1: Transport fuels via gasification
Thermal gasification produces a syngas, which can be used for the production of power and heat (see ), or further processed into transport fuels.
Feedstock
Thermal gasification is very fuel-flexible; it can in principle use any reasonably low moisture content (preferable below 15%) combustible carbon-containing material as a feedstock, presently mainly various biomasses. Conversion efficiency however will be affected when using less defined feedstocks. Possible feedstocks include forest and forest industry residues, short rotation coppice (SCR), lignocellulosic energy crops such as energy grasses and reeds, agricultural and agro-industrial residues as well as sorted municipal and industrial wastes (RDF[1], SRF[2], plastic wastes, digested sewage sludge etc.), with each of these feedstocks coming with specific challenges in terms of technical or economic feasibility. Biomass from dedicated felling of forestry wood is also possible as feedstock but is not considered sustainable.
Gasification
Gasification is a thermochemical conversion process at 800-1300°C using a sub-stoichiometric amount of oxygen (typically l = 0.2-0.5). Under these conditions the biomass is fragmented into raw gas consisting of rather simple molecules, such as: hydrogen, carbon monoxide, carbon dioxide, water, methane, H2S, NH3, HCl, etc. Solid by-products are: tar, char, and inorganic matter. After clean-up of the raw gas, the gaseous molecules are chemically re-synthesized to transport fuels..
After size reduction of the raw material, it is moved into the gasifier. Typical gasification agents are: oxygen and water/steam. The choice of the gasification agent depends on the desired raw gas composition. The combustible part of the raw gas consists of hydrogen (H2), carbon monoxide (CO), methane (CH4) and short chain hydrocarbons; the non-combustible components are inert gases. A higher process temperature or using steam as gasification agent leads to increased H2 content. High pressure, on the other hand, decreases the H2 and CO.
Entrained-flow gasifiers operate at high temperatures (1000-1300 °C) and are therefore suitable when low methane content is preferred, however feedstocks with very low particle sizes are required. Bubbling and circulating bed gasifiers in contrast are operated at lower temperatures (800-1000 °C) and have the advantage of being able to use highly heterogeneous materials.
The process heat can either come from an autothermal partial combustion of the processed material in the gasification stage or allothermally via heat exchangers or a heat transferring medium. In the latter case, the heat may be generated by the combustion of the processed material (i.e. combustion and gasification are physically separated) or from external sources.
Impurities of the raw gas depend on the gasification condition and used feedstock. They can cause corrosion, erosion, deposits and poisoning of catalysts during downstream processing. It is therefore necessary to clean the raw gas. Depending on technology, impurities such as dust, inorganic matter, bed material, tars and alkali compounds are removed through various cleaning steps.
The cleaned raw gas, now a clean fuel gas, must meet the quality requirement of the synthesis unit.
Fuel Synthesis
The technology for the use of the synthesis gas intermediate is well-established for fossil-derived synthesis gas and has immense industrial importance for producing hydrogen in refineries as well as many millions of tonnes of chemicals annually. Selective catalytic chemical reactions convert the synthesis gas to, by choice, methane, methanol, DME or Fischer-Tropsch hydrocarbons, respectively, at temperatures of 200 up to 400 °C. The synthesis gas can also be converted to ethanol by micro-organisms at ambient temperature. In addition, hydrogen as a product can be extracted directly from the gas.
Fischer-Tropsch hydrocarbon product is a mixture with a wide range of molecular weights from LPG over naphtha and distillates to waxes. The waxes are typically hydrotreated and then the combined liquid products are fractionated by distillation to gasoline, diesel and jet fuel. Synthesis gas can be used to produce methanol and DME (dimethyl ether), which, if desired, can be processed further to gasoline. Synthesis gas can also be converted to methane which can be distributed through the natural gas grid and used directly as renewable CNG or liquefied and used in heavy duty vehicles (LNG trucks). The typical energy conversion efficiency (biofuel output energy/biomass feedstock energy as received) from feedstock to advanced biofuel products ranges from 40 - 50 % for drop-in hydrocarbon fuels and 60 - 70 % for gases and methanol.
Demonstration plants
In the EU, biofuels derived via gasification are advanced biofuels as far as they are produced from biomass and biomass residues or the biogenic fraction of wastes. The fossil fraction of wastes produces a recycled carbon fuel.
There are a number of TRL6+ pilot and demonstration plants in the EU and North America[3], and the technology is fairly widely used for other purposes than biofuels, see . So far, only two advanced biofuel plants at industrial scale (TRL8) were in operation. The Enerkem plant in Edmonton, Canada, that produced methanol or ethanol from assorted wastes (RDF) at a nominal capacity of 24 000 toe/year. And up to 2018, also the GoBiGas plant in Gothenburg Sweden was in operation, producing bio-methane from forest residue pellets up to a nominal capacity of 14.000 toe/year, but was closed for technical and economic reasons. Karlsruhe Institute of Technology operates a gasification and DME-production installation of TRL 6-7 with a capacity of 600 t/y of DME in Eggenstein-Leopoldshafen in Germany. With its BioTFuel project, Total has demonstrated the full chain from torrefied biomass to FT liquids at varying scales of the process steps in Dunkirk in France in April 2021.
Another plant is under construction in Europe, Advanced Biofuels Solutions´ project GoGreenGas to produce 1 500 t/y of SNG in Swindon in the UK. There are also two plants in construction in the USA, Red Rock Biofuels and Fulcrum Sierra Biofuels that will both use the Fischer-Tropsch process to produce hydrocarbons and mainly bio-jet fuel, 45 000 toe/year from wood residues and 33 000 toe/year from refuse derived fuel (RDF), respectively. Furthermore, there are two plants in planning using the Enerkem technology to produce methanol from RDF. One in Rotterdam, the Netherlands at an annual capacity of 102 000 toe/year, and one in Tarragona, Spain, at an annual capacity of 123 000 toe/year, respectively. In California, Aemetis is planning a 23 000 toe/year ethanol plant using the InEnTec plasma gasification and the Lanzatech synthesis gas fermentation technologies, see also .
Fact Sheet: Synthetic hydrocarbons
Acknowledgement: Large parts of the texts were taken from Lars Waldheim´s contribution to the report “The Contribution of Advanced Renewable Transport Fuels to Transport Decarbonisation in 2030 and beyond”
[1] RDF: Refuse derived fuel, the fuel fraction remaining after recyclable material and non-combustible waste have been separated in a waste treatment facility however not associated with any specific quality measures.
[2] SRF: Solid recovered fuel, an RDF where certain quality parameters and procedures have been defined, for further information see standard EN 15357 and standard in development ISO/DIS 21637.
[3] SGAB Technology status and reliability of the value chains: 2018 Update. 28 December 2018. Ed. I Landälv, L Waldheim, K Maniatis. artfuelsforum.eu/news-articles/updated-sgab-report-technology-status-and-reliability-of-the-value-chains/