Europe's energy system needs to be adapted into a more sustainable one, based on a diverse mix of energy sources, in particular renewables, and among them biomass; enhancing power generation efficiency, proposing new energy vectors to improve effectiveness of renewables; addressing the pressing challenges of security of supply and climate change, whilst increasing the competitiveness of Europe's industries. Biomass gasification is a thermo-chemical conversion process utilizing air, oxygen and/or steam as gasification agents, which produces a fuel gas rich in hydrogen and carbon monoxide, with a significant content of methane; carbon dioxide, steam and nitrogen are also present in the producer gas, in addition to organic (tar) and inorganic (H2S, HCl, NH3, alkali metals) impurities, and particulate. High molecular weight hydrocarbons are an undesirable and noxious by-product (Bridgewater, 20031 ), the yield of which can be reduced by careful control of the operating conditions (temperature, biomass heating rate, etc.), appropriate reactor design, and a suitable gas conditioning system (Simell et al., 19962 ; Caballero et al., 20003 ; van Paasen and Kiel, 20044 ). Biomass gasification plants have been realised and are operating in different European countries, demonstrating the development of the integration among gasification, gas cleaning, power and biofuel generation, and the potential of these technologies to contribute significantly to sustainable energy vectors production, first of all hydrogen.
Gas cleaning is normally done by filtration and scrubbing of the producer gas, to drastically reduce particulate and tar content: in this way the clean gas is made available at temperatures close to ambient, which makes the gas not suitable for further hydrogen purification because the process efficiency would result dramatically low. This penalizes notably the possibility to exploit a renewable and abundant source like biomass to produce hydrogen. In addition, tar separation is sometimes not as effective as it should be, reduces the gas yield, originates waste streams difficult to dispose or recycle properly. It is worth mentioning here that complications in the plant scheme originated by gas treatments required to obtain a tar-free product often contribute significantly to the overall investment and operating cost. This is even more negative when it is considered that small- to medium-scale gasification plants would fit optimally with the economic context of most regions, for a number of reasons ranging from the biomass transportation cost (biomass is a dilute energy source in comparison with fossil sources), to the need to establish a distributed power generation system, to scarce social acceptability of large thermal conversion plants. As a result, process simplification and intensification could play a very important role towards a real breakthrough in the utilization of biomass in general, and specifically of gasification plants. Furthermore the conventional small-to-medium scale gasification technologies utilize fixed bed reactors and air as gasification medium. This often results in low conversion efficiency and in a syngas with a low fraction of hydrogen, because the nitrogen contained in the gasification medium dilutes the wood gas produced. Hydrogen purification of such a kind of gas would require enormous energy consumption and thus low conversion efficiency.
Steam blown indirect heated biomass gasification can solve this problem in an excellent way. The well known CHP plant in Gussing is a very successful industrial application of such gasification technology. The capacity of the Gussing plant is about 8 MW (electrical output of 2 MWel and district heating output of approximately 4.5 MWth). Gasification is carried out in a steam blown dual fluidized bed reactor and known internationally under the name FICFB-gasification system (Hofbauer et al., 20025 ; Hofbauer and Knoef, 20056 ). The basic idea behind this gasification system is to physically separate the gasification and the combustion reactions, in order to obtain a largely nitrogen-free product gas. The endothermic gasification of the fuel takes place in a stationary bubbling bed fluidized with steam. This is connected via a chute to the combustion section that is operated as a circulating bed fluidized with air. Here, any non-gasified fuel particles transported along with the bed material are fully burnt, together with additional fuel properly injected, to provide the heat required by the gasification reactions. The heated bed material is then separated by a cyclone and brought back into the gasification section. The calorific value of the producer gas is 12 a 14 MJ/Nm3; it can either be used in a gas engine or upgraded to synthesis gas. The construction of the Gussing plant was started in September 2000 and the present utilization index is above 7000 hr/year. This gasification method combines the advantages of the utilization of steam as the gasification agent (lower tar) and a nitrogen-free product gas (higher calorific value and high hydrogen content). These are made possible by the separation of gasification and combustion zones (two different gas output streams), and a strict integration between them. The result of the application of this technology is a syngas with a composition and a hydrogen content similar to that obtainable by steam methane reforming, that is actually the most straightforward way to produce hydrogen. The main differences among syngas produced by biomass gasification and methane steam reforming still remain however the tar content in the gas and the particulates derived from the utilization of a solid fuel, in addition to the different composition of biomass wastes that obviously affects the final gas composition. Still, the main drawback of the technology (FICFB) described above remains the size of the gasification plant, which is related to the complex recirculation of bed material between the combustion and gasification chambers. In order to really match steam gasification of biomass with decentralized hydrogen generation requirements, the size of the plant must be reduced to 1 MW and lower. Thus CIRPS has started in these years the realization of a small-scale steam blown indirect heated biomass gasifier (100 kWth), based upon the gasifier concept developed in the FP7 project UNIQUE (contract 211517). The main advantage of this system (DBHbF-Double Bubble HybioFlex) is its compactness :it is made of "one" reactor divided in two compartments, both containing bubbling fluidized beds that assure an easy recirculation of bed material and a great thermal efficiency, in a way to be technically and economically feasible also at small scale. This gasifier Unique 100 is now being patented and the prototype is being tested at the Hydrogen Centre of Civitavecchia7, Italy. An application of this reactor connected to a SOFC unit is already planned at Civita di Bagnoregio, Italy.
An alternative process scheme that allows to obtain a producer gas of comparable quality to that of steam blown indirect heated biomass gasifier is offered by biomass gasification with oxygen and steam, performed in a reactor with a single gaseous output stream (Gil et al., 19978 ): a well known application is the revamping of the pressurised VĂ¤rnamo plant at VVBGC (Albertazzi et al., 20059). In this case, the advantages of a gasification system less complex to design, to construct and to operate are counterbalanced by the need to utilize oxygen instead of air. The cost of oxygen has been incrementally reduced over the years today the vast majority of coal gasification processes use oxygen blown gasifiers (Shelley, 200610) â€“ so that this option will certainly become increasingly attractive in the medium term. An atmospheric steam/oxygen gasifier (1 MW load) available in the Trisaia research centre of ENEA (the Italian National Agency for Renewable Energy and the Environment) was modified to become a prototype, at pilot scale, of the integrated gasification technology, developed through the FP7 UNIQUE project, that will be used in this project. This is a stationary, bubbling fluidized bed, where internal circulation of the solid inventory around a vertical draft wall is generated by different fluidizing velocities kept on both sides of it, in order to improve light fuel particle mixing inside the much denser granular bed of mineral matter (Foscolo et al., 2007a11 ): sawdust and rice husks air gasification tests, carried out in China (Liaoning Institute for Energy Resources) with a pilot reactor of this kind, exhibited good temperature homogeneity throughout the system (Xiao-hua et al, 200512).
As mentioned above, it is universally recognised that, to increase the conversion efficiency of the utilization of thermal and chemical energy of the producer gas, hot gas cleaning and conditioning systems (abatement of particulate content and tar conversion at a temperature close to the gasification temperature) should be developed and implemented through a compact design and reliable and simple-to-operate equipment. This is even truer in the specific case of steam gasification and its coupling with a purification system for hydrogen production, to avoid loss by condensation of the significant amount of water vapour contained in the gas stream, useful to reform residual CH4, shift CO towards H2 and prevent carbon deposition on the catalytic surfaces.
The fact that hot gas cleaning is a focal point of the applied research in the gasification field is testified, beyond a large number of literature papers, by the European projects funded in FP6 (CHRISGAS, BIGPOWER, AER-GAS II, GREEN FUEL CELLS, BIOCELLUS), all dealing with this issue in one way or the other, and by the presence of research topics specifically addressed to it in the FP7 calls. Among alternative hot gas conditioning methods, catalytic cracking and steam reforming of high molecular weight hydrocarbons offer several advantages, such as thermal integration with the gasification reactor, high tar conversion and hydrogen rich syngas production. A large number of investigations deals with biomass gasification in fluidized bed reactors utilizing dolomite ((Ca, Mg)CO3) or olivine ((Mg, Fe)2SiO4), calcined dolomite, limestone and magnetite have been found able to increase the gas hydrogen content (Delgado et al., 199713). Olivine shows a slightly lower activity in biomass gasification and tar reforming, but higher attrition resistance than dolomite (RapagnĂ et al., 200014); the impregnation of olivine with additional iron leads to a substantial improvement in the reforming activity of this primary catalyst without any harmful effect for the environment and cost increase15. Ni-based reforming catalysts have shown high activity and selectivity for tar, methane and CO conversion to hydrogen-rich gas, but suffer from (i) mechanical fragility, (ii) rapid deactivation mostly due to sulphur, chlorine, alkali metals, coke, (iii) metal sintering, altogether resulting in limited lifetime (Bain et al., 200516 ).
Catalytic filters have been proposed as an alternative, very promising technology to be coupled to biomass gasification processes. The development of a suitable tar reforming catalyst for integration in a ceramic hot gas filter element has been pursued according to different and alternative procedures. The methodology developed by Pall Filtersystems GmbH Werk Schumacher requires a modification of the design of the ceramic hot gas filter candle to allow the integration of a sufficiently high amount of reforming catalyst by using impregnation techniques. Several tar reforming catalyst systems with different NiO loadings and different catalyst support materials have been tested with synthetic gases, obtaining complete conversion of naphthalene at 800Â°C and in presence of 100 ppmv of H2S (Nacken et al, 200717). Tests at real process conditions indicate that a nickel-based catalytic filter material can be used successfully in integrated high temperature reforming of tars and removal of particles from biomass gasification product, by means of a prototype candle inserted in the freeboard of a fluidized bed steam gasifier (RapagnĂ et al, 2009 and 2010a/b18). In the past 7FP UNIQUE project, the technical and economic optimisation of such nickel catalytic filters was performed, by studying the addition of a second, low cost metal as a dopant to improve catalytic activity. In fact, association of nickel and iron improved tar reforming on the filter elements to level of tar lower than 0.1 g/Nm3. UNIQUE project, 7FP-ENERGY-2007-211517, as already mentioned in point 1.1, has demonstrated the feasibility of an innovative technology for the production of clean syngas rich in hydrogen and with the specifications required for use in HT-FC in a cost-effective way.
Up to date, the most effective way to produce pure hydrogen for PEM application is the steam reforming of methane with successive purification by means of HT/LT WGS and PSA. Since 50 years these technologies are commonly used for large scale application, while since a few years they are also available for small scale application. An example is that offered by HyGear. HyGear offers a product line of on-site hydrogen generators called HGS systems. The products are based on HyGear's proprietary Reforming Technology combined with highly efficient Pressure Swing Adsorption Technology. HyGear already commercializes systems that can produce hydrogen up to 100 Nm3/h with a purity of 99.999% exploiting proprietary high selective Pressure Swing Adsorption that can work at relative low pressure (10 bar) with a small pressure gap between the upstream and downstream side of PSA.
As has been already mentioned, the UNIQUE technology can produce a syngas similar to that obtainable by steam methane reforming with no sulphur and alkali compounds thanks to optimized sorbents and with low amount of tar and methane and no particulates thanks to the catalytic filter candle: the coupling of UNIQUE technology with HGS is thus feasible.
In order to reach a hydrogen conversion efficiency higher than 66%, a WGS intermediate step (between UNIQUE and PSA) is required anyway. Today the industrial implementation of WGS takes place usually in a series of adiabatic converters where the effluent from the reformer system is converted in two steps with the second at a signi?cant lower temperature in order to shift the equilibrium towards the favoured hydrogen product. The conventional WGS reactors are used for large scale application and operate at high pressure and thus they are not suitable to be coupled with atmospheric pressure gasification. On the other hand, the enormous equipment costs make pressurized gasification a remote option for small-to-medium scale plants. A practical solution would be to pressurize the gas after gas cleaning, this however would require gas cooling (and thus the condensation of water contained in the gas). Additional energy for the vaporization of steam would be thus necessary.
In order to realize a WGS reactor working at atmospheric pressure and to increase the efficiency of the gas-solid contact (catalytic surface area) all catalysts could be impregnated and supported on ceramic foams. PALL is a global leader in the manufacturing of different porous ceramic media e.g. based on SiC, Al2O3, SiO2 and SiO2-Al2O3 of different shape and porosity, which, besides hot gas filtration applications, are also used as support materials for catalytic applications. In order to increase conversion efficiency at atmospheric pressure, a WGSR based on catalytic foam will be placed downstream of the UNIQUE gasifier. The hydrogen rich gas at the outlet of WGSR will be compressed at relatively low pressure to feed the HyGear PSA before the final compression of hydrogen for storage in the filling station. The purge gas of PSA, containing residual CO, CH4 and H2, will be recirculated to the gasifier to provide heat for endothermic reactions in the plant. The great integration of the various subsystems will guarantee conversion efficiency higher than 66%. The above mentioned gasification technologies based on steam/air and steam/oxygen, respectively, will be investigated in this project thus to obtain a wide experimental evidence on the whole process at different scales and gasification conditions.
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