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- Andersson, Johan, et al.
(författare)
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Biologisk metanisering av syngas från förgasning och pyrolys - lovande koncept mot implementering
- 2024
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Biological methanation of syngas from pyrolysis and gasification – promising concepts for implementation The need for increased biogas production is significant, and in the EU, there are plans for a substantial expansion in the coming years through the RePowerEU initiative. Part of the increase will come from the expansion of conventional digestion technology, where organic materials such as food waste, manure, and crop residues are used for biogas production. However, to meet the future increased demand, it is also necessary to utilize more difficult-to-digest substrates, such as biomass rich in lignocellulose, for biogas production. This could be forest residues such as branches and tops, sawdust, or bark. This type of substrates cannot be used in a conventional digestion process, and other technology chains are therefore required to convert such biomass into biomethane. This can be done by first converting the biomass into syngas through a thermochemical process such as gasification or pyrolysis. This is followed by a methanation process where the syngas is converted into biogas, and finally, the gas is upgraded to reach biomethane quality. These types of technology chains are not currently available on a commercial scale, but they have been demonstrated, for example, through the Gobigas project, where gasification was followed by catalytic methanation for biomethane production. As full-scale implementation of catalytic methanation of bio-syngas has not yet been achieved, thereis a need to develop alternative conversion technologies that can more cost-effectively achieve the methanation of woody biomass. One possible opportunity for to this is to apply biological methanation instead of a catalytic process. A biological process comes with several advantages, including a greater ability to handle contaminants, higher selectivity in the conversion of syngas, and operation at relatively low temperature and pressure, which simplifies material selection and reactor design. RISE, together with its partners, are developing a concept based on biological methanation of syngas. This project has examined the biological process's ability to handle contaminants in syngas through continuous experiments in carrier-filled trickle bed reactors with an active volume of 5 liters. The process's ability to handle and break down contaminants is an important parameter that can affect and simplify the design of the gas cleaning that occurs after gasification or pyrolysis. Another aspect of the project has been to put the experimental results into context at the concept and system level. Different production techniques for syngas have been mapped out, which could be combined with biological methanation. Based on the mapping, three types of plants have been selected for more detailed analyses of techno-economics, carbon footprint, and opportunities for increased carbon efficiency. The methanation experiments lasted for 552 days, and overall, it was a stable process with high turnover of syngas and high methane production over a long time. There have been some operational disturbances, mainly related to the supply of gas to the process (i.e. delivery of gas cylinders). However, biochemical inhibition or disturbances have been rare, demonstrating a high robustness for biological methanation of syngas. The breakdown of contaminants has been excellent in the process, with levels decreasing below the detection limit. At the same time, as contaminants have been continuously added to the process, microbiology has been able to maintain high turnover of hydrogen and carbon monoxide to methane. The specific methane production was high both during the reference period without contaminants and during the experimental periods with added contaminants. During long periods, the specific methane production has been around 4 L CH4/Lbed volume /day, which is about 4 times higher than our previously achieved results. The transition to thermophilic temperature and using carriers with higher effective surface area has contributed to this increase. During the project, three types of plants have been selected for more detailed analysis: 1) Gasification with Cortus process, which generates a relatively clean syngas with minimal purification needs before biological methanation. There is no need for co-location with a heating plant, but it is an advantage if there is access to the district heating network to sell waste heat. 2) Gasification with Bioshares' concept, where the gasifier is integrated into a larger cogeneration plant and where the produced syngas is purified with an RME-scrubber before biological methanation. Co-location with a larger cogeneration plant provides interesting synergies and integration opportunities, but also sets the boundaries for where the plants can be located. 3) Slow pyrolysis according to Envigas' concept, where the primary product is biochar and where the produced syngas is seen as a by-product. The syngas contains some impurities but generally requires no other purification than cooling to the right temperature (condensing out tars) before being added to biological methanation. This type of plant differs from plant types 1-2 in that the syngas formed is not the primary product, and the syngas has a relatively low energy value compared to the others. Syngas from plant types 2 and 3 contains some hydrocarbons (C1-C3) that are considered inert over the methanation step and therefore do not negatively affect the process. This means that heavier hydrocarbons do not need to be removed upstream, which would likely have been required with catalytic methanation. This leads to a higher system efficiency, and the need for reactor capacity for biological methanation decreases since there is less gas to be processed (more of the end-product consists of hydrocarbons already formed during the thermochemical conversion upstream). For all plant types, downstream of the methanation step, there is a need for further gas purification and upgrading. During the upgrading step carbon dioxide is separated to reach the product specification required by the end user. If long distance distribution is required a final process step consisting of a liquefaction plant for the production of liquid biogas (LBG) can be added to the concept. As another option, the systems can be supplemented with treatment of the carbon dioxide flow out of the upgrading plant, where the flow is processed by drying, compression, and cooling to produce liquid carbon dioxide. For plant type 2, where benzene is present in the syngas, this gas is expected to be separated with relatively high precision in the system and thereby generate a small flow of liquid benzene as a side product. The carbon dioxide emissions for the final product LBG are in the range of 1.6 to 2.6 gCO2-eq/MJLBG, which compares favorably to other types of second-generation biofuels. Compared to fossil gas, the reduction in greenhouse gas emissions is 96-97%. The carbon efficiency of the systems can be significantly increased if excess carbon dioxide is utilized either through BECCS or BECCU. If the carbon dioxide stream from the upgrading plant is processed into liquid carbon dioxide, the production cost is estimated to be 187-204 SEK/ton. If the product is to be sent to permanent storage the cost for transportation and storage would need to be added to estimate total cost of BECCS, but this is out of scope for the current project.. Assuming that BECCS is applied and that the entire carbon sink is allocated to the final product LBG, this will result in negative emissions in the range of -35 to -104 gCO2-eq/MJLBG. An alternative is to utilize excess carbon dioxide directly in the methanation process by boosting incoming gas with extra hydrogen. Hydrogen and carbon dioxide are then converted by methanogens, which generates extra methane. Since the addition of extra hydrogen is assumed to come from electrolysis, the additional methane production can likely be classified as electrofuel, so-called e-methane. The techno-economic evaluation results in a production cost ranging from 740 to 1300 SEK/MWhLBG, including all sensitivity scenarios. The lower price scenarios include a lower investment cost, which can be assumed to represent cases with public investment support. Overall, a large part of the scenarios are considered to be within the range of what can be considered market relevant production costs. This leads to the conclusion that there is techno-economic potential at this stage to justify continued development of concepts based on biological methanation of syngas. With scaling up and continued development in the right direction, the concepts may eventually lead to cost-effective utilization of forest residues for the production of biomethane at a commercially relevant scale. The next step in the development is scaling up to pilot scale, which will take place during 2023-2025 through an EU-funded project and will be carried out by RISE, Wärtsilä, Cortus and Swedish Gas Association. A pilot plant for biological methanation will then be operated with syngas from Cortus' gasifier in Höganäs.
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