Efficiency fuel processor

Heat rejection is only one aspect of thermal management. Thermal integration is vital for optimizing fuel cell system efficiency, cost, volume and weight. Other critical tasks, depending on the fuel cell, are water recovery (from fuel cell stack to fuel processor) and freeze-thaw management. 

In 2000 GM announced a breakthrough catalyst system with the current generation gasoline fuel processor that achieved more than 80 percent efficiency. 

The fuel processor efficiency is size dependent therefore, small fuel cell power plants using externally reformed hydrocarbon fuels would have a lower overall system efficiency. 

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. 

Autothermal reforming provides a fuel processor compromise that operates at a lower 0/C and lower temperature than the POX is smaller, quicker starting, and quicker responding than the SR and results in good H2 concentration and high efficiency. A catalytic POX must be used to reduce the reaction temperature to a value compatible with the SR temperature. Once started, surplus heat from other parts of the unit can be sent to the ATR to increase its efficiency. 

When the function of a fuel processor is to convert a fuel to hydrogen, the fuel conversion efficiency is... 

A fuel processor for PEFC application contains sulfur removal, an ATR-enhanced UOB reformer, advanced shift reactors, a steam generation system, a product gas cooler, a PROX system, a gas compressor, an air compressor, an anode-off gas oxidizer, and a control system. Goal efficiency (LHV H2 consumed by fuel cell/LHV fuel consumed by fuel processor) is 69 to 72%. H2 concentration is presently >50% (dry). 

Recent fuel processor performance is summarized in Table 4. The fuel processors were operated at atmospheric pressure, and the water and methanol feed mixture was about 60 wt % methanol. The typical composition of the reformate stream was 72— 74% hydrogen, 24—26% carbon dioxide, and 0.5—1.5% carbon monoxide on a dry gas basis. The carbon monoxide levels were significantly below equilibrium (5.4% at 350 °C), but they still require additional cleanup for use in fuel cells. The fuel processor efficiency was calculated using eq 5 and was reported to be greater than 80%. It is interesting to note that increasing the power 5-fold, from 20 to 100 W, only resulted in a 50% increase in volume and a 33% increase in mass. 

A compact design for a gasoline fuel processor for auxiliary power unit (APU) applications, including an autothermal reformer followed by WGS and selective oxidation stages, was reported by Severin et al. [83]. The overall fuel processor efficiency was about 77% with a start-up time of 30 min. 

TABLE 7. Fuel processor and overall fuel-processing efficiency for natural gas... 

The overall efficiency of a fuel processor is commonly defined as the ratio of the lower heating value (LHV) of the hydrogen produced to the LHV of the fuel consumed ... 

Catalyst cost may play a significant role in the overall fuel processor cost and may reach values as high as 38% [4]. Here micro structured devices offer significant possibilities for cost reduction owing to the improved mass transfer in small channel systems, which allow for a higher efficiency of the catalyst. 

System efficiency is a major aspect determining the competitiveness of fuel processors. The theoretically possible fuel processor efficiency decreases with increasing number of carbon atoms of the fuel molecule. This effect will be discussed in depth in Section 2.4. 

Few data are publicly available on fuel processor efficiency. However, a typical value of 77.3% was determined over the New European Drive Cycle (NEDC) for a fuel cell drive system supplied by methanol [8]. Johnson-Matthey reports even 89% for its HotSpot module combined with a gas purification system [9],... 

A 200 mW methanol fuel processor was presented by Hu et al. [32], A 9% efficiency was determined for the device running at 1 vol.% carbon monoxide in the reformate stream. 

The total mass of a future 50 kW fuel processor was estimated to be 55 kg (see Table 2.10), which corresponds to a total energy demand of 7 MJ for start-up heating. From this, a power demand for the air blower, which has to provide some 22 m3 min-1 of air to the system during start-up, was calculated to be about 1 kW. As this power is only required during the rapid start-up of the system (60 s), a normal battery could provide the power. An efficiency of 78% was calculated for the entire future fuel processor. 

Additionally, highly efficient heat exchangers are required in fuel processors and the micro si rue lured plate heat exchanger design seems to be the best solution so far to maintain the crucial system efficiency competitive. 

Polymers are a viable material option for the fabrication of highly efficient low-temperature heat exchangers, designed to withdraw the last portion of energy out of the fuel processor off-gases before releasing them to the environment For small-scale MEMS-like systems, the materials commonly applied in this field are silicon [20, 71] (as a material with high heat conductivity) and silicon nitride [71] (as an insulation material). 

P. S. Chintawar, B. Bowers, C. O Brien, et ah, Advanced High Efficiency Quick Start Fuel Processor for Transportation Applications. Progress Report for Hydrogen, Fuel Cells, and Infrastructure Technologies Program. U.S. Department of Energy 2003. 

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