NATURAL GAS PROCESSING TECHNOLOGIES FOR LARGE SCALE SOLID OXIDE FUEL CELLS

Robin Wang and Donald Rohr

General Electric Company 50 East Algonquin Road Des Plaines, IL 60017

Solid oxide fuel cell technology (SOFC) is one of the more

promising power generation concepts for a variety of stationary power applications. The attractive features of the SOFC are its solid-state construction (mainly ceramic), moderate operating temperature, and fuel flexibility. In general, hydrocarbon fuel is reformed to syngas before being fed to the fuel cell. Steam reforming, autothermal reforming, and partial oxidation are the most commonly used reaction technologies for producing syngas from hydrocarbon fuels. Each of these reforming technologies has received much attention in the research and patent literature. In this paper, the state of fuel reforming will be discussed. Steam Reforming (SR)

Catalytic steam reforming of natural gas is one of the most energy efficient ways to produce hydrogen and carbon monoxide. Steam reforming does not require the mixing of air in the reaction mixture and therefore produces higher H2 concentration in the reformed product. The elimination of oxygen from the initial fuel mixture improves the overall system efficiency by minimizing energy losses from catalytic combustion. Steam reforming does, however, require an external heat source due to the endothermic reactions that occur. This method for producing reformate can therefore only realize its advantage when effective heat utilization from the SOFC stack can be achieved. The cost of the conventional steam reforming catalysts is relatively low although they tend to be vulnerable to the sulfur-based catalyst poisons. SR technology is widely used in industrial syngas production at very large scale. Autothermal Reforming (ATR)

Autothermal reforming presents a flexible choice, providing reasonable hydrogen and carbon monoxide yields. The process is catalytic and involves input streams of both air and water that will react with the fuel stream to produce syngas. Effectively, an ATR combines the exothermic nature of a partial oxidation reaction (hydrocarbon fuel reacting with air) with the endothermic steam reforming reaction to balance the heat requirements. Reactions can occur on the same catalyst or on a steam reforming catalyst located in close proximity to the partial oxidation catalysts. The quality of the ATR reformate, defined in terms of hydrogen mole fraction, is thus superior to the CPOX reformate but not as good as the SR reformate. The advantage, though, is that we would have a thermally neutral system component, more responsive than a SR reformer, moderate in cost, size and weight requirements. On the downside, a more extensive control system is needed for ATRs to ensure robust operation of the fuel processing system.

Catalytic Partial Oxidation (CPOX) Catalytic partial oxidation uses reaction technology where the

hydrocarbon fuel is mixed with just enough oxygen to convert the carbon in the fuel to carbon monoxide. Fuel is reacted with air over a catalyst and the combustion is prevented from going to completion by controlling the amount of oxygen and residence time. Due to the fast reaction rates the CPOX reformer has short response times; these reactors are very compact and contact times are typically milliseconds. The CPOX system is comparatively more fuel flexible

than SR or ATR and can tolerate higher levels of sulfur contaminants in the hydrocarbon fuels. Two disadvantages of this technology are that the reformate has low hydrogen content and the high operating temperatures could lead to catalyst degradation.

Table 1. Advantages and Disadvantages of Steam Reforming

Characteristic Advantage Disadvantage Hydrogen Yield Generally higher

than 50% at T>600 oC for S/R =1

Potential high level of carbonaceous material formation

Heat Requirement Heat generated from SOFC can be used to drive SR reaction with overall higher system efficiency

External Heat transfer device is required, therefore results in system complexity and potential higher cost

Startup/transients Relative stable during transition operation.

Still needs external igniter to start up although the catalyst bed can be used for catalyst combustion tentatively. Heat transfer efficiency and higher volume makes the start-up slow.

Table 2. Advantages and Disadvantages of Autothermal

Reforming

Characteristic Advantage Disadvantage Hydrogen Yield About 50%

concentration Lower hydrogen yield than SR

Heat Requirement None May need startup heat, and control systems to switch between lean burning and ATR regimes

Startup/transients Moderate. Can be set up to fast response times by switching between CPOX and ATR ( relying in CPOX portion for the faster response time)

Transient fluctuations for load matching may be as much as 1-10 per second…. Such deviations will reflect on efficiency levels if we are switching between CPOX/ATR for responding to transients

Fuel Chemistry Division Preprints 2002, 47(2), 506

Fuel Chemistry Division Preprints 2002, 47(2), 507

Table 3. Advantages and Disadvantages of Catalytic Partial Oxidation

Characteristic Advantage Disadvantage Hydrogen Yield Relatively low yield

can be tuned by improving catalyst and convert some CO back to H2.

Heat Requirement No external heat required. The system id exothermic

The heat generated from the reaction needs to removed or utilized in the system.

Startup/Transients Startup is fast. Transient test is relatively easy to control.

High temperature startup/shutdowns may cause catalyst degradation.

Additional Startup is fast. Transient test is relatively easy to control.

High temperature startup/shutdowns may cause catalyst degradation.