Carbon monoxide as fuel

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

An experimental fuel cell has been designed that uses carbon monoxide as fuel. The overall reaction is... 

Carbon Monoxide. Carbon monoxide, a fuel in high-temperature cells (MCFC and SOFC), is preferentially absorbed on noble metal catalysts that are used in low-temperature cells (PAFC and PEFC) in proportion to the hydrogen-to-CO partial pressure ratio. A particular level of carbon monoxide yields a stable performance loss. The coverage percentage is a function of temperature, and that is the sole difference between PEFC and PAFC. PEFC cell limits are < 50 ppm into the anode major U.S. PAFC manufacturers set tolerant limits as < 1.0% into the anode MCFC cell limits for CO and H20 shift to H2 and C02 in the cell as the H2 is consumed by the cell reaction due to a favorable temperature level and catalyst. 

In practice, the power obtained from using the separated carbon monoxide as a fuel is sufficient to run all... 

Molten carbonate (MCFC). The cell operates at 650°C and uses hydrogen or carbon monoxide as anode fuel, which reacts with carbonate... 

For more than half a century this theory was accepted almost without question,3 but in 1872 Sir Lowthian Bell concluded that the theory was inadequate in so far as the combustion of coke in a blast furnace was concerned. He suggested that carbon monoxide and not carbon dioxide is the chief, if not the exclusive and immediate, action of the hot blast on the fuel. If this view is accepted, carbon dioxide is to be regarded as an oxidation product of the monoxide rather than of carbon itself, and carbon monoxide as the primary oxidation product of carbon instead of a reduction product of the dioxide. Thus ... 

Carbon dioxide and water are the main products of this reaction. However, incomplete combustion causes some emissions of unbiuned hydrocarbons, as well as intermediate oxidation products such as alcohols, aldehydes and carbon monoxide. As a result of thermal cracking reactions that take place in the flame, especially with incomplete combustion, hydrogen is formed and emitted, as well as hydrocarbons that are different from the ones present in the fuel. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

A fast, reliable, and specialized CFD model for PEM fuel cell simulation can be very useful in fuel cell design optimization and operation control. In this project, a unified PEM fuel cell simulation model has been successfully established. This project started in FY 2000 with 2-D single-phase models. In FY 2001, the 2-D models were successfully transformed into a unified 3-D model for hydrogen feed. In FY 2002, this established 3-D model was extended to include reformate feed, accounting for the poisonous effect of carbon monoxide as well as the dilution effect of the reformate gas stream on the anode side. Based on this 3-D model with the geometry of a single fuel cell, a preliminary stack model was established. Extensive experiments in our lab and industry interactions were carried out to improve and calibrate the computation model. 

The thermochemical liquefaction of wood in water to produce fuel or chemical intermediates has been studied intensively over the last decade ( l). The earlier works used either sodium carbonate as soluble catalyst and carbon monoxide as reducing gas ( ) ( ), or nickel catalyst ( U) or palladium on activated charcoal ( 5) in the presence of hydrogen. Then it has been shown that the presence of a reducing gas was not necessary if iron powder was used as additive, with moderate heating rates ( ). When the wood suspended in water was rapidly heated to 350°C, and then quenched, no catalyst was necessary and a yield in acetone solubles as high as 50 wt was obtained (T ) ( ). 

The platinum-ruthenium combination has been of interest in fuel-cell technology, because ruthenium imparts some resistance to poisoning by carbon monoxide as with platinum-iridium, it is important to see whether joining metals of very different activities in hydrogenolysis creates binary centres having new properties. The few available studies ° ° all confirmed ruthenium s superior activity for hydrogenolysis (except for cyclopentane ) and its inability to do much else under normal circumstances. It does however induce other reactions characteristic of platinum at a temperature (493 K) well below that at which that metal would be active by itself. With Pt/AlaOs,X-ray absorption spectroscopy showed... 

Whereas most of the methanol produced since the thirties until 1980 has been used to produce formaldehyde, a remarkable shift in the pattern of methanol use has occurred. Nowadays, use of methanol for chemical products other than formaldehyde has risen more steeply than for formaldehyde itself. More than all others, the increase of acetic acid production going together with its shifting from ethylene to methanol and carbon monoxide as raw materials has contributed to this increase as well as the production of fuel components such as MtBE. 

Ernst et al. (1999) suggest that the fuel processing system be comprised of a steam-reforming reactcn, water-gas shift (WGS) reactors (at high and low temperatures), and a preferential oxidation (PROX) reactor to oxidize carbon monoxide, as shown in Figure 19.17. Kinetic data are provided for the reformer and the two shift reactors. 

Typically, approximately 70% of the heating value of the feedstock fuel is associated with the carbon monoxide and hydrogen components of the gas, but can be higher depending upon the gasifier type. Hydrogen must be separated from the gas product stream (which also contains carbon dioxide and carbon monoxide as well as other trace contaminants) and polished to remove remaining sulfur, carbon monoxide, and other contaminants to meet the requirements for various end uses. 

Adachi et al. [168] developed a model for a natural gas fuel processor composed of an ATR designed as metallic foam monolith coated with catalyst and two-stage WGS reactors also designed as foam monoliths followed by two-stage ceramic monoliths for the preferential oxidation of carbon monoxide as shown in Figure 14.27. Figure 14.28 shows the course of temperature and gas composition of feed and reformate as calculated for... 

MGFGs are also able to use carbon oxides as fuel. They are not poisoned by carbon monoxide or carbon dioxide, thus MGFGs are advanced to use gases from coal so that they can be integrated with coal gasification. 

Cutillo et al. also analysed the effect of introducing a carbon monoxide tolerant fuel cell into the system, which would make the overall system less complex [443]. Because such fuel cells were expected to be less efficient, about 3% lower efficiency was assumed. Another potential simplification was the removal of one of the water-gas shift reactors. The two stage water-gas shift reactors could be replaced by a medium temperature water-gas shift reactor with higher carbon monoxide outlet concentration in combination with the high carbon monoxide tolerant fuel cell. Alternatively, a water-gas shift reactor with heat-exchange capabilities, as discussed in Section 5.2.1, could be placed into such a system and combined with preferential oxidation and low temperature PEM fuel cell technology. 

The phosphoric acid fuel cell (PAFC) was the first fuel cell to be commercialized and shares some technologies with the PEMFC, such as the porous electrodes and the platinum catalysts. The liquid phosphoric acid allows high operating temperatures, around 200 C. Fuels must be free of carbon monoxide, as with the PEMFCs. With rated power over 50 kW, PAFC systems are used for stationary applications. 

Not shown in Figure 10.14 is CO (carbon monoxide). As the air-to-fuel ratio approaches the stoichiometric mixture, small amounts of CO will be detectable in the flue gas. This will increase markedly as air rate falls below the minimum required. The CO measurement cannot be used standalone because, like O2, it only indicates over part of the operating range - showing zero no matter how much excess air is supplied. 

From the very outset of MCFC development, research workers were attracted by the fact that not only hydrogen but also carbon monoxide (CO) could be used as a reactant fuel (reducing agent). Carbon monoxide (as water gas, a mixture of CO and H2) is obtained readily by the steam gasification of coal ... 

Some of the high-temperature fuel cells described in Chapter 7 can use this carbon monoxide as a fuel. However, fuel cells using platinum as a catalyst most certainly cannot. Even very small amounts of carbon monoxide have a very great effect on the anode. If a reformed hydrocarbon is to be used as a fuel, the carbon monoxide must be shifted to carbon dioxide using more steam... 

The effect of the carbon monoxide is to occupy platinum catalyst sites - the compound has an affinity for platinum and it covers the catalyst, preventing the hydrogen fuel from reaching it. Experience suggests that a concentration of carbon monoxide as low as 10 ppm has an unacceptable effect on the performance of a PEM fuel cell. This means that the CO levels in the fuel gas stream need to be brought down by a factor of 500 or more. 

These cells operate at much higher temperature, which assists the kinetics of the electrode reactions. Also, the anode is capable of using either hydrogen or carbon monoxide as the fuel ... 

A fuel cell power generation system consists of several components besides the fuel cell such as a fuel processor and a power conditioner/inverter. The fuel processor is the first step of the conversion of fuel into an electrical current Typically, a fuel processor utilizes a combination of steam reforming (SR) and partial oxidation (POX) methods to convert hydrocarbons (methane, natural gas) into the pure hydrogen necessary as input to the fuel processor. During this process, the fuel processor also should strip the input gas of its pollutants such as carbon and carbon monoxide. The fuel processor is one of the areas in which the greatest environmental threat can occur because of this. There are a number of other considerations to be taken into account when examining the environmental impact and life cycle assessment of fuel cell power generation system such as axillary equipment and their economic and environmental impact (Kordesch and Simader 1995 van Rooijen 2006 Tromp 2002). 

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