Biomass gasification with air in a bubbling fluidized bed is studied in a small pilot plant. Variables analyzed are equivalence ratio (from 0.20 to 0.45), temperatures of the gasifier bed (750−850 °C) and of its freeboard (500−600 °C), H/C ratio in the feed, use of secondary air (10% of the overall) in the freeboard, and addition (2−5 wt %) of a calcined dolomite mixed with the biomass used as the feedstock. Using advanced tar and gas sampling and analysis methods, the gas composition and tar content in the gas are determined and their variation with the operation parameters is given. A statistical analysis of the effects of the gasification variables is also here presented.

The upgrading of the raw hot gas from a bubbling fluidized bed biomass gasifier is studied using cheap calcined minerals or rocks downstream from the gasifier. Biomass gasification is made with steam (not air) at 750-780 °C and about 0.5-1.0 kg of biomass/h. Calcined solids used are dolomite (MgO-CaO), pure calcite (CaO), and pure magnesite (MgO). Variables studied have been temperature of the secondary bed (780-910 °C), time of contact or space-time of the gas (0.08-0.32 kg‚h/m 3 n), and particle diameter (1-4 mm) and type of mineral. Their effects on tar conversion, tar amount in the exit gas, product distribution, and gas composition are presented. Using a macrokinetic model for the tar disappearance network, the activities of the stones are expressed by their apparent kinetic constant. Apparent energies of activation for tar elimination (42-47 kJ/mol) and preexponential and effectiveness factors are given for all tested solids of which the most active is the calcined dolomite.

Eight different commercial catalysts, nickel based, for steam reforming of naphthas and of natural gas are tested in biomass gasification for hot gas cleanup and conditioning. They were manufactured by BASF AG, ICI−Katalco, UCI, and Haldor Topsøe a/s. The catalysts were tested in a slip flow after a biomass gasifier of fluidized bed type at small pilot-plant scale (10−20 kg of biomass/h). The gasifying agent used is steam-oxygen mixtures. A guard bed containing a calcined dolomite is used to decrease the tar content in the gas at the inlet of the catalytic bed. Main variables studied are catalyst type, bed temperature, H2O + O2 to biomass feed ratio, and time-on-stream. All catalysts for reforming of naphthas show to be very active and useful for tar removal and gas conditioning (in biomass gasification). 98% tar removal is easily obtained with space velocities of 14 000 h-1 (n.c.). No catalysts deactivation is found in 48 h-on-stream tests when the catalyst temperature is relatively high (780−830 °C). Using a simple first-order kinetic model for the overall tar removal reaction, apparent energies of activation (of around 58 kJ/mol) and preexponential factors are obtained for the most active catalysts.

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Biomass gasification in a fluidized bed with steam−O2 mixtures has been studied in detail at pilot plant scale. The gasifier used was 15 cm i.d. and 3.2 m high, and it was fed with pine wood chips at flow rates of 5−20 kg/h. Main operating variables studied were gasifier bed temperature (780−890 °C), steam to oxygen in the feeding ratio (2−3 mol/mol), and gasifying agent (H2O + O2) to biomass fed ratio (0.6−1.6 kg/kg daf). Product distribution here shown includes gas, tar and char yields, gas composition (H2, CO, CO2, CH4, steam, ...) and heating value, tar composition and content in the flue gas, gas heating value, apparent thermal efficiency, etc. Under good operating conditions the following gas is obtained:  tar content of 5 g/Nm3, 30 vol % H2, heating value of 16.0 MJ/Nm3 (dry basis), gas yield of 1.2 Nm3 (dry basis)/kg biomass fed.

Calcined dolomites, limestones, and magnesites are active and inexpensive solids for cleaning raw hot gas from biomass gasifiers with steam. The variations of their activities with time-on-stream are studied here. Simultaneous coke formation and coke elimination by steam gasification increases the life of these “naturally occurring” catalysts under some circumstances. The lives of these solids are studied at different temperatures (800−880 °C), space times (0.08−0.32 kg of dolomite·h/nm3), particle diameters (1−4 mm), and types of solid. Not much deactivation was observed for tar concentration in the raw gas below 48 g/nm3, particle diameters of less than 1.9 mm, temperatures above 800 °C, and space times above 0.13 kg·h/nm3. The effectiveness of these calcined minerals is compared with that of an inert material (silica sand) and with a commercial steam reforming catalyst (R-67 from Haldor Topsøe).

The raw gas from a fluidized-bed biomass gasifier should have very low tar, ammonia, and particulates content, to make it easy to clean for its eventual use in gas engines or gas turbines. Besides an optimized design and operation of the gasifier, it requires the use of in-bed catalytic additives. Four available and competitive additives have been compared in this work:  a calcined dolomite (OCa·OMg), natural and sintered olivines ((Mg,Fe)2SiO4), and a Ni-olivine catalyst that was developed at the University of Strasbourg. They were tested under very similar experimental conditions in two small-scale pilot plants:  the first pilot plant was based on a circulating fluidized-bed (CFB) gasifier, and the second pilot plant was based on a bubbling fluidized-bed (BFB) gasifier. The tar content at the gasifier exit when using dolomite was, on average, only 60% (±10%) of the tar content when natural or raw olivine was used. This showed that dolomite was 1.40 times more active than olivine in biomass gasification with air. Nevertheless, dolomite generates 4−6 times more particulates or dust and also some extra NH3 in the gasification gas than olivine. Under the conditions used in this work (gasification with air), the Ni-olivine catalyst was not very active for tar elimination and it deactivated very quickly. Much of the data on gasification gas provided in this paper was obtained under operations similar to those found in commercial biomass gasifiers.