Steam reforming,

Steam Reforming. - The steam reforming reaction may be described by the general equation  

The CO formed may take part in two further reactions, the water-gas shift reaction  

Both of these reactions are exothermic and are favoured by reduction in temperature. Hence, while the products of the steam reforming reaction at higher temperatures ( 800 °C) are CO and H2, lower temperatures are used to produce methane-rich gases in this case, the overall reaction can be approximated by  

The thermodynamics of these reactions have been discussed in some detail elsewhere.2-4 

Steam Reforming. Kikuchi et al., reforming CH4, n-heptane, and other hydrocarbons, found that Rh and Ru were more active and more stable than 

which showed loss of activity at higher partial pressures of steam, probably due to oxidation.With Rh, AI2O3 was found to be a more stable support than SiOi, and the degree of dispersion of Rh on it was determined by Ha chemisorption and related to the activity of the catalyst for n-heptane reforming at 550—800 °C (Table 2). For high concentrations of Rh the activity was 

Rh cone wt % Dispersion H/Rh Surface area Rh, m Gasification Aromat 

The reactions for steam reforming of a methane and generic hydrocarbon into a mixture of hydrogen and carbon monoxide (syngas) are as follows  

Mole Percent Reformer Effluent Shifted Reformate 

Sulfur (S) is the most severe poison as it chemisorbs readily on any metal surface. A desulfurization step is carried out before the fuel is led into the reformer to avoid this poisoning. This is usually accomplished with a zinc oxide sulfur polisher and the possible use of a hydrodesulfurizer, if required. 

An application of the steam reforming reaction is the production of methane. Methane is obtained fairly readily from any hydrocarbon feedstock that can be vaporized, for example naphtha. Naphtha fractions with a final boiling point of less than 220 °C are generally considered suitable for catalytic steam reforming. In 1959 ICI started up the first large-scale pressure steam reformer using naphtha as a feedstock. 

Considering nonane as an example the reaction is C5H20 + 4H2O 7CH. -I- 2CO2 

Nanobinary (Mg-Al) and ternary metal (Ni-Mg-Al) oxy/hydroxides were synthesized by aerogel protocols from magnesium methoxide, aluminum isopropoxide and nickel acetylacetonate [19]. After supercritical drying the material obtained had 

After the DS step, the fuel is sent to a reformer. For steam reforming CH4 at 1100 K (827°C), the reaction is endothermic  

The AH without a superscript o refers to the value at the specified temperature, and the same is true for the following discussion. About 225.5 kJ of heat is needed to reform one mole of CH4. Therefore, a catalytic burner is needed (Pt/Mo and Pt/W are active catalysts). During the start of the reformer, the fuel for the burner will be CH4 itself, reacting with O2 according to Reaction 5.4. 

After the fuel cell system starts to run, some unreacted H2 will be purged from the anode, and it can be used to provide some heat for the burner. According to Reaction 5.5, 1 mole of H2 provides 248.4 kJ of heat at 1100 K, and about 0.91 (225.2/248.4 kJ mob ) moles of H2 will be needed to provide 225.5 kJ of heat. 

As presented in Chapter 3, if the reforming temperature is set at 1100 K, there will be 11.6% CH4 that is unconverted (without considering the impact of the water-gas shift reaction concurring in the reformer on the equilibrium of the reforming reaction). This amount of CH4 will be in the anode exhaust. If it can all be burned to provide heat for the burner, it can offer 93 kJ of heat (0.116 X 801.9 kJ mol ), and the remaining 132.5 kJ (225.5 - 93) of heat needed by the catalytic burner will come from 0.53 moles of H2 (132.5/248.4 kJ mob ) from the anode exhaust. 

In designing a reformer, people normally like to achieve a 100% conversion of the fuel to H2 in order to maximize the fuel utilization. In practice, due to the heat needed for the reforming process, this is not necessary. Provided the reaction rate is fast enough to meet the fuel cell peak power need, the temperature for reforming CH4 could be set at 1000 K (727°C), which will leave 

In another reactor carrying microstructured plates, a copper-based low temperature water-gas-shift catalyst was apphed [76]. The reactor took up 20 plates made of FeCr Al alloy with channel size 200 X100 pm. The kinetic measurements were carried out and expressions were determined for both a tubular fixed bed reactor containing 30 mg catalyst particles and the microreactor coated with the 

Current industrial production of hydrogen starts from methane, CH4, which is the main constituent of natural gas. A mixture of methane and water vapour at elevated temperature is undergoing the strongly endothermic reaction. 

The process is controlled by design of the reactor used for the reforming process, by input mixture (typical steam/methane ratios are 2 to 3, i.e. above the stoichiometric requirement) and, as mentioned, by reaction temperature and catalysts. Other reactions may take place in the reformer, such as the in- 

Industrial steam reformers typically use direct combustion of a fraction of the primary methane (although other heat sources could of course be used) to provide the heat required for the process (2.1), 

The elevated temperatures used in the reactor may damage the catalysts. Of particular importance is the possible carbon formation by methane cracking  

The shift reaction (2.2) employs additional sets of catalysts, traditionally Fe or Cr oxides, but in newer plants with staged-temperature processing these would be used only at the first step at around 400°C, while new catalysts such as Cu/Zn0/Al203 are used for a second low-temperature step, following heat removal for the recycling mentioned above. Carbon contamination at the catalysts can proceed via the Boudouard reaction (BasUe et ah, 2001), 

Although reaction in the right hand direction is favored by lower temperatures it is responsible for the initial carbon content of the raw synthesis gas. To maximize the hydrogen yield, this reaction is carried out in a separate step over a different catalyst at a lower temperature than the preceding gasification step (Section 4.2). 

As the nickel-containing catalysts used in the reforming reaction are sensitive to poisons, any sulfur compounds present in the hydrocarbon feedstock have to be removed by hydrodesulfurization, generally with a combination of cobalt - molybdenum and zinc oxide catalysts [413] - [415], (Eqs. 38, 39). In a few cases, especially with 

The general overall reaction for the steam reforming of hydrocarbons can be formulated as (40)  

Simultaneous with this equilibrium, the water gas shift reaction (Eq. 37) proceeds, and when this is included we arrive at the formal overall reaction (Eq. 42) 

Thermodynamics, Operation, Pressure, Steam/ Carbon Ratio 

Reaction 8.3, there are three molecules of hydrogen and one molecule of carbon monoxide produced for every molecule of methane reacted. Le ChateUer s principle therefore tells us that the equilibrium will be moved to the right (i.e. in favour of hydrogen) if the pressure in the reactor is kept low. Increasing the pressure will favour formation of methane, since moving to the left of the equilibrium reduces the number of molecules. The effect of pressure on the equilibrium position of the shift reaction (reaction 8.5) is very small. 

It is important to note at this stage that although the shift reaction 8.5 does occur at the same time as steam reforming, at the high temperatures needed for hydrogen generation, the equilibrium point for the reaction is well to the left of the equation. The result is that by no means will all the carbon monoxide be converted to carbon dioxide. For fuel cell systems that require low levels of CO, further processing will be required. This is explained in Section 8.4.9. 

It is important to realise also that steam reforming is not always endothermic. For example, in the case of steam reforming a petroleum hydrocarbon such as naphtha, with the empirical formula CH2 2, the reaction is most endothermic at the limit when the whole of the carbon is reformed to give oxides of carbon and hydrogen. This is the case when the reaction is carried out at relatively high temperatures. It is less endothermic and eventnally exothermic (liberates heat) as the temperature is lowered. This is because as the temperature is lowered, the reverse of reaction 8.1 becomes favoured, that is, a competing reaction, namely, the formation of methane, starts to dominate. This effect is illustrated in Table 8.7. 

Another type of reforming is known as dry reforming, or CO2 reforming, which can be carried out if there is no ready source of steam  

Hydrocarbons such as methane are not the only fuels suitable for steam reforming. Alcohols will also react in an oxygenolysis or steam reforming reaction, for example, methanol 

---

Steam reforming is, along with catalytic reforming, a process that can produce the additional hydrogen needed for upgrading and converting the heavy fractions of crude oil. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Steam reforming of CH is commonly carried out at 750 to 900°C, thus at the lower operating temperature of MCFCs a high activity catalyst is required. The internal reforming of methane in IRMCFCs, where the steam-reforming reaction... 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

Separation, combustion, pyrolysis, hydrogena-tion, anaerobic fermen-tation, aerobic fermen-tation, biophotolysis, partial oxidation, steam reforming, chemical hy-drolysis, enzyme hydrol-ysis, other chemical conversions, natural processes... 

Steam Reforming. When relatively light feedstocks, eg, naphthas having ca 180°C end boiling point and limited aromatic content, are available, high nickel content catalysts can be used to simultaneously conduct a variety of near-autothermic reactions. This results in the essentiaHy complete conversions of the feedstocks to methane ... 

In general, the proven technology to upgrade methane is via steam reforming to produce synthesis gas, CO + Such a gas mixture is clean and when converted to Hquids produces fuels substantially free of heteroatoms such as sulfur and nitrogen. Two commercial units utilizing the synthesis gas from natural gas technology in combination with novel downstream conversion processes have been commercialized. 

Coal gasification technology dates to the early nineteenth century but has been largely replaced by natural gas and oil. A more hydrogen-rich synthesis gas is produced at a lower capital investment. Steam reforming of natural gas is appHed widely on an iadustrial scale (9,10) and ia particular for the production of hydrogen (qv). 

J. R. Rostmp-Nielsen, Steam Reforming Catalysts Teknisk Fodag A/S, Copenhagen, 1975. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

Methane. The largest use of methane is for synthesis gas, a mixture of hydrogen and carbon monoxide. Synthesis gas, in turn, is the primary feed for the production of ammonia (qv) and methanol (qv). Synthesis gas is produced by steam reforming of methane over a nickel catalyst. 

Industrial. The main means of producing hydrogen industrially are steam reforming of hydrocarbons... 

Parameter Steam reforming (SR) Partial oxidation (POX) Texaco gasification (TG) Water electrolysis... 

Steam Reforming. In steam reforming, light hydrocarbon feeds ranging from natural gas to straight mn naphthas are converted to synthesis gas (H2, CO, CO2) by reaction with steam (qv) over a catalyst in a primary reformer furnace. This process is usually operated at 800—870°C and 2.17—2.86... 

Because hydrocarbon feeds for steam reforming should be free of sulfur, feed desulfurization is required ahead of the steam reformer (see Sulfur REMOVAL AND RECOVERY). As seen in Figure 1, the first desulfurization step usually consists of passing the sulfur-containing hydrocarbon feed at about 300—400°C over a Co—Mo catalyst in the presence of 2—5% H2 to convert organic sulfur compounds to H2S. As much as 25% H2 may be used if olefins... 

Fig. 1. Hydrogen production flow sheet, showing steam reforming, shift, hot potassium carbonate CO2 removal, and methanation.

---