Ammonia fuel requirements

This paper analyzes the sources of hydrogen for ammonia production, presents the feed and fuel requirements of the natural gas steam reforming process, estimates the relative economics of alternate feedstocks and briefly discusses the outlook for the ammonia industry. 

Current Ammonia Plant Feed And Fuel Requirements... 

Utilizing amino acids as fuel requires eventual elimination of an equimolar amount of ammonia, which may also require eliminating more water. It also requires a longer recovery time, since replacement of the hydrolyzed proteins may be slow, and requires higher dietary protein intake. In most circumstances, protein is the macronutrient in least... 

In nearly all ammonia plants the same material is used as both feedstock and fuel. The fuel requirements may be 40% of the total or mcwe, depending on the ext t to which heat recovery equipment is used. In previous years when fuel was inexpensive, many ammonia plante were built with minimum heat recovery facilities. Buividas et al. give an example of how the fuel requirement (natural gas) was decreased by 34% through more efficient energy use, which primarily included high-pressure steam generation and preheating combustion air to the reformers I7I, The decrease in total fuel plus feedstock reqiure-ment was about 15%. The increase in fuel efficiency was obtained at the expense of about 6% increase in plant investment costs. Table 6.8 shows the requirements for fuel plus feedstock that assume efficient heat recovery. 

In a natural gas-based plant 20%-30% of the gas is used for fuel and the balance for feedstock. The lower fuel values are for plants equipped with good energy-recovery systems. Fuel requirements do not include electric power generation or steam generation other than that connected with heat recovery. Modern ammonia... 

Fuel Cell and Fuel Processor Catalyst Tolerance There are major fuel requirements for the gas reformates that must be addressed. These requirements result from the effects of sulfur, carbon monoxide, and carbon deposition on the fuel cell catalyst. The activity of catalysts for steam reforming and autothermal reforming can be affected by sulfur poisoning and coke formation this commonly occurs with most fuels used in fuel cells of present interest. Other fuel constituents can also prove detrimental to various fuel cells. Examples of these are halides, hydrogen chloride, and ammonia. 

Site-Specific Condderations. The proper specification of an SCR system requires consideration of site specific information. This information includes type of fuel, flue gas flow rate and temperature, initial NO, concentrations, ash quantity and characteristics, SO2 and SO3 concentrations in the flue gas, water vapor concentration, excess oxygen concentration, plant mode of opieration, emission compliance requirements, catalyst life requirements, ammonia slip requirements, and data on the existing equipment in the flue gas train. 

High temperature steam reforming of natural gas accounts for 97% of the hydrogen used for ammonia synthesis in the United States. Hydrogen requirement for ammonia synthesis is about 336 m /t of ammonia produced for a typical 1000 t/d ammonia plant. The near-term demand for ammonia remains stagnant. Methanol production requires 560 m of hydrogen for each ton produced, based on a 2500-t/d methanol plant. Methanol demand is expected to increase in response to an increased use of the fuel—oxygenate methyl /-butyl ether (MTBE). 

Alternative approaches to nitric oxide formation include irradiation of air in a nuclear reactor (72) and the oxidation of ammonia to nitric oxide in a fuel cell generating energy (73). Both methods indicate some potential for commercial appHcation but require further study and development. 

When this reaction was first discovered, a considerably higher (ca 1300°C) temperature was required than that used in the 1990s. Thus, until Haber discovered the appropriate catalyst, this process was not commercially attractive. As of this writing (ca 1995), the process suffers from the requirement for significant quantities of nonrenewable fossil fuels. Although ammonia itself is commonly used as a fertilizer in the United States, elsewhere the ammonia is often converted into soHd or Hquid fertilizers, such as urea (qv), ammonium nitrate or sulfate, and various solutions (see Ammonium COMPOUNDS). 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

The enzyme systems responsible for fixing atmospheric N2 to form ammonia are known as the nitrogenases. These enzymes function at field temperatures and 0.8 atm N2 pressure, whereas the industrial Haber-Bosch process requires high temperatures (300-400°C) and high pressures (200-300 atm) in a capital-intensive process that relies on burning fossil fuel. Small wonder, then, that the chemistry of the nitrogenases has attracted considerable attention for many years. 

As world deposits of petroleum and coal are exhausted, new sources of hydrogen will have to be developed for use as a fuel and in the production of ammonia for fertilizer. At present, most hydrogen gas is produced from hydrocarbons, but hydrogen gas can also be generated by the electrolysis of water. Figure 19-23 shows an electrolytic cell set up to decompose water. Two platinum electrodes are dipped in a dilute solution of sulfuric acid. The cell requires just one compartment because hydrogen and oxygen escape from the cell much more rapidly than they react with each other. 

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