HYDROGEN FUEL FOR FOOD SERVICE

摘要

Hydrogen-gas fuel supplied via compressed source or a metal-hydride was demonstrated for use in food service applications. Average energy efficiencies of 33-49% and instantaneous values up to 58% were achieved with a retrofitted Army M2 field kitchen burner. The average usable heat (efficiency x heat supplied) for these experiments ranged from 0.8 kW at the lower efficiencies to 3.6 kW at the higher efficiencies (Back, 1993). This compares to commercial propane or gasoline burners which are documented or measured to perform at 10%-25% fuel efficiency with up to 4.4 kW (15,000 BTU/hr) of usable heat. An efficiently designed metal-hydride based system could also be used for refrigeration during the endothennic hydrogen-desorption process with burner operation, for example, it is estimated that 0.5 kW-1.0 kW of refrigeration energy would be available. Although weight penalties are always an issue when using metal hydrides, other beneficial factors such as safety, efficiency, auxiliary cooling, availability through electrolysis, and clean (i.e., water) byproducts must also be considered. These additional factors generally favor the use of hydrogen versus standard food-applications fuels such as propane, natural gas, and gasoline.The use of hydrogen-fuel for food service is a viable application in a hydrogen-based economy. The advantages of hydrogen fuel include: (1) hydrogen can be stored safely, (2) it is non-polluting with water being the primary combustion product, (3) hydrogen has a high energy density, (4) the fuel-to-air ratio is high relative to other fuels so that convective heat losses from nitrogen is minimized, (5) metal hydrides can be reused and hydrogen is a replenishable fuel via H_2O electrolysis, and (6) large endothennic heats of desorption from metal hydrides can be utilized for cooling.This particular effort involved the redesign of burner nozzles and aspirators for hydrogen use, taking into account safety issues pertaining to hydrogen volatility, combustion, and flame propagation velocities. The fuel-use efficiency of the systemwas then optimized using Taguchi experimental design techniques with compressed hydrogen as fuel. The experiments revealed that the burner efficiency is highly dependent upon the rate of heat supplied to the heated object (e.g., a pot filled with water), the distance between the burner head/flame and the pot, and the quantity of air aspirated into the hydrogen flow stream. The final system was then demonstrated with a LaNi_5 hydride hydrogen-supply vessel. Further optimization of a hydrogen-fueled burner would require the redesign of the burner head and pot geometry so as to: (1) minimize convective and radiation heat losses between the burner head and the heated object, (2) maximize the heat transfer area, (3) optimize the flame flow rate around the heated object to increase the heat-transfer-limiting gas-side heat transfer coefficient without overpowering the heat transfer capability of the burner/pot system, and (4) maximize the hydrogen-to-air ratio to provide complete combustion of hydrogen.