Optimization of a Scroll Expander Applied to an Ammonia/Water Combined Cycle System for Hydrogen Production - Paper No. 1645

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Optimization of a Scroll Expander Applied to an Ammonia/Water Combined Cycle System for Hydrogen Production - Paper No. 1645 ( optimization-scroll-expander-applied-an-ammonia-water-combin )

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T temperature [°R or °F] V volts [V] or volumetric flow rate [cfm] X ammonia mass fraction g vapor h enthalpy [Btu/lbm] or hour [hr] m mass flow rate [lbm/hr] v specific volume [ft3/lbm] w specific work [kW/lbm] x mass flow ratio y liquid yield ratio z nitrogen requirement ratio β coefficient of thermal expansion ε heat exchanger effectiveness η efficiency μJT Joule-Thompson expansion coefficient μs isentropic expansion coefficient ρ density [lbm/ft3] π rotational speed [rad/s] Subscripts CW cooling water H2 hydrogen NH3 ammonia vapor P isobaric or pump T isothermal ab absorber act actual ad adiabatic c compressor cool cooling load e expander f liquid g electric generator h isenthalpic in expander gas inlet max maximum min minimum o standard conditions opt optimum out expander gas outlet rect rectifier s isentropic shaft expander pulley shaft strong high ammonia concentration stream th thermoneutral v volumetric vg vapor generator 4. THEORETICAL BACKGROUND Hydrogen is the simplest, most abundant element in the universe comprising 75% of all visible matter by mass (Flynn, 1997). Currently, the majority of the hydrogen produced in the U.S. is used as a chemical in a variety of commercial applications including ammonia production, hydrogenation of fats and oils, and methanol production (National Hydrogen Association, 2004). Hydrogen has several characteristics that make it a desirable alternative fuel for transportation: • • • • • Highest energy content per unit mass of any known fuel (51,574 Btu/lbm) hydrogen produces 2.7 times more energy per unit mass than gasoline when burned. Clean – combustion of hydrogen produces no carbon dioxide or sulfur emissions. When burned with oxygen, the only byproducts are water and heat. If burned in air, nitrogen oxides may be produced. Renewable – hydrogen can be produced by a variety of methods using renewable energy sources for a virtually limitless and pollution free fuel supply. Technologically compatible – in the 1920s, German engineer Rudolf Erren successfully converted IC engines to hydrogen burning engines (National Hydrogen Association, 2004). Hydrogen can also be reacted with oxygen in a fuel cell to produce electricity to drive a motor. Efficient utilization – hydrogen IC engines are about 25% efficient, fuel cells are 45-60% efficient; typical gasoline IC engines are 18-20% efficient (National Hydrogen Association, 2004). Hydrogen fuel cell powered vehicles can be up to three times more efficient than today’s gasoline engines. The U.S. currently produces 9 million tons or 3.2 trillion cubic feet (90 billion Nm3) of hydrogen per year. Of this amount, 95% is produced by steam/methane reformation (SMR) (National Hydrogen Association, 2004). SMR operates by reacting a natural gas feedstock with steam at high temperatures (700 – 925 °C) to produce carbon monoxide and hydrogen. The carbon monoxide is then consumed in a water/gas shift reaction to create CO2 and additional hydrogen. Other hydrogen production methods are outlined in Figure 4.1. Figure 4.1 Hydrogen production technologies by energy source

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