Biogas from renewable electricity : Increasing a climate neutral fuel supply

This report summarises knowledge from the open literature on the reactivity of tars during pyrolysis and gasification of biomass. Also other mechanism of the chemical reactions involved is considered. The goal of this summary is to make the knowledge accessible not only to ECN but for a broader community and help in the development of both producer gas cleaning technology and innovative gasification processes. Gaseous biomass tars can react under inert conditions (thermal cracking) or with components in the producer gas such as H2, H2O or CO2 (gasification). The reaction rate of thermal cracking is such that high temperatures of approximately 1200°C or higher (also depending on residence time) are needed to create a producer gas with low tar concentrations. The rate of thermal cracking of tars depends on the kind of tar. The rate decreases in the series: biomass pyrolysis oils/tars > phenolic tar compounds (phenol, cresol, naphthol) > Pyrolysis tars from coal > Polycyclic aromatic tar compounds (anthracene, phenanthrene, naphthalene, benzene). The rate of thermal cracking also depends on the atmosphere in which the tars are cracked because the gas phase components H2, H2O en CO2 play a role in the cracking reactions. H2O and/or CO2 increase the decomposition rate of tars whereas H2 decreases this rate. The aromatic rings of tars can also be hydrogenated which only occurs under hydrogasification conditions at high partial pressures of H2. This leads to the production of CH4. Radical reactions are the main reactions in the mechanism of tar decomposition and the formation of methane. Radical formation is the rate-determining step in this mechanism. After radical formation, the composition of the gas phase determines what are the final products of the tar decomposition.

Tar formation in pyrolysis and gasification

Based on comparable basic assumptions, a technical, economic, and ecological assessment has been performed for the biomass/waste-related SNG production technologies, with the objective to make a selection for future implementation of the most promising options. Anaerobic digestion, upstream pressurised oxygen-blown or indirect gasification with downstream methanation, hydrogasification followed by methanation, and, in less extent, the co-production of Fischer-Tropsch-SNG have been studied within this project. Although substitute natural gas (SNG) has not yet been incorporated as an alternative fuel in the Dutch and European renewable energy policies, it can play an important role in realisation of the Dutch and European climate and renewable energy targets. Based on the modelling results, the upstream pressurised oxygenblown or indirect gasification with downstream methanation routes seemed to be the most promising options for SNG production from biomass. The RD&D trajectory for candidate SNG production processes, consisting of three development stages (Proof of Principle, Proof of Concept, demonstration of a complete integrated process on pilot scale), is expected to take about 10 years for indirect gasification and 8 years for pressurised oxygen-blown gasification. Also a period of four years is expected to be necessary for the market introduction. The indirect gasification technology, after development, will score better with respect to all aspects, compared to pressurised oxygen-blown CFB gasification, and therefore it has been selected for future development for SNG production. Through this gasification technology almost 100% carbon conversion can be achieved, resulting in a tar-free (by using the ECN OLGA process), low-nitrogen, and high-hydrocarbon content synthesis gas. The technology has also the potential of upscaling to a commercial scale on the long term. In combination with downstream methanation, SNG production efficiencies up to 70% can be achieved. The SNG production cost for this process is 7.8 €/GJ (compared to the current market price of 8.7 €/GJ). With a potential SNG production capacity of approximately 15 PJ a year, this process has the potential to fulfil approximately 5% of the Dutch renewable energy target in 2020, or even 20% of the biomassrelated target. The SNG production capacity of this process until 2020 is 128 PJ. The costs per tonne CO2 avoided and the total avoided CO2 emission until 2020 are € 83, respectively, 7.2 Mtonne.

https://www.biosng.com/fileadmin/biosng/user/documents/reports/c03066.pdf

Feasibility of biomass / waste-related sng production technologies

Renewable energy storage system via coal hydrogasification with co-production of electricity and synthetic natural gas

Gasification of wood biomass with renewable hydrogen for the production of synthetic natural gas

Hydrogen, generated from renewable sources, is likely to play a major role in the future energy supply. The storage and transport of hydrogen can take place in its free form (H2), or chemically bound, e.g. as methane. However, the storage and transport of hydrogen in its free form are more complex, and probably would require more energy than the storage and transport of hydrogen in chemical form. An additional important advantage of the indirect use of hydrogen as energy carrier is, that in the future renewable energy supply, parts of the existing large-scale energy infrastructure could still be used. Production of Substitute Natural Gas (SNG) by biomass hydrogasification has been assessed as a process for chemical storage of hydrogen. Thermodynamic analysis has shown the feasibility of this process. The product gas of the process has a Wobbe-index, a mole percentage methane, and a calorific value quite comparable to the quality of the Dutch natural gas. With a hydrogen content below 10 mol%, the produced SNG can be transported through the existing gas net without any additional adjustment. The integrated system has an energetic efficiency of 81% (LHV). In the long term, the required hydrogen for this process can be produced by water electrolysis, with electricity from renewable sources. In the short term, hydrogen may be obtained from hydrogen-rich gases available as by-product from industrial processes. Results of thermodynamic analysis of the process and experimental work, application potentials of the process in the Netherlands, and plans for future development are presented.

R.W.R. Zwart

Papers

Production of Substitute Natural Gas by Biomass Hydrogasification