Fuel Considerations

One of the great benefits of the SOFC is that it can utilise a wide range of fuels, as described in Chapter 12. The fastest reaction at the nickel anode is that of hydrogen. But other fuels can also react directly on the anode, depending on catalyst composition. For example, carbon monoxide can react on Ni/YSZ, but has a higher overpotential than hydrogen [3 5]. Also, methane can react on the 

Fuel reforming can also take place on nickel at the anode. This occurs when steam is added to the hydrocarbon fuel, typically at a ratio of 3 parts steam to 1 part of fuel. The reaction of methane is then given by 

The hj drogen and carbon monoxide released by this reaction can then react individually with oxide ions emerging from the electrolyte. Usually the CO conversion is sluggish so the shift reaction also occurs on the anode to produce more hydrogen  

It was demonstrated in the 1960s that hydrocarbons could be injected directly into SOFCs if steam was supplied [37]. The steam can beneficially be obtained from the spent fuel stream. The main problem with direct use of hydrocarbons is that coke can form to block up and contaminate the anode. There are two damaging reactions which can occur on the nickel  

When carbon formation was investigated in detail, by temperature-programmed reaction, three different types of material were discovered on the nickel, as indicated by the temperature required for oxidation [38]. The most stable carbon could not be removed below 1100 K and tended to form when current was flowing through the cell. 

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The technology for triglyceride production from microalgae has not yet been commercialized. Most of the economic analyses for the production of microalgal liquids reported in the literature indicate they are much too expensive to compete with petroleum fuels. Considerable additional research must be carried out to perfect the process despite the fact that research on microalgal fuel production has been in progress for at least the past two decades. 

Gas-solid reactions of the type disucssed in this section are of obvious importance in the combustion of coal and other solid fuels. Considerable experimental work has been done on this subject and there is an extensive literature. No attempt will be made to review this work here instead the reader is referred to a recent review article by Mulcahy and Smith [45]. 

Fuel switch. The choice of fuel used in furnaces and steam boilers has a major effect on the gaseous utility waste from products of combustion. For example, a switch from coal to natural gas in a steam boiler can lead to a reduction in carbon dioxide emissions of typically 40 percent for the same heat released. This results from the lower carbon content of natural gas. In addition, it is likely that a switch from coal to natural gas also will lead to a considerable reduction in both SO, and NO, emissions, as we shall discuss later. 

Utility costs vary enormously. This is especially true of fuel costs. Not only do costs vary considerably between different fuels (coal, oil, natural gas), but costs also tend to be sensitive to market fluctuations. Contractual relationships also have a significant effect on fuel costs. The price paid for fuel may depend very much on how much is purchased. 

The problem with this approach is that if the steam generated in the boilers is at a very high pressure and/or the ratio of power to fuel costs is high, then the value of low-pressure steam can be extremely low or even negative. This is not sensible and discourages efficient use of low-pressure steam, since it leads to low-pressure steam with a value considerably less than its fuel value. 

In the expression for heating value, it is useful to define the physical state of the motor fuel for conventional motor fuels such as gasoline, diesei fuel, and jet fuels, the liquid state is chosen most often as the reference. Nevertheless, if the material is already in its vapor state before entering the combustion system because of mechanical action like atomization or thermal effects such as preheating by exhaust gases, an increase of usefui energy resufts that is not previously taken into consideration. 

In the applications where the compactness of the energy conversion system is the determining factor as in the case of engines, it is important to know the quantity of energy contained in a given volume of the fuel-air mixture to be burned. This information is used to establish the ultimate relations between the nature of the motor fuel and the power developed by the motor it is of prime consideration in the development of fuels for racing cars. 

However, in some countries such as Germany there is considerable reservation to adding scavengers because of their possible contribution to dioxin emissions. Furthermore, for lead contents of 0.15 g/1, the need for scavengers is questionable. It is possible that the leaded fuels sold in the coming years will contain neither chlorine nor bromine. 

The gradual reduction and ultimate elimination of lead has seen considerable effort by the refiner to maintain the octane numbers at satisfactory levels. In Europe, the conventional unleaded motor fuel, Eurosuper, should have a minimum RON of 95 and a minimum MON of 85. These values were set in 1983 as the result of a technical-economic study called RUFIT (Rational Utilization of Fuels in Private Transport). A compromise was then possible between refining energy expenses and vehicle fuel consumption (Anon., 1983). 

Power output is controlled, not by adjusting the quantity of fuel/air mixture as in the case of induced spark ignition engines, but in changing the flow of diesel fuel introduced in a fixed volume of air. The work required to aspirate the air is therefore considerably reduced which contributes still more to improve the efficiency at low loads. 

For optimum combustion, the fuel should vaporize rapidly and mix intimately with the air. Even though the design of the injection system and combustion chamber play a very important role, properties such as volatility, surface tension, and fuel viscosity also affect the quality of atomization and penetration of the fuel. These considerations justify setting specifications for the density (between 0.775 and 0.840 kg/1), the distillation curve (greater than 10% distilled at 204°C, end point less than 288°C) and the kinematic viscosity (less than 8 mm /s at -20°C). 

This product, given the abbreviation FOD (fuel-oil domestique) in France, still held a considerable market share there of 17 Mt in 1993. However, since 1973 when its consumption reached 37 Mt, FOD has seen its demand shrink gradually owing to development of nuclear energy and electric heating. FOD also faces strong competition with natural gas. Nevertheless, its presence in the French, European and worldwide petroleum balance will still be strong beyond tbe year 2000. 

In 1993, French consumption of these products was around 6 Mt and 2.5 Mt respectively for use in burners and in diesel engines. The latter figure appears in the statistics under the heading, marine bunker fuel . Its consumption been relatively stable for several years, whereas heavy industrial fuel use has diminished considerably owing to the development of nuclear energy. However, it seems that heavy fuel consumption has reached a bottom limit in areas where it is difficult to replace, e.g., cement plants. 

The study of the relations between diesel fuel composition and pollution caused by the diesel engine is the focus of considerable attention, particularly in Europe where this line of thought has been rapidly developing in recent years. 

This justifies all the work undertaken to arrive at fuel denitrification which, as is well known, is difficult and costly. Moreover, technological improvements can bring considerable progress to this field. That is the case with low NO burners developed at IFF. These consist of producing separated flame jets that enable lower combustion temperatures, local oxygen concentrations to be less high and a lowered fuel s nitrogen contribution to NOj. formation. In a well defined industrial installation, the burner said to be of the low NO type can attain a level of 350 mg/Nm, instead of the 600 mg/Nm with a conventional burner. 

It also has lubricating properties similar to graphite. The hydrides are easily oxidized with considerable energy liberation, and have been studied for use as rocket fuels. Demand is increasing for boron filaments, a high-strength, lightweight material chiefly employed for advanced aerospace structures. 

Thorium, uranium, and plutonium are well known for their role as the basic fuels (or sources of fuel) for the release of nuclear energy (5). The importance of the remainder of the actinide group Hes at present, for the most part, in the realm of pure research, but a number of practical appHcations are also known (6). The actinides present a storage-life problem in nuclear waste disposal and consideration is being given to separation methods for their recovery prior to disposal (see Waste treati nt, hazardous waste Nuclear reactors, waste managet nt). 

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