Various fuels processor

Figure 4. Speciated Hydrocarbon Composition of Air Toxics Samples from the Fuel Processor or Tail Gas Combustor at Various Fuel Processor Loads...

Although a fuel cell produces electricity, a fuel cell power system requires the integration of many components beyond the fuel cell stack itself, for the fuel cell will produce only dc power and utilize only processed fuel. Various system components are incorporated into a power system to allow operation with conventional fuels, to tie into the ac power grid, and often, to utilize rejected heat to achieve high efficiency. In a rudimentary form, fuel cell power systems consist of a fuel processor, fuel cell power section, power conditioner, and potentially a cogeneration or bottoming cycle to utilize the rejected heat. A simple schematic of these basic systems and their interconnections is presented in Figure 9-1. 

There are three major gas reformate requirements imposed by the various fuel cells that need addressing. These are sulfur tolerance, carbon monoxide tolerance, and carbon deposition. The activity of catalysts for steam reforming and autothermal reforming can also be affected by sulfur poisoning and coke formation. These requirements are applicable to most fuels used in fuel cell power units of present interest. There are other fuel constituents that can prove detrimental to various fuel cells. However, these appear in specific fuels and are considered beyond the scope of this general review. Examples of these are halides, hydrogen chloride, and ammonia. Finally, fuel cell power unit size is a characteristic that impacts fuel processor selection. 

Other Although not R D, it should prove beneficial for fuel cell developers to provide species tolerance specifications to fuel processor developers established by standard definition, determination methods, and measurement procedures. This would aid the fuel processor developer to develop products compatible with various fuel cell units. Of particular importance are sulfur and CO limits. 

The following examples are presented for individual unit operations found within a fuel cell system. Unit operations are the individual building blocks found within a complex chemical system. By analyzing example problems for the unit operation, one can learn about the underlying scientific principles and engineering calculation methods that can be applied to various systems. This approach will provide the reader with a better understanding of these fuel cell power system building blocks as well as the interactions between the unit operations. For example, the desired power output from the fuel cell unit operation will determine the fuel flow requirement of the fuel processor. 

This chapter deals with development work in the field of micro structured fuel processors, which convert various fuels to hydrogen for fuel cells and other power generation modules. 

Cheap fabrication techniques feasible for mass production need to be applied for fuel processors. The various techniques available for micro structured fuel processors will be discussed in Section 2.9. 

Another option is direct heating through the flow path of the fuel by applying catalytic or conventional burners, which may be either part of the power supply system of the fuel processor or additionally installed devices (start-up burner). By direct contact of the combustion off-gas with the various devices, rapid start-up of even larger scale fuel processors seems to be feasible [10], However, a major drawback is the restriction to catalyst systems which can tolerate hot combustion off-gases. 

We have noted earlier that a refiner or fuel processor must live in an uncertain environment. He is subject to the vagaries of the supply of crude, the requirements of the market, and the perpetual question of the future markets for residual fuel. We have developed a processing approach—using the H-Oil process— which provides the degree of flexibility necessary to cope with this uncertain environment. A schematic flow diagram of such a multi-purpose plant is shown in Figure 8. The basic feature of this plant, which has been designed for the production of 0.3% sulfur fuel oil from various atmospheric residues, is its flexibility with respect to feedstock, product specifications, and future alternative uses of the plant. 

Nuvera will design, build, test, and deliver a 15 kilowatt electrical (kWe ) direct current (DC) fuel cell power module that will be specifically designed for stationary power operation using ethanol as a primary fuel. Two PEM fuel cell stacks in parallel will produce 250 amps and 60 volts at rated power. The power module will consist of a fuel processor, carbon monoxide (CO) clean-up, fuel cell, air, fuel, water, and anode exhaust gas management subsystems. A state-of-the-art control system will interface with the power system controller and will control the fuel cell power module under start-up, steady-state, transient, and shutdown operation. Temperature, pressure, and flow sensors will be incorporated in the power module to monitor and control the key system variables under these various operating modes. The power module subsystem will be tested at Nuvera and subsequently be delivered to the Williams Bio-Energy Pekin, Illinois site. 

Parallel heating of the various components is preferable to sequential heating, since parallel heating allows better temperature control of the hot gas and permits starting the fuel processor at partial capacity. 

During reforming, over 90 percent of the THC emissions were methane. The composition of the hydrocarbons was analyzed and the presence of toxic contaminants determined. Figure 3 shows the hydrocarbon fractions from fuel processor tests in terms of methane and NMHCs. Samples from these tests were also analyzed to determine the speciated hydrocarbon emissions profile over various operating loads. Figure 4 shows the results of the speciation analysis, in terms of the fraction of olefins and saturated and aromatic hydrocarbons that comprise the NMHCs. Aromatics and saturated hydrocarbons comprise almost all of the NMHC emissions. The fraction of NMHC as aromatics was close to the fraction of aromatics in the test gasoline for several samples taken from the PrOx and the tail gas combustor (TGC). The TGC burned reformer product gas. 

Figure 3. Hydrocarbon Mass Fraction of Fuel Processor Emissions at Various Stages...

Different fuels and components have been tested in automotive scale, adiabatic reactors to observe their relative reforming characteristics with various operating conditions. Ammonia (NH3) formation was monitored, and conditions were varied to observe under what conditions NH3 is made. Nitrogen-bound hydrocarbons were added to fuels to determine their effect on NH3 formation. Carbon formation was monitored during fuel processor operation by in situ laser measurements of the effluent reformate. Fuel composition effects on carbon formation were measured. 

For different applications, the power needed from the fuel cells varies from less than 1W for small applications such as sensors and mobile phones to over 100 kW for automobiles and stationary applications. With microreactors, hydrogen flows capable of producing power in the range from 0.01 W to 50 kW have been achieved [3]. Numerous applications of fuel conversion in microstructured devices have dealt with the combination with fuel cells to yield a power supply for microelectric devices and microsensors and as an alternative to a conventional battery. Thus, the resulting power output of the fuel cell has been in the low watts area, from 0.01 Wto a few watts, as in the integrated methanol fuel processors built by companies such as Casio and Motorola [4]. PNNL has developed various low-power portable fuel processor systems, from lower than 1W [5-7] to systems that could provide 15 W, such as a portable and lightweight system for a soldier portable fuel cell [8,9]. In the range of... 

When hydrocarbon distillate fuels such as gasoline, diesel or jet fuels are used, it is also necessary to include a desulfurization unit in the fuel processor system. Hence Shaaban and Campbell [26] patented a system with an effective sulfur removal process that was able to operate with various hydrocarbons. It consisted of a reformer with membrane purifier and water recovery, a catalytic combustor for fuel to provide the energy for the reforming process and the desulfurization unit. 

It is under development for various fuels such as methanol, ethanol, LPG and gasoline. The complete fuel processor was composed of a catalytic autothermal reformer reactor, a heat exchanger for cooling the reformate downstream of the CAR,... 

The Various Types of Fuel Cells and the Requirements of the Fuel Processor... 

Ahmed et al. [435] performed calculations to highlight the effect of the various operating parameters of an autothermal methane fuel processor on the overall water balance. The system considered by Ahmed et al. included an afterburner, which combusted the anode off-gas by cathode off-gas oxygen. Water was then recovered from the burner off-gas. As shown in Table 5.10, Section 5.4.1, the water balance improved when the S/C ratio was increased. 

Table 5.14 Comparison of product mass flow rates, reformer efficiency T f., fuel processor efficiency T fp and auxiliary power unit efficiency T)apu of autothermal reforming dry hydrogen molar fraction of the reformate yjj values are determined for various fuels, feed inlet...

Table 5.15 Comparison of product mass flow rates, reformer efficiency T r, fuel processor efRciency T fp and auxiliary power unit efRciency T apu of steam reforming values are determined for various, feed inlet temperatures Tjn and S/C (SCR) and O/C ratios (expressed as O2/C ratio OCR) as calculated by Specchia etal. [371]. is the fraction of the fuel which is fed to the steam reformer 1- Pr is fed to the burner yp is the dry hydrogen molar fraction of the reformate Wq shows the water balance of the systems, which is positive when the Wq exceeds unity. 

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