Catalysts can play an important role in the gasification process value chain. Catalysts can be used to speed up the gasification process while lowering the temperature needed to complete the conversion (see pages Catalytic Gasification and Great Point Energy). The synthesis gas (syngas) produced from gasification is not always the final product. The syngas can be used to create a number of additional products such as liquid fuels, synthetic natural gas, ammonia, methanol and higher molecular weight mixed alcohols and various chemicals. Catalysts are required for the economic conversion of syngas into these desired products.
Various catalyst systems are being used within a gasification value chain. Examples include ruthenium-based catalysts, methanol catalysts, Fischer-Tropsch catalysts, and molybdenum-sulfide based catalysts.
Ruthenium based catalysts are used primarily in the production of ammonia. It has been shown that ruthenium catalysts provide five to 10 times higher reactivity rates than other catalysts. However, ruthenium is not widely used because that it quickly becomes inactive due to its necessary supporting material, activated carbon. Ruthenium catalysts utilize a bed of activated carbon to achieve its most effective reactivity. During catalysis, the carbon becomes consumed, reducing the effect of the ruthenium catalyst. Given the substantially higher reactivity of ruthenium when it comes to the creation of ammonia, research is being done to reduce the speed at which it becomes inactive. Ruthenium disulfide (RuS2) is highly reactive in terms of hydrodesulphurization. However it is not commonly used due to the rate it becomes inactive. Ruthenium is also a methanation catalyst.
Methanol Catalysts are used to convert syngas into methanol. These catalysts commonly include mixtures of copper oxide, zinc oxide, alumina, and magnesia. Recent advances have also yielded a possible new catalyst composed of carbon, nitrogen, and platinum. This catalyst is based on an earlier catalyst created by Dr. Roy Periana of the Scripps Research Institute. This newer catalyst is a solid material that is suspended in sulfuric acid to aid in the catalysis. The material is easily recyclable as it can be filtered from the acid.
Fischer-Tropsch catalysts are used in the chemical conversion known as the Fischer-Tropsch (FT) process. These catalysts convert syngas into various forms of liquid hydrocarbons that can be used in other reactions to reach the desired product. These catalysts are commonly either iron or cobalt based. Nickel and ruthenium have also been used, but these materials have proven to be much less effective and cost-efficient. The ideal FT catalyst will maximize liquid products while minimizing gas production required, and LPG and carbon dioxide (CO2) production.
Cobalt is the more commonly used FT catalyst. When natural gas is the feed stock the produced syngas has a high hydrogen (H2) to carbon monoxide (CO) ratio. Cobalt is more active, and has lower water gas shift (WGS) activity. Cobalt gives better selectivity to liquid products, less light ends production and more stability.
Iron based FT catalysts tend to be used when processing coal based syngas with a low H2/CO ratio. Iron promotes the water gas shift reaction, therefore, it is more suited for a syngas with a lower H2/CO ratio. Iron is also less expensive than cobalt. Both cobalt and iron have to be reduced before being used as a catalyst. However, since iron is able to operate at a higher temperature than cobalt, iron is able to be reduced inside the reactor as opposed to cobalt which must be reduced before entering the reactor.
Molybdenum-sulfide catalysts (MoS2) are used primarily in the hydrodesulphurization, removal of sulfur, of feed gases. The most common catalysts used in this category contain a combination of cobalt and molybdenum-sulfide (Co-MoS2). These catalysts can also be doped with potassium to increase the reactivity. The Co-MoS2 variety of molybdenum-sulfide catalysts is often favored due to its increased resistance to sulfur poisoning, which renders the catalyst inactive. This allows the catalyst to function in high concentrations of sulfur. Typically, the Co-MoS2 catalysts are supported by an underlying structure of alumina (Al2O3). In addition to cobalt, nickel and tungsten are also used in conjunction with MoS2 to form the catalysts. These other MoS2 catalysts may allow for more efficient hydrodesulphurization of specific types of feed gases. MoS2 catalysts with cobalt and potassium are also used in the synthesis of C1 to C4 mixed alcohols.
The catalytic process requires that the syngas be passed through one or more reactors. The number of necessary reactors depends on the number of chemical conversions that the syngas is required to undergo.
There are two main types of reactors in use: fixed-bed reactors and fluid-bed reactors:
Fixed Bed Reactor
Fixed-bed reactors have long been used in various commercial industries. They contain the catalyst, typically in pellet form, in a fixed location. The syngas is then passed over the catalysts, allowing the reactions to take place. Originally, fixed-bed reactors were the only commercially viable reactor type due to technological limitations. However, they also presented drawbacks mainly in the form of access to the catalyst material. Since the gas has to pass over the material the reaction is limited by the available surface area. This problem can be reduced by allowing more than one “bed” in the reactor for the gas to pass over, under, and/or through. The catalysts in fixed-bed reactors do not need to be as resilient, as they do not move in the bed. Due to the reaction process being exothermic, fixed-bed reactors need to undergo a cooling process after a period of operation. If the excess heat is not dissipated, it could eventually lead to the catalyst material becoming inert. Fixed-bed reactors can be equipped with internal tubes where a heat transfer fluid, such as boiler feed water, can circulate inside the tubes to control the temperature rise in the reactor.
Liquid Bed Reactor
As opposed to fixed-bed reactors, fluid-bed reactors contain their catalyst material in a fluid state instead of fixed-beds. This fluid state can take on many different forms from having the catalyst being in an inert liquid, such as mineral oil, to having particles of the catalyst suspended as the gas is pushed through the reactor. There are several types of these reactors such as entrained bed, fluidized-bed, ebbulated bed or slurry bed depending on the velocity of the fluid in the reactor.
Fluid-bed reactors solve some of the problems that are present in fixed-bed reactors. They allow for a better access to catalyst material since the catalyst particles are thoroughly mixed with the gas allowing for the greatest possible surface area for reactions to take place. Heat issues are also reduced due to the heat transfer properties inherent in utilizing a fluid state for the catalyst bed. This also includes reducing the potential for heat gradient build ups that can occur when the catalyst is immobile as in a fixed-bed. Fluid-bed reactors can operate almost isothermally. They can also have internal tubes where heat transfer fluids can remove excess heat.
In fluid-bed reactors, catalysts can be removed on line for regeneration or replacement without shutting down the reactor. A fixed-bed reactor must be shutdown to regenerate the catalyst or add new catalyst.
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