Fig. 4.6 [1,49]. The diagram illustrates that CO2
reforming is more critical than steam reforming.
For steam to carbon ratios applied in hydrogen plants (H2O/CH4 > 1.5), there is no risk of carbon formation. The carbon limit curve can be moved to the left when using noble metal catalysts, because the noble metals show signif-icantly smaller equilibrium constants for decomposition of methane and carbon monoxide.
The principle of equilibrated gas is not a “law of nature,” but an empirical guideline. It is possible to exceed what may appear to be the thermodynamic limit. This can be done by sulfur-passivated catalyst as practiced in the SPARG process [49]. The principle of equili-brated gas predicts conditions where carbon
formation is expected (except for noble metals and SPARG). It does not guarantee that carbon is not formed if the principle predicts no poten-tial in the equilibrated gas. Methane may decompose to carbon instead of reacting with steam to form the required syngas even when there is no potential for carbon in the equili-brated gas. This is of course not possible in a closed system, but in an open system carbon may be stable at steady state and the accumula-tion of carbon may continue [1,10]. This is the situation for steam reforming of higher hydrocarbons.
4.3. STEAM REFORMING
coke). Thermodynamics predicts carbon forma-tion as long as the higher hydrocarbons are present. Carbon may be stable in a steady state in spite of the principle of equilibrated gas as defined above. The risk of carbon formation may be assessed by the critical steam to hydro-carbon (S/C) ratio [10,52]. This decreases with temperature and depends on the type of hydro-carbon and the type of catalyst. Olefins and aromatics form carbon more easily than paraffins [10]. Even traces of ethylene originating from thermal cracking in preheaters, dehydration of ethanol, or oxidative coupling of methane may be critical. Carbon formation is to be expected if the actual S/C ratio is lower than the critical ratio.
Pyrolytic coke is a high-temperature phe-nomenon resulting from thermal cracking of higher hydrocarbons [1,10]. Hence, the risk depends on the residence time/temperature exposure of the hydrocarbon in the equipment (the kinetic severity factor[32]). Pyrolytic coke may deposit on tube walls or encapsulate cata-lyst particles.
4.3.1.2. Effect of Promoters on Carbon Formation
The kinetic balance can be influenced by cata-lyst promoters [1]by enhancing the adsorption of steam or by blocking the step sites where carbon formation takes place. For example, steam adsorption is enhanced by potassium[10,52]as reflected by a negative reaction order with respect to water. The presence of alkali results in a signif-icant decrease of the activity reflected by a lower pre-exponential factor. This is not the case with active magnesia, which shows the promoting effect of enhanced steam adsorption (although smaller than that for alkali promotion), but without the loss of activity [1,10]. As a result, a Ni/MgO catalyst is able to process liquid hydro-carbons, even kerosene and diesel if properly desulfurized[50]. Similar promoter effects have been reported for La2O3and Ce2O3for CO2and steam reforming[43,53].
Another approach for inhibition of carbon formation is to retard the full dissociation of the hydrocarbon into adsorbed carbon atoms [1]. This was the explanation in a series of studies of the impact of a number of oxides such as La, Ce, Ti, Mo, and W [41,54,55].
Blockage of sites for nucleation of carbon was reported for the addition of Bi [56]or B[57]to nickel and for bimetallic catalysts such as Ni/
Au [58], Pt/Re [59], Pt/Sn [60], and Ni/Sn [61,62]. Ensemble control by means of sulfur passivation [48,49] will not work in a tubular reformer in the presence of higher hydrocarbons which will crack during heat up over the completely deactivated catalyst in the cold part of the tube.
4.3.1.3. Temperature Effects
Naphtha can be processed directly in the tubular reformer when using promoted cata-lysts [50,51,63], as practiced in many industrial units, but the control of the preheat tempera-ture and heat flux profile may be critical [51].
This is a severe constraint as the heat required in the tubular reformer (and hence the reformer costs) may be reduced by increasing preheat temperature. However, the preheater may then work as a “steam cracker” producing olefins from higher hydrocarbons in the feed [32]. The olefins easily form carbon in the reformer. Apart from the pressure, the condi-tions in the tubular steam reformer and in the preheater are not far from that of a steam cracker in an ethylene plant.
These constraints are removed when using an adiabatic pre-reformer [11,64] operating in the temperature range 350e550C. All higher hydrocarbons are converted to C1 components and hydrogen in the pre-reformer and the reforming and shift reactions are brought into equilibrium. Downstream of the pre-reformer it is possible to heat the gas to around 650C, thus reducing the size of the tubular reformer.
The pre-reforming catalyst is typically a highly active nickel catalyst. This catalyst also works
4. STEAM REFORMING FOR FUEL CELLS
60
as an effective sulfur guard for the tubular reformer and downstream catalysts, by removing any traces of sulfur still left after the desulfurization section.
However, the low-temperature operation may result in catalyst deactivation because of coke formation [65,66]. The adsorbed hydro-carbon species may slowly polymerize in competition with the CeC bond cleavage. If so, the nickel particles may slowly be encapsu-lated in a polymer film, deactivating the cata-lyst. The data in Fig. 4.7 show results from low-temperature steam reforming of heavy feedstock at adiabatic conditions [10,67]. The deactivation results in a change of the axial temperature profile. The deactivation is more pronounced with aromatic feed and may be depressed by hydrogen. In practice, coke forma-tion is overlapped by sulfur poisoning. If so, the catalyst life is related to the sulfur capacity of the pre-reforming catalyst.
With jet fuel (kerosene), the catalyst is gradu-ally poisoned by sulfur, resulting in a movement of the temperature profile. When operating on diesel, sulfur poisoning is accompanied by formation of a polymer film (gum), causing
a larger movement of the temperature profile.
Steam reforming of the sulfur-free and paraf-finic diesel from the FischereTropsch synthesis does not result in this type of deactivation.
4.3.2. Alcohols
4.3.2.1. Methanol
The thermodynamic constraints described for steam reforming disappear when methanol is used as feed [68]. The reaction takes place over a copper catalyst above 200C. This cata-lyst is not active for the methanation reaction.
This means that a methane-free gas can be produced at low temperatures and at high pres-sures and that full conversion to CO2(and CO) and hydrogen is achieved. The heat of reaction is less than for steam reforming of hydrocar-bons. In contrast, the use of nickel catalysts results in methane-rich gases [69,70] and an overall exothermic process.
Methanol reforming (decomposition) over Cu/Zn/Al catalysts [68,71,72] is a well-estab-lished technology [73], and is mainly used for small hydrogen plants (less than 1000 Nm3/h).
Since the amount of heat required per mole of hydrogen is far less than for steam reforming of natural gas, the equipment becomes much cheaper than the tubular reformer. On the other hand, the heat of evaporation on a mass basis of methanol is about four times higher than that of naphtha. The optimum choice of operating conditions [73] is around a steam to methanol ratio of 1.5 and a temperature in the range 250e300C. The pressure does not influence the reaction rate, but very high pressures limit the equilibrium conversion, which otherwise is above 99%. Like methanol, dimethyl ether (DME) is easily converted over Cu/Zn/Al cata-lysts [74,75] with little change in the layout of the plant[76].
The interest in fuel cells for automotive appli-cations has resulted in a large number of inves-tigations of reforming of methanol [72,77] for onboard reforming or for distributed units for FIGURE 4.7 Adiabatic pre-reforming of “logistic fuels.”
Temperature profiles from Topsoe RKNGR catalyst [67]
(Reproduced with permission of the authors).
STEAM REFORMING OF OTHER FEEDSTOCKS 61
hydrogen production. Compact units have been studied using microchannel reformer or plate for reformers [78,79] or a combination with selective hydrogen membranes [80]. Because copper catalysts are sensitive to air during shut-down and start-up of onboard reformers, there has been an interest in palladium catalysts for the methanol reforming process[81].
4.3.2.2. Ethanol
Ethanol has attracted interest as a feedstock for syngas production [82], thereby coupling biotechnology to classical catalysis. However, steam reforming of ethanol is not as simple as the conversion of methanol, because ethanol is easily dehydrated to ethylene, which is a coke precursor. Moreover, the reaction involves the breakage of a carbonecarbon bond. This means that copper catalysts are not suitable, as they are poor catalysts for hydrogenolysis. On the other hand, group VIII metals like nickel being active for hydrogenolysis are also active for carbon formation. Therefore, there are many attempts to identify catalysts with stable performance [82]. It appears that noble metal catalysts (Ru,Rh) [83,84], promoted Co catalysts [25,85], or bi-metallic catalysts such as Ni,Cu catalyst [86]look promising.
4.3.3. Other Oxygenates
Glycerol being a by-product from the manu-facture of biodiesel has been considered as a source for hydrogen by reforming [87].
Aqueous phase reforming (APR)[88]is a prom-ising route for converting oxygenated hydrocar-bons, such as simple alcohols, ethylene glycol, glycerol and sugars (sorbital glucose, etc.), into hydrogen and carbon dioxide. The reaction is
carried out at temperatures in the range 150e265C over a platinum catalyst or bime-tallic catalysts[88]including a non-noble metal catalyst, Ni,Sn[61].