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Gasification of Biomass Seminar Report
Although biomass is getting increased attention as a renewable energy source, one of the remaining problems still to be solved is the reduction of the high level of tar present in the product gas from gasification of biomass.
Naturally occurring catalytic substances are employed in biomass steam-gasification processes to enhance the yield of fuel gas and reduce its tar content by cracking and reforming the high molecular weight organic components. Calcined dolomite is widely used for this purpose; it exhibits good catalytic activity under the operating conditions of the gasifier. However, due to its poor mechanical strength, it gives rise to a large production of fines in a fluidized-bed environment. This work reports an investigation into the catalytic behavior of olivine, common, naturally occurring mineral containing magnesium, iron oxides and silica: iron is known to play a positive role in tar decomposition reactions. The gasification runs, performed with a laboratory scale, biomass gasification unit, show that the olivine activity is close to that exhibited by dolomite under comparable operating conditions. Olivine has the additional advantage, however, that its resistance to attrition in the fluidized bed is much greater, similar to that of sand.
The aim of is to compare the application of olivine and dolomite as catalyst in real gasification condition using a fluidized bed gasifier.
In view of the worldwide concern about the depletion of fossil fuels and environmental problems associated with the use of these sources, renewable energy sources are getting increased attention. Biomass is considered as a potential source of renewable energy.To satisfy the energy requirements by preserving the environment, biomass sources appear as a good alternative to produce electricity, heat and fuel. Among different ways of biomass energy conversion, the gasification is considered a valid process option to obtain a fuel gas composed of hydrogen and carbon monoxide called syngas. The objective is to obtain a stoichiometric cleaned mixture of these gases for different applications. Nonetheless they may also contain by-products, in particular tars and methane. Removing of tar and methane can be well performed by the help of catalysts: either placed into the reactor (primary catalyst) and/or placed outside the reactor (secondary catalyst).
Gasification reactions are endothermic overall. In order to provide the necessary heat input, some air is introduced to burn part of the available fuel (biomass, char, and gas). This can give rise to a dilution with nitrogen of the product gas, lowering its calorific value; however, process configurations have been recently developed , which attempt to separate the combustion and gasification processes. These involve in principle two parallel fluidised zones, one fed with air, the other with steam; heat transfer from the combustion to the gasification zone is achieved by means of a high circulation rate of the bed inventory through the two zones. At the same time, the design is directed at minimising the contamination of the gasifier product gas with the flue gas from the combustion section.
In order to render the gaseous product acceptable as a fuel in a wide range of practical applications, the condensable tar must be converted to permanent gases by means of cracking and reforming reactions which have the additional effect of increasing the gas yield and overall efficiency of the gasification process . For this reason a number of current research programmes are focused on catalytic processes aimed at significantly lowering the temperature of the tar reforming reactions. Both manufactured catalysts and naturally occurring minerals known to promote tar cracking and reforming have been investigated for potential incorporation in the gasification process.
These catalysts include Ni-based catalysts, calcined dolomites and magnesites, zeolites and iron catalysts. The presence of additives not only influences the gas composition, but also the heating value of the product gas. The use of catalytically active materials during biomass gasification promotes the char gasification, changes the product gas composition and reduces the tar yield. Besides these, addition of active bed materials also prevents agglomeration tendencies and subsequent choking of the bed. Dolomite and Ni-based steam reforming catalysts have been proven to be very active in terms of tar reduction. A lot is known on the behaviour of dolomite with respect to tar cracking. It has been proven to be active in reducing tars produced in biomass gasifiers. However, besides good activity in terms of tar reduction, it has some critical limitations if used inside the gasifier. Dolomite is softer and thus gets eroded. Also, some dolomite particles break during the calcination and give rise to a large production of fines, leading to an increased carryover of solids from bed.
An alternative of dolomite can be naturally occurring particles of olivine, which is a mineral containing magnesium oxide, iron oxide and silica. Olivine is advantageous in terms of its attrition resistance over that of dolomite. It is these findings that led to the present investigation into olivine as a convenient substitute for dolomite in the biomass gasification process. Olivine consists mainly of a silicate mineral in which magnesium and iron cat ions are embedded in the silicate tetrahedral; it occurs abundantly in many geographic locations. Significant experimental evidence is reported below concerning the catalytic activity of olivine in the destruction of tars, also for its outstanding mechanical strength, comparable to that of sand, even at high temperatures; in this respect its performance is certainly superior to dolomite in a fluidised-bed environment.
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