Tar removal during biomass gasification

One of the major issues in the biomass gasification process is how to deal with the tar formed during the process. Tars can be easily defined as undesirable and problematic organic products of biomass gasification. There are a large number of different operational parameters that define composition and quantity of the produced tar, concerning both mentioned methods, such as temperature, pressure, gasifying medium, catalyst and additives used, equivalence ratio (ER), gasification ratio (GR), steam-to-biomass ratio (SB), gas residence time (or space time) etc. The tars can cause quite a few problems in the different applications such as cracking in the pores of filters, forming coke and causing plugging of the filters, condensing in the cold spots and plugging the cold spots; all this resulting in serious operational interruptions and maintenance costs. Another vital issue regarding tars is that they contain carcinogenic compounds that have to be removed to achieve health and environmental demands. Both physical and chemical treatment processes can reduce the presence of tar in the product gas.
The physical processes are classified into wet and dry technologies depending on whether water is used. Various forms of wet or wet/dry scrubbing processes are commercially available, and these are the most commonly practiced techniques for physical removal of tar. Wet physical processes work via gas tar condensation, droplet filtration, and/or gas/liquid mixture separation. Cyclones, cooling towers, venturis, baghouses, electrostatic precipitators, and wet/dry scrubbers are the primary tools. The main disadvantage to using wet physical processes is that the tars are just transferred to wastewater, so their heating value is lost and the water must be disposed of in an environmentally acceptable way. Wastewater that contains tar is classified as hazardous waste; therefore, its treatment and disposal can add significantly to the over-all cost of the gasification plant.

Dry tar removal using ceramic, metallic, or fabric filters are alternatives to wet tarremoval processes. However, at temperatures above 150°C, tars can become “sticky” causing operational problems with such barriers. As a result, such dry tar removal schemes are rarely implemented. Injection of activated carbon in the product gas stream or in a granular bed may also reduce tars through adsorption and collection with a baghouse. The carbonaceous material containing the tars can be recycled back to the gasifier to encourage further thermal and catalytic decomposition.Chemical tar treatment processes are the most widely practiced in the gasificationindustry. They can be divided into four generic categories: thermal, steam, partially oxidative, and catalytic processes.

Tars can be removed from the gas stream in the fuel reformer or by separate hot gas tar removal catalysts. Thermal destruction has been shown to break down aromatics at temperatures above 1,000 degC. However, such high temperatures can have adverse effects on heat exchangers and refractory surfaces due to ash sintering in the gasification vessel. The introduction of steam does encourage reformation of primary and some secondary oxygenated tar compounds, but has little effect on tertiary aromatics.There are two methods that have been used in research on catalytic tar conversion in laboratories worldwide.

The first method is with catalyst mixed with the feed biomass in so called catalytic gasification or pyrolysis (in situ). In this case tar is removed in the gasifier itself (usually in a fluidized bed gasifier). In the second method tar is treated downstream of the gasifier in a secondary reactor, outside of the gasifier (fixed bed catalytic reactor).There are a large number of different catalysts that have been used to eliminate the tars in the product gas from the gasification process. The two most researched groups are Ni-based catalysts and dolomites. When Ni-based catalysts are used, tar concentration in the product gas can be reduced significantly by means of reforming but since this process is endothermic, a part of the chemically bound energy of the gas has to be burned to sustain this process. This effect leads to a decreased efficiency of the gasification process.In contrast, when so called tar cracking catalysts such as dolomite are used, the only thing that is reformed is the tar itself while low hydrocarbons e.g. methane, ethane and propane are left intact. Simultaneously with this transformation of tar, the gas composition (CO2, CO, H2 etc.) changes as a consequence of reactions that will be described later in the text. Tar cracking can be defined as a process that breaks down the larger, heavier and more complex hydrocarbon molecules of tar into simpler and lighter molecules by the action of heat and aided by the presence of a catalyst but without the addition of hydrogen.

Dolomite is a calcium magnesium ore with the general chemical formula CaMg(CO3)2 with some minor impurities. In order for dolomite to become active for tar conversion, it has to be calcined. Calcination involves decomposition of the carbonate mineral, eliminating CO2 to form MgO-CaO, at high temperatures (usually 800-900 degC). The effective use of dolomite as a catalyst is restricted by relatively high temperatures and the partial pressure of CO2. When it comes to the importance of dolomites composition for catalytic activity, it has been shown that an increased content of iron in dolomites, i.e. Fe2O3, can raise its activity towards tar elimination by 20%.