PVC3: Transport fuels via pyrolytic and thermolytic conversion
In this section, three different routes are discussed:
- Pyrolysis to bioliquid intermediates
- Hydrothermal liquefaction (HTL) to bioliquid intermediates
- Lignin to bioliquid intermediates
In most cases, the resulting oils are upgraded into transport fuels through co-processing in refineries; however, pyrolytic and thermolytic conversion plants could also include severe or complete hydrotreatment in their installations, directly providing transport fuels.
Co-processing in refineries is covered in the last section on this page.
Pyrolysis to bioliquid intermediates
Pyrolysis process
Pyrolysis is the chemical decomposition of organic matter by heating in the absence of oxygen. The feedstock decomposes into organic vapours, steam, non-condensable gases and char. The technology can in principle use any low moisture content (preferable below 15%) organic material as a feedstock. The feedstock potential for producing advanced biofuels lies in forest and forest industry residues, as well as agricultural and agro-industrial residues. Plastic wastes can also be used as feedstock, but the resulting fuel will be termed recycled carbon fuels, not biofuels.
The pre-treatment of the feedstock typically includes drying to less than 15 % moisture and crushing/milling to particles of less than 5 mm. The highest yield of the desired liquid fraction, up to 65 wt% on a dry feed basis, is obtained by thermal fast pyrolysis. Fast pyrolysis takes place in order of seconds at around 500 °C. The heating medium is typically circulating sand, but also other forms of heating have been used. On cooling, the organic vapours and the steam condense to a dark brown viscous liquid called fast pyrolysis oil (FPBO) or Fast Pyrolysis Bio Oil (FPBO). The char and gas are used internally to provide the process heat required, and additionally also energy for export.
The word “oil” used in this context is misleading, the energy content is only half of that of fuel oil, it contains ash solids, the oxygen content is almost as high as for biomass (35 - 40 %), it is acidic (pH usually below 2) and non-miscible with either conventional oil or with water. Nevertheless, this liquid is transportable, storable and can without upgrading to some extent be used as a fuel oil substitute, in particular when using a catalyst during pyrolysis. By using a catalyst during pyrolysis or in the vapour phase, the oxygen content and acidity of the oil can be reduced, at the expense of a lower mass and energy yield. There is also a development of a pressurised pyrolysis in a hydrogen atmosphere, whereby the bioliquid generated has a yet lower oxygen content and acidity and being more similar to hydrocarbon fuels.
There are developments of the upgrading of pyrolysis oil, either in an integrated facility at the production site or by co-feeding with fossil feeds at blend ratios of a few % in existing refineries. A concept for the pyrolysis technology is where the intermediate product, the pyrolysis oil, can be produced at smaller capacity in distributed plants and the upgrading of the oil to drop-in transport fuels is done in large plants fed by FPBO from a number of such plants.
Upgrading to transport fuels
The main routes from FPBO to a drop-in hydrocarbon biofuel is by fluid catalytic cracking (FCC) or by hydrodeoxygenation (HDO). In the case of the FCC route, oxygen is expelled from the FPBO as CO and CO2 and the H/C ratio adjusted by coke formation to result in a hydrocarbon mixture where gasoline is the main fraction. The HDO route is basically a treatment with hydrogen whereby oxygen is expelled as water, this process having similarities with the HVO process, see EVC2: Hydrotreatment to HVO, and the resulting hydrocarbon mixture predominantly gives a diesel product. In both cases, a lower yield of biofuel results from the mass loss, and also some energy loss in the case of FCC, whereas the HDO treatment has a high energy yield based on the input biomass energy, as energy from external hydrogen is consumed.
Another pathway combining these routes are an initial hydrotreatment to stabilize the FPBO followed by FCC treatment. The benefit is that both acidity and oxygen are reduced, and the blend ratio for co-processing can be increased significantly.
Demonstration plants
In the EU, biofuels derived via pyrolysis are advanced biofuels. The pyrolysis technology to produce an intermediate bio-oil, is demonstrated at TRL 8 by a handful of Ensyn plants in North America over the last decade and more recently by BTG in the Netherlands (one facility sold to and operated by Twence in 2019, another facility for Green Fuel Nordic came into operation in Lieksa, Finland, in 2020), Valmet/Fortum in Finland and Karlsruhe Institute of Technology in Germany, at capacities ranging from 10 000 -50 000 tonnes bio-oil/year, or approximately 4 000- 20 000 toe/year3. So far, the primary product is a replacement of fuel oil. Upgrading of such bio-oils to biofuels in integrated plants or by refinery co-processing at percentage blends have been and is tested at different scales, but mainly at TRL 5-6 pilots.
In 2018, Ensyn reported that they had a take-off agreement for bio-oil intermediate with Valero, but specific details were not given. In late 2019, Pyrocell in Sweden announced the start of construction of a 24 000 tonnes/year bio-oil plant co-located at a sawmill. The plant will be based on the BTG technology and the bio-oil off-take is for co-processing by the Preem Lysekil refinery, Sweden[1]. Further announcements include Biozin in Norway and Susteen Technologies in Germany.
Some general science-based information can be found in the PyroWiki that was initiated by the Empyro project.
Fact Sheet: Pyrolysis oil
Hydrothermal liquefaction (HTL) to bioliquid intermediates
Feedstock
The hydrothermal liquefaction (HTL) process can treat lignocellulosic or other biomasses as well as waste fractions. Lignocellulosic and other solid feeds must be pre-treated to allow the formation of a slurry at a reasonably high solid content by mechanical or thermomechanical pre-processing such as e.g. steam explosion. On the other hand, the great advantage of HTL is that wet fuels like sludges, algae etc. can be processed without drying, which would be necessary for other thermal processing methods.
Hydrothermal liquefaction process
Hydrothermal liquefaction (HTL) is a thermochemical conversion process of biomass (lignocellulosic or other biomasses) into a liquid intermediate by processing in a hot, pressurized water environment, typically 250 °C to 370 °C and the pressure range is usually 4 MPa to 20.06 MPa (i.e. water sub-critical conditions), for sufficient time (10-60 minutes) to break down biopolymeric structure to liquid and gaseous components. The operating conditions are quite challenging, the feed must be turned into a pumpable slurry, and this slurry and the liquids produced have an impact on the lifetime of pump, valves and construction materials, etc.
The HTL process usually produces four different product fractions, a gas phase, a solid residue, a liquid aqueous phase and a liquid oily phase, i.e., bio-crude. The produced bio-crude intermediate separates from water but still has 10 - 20 % oxygen and still has a relatively high acidity.
Upgrading to transport fuels
The HTL bio-crude has several more or less direct utilization routes e.g. low-blends into bunker fuel, but it can also be upgraded as an integrated process step or by co-feeding in refinery units to produce drop-in biofuels. The upgrading technology for this type of bio-crude is in principle similar to the upgrading of pyrolysis oil, see Pyrolysis to bioliquid intermediates.
Pilot plants
In the EU, biofuels derived via HTL processing of biomass residues and wastes are advanced biofuels. Most of the research on HTL has been done in batch processes, but several technology developers (PNNL/Genifuel, KIT, Aalborg University and Steeper Energy, Licella) have developed continuous TRL 5 pilot systems[2]. However, prototypes ranging from 4 000 up to 16 000 tonnes/year, say 3 000 to 12 000 toe/year are in construction for forest wastes (Canfor, Canada and Silva Green Fuels, Norway) and plastic wastes (ReNew ELP, UK) with the intention to have off-site upgrading of the crude intermediate in fossil refineries. Nevertheless, compared to pyrolysis technologies, the upgrading of the intermediate bio-crude has not been tried to the same extent as for pyrolysis oil, such that the overall technology when producing biofuels is at TRL 4-5.
H2020 research projects
A number of research projects on hydrothermal liquefaction are currently ongoing under the EU Horizon 2020 programme: 4Refinery, Heat to Fuel, HyFlexFuel, NextGenRoadFuels, Waste2Road and BL2F.
Lignin to bioliquid intermediates
Feedstock
Lignin, one of the three main components of lignocellulose and also one of few aromatic compounds produced by plants, is a polymeric substance composed of phenolic monomers that can be used as an intermediate for the production of biofuels. Lignin from pulping processes is dissolved in the pulping (black or brown) liquor and currently used as a fuel in the recovery boiler, where pulping chemicals are recovered for re-use. Such liquors can be gasified by procedures discussed in PVC1: Transport fuels via gasification and the chemicals can be recovered. The pulping lignin can be withdrawn up to an estimated 10 - 20 % of the total amount with limited impact on the pulping processes. Pulping lignin can be separated from the liquor by precipitation or by membrane filtration for further separate treatment. An added advantage is that removal of a part of the lignin can allow a higher pulp production as the capacity of the recovery boiler is often a process bottleneck.
Hydrolysis lignin from lignocellulosic ethanol production is also a by-product and is available as a solid after the pre-treatment or after fermentation, depending on the process configuration. Today, it is also mainly used as a fuel for the internal energy demands of the process, plus some export energy, but could possibly be better valorized as a biofuel.
Processing
The processing of the separated lignin is in the liquid phase such that precipitated lignin is dissolved. First, de-polymerization to phenolic mono- and oligomers is accomplished by chemical catalysis using bases or acids in combination with thermal or HTL processing and/or hydrogen treatment. The oligomeric and monomeric substances, depending of the level of depolymerisation and nature of the components, are then dissolved in a fossil or a triglyceride feed fraction or reacted via esterification with e.g. mixed fatty acids to allow mixing with a fossil fuel fraction. Finally, the lignin-derived feed is co-fed to a refinery and is hydro-treated to remove oxygen and to produce cyclical aromatic or aliphatic hydrocarbons, depending on the process severity.
Upgrading to transport fuels
The upgrading technology for this type of oil is in principle similar to the upgrading of pyrolysis oil, see Pyrolysis to bioliquid intermediates.
Pilot plants
In Sweden, there are a few developments (Renfuel, RISE, Inventia, SCA and Suncarbon) targeting lignin in or separated from black liquor, and a few pilot scale units at TRL 5-6 are being implemented[3]. This pathway still has to reach the demonstration phase. Renfuel has disclosed plans for building a plant in Sweden consuming 25 000-30 000 tonnes/year of lignin and the intermediate to be co-processed by refiner Preem.
There are activities for recovering lignin from cellulosic ethanol plants, but the main focus is on bio-based materials rather than on biofuels, even if there are some activities aiming at biofuels.
The value of co-processing biogenic feedstock in refineries
Definition
Co-processing refers to the simultaneous conversion of biogenic residues and intermediate petroleum distillates in existing petroleum refineries for the production of renewable hydrocarbon fuels. In contrast to the now common blending of biofuels into the finished petroleum product, co-processing makes use of biomass within the processing of petroleum.
Co-processing technologies
The requirements for the biogenic feedstocks for co-processing are high. They must have reliable material properties in order to be able to process them together with fossil fuels in a petroleum refinery. Suitable for co-processing are semi-processed biogenic feedstocks, such as pyrolysis oil or triglycerides such as vegetable oils, used cooking oils etc. Lignin and sugars can also be co-processed in existing petroleum refineries.
Co-processing involves cracking, hydrogenation, or other reformation of semi-processed biogenic oils and fats in combination with petroleum intermediates to obtain finished fuels, like diesel or gasoline.
The following refining processes may be suitable for co-processing:
- Thermal cracking – In this process, the long-chain hydrocarbons are heated under pressure to 450 – 800 °C. Due to the heating, the hydrocarbon molecules started to vibrate and the hydrocarbon chains are subsequently broken. This process produces products with a high oxygen content, which actually is not desirable in the production of fuels.
- Catalytic cracking – In this case the cracking process occurs in the presence of a catalyst. This process removes the oxygen in the feedstocks via simultaneous dehydration, decarboxylation and decarbonylation. No additional hydrogen or energy is required. This saves costs and reduce GHG emissions. The usual temperatures for this process are 350 - 500 °C. (pyrolysis oil, lignin, glycerol)
- Hydrocracking – This is also a catalytic process at high temperature and high pressure. Hydrocracking is relatively expensive, but the advantages, among others the good product quality, often prevail. (triglyceride)
- Hydrotreating – The conversion occurs through decarboxylation, decarbonylation and hydro-deoxygenation. The required temperatures for this process are 300 – 350 °C. (triglyceride, HTL bio-crude, lignin)
Motivation
As an infrastructure (transport, storage, refinery) already exists, the low-carbon and renewable fuels can be produced and sold at economically competitive prices and therefor are an option for quickly increasing the renewable content of refinery products. It is expected, that the proportion of renewable raw materials can be up to 20 %.
Challenges
Many refineries across Europe have been running co-processing tests over the past few years, and the upgrading of tall oil crude is already state of the art in Sweden. Depending on the quality of the biogenic feedstock, problems could include differences in the stability during storage and handling, and corrosion caused by water and oxygenated organic compounds in the biogenic feedstocks.
EU funded projects
4REFINERY (Scenarios for integration of bio-liquids in existing REFINERY processes) will develop and demonstrate the production of next generation biofuels from more efficient primary liquefaction routes integrated with upgraded downstream (hydro)refining processes to achieve overall carbon yields of >45%. The consortium will aim for successful deployment into existing refineries, including delivering a comprehensive toolbox for interfacing with existing refinery models. Duration: 2017 - 2021.
The BioMates project aspires in combining innovative 2nd generation biomass conversion technologies for the cost-effective production of bio-based intermediates (BioMates) that can be further upgraded in existing oil refineries as renewable and reliable co-feedstocks. The resulting approach will allow minimisation of fossil energy requirements and therefore operating expense, minimization of capital expense as it will partially rely on underlying refinery conversion capacity, and increased bio‐content of final transportation fuels. Duration: 2016 - 2021.
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] 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/
[2] 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/
[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/