We describe the operation of a laser cooled cesium fountain clock in the quantum limited regime. An ultrastable cryogenic sapphire oscillator is used to measure the short-term frequency stability of the fountain as a function of the number of detected atoms N at. For N at varying from 4 3 10 4 to 6 3 10 5 the Allan standard deviation of the frequency fluctuations is in excellent agreement with the N 21͞2 at law of atomic projection noise. With 6 3 10 5 atoms, the relative frequency stability is 4 3 10 214 t 21͞2 , where t is the integration time in seconds. This is the best short-term stability ever reported for primary frequency standards, a factor of 5 improvement over previous results. [S0031-9007(99)09299-6]

Two-staged biomass pyrolysis process consisting of torrefaction and subsequent fast pyrolysis is proposed to obtain high quality bio-oil. The purpose of this study is to evaluate the effect of torrefaction temperature on yield, composition, and physical properties of the liquid of torrefaction and bio-oil. Torrefaction of pine was conducted on an auger reactor at 240–320 °C with a residence time of 40 min to produce liquid of torrefaction and torrefied pine. Then, the torrefied pine was fast pyrolyzed in a bubbling fluidized bed reactor at 520 °C to produce bio-oil. Torrefied pine was characterized by chemical composition analysis and Fourier transform infrared (FTIR) spectroscopy. The liquid of torrefaction was determined by gas chromatography (GC); bio-oil was characterized by gas chromatography mass spectroscopy (GC-MS) and 13C nuclear magnetic resonance spectrometry (NMR). The experimental results show that water content in liquid of torrefaction and water and acetic acid content in bio-oil decreased with elevated torrefaction temperature, while the yield of liquid of torrefaction, aromaticity, higher heating value, and density of bio-oil increased. However, the yield of total liquid (sum of liquid of torrefaction and bio-oil) decreased significantly with elevated torrefaction temperature because of the cross-linking and carbonization of torrefied pine.

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