Reformate undesirable gases

That are the water gas shift (WGS) reaction [Eq. (3.4)], which is very important when fuel ricii in CO is fed the combustion reactions [Eqs. (3.5) and (3.6)] due to an undesired gas cross-over from one electrode side to the other, and finally the methane steam reforming [Eq. (3.7)], which can provide the hydrogen when the fuel contains methane. 

The presence of tars in the product gas is highly undesirable in synthesis gas for hydrogen applications. Tar formation represents a reduction in gasification efficiency since less of the biomass is converted to a fuel or synthesis gas. More importantly, tars would degrade the performance of those systems. Tars can deactivate reforming catalysts, and fuel cell toleration of tars is low. 

Low-temperature (<500 °C) reforming technologies are also under investigation. The advantages of low-temperature technologies are reduced energy intensity, compatibility with membrane separation, favorable conditions for the water-gas shift reaction, and minimization of undesirable decomposition reactions typically encountered when carbohydrates are heated to high temperatures [44]. 

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

Methanol synthesis from C02 (Equation [1]) and CO (Equation [2]) is mildly exothermic and results in volumetric contraction. Methanol steam reforming (MSR) refers to the inverse of reaction (1), and the inverse of reaction (2) is conventionally referred to as methanol decomposition - an undesired side reaction to MSR. The slightly endothermic reverse water-gas shift (rWGS) reaction (Equation [3]) occurs as a side reaction to methanol synthesis and MSR. According to Le Chatelier s principle, high pressures and low temperatures would favor methanol synthesis, whereas the opposite set of conditions would favor MSR and methanol decomposition. It should be noted that any two of the three reactions are linearly independent and therefore sufficient in describing the compositions of equilibrated mixtures. 

Indirect processes for converting natural gas to alcohols and higher hydrocarbons require the initial conversion of methane to synthesis gas (CO/H2). This is a difficult and expensive step normally carried out by steam reforming and partial oxidation (6). Subsequent synthesis gas conversion steps, such as FT synthesis and related processes (1,2), must occur with high selectivity to desired products in order to minimize extensive recycle of undesired products to the initial synthesis gas generation step. C5+ paraffins, low- and intermediate-molecular-weight olefins, and C20+ linear hydrocarbons provide useful feeds in downstream processes leading to fuels and petrochemicals. 

The concept that the removal of an undesired reaction product by selective adsorption from the reaction zone of an equilibrium-controlled reaction increases the conversion and the rate of formation of the desired component (based on Le Chatelier s principle) was used to develop a novel PSA process concept called SERP for direct production of fuel cell-grade hydrogen by steam reforming of methane (CH4 + 2H20 44 C02 + 4H2).57 61 The concept uses a physical admixture of a reforming (noble metal on alumina) catalyst and a chemisorbent (K2C03 promoted hydrotalcite), which selectively and reversibly chemisorbs C02 from a gas at a temperature of -450 °C in the presence of steam. The cyclic SERP steps consisted of the following ... 

The proportions of the two reforming reactions and shift conversion are so controlled that the gas mixture obtained contains nitrogen and hydrogen in the mole ratio (volume ratio) of 1 3. However, this mixture still contains 20-30% carbon dioxide resulting from the shift conversion reaction and traces of unconverted carbon monoxide. Carbon dioxide can yield carbonates and carbamates in the ammonia synthesis cycle, which are undesirable because they can deposit in the piping. In addition oxygen, and any of its compounds such as carbon monoxide, water, etc., are also ammonia catalyst poisons [13]. Consequently they must be removed. 

Fig. 8 shows a typical installation of a prereformer, The exit gas from the prereformer can be further preheated to up to 700°C with no risk of pyrolysis or other undesirable reactions such as methane decomposition. In this way, it is possible to replace part of the fired duty of the reformer furnace by external preheating, thus reducing the size of a tubular reformer. The advantages are illustrated in Table 4 comparing advanced reforming of natural gas with the state of the art in the 80 ties for CO-plant reformers. 

Catalyst deactivation is a common pathological phenomenon in many industrial reactions. In the case of hydrocarbon steam reforming to produce synthesis gas, catalyst activity loss may be due to coke arising from carbon deposition. Carbon lay-down usually occurs via undesired side reaction, namely ... 

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