Bubbling Circulating Fluidised-Bed (BCFB) gasifier
The biomass feedstock is first converted to raw syngas (and biochar) in the novel Bubbling Circulating Fluidised-Bed (BCFB) gasifier using a mixture of oxygen and steam as fluidising gases. The BCFB gasifier is essentially a combination of bubbling fluidised-bed (BFB) bottom and a circulating fluidised-bed (CFB) top, thus coupling the benefits of both reactor technologies. When co-producing biomethane and biochar, the gasifier bed is operated in the temperature range of 700-800°C, while in the top section the temperature is raised to 800-900°C in order to partially decompose tars and thus prevent tar-related issues (clogging, condensation) in downstream equipment. The biochar product formed in gasification is recovered from the bottom of the gasifier, and the raw gasification gas is subsequently cleaned up to remove the harmful contaminants prior to methanation. Once switched to produce only biomethane, the temperature in the gasifier is raised to approximately 900 °C, as now the target is to maximise feedstock conversion to syngas. In this operation mode, the temperature profile in the gasifier becomes more uniform resulting in similar performance to a conventional CFB gasifier.
VESTA catalytic methanation
The purified gas is routed to a compact VESTA catalytic methanation unit where carbon oxides are hydrogenated into biomethane, and CO2 is separated from the effluent stream after methanation. The biomethane product, with a methane content of 96-98%, is suitable as such for injection into the natural gas network. The main benefits of VESTA methanation technology are: 1) CO2 removal prior to synthesis is not required and thus CO2 is simply removed after methanation, b) gas recycling is not needed, which simplifies the methanation process, reduces CAPEX and allows smaller scale, and c) the integrated sweet shift conversion unit makes the VESTA process insensitive to variations in syngas composition.
Simplified gas clean-up
State-of-the-art solutions for acid gas removal from syngas streams often involve wet scrubbing processes (e.g. Rectisol™) that, despite being effective in capturing impurities, are also complex and capital intensive, which makes them suitable only for large-scale application. In the FlexSNG gas clean-up concept, wet scrubbing methods are replaced with sorbent-based sulphur removal, and CO2 removal is not necessary prior to methanation as it takes place downstream the VESTA methanation unit. Moreover, a dedicated water-gas shift reactor is typically needed for adjusting the syngas stoichiometry suitable for synthesis. In the FlexSNG concept, however, H2/CO molar ratio of around 2-3 can already be achieved through the combination of low-temperature gasification and catalytic reforming, which makes a separate water-gas shift step unnecessary. The simplified FlexSNG gas clean-up process involves the following steps: 1) filtration to remove particulates, alkali and heavy metals using novel sintered metallic filter elements, 2) catalytic reforming of tars and light hydrocarbons with minimal conversion of methane, 3) water scrubbing to remove ammonia and traces of chlorine, and 5) sorbent-based removal of sulphur compounds.
Low-cost oxygen production via oxygen transport membranes (OTMs)
The FlexSNG concept features an innovative oxygen production unit based on oxygen transport membranes that is economically attractive already at the lower end of the targeted 50-150 MW scale. The OTM technology enables the production of high purity oxygen from air at low cost utilising the heat available in the gasification process. This technology achieves 50% reduction in energy consumption compared to state-of-the-art solutions based on cryogenic air separation.